Patent Publication Number: US-2002002332-A1

Title: Sensing apparatus and methods

Description:
[0001] This invention relates to apparatus and methods for sensing parameters, especially biological parameters. In particular, the invention relates to measuring blood flow rate in a blood vessel in a non-invasive manner.  
       [0002] It is known to transmit ultrasound into a blood vessel and measure the Doppler shift of ultrasound waves scattered by the blood flow. Taking the example of a red blood cell, waves scattered from the blood cell towards a receiver will experience a Doppler shift dependent upon the component of the velocity of the blood cell in the direction of the receiver. In other words, the velocity of scatterers in a blood flow can be deduced from the Doppler shift imparted to the scattered waves. If the size of the cross-section of the stream of flowing blood is known or can be deduced, then, given the flow velocity, the flow rate can be calculated. A knowledge of the rate of blood flow is desirable in many (e.g., medical) situations.  
       [0003] It is desirable to make accurate measurements of such flow rates. To increase the accuracy of a measurement made using a Doppler technique, it is desirable to measure flow velocity at different points across the blood flow to estimate the variation of flow velocity within the flow. Measurements of flow velocity at different locations across the flow can be made by pulsing the ultrasound transmitter. At a given instant in time, the signal acquired by the receiver corresponds to ultrasound waves being scattered from the present location of the transmitted pulse. Therefore, by determining a start and an end time for reception, i.e. by time-gating the received signal, the location from which the scattered ultrasound waves are received may be dictated. Several locations on the transmission path across the blood flow can be examined by “multi-gating” the received scattered ultrasound waves.  
       [0004] Having determined the velocity profile within the blood flow, an average flow velocity may be determined. To derive the flow rate, a knowledge of the cross sectional area of the flow is required. A conventional method of measuring this cross sectional area involves scanning a gate across the blood flow and seeking the points at which the Doppler shift of the received scattered ultrasound waves first appears and then eventually disappears.  
       [0005] It is generally desirable that biological parameter sensing devices such as those using Doppler ultrasound are placed in close proximity to the actual material whose parameters are to be determined. In the case of blood flow measurement, therefore, it is advantageous to have the transmitter and receiver placed on or near the exterior surface of the appropriate blood vessel. For measurements during surgical procedures, this rarely presents a significant problem. Pre- or post-surgically, however, the measurement of, for example, blood flow in the major central vessels using such sensing techniques is very difficult because of the surrounding structures and tissues which cause attenuation of and interference with the receivable signal of interest. Thus there is a need for a method of accurately determining biological parameters such as arterial or venous blood flow and blood vessel properties in a non-surgical, non-invasive manner.  
       [0006] It is an object of this invention to provide non-invasive sensing apparatuses and methods of accurately determining biological parameters without the need for surgical intervention.  
       [0007] According to one aspect, the invention provides a method of remotely locating biological material interfaces, the method comprising transmitting a signal into the biological material using a sensing apparatus located in an exogenous conduit placed within a bodily tract with access to the extracorporeal environment, detecting the signals returned from at least one pair of locations along the path of the transmitted signal and determining from the returned signals whether there is an interface between each pair of locations.  
       [0008] According to another, and related, aspect of the invention, there is provided apparatus for remotely locating biological material interfaces, the apparatus comprising means for transmitting a signal into the biological material, means for detecting the signals returned from at least one pair of locations along the path of the transmitted signal, means for determining from the returned signals whether there is an interface between each pair of locations, means for passing at least the transmission and detection means along an exogenous conduit placed within a bodily tract with access to the extracorporeal environment and means for remotely controlling the position of the transmission and detection means within the exogenous conduit.  
       [0009] The invention thus provides a method of locating a biological material interface, such as the boundary between a blood flow and an artery wall, which is capable of improved accuracy and which is extracorporeally controllable and capable of being applied to a subject without surgical intervention.  
       [0010] In a preferred embodiment, the detecting step comprises detecting signals from a plurality of pairs of locations along the transmission path so as to notionally sweep a pair of locations (or time-gates) along the path. Preferably, the locations in each pair abut one another. The selected pairs of locations may be chosen so that the notional sweeping action is an essentially continuous sweeping action.  
       [0011] Preferably, the transmission and detection means are capable both of being moved along the exogenous conduit and of being rotated within the exogenous conduit about a longitudinal axis of the exogenous conduit.  
       [0012] Advantageously, the determining step comprises comparing the returned signals on the basis of at least one of their energy and frequency. Preferably, the transmitted signal is an ultrasonic signal.  
       [0013] In a particularly preferred embodiment, the method comprises locating two biological material interfaces, and may involve determining their separation. In this way, it is possible to determine the boundaries of, for example, a fluid flow in a biological vessel. With knowledge of the boundaries of such a flow, the flow rate may be determined.  
       [0014] According to another aspect, the invention provides a method of remotely aligning a sensor relative to a biological vessel, comprising providing a sensor capable of being passed, using passing means, along an exogenous conduit placed within a bodily tract with access to the extracorporeal environment and which sensor transmits a signal along a path, remotely positioning the sensor such that the path intersects the biological vessel, detecting the signals returned from the biological vessel, and determining from the returned signals whether the path intersects substantially the center of the biological vessel.  
       [0015] According to a further, and related, aspect, the invention also provides apparatus for remotely sensing a biological vessel, the apparatus comprising sensing means which transmits a signal along a path, detector means for detecting signals returned from the biological vessel, determining means for determining from the returned signals whether the transmission path intersects substantially the center of the biological vessel, means for passing at least the transmission and detection means along an exogenous conduit placed within a bodily tract with access to the extracorporeal environment and means for remotely controlling the position of the transmission and detection means within the exogenous conduit.  
       [0016] Thus the invention facilitates the improvement of the non-surgical remote alignment of a sensor with a biological vessel to improve the accuracy of measurements made on the latter.  
       [0017] Advantageously, the signals returning from the biological vessel are detected for several different orientations of the sensor relative to the biological vessel. These detected signals may then be used to determine the location of the center of the biological vessel relative to the sensor path and permit the sensor to be realigned so that the path substantially intersects the center of the biological vessel.  
       [0018] In a preferred embodiment, signals returned from at least one location on the path are detected and the sensor is realigned until at least one property of the returned signals is optimised. In one case, signals returned from just one location on the path are detected and the sensor is realigned until, for at least one property, the value thereof for signals returned from said one location is optimised. In an alternative arrangement, signals returned from a plurality of locations on the path are detected and the sensor is realigned until, for at least one property, the total of the values thereof for signals returned from the locations is optimised.  
       [0019] In a preferred embodiment, the transmission means provides two paths for conveying signals between the sensing means and the processing means, the two paths being arranged such that they experience substantially the same interference and/or distortion. This provides that the value of the difference between the signals on the two paths is less effected by interference and/or distortion. In one embodiment, the transmission means may comprise a pair of twisted wires.  
       [0020] Advantageously, the sensor apparatus further comprises isolating means for preventing potentially damaging signals being conveyed between the processing means and the sensing means or the living body via the transmission means. This provides for the protection of the equipment and the test subject. Where the transmission means comprises said aforementioned two paths, the isolating means ideally comprises means for providing a signal indicative of the difference between the signals on the paths of the transmission means. For example, the isolating means may be a transformer, possibly of the single-turn, air-gap kind.  
       [0021] In the preferred embodiment, the sensing means comprises at least one piezoelectric transducer. It is particularly advantageous to employ dual transmission and detection transducers with a known, fixed angle between their signal paths. As is known in the art, this arrangement obviates the need to know the angle of incidence of the path with the biological vessel in order to calculate Doppler shift-generating parameters.  
       [0022] The means for passing the transmission and detection means along the exogenous conduit may comprise a flexible cable which is capable, in use, of being rotated about its longitudinal axis without developing substantial elastic twisting forces. The remote control of the position of the sensor within the exogenous conduit may be achieved by the use of a rotatable knob connected to the extracorporeal end of the passing means. The rotation of the knob causes rotation of the sensor within the exogenous conduit. Means, such as a potentiometer based resolver, may be used to measure the angular orientation of the sensor. This angular information can be used in displaying data derived from the detected returned signals (e.g. Blood flow velocities) in a desired format, for example, a swept radar-like plot.  
       [0023] In a preferred embodiment, the exogenous conduit is an oesophageal feeding tube, such as is commonly passed through the mouth or nose, along the oesophagus and into the stomach, and which is hereafter referred to simply as a nasogastric tube. In this preferred embodiment, the biological vessel under interrogation may be the descending aorta, in which case the biological material interface of interest would be the interior wall of the descending aorta, the sensor residing in the nasogastric tube at a depth where the oesophagus is in close proximity to the descending aorta.  
       [0024] Thus, the apparatus and method of this invention provide means for determining characteristics of cardiovascular function such as blood flow velocity, flow profile, vessel diameter and cardiac output. These parameters are conveniently determined by passing the sensor along a ready-sited tube, such as a nasogastric tube, and positioning the sensor accordingly so that it is in close proximity to a blood vessel, such as the descending aorta. The apparatus can thus be readily employed by non-surgical personnel in a convenient manner on the ward, with minimal need for the attendance of physicians to assist in positioning the sensor. 
     
    
    
     [0025] By way of example only, certain embodiments of the invention will now be described with reference to the accompanying drawings, in which:  
     [0026]FIGS. 1 and 2 illustrate schematically the application of time-gated Doppler ultrasound measurements to the descending aorta using a sensor located in a nasogastric tube;  
     [0027]FIG. 3 is a plot illustrating the processing performed by the apparatus used in FIGS. 1 and 2;  
     [0028]FIGS. 4 and 5 schematically illustrate a method of aligning a Doppler ultrasound sensor located within a nasogastric tube with the descending aorta. 
    
    
     [0029] In FIG. 1, a sensor  10  comprising an ultrasonic transmitter and an ultrasonic receiver and attached to the end of a rotatable cable  11  passed along a nasogastric tube  13  sited along the oesophagus  15  transmits ultrasonic pulses along path  12  which intersects the descending aorta  14 . Note that for simplicity, this and the other Figures only show one transmission path  12  whereas, in practice, it is advantageous to use dual fixed angle paths, as described above. There is a blood flow in a direction of the arrow F in the artery  14 . The transmitted ultrasonic pulses are scattered by the blood flow. Sensor  10  acquires ultrasonic signals scattered back along path  12  to sensor  10 . As is well known in the art, a single piezoelectric transducer can be used for both transmitting and receiving ultrasonic signals.  
     [0030] The reception of the scattered ultrasonic signals by sensor  10  is time-gated to examine particular locations along path  12 . The acquisition of returned signals at sensor  10  is timed so that the returned signals correspond to ultrasonic waves scattered upon the transmitted pulse reaching the desired locations on the transmission path  12 . Two time-gated locations  16  and  18  are shown in FIG. 1. The locations abut one another on the transmission path  12 , as indicated by line A-A.  
     [0031] The energy of an acquired signal returning from a given location on path  12  is dependent upon the number of scattering particles in the blood flow at that particular location. In addition, the frequency of the signals returning from a given location will experience a Doppler shift which is dependent upon the vector velocity of the flow at that particular location relative to the reception path (in this case path  12 ). Thus, for a given location along path  12 , there are two specific measurable quantities of the returning signals: energy and Doppler shift. There is also a third, derivable quantity: the product of the energy and the Doppler shift. These three quantities can be regarding as indicator parameters. If an indicator parameter is deduced for each of locations  16  and  18 , then the ratio, R, of the indicator parameter values for locations  16  and  18 , as shown in FIG. 1, will be near unity. The ratio R will tend to unity as the extent of locations  16  and  18  along path  12  about line A-A (governed by the time-gating of the sensor  10 ) tends to zero.  
     [0032] A plot of the ratio R for distance S from the sensor  10  is shown in FIG. 3. The value of R for the distance along path  12  corresponding to line A-A in FIG. 1 is indicated A in FIG. 3.  
     [0033]FIG. 2 illustrates the situation where sensor  10  is time-gated to examine two adjacent locations  20  and  22  which meet at a point on path  12  indicated by line B-B. It should be noted also that the intersection of line B-B and path  12  lies on the wall of the descending aorta  24  furthest from sensor  10 . It will be appreciated that location  20  lies substantially within the blood flow F and that location  22  lies substantially outside the descending aorta. Thus, the energy scattered from location  20  towards sensor  10  will be considerably different to the amount of energy scattered back from location  22 . Similarly, since location  22  is outside the blood flow F, the Doppler shift of signals scattered to sensor  10  from this location will be approximately zero. On the other hand, the signals scattered to sensor  10  from location  20  will exhibit a significant Doppler shift. Considering, therefore, the ratio for any of the aforementioned indicator parameters (energy, Doppler shift or product thereof), it will deviate significantly from unity at point B-B on path  12 . Point B-B is illustrated by a notch at B in the plot of R in FIG. 3.  
     [0034] In operation, the sensor  10  is arranged to time-gate the acquisition of returning signals on path  12  so as to scan a pair of adjacent reception locations along path  12 . Effectively, this corresponds to scanning notional line A-A of FIG. 1 (or notional line B-B of FIG. 2) along path  12 . The sensor is arranged to monitor the ratio R of the values of an indicator parameter from each of the adjacent reception locations. When the ratio R deviates significantly or rapidly from unity, it is determined that the position of a wall of the descending aorta (such as  24 ) has been encountered. Hence, the separation of the aortic walls may be determined, allowing subsequent calculation of the blood flow rate at this point of the circulation.  
     [0035] It is particularly advantageous to base the ratio R under investigation upon an indicating parameter which has a Doppler shift dependence. Because the Doppler shift falls to zero beyond the blood flow, this means that the interfaces located by monitoring the ratio R are then not the walls of the artery, but the true boundaries of the blood flow therein, hence allowing a yet more accurate measure of flow rate (there may be a region adjacent the arterial walls where there is an insignificant net blood flow).  
     [0036] In making blood flow measurements based on the Doppler shift technique using pulsed-ultrasound, time-gated sensors located in, for example, a nasogastric tube, it is essential to know the position of the transmission path relative to the central axis of the blood vessel under investigation. This information is used in ensuring the correct positioning of the sensor and can be used in the calculation of an overall flow rate for the blood vessel from Doppler-shift-derived velocity measurements at discrete points along the sensor&#39;s transmission path within the blood vessel.  
     [0037]FIG. 4 illustrates, in cross-section, the descending aorta  26  which is assumed to have a circular cross-section. A sensor  28 , located within a nasogastric tube  29  sited along the oesophagus (not shown), is used to investigate the blood flow rate within the descending aorta  26 . The sensor  28  comprises a piezoelectric transmitter/receiver which transmits ultrasonic pulses along path  30 , and receives returned signals travelling in the opposite direction on the same path. The sensor  28  is time-gated to acquire returning signals which correspond to a sequence of consecutive locations along path  30 . Locations  32  and  34  are the terminal locations of this sequence. In FIG. 4, the transmission path  30  intersects substantially the central axis of the descending aorta  26 . Thus, a maximum number of the acquisition locations  32  to  34  will be within the descending aorta  26 . Thus, for any of the aforementioned indicator parameters, the cumulative total of the values for all of the locations  32  to  34  will assume a maximum value for the orientation shown in FIG. 4.  
     [0038] In FIG. 5, the transmission path  30  no longer intersects the central axis of the descending aorta  26 . Thus, a fewer number of acquisition locations  32  to  34  fall within the descending aorta  26  than in the orientation shown in FIG. 4. Thus, the cumulative total of the values of a given indicator parameter for acquisition locations  32  to  34  will be lower in the FIG. 5 orientation than in the FIG. 4 orientation. By rotating sensor  28  using the flexible cable ( 11  in FIGS. 1 and 2) to maximise the cumulative total of the values of an indicator parameter totalled across the acquisition locations  32  to  34 , misalignment can be minimised.  
     [0039] The minimisation of the cumulative total of an indicator parameter may be achieved manually by allowing a user to monitor the cumulative total of the indicator parameter in question whilst repositioning the sensor  28 . Alternatively, the sensor  28  may contain processing circuitry which monitors the cumulative total of the indicator parameter in question as the sensor  28  is repositioned, the processing circuitry noting a maximum value which indicates a minimum misalignment.  
     [0040] In the method described with reference to FIGS. 4 and 5, a consecutive sequence of gates is used. In an alternative arrangement, the sensor  28  could be arranged to acquire returning signals from a single location along path  30  within the descending aorta  26 . The previously-defined misalignment could be minimised by examining, as the sensor  28  is repositioned, the variation of a Doppler-shift-based indicator parameter of signals returned from the single location on the path  30 . A Doppler-shift-based indicator parameter will have a dependence upon the velocity of the flow at the location sensed. The distribution of velocities within the aortic vessel  26  allows these determinations to be made. The blood flow velocity is highest on the central axis of the vessel  26  and decreases radially to zero at the blood vessel walls (assuming, amongst other things, that the aorta has a circular cross-section).  
     [0041] It will be apparent to one skilled in the art that the apparatus and methods described herein would not be limited in their application to analysis of the descending aorta from an oesophageal position. Thus, other bodily tracts or cavities could be intubated and used for the passage of the transmission and detection means, such that other blood vessels or fluid transport systems within the body could be interrogated.