Energy deposition zone determination for a catheter with an ultrasound array

The invention provides for a medical apparatus (300) comprising: a magnetic resonance imaging system (302); an ultrasonic system (322) for connecting to a catheter (324, 504, 600) with an ultrasound array (400, 402, 404, 508, 602, 604). The ultrasonic system is operable for driving the ultrasound array. Machine executable instructions (354, 356, 358) cause a processor (334) for controlling the medical apparatus to: generate (100, 202) at least one acoustic radiation impulse with the ultrasonic system, wherein the generated ultrasound energy is below a predetermined level; acquire (102, 204) the magnetic resonance data using an acoustic radiation force imaging pulse sequence; reconstruct (104, 206) at least one acoustic radiation force pulse image using the magnetic resonance data; and determine (106, 208) an energy deposition zone for the catheter using at least partially the at least one acoustic radiation force pulse image.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application of International Application No. PCT/EP2014/056831, filed on Apr. 4, 2014, which claims the benefit of EP 13162504.8 filed on Apr. 5, 2013 and is incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the deposition of ultrasonic energy into a subject using a catheter, in particular the guidance of the catheter using acoustic radiation force imaging.

BACKGROUND OF THE INVENTION

Non or minimally invasive treatment of prostate tumors is a field of increasing interest. High-intensity focused ultrasound (HIFU) therapy of prostate tumors has shown great promise in reducing the side effects of well-established treatments, while still treating the tumor effectively. Most of the clinical cases performed to date have been under ultrasound guidance, but MR guidance holds several benefits that may further improve the clinical outcome of the procedures. In addition to temperature imaging, MR guidance also allows for using MRI for planning of the procedure.

Magnetic resonance acoustic radiation force imaging is a Magnetic Resonance technique which is able to map displacements produced by focused ultrasound pulses. The journal publication McDonnold et al. Med. Phys. 35 (8), August 2008, pages 3748 to 3758 provides a review of magnetic resonance acoustic radiation force imaging and how to apply the technique.

In Holbrook et. al., Med. Phys. 38 (9), September 2011, pages 5081 to 5089 discloses the use of Magnetic Resonance acoustic radiation force imaging to provide a method of visualizing the transducer focus quickly, for High Intensity Focused Ultrasound (HIFU), without damaging tissue to allow accurate execution of a treatment plan.

SUMMARY OF THE INVENTION

The invention provides for a medical apparatus, a method of operating the medical apparatus, and a computer program product in the independent claims. Embodiments are given in the dependent claims.

‘Computer memory’ or ‘memory’ is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. ‘Computer storage’ or ‘storage’ is a further example of a computer-readable storage medium. Computer storage is any non-volatile computer-readable storage medium. In some embodiments computer storage may also be computer memory or vice versa.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. A ‘user interface’ as used herein is an interface which allows a user or operator to interact with a computer or computer system. A ‘user interface’ may also be referred to as a ‘human interface device.’ A user interface may provide information or data to the operator and/or receive information or data from the operator. A user interface may enable input from an operator to be received by the computer and may provide output to the user from the computer. In other words, the user interface may allow an operator to control or manipulate a computer and the interface may allow the computer indicate the effects of the operator's control or manipulation. The display of data or information on a display or a graphical user interface is an example of providing information to an operator. The receiving of data through a keyboard, mouse, trackball, touchpad, pointing stick, graphics tablet, joystick, gamepad, webcam, headset, gear sticks, steering wheel, pedals, wired glove, dance pad, remote control, and accelerometer are all examples of user interface components which enable the receiving of information or data from an operator.

A ‘display’ or ‘display device’ as used herein encompasses an output device or a user interface adapted for displaying images or data. A display may output visual, audio, and or tactile data. Examples of a display include, but are not limited to: a computer monitor, a television screen, a touch screen, tactile electronic display, Braille screen, Cathode ray tube (CRT), Storage tube, Bistable display, Electronic paper, Vector display, Flat panel display, Vacuum fluorescent display (VF), Light-emitting diode (LED) displays, Electroluminescent display (ELD), Plasma display panels (PDP), Liquid crystal display (LCD), Organic light-emitting diode displays (OLED), a projector, and Head-mounted display.

Magnetic Resonance (MR) data is defined herein as being the recorded measurements of radio frequency signals emitted by atomic spins by the antenna of a Magnetic resonance apparatus during a magnetic resonance imaging scan. A Magnetic Resonance Imaging (MRI) image is defined herein as being the reconstructed two or three dimensional visualization of anatomic data contained within the magnetic resonance imaging data. This visualization can be performed using a computer.

A ‘pulse sequence’ as used herein encompasses a set of sequential commands used to control a magnetic resonance imaging system in order to perform a particular imaging protocol. An acoustic radiation force imaging pulse sequence is a pulse sequence is a pulse sequence which enables the magnetic resonance imaging system to acquire magnetic resonance data that can be used to produce maps of displacements caused by ultrasound pulses.

A ‘capacitive micromachined ultrasonic transducer’ (CMUT) as used herein encompasses a capacitive ultrasound transducer that has been manufactured using micromachining technologies. Micromachining technologies are thin film manufacturing techniques; typically they are performed using processes identical to or similar to those used for manufacturing integrated circuits.

Recent developments have led to the prospect that medical ultrasound transducers can be manufactured by semiconductor processes. These processes may be the same ones used to produce the circuitry needed by an ultrasound probe such as a CMOS process. These developments have produced micromachined ultrasonic transducers or MUTs. MUTs have been fabricated in two design approaches, one using a semiconductor layer with piezoelectric properties (PMUTs) and another using a diaphragm and substrate with electrode plates that exhibit a capacitive effect (CMUTs). The CMUT transducers are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge applied to the electrodes is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave.

Since these devices are manufactured by semiconductor processes the devices generally have dimensions in the 10-200 micron range, but can range up to device diameters of 300-500 microns. Many such individual CMUTs can be connected together and operated in unison as a single transducer element. For example, four to sixteen CMUTs can be coupled together to function in unison as a single transducer element. A typical two dimensional transducer array currently may have 2000-3000 piezoelectric transducer elements. When fabricated as a CMUT array, over one million CMUT cells may be used. Surprisingly, early results have indicated that the yields, from a semiconductor fabrication plant, of CMUT arrays of this size should be markedly improved over the yields for lead zirconate titanate (PZT) arrays of several thousand transducer elements.

In one aspect the invention provides for a medical apparatus comprising a magnetic resonance imaging system for acquiring magnetic resonance data from a subject. The medical apparatus further comprises an ultrasonic system operable for connecting to a catheter with an ultrasound array. The ultrasonic system is operable for driving the ultrasound array. Driving the ultrasound array as used herein encompasses providing electrical power to the ultrasound array such that it is able to generate ultrasound. The medical apparatus further comprises a memory for storing machine-executable instructions. The medical apparatus further comprises a processor for controlling the medical apparatus. Execution of the machine-executable instructions cause the processor to control the ultrasonic system to generate at least one acoustic radiation impulse by the ultrasonic system at a location of a target zone. The generated ultrasound is below a predetermined level. The predetermined level may be selected such that the generated ultrasonic energy does not cause damage locally in the subject.

For instance the predetermined may be set such that the heating of the subject in the target zone or in the vicinity around the target zone is such that tissue necrosis does not occur. Execution of the machine-executable instructions further cause the processor to acquire the magnetic resonance data by controlling the magnetic resonance imaging system with a pulse sequence at least partially during the generation of the at least one acoustic radiation impulse. The pulse sequence is an acoustic radiation force imaging pulse sequence. Execution of the machine-executable instructions further cause the processor to reconstruct at least one acoustic radiation force pulse image using the magnetic resonance data. Execution of the machine-executable instructions further cause the processor to determine an energy deposition zone for the catheter using at least partially the at least one acoustic radiation force pulse image.

This embodiment may have the benefit that the determination of where the energy deposition will be performed by the catheter can be done very rapidly and very efficiently using acoustic radiation force imaging or also known as acoustic radiation force impulse imaging.

Acoustic radiation force impulse imaging is applied using focused ultrasound pulses using transducer arrays that direct ultrasound to a focus. The images created from the acoustic radiation force imaging are typically stiffness weighted and are used to provide information about local mechanical tissue properties. In this embodiment the similar technique is used, however not with a high intensity focused ultrasound system but with the use of a catheter. By its nature the catheter does not necessarily direct the ultrasound to a focused point. This a challenge is using catheters for ultrasound therapy. Using acoustic radiation force imaging, the overall distribution of where the ultrasound energy will go can be accurately predicted. When a catheter is used the ultrasound energy will not just to a maximum or a focal point, but may be distributed throughout the subject. The acoustic radiation force imaging may enable the essentially unfocused ultrasound from a catheter to be more accurately targeted or directed within a subject. A technique like magnetic resonance thermometry could be used for targeting the ultrasound also, however the tissue would need to be heated a measureable amount. Acoustic radiation force imaging has the advantage over magnetic resonance thermometry in that it gives faster results and only minimal heating occurs.

The determination of the energy deposition zone may be determined for instance by noting where the maximum radiation force produced by the ultrasound is. This may produce more rapid and accurate information than performing say for instance a test pulse. In this embodiment the energy may be much lower than is needed to be detected by say a test pulse. When performing a test pulse a reduced energy may be used and an increase in the temperature may be noted. In the present embodiment the tissue displacement can be caused by a much smaller ultrasound impulse. This may enable more rapid or more frequent testing of the location of the energy deposition zone for the catheter. This may enable more accurate targeting using the catheter than with traditional test pulse protocols.

In another embodiment, the catheter comprises multiple ultrasound elements. Each of the multiple ultrasound elements is operable for producing ultrasound at multiple frequencies. The ultrasonic system is operable for controlling each of the ultrasound elements to generate ultrasound at the multiple frequencies. The at least one acoustic radiation impulse comprises multiple pulses or impulses with ultrasound generated using at least some of the multiple frequencies. Execution of the instructions further causes the processor to reconstruct multiple acoustic radiation force image pulse images using magnetic resonance data acquired at least partially during the multiple pulses.

Execution of the machine-executable instructions further causes the processor to receive a treatment plan descriptive of a target zone within the subject. The energy deposition zone may be determined as a function of the ultrasound frequency generated by the ultrasound elements. Execution of the instructions may further cause the processor to to determine a sonication frequency for each of the ultrasound elements using this frequency dependent energy deposition zone.

Depending upon the construction of the catheter the multiple ultrasound elements may have several or a range of frequencies which it is capable of generating ultrasound at. For instance if piezoelectric elements are used then there may be a discreet number of frequencies which are practical to use. If a so called CMUT capacitive micromachined ultrasonic transducer is used then the frequency may be freely selectable within a range that is determined by the structure of the CMUT transducers. In this embodiment the procedure is repeated using multiple frequencies and then frequencies are chosen for the various ultrasound elements such that the ultrasound is directed to the target zone.

This embodiment may be particularly beneficial because the determination of the energy deposition zone can be made for different frequencies and individual or groups of ultrasound elements using acoustic radiation force imaging. A large number of such images can be constructed very rapidly and without the use of effectively heating the tissue very much. This may enable more refined targeting of the target zone than would be possible if say a conventional test pulse were used with thermal imaging.

In another embodiment the multiple ultrasound elements comprise at least one capacitive micromachined ultrasonic transducer array. The arrays may be operated in a variety of ways. For instance the array may be operated as a single ultrasound element in some embodiments where all of the capacitive micromachined ultrasonic transducers are operated at the same frequency. In other embodiments the frequency may be varied across one CMUT array. In other embodiments the phase produced by the individual capacitive micromachined ultrasonic transducers may also be controlled. This may enable more accurate or more precise targeting using the catheter.

In another embodiment the ultrasound system is operable for adjusting the focus of the at least one capacitive micromachined ultrasonic transducer array by controlling the phase of electrical power supplied to capacitive elements of the at least one capacitive micromachined ultrasonic transducer. Execution of the machine-executable instructions causes the processor to control the phase of the electrical power supplied to capacitive elements of the at least one capacitive micromachined ultrasonic transducer to control the location of the energy deposition zone. In this embodiment controlling the frequency the phase of electrical power is also performed. This may enable a form of targeting of the target zone and reduce the amount that the catheter needs to be manipulated.

In another embodiment the multiple ultrasound elements comprise piezoelectric transducers.

In another embodiment execution of the instructions further causes the processor to control the ultrasonic system to generate ultrasound above the predetermined threshold in the energy deposition zone. For instance once the location of the energy deposition zone is known accurately the catheter may then be used for ablating tissue or for locally heating a portion of the subject.

In another embodiment the ultrasound system is operable for controlling the phase of electrical power supplied to the ultrasound array. Execution of the machine-executable instructions further causes the processor to adjust the phase of the multiple ultrasound transducer elements to modify the location of the energy deposition zone to match the target zone.

In another embodiment execution of the machine-executable instructions further causes the processor to perform a beam path evaluation using the at least one acoustic radiation force pulse image. The beam path as is used herein encompasses a path which the ultrasound takes between the transducer and the energy deposition zone. This embodiment may be beneficial because using the acoustic radiation force imaging the intensity of the ultrasound which will be generated between the transducer and the energy deposition zone can be estimated accurately by the displacement generated. This may be useful in avoiding such effects as near field heating or the burning of the surface of a subject due to high intensity ultrasound in the near field.

In another embodiment execution of the machine-executable instructions further causes the processor to determine a distance between the energy deposition zone and a predetermined volume in the subject. This for instance may be considered to be equivalent as performing a test shot using the system. The energy deposition zone is where the energy during a sonication will likely go and the predetermined volume may be a volume which is desired to be targeted within the subject. Determining the distance between the two may be useful because then a physician or other operator can reposition the catheter or electrical targeting can be changed within the catheter to reduce the distance between the energy deposition zone and the predetermined volume.

In another embodiment the medical apparatus comprises the catheter.

In another embodiment the catheter is a transurethral catheter.

In another embodiment the catheter is an interstitial catheter.

In another embodiment the catheter is an esophageal catheter.

In another aspect the invention provides for a method of operating a medical apparatus. The medical apparatus comprises a magnetic resonance imaging system for acquiring magnetic resonance data from a subject. The medical apparatus further comprises an ultrasonic system operable for connecting to a catheter with an ultrasound array. The ultrasonic system is operable for driving the ultrasound array. The method comprises the step of controlling the ultrasonic system to generate at least one acoustic radiation impulse with the ultrasonic system. The generated ultrasound energy is below a predetermined level.

The method further comprises the step of acquiring the magnetic resonance data by controlling the magnetic resonance imaging system with a pulse sequence at least partially during the generation of the at least one acoustic radiation impulse. The pulse sequence is an acoustic radiation force imaging pulse sequence. The method further comprises the step of reconstructing at least one acoustic radiation force pulse image using the magnetic resonance data. The method further comprises determining an energy deposition zone for the catheter using at least partially the at least one acoustic radiation force pulse image.

In another embodiment the method further comprises the step of determining a distance between the energy deposition zone and a predetermined volume in the subject.

In another aspect the invention provides for a computer program product comprising machine-executable instructions for execution by a processor controlling the medical apparatus. The medical apparatus comprises a magnetic resonance imaging system for acquiring magnetic resonance data from a subject. The medical apparatus further comprises an ultrasonic system operable for connecting to a catheter with an ultrasound array. The ultrasonic system is operable for driving the ultrasound array. Execution of the machine-executable instructions causes the processor to control the ultrasonic system to generate at least one acoustic radiation impulse with the ultrasonic system. The generated ultrasound energy is below a predetermined level.

Execution of the machine-executable instructions further cause the processor to acquire the magnetic resonance data by controlling the magnetic resonance imaging system with a pulse sequence at least partially during the generation of the at least one acoustic radiation impulse. The pulse sequence is an acoustic radiation force imaging pulse sequence. Execution of the machine-executable instructions further cause the processor to reconstruct at least one acoustic radiation force pulse image using the magnetic resonance data. Execution of the machine-executable instructions further cause the processor to determine an energy deposition zone for the catheter using at least partially the at least one acoustic radiation force pulse image.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1shows a flow diagram which illustrates a method according to an embodiment of the invention. In step100an acoustic radiation impulse is generated. This may be done using an ultrasonic system to control a catheter with an ultrasound array. Next in step102magnetic resonance data is acquired by controlling the magnetic resonance imaging system with a pulse sequence at least partially during the generation of the at least one acoustic radiation impulse. The pulse sequence is an acoustic radiation force imaging pulse sequence. Next in step104an acoustic radiation force pulse image is reconstructed using the magnetic resonance data. Finally in step106an energy deposition zone is determined using acoustic radiation force pulse image. By looking at the displacement of the subject's internal structure it can be determined where the majority of the energy deposition would be if for instance a sonication were performed.

FIG. 2shows a flow diagram which illustrates a further method according to an embodiment of the invention. First in step200a target zone is received. It may for instance be in the form of a treatment plan. The target zone is descriptive of a position in the internal anatomy of a subject which may be desired to be sonicated. Next in step202acoustic radiation impulses are generated at multiple frequencies. Next in step204magnetic resonance data is acquired using acoustic radiation force imaging pulse sequences and this is performed at least partially during the generation of the radiation pulses at multiple frequencies. Next in step206multiple acoustic radiation force pulse images are reconstructed using the magnetic resonance data. Next in step208the energy deposition zone is determined using the acoustic radiation force pulse images. Essentially this is a frequency-dependent energy deposition zone. When there are multiple transducers multiple frequencies may be used so that by controlling which transducers are used and/or which frequencies are used the location of the energy deposition zone can be controlled. Finally in step210a sonication frequency is determined for the transducer elements using the frequency-dependent energy deposition zone and the target zone. A frequency can be chosen for each of the transducer elements such that the energy deposition zone overlaps the target zone sufficiently well so that a sonication can be performed.

FIG. 3shows a medical apparatus300according to an embodiment of the invention. The magnetic resonance imaging system comprises a magnet304. The magnet304is a cylindrical type superconducting magnet. The magnet has a liquid helium cooled cryostat with superconducting coils. It is also possible to use permanent or resistive magnets. The use of different types of magnets is also possible for instance it is also possible to use both a split cylindrical magnet and a so called open magnet. A split cylindrical magnet is similar to a standard cylindrical magnet, except that the cryostat has been split into two sections to allow access to the iso-plane of the magnet, such magnets may for instance be used in conjunction with charged particle beam therapy. An open magnet has two magnet sections, one above the other with a space in-between that is large enough to receive a subject: the arrangement of the two sections area similar to that of a Helmholtz coil. Open magnets are popular, because the subject is less confined. Inside the cryostat of the cylindrical magnet there is a collection of superconducting coils. Within the bore of the cylindrical magnet there is an imaging zone where the magnetic field is strong and uniform enough to perform magnetic resonance imaging.

Within the bore306of the magnet304there is a magnetic field gradient coil308which is supplied current by a magnetic field gradient coil power supply310. The magnetic field gradient coil308is used to spatially encode magnetic spins within an imaging zone of the magnet during the acquisition of magnetic resonance data. The magnetic field gradient coil308is intended to be representative. Typically magnetic field gradient coils contain three separate sets of coils for spatially encoding in three orthogonal spatial directions. The current supplied to the magnetic field coil308is controlled as a function of time and may be ramped or pulsed.

Within the bore of the magnet306is an imaging zone316where the magnetic field is uniform enough for performing magnetic resonance imaging. Adjacent to the imaging zone316is an antenna312. The antenna312is connected to a transceiver314. The radio frequency antenna316is for manipulating the orientations of magnetic spins within the imaging zone316and for receiving radio transmissions from spins also within the imaging zone. The radio frequency antenna may contain multiple coil elements. The radio frequency antenna may also be referred to as a channel. The radio frequency coil is connected to a radio frequency transceiver314. The radio frequency coil312and radio frequency transceiver314may be replaced by separate transmit and receive coils and a separate transmitter and receiver. The radio frequency antenna is intended to also represent a dedicated transmit antenna and a dedicated receive antenna. Likewise the transceiver314may also represent a separate transmitter and receivers.

A subject318can be seen as reposing on the subject support320. The subject is partially within the imaging zone. A catheter324is inserted into the subject318. There is an energy deposition zone326which is shown as adjacent to the catheter324. The catheter324is connected to an ultrasonic system322which provides electrical power for powering ultrasonic transducer arrays on the catheter324. The energy deposition zone326is within the imaging zone316.

The transceiver314, the magnetic field gradient coil power supply310, and the ultrasonic system322are shown as being connected to a hardware interface332of a computer system330. The computer330also comprises a processor334. The processor334is in communication with the hardware interface332which enables the processor334to control the operation and function of the medical apparatus300. The processor334is also shown as being in communication with a user interface336, computer storage338, and computer memory340.

The computer storage is shown as containing a treatment plan342. The treatment plan342is a plan for sonicating a portion of the subject318. It contains a target zone344which is descriptive of the anatomical position which may be desirous to sonicate. The computer storage338is shown as further containing a pulse sequence346. The pulse sequence346contains a set of controls or commands which are executed in a time sequence which enables the magnetic resonance imaging system302to acquire magnetic resonance data using an acoustic radiation force imaging protocol. As such the pulse sequence346may also be used to control the ultrasound system322. Computer storage338is further shown as containing magnetic resonance data348that was acquired using the pulse sequence346. The computer storage338is further shown as containing acoustic radiation force pulse image that was reconstructed from the magnetic resonance data348. The computer storage338is further shown as containing a location of the energy deposition zone352that was determined using the acoustic radiation force pulse image350.

The computer memory340is further shown as containing a control module354. The control module354contains computer-executable code which enables the processor334to control the operation and function of the medical apparatus300. For instance it may enable the processor334to acquire the magnetic resonance data348using the pulse sequence346. The computer memory340is further shown as containing an image reconstruction module356. The image reconstruction module356contains computer-executable code which enables the processor334to reconstruct the acoustic radiation force pulse image350from the magnetic resonance data348. The computer memory340further contains imaging processing module358. The imaging processing module358contains computer-executable code which enables the processor334to determine the location of the energy deposition zone352from the acoustic radiation force pulse image350.

FIG. 4shows several examples of transducer elements400,402,404that could be used in an embodiment. InFIG. 4a conventional piezoelectric element400is shown. Next to the piezoelectric element400are two arrays402,404of capacitive micromachined ultrasound transducers. The piezoelectric element400has two electrical connections406for driving the element400.

The capacitive micromachined ultrasound transducer array402has first408and second410electrical connections. Array402is wired so that it functions as a single transducer element in the way that the piezoelectric element400does. This demonstrates how an array402may be used as a replacement for an entire piezoelectric element400. Drawing412shows a blowup of array402. The individual capacitive micromachined ultrasound transducers414can be seen. It can be seen that each of the transducers414is connected to the first408and second410electrical connections. The array404of capacitive micromachined ultrasound transducers is arranged as linear arrays. There is a set of first416and second418electrical connections for each row of transducers. Drawing420is a blowup detail of the array404. An individual capacitive micromachined ultrasound transducer422can be shown as being connected to a first424and second426electrical connection. The connections424and426are chosen from the first416and second418sets of electrical connections.

In addition to wiring the capacitive micromachined ultrasound transducers in large block arrays or in linear arrays the individual micromachined ultrasound transducers may also be individually driven by their own source.

FIG. 5shows a top view500and a side view502of a catheter504. This is purely an example of one way in which a catheter could be built. There is the flat surface506upon which is mounted a number of ultrasound transducers508. In this example the transducers are arranged as a linear array. There are electrical connections510which provide electrical power to each of the ultrasound transducers508. The ultrasound transducers508could be piezoelectric transducers or they may even be individual arrays of CMUTs.

FIG. 6shows a distal end600of a catheter according to an embodiment of the invention. In this embodiment there is a forward-looking ring array602. There is an array of capacitive micromachined ultrasound transducers surrounding a hole608. Behind the ring array602are panels of sideways-looking arrays604. The arrays604form a ring around the shaft of the catheter. Shown in this Fig. are various electrical connections606. The forward-looking ring array602may be used for such things as providing three dimensional imaging. The sideways-looking arrays604may be used for ultrasound ablation and monitoring. The individual capacitive micromachined ultrasound transducers can be used for beam steering during ultrasound ablation. Benefits of this embodiment may include that there is no or minimal need for mechanically rotating the catheter. The hole608can be used for additional instruments or for water irrigation. The embodiment shown inFIG. 6can focus in multiple directions so for such things as ablating a prostrate the entire360degrees around the probe may be performed simultaneously. This would result in less treatment time and thus also reduce costs.

FIG. 7is used to illustrate the functioning of a medical apparatus. Shown is a sketch of a catheter700with a number of transducer elements702mounted on the surface. For this example only the middle transducer element702is activated. The line704outlines the rough position of the radiation field from the ultrasound transducer702. Within the ultrasound radiation field704magnetic resonance data has been acquired using an acoustic radiation force imaging protocol and the arrows706indicate a rough measure of displacement of tissue within the subject due to ultrasound. By looking at the magnitude of the arrow706it can be determined that the maximum deposition is at point708. This may then be determined to be an energy deposition zone.

In addition to locating the maximum regions of the radiation field704can also be identified. For instance there is a beam path between the transducer element702and the maximum708. Closer to the element702is the near field712of the ultrasound radiation field. In addition to just telling the maximum there is a large amount of information which will be descriptive of how the energy will be distributed to the subject by the transducer elements702when a sonication is performed. Such measurements can be performed for a single element, multiple elements, or even at different frequencies. For instance a map such as shown inFIG. 7could be repeated for a variety of frequencies and this could be used then for accurately targeting ultrasound using the catheter700.

FIG. 8shows a further example of the catheter700. In this example the ultrasound is generated by the three middle transducer elements702. Again the energy deposition zone708can be seen. In addition a target zone800is marked on the diagram. This Fig. shows that the deposition zone708is not necessarily in the proper position for the target zone800. Such a diagram could be displayed on the display of the medical instrument and can be used as a guide for a physician or other operator to position the catheter better or else also electronic steering techniques may be used, for instance different of the transducer elements702could be activated, or also the phase and/or frequency of the ultrasound generated could be changed such that the energy deposition zone708more closely matches the position of the target zone800.

Acoustic radiation force imaging (ARFI) may be used for evaluating the acoustic environment and estimate the thermal damage that a sonication is likely to inflict without actually inducing any damage. This in turn allows for fine tuning the procedure before ablation that might further improve the safety and efficacy of the treatment.

HIFU therapy of prostate cancer is increasingly being used as a non-invasive alternative with the potential to reduce side effects such as impotence and incontinence, while still offering an efficient treatment. Most procedures to date have been made under ultrasound guidance. MR guidance offers several advantages such as temperature imaging, but also offers improvements during the planning stage.

One of the improvements that MRI offers is the potential to use acoustic radiation force imaging (ARFI) in the pre-planning step of these prostate cancer therapies. HIFU therapy of the prostate can be done either trans-rectal or sonicating through the rectal wall, or the HIFU device is transurethral and sonication occurs through the urethra wall. ARFI may in both cases be used with only minor total energy to ensure acoustic coupling from the HIFU device to the prostate. This is particularly a concern for trans-rectal HIFU as the rectal wall is very sensitive and may be damaged if the contact is poor, which would result in a local absorption of the HIFU energy potentially resulting in damaging of the rectal wall. This is less of a problem for the urethra, but will nevertheless hamper therapy if energy cannot be transmitted to where it should.

Often, HIFU therapy of prostate cancer is done as a whole gland therapy where the entire prostate is ablated. If the location of the cancer can be determined successfully within the prostate gland (a topic of active research in the MRI community) one can also do so called focal therapy where only the parts of the prostate thought to have cancer are treated. This is likely to reduce morbidity even further.

Another advantage given by ARFI, is that the pressure field can be estimated via the radiation force that the HIFU exerts on the tissue. The local pressure field is also the mechanisms via which the HIFU heats the tissue. Hence, an estimate of the distribution of the pressure field will give an idea of the distribution of the heating that may result. This can be used for improving the understanding of what is likely to happen close to sensitive structures such as the rectal wall, and in particular the nerve bundles (which are thought to control the penile functionality as well as bladder functionality). Moreover, if the transducer is capable of generating different frequencies the pressure fields of these different frequencies may be evaluated and compared using this ARFI method. This may aid in choosing the most appropriate frequency for the different parts of the prostate, thereby providing an even further improved safety.

Both piezo- and CMUT transducers can benefit from ARFI, although CMUTs can also have the benefit of optimizing the pressure field near the sensitive structures so that if cancer is found close to the nerve bundles (for example) the transducer angulation (CMUTs can be made mechanically steerable that is they can be controllably bent), frequency, and phase of the elements can be chosen so as for the pressure field to be high near the edge of the prostate but drop as fast as possible towards the sensitive nerve bundles.

ARFI of the prostate would allow for validating the acoustic path. For example, for a transurethral device (normally only around 10 elements whose individual pressure fields can be seen) one would expect to find some radiation force being exerted at the front face of each element. If not or there is a large difference between the elements, then there is likely an air pocket or similar within the urethra or catheter preventing the transducer element from performing as well as it could. This could damage the urethra (not such a big problem), damage the transducer, or lead to a suboptimal therapy if the problem is not seen and fixed. For the transrectal device, the benefits are outlined above.

The therapy planning would further be aided by allowing for a better understanding of the pressure field, and if the frequency can be changed then which frequency might be best suited to ablate which parts of the prostate. This is important for whole gland therapies as one would ideally like to fully ablate all of the prostate but nothing outside of the prostate (particularly not the nerve bundles). For focal therapy, one can analyse the different focal regions to be ablated separately.

All this can be done with ARFI at a fraction of the energy that would be needed for a test sonication. Also, ARFI can be done much more rapidly than a test sonication.

In one example, the sonication can be coupled with motion sensitizing gradients one can obtain a displacement image. Often, another image is needed with inverse motion sensitizing gradients in order to remove background information. These displacement values are dependent on the local radiation force, which are in turn dependent on the local pressure field.