Ablation monitoring system and method

Abstract

A system and method are presented for treating target tissue using cryoablation. The introducer cannula and the cryoprobe are inserted into the target tissue. The cryoprobe cools and forms ice balls. The cryoprobe is removed while the ice ball is still in freezing, and the ultrasound catheter is inserted. The ultrasound generated within the hockey is used to determine the distance from the ultrasound conduit to the outer edge of the hockey. And repeating the steps at different angles to simulate the slice of the ice hockey. The ultrasound catheter is moved radially and the process is repeated to create a model of at least a portion of the ice hockey. The hockey model may be displayed on a set of alignment images representing the target tissue to ensure that the tissue is located within the treatment area of the hockey.

Description

[0001] Cross - Reference to Related Applications

[0002] This invention is a divisional application of the application with the filing date of August 17, 2020, application number 202080069686.3, and invention title "Ablation Monitoring System and Method". The parent application is a PCT national - phase application with an international filing date of August 17, 2020, application number "PCT / US2020 / 046613", claiming the benefit of U.S. Patent Application No. US16 / 546,652 filed on August 21, 2019, the entire content and essence of which are hereby incorporated by reference in their entirety. Technical Field

[0003] A medical device ultrasound catheter system and method are presented to allow determination of the size, shape, and location of an ice ball during cryosurgery. Similar systems and methods can be used to measure the size, shape, and location of thermally ablated tissue. Background Art

[0004] Cryosurgery or cryoablation is a surgery that destroys abnormal or target tissue through a freezing process. Freezing of tissue cells causes rupture of the cells or intracellular organelles. The cryosurgery procedure requires insertion of a device ("cryoprobe") into the abnormal tissue, followed by cooling of the device. In most cases, cooling of the cryoprobe is accomplished by passing a high - pressure gas (such as argon) through the device. Cooling the cryoprobe in this way creates an "ice ball" of frozen tissue, which is roughly centered at the distal end of the cryoprobe.

[0005] It is important to accurately determine the size, shape, and location of the ice ball for the success of the surgery. If the ice ball is larger than needed, healthy tissue surrounding the target tissue will be unnecessarily damaged. If the ice ball is too small, the abnormal tissue that would be killed by the process will survive.

[0006] Traditionally, the size and location of the ice ball have been determined by ultrasound techniques. Ultrasound energy passes through normal tissue and then strikes the outer surface of the ice ball. Due to the properties of the ice ball, the ultrasound energy typically bounces back from the ice ball. This large reflection allows ultrasound imaging techniques to image the surface of the ice ball closest to the ultrasound energy source. Unfortunately, due to the "shadow" projected by this surface reflection, the ultrasound energy cannot penetrate the ice ball and cannot show the true three - dimensional size and shape of the ice ball. In essence, existing methods allow the user to find the approximate location of the closest surface of the ice ball, but not its true size, shape, or location. Summary of the Invention

[0007] An embodiment of the present invention presents a method for treating tumors or other target tissues using cryoablation. The method first identifies the location of the target tissue, for example, by performing preoperative CT imaging. The images created can be combined into a 3-D image, or into a 3-D model of the patient or the patient's organ. During the operation, an ultrasound catheter contained within an introducer sheath is inserted into the target tissue. In one embodiment, the ultrasound catheter and the sheath are inserted percutaneously. Once positioned, the ultrasound catheter can be used to image the target tissue and ensure the correct positioning of the catheter and the sheath. The ultrasound catheter may be capable of performing QUS analysis on the tissue.

[0008] Once the positions of the ultrasound catheter and the sheath are confirmed, the ultrasound catheter is removed from the sheath, and a cryoprobe is inserted into the sheath position. Alternatively, the cryoprobe and the sheath can be directly inserted percutaneously into the target tissue without using the ultrasound catheter. In this alternative embodiment, the cryoprobe is positioned with the aid of external ultrasound to ensure that the tip of the cryoprobe is correctly positioned within the target tissue. In some cases, it is necessary to insert multiple cryoprobes into the target tissue in order to provide some control over the size and shape of the ice ball generated during the cryoablation procedure.

[0009] The cryoprobe is then cooled to form an ice ball within the patient. Ideally, the ice ball will be large enough to completely surround the target tissue. To increase the effectiveness of the ice ball in killing the target tissue, the ice ball will be generated frequently, allowed to thaw, and then regenerated by cooling the cryoprobe a second time.

[0010] The cryoprobe is then removed from the introducer sheath while the ice ball is still in a frozen state. The ultrasound catheter is inserted into the sheath and into the channel left in the ice ball due to the removal of the cryoprobe. Using pulse-echo techniques and beamforming, a strong signal is emitted from the ultrasound transducer at the end of the ultrasound catheter in a single radial direction. The same direction is then monitored for the ultrasound energy reflected from the periphery of the ice ball. Using the time taken for the ultrasound signal to return, and the known speed of ultrasound in frozen tissue, the radial distance from the ultrasound transducer to the edge of the ice ball in the selected direction is known. Similar signals are emitted and received in other directions in order to generate information sufficient to simulate a slice having the same shape and size as a portion of the ice ball that has just been examined by the ultrasound beam. The ultrasound catheter can then be moved a known distance within the sheath, and the process repeated to create a second slice. When this process has been repeated enough times to calculate slices of the entire ice ball, the slices are combined into a single model showing the size and shape of the ice ball.

[0011] In one embodiment, an ultrasound catheter includes an EM sensor at its tip. Using an EM navigation system, the position and orientation of the ultrasound catheter can be determined for each slice of the created ice ball model. Assuming the EM navigation system is registered with the pre-operative CT image or model, the created ice ball model can be overlaid on the CT image. Since the CT image identifies the size and location of the target tissue, it will be evident whether the generated ice ball fully encompasses the target tissue within its effective treatment area. In some embodiments, the software compares the known size, shape, and location of the target tissue with the determined size, shape, and location of the generated ice ball, and provides a warning if the target area is not within the treatment zone of the ice ball. If necessary, a new ice ball can be created to treat the missing portion of the target area, and another model of this new ice ball can be generated to ensure the effectiveness of the treatment.

[0012] Cryoablation is just one ablation technique, and embodiments thereof can be used to determine the size, shape, and location of the ablated tissue. Similar systems and methods can be used with thermal ablation (such as microwave or radiofrequency ablation). An ultrasound catheter is inserted into the thermally ablated tissue, and pulse echo techniques can be used to create model slices of the ablated tissue. Then, multiple slices can be combined into a complete ablated tissue model, which can be displayed on a 3-D image of the target tissue to determine the effectiveness of the ablation. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a side perspective view of a cryoprobe inserted percutaneously under ultrasound guidance.

[0014] Figure 2 is Figure 1 a schematic diagram of the cryoprobe penetrating the target tissue and generating an ice ball.

[0015] Figure 3 is a schematic diagram of the generated ice ball, showing three isotherms.

[0016] Figure 4 is after the second cryoablation Figure 3 a schematic diagram of the generated ice ball.

[0017] Figure 5 is a schematic diagram of an irregular ice ball generated using three cryoprobes.

[0018] Figure 6 is the Figure 5 irregular ice ball seen through standard ultrasound techniques.

[0019] Figure 7 is a schematic diagram of an Figure 5 irregular ice ball with one cryoprobe removed.

[0020] Figure 8where an ultrasonic probe is inserted to replace the removed cryoprobe Figure 5 Schematic diagram of an irregular ice ball.

[0021] Figure 9 is a plan view of the distal tip of an embodiment of an ultrasonic probe within a guide cannula.

[0022] Figure 10 is a first graph showing the reflected ultrasonic signal versus time.

[0023] Figure 11 is a second graph showing the reflected ultrasonic signal versus time.

[0024] Figure 12 is a schematic diagram of the calculated dimensions of a single slice of an ice ball.

[0025] Figure 13 is a schematic diagram of the calculated dimensions of an ice ball including multiple slices.

[0026] Figure 14 is a flowchart showing a method for implementing an embodiment of the present invention.

[0027] Figure 15 is a schematic diagram of an irregularly shaped area of ablated tissue ablated using thermal ablation.

[0028] Figure 16 is a plan view of the distal tip of a second embodiment of an ultrasonic probe within a guide cannula.

[0029] Figure 17 is a side perspective view of the percutaneous insertion of an ultrasonic probe rotated by an electric stepper motor. Detailed Description

[0030] Ice Ball Formation

[0031] Cryoablation is commonly used to kill abnormal tissue that has been identified in a patient prior to surgery. In most cases, the exact location of the abnormal tissue is identified by imaging using conventional techniques such as CT or MRI imaging. After determining that cryoablation is suitable for the abnormal tissue, the patient is prepared and the abnormal tissue is repositioned prior to the start of the procedure. In Figure 1 this, an external ultrasonic device 100 is used to identify the location of the target tissue. The same ultrasonic device is then used to monitor the insertion of the tip of the cryoprobe 110 into the patient 120. The tip of the cryoprobe 110 can be specifically designed to enhance the tip's identifiability under ultrasound, for example, by applying grooves or other physical distortions that are highly visible to ultrasonic energy, thereby enhancing the tip's identifiability under ultrasound. In this way, the tip of the cryoprobe 110 is percutaneously directed towards the target tissue. As described below in connection with Figure 14As explained, an inserted ultrasound catheter can also be used to identify the location of the target tissue and position the cryoprobe 110.

[0032] Figure 1 A computer system (not shown) determines which signals and how much power to send to the ultrasound device 100 and the cryoprobe 110, sends signals and power to these devices 100, 110, receives signals from these devices 100, 110, analyzes these signals, and then displays the analysis results to the user. The computer system that controls and analyzes the signals and power sent to the devices used in the disclosed embodiments and the signals and power sent from the devices used in the disclosed embodiments is a standard computer system that includes a CPU, short-term and long-term memories, computer programming, a display system, and an interface for communicating with the devices (such as devices 100, 110). A computer system such as this also controls the signals sent to the devices described below and the signals sent from the devices described below, and is responsible for performing the calculation and rendering steps in the methods described below, including Figure 14 method 1400.

[0033] Figure 2 The distal end or tip 112 of the cryoprobe 110 is shown after insertion into the abnormal target tissue 200 of the patient 120. In Figure 2 the cryoprobe 110 passes through the lumen of a guide cannula 210 that has been positioned at the target tissue 200. Although a guide cannula 210 is not always required during a cryoablation procedure, the ability to position additional catheter devices at the target tissue 200 makes the use of the guide cannula 210 beneficial for most embodiments of the present invention.

[0034] Once the tip 112 of the cryoprobe 110 is inserted into the target tissue 200, argon gas is passed through the probe 110. The design of the probe 110 causes the gas to expand at or near the tip 112. Since argon cools when it expands, this expansion causes the tip 112 of the probe 110 to cool very rapidly. In a conventional cryoprobe 110, the injection of argon gas will cause the temperature of the tissue near the tip 112 to reach between -170 and -160 degrees Celsius (°C). This temperature will rapidly cause an ice ball 220 of frozen tissue to form near the tip 112 and expand into the target tissue 200.

[0035] Although the temperature of the ice ball 220 formed near the probe may be lower than -160 °C, the surface temperature of the ice ball 220 will remain at 0 °C. To ensure tissue destruction, it is generally considered that the temperature of the tissue should reach -40 °C or reach a lower temperature for about 3 minutes. This temperature causes intracellular ice formation, which is destructive to most cells. Therefore, abnormal tissue is typically frozen for three to five minutes during a cryoablation procedure. At this time, as Figure 3As shown, the ice ball 220 has grown. After this time period, at least half of the diameter of the ice ball 220 will reach -40 °C. This is schematically shown by the shaded region 300 in Figure 3 . The wider shaded region 310 shows the approximate position of the -20 °C isotherm, while the outer surface 320 of the ice ball 220 will have a temperature of -0 °C.

[0036] Since only that part of the ice ball 220 with a sustained temperature of -40 °C can ensure that it has been destroyed, most cryoablation practitioners perform the procedure twice. After the first ice ball 220 is formed, the ice ball is allowed to thaw. The slow thawing of the frozen tissue in the ice ball 220 will cause further cell damage because the thawed ice crystals will fuse to form larger crystals, resulting in further cell damage. The thawing process can be accelerated by passing helium gas through the cryoprobe 110. Different from refrigerating gases such as argon, helium gets hot when it expands. When helium passes through the cryoprobe 110, it will have the opposite effect to argon and will heat the tip 112 of the cryoprobe 110.

[0037] The standard technique for freezing abnormal tissue a second time after thawing will cause the freezing of the tissue to occur faster (which is more destructive to the tissue). This allows tissue destruction to be completed at a slightly higher temperature (for example, between -30 °C and -20 °C). As a result, the effective treatment area of the procedure moves closer to the periphery 320 of the ice ball 220. As Figure 4 shown, the shaded kill region will expand to approximately the -20 °C isotherm 310. In most cases, the distance between the kill region and the periphery of the ice ball is considered to be between 4 mm and 10 mm. Since the outer region of the ice ball 220 will be outside the guaranteed treatment area 310, it is usually necessary to create an ice ball larger than the tissue 200 that is desired to be destroyed during cryosurgery.

[0038] In some cases, it is necessary to create a different shape for the ice ball 220 to match the shape and size of the target tissue 200. In this case, multiple cryoprobes can be inserted into different parts of the tissue 200. In Figure 5In it, the first cryoprobe 110 is connected to the second cryoprobe 510 and the third cryoprobe 520. Although these two additional cryoprobes 510, 520 can be inserted into the target tissue 200 using a guide cannula, only a single probe 110 uses the sheath 210. When the three cryoprobes 110, 510, 520 are cooled, they work together to form a single ice ball 530 having a surface 532 with a uniform but irregular shape. It is possible that some of the cryoprobes 110, 510, 520 will operate at different temperatures, where slightly higher temperatures have a smaller freezing effect. In addition, the manufacture of the cryoprobes 110, 510, 520 will affect the final shape of the ice ball (for example, some probes will form a more spherical shape). By using different designs and temperatures between the cryoprobes 110, 510, 520, the resulting ice ball 530 can be intentionally made to conform to a shape that more effectively kills the target tissue 200 while minimizing damage to surrounding tissue. In Figure 5 it, the resulting ice ball 530 does not destroy all of the target tissue 200 because some of the tissue 200 that is far from the tip of the third cryoprobe 520 remains outside the boundary of the formed ice ball 530. This may be caused by the third cryoprobe 520 not being inserted far enough into the target tissue 200 before the ice ball 530 is formed.

[0039] Although the ultrasound device 100 used to guide the cryoprobes 110, 510, and 520 can be used to monitor the size and position of the ice ball once it is formed, the visible range of the device 100 is limited. As Figure 6 shown, when the ultrasound device 100 is used to image the ice ball 530, the ultrasound sound energy 600 emitted from the device 100 will pass through the unfrozen tissue 620 before hitting the ice ball 530. The device 100 is designed to monitor the returned ultrasound energy. Using the time and intensity information associated with this returned energy, the ultrasound imaging device can create a three-dimensional image of the different tissues of the patient 120 that the energy 600 encounters.

[0040] Unfortunately, the frozen nature of the ice ball 530 makes it highly echogenic to ultrasound 600. In fact, the different physical properties between thawed and frozen tissue (including changes in tissue density and the resulting changes in the speed of sound passing through the tissue) create an acoustic impedance mismatch, which causes the ultrasound energy to bounce back from the ice ball. In addition, the ice ball itself will absorb the ultrasound energy more effectively than the unfrozen tissue. Although using ultrasound, the reflective nature of the ice ball 530 creates a clear image of the ice ball surface 532, the ultrasound energy 600 cannot effectively penetrate this surface 532. This creates an acoustic shadow behind the surface 532, which prevents any tissue or structure behind the surface 532 from appearing in the resulting ultrasound image.

[0041] In addition, because the ultrasonic energy 600 emanates from a single device 100, the energy 600 essentially creates a viewing plane 610 that defines that portion of the surface of the ice ball 532 that will be seen in the ultrasonic image. This is the case even if the device 100 uses a curved ultrasonic array that emits an arcuate ultrasonic energy pattern, or even if a phased array probe that emits a pie-shaped energy pattern is used. In each of these cases, the ultrasonic energy 600 emanates from a single device that will define the effective viewing plane 610. This means that a practitioner using the ultrasonic device 100 will be able to see that the ice ball 530 forms at the correct proximal location, that the width of the ice ball 530 is wide enough to surround the target tissue, and that the proximal surface 632 is sufficiently outside of the target tissue 200 to ensure destruction of the proximal portion of the tissue 200. However, because the practitioner cannot see the shadow formed beyond the nearest surface 632 of the ice ball 530, she cannot determine whether the third cryoprobe 520 has been inserted deep enough into the target tissue 200.

[0042] Ultrasonic catheter

[0043] To overcome this problem, the cryoprobe 110 can be removed from the introducer sheath 210 while the ice ball 530 is still frozen. Although the cryoprobe 110 may initially be frozen in place, a brief application of helium will sufficiently heat the cryoprobe 110 to release the probe 110 without any significant thawing of the ice ball 530. Figure 7 The ice ball 530 from which the cryoprobe 110 has been removed is shown. As shown in this figure, the removal of the cryoprobe 110 leaves an opening or channel 700 within the ice ball 530. Figure 5 The fact that the channel 700 communicates with the sheath 210 means that an ultrasonic catheter 800 can be inserted into the ice ball 530, as

[0044] shown. The catheter 800 will include a plurality of ultrasonic transducers 810 at its distal end. By inserting the ultrasonic catheter 800 directly into the formed ice ball 530, the size and shape of the ice ball 530 can be better understood. Figure 8 The catheter 800 can be constructed in accordance with the disclosures filed as U.S. Provisional Application Nos. 62 / 776,667 and 62 / 776,677, both of which were filed by the owner of the present application on December 7, 2018. The entire contents of these two provisional applications are incorporated herein by reference.

[0045]

[0046] Figure 9 ​An embodiment of an ultrasound catheter 900 is shown. The catheter 900 has a plurality of ultrasound transducer elements 910 near the distal end 902 of the catheter. In a preferred embodiment, a 64-element annular array of ultrasound transducers 910 is arranged around the periphery of the catheter 900. These transducers 910 can be PZT-based, pMUT-based, or cMUT-based transducers and are capable of emitting and detecting ultrasound energy at different frequencies (e.g., frequencies operating from 4 to 50 MHz). Since Figure 9 A plan view of the end of the ultrasound catheter 900 is shown, where the individual transducers 910 are coplanar and arranged on the flat surface of the catheter 900. While this is one possible configuration, in Figure 9 the configuration shown, the transducers 910 are positioned around the periphery of a non-flat surface (e.g., an annular or cylindrical surface, such as a surface formed by using a catheter having a circular, oval, or other circular cross-section).

[0047] The individual transducers 910 can form a phased array, which means that the energy from multiple transducers 910 can work together to form a single directed beam of ultrasound energy. This is typically performed by timing the transmission of ultrasound energy from multiple transducers so as to produce an interference pattern in a single controllable direction. The ultrasound energy emitted in that direction will be greater than the energy that can be emitted from a single transducer. The same principle works when receiving energy, allowing the energy reception at multiple transducers 910 to be individually and carefully delayed and analyzed so as to maximize the signal received by the transducers 910 from a single direction. Using this technique and Figure 9 the annular array of transducers 910 shown, ultrasound signals can be emitted and received from the transducers 910 in a single radial direction.

[0048] In other embodiments, synthetic aperture techniques are used. In this case, a single transmit pulse is sent and received non-focusedly. Then, beamforming algorithms analyze the previously non-focused signals post hoc in order to concentrate the emission / reception of ultrasound energy in a single direction.

[0049] Preferably, the size of the ultrasound catheter 900 is reduced to the smallest possible device (preferably with a diameter less than 2 mm) to allow the transducers 910 to enter the channel 700 created by removing the cryoprobe 110. Additionally, while one embodiment contemplates having at least 64 imaging elements 910, other configurations from 16 elements to more than 256 elements are possible. In fact, Figure 16 (as described below) shows an embodiment having only a single transducer element 1610.

[0050] In one embodiment, the ultrasound catheter 900 is capable of creating images using conventional ultrasound imaging techniques. A variety of ultrasound imaging techniques can be applied, including gray-scale "B-mode" imaging to display echo amplitudes in the scan plane; M-mode imaging to track motion at a given fixed location over time; duplex, color, and power Doppler imaging to display motion in the scan plane; harmonic imaging to display the non-linear response to incident ultrasound; elastography to display relative tissue stiffness; and contrast agent imaging using contrast agents to display blood-filled spaces, or contrast agent imaging using targeted agents to display specific agent-binding tissue types.

[0051] A little-known ultrasound imaging technique is based on quantitative ultrasound or (QUS), which analyzes the power distribution as a function of frequency in the received echo signals backscattered from tissue. QUS utilizes the resulting spectral parameters to characterize and differentiate tissue. The use of QUS allows for the analysis of very small target tissue samples, which effectively creates an "acoustic biopsy" (AB) or can be performed on in-situ tissue (sonic biopsy). Additionally, QUS can be used to analyze the properties of the tumor stroma and microvasculature to provide parameters related to cell death and / or apoptosis, thus providing confirmation or monitoring data for treatments such as chemotherapy, brachytherapy, cytotoxic agents (drugs), or ablation. The analysis can provide interim feedback on the tumor's response to treatment using parameters such as the effective scattering diameter and effective acoustic concentration. The heterogeneity of the tumor or tissue stiffness can be analyzed by evaluating the nodule from multiple different directions and determining the penetration depth of the ultrasound signal.

[0052] A preferred embodiment of the ultrasound catheter 900 further includes an embedded electromagnetic (EM) sensor 920 at the distal end 902. These sensors 920 can be used to guide the catheter 900 within the patient 120. In practice, at least two sensors 920 are arranged adjacent to each other, but in different orientations within the catheter 900 to maximize the available position and orientation information. Veran Medical Technologies has developed a catheter system that uses EM sensors and EM navigation to accurately locate and reach very small tissue masses. This technology is described in detail in U.S. Patent No. 8,696,549, entitled "Apparatus and Method for Four Dimension Soft Tissue Navigation in Endoscopic Applications," the entire contents of which are incorporated herein by reference. This document explains that in most cases, a preoperative computed tomography X-ray (CT) scan can be used to construct a model of the organs within the patient (e.g., the lung airways). Then, during the surgery, electromagnetic navigation uses the sensors 920 on the catheter 900 to provide position and orientation information in 3D space. The EM 3D space is registered with the created CT model, allowing the real-time display of the position of the catheter on the organ model. The Veran system also provides a fourth dimension of time-varying tracking information. Respiratory tracking is performed, changing the apparent position of the probe in the virtual display to match the physical position of the EM sensors as they move with the respiratory motion of the body, which is very useful in this device.

[0053] In the catheter 900, both the EM sensors 920 and the ultrasound transducer array 910 are coupled to an electronics package 930. This electronics package is responsible for operating the individual transducers 910 and for transmitting the received signals along a data transmission path (not shown) through the catheter 900 for digital analysis and display to the practitioner. In one embodiment, the electronics package 930 is responsible for multiplexing the signals from both the EM sensors 920 and the transducers 910 so that they can share a single data path along the catheter 900.

[0054] Puck size determination

[0055] Although the ultrasound catheter 900 is designed to be able to create standard ultrasound images (e.g., by using B-mode imaging) and is designed to use QUS to analyze specific tissues, this imaging technique is not used to determine the size of the ice ball 530. Due to the nature of frozen tissue, the ultrasonic energy transmitted through the tissue will propagate faster than the ultrasonic energy transmitted through normal tissue. In addition, the absorption properties of frozen tissue, as well as the risk of signal reflection even before the ultrasound enters the ice ball, will make it almost impossible to create a normal ultrasound image. In addition, the fact that all the problematic tissues have been frozen may make it difficult to detect the normal tissue differences that can be seen in ultrasound.

[0056] Therefore, the catheter 900 will use a pulse-echo modality that effectively acts as sonar rather than generating an image of the ice ball 530. First, the annular array of the transducer 910 is controlled to send an ultrasonic signal in a single direction. Multiple transducers 910 can be used to send this signal by creating a beamformed signal. Alternatively, a single transducer 910 in the annular array can be used to send an ultrasonic signal in this single direction. The amount of energy emitted in this signal pulse can be greater than the amount of acoustic energy normally transmitted during imaging. In fact, this pulse-echo technique does not require subtlety - rather, the transmitted energy should be maximized.

[0057] The same transducer or multiple transducers 910 that emitted the pulse will also detect the ultrasound after the ultrasound bounces back from the surface of the ice ball 530. As described above, the impedance mismatch between the ice ball 530 and the surrounding unfrozen tissue 620 will cause the ultrasonic signal to reflect when it encounters the periphery 532 of the ice ball 530. This reflection will cause the signal to return to the transducer 910, and they will be detected after the time it takes for the acoustic energy to pass through the ice ball 530 to the surface 532 and then return. Using the phased array technique described above, multiple transducers 910 can receive this energy, where the received energy is filtered to display only the energy received from the same direction as the direction in which the energy was transmitted. Alternatively, the same single transducer that transmitted the signal in a single direction can receive the returned energy. As Figure 10As shown in the graph 1000, the amplitude of the received acoustic energy can be plotted against time. The graph 1000 shows that a large amount of ultrasonic energy is received at the peak 1010 of the graph 1000, and this peak 1010 corresponds to the time 1012. If the time 1012 is T and V is the ultrasonic velocity through the cryo-ball 530, the total distance traveled by the sound received at the peak 1010 is D = V * T. Since the ultrasonic wave must travel to the edge 532 of the cryo-ball 530 and return, the actual distance from the catheter 900 to the edge of the cryo-ball 530 in this direction is 1 / 2V * T. Ultrasonic waves typically travel through unfrozen tissue at a speed of approximately 1540 m / s. However, in frozen tissue, the speed of sound is significantly greater and is between 2500 m / s and 4000 m / s. In addition, it is known that the speed of sound in frozen water increases as the temperature of the ice decreases, and it is expected that the same will be true for the tissue frozen during cryoablation treatment.

[0058] The graph 1000 shows a slightly idealized result of the received ultrasonic energy. In practice, most of the ultrasonic energy emitted by the transducer 910 is likely to be immediately reflected back at the initial boundary of the cryo-ball 530 within the channel 700. In this case, the received ultrasonic energy may appear to be closer to Figure 11 the graph 1100. In this case, the outer boundary 532 of the cryo-ball 530 is visible at the peak 1110 at time 1112. A significant peak 1120 is also seen at time 1122, and the peak 1120 shows an immediate reflection from within the channel 700. This peak 1120 can be ignored when determining the size of the cryo-ball 530, and if the peak 1120 appears too quickly after the pulse transmission, it may not even be detected. Preferably, the ultrasonic transducer 910 is in contact with the frozen tissue of the cryo-ball 530 when inserted into the channel 700. This should reduce any initial reflection of the ultrasonic energy.

[0059] As described above, the ultrasonic pulse generated by the transducer 910 will be emitted in a single direction. This is regarded as Figure 12 the direction 1200 in the schematic diagram. Using the time analysis of the graphs 1000 / 1100 and the above formula, the distance from the catheter 900 to the outer wall 532 of the cryo-ball 530 can be calculated. Once this has occurred, the transducer 910 will send another pulse in a different direction, such as direction 1210, and then determine the distance to the outer boundary 532 in the direction 1210. This is repeated in a third direction 1220 and then repeated over the entire 360-degree range of the transducer 910. In each direction, the distance to the outer boundary 532 is determined by the known position of the catheter 900 ( Figure 12 the position 1202 in). As Figure 12As shown, when these distances are combined, a slice 1230 representing the size and shape of the outer edge 532 of the ice ball is created. In this embodiment, each pulse is radially emitted away from the transducer 910 (position 1202) of the catheter 900, which means that the created slice 1230 will only represent the size of the ice ball 530 at the current position of the transducer 910.

[0060] The process of creating slices 1230 at different positions within the ice ball 530 can be repeated by physically sliding the catheter 900 within the guide cannula 210. In one embodiment, the catheter 900 starts at the farthest position within the channel 700 and then moves a distance between 2 mm and 10 mm along the channel 700 between each created slice (e.g., slice 1230). When these slices are combined, a relatively complete model 1300 of the size and shape of the ice ball 530 is created, as Figure 13 shown. To create these different slices, the catheter 900 moves along the Figure 13 path shown as element 1302 with respect to the channel 700 and the guide cannula 210. Note that the path 1302 is not the "axis" or "center point" of the slices that make up the model 1300, as there is no requirement that the outer wall 532 of the ice ball 530 be centered on the channel 700 created by the cryoprobe 110.

[0061] As described above, the physical position of the catheter tip 902 can be identified at any time using EM navigation and the signals created by the EM sensor 920. Thus, the 3D model 1300 of the ice ball 530 can be positioned in 3D space and then superimposed on a registered CT image showing the target tissue 200. Using this technique, a practitioner can identify areas where the kill zone of the ice ball 530 fails to enclose the target tissue 200. In some embodiments, the model of the target tissue 200 in the CT image is automatically compared with the 3D model 1300 of the ice ball 530, and the practitioner is automatically warned that the target tissue 200 remains outside the effective zone of the ice ball 530. Using this feedback, the practitioner can reinsert the cryoprobe 110 and refreeze the tissue using the existing positions of the probes 110, 510, 520, while using a longer or more intense freezing cycle. Alternatively, the practitioner can insert additional probes to treat the unfrozen areas of the target tissue 200.

[0062] Process 1400

[0063] As Figure 14 shown in the flowchart of, the various steps described above can be combined into a process or method 1400. The first step 1405 in this process is the only step that is significantly different from the steps described above. In combination with Figure 1 and Figure 2, it is explained that the ultrasound device 100 can assist in guiding the cryoprobe 110 into the target tissue 200. In an alternative embodiment, using EM navigation, the EM sensor 920 on the ultrasound catheter 900 can be guided to the target tissue 200. Once the ultrasound transducer 910 is positioned within the (unfrozen) target tissue 200, an image of the tissue 200 can be obtained. In one embodiment, QUS is used to assist in diagnosing or otherwise analyzing the target tissue. The ultrasound imaging created by the sensor 910 at step 1405 can be used to ensure that the introducer sheath 210 is properly placed for inserting the cryoprobe 110. At step 1410, the ultrasound catheter 900 is removed, and at step 1415, the cryoprobe 110 is inserted through the introducer sheath 210 into the same position. In some cases, additional cryoprobes 510, 520 can be inserted into the target tissue 200 to create an ice ball 530 of an appropriate size and shape.

[0064] At step 1420, the target tissue is frozen using the cryoprobe. As described above, the freezing process often involves two different freezing operations separated by thawing of the ice ball 530 to improve the effectiveness of the ice ball 530.

[0065] At step 1425, the cryoprobe 110 is removed, creating a channel within the ice ball 530 that can accommodate the introducer sheath 210. At step 1430, the ultrasound catheter 900 is inserted into this channel.

[0066] Then a 3D model of the ice ball size is created using the ultrasound catheter 900. This is carried out through steps 1435 to 1455 of method 1400. At step 1435, a single direction for emitting an ultrasound energy pulse is selected. Then the echo from the outer edge 532 of the ice ball 530 in that direction is detected. Using the known ultrasound speed in the frozen tissue, the distance from the catheter 900 to the outer edge 532 in that direction is determined. Then this process is repeated at different angles until the size and shape of a single slice of the ice ball 530 are determined at step 1440. Laboratory tests are used to ideally determine the angle between each pulse so as to obtain a good compromise between enhanced detail in the model of a single slice obtained with small angles between the pulses and the speed of model generation obtained by using larger angles between the pulses. As the speed of analyzing each pulse increases with the improvement of computing technology, the preferred angle will decrease. In one embodiment, the angle between the pulses is selected to be between 5 and 20 degrees. At step 1445, the catheter 900 is moved, and steps 1430 to 1445 are repeated to generate multiple slices, each slice showing the shape and size of the ice ball 530 at the position along the movement path of the catheter 900 (step 1450). Then at step 1455, these multiple slices are combined into a single 3D model of the ice ball 530.

[0067] Because the catheter 900 includes the EM sensor 920, the 3D model can be positioned, sized, and oriented in the CT images for EM navigation. Step 1460 displays the 3D model on these images.

[0068] At step 1465, any portions of the target tissue 200 that have been missed by the effectiveness of the created and modeled hockey puck 530 are determined. This can be done using computer software that compares the size, shape, and orientation of the 3D model with the known size, shape, and orientation of the target tissue 200. Since both the 3D model and the target tissue can be displayed simultaneously on the display, some type of distinguishable visual feature can be used to present the viable portions of the target tissue. For example, viable tissue can be presented in a unique color or using a different brightness (brighter or darker) than the surrounding tissue. Whatever feature is used, it is important for the practitioner to be able to immediately see and identify which portions of the target tissue are not within the kill zone of the modeled hockey puck 530. The viable tissue can then be processed at step 1470. In some cases, the entire process 1405 - 1465 can be repeated through step 1470 to ensure that all of the target tissue 200 has been destroyed. In other cases, it may only be necessary to refreeze one or more of the cryoprobes 110, 510, 520 in a manner that changes the size and shape of the hockey puck 530 in order to effectively freeze the viable target tissue. The method then ends at step 1475.

[0069] Thermal ablation applications

[0070] The novel process for visualizing ablated tissue described above has applications beyond cryosurgery. Target tissue can be ablated by a variety of techniques such as microwave ablation and radiofrequency ablation. Microwave ablation applies electromagnetic waves in the microwave spectrum (from 300 MHz to 300 GHz) to kill tissue in the target area. Water in the tissue absorbs the microwave radiation, thereby heating and killing the tissue. Radiofrequency ablation is similar in that (this time in the radiofrequency spectrum) electromagnetic waves are used to heat and kill the target tissue. In both cases, the electromagnetic waves are emitted through a needle that is inserted directly into the target tissue. The needle is guided to the target tissue in the same manner as the cryoprobe 110 described above, and then a heating signal is emitted from the tip of the needle. This means that it needs to be insertable percutaneously, laparoscopically, or during surgery. In each case, a introducer cannula such as the cannula 210 described above can be used to insert the needle into the target tissue.

[0071] To apply the above techniques in the case of thermal ablation, an RF or microwave ablation needle 1510 is inserted through a guide cannula 1520 and into the target tissue 1500. In the case of microwave ablation, additional microwave needles 1512, 1514 can also be inserted into the target tissue 1500 to conform to the final shape of the ablation zone 1530. It is generally not possible to activate multiple radiofrequency ablation needles simultaneously. Nevertheless, by using multiple insertions of the same ablation needle, or by using different needles while ensuring that no two needles are activated simultaneously, multiple ablation source locations can still be achieved in radiofrequency ablation. Whether multiple needles or multiple insertions are used, the use of multiple ablation source locations will create an irregularly shaped zone of ablated tissue 1530.

[0072] As described above, the physician performing the ablation procedure needs to know whether the zone 1530 of killed / ablated tissue has successfully killed the patient's target tissue 1500. To determine this, the ablation needle 1510 inserted through the guide cannula 1520 is removed, and an ultrasound catheter is inserted through the same guide cannula 1510 into the middle of the ablated tissue 1530. Using the same techniques described above, the size, shape, location, and orientation of the ablated tissue 1530 relative to the target tissue 1500 can be determined. Obviously, since the ablated tissue 1530 is heated rather than frozen, the above calculations are slightly modified. It is well known that ultrasound energy travels much faster in frozen tissue than in normal tissue, making it difficult to perform three-dimensional imaging of the frozen ice ball using standard ultrasound imaging techniques. It is also true that ultrasound energy propagates very differently through the heated ablated tissue 1530, and this difference again makes it difficult to image the ablated tissue 1530 using standard ultrasound techniques. By using pulse-echo techniques and beamforming, and by modifying the above algorithms to use the speed of ultrasound energy in thermally ablated tissue rather than the speed within the ice ball, a model of the ablated tissue 1530 can be generated and compared to the size and location of the target tissue 1500. As described above in connection with cryotherapy, this process can determine that the thermally ablated tissue 1530 fails to include all of the target tissue 1500, causing the physician to re-perform RF or microwave ablation to ensure that portions of the target tissue 1500 outside the original thermal ablation zone 1530 are properly treated.

[0073] Single transducer catheter

[0074] Figure 16An alternative embodiment of the ultrasound catheter 1600 is shown, where there is only one transducer element 1610 near the tip 1602 of the catheter 1600. By using a single larger transducer 1610, the amount of ultrasound energy transmitted perpendicular to the transducer 1610 can be maximized. In terms of receiving the reflected ultrasound energy from the distal boundary 532 of the puck 530, the same large transducer 1610 will also be more sensitive. The unidirectional, single-transducer pulse optimizes the ultrasound catheter 1600 for the sonar-like pulses required to perform the above method 1400.

[0075] The transducer 1610 is preferably flat. In one embodiment, the transducer is located on the flat surface 1620 of the tip of the ultrasound catheter 1600. The flat surface 1620 can extend throughout the length of the catheter 1600, or as Figure 16 shown, the flat surface 1620 can terminate at position 1622. The position 1622 separates the tip portion having the flat surface 1620 from the remaining portion 1630 of the catheter 1600. Thus, the remaining portion 1630 can have a circular or substantially circular cross-section to facilitate movement within the introducer sheath 210. The cross-section of the tip portion having the flat surface 1620 can be semi-circular, with its circular bottom (not shown) located above the flat surface 1620, and the flat surface 1620 supports the transducer 1610.

[0076] As Figure 16 shown, the catheter 1600 can also include an embedded electromagnetic (EM) sensor 1640 at the distal end 1602. The functions of these EM sensors 1640 are as described above in connection with the sensor 920. The electronics package 1642 is coupled to the single transducer 1610 and the EM sensors 1640 to control the signals sent to and received from these components.

[0077] The use of an ultrasound catheter with a single transducer 1610 allows for greater energy transmission in a single direction and better signal detection. Although the use of a single transducer 1610 significantly reduces the ability of catheter 1600 to generate ultrasound images, this reduced functionality is irrelevant to the context of method 1400. However, the lack of an annular array of transducers 910 used on catheter 900 means that single-transducer catheter 1600 must be rotated to generate the aforementioned slices (e.g., slice 1230). This rotation can be performed manually in the same manner as a practitioner rotates other catheters. The requirement to manually perform a 360-degree rotation highlights the importance of accurate position measurement, which can only be obtained through the use of EM sensor 1640. Manual rotation of catheter 1600 by a practitioner may result in unintentional translational movement of catheter 1600 relative to introducer sheath 210. Such unintentional position changes can be registered by detecting the position of EM sensor 1602. For each angular distance measurement (step 1435 ), the accurate current position of catheter 1600 may result in less uneven “slices” but will result in an equally accurate overall model of puck 530 .

[0078] Figure 17 An alternative embodiment is shown in FIG, which illustrates the use of a single-transducer catheter 1600 for a patient, inserted through an introducer sheath 210. An electric stepper motor 1700 is attached to the catheter 1600. The stepper motor is physically engaged with the catheter 1600, enabling the motor 1700 to control the rotation of the catheter 1600. The number of steps of the motor 1700 is correlated with a given degree of rotation of the catheter, allowing a control signal to be sent along a control line 1710 to cause the motor 1700 to rotate the catheter 1600 as commanded. In this way, the practitioner does not need to physically rotate the catheter 1600. Instead, under the control of the control line 1710, the motor 1700 can time the rotation of the single transducer 1600 so that an entire slice measurement of the puck 530 can be performed as quickly and efficiently as possible with minimal translational movement of the transducer 1600 during the rotation cycle.

[0079] Many features and advantages of the present invention are apparent from the above description. Those skilled in the art will readily appreciate numerous modifications and variations. While such modifications are possible, the present invention is not limited to the exact construction and operation shown and described. Rather, the present invention should be limited only by the following claims.

Claims

1. A computer-implemented method for generating a model of an ice ball formed in a patient's body, comprising: Receiving, by a computer, input data indicative of a position of a guide cannula of an ultrasound catheter inserted into a target tissue; Triggering, by the computer, emission of ultrasound energy in a direction radially away from the ultrasound catheter within the ice ball and detecting reflected ultrasound energy from a periphery of the ice ball; And Calculating, by the computer, a distance between the ultrasound catheter and the periphery of the ice ball based on a time between emission and reception of the ultrasound energy and a known speed of ultrasound propagation in frozen tissue.

2. The computer-implemented method according to claim 1, wherein, Emitting the ultrasound energy includes emitting a plurality of directional ultrasound pulses in directions radially away from the ultrasound catheter in a plurality of radial directions within the ice ball, and determining, by the computer, a distance from each radial direction to the periphery of the ice ball.

3. The computer-implemented method according to claim 2, wherein, The computer calculates the distance by analyzing a time between emission and reception of each pulse and applying a known speed of ultrasound propagation in frozen tissue to determine a radial distance of each pulse.

4. The computer-implemented method according to claim 2, further comprising moving the ultrasound catheter to a plurality of different translation positions within the ice ball, emitting directional ultrasound pulses at each translation position, and determining, by the computer, a distance from each translation position to the periphery of the ice ball.

5. The computer-implemented method according to claim 4, further comprising generating, by the computer, a model of a slice of the ice ball at each translation position based on the determined distances, and combining, by the computer, the slices into a three-dimensional model of the ice ball.

6. The computer-implemented method according to claim 5, further comprising displaying, by the computer, the three-dimensional model of the ice ball on a set of registered images representing the target tissue, and comparing, by the computer, the three-dimensional model of the ice ball with a known size and shape of the target tissue to identify portions of the target tissue outside an effective treatment area of the ice ball.

7. The computer-implemented method according to claim 2, further comprising generating, by the computer, a three-dimensional model of the ice ball based on the determined distances, comparing, by the computer, the three-dimensional model of the ice ball with a known size and shape of the target tissue, and identifying, by the computer, portions of the target tissue outside an effective treatment area of the ice ball.

8. The computer-implemented method according to claim 2, wherein, Rotating the ultrasound catheter to emit the plurality of directional ultrasound pulses, and the computer determining a distance from each rotation position to the periphery of the ice ball.

9. The computer-implemented method according to claim 2, wherein, The ultrasound catheter includes a plurality of ultrasound transducer elements near a distal end, and the computer controls the transducer elements to emit the directional ultrasound pulses in a direction radially away from the ultrasound catheter.

10. The computer-implemented method according to claim 9, wherein, The computer controls the transducer elements to form a phased array such that the plurality of transducer elements work together to emit a directional beam of ultrasound energy in a single direction, wherein the computer analyzes the received energy to maximize a signal received from the single direction.

11. The computer-implemented method according to claim 10, wherein, The plurality of transducer elements work together to form a directed beam of the ultrasonic energy in the single direction, and also receive energy previously emitted by a specific directed ultrasonic pulse, wherein the received energy is analyzed to maximize the signal received by the plurality of transducer elements from the single direction.

12. A medical device for generating a model of an ice ball formed in a patient's body, the medical device comprising: a) an ultrasonic catheter configured to emit a plurality of directed ultrasonic pulses in a direction radially away from the ultrasonic catheter within the ice ball, wherein the plurality of directed ultrasonic pulses: i) are emitted in a plurality of radial directions, and ii) are emitted from a plurality of different translation positions; and b) a computing device configured to: i) determine a plurality of distances from the ultrasonic catheter to the edge of the ice ball by using the time between the emission and reception of each ultrasonic pulse and by using a known speed of ultrasonic propagation in the ice ball; and ii) create a model of the ice ball using the plurality of distances based on the radial direction and translation position of each ultrasonic pulse.

13. The medical device according to claim 12, wherein, The medical device is further configured to: e) display the model of the ice ball on a three-dimensional image of a region of the patient's body showing the target tissue.

14. The medical device according to claim 13, wherein, The computing device is further configured to: f) compare the model of the ice ball with a known size and shape of the region of the target tissue to identify portions of the target tissue outside an effective treatment region for ablating the tissue region; and g) display the identified portions of the target tissue using distinguishable visual features.

15. The medical device according to claim 14, wherein, The distal end of the ultrasonic catheter further includes an electromagnetic sensor for receiving an electromagnetic signal that positions the distal end in an electromagnetic field, and also includes displaying the model of the ice ball on a three-dimensional image of the patient using the received electromagnetic signal.

16. The medical device according to claim 12, wherein, The ultrasonic catheter has a plurality of ultrasonic transducers at the distal end. Further, fewer than all subsets of the plurality of ultrasonic transducers are used to generate each ultrasonic pulse, and further, the subset is selected based on a specific radial direction of each ultrasonic pulse.

17. The medical device according to claim 16, wherein, The plurality of ultrasonic transducers are PZT-based transducers.

18. The medical device according to claim 16, wherein, The plurality of ultrasonic transducers are pMUT-based transducers.

19. The medical device according to claim 16, wherein, The plurality of ultrasonic transducers are cMUT-based transducers.

20. A medical device for generating a model of an ablated tissue region formed in a patient's body, comprising: c) an ultrasonic catheter configured to be positioned inside the ablated tissue region and emit a plurality of directed ultrasonic pulses in a direction radially away from the ultrasonic catheter within the ablated tissue region, wherein the plurality of directed ultrasonic pulses: i) are emitted in a plurality of radial directions, and ii) are emitted from a plurality of different translation positions; and d) a computer configured to: i) determine a plurality of distances from the ultrasonic catheter to the edge of the ablated tissue region by using the time between the emission and reception of each ultrasonic pulse and by using a known speed of ultrasonic propagation in the ablated tissue region; ii) Create a model of the ablated tissue region using the plurality of distances based on the radial direction and translational position of each ultrasound pulse; iii) Display the model of the ablated tissue region on a three-dimensional image of the region of the patient showing the target tissue; iv) Compare the model of the ablated tissue region with the known size and shape of the region of the target tissue to identify portions of the target tissue outside the effective treatment region of the ablated tissue region; and v) Display the identified portions of the target tissue using distinguishable visual features.

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