Ablation Monitoring System and Method

Abstract

A system and method are provided for treating a target tissue using cryoablation. A guide cannula and a cryoprobe are inserted into the target tissue. The cryoprobe is cooled and an ice ball is formed. The cryoprobe is removed while the ice ball is still frozen and an ultrasound catheter is inserted. Ultrasound generated within the ice ball is used to determine the distance from the ultrasound catheter to the outer edge of the ice ball. The above steps are repeated at different angles to simulate slices of the ice ball. The ultrasound catheter is moved radially and the process is repeated to create a model of at least a portion of the ice ball. The ice ball model can be displayed on a set of registered images representing the target tissue to ensure that the tissue is within the treatment area of the ice ball.

Description

Technical Field

[0001] 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

[0002] Cryosurgery or cryoablation is a surgical procedure 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 manner creates an "ice ball" of frozen tissue, which is generally centered at the distal end of the cryoprobe.

[0003] 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 procedure will survive.

[0004] 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" cast by this surface reflection, the ultrasound energy cannot penetrate the ice ball and the true three-dimensional size and shape of the ice ball cannot be shown. 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

[0005] One embodiment of the present invention presents a method for treating tumors or other target tissue using cryoablation. The method first identifies the location of the target tissue, for example, by performing preoperative CT imaging. The created images can be combined into a 3-D image, or combined into a 3-D model of the patient or the patient's organ. During the surgery, an ultrasound catheter contained within an introducer sheath is inserted into the target tissue. In one embodiment, the ultrasound catheter and 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 sheath. The ultrasound catheter may be capable of performing QUS analysis on the tissue.

[0006] Once the positions of the ultrasound catheter and cannula are confirmed, the ultrasound catheter is removed from the cannula and the cryoprobe is inserted into the cannula position. Alternatively, the cryoprobe and cannula can be inserted percutaneously directly 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.

[0007] The cryoprobe is then cooled in order 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, allowing the ice ball to thaw and then regenerating the ice ball by cooling the cryoprobe a second time.

[0008] The cryoprobe is then removed from the introducer cannula while the ice ball is still in the frozen state. The ultrasound catheter is inserted into the cannula 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 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 cannula 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.

[0009] In one embodiment, the 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 ice ball model created. Assuming that the EM navigation system is registered with preoperative CT images or models, the created ice ball model can be superimposed on the CT images. Since the CT images identify the size and location of the target tissue, it will be obvious whether the generated ice ball encompasses the target tissue completely within its effective treatment area. In some embodiments, 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 if the target area is not within the treatment zone of the ice ball, the software provides a warning. 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.

[0010] Cryoablation is just an ablation technique, and its embodiments can be used to determine the size, shape, and location of the ablated tissue. Similar systems and methods can be used in conjunction 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. Multiple slices can then be combined into a complete model of the ablated tissue, 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

[0011] Figure 1 is a side perspective view of a cryoprobe inserted percutaneously under ultrasound guidance.

[0012] Figure 2 is Figure 1 schematic diagram of the cryoprobe penetrating the target tissue and generating an ice ball.

[0013] Figure 3 is a schematic diagram of the generated ice ball, showing three isotherms.

[0014] Figure 4 is after the second cryoablation Figure 3 schematic diagram of the generated ice ball.

[0015] Figure 5 is a schematic diagram of an irregular ice ball generated using three cryoprobes.

[0016] Figure 6 is seen through standard ultrasound techniques Figure 5 schematic diagram of the irregular ice ball.

[0017] Figure 7 is one in which a cryoprobe has been removed Figure 5 schematic diagram of the irregular ice ball.

[0018] Figure 8 is an ultrasound probe inserted to replace the removed cryoprobe Figure 5 schematic diagram of the irregular ice ball.

[0019] Figure 9 is a plan view of the distal tip of an embodiment of an ultrasound probe within a guide cannula.

[0020] Figure 10 is a first graph showing the reflected ultrasound signal versus time.

[0021] Figure 11 is a second graph showing the reflected ultrasound signal versus time.

[0022] Figure 12 is a schematic diagram of the calculated dimensions of a single slice of the ice ball.

[0023] Figure 13 Schematic diagram of the calculated dimensions of a hockey puck including multiple slices.

[0024] Figure 14 Flowchart showing a method for implementing an embodiment of the present invention.

[0025] Figure 15 Schematic diagram of an irregularly shaped region of ablated tissue ablated using thermal ablation.

[0026] Figure 16 Plan view of the distal tip of a second embodiment of an ultrasound probe within a guide cannula.

[0027] Figure 17 Side perspective view of the percutaneous insertion of an ultrasound probe rotated by an electric stepper motor. Detailed Description

[0028] Hockey Puck Formation

[0029] Cryoablation is typically 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 traditional 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 example, an external ultrasound device 100 is used to identify the location of the target tissue. The same ultrasound 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 specially designed to enhance its visibility under ultrasound, for example, by applying grooves or other physical distortions that are highly visible to ultrasound energy, thereby enhancing the visibility of the tip under ultrasound. In this way, the tip of the cryoprobe 110 is directed percutaneously towards the target tissue. As explained below in connection with Figure 14 it is also possible to use an inserted ultrasound catheter to identify the location of the target tissue and position the cryoprobe 110.

[0030] Figure 1A computer system, not shown, determines which signals and how much power to send to ultrasound device 100 and 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 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.

[0031] Figure 2 The distal end or tip 112 of cryoprobe 110 is shown after insertion into abnormal target tissue 200 of patient 120. In Figure 2 , cryoprobe 110 passes through the lumen of introducer sheath 210 that has been positioned at target tissue 200. Although an introducer sheath 210 is not always required in cryoablation procedures, the ability to position additional catheter devices at target tissue 200 makes the use of introducer sheath 210 beneficial for most embodiments of the present invention.

[0032] Once the tip 112 of cryoprobe 110 is inserted into 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 gas 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 target tissue 200.

[0033] 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 3 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 in Figure 3is schematically shown by the shaded region 300. The wider shaded region 310 shows the approximate location of the -20 °C isotherm, while the outer surface 320 of the ice ball 220 will have a temperature of -0 °C.

[0034] 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 ice ball 220 is first 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.

[0035] The standard technique of freezing abnormal tissue a second time after thawing will cause the tissue to freeze more quickly (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.

[0036] 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 [description], 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 with a uniform but irregularly shaped surface 532. 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. Additionally, the manufacture of the cryoprobes 110, 510, 520 can affect the final shape of the ice ball (e.g., 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 conformed to a shape that more effectively kills the target tissue 200 while minimizing damage to surrounding tissue. In Figure 5 In [description], 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 due to the third cryoprobe 520 not being inserted far enough into the target tissue 200 before the ice ball 530 is formed.

[0037] 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.

[0038] Unfortunately, the freezing 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. Additionally, 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.

[0039] In addition, because the ultrasonic energy 600 emanates from a single device 100, the energy 600 substantially 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 position, 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 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.

[0040] Ultrasonic catheter

[0041] 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 is shown Figure 5 the ice ball 530 from which the cryoprobe 110 has been removed. As shown in this figure, the removal of the cryoprobe 110 leaves an opening or channel 700 within the ice ball 530.

[0042] 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 Figure 8 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.

[0043] The catheter 800 can be constructed in accordance with the disclosures set forth in U.S. Provisional Application Nos. 62 / 776,667 and 62 / 776,677, both of which were filed on December 7, 2018 by the owner of the present application. The entire contents of both of these provisional applications are incorporated herein by reference.

[0044] Figure 9An embodiment of an ultrasonic catheter 900 is shown. The catheter 900 has a plurality of ultrasonic transducer elements 910 located near the distal end 902 of the catheter. In a preferred embodiment, a 64-element annular array of ultrasonic 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 transmitting and detecting ultrasonic energy at different frequencies (e.g., frequencies operating from 4 to 50 MHz). Since Figure 9 A plan view of the end of the ultrasonic catheter 900 is shown, and the individual transducers 910 are coplanar and arranged on the flat surface of the catheter 900. While this is a possible configuration, in Figure 9 the configuration shown, the transducers 910 are positioned around 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).

[0045] 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 ultrasonic energy beam. This is typically performed by timing the transmission of ultrasonic energy from multiple transducers so as to produce an interference pattern in a single controllable direction. The ultrasonic 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, ultrasonic signals can be transmitted and received from the transducers 910 in a single radial direction.

[0046] In other embodiments, synthetic aperture techniques are used. In this case, a single transmit pulse is sent and received unfocused. Then, beamforming algorithms analyze the previously unfocused signals post hoc in order to focus the transmission / reception of ultrasonic energy in a single direction.

[0047] Preferably, the size of the ultrasonic 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.

[0048] 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 the echo amplitude in the scan plane; M-mode imaging to track motion at a given fixed position 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 a contrast agent to display blood-filled spaces, or contrast agent imaging using a targeting agent to display specific agent-binding tissue types.

[0049] 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 create an “acoustic biopsy” (AB) or a (sonic biopsy) that can be performed on in-situ tissue. In addition, 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.

[0050] 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, preoperative computed tomography X-ray (CT) scans can be used to construct a model of the organs within the patient (such as the lung airways). Then, during the surgery, electromagnetic navigation uses the sensors 920 on the catheter 900 to provide position and orientation information in a 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.

[0051] In the catheter 900, both the EM sensor 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 sensor 920 and the transducer 910 so that they can share a single data path along the catheter 900.

[0052] Puck size determination

[0053] 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.

[0054] 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 typically transmitted during imaging. In fact, this pulse-echo technique does not require nuances - rather, the transmitted energy should be maximized.

[0055] The same transducer or multiple transducers 910 that emit 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 transmits 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. Graph 1000 shows that a large amount of ultrasonic energy is received at the peak 1010 of graph 1000, and this peak 1010 corresponds to time 1012. If time 1012 is T and V is the ultrasonic velocity through the frozen ice ball 530, then the total distance traveled by the sound received at peak 1010 is D = V * T. Since the ultrasonic wave must travel to the edge 532 of the ice ball 530 and return, the actual distance from the catheter 900 to the edge of the ice 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 therapy.

[0056] 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 ice ball 530 within the channel 700. In this case, the received ultrasonic energy may appear closer to Figure 11 the graph 1100. In this case, the outer boundary 532 of the ice ball 530 is visible at the peak 1110 at time 1112. A significant peak 1120 is also seen at time 1122, and peak 1120 shows an immediate reflection from within the channel 700. This peak 1120 can be ignored when determining the size of the ice ball 530, and it may not even be detected if it appears too quickly after the pulse transmission. Preferably, the ultrasonic transducer 910 is in contact with the frozen tissue of the ice ball 530 when inserted into the channel 700. This should reduce any initial reflection of the ultrasonic energy.

[0057] 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 graph table 1000 / 1100 and the above formula, the distance from the catheter 900 to the outer wall 532 of the ice 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 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.

[0058] 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, because 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.

[0059] 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 surround 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.

[0060] Process 1400

[0061] 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 positioned 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 location. 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.

[0062] 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 enhance the effectiveness of the ice ball 530.

[0063] 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.

[0064] 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 details in the model of the 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 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.

[0065] Because catheter 900 includes EM sensor 920, the 3D model can be positioned, sized, and oriented in CT images for EM navigation. Step 1460 displays the 3D model on these images.

[0066] At step 1465, any portions of target tissue 200 that have been missed by the efficacy of the created and modeled hockey puck 530 are identified. This can be done using computer software that compares the size, shape, and orientation of the 3D model to the known size, shape, and orientation of target tissue 200. Since both the 3D model and the target tissue can be displayed simultaneously on a monitor, 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 at step 1470 to ensure that all of 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.

[0067] Thermal ablation applications

[0068] 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 a 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 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 guiding cannula such as cannula 210 described above can be used to insert the needle into the target tissue.

[0069] 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.

[0070] 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 cryo-balloon using standard ultrasound imaging techniques. It is also true that ultrasound energy travels very differently through 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 cryo-balloon, 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.

[0071] Single transducer catheter

[0072] Figure 16An alternative embodiment of the ultrasonic 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 ultrasonic energy transmitted perpendicular to the transducer 1610 can be maximized. In terms of receiving the reflected ultrasonic energy from the distal boundary 532 of the puck 530, the same large transducer 1610 will also be more sensitive. The one-way, single-transducer pulse optimizes the ultrasonic catheter 1600 for the sonar-like pulses required to perform the above method 1400.

[0073] The transducer 1610 is preferably flat. In one embodiment, the transducer is located on the flat surface 1620 of the tip of the ultrasonic 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 remainder 1630 of the catheter 1600. Thus, the remainder 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.

[0074] 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.

[0075] The use of an ultrasound catheter with a single transducer 1610 allows for greater energy transmission and better signal detection in a single direction. Although the use of a single transducer 1610 significantly reduces the ability of the catheter 1600 to generate ultrasound images, this reduced functionality is irrelevant in the context of method 1400. However, the lack of an annular array of transducers 910 on catheter 900 means that the single-transducer catheter 1600 must be rotated to generate the above-described 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 precise position measurement, which can only be obtained by using the EM sensor 1640. Manual rotation of the catheter 1600 by the practitioner may result in unintentional translational movement of the catheter 1600 relative to the introducer sheath 210. This unintentional change in position can be registered by detecting the position of the EM sensor 1602. For each angular distance measurement (step 1435), the accurate current position of the catheter 1600 may result in less non-uniform "slices", but will result in an equally accurate overall model of the ice ball 530.

[0076] Figure 17 An alternative embodiment is shown in which the single-transducer catheter 1600 is used for a patient by inserting the catheter through the introducer sheath 210. An electric stepper motor 1700 is attached to the catheter 1600. The stepper motor is physically engaged with the catheter 1600 such that the motor 1700 is able to control the rotation of the catheter 1600. The number of steps of the motor 1700 is associated with a given number of degrees of rotation of the catheter, such that a control signal can be sent along the control line 1710 to cause the motor 1700 to rotate the catheter 1600 according to a command. 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 such that the entire slice measurement of the ice ball 530 can be performed as quickly and efficiently as possible with minimal translational movement of the transducer 1600 during the rotation cycle.

[0077] Many features and advantages of the invention are apparent from the above description. Those skilled in the art will readily conceive of many modifications and variations. Since such modifications are possible, the invention is not limited to the exact structures and operations shown and described. Instead, the invention should be limited only by the following claims.

Claims

1. A monitoring system for generating an ice ball model using a computer, comprising: A cryoprobe configured to (i) be inserted into a target tissue of a patient through a guide cannula; And (ii) generate an ice ball at a distal tip of the cryoprobe by cooling the distal tip of the cryoprobe; An ultrasound catheter having an ultrasound transducer at a distal end and configured to be inserted into the guide cannula after removing the cryoprobe from the guide cannula; An ultrasound device configured to (i) emit directional ultrasound signals from within the ice ball in different directions using the ultrasound transducer, and (ii) monitor corresponding ultrasound reflections from the surface of the ice ball by the ultrasound transducer in different directions; A computer configured to calculate the distance from the ultrasound catheter to the surface of the ice ball based on the time taken to receive each ultrasound reflection and the ultrasound speed in the frozen tissue, to generate information sufficient to model a first slice of the ice ball; Wherein the ultrasound catheter is further configured to move a known distance within the guide cannula to obtain at least a second slice of the ice ball; And The computer is further configured to combine at least the first slice and the second slice into an ice ball model showing the size and shape of a portion of the ice ball.

2. The monitoring system according to claim 1, wherein, The computer is further configured to: Determine an effective treatment area of the ice ball; Compare the ice ball model with the known size and shape of the target tissue to identify portions of the target tissue outside the effective treatment area of the ice ball.

3. A system for creating a model of an ablated tissue region, comprising: An ablation catheter configured to be inserted into a target tissue of a patient through a guide cannula; And ablating the target tissue to create an ablated tissue region having a known ultrasound transmission speed different from the normal ultrasound transmission speed in the non-ablated tissue; An ultrasound catheter configured to be inserted into the guide cannula after removing the ablation catheter, such that at least one ultrasound transducer on the distal end of the ultrasound catheter is positioned within the ablated tissue; and emit a plurality of directional ultrasound pulses radially away from the ultrasound catheter within the ablated tissue, wherein the plurality of directional ultrasound pulses: i) Are respectively directed from a specific radial direction of the ultrasound catheter and received from the specific radial direction; ii) Are emitted jointly in a plurality of radial directions; iii) Are emitted jointly with the ultrasound catheter positioned at a plurality of different translation positions relative to the guide cannula; A computer configured to determine a plurality of distances from the ultrasound catheter to the edge of the ablated tissue region by using the time between the emission and reception of each pulse and by using the known speed of ultrasound transmission in the ablated tissue region; And create a model of the ablated tissue region using the determined distances in the radial direction and the translation position of each pulse.

4. The system according to claim 3, wherein The computer is further configured to: Display the model of the ablated tissue region on a three-dimensional image of the patient showing the target tissue.

5. The system according to claim 4, wherein, The computer is further configured to: Compare the model of the ablated tissue region with the known size and shape of the target tissue to identify portions of the target tissue that are outside the effective treatment region of the ablated tissue region; Display the identified portions of the target tissue using distinguishable visual features.

6. The system according to claim 4, wherein The distal end of the ultrasound transducer further includes an electromagnetic sensor that receives an electromagnetic signal that positions the distal end in an electromagnetic field, and the received electromagnetic signal is used to display a model of the ablated tissue region on a three-dimensional image of the patient.

7. The system according to claim 3, wherein, The ablation catheter is a cryoprobe, and further, the ablated tissue region includes an ice ball.

8. The system according to claim 3, wherein, The ablation catheter is a thermal ablation catheter, and further, the ablated tissue region includes tissue killed by heating.

9. The system according to claim 8, wherein, The thermal ablation catheter is a microwave ablation catheter.

10. The system according to claim 8, wherein, The thermal ablation catheter is a radiofrequency ablation catheter.

11. The system according to claim 3, wherein The ultrasound catheter has a plurality of ultrasound transducers at the distal end, and further, fewer than all subsets of the ultrasound transducers are used to generate each of the ultrasound pulses, and further, the subset is selected based on the specific radial direction of each ultrasound pulse.

12. The system according to claim 11, wherein, The plurality of ultrasound transducers are arranged in an annular array around the periphery of the ultrasound catheter at the distal end of the ultrasound catheter.

13. The system according to claim 12, wherein, The subset of ultrasound transducers forms a phased array transmitter that emits each directional ultrasound pulse.

14. The system according to claim 13, wherein, The subset of the same ultrasound transducers that form the phased array transmitter also forms a phased array receiver that receives each directional ultrasound pulse.

15. The system according to claim 12, wherein, The subset of ultrasound transducers forms a phased array receiver that receives each directional ultrasound pulse.

16. The system according to claim 3, wherein, The ultrasound catheter has a single ultrasound transducer at the distal end, where the single ultrasound transducer emits and receives the directional ultrasound pulse.

17. The system according to claim 16, wherein, The ultrasound catheter and the single ultrasound transducer are physically rotated to emit a plurality of ultrasound pulses in a plurality of radial directions.

18. The system according to claim 17, wherein A stepper motor physically rotates the ultrasound catheter.

19. A system for determining the distance between an ultrasound catheter and the periphery of an ice ball, comprising: An ultrasound catheter configured to be inserted into a target tissue of a patient through a guide sheath; And performing QUS analysis on the target tissue; A cryoprobe configured to replace the ultrasound catheter without moving the guide sheath; and generating an ice ball by cooling the cryoprobe; Wherein the ultrasound catheter is further configured to replace the cryoprobe without moving the guide sheath; and determine the distance between the ultrasound catheter and the periphery of the ice ball by generating ultrasound energy within the ice ball.

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