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 . fig1 shows a diagram which illustrates a radiation force balance calibrator 100 according to an embodiment of the invention . the radiation force balance calibrator 100 is mounted on a subject support 102 . below the subject support 102 is an ultrasound transducer system 104 . the ultrasound transducer system 104 has a high intensity focused ultrasound transducer 106 within a fluid filled chamber 108 . the fluid filled chamber 108 is located below the subject support 102 . there is an ultrasound window 110 which allows ultrasound to leave the fluid filled chamber 108 . the dashed lines marked 112 illustrate the path of ultrasound from the high intensity focused ultrasound transducer 106 through the ultrasound window 110 and into the radiation force balance calibrator 100 . the path of the ultrasound radiation 112 is focused into a sonication zone 114 . the sonication zone 114 is shown as being within an oil target phantom 116 . the oil target phantom 116 is suspended in a bath of water 118 . the water 118 is held by a basin 120 . the basin 120 is fit such that it fits into the patient support 102 to cover the ultrasound window 110 . above the basin 120 there is a balance 124 . the oil target phantom 116 is connected to the balance 124 by small wires 126 . the balance 124 has a first end 132 and a second end 134 . the first end 132 has a pivot 128 which connects the balance 124 to a balance frame 130 . the pivot 128 allows the balance 124 to pivot relative to the balance frame 130 . the second end 134 of the balance 124 exerts force on a force sensor 138 . the force sensor 138 is shown as having a force sensitive or measuring region 140 on which the second end 134 exerts force . the arrow 142 illustrates the direction of pivoting of the balance 124 . the arrow labeled 144 indicates the direction of force applied by the second end 134 of the balance 124 on the force sensitive region 140 . fig2 shows a flow diagram which illustrates a method according to an embodiment of the invention . in step 200 a first dataset is measured with the force sensor . at least a portion of the first dataset is acquired with a calibration weight in the calibration weight holder . in step 202 a second dataset is measured with the force sensor . a calibration weight is placed in the calibration weight holder when the second dataset is measured . step 204 is measuring a third dataset with the force sensor . at least a portion of the third dataset is acquired with a calibration weight in the calibration weight holder . finally in step 206 a calibration for the high intensity focused ultrasound transducer is calculated . fig3 shows a flow diagram which illustrates a further method according to an embodiment of the invention . in step 300 a first dataset is measured with the force sensor . step 300 corresponds to step 200 of fig2 . step 302 is to measure a second dataset with the force sensor . step 302 corresponds to step 202 of fig2 . in step 304 a third dataset is measured with the force sensor . step 304 corresponds to step 204 of fig2 . steps 306 , 308 , 310 , 312 and 314 of fig3 correspond to step 206 of fig2 . in step 306 ultrasonic pulse timing data is received . the ultrasonic pulse timing data is descriptive of the power of the ultrasonic radiation applied by the high intensity focused ultrasound transducer as a function of time . this is used in the calibration calculation . in step 308 a model of an expected second dataset is calculated using the ultrasonic pulse timing data . in other words using the ultrasonic pulse timing data a simulated second dataset which is referred to here as an expected second dataset is calculated . next in step 310 a residual is calculated between the expected second dataset and the second dataset which was measured . in step 312 convergence to a solution or a minimum threshold for the residual is checked . if the solution does not converge then the second dataset or the simulated or expected second dataset is recalculated and the method goes back to step 308 . if the solution did converge then the calibration is finished in step 314 . fig4 shows a block diagram which illustrates an embodiment of an ultrasound apparatus 400 according to the invention . the ultrasound apparatus 400 comprises a magnetic resonance imaging system , a radiation force balance calibrator 100 and an ultrasound transducer system 104 . the magnetic resonance imaging system comprises a magnet 402 . the magnet 402 has an imaging zone 404 where the magnetic field is uniform enough for performing magnetic resonance imaging . the subject support 102 , the ultrasound transducer system 104 and the radiation force balance calibrator 100 are all shown as being within the bore of the magnet 402 . in many embodiments the subject support 102 is able to be removed from the magnet 402 . in some embodiments the radiation force balance calibrator 100 is installed into the subject support 102 after the subject support 102 has been removed or partially removed from the magnet 402 . likewise in this figure not all details of the radiation force balance calibrator 100 that are shown in fig1 are shown in this figure . above the imaging zone 404 is a radio frequency coil 406 . the radio frequency coil is for acquiring magnetic resonance data from the imaging zone 404 . the radio frequency coil 406 is connected to a radio frequency transceiver 408 . also within the bore of the magnet 410 is a magnetic field gradient coil . the magnetic field gradient coil 410 is connected to a magnet field gradient coil power supply 412 . the magnetic field gradient coil power supply 412 supplies electrical current to the magnetic field gradient coil 410 . also shown is electronics 416 for the radiation force balance calibrator 100 . not shown in the diagram is a power supply and control electronics and possibly an actuator for the ultrasound transducer system 104 . the ultrasound transducer system 104 , the radio frequency transceiver 408 , the electronics 416 and the magnetic field gradient coil power supply 412 are all shown as being connected to a hardware interface 418 of a computer system 420 . the computer system 420 further comprises a processor 422 which is connected to both the hardware interface 418 and a user interface 424 . the hardware interface 418 comprises electronics which enable the processor 422 to send and receive messages to various pieces of hardware and for controlling them . the human interface hardware 424 comprises electronic and mechanical components which allow an operator to interact with the computer system 420 . the human interface hardware 424 may comprise but is not limited to a keyboard , a mouse , and a display . the display of 424 may be used for operating graphical user interfaces for controlling the operation of the ultrasound apparatus 400 and also for displaying data or for rendering images such as magnetic resonance images . the processor 422 is also connected to computer storage 426 and computer memory 428 . the computer storage 426 is shown as containing magnetic resonance data 430 which has been acquired by the magnetic resonance imaging system . the computer storage is further shown as containing a magnetic resonance image 432 which has been reconstructed from the magnetic resonance data 430 . also within the computer storage 426 is stored a treatment plan 434 which contains instructions and / or control sequences for controlling the operation of the ultrasound transducer system 104 during the treatment of a subject . also within the computer storage 426 is stored a first dataset 436 . also stored within the computer storage 426 is a second dataset 438 . also stored within the computer storage is a third dataset 440 . the first dataset 436 , second dataset 438 , and third dataset 440 were acquired during the course of calibrating the high intensity focused ultrasound transducer 106 . also stored within the computer storage is an expected second dataset 442 . also stored within the computer storage is a residual 444 between the second dataset 438 and the expected second dataset 442 . also stored within the computer memory is a calibration 445 for the high intensity focused ultrasound transducer 106 . the calibration 445 contains a mapping of electrical power delivered to the high intensity focused ultrasound transducer 106 to a power or intensity of ultrasonic radiation generated by the high intensity focused ultrasound transducer 106 . within the computer memory 428 there is a magnetic resonance control module 446 . the magnetic resonance control module contains computer executable code for controlling the operation and functioning of the magnetic resonance imaging system . also within the computer memory 428 is an ultrasound system control module which contains computer executable code for controlling the operation and functioning of the ultrasound transducer system 104 . the ultrasound system control module 448 uses the treatment plan 434 to generate the control sequences for the ultrasound system . also stored within the memory is a finite difference module 450 . the finite difference module contains computer executable code for performing finite difference calculations . also stored within the memory is a calibration calculation module 452 . the calibration calculation module 452 uses the finite difference module 450 and is able to calculate the calibration 445 using the first dataset 436 , the second dataset 438 , and the third dataset 440 . the calibration calculation module 452 uses the finite difference module 450 for calculating the expected second dataset 442 and to calculate the residual 444 . the calibration calculation module 452 may also contain computer executable code for automating the calibration procedure . the computer memory 428 is also shown as containing a magnetic resonance imaging reconstruction module 454 . the magnetic resonance imaging reconstruction module 454 contains computer executable code for reconstructing the magnetic resonance image 432 from the magnetic resonance data 430 . fig5 a , 5 b , 5 c , and 5 d show different views of an embodiment of a radiation force balance calibrator according to the invention . fig5 a shows an isometric view , fig5 b shows a side view , fig5 c shows a top view , and fig5 d shows an exploded isometric view . shown in these figures is arrestor block 500 . the arrestor block 500 can be used to prevent the balance 124 from contacting the force sensitive region 140 when the radiation force balance calibrator 100 is not in use . fig6 a , 6 b , and 6 c show drawings which illustrate the same radiation force balance calibrator 100 mounted onto a basin 120 for connecting to a subject support . fig6 a shows an isometric view , fig6 b shows a cut - away or cross - sectional side view , and fig6 c shows an exploded part view in isometric . connected to the basin is a drain plug 600 . the basin 120 is shown as fitting into an adaptor 602 which allows installation of the radiation force balance calibrator 100 into a subject support over an ultrasonic window . between the oil target phantom and the balance frame 130 there is a plate 608 . the plate is adjusted such that the plate is exactly at the height of the water filling the basin 120 . the plate stabilizes the water and reduces vibrations during measurement . the plate 608 is suspended or held on the basin 120 by clamps 610 . one embodiment of a method for performing a calibration using the apparatus of fig5 and 6 is : a . remove all the calibration weights . b . remove all personnel from the vicinity of the bed and let the system settle for a minute . record about 20 seconds of data and pause recording . c . insert the 10 g weight in the bottom slot of the weight cage . d . repeat 1 . b e . insert the 5 g weight in the middle slot of the weight cage . f . repeat 1 . b g . insert the 2 g weight in the top slot of the weight cage . h . repeat 1 . b 2 . perform a sonication with the ultrasound transducer and record the second data set . a . start recording data from the force sensor b . start the sonication protocol . the protocol will run to completion or it can stop with an error , typically when reaching the highest power levels . if this happens , make a note of the error message and include a description of the comments . c . pause the recording of data from the force sensor . a . let the system cool down from the sonications for about 5 minutes , this will mitigate the effects of the buoyancy drift . ( some drift is inevitable and acceptable , this data serves as only a rough check of integrity after sonication ). b . record about 20 seconds of data and pause recording . c . remove the 2 g weight from the top slot of the weight cage . d . remove all personnel from the vicinity of the bed and let the system settle for a minute . record about 20 seconds of data and pause recording . e . remove the 5 g weight from the middle slot of the weight cage . f . repeat 3 . d g . remove the 10 g weight from the bottom slot of the weight cage . h . repeat 3 . d fig7 illustrates a method of performing a calibration with a radiation force balance calibrator according to the invention . the method simulates the radiation force balance ( rfb ) target behavior over the measurement trace to withstand data which is noisy or distorted by large buoyancy effects . in step 700 , every iteration first produces an array of electrical power values according to p = a *( ampval + e ) 2 + b where a , b and e are fit parameters . then this assumed electrical power ( as function of time ) is fed to a difference equation module 702 , which takes in fit parameters for heat gain and loss as well as parameters to account for non - equilibrium buoyancy . the module outputs an array of assumed balance indicated weight readings , which goes to levenberg - marquardt nonlinear least squares minimization algorithm . in step 704 , a residual is calculated between the simulated and the measured data . if convergence to a solution is not achieved , the algorithm varies the fit parameters and starts the next iteration round . this is contrasted with the traditional way of taking generator reported power as the fit target data and fitting a function in the form p = a *( ampval + e ) 2 + b to it . this formula is used to predict electrical power , and acoustic power is then obtained by multiplying with a constant w el to w ac . the main difference of these methods is the fit target : the traditional way fits to reported generator power , the new method fits to a balance acoustic power trace . 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 . for example , it is possible to operate the invention in an embodiment wherein the calibration weights are automatically added to or removed from the calibration weight holder . for instance a robotic system may be used to accomplish this . 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 .
a radiation force balance calibrator for calibrating a high intensity focused ultrasound transducer , the radiation force balance calibrator comprising : a balance frame ; a force sensor for measuring force , wherein the force sensor is attached to the balance frame ; a balance with first and second ends , wherein the first end is attached to a pivot for enabling the balance to pivot relative to the balance frame , wherein the second end is adapted for exerting force onto the force sensor ; an oil target phantom connected to the balance for absorbing ultrasound radiation from the high intensity focused ultrasound transducer , wherein the absorption of ultrasonic radiation causes a reduction in the force exerted by the balance on the force sensor ; and wherein the radiation force balance calibrator is constructed of non - magnetic materials .