Medical Instrument For Sonicating A Set Of Target Volumes

Abstract

The invention provides for a medical instrument (100) comprising: a high intensity focused ultrasound system (104) a magnetic resonance imaging system (102). Machine executable instructions (180, 182, 184, 186) cause a processor (144) controlling the medical instrument to: receive (300) sonication commands (160), wherein the sonication commands specify a set of multiple target volumes (202) within the target zone; and receive (302) a selection of a current target volume (200) selected from the set of multiple target volumes. The machine executable instructions further cause the processor to repeatedly: acquire (304) the thermal magnetic resonance data by controlling the magnetic resonance imaging system with the thermometry pulse sequence commands (164); calculate (306) a temperature map (168) using the thermal magnetic resonance data; control (308) the high intensity focused ultrasound system to sonicate the current target volume by steering the sonication location to the current target volume; remove (310) the current target volume from the set of multiple target volumes after controlling the high intensity focused ultrasound system to sonicate the current target volume; calculate (312) a sonication energy (172) for each of the multiple target volumes by using the temperature map; select (314) a next target volume from the multiple target volumes using the calculation of the sonication energy for each of the multiple target volumes, wherein the selection of the next target volume comprises searching for the sonication energy with a minimum value; and set (316) the next target volume as the current target volume.

Claims

1. A medical instrument comprising: a high intensity focused ultrasound system comprising an ultrasonic transducer, wherein the ultrasonic transducer comprises multiple transducer elements for sonicating a target zone, wherein the high intensity focused ultrasound system is operable for electronically steering a sonication location by controlling supply of electrical power to each of the multiple transducer elements; a magnetic resonance imaging system for acquiring thermal magnetic resonance imaging data from an imaging zone, wherein the target zone is within the imaging zone; a processor for controlling the medical instrument; a memory containing machine executable instructions and thermometry pulse sequence commands, wherein the thermometry pulse sequence commands cause the magnetic resonance imaging system to acquire the thermal magnetic resonance imaging data according to a magnetic resonance imaging thermometry protocol; wherein execution of the machine executable instructions causes the processor to: receive sonication commands, wherein the sonication commands specify a set of multiple target volumes within the target zone; and receive a selection of a current target volume selected from the set of multiple target volumes; wherein execution of the machine executable instructions causes the processor to repeatedly: acquire the thermal magnetic resonance data by controlling the magnetic resonance imaging system with the thermometry pulse sequence commands; calculate a temperature map using the thermal magnetic resonance data; control the high intensity focused ultrasound system to sonicate the current target volume by steering the sonication location to the current target volume; remove the current target volume from the set of multiple target volumes after controlling the high intensity focused ultrasound system to sonicate the current target volume; calculate a sonication energy that needs to be deposited at a target volume for each of the multiple target volumes by using the temperature map; select a next target volume from the multiple target volumes based on the calculated sonication energy, wherein the next target volume is a target volume, which will require a minimum sonication energy to finish the sonication; and set the next target volume as the current target volume.

2. The medical instrument of claim 2, wherein execution of the machine executable instructions further causes the processor to repeatedly: calculate an estimated near field temperature map for each of the multiple target volumes using the temperature map and an ultrasonic transducer model, and select the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes.

3. The medical instrument of claim 2, wherein selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes comprises searching the estimated near field temperature map for a high temperature zone which has a temperature above a predetermined threshold.

4. The medical instrument of claim 3, wherein selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes comprises excluding a chosen target volume from being selected as the next target volume if the high temperature zone is found.

5. The medical instrument of claim 3, wherein selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes comprises modifying the sonication commands to shut off transducer elements selected from multiple transducer elements that contribute to the heating of the high temperature zone.

6. The medical instrument of claim 1, wherein the magnetic resonance imaging thermometry protocol is a proton resonance frequency shift magnetic resonance protocol, wherein the memory further contains calibration pulse sequence commands, wherein the calibration pulse sequence commands cause the magnetic resonance imaging system to acquire phase calibration magnetic resonance data according to the magnetic resonance imaging thermometry protocol, wherein execution of the machine executable instructions further causes the processor to: acquire the phase calibration magnetic resonance data by controlling the magnetic resonance imaging system with the calibration pulse sequence commands before controling the high intensity focused ultrasound system to sonicate the current target volume, and calculate a phase calibration using the phase calibration magnetic resonance data, wherein the temperature map is calculated using the thermal magnetic resonance data and the phase calibration.

7. The medical instrument of claim 6, wherein the medical instrument further comprises an actuator system for moving the ultrasonic transducer, wherein the sonication commands specify an actuator position for each of the set of multiple target volumes, wherein execution of the instructions further causes the processor to create a list of possible actuator positions from the actuator position for each of the set of multiple target volumes, wherein the phase calibration magnetic resonance data is acquired by acquiring the phase calibration magnetic resonance data for each actuator position in the list of possible actuator positions, wherein the phase calibration calculated by calculating the phase calibration for each actuator position in the list of possible actuator positions.

8. The medical instrument of claim 7, wherein execution of the machine executable instructions cause the processor to calculate the temperature map for each actuator position in the list of possible actuator positions using the thermal magnetic resonance data for each actuator position in the list of possible actuator positions, wherein controling the high intensity focused ultrasound system to sonicate the current target comprises controlling the actuator system to move the ultrasonic transducer to the actuator position of the current target volume.

9. A method of operating a medical instrument, wherein the medical instrument comprises a high intensity focused ultrasound system comprising an ultrasonic transducer; wherein the ultrasonic transducer comprises multiple transducer elements for sonicating a target, wherein the high intensity focused ultrasound system is operable for electronically steering a sonication location by controlling supply of electrical power to each of the multiple transducer elements, wherein the medical instrument further comprises a magnetic resonance imaging system for acquiring thermal magnetic resonance imaging data from an imaging zone, wherein the target zone is within the imaging zone, wherein the method comprises: receiving sonication commands, wherein the sonication commands specify a set of multiple target volumes within the target zone; receiving a selection of a current target volume selected from the set of multiple target volumes; wherein the method comprises repeatedly: acquiring the thermal magnetic resonance data by controlling the magnetic resonance imaging system with thermometry pulse sequence commands, wherein the thermometry pulse sequence commands cause the magnetic resonance imaging system to acquire the thermal magnetic resonance imaging data according to a magnetic resonance imaging thermometry protocol; calculating a temperature map using the thermal magnetic resonance data; controlling the high intensity focused ultrasound system to sonicate the current target volume by steering the sonication location to the current target volume; removing the current target volume from the set of multiple target volumes after controlling the high intensity focused ultrasound system to sonicate the current target volume; calculating a sonication energy for each of the multiple target volumes by using the temperature map; selecting a next target volume from the multiple target volumes using the calculation of the sonication energy for each of the multiple target volumes, wherein the selection of the next target volume comprises searching for the sonication energy with a minimum value; and setting the next target volume as the current target volume.

10. The method of claim 9, wherein the method further comprises repeatedly: calculating an estimated near field temperature map for each of the multiple target volumes using the temperature map and an ultrasonic transducer model, and selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes.

11. The method of claim 10, wherein selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes comprises searching the estimated near field temperature map for a high temperature zone which has a temperature above a predetermined threshold.

12. The method of claim 11, wherein selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes comprises excluding a chosen target volume from being selected as the next target volume if the high temperature zone is found.

13. The method of claim 11, wherein selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes comprises modifying the sonication commands to shut off transducer elements selected from multiple transducer elements that contribute to the heating of the high temperature zone.

14. The method of claim 9, wherein the magnetic resonance imaging thermometry protocol is a proton resonance frequency shift magnetic resonance protocol, wherein the method further comprises: acquiring the phase calibration magnetic resonance data by controlling the magnetic resonance imaging system with calibration pulse sequence commands before controling the high intensity focused ultrasound system to sonicate the current target volume, wherein the calibration pulse sequence commands cause the magnetic resonance imaging system to acquire phase calibration magnetic resonance data according to the magnetic resonance imaging thermometry protocol; and calculating a phase calibration according with the phase calibration magnetic resonance data, wherein the temperature map is calculated using the thermal magnetic resonance data and the phase calibration.

15. A computer program product comprising machine executable instructions for execution by a processor controlling a medical instrument, wherein the medical instrument comprises a high intensity focused ultrasound system comprising an ultrasonic transducer; wherein the ultrasonic transducer comprises multiple transducer elements for sonicating a target zone, wherein the high intensity focused ultrasound system is operable for electronically steering a sonication location by controlling supply of electrical power to each of the multiple transducer elements, wherein the medical instrument further comprises a magnetic resonance imaging system for acquiring thermal magnetic resonance imaging data from an imaging zone, wherein the target zone is within the imaging zone, wherein execution of the machine executable instructions causes the processor to: receive sonication commands, wherein the sonication commands specify a set of multiple target volumes within the target zone; receive a selection of a current target volume selected from the set of multiple target volumes; wherein execution of the machine executable instructions causes the processor to repeatedly: acquire the thermal magnetic resonance data by controlling the magnetic resonance imaging system with thermometry pulse sequence commands, wherein the thermometry pulse sequence commands cause the magnetic resonance imaging system to acquire the thermal magnetic resonance imaging data according to a magnetic resonance imaging thermometry protocol; calculate a temperature map using the thermal magnetic resonance data; control the high intensity focused ultrasound system to sonicate the current target volume by steering the sonication location to the current target volume; remove the current target volume from the set of multiple target volumes after controlling the high intensity focused ultrasound system to sonicate the current target volume; calculate a sonication energy for each of the multiple target volumes by using the temperature map; select a next target volume from the multiple target volumes using the calculation of the sonication energy for each of the multiple target volumes, wherein the selection of the next target volume comprises searching for the sonication energy with a minimum value; and set the next target volume as the current target volume.

16. The medical instrument of claim 2, wherein the magnetic resonance imaging thermometry protocol is a proton resonance frequency shift magnetic resonance protocol, wherein the memory further contains calibration pulse sequence commands, wherein the calibration pulse sequence commands cause the magnetic resonance imaging system to acquire phase calibration magnetic resonance data according to the magnetic resonance imaging thermometry protocol, wherein execution of the machine executable instructions further causes the processor to: acquire the phase calibration magnetic resonance data by controlling the magnetic resonance imaging system with the calibration pulse sequence commands before controling the high intensity focused ultrasound system to sonicate the current target volume, and calculate a phase calibration using the phase calibration magnetic resonance data, wherein the temperature map is calculated using the thermal magnetic resonance data and the phase calibration.

17. The medical instrument of claim 3, wherein the magnetic resonance imaging thermometry protocol is a proton resonance frequency shift magnetic resonance protocol, wherein the memory further contains calibration pulse sequence commands, wherein the calibration pulse sequence commands cause the magnetic resonance imaging system to acquire phase calibration magnetic resonance data according to the magnetic resonance imaging thermometry protocol, wherein execution of the machine executable instructions further causes the processor to: acquire the phase calibration magnetic resonance data by controlling the magnetic resonance imaging system with the calibration pulse sequence commands before controling the high intensity focused ultrasound system to sonicate the current target volume, and calculate a phase calibration using the phase calibration magnetic resonance data, wherein the temperature map is calculated using the thermal magnetic resonance data and the phase calibration.

18. The medical instrument of claim 4, wherein the magnetic resonance imaging thermometry protocol is a proton resonance frequency shift magnetic resonance protocol, wherein the memory further contains calibration pulse sequence commands, wherein the calibration pulse sequence commands cause the magnetic resonance imaging system to acquire phase calibration magnetic resonance data according to the magnetic resonance imaging thermometry protocol, wherein execution of the machine executable instructions further causes the processor to: acquire the phase calibration magnetic resonance data by controlling the magnetic resonance imaging system with the calibration pulse sequence commands before controling the high intensity focused ultrasound system to sonicate the current target volume, and calculate a phase calibration using the phase calibration magnetic resonance data, wherein the temperature map is calculated using the thermal magnetic resonance data and the phase calibration.

19. The medical instrument of claim 5, wherein the magnetic resonance imaging thermometry protocol is a proton resonance frequency shift magnetic resonance protocol, wherein the memory further contains calibration pulse sequence commands, wherein the calibration pulse sequence commands cause the magnetic resonance imaging system to acquire phase calibration magnetic resonance data according to the magnetic resonance imaging thermometry protocol, wherein execution of the machine executable instructions further causes the processor to: acquire the phase calibration magnetic resonance data by controlling the magnetic resonance imaging system with the calibration pulse sequence commands before controling the high intensity focused ultrasound system to sonicate the current target volume, and calculate a phase calibration using the phase calibration magnetic resonance data, wherein the temperature map is calculated using the thermal magnetic resonance data and the phase calibration.

20. The medical instrument of claim 4, wherein selecting the next target volume at least partially using the estimated near field temperature map for each of the multiple target volumes comprises modifying the sonication commands to shut off transducer elements selected from multiple transducer elements that contribute to the heating of the high temperature zone.

21. The medical instrument of claim 20, wherein the magnetic resonance imaging thermometry protocol is a proton resonance frequency shift magnetic resonance protocol, wherein the memory further contains calibration pulse sequence commands, wherein the calibration pulse sequence commands cause the magnetic resonance imaging system to acquire phase calibration magnetic resonance data according to the magnetic resonance imaging thermometry protocol, wherein execution of the machine executable instructions further causes the processor to: acquire the phase calibration magnetic resonance data by controlling the magnetic resonance imaging system with the calibration pulse sequence commands before controling the high intensity focused ultrasound system to sonicate the current target volume, and calculate a phase calibration using the phase calibration magnetic resonance data, wherein the temperature map is calculated using the thermal magnetic resonance data and the phase calibration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

[0060] FIG. 1 illustrates an example of a medical instrument;

[0061] FIG. 2 shows an enlarged view from FIG. 1;

[0062] FIG. 3 shows a flow chart that illustrates a method of operating the medical instrument of FIG. 1;

[0063] FIG. 4 shows a flow chart that illustrates a further method of operating the medical instrument of FIG. 1; and

[0064] FIG. 5 shows a flow chart that illustrates a further method of operating the medical instrument of FIG. 1;

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0065] 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.

[0066] FIG. 1 shows an example of a medical instrument 100. The medical instrument comprises a magnetic resonance imaging system 102 and a high-intensity focused ultrasound system 204. The magnetic resonance imaging system comprises a magnet 106. The magnet shown in FIG. 1 is 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 108 of the cylindrical magnet 106 there is an imaging zone 118 where the magnetic field is strong and uniform enough to perform magnetic resonance imaging.

[0067] Within the bore 108 of the magnet there is also a magnetic field gradient coil 110 which is used to spatially encode magnetic spins within an imaging zone of the magnet during the acquisition of magnetic resonance data. The magnetic field gradient coil 110 is connected to a magnetic field gradient coil power supply 112. The magnetic field gradient coil is intended to be representative. Typically magnetic field gradient coils contain three separate sets of coils for spatially encoding in three orthogonal spatial directions. A magnetic field gradient power supply supplies current to the magnetic field gradient coils. The current supplied to the magnetic field coils is controlled as a function of time and may be ramped or pulsed.

[0068] In the center of the bore 108 is an imaging zone 118. Adjacent to the imaging zone is a radio-frequency coil 114 which is connected to transceiver 116. Also within the bore 108 is a subject 120 reposing on a subject support 122. The radio-frequency coil 114 is adapted for manipulating the orientations of magnetic spins within the imaging zone and for receiving radio transmissions from spins also within the imaging zone. The radio-frequency coil 114 may contain multiple coil elements. The radio-frequency coil may also be referred to as a channel or an antenna. The radio-frequency coil 114 and radio frequency transceiver 116 may be replaced by separate transmit and receive coils and a separate transmitter and receiver. It is understood that the radio-frequency coil 114 and the radio frequency transceiver 116 are representative. The radio-frequency coil 114 is intended to also represent a dedicated transmit antenna and a dedicated receive antenna. Likewise the transceiver may also represent a separate transmitter and receivers.

[0069] The high-intensity focused ultrasound system 104 comprises a fluid-filled chamber 124 which houses an ultrasound transducer 126. The ultrasound transducer 126 is mechanically positioned by a mechanical positioning system 128. There is an actuator 130 for actuating the mechanical positioning system. In alternative embodiments the ultrasound transducer may be a manually positioned external transducer without the fluid-filled chamber 124 or mechanical positioning system 128.

[0070] The ultrasonic transducer 126 may also contain multiple elements for emitting ultrasound. A power supply which is not shown may control the amplitude and/or phase and/or frequency of alternating current electric power supplied to the elements of the ultrasonic transducer 126. The dashed lines 132 show the path of ultrasound from the ultrasonic transducer 126. The ultrasound 132 first passes through the fluid-filled chamber 124. The ultrasound then passes through an ultrasound window 134. After passing through the ultrasound window 134 the ultrasound passes through an optional gel pad 136 or a layer of ultrasound conductive gel which may be used to conduct ultrasound between the window 134 and the subject 120. The ultrasound 132 then enters the subject 120 and is focused into a focus 138 or sonication point. There is a region 140 which is a target zone. Through a combination of electronic and mechanical positioning of the focus 138 the entire target zone 140 can be heated. The target zone 140 is within the imaging zone 118. The high-intensity focused ultrasound system 104, the transceiver 116, and the magnetic field gradient coil power supply 112 are all connected to a hardware interface 146 of computer system 142. The hardware interface 146 is connected to processor 144. The processor 144 is also connected to a user interface 148, computer storage 150, and computer memory 152.

[0071] The computer storage 150 is shown as containing sonication commands 160. The sonication commands could for example been previously stored on the computer storage device 150, could have been entered through the user interface 148, or may have even been received via a network interface of some kind. The computer storage 150 is further shown as containing a selection of a current target volume 162. The selection of the current target volume 162 is where the high-intensity focused ultrasound system 104 focuses the focal point 138. The computer storage 150 is further shown as containing thermometry pulse sequence commands 164. The thermometry pulse sequence commands 164 enable the medical instrument 100 to perform magnetic resonance thermometry. The computer storage 150 is further shown as containing thermal magnetic resonance imaging data 166 that was acquired using the thermometry pulse sequence commands 164.

[0072] The computer storage 150 is further shown as containing a temperature map 168 that was reconstructed from the thermal magnetic resonance imaging data 166. The computer storage 150 is further shown as containing a set of target volumes 170 which are used to define where the focal point 138 is placed at different locations within the target zone 140. The computer storage 150 is further shown as containing a map with calculated sonication energies 172. These are the energy which is necessary to complete the sonication of each point in the set of target volumes 170. The calculated sonication energies 172 are used at least partially to select the next target volume that is sonicated.

[0073] The computer memory 152 is shown as containing a control module 180. The control module 180 contains computer-executable code which enables the processor 144 to control the operation and function of the various components of the medical instrument 100. The computer storage 152 is further shown as containing an image reconstruction module 182. The image reconstruction module 182 enables the processor 144 to reconstruct magnetic resonance images using magnetic resonance data that is acquired. The image reconstruction module 182 may also contain commands and routines which enable the processor 144 to perform various image processing operations.

[0074] The computer storage 152 is further shown as containing a magnetic resonance thermometry module 184. The magnetic resonance thermometry module 184 enables the processor 144 to analyze and process the thermal magnetic resonance imaging data 166 into the temperature map 168. Depending upon the exact magnetic resonance thermometry method used the magnetic resonance thermometry module 184 may also be adapted for performing various types of calibration for magnetic resonance thermometry also. The computer memory 152 is further shown as containing an ultrasound model module 186 which enables the processor 144 to model the ultrasound transducer 126 and any individual transducer elements. This for instance may be useful for performing ray tracing for ultrasound images by various transducer elements. This may be useful in determining which and possibly turning off various transducer elements such that they do not contribute to overheating of the subject's near field zone.

[0075] FIG. 2 shows an enlarged view of the imaging zone 118. In particular the target zone 140 can be seen in greater detail. It can be seen that the focal point 138 is focused on a current target volume 200. The target zone is divided into a number of discreet set of target volumes 202. After the location 200 has been sonicated the temperature map is then used to determine which of the remaining target volumes 202 should be sonicated next. This for instance may require electronic and/or mechanical steering of the transducer 126. Between the target volume 202 and the transducer 126 is the near field region 204 of the subject 120. In some examples individual elements of the transducer 126 may be turned off to prevent overheating of the near field region 204.

[0076] FIG. 3 shows a flowchart which illustrates an example of a method of operating the medical instrument 100 illustrated in FIGS. 1 and 2. First in step 300 sonication commands 160 are received. The sonication commands specify a set of target volumes 202 within the target zone 140. Next in step 302 the selection of the current target volume 200 is received. This is the location where the first sonication will be performed. Next in step 304 thermal magnetic resonance data 166 is acquired by controlling the magnetic resonance imaging system 102 with the thermometry pulse sequence commands 164. Next in step 306 a temperature map 168 is calculated using the thermal magnetic resonance imaging data 166. Next in step 308 the high-intensity focused ultrasound system is controlled to sonicate the current target volume 200 by steering the sonication location 138 to the current target volume 200.

[0077] Next in step 310 the current target volume 200 is removed from the set of multiple target volumes 270 after controlling the high-intensity focused ultrasound system 104 to sonicate the current target volume 200. In step 312 a sonication energy 172 is calculated for each of the multiple target volumes 202 using the temperature map 168. Next in step 314 a next target volume is selected from the multiple target volumes using the calculation of the sonication energy 172 for each of the multiple target volumes 202. The selection of the next target volume comprises searching for a sonication energy with a minimum value.

[0078] Next in step 316 the next target volume is set as the current target volume. 318 is a decision box and the question is, have all target volumes been sonicated. If the answer is yes then the method proceeds to step 320 and the method of FIG. 3 ends. If no, then the method returns back to step 304. It should be noted that the steps shown in FIG. 3, various steps may be re-arranged and placed in a different order. Also during the execution of the method shown in FIG. 3, several of the steps may be performed at the same time. For example the acquisition of the thermal magnetic resonance data may be done continuously and as the data is acquired the various thermal maps and corrections are done as they become available. The sonication of the current target volume may also be done during the execution of some steps. For example the acquisition of the thermal magnetic resonance data and the sonication of the current target volume may be performed at the same time.

[0079] FIG. 4 shows a flowchart which illustrates a further method of operating the medical instrument shown in FIGS. 1 and 2. The method shown in FIG. 4 is similar to that in FIG. 3. However, in the method shown in FIG. 4 the method proceeds from step 312 to step 400. In step 400 an estimated near field temperature map is calculated for each of the multiple target volumes 202 using the temperature map 168 and an ultrasonic transducer module model 186. The target volume is then at least partially selected using the near field temperature map for each of the multiple target volumes. After step 400 the method proceeds to step 402. In step 402 the estimated near field temperature maps are searched for a high temperature zone which has a temperature above a predetermined threshold. In other words the near field temperature maps are searched for regions which have a temperature above an allowed or predetermined value. The model is then used to predict regions where there will be overheating of the subject.

[0080] The method then proceeds to step 404, where the sonication commands are modified to shut off transducer elements of the ultrasonic transducer 126 that contribute to the hearing of the high temperature zone. In this step high temperature regions that have been identified are then used to modify the sonication commands and individual transducer elements are shut off in an effort to reduce the heating of these high temperature zones. This may be beneficial because it enables the sonication of the various target volumes to proceed with little or no delay. After step 404 the method returns normally to step 314 and the method of FIG. 4 is then identical with the method of FIG. 3.

[0081] FIG. 5 shows a further example of a flowchart of a method of operating the medical instrument of FIGS. 1 and 2. The method of FIG. 5 is similar to that of FIG. 3 with several additions. After step 302 is performed the method proceeds to step 500. In step 500 a list of possible actuator positions of the ultrasound transducer 126 is performed. The sonication commands specify an actuator position for each of the set of multiple target volumes. This is then used to make the list of possible actuator positions. Next in step 502 phase calibration magnetic resonance data is acquired by controlling the magnetic resonance imaging system with calibration pulse sequence commands. The phase calibration magnetic resonance data is acquired for each of the various positions in the list of possible actuator positions.

[0082] Finally in step 404, a phase calibration for each of the possible actuator positions is calculated using the phase calibration magnetic resonance data at each of those locations. This enables the temperature map to be calculated using a different calibration for each actuator position that is used when positioning the ultrasonic transducer 126. This may result in more accurate temperature measurements during use of the medical instrument 100.

[0083] The steps of FIGS. 4 and 5 may be combined.

[0084] MR-guided High Intensity Focus Ultrasound (HIFU) therapies that use proton resonance frequency method for monitoring and controlling the heating typically consist of several separate sonications. Each sonication begins with a gathering of reference phase image, after which applying of the ultrasound energy is started. Phase images are gathered during the sonication and used to reconstruct the temperature maps, which are then used to control the heating. After the heating is finished a cooling period is required before the next sonication can be started. This is due to the fact that in focused ultrasound sonications also the near field, i.e. intervening tissues such as skin and fat layer, are inevitably heated. In order to prevent the overheating of near field tissues cooling periods need to be added between sonication. This will make the total treatment time longer and reduces the therapy efficiency. The length of the cooling period depends on the applied energy in the sonication, which is defined by sonication size, duration, and power. Making cooling periods shorter would improve the total therapy efficiency.

[0085] Examples may help in reducing the cooling time and thus the chance of overheating the non-targeted tissues and/or increasing the therapy efficiency.

[0086] Certain examples may have one or more of the following features:

1. MR temperature mapping that can compensate for the susceptibility artefacts caused the change in the transducer position.
2. Sonication algorithm that can exploit the heat that diffuses from the neighboring sonication targets.
3. Element switch-off algorithm to control the near field heating.

[0087] Various example may enable performing sonication cells from multiple sonication positions without a cooling period and new reference image gathering phase between sonications. In the beginning the user may set for example three sonication cells that are close to each other but which require transducer movement in between. Reference images are gathered separately for each cell by moving the transducer to each location one at a time prior starting of the sonication. When the reference images have been gathered the sonication starts from one cell and after it has been treated, the transducer is moved to another position. After movement the temperature mapping needs to change the used reference image that corresponds to the new transducer position.

[0088] Since the cells are positioned closely to each other, the heating of the second cell may now exploit the heat that diffuses from the first cell. This reduces the amount of energy needed to heat the second cell and thus increases the heating efficiency respect the near field heating. After the second cell has been treated, the sonication is continued from the next cell. The maximum amount of subsequently treatable cells will be eventually limited by for example tissue perfusion and size of cells. Since the acoustic fields for each position may overlap in the near field causing hot spots, the near field heating may be controlled by using element switch-off. The cells can be positioned for example on a same plane or along the same beam axis. In the latter choice the temperature mapping slices should be moved accordingly to the transducer movement.

[0089] 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.

[0090] 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

[0091] 100 medical instrument [0092] 102 magnetic resonance imaging system [0093] 104 high-intensity focused ultrasound system [0094] 106 magnet [0095] 108 bore of magnet [0096] 110 magnetic field gradient coil [0097] 112 magnetic field gradient coil power supply [0098] 114 radio frequency coil [0099] 116 transceiver [0100] 118 imaging zone [0101] 120 subject [0102] 122 subject support [0103] 124 fluid filled chamber [0104] 126 ultrasonic transducer [0105] 128 mechanical positioning system [0106] 130 actuator [0107] 132 path of ultrasound [0108] 134 ultrasound window [0109] 136 gel pad [0110] 138 focus [0111] 140 target zone [0112] 142 computer system [0113] 144 processor [0114] 146 hardware interface [0115] 148 user interface [0116] 150 computer storage [0117] 152 computer memory [0118] 160 sonication commands [0119] 162 selection of current target volume [0120] 164 thermometry pulse sequence commands [0121] 166 thermal magnetic resonance data [0122] 168 temperature map [0123] 170 set of target volumes [0124] 172 calculated sonication energies [0125] 180 control module [0126] 182 image reconstruction module [0127] 184 MR thermometry module [0128] 186 ultrasound model module [0129] 200 current target volume [0130] 202 target volumes [0131] 204 nearfield region [0132] 300 receive sonication commands, wherein the sonication commands specify a set of multiple target volumes within the target zone [0133] 302 receive a selection of a current target volume selected from the set of multiple target volumes [0134] 304 acquire the thermal magnetic resonance data by controlling the magnetic resonance imaging system with the thermometry pulse sequence commands [0135] 306 calculate a temperature map using the thermal magnetic resonance data [0136] 308 control the high intensity focused ultrasound system to sonicate the current target volume by electronically steering the sonication location to the current target volume [0137] 310 remove the current target volume from the set of multiple target volumes after controlling the high intensity focused ultrasound system to sonicate the current target volume [0138] 312 calculate a sonication energy for each of the multiple target volumes by using the temperature map [0139] 314 select a next target volume from the multiple target volumes using the calculation of the sonication energy for each of the multiple target volumes [0140] 316 set the next target volume as the current target volume [0141] 318 have all of the multiple target volumes been sonicated? [0142] 320 end [0143] 400 calculate an estimated near field temperature map for each of the multiple target volumes using the temperature map and an ultrasonic transducer model [0144] 402 search the estimated near field temperature map for a high temperature zone which has a temperature above a predetermined threshold [0145] 404 modify the sonication commands to shut off transducer elements selected from multiple transducer elements that contribute to the heating of the high temperature zone [0146] 500 create a list of possible actuator positions from the actuator position for each of the set of multiple target volume [0147] 502 acquire the phase calibration magnetic resonance data for each of the possible actuator positions by controlling the magnetic resonance imaging system with the calibration pulse sequence commands [0148] 504 calculate a phase calibration according with the phase calibration magnetic resonance data for each of the possible actuator positions