WO2012049612A2

WO2012049612A2 - High intensity focused ultrasound system, computer-implemented method, and computer program product

Info

Publication number: WO2012049612A2
Application number: PCT/IB2011/054459
Authority: WO
Inventors: Reko Tapio Vuorinen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-10-14
Filing date: 2011-10-10
Publication date: 2012-04-19
Also published as: WO2012049612A3

Abstract

A high intensity focused ultrasound system (300, 500) comprising: an ultrasound transducer (306, 506) with multiple transducer elements (312) for focusing ultrasound energy into a focal volume (320, 420, 520); an ultrasound transducer power supply (422) for the ultrasound transducer, a mechanical positioning system (310, 510) for the ultrasound transducer; a processor (428); and a memory (432, 434) containing machine executable instructions. Execution of the instructions causes the processor to: determine (100, 204) a location of an acoustic reflection (324, 328, 420, 540, 546) of the focal volume; and adjust (102, 206) the location of the acoustic reflection if the acoustic reflection is within a predetermined distance from any one of the multiple transducer elements, the location of the acoustic reflection is adjusted by moving the focal volume by controlling the ultrasound transducer power supply and by controlling the ultrasound transducer using the mechanical positioning system.

Description

High intensity focused ultrasound system, computer-implemented method, and computer program product

TECHNICAL FIELD

The invention relates to high intensity focused ultrasound, in particular a high intensity focused ultrasound system which avoids reflected ultrasonic energy from being focused on transducer elements.

BACKGROUND OF THE INVENTION

The selective heating of tissue in a subject may have therapeutic value. High intensity focused ultrasound therapy, which uses highly focused ultrasound has been successfully used to either heat or ablate tissue.

In high intensity focused ultrasound an array of transducer elements are used to form an ultrasonic transducer. Supplying alternating current electrical power to the transducer elements causes them to generate ultrasonic waves. The ultrasonic wave from each of the transducer elements either adds constructively or destructively. By controlling the phase of alternating current electrical power supplied to each of the transducer elements the focal point or volume into which the ultrasound power is focused may be controlled.

SUMMARY OF THE INVENTION

The invention provides for a high-intensity focused ultrasound system, a computer-implemented method, and a computer program product in the independent claims. Embodiments are given in the dependent claims.

One of the problems of HIFU is the limited power handling of the transducer. Transducer piezo material and/or matching layer's safe operating temperature range is limited clearly below 100°C. This is not a problem in free field, but in confined space as in HIFU the acoustical reflection from the membrane reflects part of the emitted US beam back to the transducer surface which then heats up. Depending on conditions, this may produce localized heating many times higher than elsewhere on the transducer face.

Although the power reflection coefficient R may be very low, the reflected beam may produce very high acoustical intensities locally at the surface due to the focusing effect. The worst case is when the distance from transducer to the membrane is exactly f/2 where f is the focal length of the transducer. For transducer aperture D, inherent focus diameter df (Focus area Af = ndf/4), the acoustical intensity could increase by R^m(D/d/)². Taking D= 127.8 mm, d_j=2 mm, and R = 1.5%, the power deposited on the transducer may increase by a factor of 0.015· (127.8/2)2 = 61.

In some embodiments electronic deflection, which is normally used to create the desired sonication trajectories, to deflect the sonication spot in order to be able to mechanically move the transducer so that the localized heating is reduced.

Electronic deflection may be performed by using an ultrasound transducer power supply that is adapted for adjusting the phase of electrical power supplied to each of the multiple transducer elements. The ultrasound transducer power supply supplies alternating electrical current to each of the multiple transducer elements. When supplied with alternating current each of the multiple transducer elements vibrate and produce ultrasound. The ultrasound produced by each of the multiple transducer elements can either add constructively or destructively with the ultrasound produced by the other transducer elements. The multiple transducer elements may be arranged such that the ultrasound produced is concentrated into a focal volume. By adjusting the phase of the various multiple transducer elements the location of the focal volume may be adjusted or changed.

In principle other means could also be used to deflect the focus. For example mechanical deformation of the transducer face or assembly may also deflect the focus.

Generally the beam diameter d at distance x from the focus (towards the transducer) can be approximated by:

d(x) = d/ + D · x/L

where L is the transducer height i.e., distance from the focus to the transducer aperture (L<f). If the beam is reflected from a flat membrane, perpendicular to the focal axis, at distance / (/ < ) from the focus, the resulting spot diameter at transducer surface would be approximately:

If the transducer is physically moved away from the worst case position (l=f/2) by δ, while using the electronic deflection to keep the local heating at the original location, the spot size (produced by membrane reflection) at the surface of the transducer would be:

d_R,d(l,S) = d_f + D ' (2 ' \ l + d\ -j) /L

The ratio of the reflected acoustical intensity towards the outgoing wave is given by I_r/Io = R^m(D/d/)². From Table 1 we find that with deflection as small as 5 mm we can guarantee that the heating effect coming from the reflection is comparable to, or less than the heating caused by the outgoing wave.

Table 1 :

60 mm 2.0 61.2

62.5 mm 8.3 3.6

65 mm 14.6 1.2

67.5 mm 20.9 0.6

70 mm 27.2 0.3

Table 1 illustrates shows calculated reflected spot diameters and additional deposited intensity at the transducer surface as a function of transducer position and beam deflection. Although the example in Table 1 is for a geometry where the reflecting surface is flat and perpendicular to the transducer, and the transducer surface is spherical , the same principle can be generalized to any geometry. The only requirement is that the reflection produces localized heating on the transducer face, and that the heating pattern is sensitive to the position of the transducer.

A computer-readable storage medium as used herein is any storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be a computer-readable non-transitory storage medium. The computer-readable storage medium may also be a tangible computer readable medium. A computer-readable storage medium may also be referred to as 'memory.' In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device. An example of a computer- readable storage medium include, but are not limited to: a floppy disk, a magnetic hard disk drive, a solid state hard disk, flash memory, a USB thumb drive, Random Access Memory (RAM) memory, Read Only Memory (ROM) memory, an optical disk, a magneto-optical disk, and the register file of the processor. Examples of optical disks include Compact Disks (CD) and Digital Versatile Disks (DVD), for example CD-ROM, CD-RW, CD-R, DVD- ROM, DVD-RW, or DVD-R disks. The term computer readable-storage medium also refers to various types of recording media capable of being accessed by the computer device via a network or communication link. For example a data may be retrieved over a modem, over the internet, or over a local area network. Computer memory is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. Examples of computer memory include, but are not limited to: RAM memory, registers, and register files.

Computer storage is an example of a computer-readable storage medium. Computer storage is any non- volatile computer-readable storage medium. Examples of computer storage include, but are not limited to: a hard disk drive, a USB thumb drive, a floppy drive, a smart card, a DVD, a CD-ROM, and a solid state hard drive. In some embodiments computer storage may also be computer memory or vice versa.

A 'processor' as used herein encompasses an electronic component which is able to execute a program or machine executable instruction. References to the computing device comprising "a processor" should be interpreted as possibly containing more than one processor. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor. Many programs have their instructions performed by multiple processors that may be within the same computing device or which may even distributed across multiple computing device.

A medical imaging device as used herein encompasses a device or apparatus for acquiring medical imaging data. Medical imaging data as used herein encompasses data, images, or information which is descriptive of the internal structure and/or is descriptive of the anatomical structure of a subject.

Magnetic resonance 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. Magnetic resonance data is an example of medical imaging data. 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.

An ultrasound model as used herein encompasses a model which models the ultrasonic properties of a subject. For instance, an ultrasonic model may model the reflection of ultrasound at boundaries between different regions of a subject. An ultrasonic model may also model the transmission and/or attenuation of ultrasound by different regions of a subject. An ultrasonic model may model the local absorption of ultrasonic energy by a subject.

Described in other terms, an ultrasonic model is a model which may be used to

mathematically predict the transmission, attenuation, energy absorption, and/or reflection of ultrasound in a subject. In one aspect the invention provides for a high-intensity focused ultrasound system which comprises an ultrasound transducer with multiple transducer elements for focusing ultrasound energy into a focal volume. The high-intensity focused ultrasound system further comprises an ultrasound transducer power supply for supplying electrical power to each of the multiple transducer elements. The ultrasound transducer power supply is adapted for adjusting the phase of electrical power supplied to each of the multiple transducer elements.

In some embodiments the high-intensity focused ultrasound system further may comprises a fluid- filled volume in contact with the multiple transducer elements. The fluid in the fluid- filled volume conducts the ultrasound generated by the multiple transducer elements. In some embodiments the fluid-filled volume may also be in contact with an ultrasound window. An ultrasound window as used herein is a window which transmits ultrasound. Typically a thin film or membrane is used as an ultrasound window. The ultrasound window may for example be made of a thin membrane of BoPET (Biaxially- oriented polyethylene terephthalate). The ultrasound transducer is adapted for directing ultrasound energy through the ultrasound window into a subject. The focal volume is within the subject. The high-intensity focused ultrasound system further comprises a mechanical positioning system for mechanically positioning the ultrasound transducer relative to the ultrasound window. The mechanical positioning system is essentially a mechanical system which is used to physical move the location or orientation of the ultrasound transducer. In some embodiments the mechanical positioning system may be manually actuated. In other embodiments a mechanism or system of mechanisms may be used to automatically move the ultrasound transducer.

The high-intensity focused ultrasound system further comprises a processor for controlling the high-intensity focused ultrasound system. The high-intensity focused ultrasound system further comprises a memory containing machine executable instructions for execution by the processor. Execution of the instructions causes the processor to perform the step of determining a location of an acoustic reflection of the focal volume. Ultrasound waves may be deflected when the ultrasound impedance, the medium through which the ultrasound wave is traveling changes. For instance ultrasound may be reflected by the ultrasound window. If the subject is in contact with the ultrasound window using a gel pad then the ultrasound may also be reflected by the boundary between the subject and the gel pad. Further, the subject itself may have different regions also. For instance a subject may comprise fatty tissue, muscle tissue, skin and/or bone tissue. Each of these various tissues has a different ultrasonic property which means that their interfaces may reflect ultrasonic waves. Execution of the instructions further causes the processor to perform the step of adjusting the location of the acoustic reflection if the acoustic reflection is within a predetermined distance from any one of the multiple transducer elements. The location of the acoustic reflection is adjusted by moving the focal volume by controlling the ultrasound transducer power supply and by moving the ultrasound transducer by controlling the mechanical positioning system. Depending upon the situation the ultrasound transducer may be moved either closer or further away from the ultrasound window. If the acoustic reflection concentrates ultrasonic energy too close to a transducer element then the transducer element may be damaged. This is because the concentrated ultrasonic waves may heat the transducer element. This may damage the transducer element causing it to fail in functionality.

The determination of a predetermined distance may be determined in several different ways. In some embodiments the predetermined distance may be a fixed distance which defines a distance at which the reflection of the focus volume damages, degrades, or destroys transducer elements. Alternatively the predetermined distance may be a function of the distance between the transducer elements on the ultrasonic transducer and the ultrasonic window. For this alternative cost functions may be used. Cost functions may be used to balance between transducer heating versues other properies such as: deterioration of focal properties such as intensity focus size, near field heating, far field heating, and avoidance of sensitive body parts.

One consideration is that there may be more than one reflection of the focal volume. For instance, there may be a reflection due to the ultrasound window and there may be another reflection due to the boundary between fatty and muscle tissue. In some embodiments a cost function could provide a way of minimizing the heating of the transducer elements.

The location of the acoustic reflection is adjusted by moving the focal volume using the ultrasound transducer power supply and by moving the ultrasound transducer using the mechanical positioning system. The ultrasound transducer power supply can be used to move the focal volume because the phase of the electrical power or alternating current which is applied to each of the transducer elements may be modified or changed. By moving the focal volume, by also physically moving the ultrasound transducer the focal volume can be directed into a region of the subject without excess energy being directed onto a transducer element. This reduces the likelihood that a transducer element will be damaged and extends the lifetime of the ultrasound transducer. In another embodiment the location of the acoustic reflection is adjusted to avoid reflected ultrasonic energy from being focused onto the multiple transducer elements. This is advantageous because the reflected ultrasonic energy may damage one or more of the transducer elements.

In another embodiment the predetermined distance is a fixed distance. This is advantageous because a zone where damage to the transducer elements may be defined. If the reflection of the focus volume is within predetermiend distance then damange to transducer elements may occur, preventing the reflection fo the focus volume from being within this zone eliminates the possiblity of damage to transducer elements.

In another embodiment the predetermined distance is determiend by a a cost function. In some embodiments the predetermined distance may be a function of the ultrasonic transducer distance to the ultrasonic window. This may be adventageous because as with the previous embodiment dammage to transducer elements may be prevented. In addition an optimum distance between the ultrasounic transducer and the ultrasonic window may also be selected. Using a cost function may have the benefit of maininting a consistently lower temperature on the transducer surface to increase lifetime of the transducer.

Using a cost function may also have the benefit of decreasing the near field heating when the distance between the transducer element and the focal volume is decreased electronically. This is because the ultrasonic radiation or energy in the near field is directed towards the focal volume from a larger solid angle. In the near field the energy density is therefore reduced. Moving the focal volume closer to the transducer element is performed electronically by controlling the phase of alternating electrical power to the individuatl transducer elements.

In another embodiment the step of determining a location of an acoustic reflection of the focal volume is performed by calculating a position of the acoustic reflection of the focal volume by the ultrasound window. This embodiment is advantageous because the mechanical positioning system which positions the ultrasound transducer and the relation of the ultrasound transducer to the ultrasound window are a known quantity. A model or a table of positions which indicate when ultrasound may be reflected onto a transducer element can be constructed.

In another embodiment the step of determining a location of an acoustic reflection of the focal volume is performed by calculating a position of the acoustic reflection of the focal volume by the ultrasound window and/or a gel pad and/or an external surface of the subject. This embodiment is advantageous because the mechanical positioning system which positions the ultrasound transducer and the relation of the ultrasound transducer to the ultrasound window is a known quantity. In some embodiments when a subject is in contact with the ultrasound window the relation of the ultrasound transducer to the location of an external surface of the subject is a known quantity. In some embodiments a gel pad is between an external surface of a subject and the ultrasound window. In this embodiment the location of the external surface of the subject, the gel pad, and the location of the ultrasound window in relation to the ultrasound transducer are a known quantity. For these various embodiments a model or a table of positions which indicate when ultrasound may be reflected onto a transducer element can be constructed.

In another embodiment the focal volume is moved by controlling the phase of the electrical power supplied to each of the multiple transducer elements. As was mentioned before, the transducer elements are supplied with alternating current. By controlling the phase of the electrical power supplied to each of the multiple transducer elements the position of the focal volume can be adjusted. This is because the ultrasonic waves add constructively or destructively in such a way that the focal volume is moved or transmitted.

In another embodiment execution of the instructions further causes the processor to perform the step of calculating a set of phases of electrical power to be supplied to each of the multiple transducer elements by using ray tracing. Ray tracing as used herein encompasses calculating the intensity of ultrasonic energy within the location of a subject or in a path to the subject taking into account the propagation of the ultrasound and its phase. In other words ray tracing encompasses taking into account the phase of the ultrasound energy from the different transducer elements of the ultrasound transducer. The focal volume is moved by controlling the ultrasound transducer power supply in accordance with the set of phases.

In another embodiment the high-intensity focused ultrasound system further comprises a medical imaging system for acquiring medical imaging data. The instructions further cause the processor to acquire medical image data using the medical imaging system. The ray tracing is performed in accordance with the medical image data.

In another embodiment the medical imaging system is a magnetic resonance imaging system.

In another embodiment the medical imaging system is a computed tomography system.

In another embodiment the medical imaging system is an ultrasound imaging system. In some embodiments a separate ultrasound imaging system is used. In other embodiments the same ultrasound transducer is used for both imaging and for generating ultrasound which is directed into the focal volume.

In another embodiment the medical imaging system is a positron emission tomography system.

In another embodiment the medical imaging system is a combined magnetic resonance imaging system and/or a computed tomography system and/or ultrasound imaging system, and/or positron emission tomography system.

In another embodiment execution of the instructions further cause the processor to perform the step of constructing an ultrasound model using the medical image data. For instance the medical image data may be segmented using known segmentation techniques to identify various regions or anatomical structures of the subject. This may be used to generate or construct an ultrasound model. In the case where ultrasound imaging is used the ultrasound data may be used to directly construct an ultrasound model. The step of determining a location of an acoustic reflection of the focal volume is performed using the ultrasound model. Using a knowledge of the locations where the ultrasound impedance changes may be used to determine the location of acoustic reflections of the focal volume.

In another embodiment the ultrasound impedance interfaces form the boundary of any one of the following: an ultrasound window, a membrane, a gel pad, a skin tissue region, a fatty tissue region, a muscle tissue region, and a bony tissue region. A skin tissue region is a region of tissue comprised primarily of skin. A fatty tissue region is a region of tissue which is comprised primarily by fat. A muscle tissue region is a region of tissue which is comprised primarily of muscle tissue. A bony tissue region is a region of tissue which is comprised primarily of bone.

In another embodiment the high intensity focused ultrasound system further comprises an ultrasound window. The high intensity focused ultrasound system further comprises a fluid filled volume in contact with the multiple transducer elements and the ultrasound window. The ultrasound transducer is adapted for directing ultrasound energy through the ultrasound window into the subject. Execution of the instructions further causes the processor to perform the steps of constructing an ultrasound model using a knowledge of the geometry of the ultrasound transducer, the fluid- filled volume, and the ultrasound window. The step of determining a location of an acoustic reflection of the focal volume is performed using the ultrasound model.

In another embodiment execution of the instructions further comprise performing a sonication of the focal volume by controlling the ultrasound transducer. Ultrasound energy may be directed into the focal volume which causes the focal volume to be heated which may cause the mechanical damage of cells located within the focal volume. This may be used to perform therapeutic operations on the subject.

In another embodiment there is a target volume. The target volume may be larger than the focal volume. Execution of the instructions further causes the processor to perform the step of performing a volumetric sonication of the target volume by dynamically controlling the phase of electrical power supplied to the multiple transducer elements. In some embodiments the ultrasound transducer may also be mechanically or physically moved by controlling the mechanical positioning system as part of the volumetric sonication.

In another aspect the invention provides for a computer-implemented method of controlling a high-intensity focused ultrasound system according to an embodiment of the invention. The method comprises the step of determining a location of an acoustic reflection of the focal volume. The method further comprises the step of adjusting the location of the acoustic reflection if the acoustic reflection is within a predetermined distance from any one of the multiple transducer elements. The location of the acoustic reflection is adjusted by moving the focal volume by controlling the ultrasound transducer power supply and by moving the ultrasound transducer by controlling the mechanical positioning system.

In another aspect the invention provides for a computer program product comprising machine executable instructions for execution by a processor of a high-intensity focused ultrasound system according to an embodiment of the invention. The computer program product may for instance be stored on a computer-readable storage medium.

Execution of the instructions causes the processor to perform the step of determining a location of an acoustic reflection of the focal volume. Execution of the instructions further causes the processor to perform the step of adjusting the location of the acoustic reflection if the acoustic reflection is within a predetermined distance from any one of the multiple transducer elements. The location of the acoustic reflection is adjusted by moving the focal volume and by controlling the ultrasound transducer power supply and then moving the ultrasound transducer by controlling the mechanical positioning system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

Fig. 1 shows a block diagram which illustrates a method according to an embodiment of the invention; Fig. 2 shows a block diagram which illustrates a method according to a further embodiment of the invention;

Figs. 3a and 3b show a high-intensity focused ultrasound system according to an embodiment of the invention;

Fig. 4 shows a high-intensity focused ultrasound system according to a further embodiment of the invention;

Fig. 5 shows a high-intensity focused ultrasound system according to a further embodiment of the invention; and

Fig. 6 shows a graph which illustrates cost functions used to determine the predetermined distance.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Like numbered elements in these figures are either equivalent elements or perform the same function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent.

Fig. 1 shows a block diagram which illustrates a method according to an embodiment of the invention. In step 1 a location of the acoustic reflection of the focal volume is determined. It may be determined using modeling or if the geometry of the system is already known it may be determined using a lookup table or any equivalent. In step 102 the location of the acoustic reflection is adjusted if the acoustic reflection is within a

predetermined distance from any transducer element. The location of the acoustic reflection may be adjusted by physically moving the ultrasound transducer and/or adjusting the phase of electrical power delivered to each of the transducer elements used for constructing the ultrasound transducer.

Fig. 2 shows a block diagram which illustrates a method according to a further embodiment of the invention. In step 200 medical image data is acquired. In step 202 an ultrasound model is constructed using the medical image data. In step 204 a location of an acoustic reflection of the focal volume is determined using the ultrasound model. In step 206 the location of the acoustic reflection is adjusted if the acoustic reflection is within a predetermined distance from any transducer element.

Fig. 3a shows an embodiment of a high-intensity focused ultrasound system 300 according to an embodiment of the invention. The processor for controlling the high- intensity focused ultrasound system and the associated control electronics are not shown in this Fig. In Fig. 3a a subject 302 is shown as resting upon a subject support 304. There is an ultrasound transducer 306 positioned below the subject 302 and the subject support 304. The ultrasound transducer 306 is within a fluid-filled volume 308. There is a mechanical positioning system 310 for moving the ultrasound transducer 306 within the fluid- filled volume 308. On the ultrasound transducer 308 there is a plurality of transducer elements 312. When the transducer elements 312 are supplied with alternating current electrical power they produce ultrasound.

The lines labeled 314 shows the path of ultrasound 314 from the ultrasound transducer 306 to the subject 302. The ultrasound 314 passes through an ultrasound window 316. After passing through the ultrasound window 316 the ultrasound passes through a gel pad 318. The gel pad 318 is located within an empty volume of the subject support 304. The gel pad 318 is in contact with the subject 302. The ultrasound 314 is shown as being focused into a focal volume 320. The dotted lines 322 show the reflection of the path of ultrasound 314. The dashed circle 324 shows the location of an image of the focal volume 320. In the example shown in Fig. a it can be shown that the reflection of the focal volume 324 caused by the ultrasound window 316 is located adjacent to the transducer elements 312. During sonication of the focal volume 320 large amounts of ultrasonic energy may be directed towards one or more of the transducer elements 312. This may result in the damaging of a transducer element 312.

Fig. 3b shows the same high-intensity focused ultrasound system 300 that was shown in Fig. 3a. However, the ultrasound transducer 306 has been moved closer to the ultrasound window 316. Relative to the ultrasound transducer 306 the position of the focal volume has changed. However, relative to the subject 302 the position of the focal volume 320 has not changed. The focal volume 320 was kept relative to the subject 302 by adjusting the phase of electrical energy delivered to the transducer elements 312. The dashed lines 326 show the reflections of the path of the ultrasound 314. The dashed circle 328 shows the location of a reflection of the focal volume 320. In the example shown in Fig. 3b it is shown that the reflection 328 of the focal volume does not form adjacent to the transducer elements 312. Furthermore, since the ultrasound is not able to pass through the ultrasonic transducer 306 the reflection of the focal volume 328 would not form in the first place. The Fig. 3b illustrates how performing an embodiment of the invention can eliminate heating of transducer elements by using a combination of moving the ultrasonic transducer 306 and by adjusting the phase of electrical energy delivered to the transducer elements 312.

Fig. 4 shows a diagram with a combined high-intensity focused ultrasound system 300 and a magnetic resonance imaging system 400. The high-intensity focused ultrasound system 300 is equivalent to the high-intensity focused ultrasound system 300 shown in Figs. 3a and 3b. However, not all detail shown in Fig. 3a and 3b are shown in this figure. In this example a magnetic resonance imaging system 400 is used. However other medical imaging systems may also be integrated into the high intensity focused ultrasound system 300. For instance the magnetic resonance imaging system may be replaced by a computed tomography system, an ultrasound imaging system, a positron emission

tomography system, or a combined magnetic resonance imaging system and/or computed tomography system and/or ultrasound imaging system and/or positron emission tomography system.

The magnetic resonance imaging system comprises a magnet 402 for generating a magnetic field. The type of magnet shown in this Fig. is a cylindrical bore superconducting magnet. However, other varieties of magnets may be used such as a so- called open magnet which resembles a magnet generated by a Helmholtz coil. Within the bore of the magnet there is an imaging zone 404 where the magnetic field is sufficiently strong and uniform enough for acquiring magnetic resonance data. Also within the bore of the magnet 406 is a magnetic field gradient coil power supply. Although a single magnetic field gradient coil power supply is shown it is understood that this represents three separate sets of magnetic field gradient coils. The magnetic field gradient coil 406 is connected to a magnetic field gradient coil power supply 408. The magnetic field gradient coil power supply 408 supplies current to the magnetic field gradient coil 406.

There is a radio frequency transmitter 410 connected to a radio frequency coil 412. The radio frequency coil 412 is adjacent to the imaging zone 404. It is understood that the radio frequency receiver 410 and coil 412 are equivalent to individual transmitters and receivers and independent transmit and receive coils respectively. The magnetic field gradient coil 406 and the radio frequency coil 412 are used for manipulating magnetic spins within the imaging volume 404 and for acquiring magnetic resonance data.

The dashed lines 314 show the path of ultrasound from the ultrasound transceiver 306 to a focal volume 414. There is a target volume 416 which is shown as being larger than the focal volume 414. During sonication the focal volume 414 will be moved such that the entire target volume 416 is treated. This may be accomplished using a combination of moving the ultrasonic transducer 306 with the medical positioning system 310 or by using a high-intensity focused ultrasound power supply 422 to control the phase of electrical power delivered to individual transducer elements 312 on the surface of the ultrasound transducer 306. The solid lines 418 show the path of ultrasound reflected by the boundary of the gel pad 318 and an external surface the subject 419. The dashed circle 420 is a reflection of the focal volume 414 caused by the interface between the subject 302 and the gel pad 318. Using the embodiment of a method according to the invention the image of the focal volume 420 may be shifted away from the ultrasound transducer 306 by a combination of moving the ultrasound transducer and by controlling the phase of power generated by the high- intensity focused ultrasound power supply 422.

The radio frequency transmitter 410, the high- intensity focused ultrasound power supply 422 and the magnetic field gradient coil power supply 408 are shown as being connected to a hardware interface 426 of a computer system 424. The computer system 424 further comprises a processor 428 which is connected to the hardware interface 426, a user interface 430, computer storage 432 and computer memory 434. The hardware interface 426 is used for controlling the high-intensity focused ultrasound system 300 and the magnetic resonance imaging system 400. It is understood that although this is shown as a single computer system 424 and a single processor 428 multiple computer systems and/or processors may be used and are equivalent. The user interface 430 is an interface which allows an operator to interact with and/or control the computer system 424. The user interface 430 may comprise such devices as a display, a mouse, a keyboard, a tablet, and/or a light pen.

The computer storage 432 is shown as containing magnetic resonance data 436 which is acquired using the magnetic resonance imaging system 400. The computer storage 432 is further shown as containing a treatment plan 438. The treatment plan 438 contains instructions or details created by a physician or care giver which may be used to create a set of control sequences 446 for performing the volumetric sonication of the target volume 416. The computer storage 432 is further shown as containing an ultrasound model 444. The ultrasound model 444 is constructed by segmenting the magnetic resonance image 440. The computer storage 432 is further shown as containing a control sequence 446. The control sequence 446 contains machine executable instructions which the processor 428 may send to the high intensity focused ultrasound system 300 and/or magnetic resonance imaging system 400 during sonication of the target zone 416. The control sequence may comprise a set of phases for setting the phase of alternating electrical power delivered to the transducer elements during sonification. In some embodiments the system is able to modify the control sequence 446 during the sonication of the target volume 416. The computer memory 434 is shown as containing a magnetic resonance control module 448. The magnetic resonance control module 448 contains machine executable instructions usable by the processor 428 for generating control sequences for controlling the operation of the magnetic resonance imaging system 400. The computer memory 434 is further shown as containing an ultrasound control module 450. The ultrasound control module 450 contains machine executable instructions for use by the processor 428 for generating control commands for operating the high intensity focused ultrasound system 400. The computer memory 434 is further shown as containing a magnetic resonance image reconstruction module 452. The magnetic resonance image reconstruction module 452 contains machine executable instructions for reconstructing the magnetic resonance data 436 into a magnetic resonance image 440. The computer memory 434 is further shown as containing an ultrasound model creation module 454. The ultrasound model creation module 454 contains machine executable instructions which enable to segment the magnetic resonance image 440 and create the ultrasound module 444. Finally the computer memory 434 is shown as containing a control sequence creation module 456. The control sequence creation module 456 is able to use the treatment plan 438 and the ultrasound model 444 for generating the control sequence 446.

Fig. 4 shows a functional diagram of a high intensity focused ultrasound system 400 according to a further embodiment of the invention. In this embodiment there is an ultrasound transducer 406 immersed in a fluid filled volume 408. The ultrasound transducer 406 is positioned using a robotic positioning system 410. The robotic positioning system 410 moves the ultrasound transducer 406 relative to an ultrasound window 416. In this embodiment the fluid filled volume 408 is surrounded by a membrane 426. The ultrasound window 416 is connected to a support ring 428 as is the membrane 426. The ultrasound window 416 is in contact with a subject 402. In this embodiment the subject 402 has several different regions.

The ultrasound window 416 is in contact with a first layer of the subject 430. The first layer of the subject is in contact with a second layer of the subject 432. The first layer 430 and the second layer 432 comprise different materials or tissue types so there is an ultrasound impedance interface 434 between the two of them. For calculation of the phases for transducer elements in this figure Rays 436, which trace the path from the ultrasound transducer 406 to a focal volume 420, are shown. To calculate the phase accurately in the focal volume 420 transmission of ultrasound through the first layer 430 and the second layer 432 both need to be considered. By accurately calculating the phase in the focal volume 420 for each of the transducer elements the velocity of the ultrasound in the first layer 430, the second layer 432 and the fluid filled volume 408 are all considered.

Fig. 5 shows a functional diagram of a high intensity focused ultrasound system 500 according to a further embodiment of the invention. In this embodiment there is an ultrasound transducer 506 immersed in a fluid filled volume 508. The ultrasound transducer 506 is positioned using a robotic positioning system 510. The robotic positioning system 510 moves the ultrasound transducer 506 relative to an ultrasound window 516. In this embodiment the fluid filled volume 508 is surrounded by a membrane 526. The ultrasound window 516 is connected to a support ring 528 as is the membrane 526. The ultrasound window 516 is in contact with a subject 502. In this embodiment the subject 502 has several different regions.

The ultrasound window 516 is in contact with a first layer of the subject 530. The first layer of the subject is in contact with a second layer of the subject 532. The first layer 530 and the second layer 532 comprise different materials or tissue types so there is an ultrasound impedance interface 534 between the two of them. For calculation of the phases for transducer elements in this figures Rays 536, which trace the path from the ultrasound transducer 506 to a focal volume 520, are shown. To calculate the phase accurately in the focal volume 520 transmission of ultrasound through the first layer 530 and the second layer 532 both need to be considered. By accurately calculating the phase in the focal volume 520 for each of the transducer elements the velocity of the ultrasound in the first layer 530, the second layer 532 and the fluid filled volume 508 are all considered.

The dashed lines 438 trace the path of the ultrasound to a reflection of the focal volume 440 caused by the ultrasound window 516. The dashed lines 442 trace the path of the ultrasound to a reflection of the focal volume 446 caused by the boundary between the first layer 530 and the second layer 532. Fig. 5 illustrates that when the subject is considered there may be multiple reflections 440, 446 of the focal volume 520.

Fig. 6 shows a plot which illustrates several different ways of selecting the predetermined distance. The x-axis 600 is the distance between the active surface of the ultrasonic transducer and the ultrasonic window. The active surface of the ultrasonic transducer is the surface of the ultra sonic transducer which has the transducer elements. The y-axis 602 is the distance between the reflection of the focal volume and the active surface of the ultrasonic transducer. The lines 604 and 606 define a distance at which the image of the focal volume will cause damage to the transducer elements. The lines 604 and 606 define a zone 607 where there is "immediate danger" to the transducer elements, and are denoted by range from -d to +d.

The distance at which the reflection of the focal volume hits the active surface of the ultrasonic transducer element exactly is the line labeled 608. Line 608 passes through the x-axis 600 at f/2. Also sown in the figure are two cost functions. The first cost function 610 approximates the lines 604 and 606.

The second cost function 612 is smoother than the first const function 610. The second cost function may have the advantage that a consistently lower temperature on the transducer surface is maintained. This has the advantage of increasing the lifetime of the transducer elements. The second cost function may also have the advantage that the near field heating is decreased when x <f/2.

In other embodiments several predefined zones where the first one is strictly forbidden area, and on further ones one would employ less and less deflection. This may have the same effect as the cost function 612.

In another embodiment the cost function 612 when x < f/2 and the distances 604 and 606 could be used when x > f/2 . This may be advantageous because the near field heating decreases when this reflected focus avoidance is employed at x < f/2, while it increases at x > f/2-

Electronic deflection of the focal volume also affects the shape and properties of the cone, which means that it may have an effect on treatment planning. This may allow avoiding sensitive structures on the beam path and may affect far field heating. Some properties of the cost function may therefore be user selectable.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE NUMERALS

300 high intensity focused ultrasound system

302 subject

304 subject support

306 ultrasound transducer

308 fluid filled volume

310 mechanical positioning system

312 transducer element

314 path of ultrasound

316 ultrasound window

318 gel pad

320 focal volume

322 path of reflected ultrasound

324 reflection of focal volume

326 path of reflected ultrasound

328 reflection of focal volume

400 magnetic resonance imaging system

402 magnet

404 imaging volume

406 magnetic field gradient coil

408 magnetic field gradient coil power supply

410 radio frequency transmitter

412 radio frequency coil

414 focal volume

416 target volume

418 reflected ultrasound

419 external surface of the subject

420 image of focal volume

422 high intensity focused ultrasound power si

424 computer system

426 hardware interface

428 processor

430 user interface

432 storage 434 memory

436 magnetic resonance data

438 treatment plan

440 magnetic resonance image

444 ultrasound model

446 control sequence

448 magnetic resonance control module

450 ultrasound control module

452 magnetic resonance image reconstruction module

454 ultrasound model creation module

456 control sequence creation module

500 high intensity focused ultrasound system

502 subject

506 ultrasound transducer

508 fluid filed volume

510 robotic positioning system

516 ultrasound window

520 focal volume

526 membrane

528 support ring

530 first layer of subject

532 second layer of subject

534 ultrasound impedance interface

536 ray

538 path of reflected ultrasound

540 image of focal volume

542 path of reflected ultrasound

546 image of focal volume

600 distance between the active surface of the ultrasonic

transducer and the ultrasonic window

602 distance between the reflection of the focal volume and the active surface of the ultrasonic transducer

604 distance at which transducer elements are damaged

606 distance at which transducer elements are damaged zone of immediate damage to transducer elements distance at which the reflection of the focal volume hits the active surface of the ultrasonic transducer element exactly cost function

cost function

Claims

CLAIMS:

1. A high intensity focused ultrasound system (300, 500) comprising:

an ultrasound transducer (306, 506) with multiple transducer elements (312) for focusing ultrasound energy into a focal volume (320, 420, 520) within a subject;

an ultrasound transducer power supply (422) for supplying electrical power to each of the multiple transducer elements, wherein the ultrasound transducer power supply is adapted for adjusting the phase of electrical power supplied to each of the multiple transducer elements;

a mechanical positioning system (310, 510) for mechanically positioning the ultrasound transducer;

- a processor (428) for controlling the high intensity focused ultrasound system; and

a memory (432, 434) containing machine executable instructions for execution by the processor, wherein execution of the instructions causes the processor to:

- determine (100, 204) a location of an acoustic reflection (324, 328, 420, 540, 546) of the focal volume; and

- adjust (102, 206) the location of the acoustic reflection if the acoustic reflection is within a predetermined distance from any one of the multiple transducer elements, wherein the location of the acoustic reflection is adjusted by moving the focal volume by controlling the ultrasound transducer power supply and by moving the ultrasound transducer by controlling the mechanical positioning system.

2. The high intensity focused ultrasound system of claim 1, wherein the location of the acoustic reflection is adjusted such that reflected ultrasonic energy avoids being focused on the multiple transducer elements.

3. The high intensity focused ultrasound system of claim 1 or 2, wherein the predetermined distance is any one of the following: a fixed distance and a distance determined by a cost function.

4. The high intensity focused ultrasound system of claim 1,2, or 3, wherein the location of an acoustic reflection of the focal volume is determined by calculating a position of the acoustic reflection of the focal volume by an ultrasound window and/or a gel pad (318) and/or an external surface of the subject (419).

5. The high intensity focused ultrasound system of any one of the preceding claims, wherein the focal volume is moved by controlling the phase of the electrical power supplied to each of the multiple transducer elements.

6. The high intensity focused ultrasound system of claim 5, wherein execution of the instructions further causes the processor to calculate a set of phases of electrical power to be supplied to each of the multiple transducer elements by using ray tracing, wherein the focal volume is moved by controlling the ultrasound transducer power supply in accordance with the set of phases.

7. The high intensity focused ultrasound system of claim 6, wherein the high intensity focused ultrasound system further comprises a medical imaging system for acquiring medical imaging data, wherein the instructions further cause the processor to acquire (200) medical image data using the medical imaging system, and wherein the ray tracing is performed in accordance with medical image data.

8. The high intensity focused ultrasound system of claim 7, wherein the medical imaging system is any one of the following: a magnetic resonance imaging system, a computed tomography system, an ultrasound imaging system, a positron emission

tomography system, and a combined magnetic resonance imaging system and/or

computedtomography system and/or ultrasound imaging system and /or positron emission tomography system.

9. The high intensity focused ultrasound system of claim 7 or 8, wherein execution of the instructions further causes the processor to construct (202) an ultrasound model using the medical image data, wherein the ultrasound model models ultrasound impedance interfaces, and wherein the location of an acoustic reflection of the focal volume is determined using the ultrasound model.

10. The high intensity focused ultrasound system of claim 9, wherien the ultrasound impednace interface is formed at the boundary of any one of the following: an ultrasound window, a membrane, a gelpad, a skin tissue region, a fatty tissue region, and a muscle tissue region, and a bony tissue region.

11. The high intensity focused ultrasound system of any one of claims 1 through 6; wherein the high intensity focused ultrasound system further comprises an ultrasound window (316, 526); wherein the high intensity focused ultrasound system further comprises a fluid filled volume (308, 508) in contact with the multiple transducer elements and the ultrasound window; wherein the ultrasound transducer is adapted for directing ultrasound energy through the ultrasound window into the subject (302, 502); wherein execution of the instructions further causes the processor to construct an ultrasound model using a knowledge of the geometry of the ultrasound transducer, the fluid filled volume, and the ultrasound window; and wherein the location of an acoustic reflection of the focal volume is determined using the ultrasound model.

12. The high intensity focused ultrasound system of any one of the preceding claims, wherein execution of the instructions further causes the processor to perform a sonication of the focal volume by controlling the ultrasound transducer.

13. The high intensity focused ultrasound system of claim 12, wherein there is a target volume (416), wherein execution of the instructions further cause the processor to perform a volumetric sonication of the target volume by dynamically controlling the phase of electrical power supplied to the multiple transducer elements.

14. A computer-implemented method of controlling a high intensity focused ultrasound system according to any one of claims 1 through 13, wherein the method comprises:

- determining (100, 204) a location of an acoustic reflection of the focal volume; and

adjusting (102, 206) the location of the acoustic reflection if the acoustic reflection is within a predetermined distance from any one of the multiple transducer elements, wherein the location of the acoustic reflection is adjusted by moving the focal volume by controlling the ultrasound transducer power supply and by moving the ultrasound transducer by controlling the mechanical positioning system.

15. A computer program product comprising machine executable instructions for execution by a processor of a high intensity focused ultrasound system according to any one of claims 1 through 13, wherein execution of the instructions causes the processor to:

determine (100, 204) a location of an acoustic reflection of the focal volume; and

adjust (102, 206) the location of the acoustic reflection if the acoustic reflection is within a predetermined distance from any one of the multiple transducer elements, wherein the location of the acoustic reflection is adjusted by moving the focal volume by controlling the ultrasound transducer power supply and by moving the ultrasound transducer by controlling the mechanical positioning system.