Patent Publication Number: US-2019175032-A1

Title: Physiological sensor delivery device and method

Description:
RELATED APPLICATION 
     This application is a divisional of, and claims priority to, U.S. patent application Ser. No. 14/300,654, filed Jun. 10, 2014, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This application relates generally to the field of medical device technology and, more particularly, to devices and methods for positioning and utilizing physiological sensors in anatomical (e.g., vascular) structures of patients, such as in blood vessels or across heart valves. 
     BACKGROUND 
     Certain physiological measurements may be made by positioning a sensor within a patient. Such physiological measurements may include, for example, measurements of blood parameters, such as blood pressure, oxygen saturation levels, blood pH, etc. Some such measurements may have diagnostic value and/or may form the basis for therapy decisions. 
     A technique for evaluating the degree to which a stenotic lesion obstructs flow through a blood vessel is called the Fractional Flow Reserve measurement (FFR). To calculate the FFR for a given stenosis, two blood pressure readings are taken. One pressure reading is taken on the distal side of the stenosis (e.g., downstream from the stenosis), the other pressure reading is taken on the proximal side of the stenosis (e.g., upstream from the stenosis, towards the aorta). The FFR is defined as the ratio of maximal blood flow in a stenotic artery, taken distal to the lesion, to normal maximal flow, and is typically calculated based on a measured pressure gradient of the distal pressure to the proximal pressure. The FFR is therefore a unitless ratio of the distal and proximal pressures. The pressure gradient, or pressure drop, across a stenotic lesion is an indicator of the severity of the stenosis, and the FFR is a useful tool in assessing the pressure drop. The more restrictive the stenosis is, the greater the pressure drop, and the lower the resulting FFR. The FFR measurement may be a useful diagnostic tool. For example, clinical studies have shown that an FFR of less than about 0.75 may be a useful criterion on which to base certain therapy decisions. Pijls, DeBruyne et al.,  Measurement of Fractional Flow Reserve to Assess the Functional Severity of Coronary Artery Stenoses,  334:1703-1708, New England Journal of Medicine, Jun. 27, 1996. A physician might decide, for example, to perform an interventional procedure (e.g., angioplasty or stent placement) when the FFR for a given stenotic lesion is below 0.75, and may decide to forego such treatment for lesions where the FFR is above 0.75. Thus, the FFR measurement could become a decision point for guiding treatment decisions. 
     One method of measuring the pressure gradient across a lesion is to use a small catheter connected to a blood pressure measurement sensor. The catheter would be passed over the guidewire which has already been placed across the lesion. The catheter would be advanced down the guidewire until the tip of the catheter crosses the lesion. The blood pressure on the distal side of the lesion is recorded. This pressure would be divided by the pressure value recorded in the aorta. A disadvantage of using this method is that some error may be introduced due to the cross sectional size of the catheter. As the catheter crosses the lesion, the catheter itself introduces blockage, in addition to that caused by the lesion itself. The measured distal pressure would therefore be somewhat lower than it would be without the additional flow obstruction, which may exaggerate the measured pressure gradient across the lesion. 
     Pressure drop can also be measured across a heart valve. When a heart valve is regurgitant, a less than optimal pressure drop is typically observed. Using a catheter to measure pressure drop is common across a heart valve. However, because of the catheter size, the heart valve may not seal well around the catheter. Leakage might also result from the presence of the catheter and may contribute to an inaccurate pressure drop reading. One example of where this could occur is in the mitral valve (e.g., mitral valve regurgitation). 
     One method of measuring blood pressure in a patient is to use a pressure sensing guidewire. Such a device has a pressure sensor embedded within the guidewire itself. A pressure sensing guidewire could be used in the deployment of interventional devices such as angioplasty balloons or stents. Prior to the intervention, the pressure sensing guidewire would be deployed across a stenotic lesion so the sensing element is on the distal side of the lesion and the distal blood pressure is recorded. The guidewire may then be retracted so the sensing element is on the proximal side of the lesion. The pressure gradient across the stenosis and the resulting FFR value could then be calculated. 
     To use a guidewire-based pressure sensor in certain applications, the guidewire must be repositioned so the sensing element of the guidewire is correctly placed with respect to a stenotic lesion, for example. Blood pressure measurements for calculating FFR, for example, are generally taken on both sides of a given stenosis, so the guidewire is typically retracted across the stenosis to make the upstream measurement. After retracting the guidewire to make the proximal pressure measurement (aortic pressure or upstream coronary pressure), the guidewire may again be repositioned downstream of the lesion, for example, if it is determined (e.g., based on the FFR calculation) that an interventional device should be deployed. In cases where there are multiple lesions, the sensing element of a pressure sensing guidewire would need to be advanced and retracted across multiple lesions, and would potentially have to be advanced and repositioned again for each such lesion. Advancing and maneuvering a pressure sensing guidewire though stenotic lesions and the vasculature, for example, can be a difficult and/or time consuming task. 
     Physician preference is another factor that may influence the choice of diagnostic tools or techniques used for certain applications. For example, some physicians may tend to become accustomed to using certain specific guidewires for certain applications. “Standard” (e.g., commercially available) medical guidewires may vary in size, flexibility, and torque characteristics. A physician may prefer to use different guidewires for different tasks, for example, to access hard-to-reach anatomical areas, or when encountering bifurcations in arteries. Certain guidewires may therefore be better suited for specific tasks because of the torque and flexing characteristics, and a physician may display a strong preference for using a certain guidewire based on the specific task (or tasks) he or she is facing. A pressure sensing guidewire may have torque and flexing characteristics that are either unknown to the physician, or that are unsuitable for a particular task, because such a guidewire is specifically constructed to have a pressure sensor incorporated as part of the guidewire itself. As a result, a physician may find it difficult to maneuver a pressure sensing guidewire into an anatomical location of interest, as compared to a “standard” (e.g., non-pressure sensing) medical guidewire. 
     Having grown accustomed to the handling characteristics of a particular non-pressure sensing guidewire, a physician may be reluctant to employ a pressure sensing guidewire, which may increase the time and difficulty of positioning and repositioning the pressure sensing guidewire across a stenotic lesion, for example. In such cases, a physician may choose to forego the benefit of a diagnostic measurement, such as FFR, and simply choose to deploy some form of interventional therapy as a conservative approach to such decisions. If the diagnostic measurement techniques and the associated devices were simple enough to use, more physicians would use them and thereby make better therapy decisions. 
     SUMMARY 
     Physiological sensor delivery devices and methods according to embodiments of the invention may be used in diagnostic applications, such as cardiovascular procedures in coronary arteries, interventional radiology applications in peripheral arteries, and structural heart applications in heart valves. 
     In some embodiments, the methods include measuring FFR in a patient using a sensor delivery device. The sensor deliver device may include a distal sleeve, a distal sensor located on the distal sleeve, a proximal sensor located on the distal sleeve and proximal to the distal sensor, a proximal portion, and a communication channel, the sensors adapted to generate signals proportional to fluid pressure. The method may include advancing a guidewire through a patient&#39;s vasculature to a location of interest in a patient, advancing a guiding catheter over the guidewire to the location of interest, advancing a sensor delivery device within the guiding catheter to the location of interest, advancing only a distal portion of the distal sleeve outside of the guiding catheter such that the distal sensor is outside of the guiding catheter and downstream of the location of interest and the proximal sensor is inside of the guiding catheter, measuring a distal fluid pressure with the distal sensor distal to the location of interest, measuring a reference fluid pressure with the proximal sensor inside of the guiding catheter, and calculating FFR using the measured distal fluid pressure and the reference fluid pressure. The FFR may be calculated as a ratio of the sensor signal to the fluid pressure signal. The method may further include making a therapy decision based on the calculated FFR being below a threshold value. For example, the therapy decision may be selecting an interventional therapy if the calculated FFR is less than about 0.75. In some embodiments, the method further includes withdrawing the device and deploying an interventional therapy device using the same guidewire used to position the sensor downstream of the location of interest. 
     In some embodiments, the steps of measuring a distal fluid pressure with the distal sensor distal to the location of interest and of measuring a reference fluid pressure with the proximal sensor inside of the guiding catheter are performed after the step of advancing only a distal portion of the distal sleeve, without advancing or retracting the distal sleeve. In some embodiments, the method also includes normalizing the sensor signal to the fluid pressure signal prior to positioning the sensor downstream of the location of interest. 
     In some embodiments, one or both of the proximal and sensors has a low thermal coefficient. In some embodiments, only one of the proximal and distal sensors has a low thermal coefficient, and the method includes, before advancing the sensor delivery device to the location of interest, calibrating the sensor having a low thermal coefficient to atmospheric pressure, and after advancing the sensor delivery device to the location of interest and before measuring a distal fluid pressure or a reference fluid pressure, calibrating the sensor which is does not have a low thermal coefficient to be equal to the sensor having a low thermal coefficient. 
     In other embodiments, the method includes calibrating a low thermal coefficient sensor of a sensor delivery device to atmospheric pressure while located outside of the patient, wherein the sensor delivery device comprises the low thermal coefficient sensor and a non-low thermal coefficient sensor, advancing the sensor delivery device to a location of interest within the patient&#39;s body, equilibrating the non-low thermal coefficient sensor to the low thermal coefficient sensor, after equilibrating the sensors, positioning one of the sensors distal to the location of interest and measuring a distal pressure and measuring a proximal pressure with the other of the sensors, and calculating FFR using the measured distal fluid pressure and the reference fluid pressure. The method may further include measuring the proximal pressure while the sensor is located within a guiding catheter. 
     In some embodiments, the device includes a sensor delivery device including a distal sleeve configured to be advanced through a patient&#39;s vasculature over a guidewire, the distal sleeve including a distal sensor located on the distal sleeve, a proximal sensor located on the distal sleeve and proximal to the distal sensor, a proximal portion, and a communication channel, the sensors adapted to generate signals proportional to fluid pressure. One or both of the distal sensor and the proximal sensor may be comprised of a material having a low thermal coefficient. For example, the thermal coefficient of the material may be less than or equal to approximately 0.1 mmHg/° C. The thermal coefficient of the material of the sensor allows the sensor to be calibrated to atmospheric pressure outside of a patient&#39;s body at approximately room temperature and to remain calibrated to atmospheric pressure after insertion into a patient&#39;s body. In some embodiments, both the proximal and distal sensors are comprised of material having a low thermal coefficient. The device may further include a guidewire and/or a guiding catheter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a sensor delivery device according to an embodiment of the invention; 
         FIG. 2  is a conceptual perspective view of a sensor delivery device for making physiological measurements according to an embodiment of the invention; 
         FIG. 3  is an conceptual plot of a patient&#39;s blood pressure as a function of time; 
         FIG. 4( a )  is a side view of a sensor delivery device according to an embodiment of the invention having one or more flow holes disposed along a side portion; 
         FIG. 4( b )  is a cross-sectional view of a sensor delivery device according to an embodiment having one or more flow holes; 
         FIG. 5( a )  is a cut-away side view of a sensor delivery device with a sensor housing according to one embodiment of the invention; 
         FIG. 5( b )  is a cut-away side view of a sensor delivery device with a sensor housing according to one embodiment of the invention; 
         FIGS. 5( c ) and 5( d )  are side views of a sensor delivery device with radiopaque marker band according to certain embodiments of the invention; 
         FIG. 5( e )  is a cut-away side view of a sensor delivery device with a strain relief spacer according to one embodiment of the invention; 
         FIGS. 6( a )-6( g )  are enlarged side views of a distal transition of a sensor delivery device according to certain embodiments of the invention; 
         FIGS. 7( a ) and 7( b )  are perspective views of a sensor delivery device having a second sensor disposed on a proximal sleeve according to an embodiment of the invention; 
         FIG. 8  is a perspective view of a sensor delivery device having a furcation tube according to an embodiment of the invention; 
         FIG. 9  is a cross-sectional side view of a sensor delivery device having a dual lumen configuration according to one embodiment of the invention; 
         FIG. 10( a )-( c )  is a side view of a sensor delivery device having an over-the-wire configuration according to one embodiment of the invention; 
         FIG. 11  is a flow diagram showing a method of using a sensor delivery device according to certain embodiments of the invention; 
         FIG. 12  is a perspective view of a fluid injection system that may be used to interact with a sensor delivery device according to an embodiment of the invention; 
         FIG. 13  is a perspective view of a fluid injection system that may be used to interact with a sensor delivery device according to an embodiment of the invention; 
         FIG. 14  is a flow diagram of a method of using a sensor delivery device in conjunction with a fluid injection system according to certain embodiments of the invention; 
         FIG. 15  is a flow diagram of a method of using a sensor delivery device according to an embodiment of the invention; 
         FIG. 16  is a perspective view of a powered injection system adapted to be coupled to a physiological sensor delivery device according to certain embodiments of the invention; and 
         FIG. 17  is an idealized view of a user interface screen containing information that may be displayed to an operator, according to certain embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description should be read with reference to the accompanying drawings, in which like numerals denote like elements. The drawings, which are not necessarily to scale, depict selected embodiments of the invention—other possible embodiments may become readily apparent to those of ordinary skill in the art with the benefit of these teachings. Thus, the embodiments shown in the accompanying drawings and described below are provided for illustrative purposes, and are not intended to limit the scope of the invention as defined in the claims appended hereto. 
     An example of a sensor delivery device according to certain embodiments of the invention is shown in  FIG. 1 . The sensor delivery device  10  of  FIG. 1  includes a distal sleeve  20  having a guidewire lumen  22  for slidably receiving a medical guidewire  30 . A sensor  40  is coupled to the distal sleeve  20 , sensor  40  being capable of sensing and/or measuring a physiological parameter of a patient and generating a signal representative of the physiological parameter. Thus, the distal sleeve  20 , and hence, the sensor  40 , may be positioned within a patient (e.g., within an anatomical structure of a patient, such as within a vein, artery, or other blood vessel, or across a heart valve, for example) by causing the distal sleeve  20  to slide over the medical guidewire  30  to the desired position. 
     The sensor delivery device  10  of  FIG. 1  also includes a proximal portion  50 , which is coupled to the distal sleeve  20 . The proximal portion  50  includes a communication channel  60  for communicating the signal from the sensor  40  to a location outside of the patient (e.g., to a processor, display, computer, monitor, or to another medical device). Communication channel  60  may comprise a fiber optic communication channel in certain preferred embodiments, such as where the sensor  40  is a fiber optic pressure sensor. Alternately, communication channel  60  may comprise an electrically conductive medium, such as one or more electrical conducting wires. Of course, many other forms of communication media may be suitable for transmitting the signal generated by sensor  40  to a location outside of the patient. In some embodiments of the invention, the communication channel  60  may comprise any of a variety of fluid and/or non-fluid communication media, such as a wireless communication link, or an infrared capability, or acoustic communications such as ultrasound, as possible examples. 
     The proximal portion  50  is also adapted to assist an operator (e.g., a physician or other medical staff) in positioning the distal sleeve  20  and the sensor  40  within an anatomical (e.g., vascular) structure of the patient. This is typically accomplished by an operator first inserting a “standard” medical guidewire  30  into a patient&#39;s vasculature and advancing it past an area of interest. The sensor delivery device  10  is then deployed by “threading” the distal sleeve  20  onto the guidewire  30  such that the lumen  22  slides over the guidewire  30 , and advancing the distal sleeve  20  (and the associated sensor  40 ) by moving (e.g., pushing and/or pulling) the proximal portion  50  until sensor  40  is in the desired location. 
     The device  10  and the guidewire  30  are typically manipulated inside a guiding catheter  32 , which has been placed in the anatomical (e.g., vascular) structure of interest. In certain preferred embodiments of the invention, the guidewire lumen  22  may be sized to slide over “standard” sized medical guidewires. For example, a number of manufacturers make medical guidewires that range in size from less than about 0.014 inches outer diameter to more than about 0.038 inches outer diameter, typically having a finite number of common sizes within this range. “Standard” size medical guidewires might, for example, have outer diameters of 0.010, 0.014, 0.018, 0.021, 0.025, 0.028, 0.032, 0.035, and 0.038 inches. Thus, in certain preferred embodiments of the invention, the guidewire lumen  22  may be sized appropriately to slide over a particular standard size medical guidewire. A device according to preferred embodiments of the invention may therefore be made available in a range of sizes corresponding to standard medical guidewire sizes. 
     One potential advantage of a sensor delivery device  10  according to embodiments of the invention is that it allows a physician to use the guidewire of their choice. Sensor delivery device  10  can be sized to be used with any guidewire. The physician may, for example, choose a particular guidewire based on its unique flexing and torque characteristics for certain procedures. Delivery device  10  according to various embodiments of the invention provides the physician with the ability to use whichever guidewire is deemed best suited for the particular application. 
     Another potential advantage of the sensor delivery device  10  is that it does not require repositioning of the guidewire in order to make sensor readings. Once the guidewire has been positioned across a stenotic lesion, for example, the sensor delivery device  10  can be positioned (e.g., advanced and/or retracted) over the guidewire and the sensor  40  can therefore be advanced and retracted across lesions to make pressure readings, for example, without moving the guidewire. A physician may also save time by not having to reposition the guidewire across the lesion or lesions to make such measurements. 
     In the example shown in  FIG. 1 , the device  10  is being deployed using guiding catheter  32 , which has been placed within a vascular structure of interest (in this example, blood vessel  34 , which could be, for example, a coronary artery of the patient). In certain embodiments of the invention, the size or “footprint” (e.g., the width and/or the cross-sectional area) of device  10  may allow it to fit within certain standard sized guiding catheters. For example, in certain diagnostic applications, it would be desirable to have device  10  deployed within a certain sized guiding catheter (e.g., smaller than about 4 or 5 French (FR)). 
     In certain embodiments of the invention, the distal sleeve  20  of the device may be substantially concentric with the guidewire  30 . The coupling of the proximal portion  50  to the distal sleeve  20  allows the guidewire  30  to separate from the rest of device  10  (e.g., in what is sometimes referred to as a “monorail” catheter configuration); this would typically occur inside the guiding catheter  32 . The guidewire  30  and device  10  would both exit the patient at the proximal end of the guiding catheter  32  as separate devices. Having the device  10  and guidewire  30  separate allows the physician to independently control device  10  and guidewire  30 , as necessary. It may also allow a physician to use a shorter guidewire for catheter exchange. For example, a monorail-type configuration may allow for the use of a guidewire that is approximately 170 to 200 cm long, whereas an “over-the-wire” configuration might require the use of a much longer (e.g., up to 300 cm or more) guidewire. Having the device  10  and guidewire  30  separate (except at the distal sleeve  20 ) may also result in less friction (e.g., within the guiding catheter  32 ) than if the device  10  and guidewire  30  had to be moved together as a unit. In some embodiments, a hydrophilic coating may be applied to various portions of the device to further reduce the amount of friction encountered, for example, when advancing or retracting device  10 . 
     One diagnostic application in which various embodiments of the invention may be well-suited is the measurement of Fractional Flow Reserve (FFR). As noted above, the FFR measurement quantifies the degree to which a stenotic lesion, for example, obstructs flow through a blood vessel. To calculate the FFR for a given stenosis, two blood pressure measurements are needed: one pressure reading is taken on the distal side of the stenosis (downstream side), the other pressure reading is taken on the proximal side of the stenosis (upstream side). The FFR is therefore a unitless ratio of the distal pressure to the proximal pressure. The pressure gradient across a stenotic lesion is an indicator of the severity of the stenosis. The more restrictive the stenosis is, the more the pressure drop, and the lower the FFR. 
     To add clarity and context to the disclosure, several embodiments of the invention will now be described below in the context of making FFR measurements. However, it should be realized that there are other applications in which physiological parameter measurements could be facilitated with the devices and/or methods described herein. 
       FIG. 2  is a perspective view of a sensor delivery device for measuring a physiological parameter in a patient according to an embodiment of the invention. The embodiment shown in  FIG. 2  might, for example, be deployed to make an FFR measurement in a blood vessel of a patient.  FIG. 2  shows a sensor delivery device  210  being deployed in a blood vessel of a patient (e.g., coronary artery  234 ) across a stenosis (e.g., stenotic lesion  236 ). To make an FFR measurement, for example, first sensor  240  may be positioned to measure distal (downstream) blood pressure, P d , at a location  231  downstream of a location of interest (e.g., stenotic lesion  236 ). First sensor  240  may then be positioned to measure proximal (upstream) blood pressure, P p , at a location  233  upstream of a location of interest (e.g., stenotic lesion  236 ). FFR is simply calculated as the ratio of distal pressure to proximal pressure, or FFR=(P d /P p ). The use of the terms “downstream” and “upstream” are with respect to the normal direction of blood flow, “D,” as shown in  FIG. 2 . 
     In  FIG. 2 , first sensor  240  is coupled to distal sleeve  220 . In the embodiment shown in  FIG. 2 , first sensor  240  is coupled to an outer surface of distal sleeve  220 . The first sensor  240  is adapted to measure a physiological parameter of a patient, such as a blood parameter (e.g., blood pressure, temperature, pH, blood oxygen saturation levels, etc.), and generate a signal representative of the physiological parameter. In certain preferred embodiments of the invention, the first sensor  240  is a fiber optic pressure sensor adapted to measure blood pressure. An example of a fiber optic pressure sensor is a Fabry-Perot fiber optic pressure sensor, which is a commercially available sensor. Examples of Fabry-Perot fiber optic sensors are the “OPP-M” MEMS-based fiber optic pressure sensor (400 micron size) manufactured by Opsens (Quebec, Canada), and the “FOP-MIME” sensor (515 micron size) manufactured by Fiso Technologies, Inc. (Quebec, Canada). In certain alternate embodiments, first sensor  240  may be a piezo-resistive pressure sensor (e.g., a MEMS piezo-resistive pressure sensor), and in other embodiments, first sensor  240  may be a capacitive pressure sensor (e.g., a MEMS capacitive pressure sensor). A pressure sensing range from about −50 mm Hg to about +300 mm Hg (relative to atmospheric pressure) is desired for making most physiological measurements with sensor  240 , for example. 
     In embodiments of the invention using the Fabry-Perot fiber optic pressure sensor as the sensor  240 , such a sensor works by having a reflective diaphragm that varies a cavity length measurement according to the pressure against the diaphragm. Coherent light from a light source travels down the fiber and crosses a small cavity at the sensor end. The reflective diaphragm reflects a portion of the light signal back into the fiber. The reflected light travels back through the fiber to a detector at the light source end of the fiber. The two light waves, the source light and reflected light travel in opposite directions and interfere with each other. The amount of interference will vary depending on the cavity length. The cavity length will change as the diaphragm deflects under pressure. The amount of interference is registered by a fringe pattern detector. 
       FIG. 2  shows proximal portion  250  coupled to the distal sleeve  220 . The proximal portion  250  includes a communication channel  260  for communicating the physiological signal from the sensor  240  to a location outside of the patient (e.g., to a processor, display, computer, monitor, or to another medical device). The proximal portion  250  may preferably be formed of a material of sufficient stiffness in order to assist an operator (e.g., a physician or other medical staff) in positioning the distal sleeve  220  and the sensor  240  within an anatomical (e.g., vascular) structure of the patient. 
     One suitable material for the proximal portion  250  may be a stainless steel hypotube, for example. Depending on the application, the proximal portion  250  (sometimes also referred to as the “delivery tube”) should typically be stiffer and more rigid than the distal sleeve  220  in order to provide a reasonable amount of control to push, pull and otherwise maneuver the device to a physiological location of interest within the patient. In interventional cardiology procedures, for example, at least a portion of the proximal portion  250  will be maneuvered within a guiding catheter positioned within the aortic artery. The proximal portion  250  in such an application should therefore be flexible enough to accommodate the arch of the aorta, while being rigid enough to push and pull the device. Accordingly, suitable materials for proximal portion  250  may also include (in addition to the aforementioned stainless steel hypotube) materials such as nitinol, nylon, and plastic, for example, or composites of multiple materials. 
     The communication channel  260  may be disposed along an outer surface of proximal portion  250 , or may be formed within the proximal portion  250 , as shown in  FIG. 2 . For example, communication channel  260  may comprise a communication lumen that extends longitudinally through proximal portion  250  in some embodiments. Communication channel  260  may comprise a fiber optic communication channel in certain embodiments, such as where the sensor  240  is a fiber optic pressure sensor. Alternately, communication channel  260  may comprise an electrically conductive medium, such as electrical conducting wires, or other communication media suitable for transmitting the signal generated by sensor  240 . In preferred embodiments of the invention, the communication channel  260  comprises a non-fluid communication medium. In the embodiment shown in  FIG. 2 , communication channel  260  (e.g., a fiber optic cable) extends distally beyond proximal portion  250  and is coupled to sensor  240 . The communication channel  260  in such an embodiment is at least partially housed within a communication lumen of the proximal portion  250  (e.g., a stainless steel hypotube). 
       FIG. 2  also shows an optional embodiment of the invention in which a second sensor  242  may be coupled to the device  210 . For example, a second sensor  242  may be coupled to proximal portion  250  such that the first and second sensor  240 ,  242  are spaced apart sufficiently (e.g., a fixed distance apart) to span a stenotic lesion. This embodiment may offer the ability to measure FFR without having to reposition device  210 , since first sensor  240  could be placed distal of the stenotic lesion  236  to measure P d , and second sensor  242  could be placed proximal of the stenotic lesion  236  to measure P p . Second sensor  242  may have a communication channel  262 , which could be housed within proximal portion  250 , or could be disposed along an outside surface of proximal portion  250 , as shown in  FIG. 2 , for example. Further, the ability to measure P d  and P p  substantially simultaneously may improve accuracy and/or reduce the effects of certain types of errors illustrated and described below with reference to  FIG. 3 . 
     It should be noted that certain embodiments could have more than 2 sensors, and that the spacing between adjacent sensors in such embodiments may be varied to provide a variable spacing capability. In certain alternate embodiments of the invention, one or more sensors could be disposed on the proximal portion  250  with no sensors disposed on the distal sleeve  220 , for example. In some alternate embodiments, it may be desirable to have a plurality of sensors (two, or three, or four, or more sensors) spaced at known, fixed distances, disposed along the proximal portion  250 . This could, for example, provide the ability to measure P d  and P p  substantially simultaneously, regardless of lesion length, by selecting an appropriate pair of sensors (from among the plurality of sensors) placed across the lesion from which to obtain the P d  and P p  signals. Further, the sensors could have some form of radiopaque markings incorporated thereon (e.g., marker bands), which could provide a visual estimate of lesion size in conjunction with the measurement of physiological parameters (e.g., P d  and P p ). 
     In some embodiments, it may be desirable to measure the distal pressure P d  with the sensor outside of (extended distal to the distal end of) the guiding catheter, while the proximal pressure P p  is measured by sensor  240  or  242  within the guiding catheter. In this way, it can be assured that the proximal measurement is taken proximal to the lesion and not mistakenly taken within the lesion. If the proximal pressure P p  measurement were obtained within the location of interest such as a lesion, the measurement could be inaccurate, resulting in an erroneous FFR and potentially inaccurate treatment recommendations. Embodiments in which the P p  is taken within the guiding catheter therefore minimize or prevent this type of error, improving the accuracy of FFR calculations and subsequent treatment decisions. 
     In embodiments having only a single sensor  240 , this may be achieved by either first measuring P p  within the guiding catheter, and then extending the distal sleeve to position the sensor  240  downstream of the location of interest to measure P d . Alternatively, the distal sleeve may be extended to position the sensor  240  downstream of the location of interest to measure P d  and may then be retracted across the location of interest and into the guiding catheter to measure P p . In embodiments having two or more sensors, the first sensor  240  may be positioned downstream of the location of interest to measure P d  while the second sensor  242  or other more proximal sensor is positioned proximal to the location of interest and within the guiding catheter to measure P p . The device  210  may be configured such that the length of the guiding catheter and the distance between the first sensor  240  and the second sensor  242  or other more proximal sensor are adequate to allow the distal sensor to be located distal to the location of interest while the proximal sensor remains proximal and within the guiding catheter without adjusting position, with the location of interest at a varying distance from the distal end of the guiding catheter. For example, location of interest may be located between about 10 and about 30 centimeters distal to the distal end of the guiding catheter. The device  210  may be configured such that the spacing between the first sensor  240  and the second sensor  242  or other sensor, and the length of the guiding catheter within which the second sensor  242  or other sensor measures P p , is adequate to allow the first sensor  240  to be positioned distal to the location of interest to measure P d , with the proximal sensor  242  or other sensor within the guiding catheter and at a variable distance from location of interest. 
     As mentioned above, the sensors such as sensors  240  and  242  and/or other additional sensors may include radiopaque markings. In some embodiments, the guidance catheter may also include radiopaque markings, such as in a distal portion including the distal end of the guidance catheter. In this way, the location of the sensor  240 ,  242  within the guidance catheter can be seen by the clinician performing the procedure while measuring P p . 
       FIG. 3  graphically illustrates several other possible sources of error in measuring blood pressure, particularly as they may affect the calculation of FFR, for example.  FIG. 3  is a conceptual plot of blood pressure,  340 , as a function of time for a given patient, P(t). One potential error in calculating FFR is due to the fluctuations in blood pressure due to the systolic and diastolic phases of the cardiac cycle  342 . Unless P d  and P p  are measured at substantially the same phase of the cardiac cycle  342 , there may be some amount of error introduced. Similarly, a more slowly varying source of error can also be introduced by the effect of the respiratory cycle (e.g., inspiration and expiration) on blood pressure, as illustrated at  344  in  FIG. 3 . A third source of error could be introduced by changes in the patient&#39;s posture, which could either raise or lower the overall pressure profile as indicated at  346  in  FIG. 3 . Embodiments of the invention which have the ability to measure P d  and P p  substantially simultaneously, such as the two-sensor embodiment shown in  FIG. 2 , may be able to minimize or eliminate the effects of such “timing errors” on the FFR calculation. Another method of addressing the effects of such “timing errors” will be discussed below in the context of using a contrast injection system in conjunction with a sensor delivery device, according to some embodiments of the invention. 
     In some embodiments, one or more of the sensors may be comprised of one or more materials that together have a low thermal coefficient, such as a thermal coefficient of approximately zero. In some embodiments, the sensor is comprised of one or more materials that together have a thermal coefficient of pressure of between approximately −0.1 and 0.1 mmHg/° C. The portions of the sensor which contribute to the pressure reading may be comprised of a material having a low thermal coefficient. This may include, for example, any component which could change the width of the sensor cavity, which can change with pressure but also could change with temperature. This may be accomplished by constructing the sensor from a set of materials with opposing thermal coefficients so that the net thermal coefficient of the cavity is close to zero, such as between approximately −0.1 and 0.1 mmHg/° C. The use of one or more sensors having a low thermal coefficient may avoid errors which could be caused by the sensors moving out of calibration with atmospheric pressure as they are moved into the warmer environment of the patient&#39;s body, which could in turn result in erroneous calculations of FFR. In some embodiments, one or more of the sensors may be comprised of a material having a thermal coefficient that is sufficiently low such that the sensor remains calibrated to atmospheric temperature after the temperature of the sensor is changed from approximately room temperature (such as approximately 70-72 degrees F.) to approximately normal body temperature (such approximately 96-99 degrees F.) by insertion into the blood stream of a patient. The sensor or sensors comprised of a material having a low thermal coefficient remain calibrated to atmospheric pressure after introduction into the bloodstream, while any sensors that are comprised of a material having a higher thermal coefficient may be moved out of calibration by the temperature of the blood stream. Because at least one of the sensors is comprised of a material having a low thermal coefficient and remains calibrated to atmospheric pressure despite being introduced into the body, any other device sensors which are not comprised of a low thermal coefficient material can be corrected by equilibrating them to the sensor or sensors comprised of the material having a low thermal coefficient. 
     Prior to use in a procedure, the sensor or sensors comprised of a low thermal coefficient material may be calibrated to atmospheric pressure outside of the patient&#39;s body. The sensors may then be interested into the patient&#39;s blood stream as part of insertion of the device for the procedure. If one of more of the sensors is not comprised of a low thermal coefficient material, the sensors can then be equilibrated, with the any sensor which is not comprised of a low thermal coefficient calibrated to match a sensor having a low thermal coefficient. This may be done after the sensors are positioned at a location proximal to the location of interest, before making pressure measurements. If all sensors are comprised of a material having a low thermal coefficient, the equilibration step may not be needed. 
     The benefit of using a sensor having a low thermal coefficient may be understood by considering the change in pressure due to temperature dependent pressure sensing. This can be appreciated by the following equation: 
     
       
         
           
             FFR 
             = 
             
               
                 
                   P 
                   d 
                 
                 
                   P 
                   A 
                 
               
               = 
               
                 
                   
                     P 
                     d 
                   
                   + 
                   
                     P 
                     d 
                     T 
                   
                   + 
                   Δ 
                 
                 
                   
                     P 
                     A 
                   
                   + 
                   
                     P 
                     A 
                     T 
                   
                 
               
             
           
         
       
     
     where P d   T  is the pressure shift due to temperature of the distal sensor, P A   T  is the pressure shift due to temperature of the proximal sensor, such as the aortic sensor, and Δ is the offset introduced when pressures are equalized. If the sensors are zeroed outside of the body, at room temperature, and equalized while reading the same pressure, then Δ=P A   T −P d   T  and 
     
       
         
           
             FFR 
             = 
             
               
                 
                   
                     P 
                     d 
                   
                   + 
                   
                     P 
                     A 
                     T 
                   
                 
                 
                   
                     P 
                     A 
                   
                   + 
                   
                     P 
                     A 
                     T 
                   
                 
               
               . 
             
           
         
       
     
     The error in FFR due to the temperature dependence of the sensor is therefore: 
     
       
         
           
             
               
                 δ 
                  
                 
                     
                 
                  
                 FFR 
               
               
                 δ 
                  
                 
                     
                 
                  
                 
                   P 
                   A 
                   T 
                 
               
             
             = 
             
               
                 
                   P 
                   A 
                 
                 - 
                 
                   P 
                   d 
                 
               
               
                 
                   ( 
                   
                     
                       P 
                       A 
                     
                     + 
                     
                       P 
                       A 
                       T 
                     
                   
                   ) 
                 
                 2 
               
             
           
         
       
     
     The use of a sensor having a low thermal coefficient of pressure can therefore be used in various embodiments to minimize the impact of temperature changes upon the pressure measurement. The value of FFR at which the most accuracy is desired is at the treatment decision making threshold, which is typically an FFR of 0.8. In addition, the FFR is measurement may be obtained at a pressure of about 100 mmHG, which is a typical aortic pressure. In some embodiments, the sensor is designed to have a thermal coefficient of pressure which results in an FFR error of less than 0.01 due to the effect temperature at an FFR of 0.8 and a pressure of 100 mmHg. 
     Referring again to  FIG. 2 , distal sleeve  220  may be substantially tubular, as shown, or may have any shape that allows distal sleeve  220  to slide over a medical guidewire  230  in an anatomical (e.g., vascular) structure of interest. In the context of measuring FFR in a coronary artery, for example, it may be desirable that distal sleeve  220  be substantially cylindrical in cross-section to minimize the total cross-sectional area of the device. Distal sleeve  220  may be preferably formed of a flexible material in some embodiments to facilitate positioning and placement of the distal sleeve  220  (and sensor  240 ) over a guidewire  230  through narrow vascular structures such as coronary arteries. In certain preferred embodiments, the distal sleeve  220  comprises a flexible polyimide tube sized for placement in anatomical (e.g., vascular) structures of interest, such as in coronary arteries or peripheral arteries. In some embodiments, the distal sleeve  220  may comprise a flexible microcoil tube. In some embodiments, flexibility may be achieved and/or enhanced by applying a series of cuts along the surface of the tube. For example, a plurality of cuts or notches along a length of the outer surface of distal sleeve  220  may be applied (e.g., by laser cutting techniques known to those of ordinary skill in this field). Such cuts or notches may be substantially circumferentially directed, and may extend at least partially around the circumference of the distal sleeve. Successive cuts may be angularly offset from each other to provide flexibility in all directions according to some embodiments. 
     The length of distal sleeve  220  may vary. In embodiments to be used in coronary arteries, for example, distal sleeve  220  may be up to about 15 inches long, and in some preferred embodiments may be 11 inches long (e.g., to facilitate use deep within certain coronary arteries). In some embodiments, the distal sleeve  220  may also include a thin covering to provide additional structural support and/or improve handling characteristics of the device. Such a covering may comprise, for example, polyester (PET) shrink tubing that substantially covers the distal sleeve. 
     Distal sleeve  220  has a guidewire lumen  222  that is sized to slidably receive a guidewire  230  having an outer diameter between about 0.010 inches and 0.050 inches. For making an FFR measurement in a coronary artery  234 , for example, the guidewire  230  may preferably have an outer diameter of 0.014 inches, and guidewire lumen  222  would therefore need to have an inner diameter slightly larger than this to facilitate slidable movement of the distal sleeve  220  over the guidewire  230 . 
       FIG. 4( a )  shows an embodiment of the invention in which one or more flow holes  224  are disposed along a side portion of the distal sleeve  220  (e.g., along the length of distal sleeve  220 ). Flow holes  224  could allow blood to flow into the guidewire lumen  222  if an operator were to pull back (e.g., withdraw) the guidewire  230  as shown in  FIG. 4( a ) . Such an embodiment may provide an improvement in accuracy in measuring the pressure drop across a stenosis, since the pressure drop attributable to the device itself would be lessened by decreasing the effective cross-sectional area of the device. 
       FIG. 4( b )  is a cross-sectional view of an embodiment of the invention, illustrating the potential reduction in cross-sectional area that could be obtained by employing flow holes  224  in a side portion of distal sleeve  220 . For example, by allowing blood to flow through flow holes  224  into guidewire lumen  222 , the effective cross-sectional area of the device  210  is reduced by the area of guidewire lumen  222 , and any error in blood pressure measurements caused by the flow obstruction of the device  210  itself would be accordingly reduced. 
       FIG. 5( a )  is a cut-away side view of a portion of the device  210  according to certain embodiments of the invention.  FIG. 5( a )  shows the distal sleeve  220  and first sensor  240  of an embodiment in which sensor  240  is provided with a certain degree of protection by being at least partially covered by a sensor housing  270  disposed on distal sleeve  220 . Sensor housing  270  may be substantially tubular, or may be semi-circular, or may be any other shape that provides suitable protection for sensor  240 . Sensor housing  270  may be constructed of tubing such as polyimide, which is capable of being formed with a relatively thin wall thickness. 
     The sensor housing  270  may be constructed in several different ways, as described with reference to  FIGS. 5( a ) through 5( e ) . Fiber optic sensors, for example, may be somewhat fragile, and should typically be provided with some form of mechanical protection from stress and/or strain relief. The sensing head of sensor  240  is generally attached to the communication channel  260  (e.g., a fiber optic cable) with an adhesive. The sensing head can be prone to being pulled away from (e.g., disconnected from) the fiber optic without much force because the bonding area is typically very small.  FIGS. 5( a ) through 5( e )  illustrate several techniques that utilize a protective sensor housing  270  surrounding the sensor  240  to minimize or eliminate the effects of such stresses on the sensor  240 . 
     One material which may be used to construct the sensor housing  270  is a heavy metal that is x-ray visible, such as platinum. A sensor housing  270  formed of platinum may provide an x-ray marker band to facilitate the placement and positioning of the sensor  240 . A platinum sensor housing  270  may be formed so it is generally thin, for example, approximately 0.001 inches in thickness. Such a thin-walled platinum sensor housing  270  may provide suitable protection to the sensor  240  from stresses that might otherwise cause it to detach from the communication channel  260 . 
     In some embodiments, sensor housing  270  may be shaped to facilitate movement and placement of the device in the anatomical (e.g., vascular) structure of the patient. For example, as shown in  FIG. 5( a ) , the forward and rearward portions  274  of sensor housing  270  may be formed at an angle (e.g., cut at an angle) to present a smoother, tapered structure that is easier to navigate through anatomical (e.g., vascular) structures and passages in a patient (e.g., it allows the device  210  to slide through vascular passages such as arterial walls without catching or snagging). 
     In some embodiments, sensor housing  270  may be formed as part of the process of forming distal sleeve  220 . For example, a substantially cylindrical mandrel may be used to form a distal sleeve  220  made of a thermoset polymer (e.g., polyimide) by employing a dipping process. A slight modification of this manufacturing process could employ a “housing forming element” located alongside the mandrel at the distal end of the mandrel. A single dipping process could thereby form sensor housing  270  as an integral part of distal sleeve  220 . 
     In some embodiments, an optional covering  226  may be applied over the sensor housing  270  and distal sleeve  220 . Such a covering  226  may facilitate movement and positioning of the device  210  within an anatomical (e.g., vascular) structure of a patient. The covering  226  may also provide additional structural stability to the sensor  240 , housing  270 , and distal sleeve  220  arrangement. An example of a class of materials that may be suitable for forming covering  226  are thermoplastics. Such materials may sometimes be referred to as thin-walled heat-shrink tubing, and include materials such as polyolefin, fluoropolymers (PTFE), polyvinyl chloride (PVC), and polyester, specifically polyethylene terephthalate (PET). For simplicity, the term “PET tubing” will be used herein in reference to embodiments that incorporate such thin covering materials. The use of PET tubing could be employed, for example, in embodiments with or without a housing  270 . 
     PET tubing is a heat shrink tube made from polyester that exhibits excellent tensile strength characteristics, while having a wall thickness as little as 0.0002 inches. PET tubing may be used in some embodiments of the invention to encapsulate the distal sleeve  220 . This may include, for example, encapsulating the sensor housing  270  and/or a portion of the communication channel  260  (e.g., the fiber optic cable), to the extent the communication channel  260  extends from the proximal portion  250 . In some embodiments, the PET tubing may also extend to cover part of the proximal portion  250 , for example, where it is coupled to the distal sleeve  220 . In some embodiments, PET tubing may be used to hold a fiber optic communication channel  260  in place around the distal sleeve  220 . After the PET tubing has been heat shrunk, one or more openings may be cut in the PET tubing, for example, to allow an exit port for the guidewire  230 . 
       FIG. 5( a )  shows a fluid opening  272  formed in one of the portions  274  (e.g., the forward portion in this example) of the sensor housing  270 . Fluid opening  272  allows fluid (e.g., blood) to enter the sensor housing  270  and come into fluid contact with sensor  240 . In embodiments that incorporate a covering  226  (such as PET tubing), fluid opening  272  may be formed in the covering  226 . 
       FIG. 5( b )  shows an embodiment of the invention where the fluid opening  272  is formed in a side portion of the housing  270 . This arrangement may provide a reduced likelihood of “clogging” within sensor housing  270 , and/or a reduced likelihood of catching or snagging on any obstructions or bends encountered while positioning device  210 . For example, plaque or calcium from arterial walls may enter the housing  270  as the device is moved through an artery; having the fluid opening  272  in a side portion of housing  270  may reduce this effect. In some embodiments, allowing the PET tubing covering  226  to remain intact at the distal end of the housing  270  may prevent foreign material from entering the housing  270  and possibly damaging the sensor  240 , or affecting the accuracy of pressure measurements. After the PET tubing covering  226  has been heat shrunk over the device  210 , holes can be punched through the covering  226  as needed to form fluid openings  272  to allow fluid access (e.g., blood flow) inside the sensor housing  270 . 
     In some embodiments of the invention, the inside portion of the sensor housing  270  may be filled with a gel  278 , such as a silicone dielectric gel. Silicone dielectric gels are often used with solid state sensors to protect the sensor from the effects of exposure to a fluid medium, for example. If the sensor housing  270  is filled with a gel  278  in front of the sensor diaphragm  279 , then foreign material would be less likely to penetrate inside the housing  270 . The gel  278  may also offer added structural stability to the sensor  240 , and/or may enhance the pressure-sensing characteristics of the sensor  240 . A gel  278  may be used in any of the embodiments of sensor housing  270  illustrated in  FIGS. 5( a ) to 5( d )  and their equivalents. 
     In  FIGS. 5( c ) and 5( d ) , embodiments of the invention are shown which include an optional marker band. If the sensor housing  270  is made from polyimide tubing, for example, the device  210  may not show up as well under x-ray. An optional marker band  276  could be placed near the end of the distal sleeve  220 . Marker band  276  may provide a visible indication of the location of the sensor  240  when viewed under x-ray. As shown in  FIG. 5( c ) , the marker band  276  on the end of the distal sleeve  220  may provide some structural reinforcement to the end of the distal sleeve  220 . In the alternative embodiment shown in  FIG. 5( d ) , a marker band  276  on the distal sleeve  220  located proximal of the sensor housing  270  may reduce the likelihood of the marker band  276  becoming dislodged from the device  210 . In some embodiments, it may be desirable to include a number of such marker bands spaced at known distances (e.g., every 10 mm along distal sleeve  220 , for example), such that the marker bands could be used to provide visual estimates of length or distance (e.g., to measure lesion length). 
       FIG. 5( e )  shows an embodiment where a spacer  278  is used to provide strain relief at the connection between the sensor  240  and the communication channel  260 . This strain relief may be made of any suitable material, such as polyetheretherketone (PEEK), for example. In some embodiments, spacer  278  may also be formed so as to serve as a marker band  276 , substantially as described above. Spacer  278  could be employed in embodiments with a sensor housing  270 , or in embodiments without a sensor housing. 
       FIG. 6( a )  shows an enlarged side view of a portion of the device  210  according to one embodiment of the invention. The delivery tube (proximal portion  250 ) and distal sleeve  220  are preferably coupled together using a flexible bond method (medical adhesive) to maintain flexibility of the device  210 . In some preferred embodiments, for example, the proximal portion  250  will be bonded to an outer surface  221  of the distal sleeve  220  in a bonding area  223 . Bonding area  223  is preferably disposed on distal sleeve  220  sufficiently proximal of the sensor  240  so that bonding area  223  is not within the vascular structure or passage of interest (e.g., it is not within the arterial vessel near a stenosis), but would still be inside the guiding catheter  232 . The joining or bonding area  223  preferably maintains a degree of flexibility in order to accommodate bends such as that in the aortic arch. As previously noted, it may be desirable to minimize the width of the device  210  so that it can be passed through a relatively small guiding catheter  232 , for example. This goal may be achieved, at least in part, by causing the bonding area  223  to be as narrow as possible. In some embodiments, it is desirable to use the sensor delivery device  210  inside a diagnostic guiding catheter  232 , which are generally 4 Fr. 
     In some embodiments, the use of a distal transition  254  to couple the proximal portion  250  to the distal sleeve  220  may obtain a significant reduction in the width of the device  210 . In certain preferred embodiments of the invention, the device  210  will be able to pass through a 4 Fr guiding catheter  232 . The embodiment of  FIG. 6( a )  has a proximal portion  250  that comprises a main section  252  and a distal transition  254 . Distal transition  254  extends distally from main section  252  and is coupled to an outer surface  221  of distal sleeve  220  at bonding area  223 . As shown in  FIG. 6( a ) , the use of a distal transition  254  to couple the proximal portion  250  to the distal sleeve  220  may cause a reduction in the width of the device  210  as compared to a device  210  without the distal transition  254 . This may be accomplished, for example, in embodiments where the distal transition  254  is smaller in cross-sectional area than main section  252 . (Of course, the distal transition  254  is optional and may not be required in all embodiments of the invention; the embodiments shown in  FIGS. 1, 2, and 4 , for example, do not include a distal transition. Such embodiments may result in a simpler manufacturing process, for example.) 
     In the embodiment shown in  FIG. 6( a ) , distal transition  254  may be substantially coaxial and/or concentric with main section  252 , and is smaller in diameter than main section  252 . In some embodiments, distal transition  254  may be formed by inserting a hypotube inside the end of the proximal portion  250 , the hypotube being of somewhat smaller diameter than the proximal portion  250 . The hypotube distal transition  254  and the proximal portion may then be soldered together, as shown at  256 . The distal sleeve  220 , which may comprise a thin walled tube formed of a material such as polyimide, may then be bonded to the smaller diameter distal transition  254 . Alternately, the distal sleeve  220  could be formed from a flat wire wound microcoil with PET tubing heat shrunk over the microcoil. An embodiment using a stainless steel microcoil for the distal sleeve  220  might provide a lower coefficient of friction (than polyimide, for example) to reduce the sliding friction. However, such a microcoil embodiment would probably benefit from the use of a PET tubing covering  226  to provide reinforcement and/or a smooth surface. PET tubing may be used to form covering  226 , as shown in  FIG. 6( a ) , and substantially as described above. Once the PET tubing covering  226  has been heat shrunk in the area of distal transition  254 , for example, covering  226  may have one or more openings  227  formed in the PET tubing, for example, to create an exit port  227  for the guidewire  230 , as shown. Note that, although only shown in  FIG. 6( a ) , the embodiments shown in  FIGS. 6( a ), 6( b ), and 6( c )  may all include an optional covering  226  (e.g., PET tubing), according to certain embodiments of the invention. 
       FIG. 6( b )  shows an embodiment of the invention in which the longitudinal axis of distal transition  254  is offset radially some distance “R” from the longitudinal axis of main section  252  to provide a further potential reduction in the width of device  210 , for example, to minimize the footprint of device  210  and allow the use of a relatively small guiding catheter.  FIG. 6( c )  shows an embodiment where the radial offset “R” is in an opposite direction from the offset “R” shown in  FIG. 6( b ) . This arrangement may provide more clearance for guidewire  230  as it exits distal sleeve  220  in the area near distal transition  254 . 
       FIGS. 6( a ) and 6( b )  also illustrate techniques that may be employed to form the distal transition  254 . For example, the distal transition  254  may be formed by welding or soldering a tubular member to the main section  252  as shown at  256 . As shown, the tubular member  254  may extend into the end of main section  252 , and may include a communication channel  260  (e.g., an extension of communication channel  260  within main section  252 ). Alternately, the distal transition  254  may be formed by “swaging” a distal end of the main section  252 , as shown at  256 . “Swaging,” as that term is used herein, encompasses a number of manufacturing processes that reduce the diameter of a workpiece, for example, by forcing the workpiece (or a portion thereof) through a confining die, or by hammering a round workpiece into a smaller diameter workpiece (e.g., rotary swaging or radial forging, for example). 
     Other methods of forming the distal transition  254  may include grinding (e.g., to reduce the outer diameter of a single piece from that of main section  252  to that of distal transition  254 ), or the use of adhesives or glue (e.g., epoxy, ultraviolet adhesives, cyanoacrylates, etc.), or thermoforming, and/or other techniques known to those of ordinary skill in this area.  FIGS. 6( d ) and 6( e )  show exemplary embodiments that may be formed by grinding or other comparable techniques, for example. Further, distal transition  254  need not extend into the main section  252  and could instead be held in an abutting relationship to main section  252  using certain of the aforementioned techniques. 
       FIGS. 6( a ) and 6( b )  happen to show embodiments of the invention in which a distal transition  254  is employed to “setback” the main section  252  from the distal sleeve  220  a distance “S” as shown. This may, for example, be advantageous in creating additional “clearance” for the guidewire  230  as it exits the distal sleeve  220 . However, the setback is not a requirement, and embodiments of the invention may be employed with a zero setback, as shown in  FIG. 6( c )  (e.g., S=0). 
       FIG. 7( a )  shows one possible embodiment of the invention in which a second sensor  242  is coupled to a proximal sleeve  280 , which thereby allows the first and second sensors  240 ,  242  to be spaced apart a variable distance, “V,” as shown. Proximal sleeve  280  in such an embodiment is adapted to be moved longitudinally (e.g., advanced and/or retracted) by an operator by sliding over proximal portion  250  to achieve the desired spacing, “V,” as shown. 
       FIG. 7( b )  shows an alternate embodiment in which a multilumen shaft  290  (e.g., formed of a polymer) includes a guidewire lumen  292 , a sensor lumen  294  for an extendible/retractable first sensor  240  disposed on a distal end of an extendible/retractable sensor shaft  296 , the sensor shaft  296  being slidably received within sensor lumen  294 , and a second sensor  242  coupled to an outer portion of the multilumen shaft  290 . The first and second sensors  240 ,  242  may be spaced a variable distance apart (e.g., across a stenotic lesion of other anatomical location of interest in a patient) by slidably moving the sensor shaft  296  with respect to the multilumen shaft  290  (e.g., by moving sensor shaft  296  within sensor lumen  294 ). 
       FIG. 8  shows a device  210  according to an embodiment of the invention in which a proximal end of proximal portion  250  interconnects with a fiber optic furcation tube  290  (e.g., in embodiments of the invention employing a fiber optic sensor). A fiber optic furcation tube  290  provides an extension of the fiber optic communication channel  260  (from the sensor  240  through the proximal portion  250 ), to an optional connector  294 , such as an “SC” fiber optic connector. (An SC connector is a fiber optic connector with a push-pull latching mechanism which provides quick insertion and removal while also ensuring a positive connection. It also follows certain industry standards, allowing interconnection with a variety of fiber optic devices which follow the same standards.) Furcation tube  290  may, for example, be provided with SC connector  294  to allow the device  210  to send a signal from sensor  240 , for example, to other devices, monitors, fluid injection devices, display and control units, etc. Furcation tube  290  may comprise a Kevlar fiber reinforced tube (e.g., for strength) according to some embodiments. In some alternate embodiments, furcation tube  290  could be formed of coaxial tubing. 
     The length of furcation tube  290  may be chosen to extend from the device  210  in the sterile field (e.g., where the patient is) to a location outside of the patient, such as a medical fluid injector, or to a standalone display device, or to some other processing or computing equipment  296  positioned some distance from the patient. The SC connector  294  is adapted to interconnect with an injector (or other signal processing unit) appropriately configured. If signal processing is done within the injector, then the injector display could be utilized to display pressure waveforms and/or to calculate and display FFR values. 
     An alternate embodiment of the invention would be to construct a distal portion  300  of the sensor delivery device  210  using a dual lumen configuration. An example of such an embodiment is illustrated in  FIG. 9 . One lumen of the distal portion  300  would accommodate the fiber optic communication channel  260  from the sensor  240  (and from sensor housing  270 , in some embodiments). The other lumen (e.g., guidewire lumen  222 ) would be adapted to slide over the guidewire  230  as shown. The guidewire  230  in such an embodiment would exit from the dual lumen distal portion  300  a certain distance (e.g., about 10-12 inches) back from (e.g., proximal to) the sensor  240  through an opening  320  in the device  210 . In some embodiments, a stiffening wire  310  could be placed in the remaining proximal portion of the lumen  222  (that is, the portion of the guidewire lumen  222  in the proximal portion  250  of device  210 ). The stiffness of the stiffening wire  310  could be varied, for example, to aid a physician in deploying and positioning the device  210  through a catheter and into a particular anatomical (e.g., vascular) structure of interest. The stiffening wire  310  could be part of the dual-lumen device  210 , or could be an optional, removable item selected by a physician to obtain the desired amount of stiffness according to some embodiments. 
     Another alternate embodiment of the invention would be an entirely over-the-wire (OTW) device, substantially as shown in  FIG. 10 .  FIG. 10  illustrates an embodiment of the invention in which both the distal sleeve  220  and the proximal portion  250  of sensor delivery device  210  are adapted to slide over a guidewire  230 . The guidewire  230  in such an embodiment would not exit from or separate from the device  210  at some point along the length of device  210 . Instead, the entire length of the proximal portion  250  of device  210  would slide over the guidewire  230  within a guiding catheter (not shown). The design of the device may incorporate two different sizes of tubes, for example, to form the distal sleeve  220  and proximal portion  250 . For example, a smaller diameter thin-walled tube could form the distal sleeve  220 , where the sensor  240  resides (optionally, within a sensor housing  270 ). Back some distance from the location of sensor  240  on the distal sleeve  220 , the smaller diameter tube of the distal sleeve  220  would transition into a larger diameter portion (e.g., proximal portion  250 ), with sufficient clearance between the inner wall of both tubes and the guidewire. Such clearance may provide less friction and sliding resistance while positioning the sensor  240 , for example. The larger diameter tube of the proximal portion  250  could be made, for example, from a material with a low coefficient of friction to lower the sliding force. The sensor  240  (and sensor housing  270 , where applicable) could be of similar construction to that described above with respect to  FIGS. 5( a )-5( d ) . 
       FIG. 10  is an example of an embodiment of the invention that illustrates the over-the-wire concept. The larger diameter tubing of the proximal portion  250  could be formed of a single lumen tube or a dual lumen tube. With a single lumen tube, the communication channel  260  (e.g., fiber optic) could be disposed on an outer surface of the proximal portion  250 , for example, and could extend toward a connector at a proximal end of the device  210 . In embodiments with a dual lumen tube forming the proximal portion  250 , the communication channel  260  could extend toward a connector at a proximal end of the device  210  within the second lumen. This could, for example, provide added protection for the communication channel  260  (e.g., fiber optic). 
       FIG. 11  is a flow diagram showing a method of using a sensor delivery device according to certain embodiments of the invention. In a preferred embodiment of the invention, for example, the method may be used to assess the severity of a stenotic lesion in a patient&#39;s vasculature. Step  1105  comprises placing a guidewire in a patient to a location of interest. In some embodiments, this may be a diagnostic guidewire, and a guiding catheter may also be inserted into the patient in conjunction with the guidewire. Step  1110  comprises deploying a sensor delivery device over the guidewire and out of the guiding catheter such that the sensor is positioned downstream of the location of interest (e.g., downstream of a stenotic lesion). In some embodiments, the sensor delivery device will have a sensor mounted to a distal sleeve that slides over the guidewire, and a proximal portion that is used to advance the distal sleeve over the guidewire without having to move the guidewire. Step  1115  comprises using the sensor of the sensor delivery device to measure a physiological parameter of interest at the location of interest. In some embodiments, the physiological parameter is blood pressure downstream of a stenotic lesion, P d . Step  1120  comprises measuring a reference value of the physiological parameter of interest. In some embodiments, this step comprises measuring blood pressure upstream of a stenotic lesion, P p . This could be done, for example, with a separate blood pressure monitoring apparatus, according to some embodiments, or could be done by repositioning the sensor delivery device to a location upstream of the stenotic lesion and making a second pressure measurement with the sensor of the device. In some embodiments, the sensor may be positioned within the lumen of the guiding catheter for measuring P p . Step  1125  may be an optional step which comprises comparing the physiological parameter of interest measured at the location of interest to the reference value measured in step  1120 . In some embodiments, this may comprise calculating a ratio of the two measured values. In one preferred embodiment of the invention, step  1125  comprises calculating FFR as the ratio of downstream to upstream blood pressures, P d /P p . Step  1130  may be an optional step which comprises providing an indication of the result obtained in step  1125 . For example, step  1130  may comprise providing a visual indication of the calculated FFR value, or may provide other visual cues (e.g., providing a color-coded indication of the severity of a stenotic lesion, such as a red indicator for FFR values less than 0.75, and a green indicator for FFR values equal to or greater than 0.75, as possible examples). 
     It may be desirable, as mentioned above with respect to  FIG. 8 , to have the sensor delivery device  210  interact with other devices and/or display equipment. For example, a furcation tube  290  and a connector  294  may be used to send the signal (e.g., the measured physiological parameter signal) from sensor  240  to processing device  296 . Processing device  296  could be, for example, a standalone display monitor to show signal waveforms and/or numerical values of the physiological parameter signal from sensor  240 . Processing device  296  could include data recording capabilities in some embodiments. In certain preferred embodiments of the invention, processing device  296  could comprise a medical fluid injection system, such as a powered fluid injector used to inject contrast media and/or saline during certain imaging procedures (e.g., angiography, computed tomography, MRI, ultrasound, etc.).  FIGS. 12 and 13  illustrate exemplary powered injection systems which may be used with a sensor delivery device according to various embodiments of the invention. 
       FIG. 12  is a perspective view of one embodiment of a powered injection system  1200  that may be used to perform various functions and, when operable, may be coupled to a physiological sensor delivery device, such as the various embodiments of a sensor delivery device described above. The powered injection system  1200  shown in  FIG. 12  may be used to inject medical fluid, such as contrast media or saline, into a patient within the sterile field during a medical procedure (such as during an angiographic or CT procedure). A physiological sensor delivery device may be coupled to the system  1200  and used within the sterile field during a patient procedure, according to one embodiment. The system  1200  includes various components, such as a control panel  1202 , a hand-controller connection  1204 , a hand controller  1212 , a fluid reservoir  1206 , tubing  1208 , a pump  1210 , a pressure transducer  1218 , a fluid reservoir  1214 , an injection syringe  1216 , high pressure injection tubing  1222 , a valve  1220 , an air detector  1224 , and a stopcock  1226 . In one embodiment, described in more detail below, the fluid reservoir  1206  comprises a container such as, for example, a bag or bottle of diluent (such as saline), the fluid reservoir  1214  comprises a container such as, for example, a bag or bottle of contrast media, and the pump  1210  comprises a peristaltic pump. In other embodiments, the pump  1210  may comprise other forms of pumping devices, such as a syringe, a gear pump, or other form of displacement pump. In some embodiments, the injection syringe  1216  (along with its associated plunger), which is a pumping device, may be replaced with another form of pumping device that delivers high-pressure fluid injections to a patient. An individual pumping device is capable of operating or functioning in different, or multiple, operational modes. For example, a pumping device may be operable to pump fluid when actuated, or driven, to move in a first direction (e.g., forward), while it may also be operable to move in a second direction (e.g., an opposite direction, backward) to carry out certain functions. 
     The system  1200  of  FIG. 12  also shows a hand controller  1212  and an air detector  1224 . An operator may use the hand controller  1212  to manually control injection of saline and/or contrast media. The operator may push a first button (not shown) on the hand control  1212  to inject saline, and may push a second button (not shown) to inject contrast, for example. In one embodiment, the operator may push on the contrast button to deliver contrast at a variable flow rate. The harder the operator pushes on the button, the greater the flow rate of contrast media delivered to the patient. Other controllers, such as foot pedal controllers, may also be used. The air detector  1224  is able to detect potential air bubbles or columns within the high-pressure tubing  1222 . In one embodiment, the air detector  1224  is an ultrasonic or acoustic-based detector. In other embodiments, the air detector  1224  may use infrared or other detection means (such as optical). If the air detector  1224  detects the presence of air in the high-pressure tubing  1222 , it generates a signal that is used to warn the operator and/or halt an injection procedure. 
     An operator may use the control panel  1202  to view and/or select various parameters and/or protocols to be used during a given procedure. The control panel  1202  may be used to display information to an operator about the status of the equipment and/or the patient. The pump  1210  may be used to pump saline from the bag into the patient via the saline tubing  1208 , the valve  1220 , and the high-pressure tubing  1222 . In one embodiment, the valve  1220  comprises a spring-based spool valve, as is known in the art. In one embodiment, the valve  1220  comprises an elastomeric-based valve. 
     In one embodiment, the syringe  1216  is used to draw contrast from the reservoir  1214  into the syringe  1216 , and to inject contrast from the syringe  1216  into the patient via the valve  1220  and high-pressure tubing  1222 . In one embodiment, the syringe  1216  is a self-purging syringe that has one port for filling of contrast and purging of air, and a second port for injection of contrast. 
     The valve  1220  may be used to control coupling between input ports to the valve  1220  and an output port. In one embodiment, the valve includes two input ports, one which is coupled to the contrast fluid line and another which is coupled to the saline fluid line. The saline fluid line also includes a pressure transducer  1218  for providing a signal representative of patient blood pressure, for example. 
     The stopcock  1226  regulates the flow of fluids to the patient. In one embodiment, the valve  1220  allows either the saline line or the contrast line to be coupled to the patient (high-pressure tubing) line  1222 . When the syringe  1216  is used to inject contrast media, for example, the valve  1220  may allow the contrast media to flow to the patient line  1222  while blocking the flow of saline to the patient line  1222 . Valve  1220  may operate such that the pressure transducer  1218  may also be blocked or isolated from the patient line  1222  during high-pressure injections, for example, to protect the transducer  1218  from high injection pressures that may accompany a contrast injection. When there is no injection of contrast from the syringe  1216 , the valve  1220  may operate to block the contrast line from the patient line  1222 , while opening the fluid connection between the saline line (tubing)  1208  and the patient line  1222 . In this state, the pump  1210  is capable of injecting saline into the patient, and the pressure transducer  1218  is also capable of monitoring hemodynamic signals coming from the patient via the patient line  1222  and generating representative signals based upon the measured pressures. 
     As noted above, the system  1200  of  FIG. 12  may be adapted to be coupled to a physiological sensor delivery device according to certain embodiments of the invention. System  1200  may, for example, be adapted to receive the physiological signal generated by the sensor  240  of device  210 . In embodiments where the physiological signal from device  210  is a pressure signal measured downstream of a stenotic lesion (e.g., P d ), system  1200  may facilitate calculation of FFR, for example, since P p  may already be provided by pressure transducer  1218  of system  1200 . A visual or graphical display of the calculated FFR value could be presented to an operator via control panel  1202 , for example. Since instantaneous values of P p  and P d  are available in such an arrangement, the timing effects and associated errors noted above with respect to  FIG. 3  would not pose a problem—simultaneous measurement of P p  and P d  would reduce or eliminate such errors. In addition, time averaging or other signal processing could be employed by system  1200  to produce mathematical variants of the FFR calculation (e.g., mean, max, min, etc.). Alternately, a time-varying display or plot of the calculated FFR value could be displayed as a waveform (e.g., as a function of time). 
       FIG. 13  is a perspective view of another embodiment of a powered injection system  1300  that may be used to perform various functions and, when operable, may be coupled to a physiological sensor delivery device, such as the embodiments described above. The powered injection system  1300  shown in  FIG. 13  may be used to inject medical fluid, such as contrast media or saline, into a patient within the sterile field during a medical procedure (such as during an angiographic or CT procedure). A physiological sensor delivery device may be coupled to the system  1300  and used within the sterile field during a patient procedure, according to one embodiment. 
     The system  1300  of  FIG. 13  is a dual-syringe system that includes a control panel  1302  and two motor/actuator assemblies  1303   a  and  1303   b . Each motor drives one of the linear actuators in the assemblies  1303   a ,  1303   b . Each linear actuator drives a plunger of one syringe  1308   a  or  1308   b . An individual plunger moves within the syringe barrel of the syringe  1308   a  or  1308   b  in either a forward or rearward direction. When moving in a forward direction, the plunger injects liquid into the patient line or purges air out of the syringe and into a liquid container (e.g., bottle). When moving in a rearward direction, the plunger fills liquid into the syringe  1308   a ,  1308   b  from a liquid container.  FIG. 13  shows examples of two such liquid containers  1304  and  1306 . In one embodiment, the container  1304  is a bag or bottle containing contrast agent, and the container  1306  is a bag or bottle containing diluent, such as saline. In other embodiments, the syringes  1308   a ,  13808   b  (along with associated plungers), which are each pumping devices, may either separately or together comprise another form of pumping device that is capable of injecting fluids at appropriate flow rates/pressures/etc., such as, for example, a peristaltic pump or another form of displacement pump. An individual pumping device is capable of operating or functioning in different, or multiple, operational modes. For example, a pumping device may be operable to pump fluid when actuated, or driven, to move in a first direction (e.g., forward), while it may also be operable to move in a second direction (e.g., an opposite direction, backward) to carry out certain functions. Multiple sets of pinch valve/air detect assemblies are shown both  FIG. 13 . One pinch valve/air detect assembly  1310   a  is coupled between the liquid container  1306  and a syringe input port of the syringe  1308   a , and a second pinch valve/air detect assembly  1312   a  is coupled between a syringe output port of the syringe  1308   a  and the patient connection. A third pinch valve/air detect assembly  1310   b  is coupled between the liquid container  1304  and a syringe input port of the syringe  1308   b , and a fourth pinch valve/air detect assembly  1312   b  is coupled between a syringe output port of the syringe  1308   b  and the patient connection. In the embodiment shown in  FIG. 13 , each syringe  1308   a ,  1308   b  is a dual-port syringe. Fluid flows and is drawn into the syringe  1308   a  or  1308   b  from a container via the syringe input port, and fluid flows out of and is injected from the syringe  1308   a  or  1308   b  via the syringe output port. 
     Each pinch valve is a pinch valve/air detect assembly  1310   a ,  1310   b ,  1312   a ,  1312   b  may be opened or closed by the system  1300  to control the fluid connections leading to or away from each of the syringes  1308   a ,  1308   b . The air detect sensors in the assemblies  1310   a ,  1310   b ,  1312   a ,  1312   b  may be optical, acoustic, or other form of sensor. These sensors help detect air that may be present in the fluid connections leading to or away from the syringes  1308   a ,  1308   b . When one or more of these sensors generates a signal indicating that air may be present in a fluid line, the system  1300  may warn the user or terminate an injection procedure. The use of multiple pinch valves within the system  1300  allows the system  1300  automatically, or through user interaction, selectively control the flow of fluid into or out of the syringes  1308   a ,  1308   b  by opening or closing fluid tubing. In one embodiment, the system  1300  controls each of the pinch valves. The use of multiple air-detect sensors helps improve the overall safety of the system  1300  by detecting possibly air (e.g., columns, bubbles) within fluid (in the tubing) leading to or away from the syringes  1308   a ,  1308   b . Signals from the air detectors are sent to and processed by the system  1300 , such that the system  1300  may, for example, provide a warning, or terminate an injection procedure, if air is detected. In the example of  FIG. 13 , the fluid tubing first flows through a pinch valve and then flows through an air detector within the assemblies  1310   a ,  1310   b ,  1312   a ,  1312   b . In other embodiments, other configurations, ordering, and the like may be used for the pinch valves and air detectors within these assemblies. Moreover, other types of valves may be substituted for the pinch valves. 
     An operator may use the control panel  1302  to initialize, or setup, the injection system  1300  for one or more injection procedures, and may further use the control panel  1302  to configure one or more parameters (e.g., flow rate, volume of fluid to be delivered, pressure limit, rise time) of an individual injection procedure. The operator may also use the panel  1302  to pause, resume, or end an injection procedure and begin a new procedure. The control panel also displays various injection-related information to the operator, such as flow rate, volume, pressure, rise time, procedure type, fluid information, and patient information. In one embodiment, the control panel  1302  may be connected to a patient table, while being electrically coupled to the main injector of the system  1300 . In this embodiment, the operator may manually move the control panel  1302  to a desirable location, while still having access to all functionality provided by the panel  1302 . 
     The system of  FIG. 13  also includes a valve  1314  coupled to both output lines coming from the syringes  1308   a  and  1308   b . Each syringe output provides fluid injected through tubing that passes through a pinch valve/air detect assembly  1312   a  or  1312   b  and that then leads to an input of the valve  1314 . In one embodiment, one fluid line to the valve  1314  also includes a pressure transducer. The valve output port of the valve  1314  is coupled to high-pressure tubing line, which is used to direct fluid to the patient. In one embodiment, the valve  1314  is made of a flexible material, such as an elastomeric material. The valve  1314  allows one of the fluid lines (e.g., the contrast line or the saline line) to be coupled to the patient (high-pressure tubing) line. When saline and contrast are contained within the syringes  1308   a  and  1308   b , respectively, the valve  1314  allows the contrast media to flow from the syringe  1308   b  to the patient line (assuming the pinch valve in the assembly  1312   b  is open and there has been no air detected), but blocks the flow of saline from the syringe  1308   a  to the patient line. The pressure transducer coupled to the saline line (according to one embodiment) is also blocked from the patient line, thereby protecting the transducer from high injection pressures that may accompany a contrast injection. When there is no injection of contrast from the syringe  1308   b , the valve  1314  blocks the contrast line from the patient line, but allows a connection between the saline line from the syringe  1306  to the patient line. The syringe  1308   a  is capable of injecting saline into the patient (assuming the pinch valve in the assembly  1312   a  is open and there has been no air detected), and the pressure transducer is also capable of monitoring hemodynamic signals coming from the patient via the patient line, and generating representative electronic signals based upon the measured pressures that can be processed by the system  1300 . 
     In one embodiment, a secondary control panel (not shown) provides a subset of functions provided by the main panel  1302 . This secondary control panel (also referred to herein as the “small” control panel) may be coupled to the injector within the system  1300 . In one scenario, the operator may use the small panel to manage injector setup. The small panel may display guided setup instructions that aid in this process. The small panel may also display certain error and troubleshooting information to assist the operator. For example, the small panel may warn the operator of low contrast or saline fluid levels in the liquid reservoirs and/or syringes. 
     As with the system  1200  of  FIG. 12 , system  1300  of  FIG. 13  may be adapted to be coupled to a physiological sensor delivery device according to certain embodiments of the invention. System  1300  may, for example, be adapted to receive the physiological signal generated by the sensor  240  of device  210 . Processing of the physiological signal from sensor  240  (and/or from additional sensors of the sensor delivery device  210 , if applicable) may be performed within the injection system  1200  or  1300 , for example. Signal conditioning and/or processing may, for example, be performed by a circuit board or card that may be an add-on feature to system  1200  or  1300 . Such a signal conditioning board or card may process a “raw” signal from sensor  240  and convert the signal into a standard analog and/or digital signal, which can be used by processors of the injector system, according to some embodiments. The processed signal may enable injector system  1200  or  1300  to display the signal data (e.g., as pressure waveforms), and/or perform algorithms and/or calculations and display the results. 
     In embodiments where the physiological signal from device  210  is a pressure signal measured downstream of a stenotic lesion (e.g., P d ), system  1300  may facilitate calculation of FFR, for example, since P p  is already provided by the pressure transducer of system  1300 . A visual or graphical display of the calculated FFR value, for example, could be presented to an operator via control panel  1302 , for example, or via a small control panel (not shown) having a subset of the functions provided by control panel  1302 . Since instantaneous values of P p  and P d  are available in such an arrangement, the timing effects noted above with respect to  FIG. 3  would not pose a problem. In addition, time averaging or other signal processing could be employed by system  1300  to produce mathematical variants of the FFR calculation (e.g., mean, max, min, etc.). 
       FIG. 14  is a flow diagram of a method that may be performed according to one embodiment of the invention. The methods described herein may be performed in varying degrees of automation, for example, by having instructions stored in a computer-readable medium and/or performed by a computer or processor associated with a powered injection system (such as the ones described above with respect to  FIGS. 12 and 13 , or other comparable fluid injection systems). The method of  FIG. 14  may, for example, be used to assess the severity of a fluid flow restriction in a patient according to some embodiments of the invention. This method may be performed using various powered injection systems, such as the system  1200  shown in  FIG. 12 , or the system  1300  shown in  FIG. 13 . The ordering of the actions shown in  FIG. 14  is for exemplary purposes only. In one embodiment, a powered injection system may be capable of performing some of the steps of the method shown in  FIG. 14  automatically, or alternately, after the operator has requested that the method be commenced through manual activation on the control panel (or secondary panel, if available). 
     Step  1405  in  FIG. 14  comprises placing a guidewire in a patient to a location of interest, such as a stenotic lesion, or across a heart valve, for example. In some embodiments, this may be a diagnostic guidewire, and a guiding catheter may also be inserted into the patient in conjunction with the guidewire. Step  1410  comprises deploying a sensor delivery device over the guidewire such that the sensor is positioned upstream of the location of interest (e.g., upstream of a stenotic lesion, or on the high pressure side of a valve). In some embodiments, the sensor delivery device will have a sensor mounted to a distal sleeve that slides over the guidewire, and a proximal portion that is used by an operator to advance the distal sleeve over the guidewire to the desired location without having to move the guidewire. Step  1415  comprises using the sensor of the sensor delivery device to measure a value of the physiological parameter upstream of the location of interest. In some embodiments, the physiological parameter is blood pressure, and the pressure measured by the sensor upstream of a stenotic lesion is the proximal pressure, P p . 
     Step  1420  in  FIG. 14  comprises “normalizing” the P p  measurement made in step  1415  to the P p  measurement obtained from an independent source. “Normalizing” the P p  measurement refers to the fact that an independent source (e.g., a fluid sensor for monitoring patient blood pressure during a procedure) will be used to obtain the P p  value that will be used for later comparisons or calculations with the P d  value (e.g., the downstream pressure) measured with the sensor of the sensor delivery device. The normalizing step basically ensures that the P p  value measured with the sensor equals the P p  value measured using the independent source so that no error is introduced (or that any error is minimized) when a subsequent downstream pressure measurement (e.g., P d ) is made. An adjustment, if needed, could be made to either P p  value, although it may often be simpler to adjust the sensor-based P p  value to match the independent source&#39;s P p  value. 
     Step  1425  comprises deploying the sensor delivery device over the guidewire such that the sensor is downstream of the location of interest (e.g., downstream of the stenotic lesion). Step  1430  comprises using the sensor of the sensor delivery device to measure a downstream value of the physiological parameter. In some embodiments, this step comprises measuring blood pressure downstream of the stenotic lesion, P d . Step  1435  comprises comparing the measured value downstream of the location of interest (e.g., P d , downstream blood pressure) to a value measured upstream of the location of interest using the independent source (e.g., P p ). In some embodiments, the comparison made in step  1435  may comprise calculating a ratio of the two measured values. In one preferred embodiment of the invention, step  1435  comprises calculating FFR as the ratio of downstream to upstream blood pressures, P d /P p . Step  1440 , which may be an optional step, comprises providing an indication of the result of the comparison made in step  1435 . For example, step  1440  may comprise providing an indication of the calculated FFR value (e.g., numerical or graphical display or plot), and/or other cues may be provided to an operator. A color-coded indication of the severity of a stenotic lesion may be provided, for example, a RED indicator for FFR values less than 0.75, and/or a GREEN indicator for FFR values equal to or greater than 0.75. Other examples of indicators are possible, including non-visual indicators—an audible indication, an alarm sound for example, could alert an operator of an FFR value that is less than 0.75, which may prompt the operator to make a therapy decision. 
       FIG. 15  is a flow diagram of a method that may be performed according to an embodiment of the invention. The method of  FIG. 15  may, for example, be used to assess the severity of a fluid flow restriction in a patient according to some embodiments of the invention. The method of  FIG. 15  employs a sensor delivery device  210  having a first and second sensor  240 ,  242 , such as the devices  210  shown in  FIGS. 2 and 7 . This method may also be performed in conjunction with various powered injection systems, such as the system  1200  shown in  FIG. 12 , or the system  1300  shown in  FIG. 13 . The ordering of the actions shown in  FIG. 15  is for exemplary purposes only. 
     Step  1505  in  FIG. 15  comprises placing a guidewire in a patient to a location of interest, such as a stenotic lesion, or across a heart valve, for example. In some embodiments, the guidewire may be a diagnostic guidewire, and a guiding catheter may also be inserted into the patient in conjunction with the guidewire. Step  1510  comprises deploying a sensor delivery device over the guidewire such that a first sensor of the sensor delivery device is positioned upstream of the location of interest, and a second sensor of the sensor delivery device is positioned downstream of the location of interest. The second sensor may be positioned such that it is located within the guiding catheter. In an embodiment such as that described above with respect to  FIG. 7 , an optional step may next be performed wherein a proximal sleeve  280  is moved by an operator relative to the rest of device  210  in order to vary the distance, V, between first sensor  240  and second sensor  242  while positioning the second sensor  242  within the guiding catheter. In an embodiment such as that described above with respect to  FIG. 2 , it should be noted that more than two sensors could be mounted along device  210 , and that the spacing between adjacent sensors could vary as well, according to some embodiments of the invention. Step  1515  comprises using the first sensor to measure an upstream value of the physiological parameter, and using the second sensor to measure a downstream value of the physiological parameter, such as from within the guiding catheter. 
     Step  1535  comprises comparing the measured value downstream of the location of interest (e.g., P d , downstream blood pressure) to the value measured upstream of the location of interest (e.g., P p ). In some embodiments, the comparison made in step  1535  may comprise calculating a ratio of the two measured values. In one preferred embodiment of the invention, step  1535  comprises calculating FFR as the ratio of downstream to upstream blood pressures, P d /P p . Step  1540 , which may be an optional step, comprises providing an indication of the result of the comparison made in step  1535 . For example, step  1540  may comprise providing an indication of the calculated FFR value (e.g., numerical or graphical display or plot), and/or other cues may be provided to an operator. A color-coded indication of the severity of a stenotic lesion may be provided, for example, a RED indicator for FFR values less than 0.75, and/or a GREEN indicator for FFR values equal to or greater than 0.75. Other examples of indicators are possible, including non-visual indicators—an audible indication, an alarm sound for example, could alert an operator of an FFR value that is less than 0.75, which may prompt the operator to make a therapy decision. 
     Although not shown in  FIGS. 11, 14, and 15 , any of these methods could be performed with an embodiment of device  210  having flow holes  224 , such as the device of  FIGS. 4( a ) and 4( b ) . Using such a device, the methods may optionally include a step wherein an operator retracts the guidewire  230  to allow fluid flow (e.g., blood flow) through flow holes  224  into the guidewire lumen  222  of the distal sleeve  220 . Performing this optional step prior to measuring downstream pressure, P d , may reduce the amount of flow restriction caused by the device  210  itself, and may thereby reduce the measurement error. 
     In some embodiments, a method may include basing a therapy decision on the calculated FFR value, e.g., if the calculated FFR is less than 0.75, an interventional therapy is recommended and/or performed. In some embodiments, an interventional therapy device may be deployed by withdrawing sensor delivery device  210 , and using the same guidewire  230  to deploy the interventional therapy device. 
       FIG. 16  is a perspective view of a powered injection system adapted to be coupled to a physiological sensor delivery device according to certain embodiments of the invention.  FIG. 16  shows a sensor delivery device  210  connected to a powered injection system  1630  via furcation tube  290  and connector  294 . Injection system  1630  is adapted to receive a physiological measurement signal (e.g., blood pressure) from device  210  via input port  1650 . In preferred embodiments, the signal is an optical signal, and connector  294  is an SC fiber optic connector adapted to mate with port  1650  to receive the optical signal. 
     As shown in  FIG. 16 , system  1630  has 2 fluid containers  1632 ,  1634 , which are adapted to deliver fluid through lines  1633  and  1635 . Fluid in line  1633  (e.g., contrast solution) may be delivered at significantly higher pressures than fluid in line  1635  (e.g., saline solution), for example. Valve  1620  may be used to control coupling between input ports to the valve  1620  and to an output port which ultimately leads to a patient via patient line  1622 . In one embodiment, valve  1620  includes two input ports, one which is coupled to a contrast fluid line  1633  and another which is coupled to a saline fluid line  1635 . The saline fluid line is also coupled to a pressure transducer  1618  for providing a signal representative of patient blood pressure, for example. The signal from pressure transducer  1618  may be communicated to system  1630  via communication path  1640  and connector  1642 , or via other equivalent means (e.g., infrared, optical, etc.). 
     In one embodiment, the valve  1620  allows either the saline line or the contrast line to be coupled to the patient (high-pressure tubing) line  1622 . When the system  1630  is injecting contrast media, for example, the valve  1620  may allow the contrast media to flow to the patient line  1622  while blocking the flow of saline to the patient line  1622 . Valve  1620  may operate such that the pressure transducer  1618  may also be blocked or isolated from the patient line  1622  during high-pressure injections, for example, to protect the transducer  1618  from high injection pressures that may accompany a contrast injection. When there is no injection of contrast from the system  1630 , the valve  1620  may operate to block the contrast line from the patient line  1622 , while opening the fluid connection between the saline line (tubing)  1635  and the patient line  1622 . In this state, the system  1630  may be capable of injecting saline into the patient, while the pressure transducer  1618  is capable of monitoring hemodynamic signals coming from the patient via the patient line  1622 , and generating representative signals based upon the measured pressures. 
       FIG. 16  shows control panel  1602  connected to injection system  1630  via communication path  1660 . An operator may interact with system  1630  via control panel  1602  (or via a secondary panel, if available) to review and/or modify injection parameters, for example. In a preferred embodiment of the invention, system  1630  is adapted to receive pressure signals simultaneously from pressure transducer  1618  and from device  210 , representative of downstream and upstream pressures (e.g., P d , P p ), respectively. Thus, in a preferred embodiment, system  1630  receives P d  and P p  signals substantially simultaneously, compares the two signals (e.g., calculates FFR=P d /P p ), and provides an indication of the result of the comparison to an operator via a display screen  1670  of control panel  1602 . As noted above, the indication of the result of the comparison may take a number of different forms, including numerical, graphical, time plots, etc. The indication may be of the pass/fail variety, for example, indicating one color-coded pattern (e.g., a RED icon) for an FFR value below a certain value (e.g., 0.75), and/or a different color-coded pattern (e.g., a GREEN icon) for an FFR value at or above a certain value (e.g., 0.75). The indication may also be an audible alarm according to some embodiments of the invention. 
       FIG. 17  is an idealized view of information that may be displayed (e.g., via an interactive graphical user interface, or “GUI interface”) to an operator, according to certain embodiments of the invention.  FIG. 17  shows a GUI screen that may be displayed either via a control panel that is unique to the sensor delivery device  210 , or via a control panel of a device adapted for use with device  210 , such as the powered fluid injection systems described above with respect to  FIGS. 12, 13, and 16 . (The GUI interface could be implemented in software such that a user might see a very similar screen regardless of whether a stand-alone display device or an integrated injector system was being used, according to various embodiments of the invention.) 
     In  FIG. 17 , screen  1702  is adapted to display data in various forms (e.g., waveform data, numerical data, calculated values, patient information, device status information, etc.). For example, in a preferred embodiment of the invention useful for making FFR measurements, blood pressure waveforms may be displayed as a function of time for both proximal pressure, P p (t)  1704 , and distal pressure, P d (t)  1706 . In some embodiments, systolic and diastolic blood pressure measurements may be superimposed on the time plot for the proximal (e.g., aortic) pressure waveform, as shown at  1708  and  1710 , respectively, and/or may be calculated as average values and displayed substantially as shown at  1712 . Similarly, average values for proximal pressure  1704  and distal pressure  1706  may be calculated (e.g., these could be time-weighted averages, moving averages, etc.) and displayed as shown at  1714  and  1716 , respectively. A calculation of FFR based on proximal pressure  1704  and distal pressure  1706  may also be calculated and displayed as shown at  1718 , for example (e.g., FFR equals P p /P d , and the values used for P p  and P d  could be averages or other forms of statistical or numerical representation), according to some embodiments of the invention. Further, some embodiments may include a feature to alert an operator to an FFR value that lies outside of a normal range (e.g., less than 0.75) to indicate, for example, that some other action should be taken (e.g., select and perform an interventional therapy). This could be a visual cue (such as a colored light, as shown at  1720 ), or could be an audible cue (such as an alarm sound, for example). 
     The screen  1702  of  FIG. 17  shows various additional features which may be (optionally or alternately) incorporated in various embodiments. Status area  1722 , for example, may provide information about the patient, date/time, the site within a particular patient, the status of the sensor, and an indication of whether the sensor signal has been “normalized” to another pressure monitoring signal. A normalization button  1724  may be included in some embodiments, and could be used, for example, to normalize the pressure signal from a sensor of sensor delivery device  210 . Normalization might be done during a procedure in which an FFR measurement is desired (e.g., to assess the severity of a stenosis). When a sensor of sensor delivery device  210  is positioned upstream of the stenosis, the measured pressure using the sensor should be equal to the proximal pressure measured using normal blood pressure monitoring equipment (e.g., via the pressure transducer  1618  of the injection system shown in  FIG. 16 , for example). In one embodiment, an operator would position the sensor  240  of sensor delivery device  210  upstream of a location of interest and press the normalization button  1724  of screen  1702 , which could then automatically adjust or calibrate the pressure signal from sensor  240  to match the proximal pressure measured using normal blood pressure monitoring equipment. 
     The screen  1702  of  FIG. 17  may also include navigational features, in some embodiments, which may allow an operator to view and record information that may be of interest. For example, a cursor button  1726  may allow an operator to position a marker or cursor  1727  to a point of interest on the waveforms  1704 ,  1706 , which could provide instantaneous measured values of P p (t)  1704  and P d (t)  1706  at a selected point in time. In some embodiments, an operator may elect to save the cursored data by pressing a “save” button  1728 , which could save the highlighted data for review at a later point in time. A review button  1730  may be provided for this purpose in some embodiments, allowing a user to compare previous historical measurements to current ones and use this information to make diagnostic and therapeutic decisions. In some embodiments, it may be desirable to include a “zoom” feature, for example, to analyze the data. For example, an operator may wish to zoom in (e.g., via the +arrow of zoom  1732 ) to look more closely at certain data, or may instead wish to zoom out (e.g., via the −arrow of zoom  1732 ) to evaluate overall trends, for example. 
     A Physiological Sensor Delivery Device has been described in connection with exemplary embodiments and exemplary preferred embodiments and implementations, as examples only. It will be understood by those having ordinary skill in the pertinent art that modifications to any of the embodiments or preferred embodiments may be easily made without materially departing from the scope of the appended claims.