Patent Publication Number: US-2020297313-A1

Title: Tissue and vascular pathway mapping utilizing photoacoustic and ultrasound techniques

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to imaging and mapping vascular pathways and surrounding tissue with photoacoustic and ultrasound modalities. 
     BACKGROUND 
     Innovations in diagnosing and verifying the level of success of treatment of disease have migrated from external imaging processes to internal diagnostic processes. In particular, diagnostic equipment and processes have been developed for diagnosing vasculature blockages and other vasculature disease by means of ultra-miniature sensors placed upon the distal end of a flexible measurement apparatus such as a catheter, or a guide wire used for catheterization procedures. For example, known medical sensing techniques include angiography, intravascular ultrasound (IVUS), forward looking IVUS (FL-IVUS), fractional flow reserve (FFR) determination, a coronary flow reserve (CFR) determination, optical coherence tomography (OCT), trans-esophageal echocardiography, and image-guided therapy. 
     For example, intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for assessing a diseased vessel, such as an artery, within the human body to determine the need for treatment, to guide the intervention, and/or to assess its effectiveness. There are two general types of IVUS devices in use today: rotational and solid-state (also known as synthetic aperture phased array). For a typical rotational IVUS device, a single ultrasound transducer element is located at the tip of a flexible driveshaft that spins inside a plastic sheath inserted into the vessel of interest. In side-looking rotational devices, the transducer element is oriented such that the ultrasound beam propagates generally perpendicular to the longitudinal axis of the device. In forward-looking rotational devices, the transducer element is pitched towards the distal tip so that the ultrasound beam propagates more towards the tip (in some devices, being emitted parallel to the longitudinal centerline). The fluid-filled sheath protects the vessel tissue from the spinning transducer and driveshaft while permitting ultrasound signals to propagate from the transducer into the tissue and back. As the driveshaft rotates, the transducer is periodically excited with a high voltage pulse to emit a short burst of ultrasound. The same transducer then listens for the returning echoes reflected from various tissue structures. The IVUS medical sensing system assembles a two dimensional display of the tissue, vessel, heart structure, etc. from a sequence of pulse/acquisition cycles occurring during a single revolution of the transducer. In order to image a length of a vessel, the transducer element is drawn through the vessel as it spins. 
     In contrast, solid-state IVUS devices utilize a scanner assembly that includes an array of ultrasound transducers connected to a set of transducer controllers. In side-looking and some forward-looking IVUS devices, the transducers are distributed around the circumference of the device. In other forward-looking IVUS devices, the transducers are a linear array arranged at the distal tip and pitched so that the ultrasound beam propagates closer to parallel with the longitudinal centerline. The transducer controllers select transducer sets for transmitting an ultrasound pulse and for receiving the echo signal. By stepping through a sequence of transmit-receive sets, the solid-state IVUS system can synthesize the effect of a mechanically scanned transducer element but without moving parts. Since there is no rotating mechanical element, the transducer array can be placed in direct contact with the blood and vessel tissue with minimal risk of vessel trauma. Furthermore, because there is no rotating element, the interface is simplified. The solid-state scanner can be wired directly to the medical sensing system with a simple electrical cable and a standard detachable electrical connector. While the transducers of the scanner assembly do not spin, operation is similar to that of a rotational system in that, in order to image a length of a vessel, the scanner assembly is drawn through the vessel while stepping through the transmit-receive sets to produce a series of radial scans. 
     Rotational and solid-state state IVUS are merely some examples of imaging modalities that sample a narrow region of the environment and assemble a two- or three-dimensional image from the results. Other examples include optical coherence tomography (OCT), which has been used in conjunction with ultrasound systems. One of the key challenges using these modalities with in a vascular pathway is that they are limited in gathering data on anatomy beyond the vessel walls. Although OCT imaging may yield higher resolution than IVUS imaging, OCT has particularly limited penetration depth and may take more time to image a region of tissue. 
     Another recent biomedical imaging modality is photoacoustic imaging. Photoacoustic imaging devices deliver a short laser pulse into tissue and monitor the resulting acoustic output from the tissue. Due to varying optical absorption throughout the tissue, pulse energy from the laser pulse causes differential heating in the tissue. This heating and associated expansion leads to the creation of sound waves corresponding to the optical absorption of the tissue. These sound waves can be detected and an image of the tissue can be generated through analysis of the sound waves and associated vascular structures can be identified, as described in U.S. Patent Publication 2013/0046167 titled “SYSTEMS AND METHODS FOR IDENTIFYING VASCULAR BORDERS,” which is hereby incorporated by reference in its entirety. 
     Accordingly, for these reasons and others, the need exists for improved systems and techniques that allow for the mapping of vascular pathways and surrounding tissue. 
     SUMMARY 
     Embodiments of the present disclosure provide devices, systems, and methods that combine photoacoustic and IVUS imaging techniques. The devices, systems, and methods may be used to image and/or map vascular pathways and surrounding tissue. 
     In some embodiments, a medical sensing system is provided comprising one or more laser emitters configured to emit laser pulses to tissue of a patient in a region of interest; a measurement apparatus configured to be placed within a vascular pathway in the region of interest, wherein the measurement apparatus comprises at least one transducer, wherein the measurement apparatus is configured to: receive sound waves generated by the tissue as a result of interaction of the laser pulses with the tissue; transmit ultrasound signals; and receive ultrasound echo signals based on the transmitted ultrasound signals; a processing engine in communication with the measurement apparatus, the processing engine operable to produce an image of the region of interest based on the received sound waves and the received ultrasound echo signals; and a display in communication with the processing engine, the display configured to visually display the image of the region of interest. 
     In some embodiments, the measurement apparatus further comprises at least one ultrasound transducer configured to transmit ultrasound signals and receive ultrasound echo signals based on the transmitted ultrasound signals. The at least one ultrasound transducer may be further configured to receive sound waves generated by the tissue as a result of interaction of the laser pulses with the tissue. In some embodiments, the measurement apparatus further comprises at least one photoacoustic transducer configured to receive sound waves generated by the tissue as a result of interaction of the laser pulses with the tissue. The measurement apparatus may further comprise at least one photoacoustic transducer configured to receive sound waves generated by the tissue as a result of interaction of the laser pulses with the tissue. 
     In some embodiments, one or more laser emitters are disposed outside the body of the patient. The region of interest may comprise the vascular pathway and a region of tissue surrounding the vascular pathway. In some embodiments, the at least one photoacoustic transducer and the at least ultrasound transducer are configured to alternate in measuring sound waves and ultrasound echo signals, wherein during the time that the at least one photoacoustic transducer and the at least one ultrasound transducer is measuring sound waves or ultrasound echo signals, the other of the at least one photoacoustic transducer and the at least one ultrasound transducer does not measure sound waves or ultrasound echo signals. The processing engine may be further operable to control the operation of the at least one transducer, may be operable to activate the at least one transducer to transmit ultrasound signals, and may be operable to activate the at least one transducer to receive at least one of the sound waves and the ultrasound echo signals. 
     In some embodiments, the at least one transducer is disposed circumferentially around a distal portion of the measurement apparatus. The at least one transducer may be coupled to a drive member that rotates the at least one transducer around a longitudinal axis of the measurement apparatus. The apparatus may comprise two or more laser emitters configured to emit laser pulses to tissue of the patient in the region of interest, and the two or more laser emitters may be configured to emit laser pulses simultaneously. 
     In some embodiments, at least one of the two or more laser emitters is configured to emit laser pulses at an oblique angle with respect to a longitudinal axis of the measurement apparatus. The one or more laser emitters may be disposed on an array outside the body of the patient. Furthermore, the array may have an arcuate shape. 
     In some embodiments, method of producing an image of a region of interest is provided, comprising: transmitting, with a laser source disposed outside a body of a patient, a focused laser pulse on tissue in a region of interest having at least one vascular pathway; receiving, with at least one photoacoustic sensor positioned within the vascular pathway of the region of interest, sound waves generated by the tissue in response to an interaction of the focused laser pulse with the tissue; transmitting, with at least one ultrasound transducer positioned within the vascular pathway of the region of interest, ultrasound signals toward the tissue in the region of interest; receiving, with the at least one ultrasound transducer positioned within the vascular pathway of the region of interest, ultrasound echo signals of the transmitted ultrasound signals; producing an image of the region of interest based on the received sound waves and the received ultrasound echo signals; and outputting the image of the region of interest to a display. 
     In some embodiments, the method further comprises rotating at least one of the at least one photoacoustic sensor and the at least one ultrasound transducer within the vascular pathway about a longitudinal axis of an intravascular device to which the at least one photoacoustic sensor and/or the at least one ultrasound transducer is coupled. The method may also comprise positioning two or more laser sources outside the body of the patient around the region of interest. In some embodiments, the method further comprises positioning the two or more laser sources in an arcuate array outside the body of the patient around the region of interest. The two or more laser sources may transmit focused laser pulses simultaneously. 
     Additional aspects, features, and advantages of the present disclosure will become apparent from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the present disclosure will be described with reference to the accompanying drawings, of which: 
         FIG. 1A  is a diagrammatic schematic view of a medical sensing system according to some embodiments of the present disclosure. 
         FIG. 1B  is a diagrammatic schematic view of a medical sensing system according to some embodiments of the present disclosure. 
         FIG. 1C  is a diagrammatic schematic view of a medical sensing system with an exemplary sensor array according to some embodiments of the present disclosure. 
         FIG. 1D  is a diagrammatic schematic view of a medical sensing system with another exemplary sensor array according to some embodiments of the present disclosure. 
         FIG. 2  is a diagrammatic, perspective view of a vascular pathway and surrounding tissue with an instrument positioned within the pathway and an external emitter according to an embodiment of the present disclosure. 
         FIG. 3  is a diagrammatic, perspective view of a vascular pathway and surrounding tissue with an instrument positioned within the pathway and two external emitters according to an embodiment of the present disclosure. 
         FIG. 4  is a diagrammatic, perspective view of a vascular pathway and surrounding tissue with an instrument positioned within the pathway and an external emitter array according to an embodiment of the present disclosure. 
         FIG. 5  is a flow diagram of a method for mapping a vascular pathway with photoacoustic and ultrasound modalities according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It is nevertheless understood that no limitation to the scope of the disclosure is intended. Any alterations and further modifications to the described devices, systems, and methods, and any further application of the principles of the present disclosure are fully contemplated and included within the present disclosure as would normally occur to one skilled in the art to which the disclosure relates. For example, while the intravascular sensing system is described in terms of cardiovascular imaging, it is understood that it is not intended to be limited to this application. The system is equally well suited to any application requiring imaging within a lumen or cavity of a patient. In particular, it is fully contemplated that the features, components, and/or steps described with respect to one embodiment may be combined with the features, components, and/or steps described with respect to other embodiments of the present disclosure. For the sake of brevity, however, the numerous iterations ofthese combinations will not be described separately. 
       FIG. 1A  is a diagrammatic schematic view of a medical sensing system  100  according to some embodiments of the present disclosure. The medical sensing system  100  may include a measurement apparatus  102  (such as a catheter, guide wire, or guide catheter). As used herein, “measurement apparatus” or “flexible measurement apparatus” includes at least any thin, long, flexible structure that can be inserted into the vasculature of a patient. While the illustrated embodiments of the “measurement apparatus” of the present disclosure have a cylindrical profile with a circular cross-sectional profile that defines an outer diameter of the flexible measurement apparatus  102 , in other instances, all or a portion of the flexible measurement apparatus  102  may have other geometric cross-sectional profiles (e.g., oval, rectangular, square, elliptical, etc.) or non-geometric cross-sectional profiles. Flexible measurement apparatus  102  may include, for example, guide wires, catheters, and guide catheters. In that regard, a catheter may or may not include a lumen extending along all or a portion of its length for receiving and/or guiding other instruments. If the catheter includes a lumen, the lumen may be centered or offset with respect to the cross-sectional profile of the device. 
     The medical sensing system  100  may be utilized in a variety of applications and can be used to assess vessels and structures within a living body. To do so, the measurement apparatus  102  is advanced into a vessel  104 . Vessel  104  represents fluid filled or surrounded structures, both natural and man-made, within a living body that may be imaged and can include for example, but without limitation, structures such as: organs including the liver, heart, kidneys, as well as valves within the blood or other systems ofthe body. In addition to imaging natural structures, the images may also include man-made structures such as, but without limitation, heart valves, stents, shunts, filters and other devices positioned within the body. The measurement apparatus  102  includes one or more sensors  106  disposed along the length of the apparatus  102  to collect diagnostic data regarding the vessel  104 . In various embodiments, the one or more sensors  106  correspond to sensing modalities such as IVUS imaging, pressure, flow, OCT imaging, transesophageal echocardiography, temperature, other suitable modalities, and/or combinations thereof. 
     In the exemplary embodiment of  FIG. 1A , the measurement apparatus  102  includes a solid-state IVUS device, and the sensors  106  include one or more IVUS ultrasound transducers and/or photoacoustic transducers and associated control. As used herein, a “photoacoustic transducer” includes at least a sensor configured to detect photoacoustic waves generated as a result of the interaction of optical pulses with tissue. In one embodiment, a photoacoustic transducer utilizes the same ultrasound detection mechanism as an IVUS ultrasound transducer. In some implementations, a single transducer can serve as both an IVUS transducer and a photoacoustic transducer. In another embodiment, a photoacoustic transducer uses a dedicated photoacoustic wave detection mechanism distinct from that of an IVUS ultrasound transducer. The system of  FIG. 1A  may include aspects of phased-array IVUS devices, systems, and methods associated with the Eagle Eye® Platinum catheter available from Volcano Corporation as well as those described in U.S. Pat. No. 7,846,101 and/or U.S. patent application Ser. No. 14/812,792, filed Jul. 29, 2015, each of which is hereby incorporated by reference in its entirety. 
     The sensors  106  may be arranged around the circumference of the measurement apparatus  102  and positioned to emit ultrasound energy radially  110  in order to obtain a cross-sectional representation of the vessel  104  and the surrounding anatomy. When the sensors  106  are positioned near the area to be imaged, the control circuitry selects one or more IVUS transducers to transmit an ultrasound pulse that is reflected by the vessel  104  and the surrounding structures. The control circuitry also selects one or more transducers to receive the ultrasound echo signal. By stepping through sequences of transmit-receive sets, the medical sensing system  100  system can synthesize the effect of a mechanically scanned transducer element without moving parts. 
     In one embodiment, the sensors  106  are disposed circumferentially around a distal portion of the measurement apparatus  102 . In another embodiment, the sensors  106  are contained within the body of the measurement apparatus  102 . In other embodiments, the sensors  106  are disposed radially across the measurement apparatus  102 , on a movable drive member connected to the measurement apparatus  102 , or on one or more planar arrays connected to the measurement apparatus  102 . 
     In some embodiments, the processing engine  134 , which may be included in the console  116 , combines the imaging data acquired from both the IVUS and photoacoustic modalities into a single visualization. This use of both IVUS and photoacoustic modalities may provide a number of advantages over traditional systems using a single modality. First, the addition of photoacoustic sensors may allow for higher resolution mapping than traditional IVUS methods alone. Second, the combination of IVUS and photoacoustic modalities may allow for faster imaging speeds than OCT imaging or other methods. Third, the combination may allow for two-dimensional and/or three-dimensional imaging of the tissue surrounding vascular pathways. Fourth, the use of photoacoustic imaging may expand the diagnostic scope of an IVUS mapping procedure by including more of the surrounding tissue. In particular, the combined IVUS and photoacoustic mapping can allow for detection of certain types of cancers, tissue damage, and the mapping of multiple vascular pathways without sacrificing the dependability of ultrasound in detecting plaques, stenosis, and other forms of vascular diseases. Fifth, combining these two modalities may allow substantial costs savings because existing IVUS systems may be adapted to mapping systems using both modalities. Sixth, due to the interaction of optical pulses with tissue and the omni-directional emission of photoacoustic waves from the tissue, an optical pulse need not be emitted along the same axis as the transducer. This allows for more flexibility in carrying out combined photoacoustic and IVUS procedures, and may allow for precise mapping procedures even along deep or convoluted vascular pathways. Seventh, the mapping capabilities of the present disclosure may be integrated with some forms of laser therapy. For example, diagnosis of diseases in tissue may be accomplished using the optical emitter in diagnostic mode. After a diagnosis, the optical emitter can be switched to a treatment mode. In this regard, the map of the vasculature and surrounding tissue may be used to guide the application of the treatment. After the optical treatment is finished, the optical emitter can be switched back to diagnostic mode to confirm treatment of the diseased portion of tissue. 
     Sensor data may be transmitted via a cable  112  to a Patient Interface Module (PIM)  114  and to console  116 , as well as to the processing engine  134  which may be disposed within the console  116 . Data from the one or more sensors  106  may be received by a processing engine  134  of the console  116 . In other embodiments, the processing engine  134  is physically separated from the measurement apparatus  102  but in communication with the measurement apparatus (e.g., via wireless communications). In some embodiments, the processing engine  134  is configured to control the sensors  106 . Precise timing of the transmission and reception of signals may be used to map vascular pathways  104  in procedures using both IVUS and photoacoustic modalities. In particular, some procedures may involve the activation of sensors  106  to alternately transmit and receive signals. In systems using one or more IVUS transducers that are configured to receive both photoacoustic and ultrasound signals, the processing engine  134  may be configured to control the state (e.g., send/receive) of one or more transducers during the mapping of the vascular pathway and surrounding tissue. 
     Moreover, in some embodiments, the processing engine  134 , PIM  114 , and console  116  are collocated and/or part of the same system, unit, chassis, or module. Together the processing engine  134 , PIM  114 , and/or console  116  assemble, process, and render the sensor data for display as an image on a display  118 . For example, in various embodiments, the processing engine  134 , PIM  114 , and/or the console  116  generates control signals to configure the sensor  106 , generates signals to activate the sensor  106 , performs amplification, filtering, and/or aggregating of sensor data, and formats the sensor data as an image for display. The allocation of these tasks and others can be distributed in various ways between the processing engine  134 , PIM  114 , and the console  116 . 
     In addition to various sensors  106 , the measurement apparatus  102  may include a guide wire exit port  120  as shown in  FIG. 1A . The guide wire exit port  120  allows a guide wire  122  to be inserted towards the distal end in order to direct the member  102  through a vascular structure (i.e., the vessel)  104 . Accordingly, in some instances the measurement apparatus  102  is a rapid-exchange catheter. Additionally or in the alternative, the measurement apparatus  102  can be advanced through the vessel  104  inside a guide catheter  124 . In an embodiment, the measurement apparatus  102  includes an inflatable balloon portion  126  near the distal tip. The balloon portion  126  is open to a lumen that travels along the length of the IVUS device and ends in an inflation port (not shown). The balloon  126  may be selectively inflated and deflated via the inflation port. In other embodiments, the measurement apparatus  102  does not include balloon portion  126 . 
       FIG. 1B  is a schematic view of a system that includes an alternative measurement apparatus  102  according to some embodiments of the present disclosure. The measurement apparatus  102  of  FIG. 1B  is typical of a rotational device such as a rotational IVUS ultrasound system and the one or more sensors  106  include one or more IVUS transducers arranged to emit ultrasound energy in a radial direction  110 , as well as one or more photoacoustic transducers. Again, a single transducer may serve as both an IVUS transducer and a photoacoustic transducer. In such an embodiment, the one or more sensors  106  may be mechanically rotated around a longitudinal axis of the measurement apparatus  102  to obtain a cross-sectional representation of the vessel  104 . The system of  FIG. 1B  may include aspects of rotational IVUS devices, systems, and methods associated with the Revolution® catheter available from Volcano Corporation as well as those described in U.S. Pat. Nos. 5,243,988, 5,546,948, and 8,104,479 and/or U.S. patent application Ser. No. 14/837,829, filed Aug. 27, 2015, each of which is hereby incorporated by reference in its entirety. 
       FIGS. 1C and 1D  show alternative sensor arrays  128  that may be used in conjunction with the measurement apparatus  102  according to some embodiments of the present disclosure. In particular, the sensor array  128  may include one or more sensors  106  and emitters including IVUS transducers, IVUS emitters, photoacoustic transducers, and optical emitters. In  FIG. 1C , the sensor array  128  is disposed around the circumference of the measurement apparatus  102 . Sensors  106  of two more different types are placed in the sensor array  128 . In particular, sensors of a first type  130  are placed in the sensor array  128  with sensors of a second type  132 . In the example of  FIG. 1C , the sensors of the first and second types  130 ,  132  are disposed on the array  128  in an alternating manner. In some embodiments, sensors of the first and second types  130 ,  132  are disposed on the array  128  in a checkerboard configuration such that individual sensors of the first type  130  are not adjacent to each other. Additionally, sensors of the first and second types  130 ,  132  may take up roughly equal proportions of the area of the array  128 . Although they appear as square or rectangular in the example of  FIG. 1C , sensors of the first and second types  130 ,  132  may have circular, elliptical, polygonal, or other shapes. Sensors of the first and second types  130 ,  132  may be spaced across the measurement apparatus  120  or they may be placed flush against each other. 
     In the example of  FIG. 1D , a sensor array  128  is shown with sensors of two or more different types  130 ,  132  disposed in alternating rows. These rows may be disposed axially and may extend part way or completely around the measurement apparatus  102 . In some embodiments, rows of sensors placed in a staggered formation and the ends of individual rows are not flush. In some embodiments, rows of sensors are placed adjacent to each other with no space in between. Alternatively, rows of sensors are spaced across the measurement apparatus  102  with space therebetween. In some cases, 2, 3, 4, or 5 rows of alternating sensors are disposed on the measurement apparatus  102 . As discussed above, the array  128  may be configured to rotate around an axis of the measurement apparatus  102 . 
     The systems of the present disclosure may also include one or more features described in U.S. Provisional patent application Ser. No. ______ (Attorney Docket No. IVI-0083-PRO/44755.1587PV01), ______ (Attorney Docket No. IVI-0086-PRO/44755.1592PV01), ______ (Attorney Docket No. IVI-0087-PRO/44755.1590PV01), and/or (Attorney Docket No. IVI-0088-PRO/44755.1589PV01), each of which is filed on the same day herewith and incorporated by reference in its entirety. 
       FIG. 2  is a diagrammatic, perspective view of a vascular pathway  104  and surrounding tissue  210  with a measurement apparatus  102  such as that depicted in  FIGS. 1A -ID disposed within the vascular pathway  104 . An optical emitter  220  is also shown emitting an optical pulse  230  toward an area of interest within the tissue. In some embodiments, the area of interest includes part of a vascular pathway  104  as well as adjacent tissue. In some embodiments, the optical emitter  220  is a laser source that emits short laser pulses toward the area of interest. These laser pulses interact with the tissue  210 , generating a series of photoacoustic waves  240  that propagate through the tissue  210  as well as through the vascular pathway  104 . The photoacoustic waves  240  are received by the sensors  106  connected to the measurement apparatus  102 . The sensors  106  may also image and/or map the vascular pathway  104  independently of the photoacoustic waves  240 , by transmitting ultrasound signals toward the vessel walls and receiving the corresponding reflected ultrasound echoes. 
     An operator may move the measurement apparatus  102  through the vascular pathway  104  to image and/or map the vascular pathways  104 . In some cases, the optical emitter  220  is configured to emit optical pulses  230  toward the sensors  106  of the measurement apparatus. Accordingly, the optical emitter  220  may be moved at a similar speed and direction as the measurement apparatus  102 . 
       FIG. 3  shows a mapping system using two optical emitters  220  that emit optical pulses toward the sensors  106  of the measurement apparatus  102 . In some embodiments, at least one of the optical emitters  220  is configured to emit optical pulses at an oblique angle with respect to a longitudinal axis of the measurement apparatus  102 . The use of two or more optical emitters  220  may allow more accurate mapping of the area of interest. In particular, the emission of optical pulses  230  from different sources may generate interference patterns  250  between the photoacoustic waves  240  emanating from the tissue  210 . These interference patterns may be analyzed by the processing engine  134  to produce additional data points for use in tissue mapping. In some embodiments, the optical emitters emit optical pulses into the tissue  210  in different patterns. In some embodiments, three, four, five, or six optical emitters are used together to map a region of interest. 
       FIG. 4  shows a measurement apparatus  102  disposed within a vascular pathway  104  and surrounding tissue  210 . An emitter array  400  comprising a plurality of optical emitters  410  is disposed around the tissue  210 . The example of  FIG. 4  shows an emitter array  400  with an arcuate ring shape. The emitter array  400  may also have a hexagon, octagon, or other polygonal shape, etc. This shape may allow for placement of the emitter array  400  around an extremity of a patient, such as around an arm or leg. In other embodiments, the emitter array  400  has one or more flat surfaces that allow at least a portion of the emitter array  400  to be placed parallel to a flat portion of tissue  210 , such as the abdomen of a patient. In some embodiments, the optical emitters  410  simultaneously emit optical pulses  240  into the tissue  210 . In other embodiments, the optical emitters  410  alternatively emit optical pulses  240  into the tissue  210 . For example, the emitters  420  may emit pulses consecutively around the circumference of the emitter array  400 . The use of such an emitter array  400  may allow for a faster photoacoustic imaging rates by providing imaging and/or mapping of a large swath of tissue without rotation of an optical emitter itself, which may be particularly useful for imaging and/or mapping extremities. Additionally, the use of an emitter array  400  may allow for the simultaneous production of photoacoustic waves in different areas of tissue  210  while avoiding destructive interference between waves. 
     In one embodiment, a plurality of transducers, each corresponding to an optical emitter  410  of the emitter array  400 , are included in the sensors. In this case, the emitter array  400  and measurement apparatus  102  are moved at a similar speed to ensure that each transducer receives photoacoustic signals generated as a result of the corresponding emitter  410 . In an alternative embodiment, the individual emitters  410  do not correspond to individual sensors. In this case, the emitter array  400  can contain a different number of emitters  410  than the number of sensors. Each sensor instead receives the signals that are directed toward its respective location regardless of the emitter  410  that caused the generation of the signals. 
       FIG. 5  is a flow chart showing a method  500  of mapping an area of interest using both photoacoustic and IVUS modalities. It is understood that additional steps can be provided before, during, and after the steps of method  500 , and that some of the steps described can be replaced or eliminated for other embodiments of the method. In particular, steps  504 ,  506 ,  508 , and  510  may be performed simultaneously or in various sequences as discussed below. 
     At step  502 , the method  500  can include activating an external laser source. This laser source may be the optical emitter  220  of  FIG. 3 . In some cases, the external laser source is activated by a communication system  250  by means of an electronic or optical signal. This signal may be sent wirelessly, and the external laser source may be equipped with a wireless signal receiver. 
     At step  504 , the method  500  can include focusing a laser pulse on tissue in a region of interest having a measuring device and one or more sensors within a vascular pathway. In some embodiments, the region of interest includes a portion of tissue including a portion of at least one vascular pathway  104 , and the measuring device may be disposed within the vascular pathway  104 . The region of interest may be chosen based on a suspected or diagnosed problem in the tissue, or based on the proximity of a region of tissue to problems within a vascular pathway  104 . In other embodiments, the region of interest is part of a more general mapping plan. For example, a mapping plan for a section of a vascular pathway  104  may involve the mapping of tissue surrounding the vascular pathway  104  along its length. The one or more photoacoustic sensors may be disposed on or within the measurement apparatus. In some cases, the one or more photoacoustic signals are disposed circumferentially around the measurement apparatus. The interaction of the emitted laser pulse and tissue in the region of interest creates a number of photoacoustic waves  240  that emanate from the tissue. 
     In some embodiments, the measuring device is the measurement apparatus  102  depicted in  FIGS. 1A, 1B, 2A-2E, 3, and 4 . In some embodiments, sensors are those depicted in  FIGS. 1A, 1B, and 2-4 , and can be included in the sensor array  128  depicted in  FIGS. 2A-2E . The sensors can include IVUS transducers, IVUS emitters, OCT transducers, photoacoustic transceivers, and optical emitters. The sensors can be arranged in any of the examples depicted in  FIGS. 2A-2E . In some embodiments, the sensors do not rotate as they travel through the vascular pathway  104 . In other embodiments, the sensors are included in a rotational array disposed on a revolving portion of the measurement device. In some embodiments, the sensors are disposed circumferentially around the measurement device. 
     At step  506 , the method  500  can include receiving sound waves generated by the interaction of the laser pulse and tissue with one or more photoacoustic sensors. In some cases, the one or more photoacoustic sensors can function as a traditional IVUS imaging element. In other cases, the one or more photoacoustic sensors are dedicated to receiving photoacoustic waves without IVUS functionality. In some embodiments, the one or more photoacoustic sensors are controlled by a communication system  250  like that depicted in  FIGS. 3 and 4 . In another embodiment, a processing engine  130  or a PIM  114  may control sensors on a sensor array  128 . Signals may be sent from processing engine  130  or the PIM  114  to the sensors, including the one or more photoacoustic sensor, via connector  234 , causing the one or more photoacoustic sensor to receive diagnostic information such as sound waves or ultrasound signals. 
     At step  508 , the method  500  can include transmitting ultrasound signals into the vascular pathway  104  with one or more ultrasound transducers. The ultrasound signals may be transmitted toward the walls of the vascular pathway  104  and may be deflected off the walls of the vascular pathway  104  and propagate through the vascular pathway  104  as ultrasound echo signals. In some embodiments, the one or more ultrasound transducers are disposed on a solid-state array positioned within the vascular pathway. The one or more ultrasound transducers may be coupled to a drive member that rotates around a longitudinal axis of the measurement apparatus. The transmitted ultrasound signals are deflected off the walls of the vascular pathway  104  and propagate through the vascular pathway  104  as ultrasound echo signals. 
     At step  510 , the method  500  can include receiving the ultrasound echo signals with the one or more ultrasound transducers. In some embodiments, the one or more ultrasound transducers may be operable to receive sound waves as well as ultrasound signals. The one or more ultrasound transducers of step  508  and the one or more ultrasound transducers of step  510  may be combined in a single element, or the transducer elements may be separate. 
     Steps  504 ,  506 ,  508 , and  510  may be coordinated in the method  500  and occur in various orders based on the desired outcome of a medical procedure. For example, transmission of ultrasound signals and reception of ultrasound echo signals can occur at regular intervals throughout the method  500 , while reception ofphotoacoustic waves may occur sporadically. This may be the case in a medical procedure to map a vascular pathway  104  and spot-check trouble areas of tissue surrounding sections of the vascular pathway  104 . Alternatively, steps  504 ,  506 ,  508 , and  510  are performed successively. For example, steps  506 ,  508 , and  510  may be performed individually before proceeding to the next step to avoid signal noise and allow for adequate signal processing when method  500  is used in a system where a photoacoustic sensor and ultrasound transducer are each included in a transducer array. Furthermore, the steps of method  500  may be interleaved in various orders. 
     At step  512 , the method  500  can include producing an image of the region of interest, including the vascular pathway  104  and surrounding tissue, based on the sound waves and the ultrasound echo signals. In some embodiments, a processing engine (such as the processing engine  130  of  FIG. 1A ) in communication with the sensors produces the image of the region of interest. This image can include both two-dimensional and three-dimensional images based on the received sensor data. In some cases, the image includes a number of two-dimensional cross sections of the vascular pathway  104  and surrounding tissue. 
     At step  514 , the method  500  can include outputting the image of the region of interest to a display  118 . This display  118  can include a computer monitor, a screen on a patient interface module (PIM)  114  or console  116 , or other suitable device for receiving and displaying images. 
     In an exemplary embodiment within the scope of the present disclosure, the method  500  repeats after step  514 , such that method flow goes back to step  504  and begins again. Iteration of the method  500  may be utilized to map a vascular pathway and surrounding tissue. 
     Persons skilled in the art will recognize that the apparatus, systems, and methods described above can be modified in various ways. Accordingly, persons of ordinary skill in the art will appreciate that the embodiments encompassed by the present disclosure are not limited to the particular exemplary embodiments described above. In that regard, although illustrative embodiments have been shown and described, a wide range of modification, change, and substitution is contemplated in the foregoing disclosure. It is understood that such variations may be made to the foregoing without departing from the scope of the present disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the present disclosure.