Abstract:
An apparatus ( 300, 400, 500 ) comprising a magnetic resonance imaging system ( 302 ), the magnetic resonance imaging system comprising: a magnet ( 306 ) adapted for generating a magnetic field for orientating the magnetic spins of nuclei of a subject ( 310 ) located within an imaging volume ( 308 ); a radio frequency transceiver ( 320 ) adapted for acquiring magnetic resonance data ( 346 ) using a radio frequency coil ( 318 ); a computer system ( 336 ) comprising a processor ( 338 ), wherein the computer system is adapted for controlling the apparatus; and a memory ( 342, 344 ) containing machine readable instructions ( 354, 356, 358, 360, 362 ), wherein execution of the instructions cause the processor to perform the steps of: acquiring ( 100, 204 ) magnetic resonance data using the magnetic resonance imaging system, wherein the magnetic resonance data comprises transverse relaxometry data, and calculating ( 102, 206 ) the temperature of the subject within a temperature measurement volume ( 332 ) in accordance with the transverse relaxometry data.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to magnetic resonance imaging, in particular to the calculation of temperature using transverse relaxometry data. 
       BACKGROUND OF THE INVENTION 
       [0002]    The localized heating of tissues may be useful for therapeutic treatments. For example increased tissue temperature may be used to induce necrosis or induce other physiological changes in the target tissue. There exists a variety of means for localized heating of tissue. High Intensity Focused Ultrasound (HIFU), the ablation of tissue with lasers, and the heating of tissue with electromagnetic fields are some examples of how tissue may be heated locally. 
         [0003]    When heating tissue locally, it is beneficial to either measure the temperature of the region being heated and/or tissue surrounding the tissue being heated. For instance, during a HIFU procedure it may be beneficial to monitor the temperature of surrounding tissue to ensure that damage to sensitive organs or tissue does not result. 
         [0004]    Magnetic resonance imaging has been combined with HIFU to monitor the heating effect caused by HIFU. The combination of magnetic resonance imaging and HIFU is also beneficial because the magnetic resonance imaging may be used to provide anatomical data as well as to measure the temperature within a subject. 
         [0005]    U.S. Pat. No. 7,542,793 discloses a MRI system used to guide an ultrasonic transducer at tissues to be treated and to produce images which enable the treatment of tissues to be monitored. Changes in amplitude of a spin-echo NMR signal from a reference spin-echo NRM signal are used to produce image which indicate temperature changes in both fat and water. The method disclosed in this patent is a proton resonance frequency shift (PRF) method of determining the temperature. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides for an apparatus, a computer-implemented method, and a computer program product in the independent claims. Embodiments are given in the dependent claims. 
         [0007]    A difficulty with the current state of the art is that the PRF method of measuring temperature with magnetic resonance imaging is that the PRF shift in fat is difficult to measure. Fast and effective T1 weighted FFE sequences may be used for the purpose, but their accuracy often suffers from motion artifacts and B1 distortions (especially at 3 Tesla). The results obtained do not produce accurately measure the temperature of fat. However, temperature measurement of fat may be very useful, for example to avoid excessive near field US heating of subcutaneous fat, or monitor the fat close to breast lesions. Today, without practical solutions for fat temperature monitoring, e.g. heating of subcutaneous fat cannot be monitored, which is a safety risk. In the future, there will also be applications where there is often fatty tissue very close to the treated lesion (e.g. breast, bones, etc). 
         [0008]    Embodiments of the invention address this problem by using transverse relaxometry. Transverse relaxometry may be interpreted as the measurement of either the T2 or T2-star relaxation rate. Transverse relaxometry data may also be interpreted as the intensity of a T2 or a T2-star weighted image. T2 is the so called spin-spin relaxation rate and is time that atomic spins need to de-phase in the transverse plane. T2-star is a combination of the spin-spin relaxation time plus de-phasing in the transverse plane due to local magnetic field inhomogeneity. 
         [0009]    It is not generally understood that the T2 and T2-star of fat have quite significant temperature dependence. This dependence offers possibilities to utilize fast T2 or T2-star dependent sequences for qualitative or quantitative temperature measurement. 
         [0010]    For HIFU, it is beneficial if the temporal resolution of the temperature measurement is on the order of a few seconds. For commercial systems presently used in clinical settings the temporal resolution is typically 2 to 3 seconds. Also, the selected method shall be robust enough, e.g. against motion and flow. There are several alternatives for fast and relatively robust T2 or T2-star weighted sequences, like single-shot TSE or single-shot EPI. 
         [0011]    Experimental measurements show that both T2 and T2-star of in in-vitro fat samples are temperature dependent. This is also true for some oils that have been used for Magnetic Resonance (MR) or Ultra Sound (US) phantoms. The experimental data also shows that the apparent T2 temperature chance varies when measured with different sequences, indicating that the T2 relaxation mechanisms may depend in different ways on temperature. Especially J-coupling behaves differently compared to other mechanisms. Also, for T2-star weighted sequences there is interference pattern as a function of the Time to Echo (TE) due to the fat NMR spectrum. This pattern might also be temperature dependent. 
         [0012]    The T2 relaxation time change of fat seems to be roughly linear within the studied temperature region. It is probable that close to phase transition temperature of fat the T2 dependence becomes very non-linear, and the T2 change is irreversible. 
         [0013]    Embodiments of the invention may produce qualitative or quantitative temperature information of fat based on optimized T2 or T2-star weighted sequence. This information can then be combined in applications like HIFU or other MR guided ablation methods. In practice it might be useful to combine PRF and T2 weighted methods to get temperature information from both water and fat-containing tissues. 
         [0014]    Typically T2 or T2* weighted sequences depend on other physical, physiological or technical parameters which may not be stable during the whole treatment procedure. Therefore, it may be essential to compare the signal intensity of the sequence with a reference image obtained just before each heating event. Then the temperature information is the temperature change compared the situation before heating. While this information is very useful, it could beneficial to get measurement of the absolute temperature also. Example of such case is the accumulation of the heat in subcutaneous fat after several heating events in HIFU treatment. In those cases measuring the change in T2 or T2-star compared to the situation just before the treatment would provide temperature in more absolute scale. 
         [0015]    Embodiments of the invention may involve performing the following steps:
       In MR guided HIFU ablation, PRF sequence is applied during the US treatment.   In predefined time intervals, a fat-selected single-shot T2 weighted TSE sequence is obtained in one or more slice positions.   The signal intensity obtained is compared to the images done before the treatment   Pre-calibrated look-up table is used to convert the signal intensity change in temperature scale   The temperature estimate is shown on the images, for example as a color overlay to indicate to the user of possible safety hazard       
 
         [0021]    There are numerous variations of the invention. Some of basic elements of the methods are listed below: 
         [0022]    Utilization of T2 temperature dependence of fat for qualitative or quantitative temperature mapping in HIFU or other ablation experiment with T2 weighted sequences like multi-shot or single-shot TSE, multi-shot or single-shot GRASE, spine echo, spin echo EPI. 
         [0023]    Utilization of T2-star temperature dependence of fat for qualitative or quantitative temperature mapping with T2-star weighted sequences like FFE, or field echo EPI. 
         [0024]    Combine T2-star dependence and temperature dependent shift of fat and water peaks for qualitative or quantitative temperature mapping by selecting TE in FFE based sequence in such a way these effects are synergistic. 
         [0025]    Use relative long repetition time compared to T1 of the tissue under interest to get optimized T2 weighting in the TSE, GRASE, EPI or FFE sequence. Alternatively or additionally use low excitation flip angle for gradient echo EPI or FFE. 
         [0026]    Use single-shot sequence or a sequence with low T1 saturation to avoid the effects due T1 temperature dependence. 
         [0027]    Utilize T2 weighted IR sequence in such a way that T1 and T2 or T2-star temperature effects are synergistic. Typically this means that short Inversion Time (T1) and long TE are used. 
         [0028]    Combine PRF and T2 dependent sequence sequentially to almost simultaneously obtain data for water and fat containing tissues. 
         [0029]    Combine PRF and T2 or T2-star dependent sequences sequentially by using fat selective excitation for the T2 or T2-star weighted sequence and water selective sequence for the PRF measurement. With this arrangement the interference between the sequences due to saturation effects are minimized. 
         [0030]    Use predefined function or look-up table to convert the intensity data of T2 or T2-star dependent sequence to estimated temperature data. This look-up table or function can take to account other temperature dependent factors (like magnetization or T1). 
         [0031]    Obtain images with several TE values to get quantitative T2 estimation. This would allow more absolute temperature scale. 
         [0032]    Embodiment of the invention may be used in MR applications where the temperature monitoring of tissue is essential. Examples of such are MR guided HIFU, MR guided RF ablation, or MR guided laser ablation. 
         [0033]    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. 
         [0034]    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. 
         [0035]    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. 
         [0036]    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. 
         [0037]    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. 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. 
         [0038]    Magnetic resonance thermometry 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 which contains information which may be used for magnetic resonance thermometry. Magnetic resonance thermometry is enabled by measuring changes in temperature sensitive parameters. Examples of parameters that may be measured during magnetic resonance thermometry are: the proton resonance frequency shift, the diffusion coefficient, or changes in the T1 and/or T2 relaxation time may be used to measure the temperature using magnetic resonance. The proton resonance frequency shift is temperature dependent, because the magnetic field that individual protons, hydrogen atoms, experience depends upon the surrounding molecular structure. An increase in temperature decreases molecular screening due to the temperature affecting the hydrogen bonds. This leads to a temperature dependence of the proton resonant frequency. 
         [0039]    In one aspect the invention provides for an apparatus comprising a magnetic resonance imaging system. The magnetic resonance imaging system comprises a magnet adapted for generating a magnetic field for orienting the magnetic spins of nuclei of a subject located within an imaging volume. An imaging volume as used herein encompasses a region of the magnetic field of the magnet which is uniform enough for performing the acquisition of magnetic resonance imaging data. The magnetic resonance imaging system further comprises a radio frequency transceiver adapted for acquiring magnetic resonance data using a radio frequency coil. It is understood herein that the radio frequency transceiver may also be a separate radio frequency transmitter and a separate radio frequency receiver. Likewise a radio frequency coil also encompasses separate transmit and receive coils or antennas. The magnetic resonance imaging system further comprises a magnetic field gradient coil adapted for spatial encoding of the magnetic spins of nuclei within the imaging volume. The term magnetic field gradient coil encompasses all or a collection of magnetic field gradient coils. Typically magnetic resonance imaging systems have three separate gradient coil systems. 
         [0040]    The magnetic resonance imaging system further comprises a magnetic field gradient coil power supply for supplying current to the magnetic field gradient coil. When the magnetic field gradient coil power supply supplies current to the magnetic field gradient coil the magnetic field gradient coil generates a magnetic field. The magnetic resonance imaging system further comprises a computer system comprising a processor. The computer system is adapted for controlling the apparatus. That is to say the computer system functions as a control system for the apparatus. It is understood that a computer system and a processor may actually be comprised of multiple computer systems and/or multiple processors. The magnetic resonance imaging system further comprises a memory containing machine readable instructions for execution by the processor. Execution of the instructions cause the processor to perform the step of acquiring magnetic resonance data using the magnetic resonance imaging system. The magnetic resonance data comprises transverse relaxometry data. Transverse relaxometry data as used herein encompasses data which can be used to infer or calculate the T2 or T2-star relaxation time or the signal intensity of T2 or T2-star weighted images. 
         [0041]    Execution of the instructions further cause the processor to perform the step of calculating the temperature of the subject within a temperature measurement volume in accordance with the transverse relaxometry data. The temperature measurement volume is within the imaging volume. Embodiments of this apparatus may be particularly beneficial because transverse relaxometry data is used to calculate the temperature within the temperature measurement volume. This is particularly beneficial when fatty tissue is within the temperature measurement volume. This is because the T2 or T2-star relaxation time may be used to accurately measure the temperature in fatty tissue. 
         [0042]    In another embodiment the temperature within the temperature measurement volume is calculated by any one of the following: by determining the change of signal intensity in a T2 weighted magnetic resonance image, by calculating the T2 relaxation time, and by calculating the T2-star relaxation time. This embodiment is advantageous because any of these methods may be used to use the transverse relaxometry data to determine the temperature. 
         [0043]    In another embodiment the temperature measurement volume contains fat tissue. 
         [0044]    In another embodiment the therapeutic apparatus further comprises a tissue heating system for heating a target volume of the subject. The target volume is within the imaging volume. The computer system is further adapted for controlling the tissue heating system. Depending upon the embodiment the target volume may be identical with the temperature measurement volume or they may be different. For instance a tissue heating system may be used to heat a target volume and the temperature of the target volume may be measured directly with the magnetic resonance imaging system. In other cases the target volume may be heated with a tissue heating system and a volume adjacent to the target volume may be monitored to see or ensure that the temperature measurement volume does not exceed a maximum threshold temperature. For instance a laser system may be used to ablate tissue and the magnetic resonance imaging system may be used to monitor an adjacent volume of tissue. This is particularly beneficial because this can be used to ensure that healthy tissue is not destroyed or damaged during a therapy session. 
         [0045]    In another embodiment of the tissue heating system a high intensity focused ultrasound system for sonicating the target volume. 
         [0046]    In another embodiment the tissue heating system is a radio frequency tissue heating system for heating the target volume with radio frequency energy. 
         [0047]    In another embodiment the tissue heating system is a laser ablation system for ablating tissue within the target volume. 
         [0048]    In another embodiment the instructions further cause the processor to perform the step of receiving a treatment plan specifying a target volume within the imaging volume. The treatment plan comprises instructions for controlling the operation of the tissue heating system. A ‘treatment plan’ as used herein encompasses a set of instructions which may either be interpreted into control signals or are control signals for the tissue heating system. 
         [0049]    The instructions further cause the processor to perform the step of heating the target volume using the tissue heating system by executing the treatment plan. Execution of the treatment plan causes the processor to send control signals to the tissue heating system which causes the tissue heating system to heat the target volume. The instructions further cause the processor to perform the step of repeatedly acquiring magnetic resonance data using the magnetic resonance imaging system during execution of the treatment plan. The instructions further cause the processor to perform the step of repeatedly calculating the temperature of the subject within the temperature measurement volume. 
         [0050]    The instructions further cause the processor to perform the step of modifying the treatment plan during execution of the treatment in accordance with the temperature within the temperature measurement volume. This embodiment is particularly advantageous because the temperature within the temperature measurement volume is repeatedly measured or monitored using the magnetic resonance imaging system. As this is done during execution of the treatment plan modifications to the treatment plan may be performed on the fly to enhance the effectiveness or safety of executing the treatment plan. 
         [0051]    In another embodiment the memory contains a pulse sequence for controlling the operation of the magnetic resonance imaging system. A pulse sequence as used herein is a set of commands or a timing diagram which is used to control the acquisition of magnetic resonance data using a magnetic resonance imaging system. The pulse sequence is an inversion recovery T2 weighted turbo spin echo or an inversion recovery T2 weighted spin echo pulse sequence. The pulse sequence is adapted for acquiring both transverse relaxometry data and longitudinal relaxometry data using an inversion time which is short in comparison to the T1 relaxation time. As used herein the longitudinal relaxation data encompasses magnetic resonance data which contains information which should be used to infer or calculate the T1 relaxation time. The T1 relaxation time referred to in this embodiment is the T1 relaxation time within the temperature measurement volume. This embodiment is advantageous because signal intensity changes due to temperature dependence of both T1 and T2 are additive which improves the sensitivity of the method. 
         [0052]    In another embodiment the magnetic resonance data further comprises proton resonance frequency shift data. As used herein proton resonance frequency shift data encompasses magnetic resonance data which may use to determine the proton resonance frequency shift data which is dependent upon temperature. Execution of the instructions further cause the processor to perform the step of calculating the temperature of the subject within the temperature measurement volume in accordance with the proton resonance frequency shift data. It is advantageous because in addition to using the transverse relaxation relaxometry data and/or longitudinal relaxometry data to determine the temperature also the proton resonance frequency shift data may also be used to determine the temperature within the temperature measurement volume. 
         [0053]    In another embodiment the memory contains a pulse sequence for controlling the operation of the magnetic resonance imaging system. The pulse sequence is automized during alternating time periods to attain the transverse relaxometry data and the proton resonance frequency shift data. This is advantageous because the pulse sequence may be modified such that these two different types of data are obtained both of which may be used for determining the temperature. 
         [0054]    In another embodiment the temperature of each of the temperature measurement volumes is determined individually in accordance with any one of the following: the transverse relaxometry data and the proton resonance frequency shift data. If the longitudinal relaxometry data is available also then this may also be used to determine the temperature in the temperature measurement volume. This embodiment is advantageous because for different types of tissue different methods of determining a temperature may be beneficial. For instance for water or tissue containing mostly water the proton resonance frequency shift may provide an accurate measurement or determination of the temperature. 
         [0055]    However this is not necessarily in fatty tissue. In this case it may be beneficial to use the transverse relaxometry data to determine the temperature. 
         [0056]    In another embodiment execution of the instructions causes the processor to perform the step of calculating the temperature of the subject within multiple temperature measurement volumes in accordance with the magnetic resonance data. Instead of just determining the temperature within a single temperature measurement volume the temperature within multiple volumes is determined. 
         [0057]    In another embodiment execution of the instructions further causes the processor to perform the step of constructing a temperature map in accordance with the temperature measurement volumes. Since the temperature has been determined in multiple temperature measurement volumes a mapping of these various temperatures may be constructed. Execution of the instructions further causes the processor to perform the step of reconstructing a magnetic resonance image in accordance with the magnetic resonance data. 
         [0058]    Execution of the instructions further causes the processor to perform the step of displaying a thermal magnetic resonance image on a display by superimposing the temperature map on the magnetic resonance image. This is particularly advantageous because the magnetic resonance image may display anatomical features and the temperature map may be used to map the temperature of these various anatomical features. This may be useful for an operator or physician to determine the effectiveness of a therapy using the apparatus or for manually altering the treatment plan during therapy. 
         [0059]    In another embodiment the memory contains a pulse sequence for controlling the operation of the magnetic resonance imaging system. The pulse sequence is any one of the following: a turbo spin echo sequence, an echo planar imaging sequence, a spin echo pulse sequence, and a gradient echo sequence. 
         [0060]    In another aspect the invention provides for a computer-implemented method of acquiring magnetic resonance thermometry data. The method comprises the step of acquiring magnetic resonance data using the magnetic resonance imaging system. The magnetic resonance data comprises transverse relaxometry data. The method further comprises the step of calculating the temperature of the subject within a temperature measurement volume in accordance with the transverse relaxometry data. The temperature measurement volume is within the imaging volume. 
         [0061]    In another aspect the invention provides for a computer program product comprising machine executable instructions for execution by the processor of a computer system of an apparatus comprising a magnetic resonance imaging system. The computer program part may for instance be computer executable code stored on a computer-readable storage medium. Execution of the instructions causes the processor to perform the step of acquiring magnetic resonance data using the magnetic resonance imaging system. The magnetic resonance data comprises transverse relaxometry data. Execution of the instructions further cause the processor to perform the step of calculating the temperature of the subject within a temperature measurement volume in accordance with the transverse relaxometry data. The temperature measurement volume is within the imaging volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0062]    In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which: 
           [0063]      FIG. 1  shows a flow diagram which illustrates a method according to an embodiment of the invention; 
           [0064]      FIG. 2  shows a flow diagram which illustrates a method according to a further embodiment of the invention; 
           [0065]      FIG. 3  shows a functional diagram of an apparatus according to an embodiment of the invention; 
           [0066]      FIG. 4  shows a functional diagram of an apparatus according to a further embodiment of the invention; 
           [0067]      FIG. 5  shows a functional diagram of an apparatus according to a further embodiment of the invention; 
           [0068]      FIG. 6  shows a plot which illustrates how temperature may be determined using the value of the T1 relaxation rate; 
           [0069]      FIG. 7  shows a plot which illustrates how the T2 relaxation rate may be used to determine the temperature of fat or fatty tissue; 
           [0070]      FIG. 8  shows a timeline which may be used to illustrate how a Proton resonance frequency shift and T2 weighted pulse sequences may be integrated; 
           [0071]      FIG. 9  shows an alternative method of integrating the proton resonance frequency shift and T2 weighted pulse sequences; 
           [0072]      FIG. 10  shows a timeline which illustrates the interleaving of the proton resonance frequency shift pulse sequence with the T2 weighted pulse sequence and when heating is performed by the tissue heating system; 
           [0073]      FIG. 11  shows a timing diagram of a gradient echo EPI pulse sequence which may be used for both proton resonance frequency shift and T2-star weighted imaging; and 
           [0074]      FIG. 12  shows a diagram for a turbo spin echo pulse sequence which can be used for T2 weighted imaging. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0075]    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. 
         [0076]      FIG. 1  shows a flow diagram which illustrates a method according to an embodiment of the invention. Step  100  magnetic resonance data is acquired which comprises transverse relaxometry data. In step  102  the temperature of a subject is calculated within a temperature volume in accordance with the transverse relaxometry data. 
         [0077]      FIG. 2  shows a flow diagram which illustrates a method according to a further embodiment of the invention. In step  200  a treatment plan received which specifies a target volume. In step  202  the target volume is heated by the tissue heating system by executing the treatment plan. In step  204  magnetic resonance data is repeatedly acquired by the apparatus and the magnetic resonance data comprises transverse relaxometry data. In step  206  the temperature of a subject within a temperature volume is repeatedly calculated in accordance with the transverse relaxometry data. In step  208  the treatment plan is modified during execution of the treatment plan in accordance with the temperature that was repeatedly calculated. 
         [0078]      FIG. 3  shows an embodiment of an apparatus  300  according to an embodiment of the invention. The apparatus  300  comprises a magnetic resonance imaging system  302  and a high intensity focused ultrasound system  304 . The magnetic resonance imaging system  302  may be used to guide therapy using the high intensity focused ultrasound system  304 . 
         [0079]    The magnetic resonance imaging system  302  comprises a magnet  306 . In this figure a cylindrical magnet with a bore through the center is shown. However other types of magnets may be used such as so-called open coil magnets or toroidal magnets. Within the bore of the magnet  306  there is an imaging volume  308  where the magnetic field is uniform enough for performing magnetic resonance imaging. A subject  310  is also located within the bore of the magnet  306  and a portion of the subject  310  is within the imaging volume  308 . The subject  310  is reposing on a subject support  312 . Also within the bore of the magnet is a magnetic field gradient coil  314 . The magnetic field gradient coil  314  is connected to a magnetic field gradient coil power supply  316  which supplies current to the magnetic field gradient coil  314 . Adjacent to the imaging volume  308  is a radio frequency coil  318 . The radio frequency coil  318  is connected to a radio frequency transceiver  320 . 
         [0080]    The high intensity focused ultrasound system  304  comprises an ultrasound transducer  322  which is immersed in a fluid filled chamber  324 . Not shown in this figure is a power supply for supplying power to the ultrasound transducer  322 . There may also be a mechanism for physically moving the ultrasound transducer  322  in some embodiments. The fluid filled chamber  324  is for good coupling of ultrasound energy to the subject  310 . The fluid filled chamber  324  has a ultrasound window  325  which seals it but however it allows ultrasound to pass through. The subject support  312  has an opening with which a gel pad  327  has been placed on the ultrasound window  325 . The gel pad  327  provides ultrasound coupling between the ultrasound window  325  and the subject  310 . The dotted lines  326  show the rough direction of travel of the focused ultrasound towards a sonication zone  328  within the subject  310 . The sonication zone  328  is shown as being within target zone  330 . In the course of a therapy focused ultrasound is generated in the sonication zone  328  which heats the target zone  330 . Adjacent to the target zone  330  is a temperature measurement volume  332 . The magnetic resonance imaging system  302  is used to monitor the temperature in the temperature measurement volume  332 . 
         [0081]    The high intensity focused ultrasound system  304  the radio frequency transceiver  320  and the magnetic field gradient coil power supply  316  are all connected to a hardware interface  334  of a computer system  336 . The computer system  336  is used to control the apparatus  300  and comprises a processor  338 . The processor  338  is connected to the hardware interface  334  and a user interface  340 . The user interface comprises devices which an operator may use to interact with the computer system  336 . For instance the hardware interface may comprise a mouse, a keyboard and a display. Images which are rendered by the computer system  336  may be displayed on a display of the user interface  340 . The processor  338  is further shown as being connected to a computer storage  342  and a computer memory  344 . 
         [0082]    The computer storage  342  is shown as containing magnetic resonance data  346  that has been acquired by the magnetic resonance imaging system  302 . The computer storage  342  further comprises a magnetic resonance imaging image which has been reconstructed from the magnetic resonance data  346 . The computer storage  342  further contains a treatment plan  350 . The computer storage  342  further contains a pulse sequence  352  which contains a timing diagram which is used for operating the magnetic resonance imaging system  302 . Computer memory  344  as shown as containing a magnetic resonance imaging control module. The magnetic resonance imaging control module  354  contains computer executable code for controlling the operation of the magnetic resonance imaging system  302 . The magnetic resonance imaging control module  354  uses the pulse sequence  352  for generating control commands for the magnetic resonance imaging system  302 . 
         [0083]    The computer memory  344  further contains a tissue heating system control module  356 . The tissue heating system control module  356  contains executable instructions for controlling the operation of the tissue heating system  304 . In other embodiments the tissue heating system control module  356  contains codes for operating different types of tissue heating systems. The computer memory  344  further contains a temperature calculation module  358 . The temperature calculation module contains computer executable code which uses the magnetic resonance data  346  to calculate a temperature of the temperature measurement volume  332 . The memory  344  is further shown as containing a treatment plan modification module  360 . The treatment plan modification module  360  contains computer executable code which uses the value of the temperature of the temperature measurement volume  332  as an input to modify the treatment plan  350 . The computer memory  344  is shown as further containing an image reconstruction module  362 . The image reconstruction module contains computer executable code which is able to generate the magnetic resonance imaging image  348  using the magnetic resonance data  346 . 
         [0084]      FIG. 4  shows an apparatus  400  according to a further embodiment of the invention. The apparatus  400  shown in  FIG. 4  is very similar to the apparatus shown in  FIG. 3 . Instead of using a high intensity focused ultrasound system the target volume  330  is heated using a radio frequency coil  402  and a radio frequency generator  404 . The target zone  330  is essentially heated with radio frequency energy. As with the previous embodiment the temperature within the temperature measurement volume  332  is measured using the magnetic resonance imaging system  302 . In this embodiment the tissue heating system control module  356  is used for controlling the radio frequency generator  404 . 
         [0085]      FIG. 5  shows an apparatus  500  according to a further embodiment of the invention. The apparatus  500  shown in  FIG. 5  is similar to the apparatuses shown in  FIGS. 3 and 4 . In the embodiment shown in  FIG. 5  a laser  502  is used to heat the target zone  330  instead. There is a laser  502  which is controlled by the processor  338  via hardware interface  334 . The laser  502  is connected to a laser catheter  506  via a fiber optic  504 . The laser catheter  506  is inserted invasively into the subject  310 . The laser catheter  506  is used to focus and ablate tissue in the target zone  330 . The magnetic resonance imaging system  302  is used to monitor the temperature  332  in the temperature measurement volume  332 . In this embodiment the tissue heating system control module  356  is used for controlling the laser  502 . 
         [0086]      FIG. 6  shows a plot which illustrates how the temperature in the temperature measurement volume may be determined using the value of the T1 relaxation rate. Performance measurement an in-vitro fat sample was warmed in a water bath within a magnetic resonance imaging system. Thermocouples were used to measure the actual temperature of the fat sample. The T1 value was measured with a IR pulse sequence. The X axis is the temperature in degrees Celsius. The Y axis is the R1 value. The R1 value is one over the T1 relaxation rate. Points are measured values and the solid line is a linear fit to the data. The plot in  FIG. 6  illustrates that the T1 value may be used for measuring the temperature of fat using magnetic resonance imaging. 
         [0087]      FIG. 7  shows a plot which illustrates how the T2 relaxation rate may be used to govern the temperature of fat or fatty tissue using magnetic resonance imaging. The T2 values shown in this plot were measured using SE and TSE pulse sequences. There are inter-echo spacings of 5, 13, and 20 milliseconds respectively. As with  FIG. 6  the X axis shows the temperatures in degrees Celsius. The Y axis shows the R2 value. R2 is one over the T2 value. There are three sets of data shown in  FIG. 10 . The points and line labeled  700  correspond to the five milliseconds inter-echo spacing. The points are the actual data and the line is the linear fit. The same is shown with the line and points labeled  702 . The line and points labeled  702  are for 13 milliseconds. The line and points labeled  704  are for 20 milliseconds for the value of the echo spacing. 
         [0088]      FIG. 8  shows a timeline which may be used to illustrate how a Proton Resonance Frequency shift (PRF) and T2 weighted (T2W) pulse sequences may be integrated. The regions labeled  800  are when a pulse sequence which can measure the PRF or proton resonance frequency shift can be measured. The sections of the timeline labeled  802  represent when a pulse sequence which acquires data which may be used to determine the T2 relaxation rate is measured. 
         [0089]    One possible timing of interleaving PRF and T2 weighted sequences. N1 slices are obtained with PRF sequence, after which after N2 slices (may be partly the same or completely different slices as with PRF) with T2w (or T2w-star) sequence, where N1 is a predetermiend number of slices and N2 is a second predetermiend number of slices. After that, N1 slices with PRF is obtained and so on. In this arrangement both sequences are updated with the same interval, i.e. temperature information of both sequences has the same temporal resolution. 
         [0090]      FIG. 9  shows an alternative method of interleaving the PRF and T2 weighted pulse sequences. The sections of the timeline labeled  900  are when the PRF pulse sequence is performed. The section of the timeline labeled  902  is when the T2 weighted pulse sequence is performed. 
         [0091]    One possible timing of interleaving PRF and T2 weighted (T2w) sequences. T2w-star is an abreviation for a T2-star weighted image. PRF sequence is repeated N times, after which T2w (or T2w-star) sequence is obtained. After that, PRF is obtained again N times etc. In this arrangement PRF image is updated more often than T2w image. Yet another modification is to collect only part of the k-space in each time period to further adjust the image quality vs. temporal resolution point. 
         [0092]      FIG. 10  shows a timeline which illustrates the interleaving of the PRF pulse sequence with the T2 weighted pulse sequence along with when heating is performed by the tissue heating system. The sections of the timeline labeled  1000  are when the PRF pulse sequence is performed. The section of the timeline labeled  1002  are when the T2W pulse sequence is performed. Initially it is seen that a reference measurement  1004  is performed before the heating  1006  is activated. Reference measurement  1004  is to take a baseline measurement. After the heating  1006  is activated additional PRF pulse sequences and T2W pulse sequences are performed to measure heating during the heating process. 
         [0093]    Timing diagram showing how one or more reference images are obtained before switching on the heating device (e.g. focused ultrasound). 
         [0094]      FIG. 11  shows a timing diagram of a gradient echo EPI pulse sequence which may be used for both PRF and T2W-star imaging. The timing diagram for the RF is labeled  1100 . The gradient slice is labeled  1102 . The gradient phase is labeled  1104  and the gradient read is labeled  1106 . The timed repetition is indicated by a bar labeled  1108 . The time to echo is represented by a bar labeled  1110 . The bracket  1112  indicates that this pulse sequence is repeat n times. Corresponding echoes are labeled E1 through En. 
         [0095]    Timing diagram of gradient echo EPI sequence which can be used both for PRF and T2w-star imaging. Effective echo time TE (essentially referring to the echo where the center of the k-space is acquired) defines the sensitivity of the sequence o for both purposes. The sequence can be either single shot, when all the phase encodings are collected after a single excitation (alpha) pulse, or multishot, when part of the encodings are collected and the sequence is then repeated with different phase encodings. 
         [0096]      FIG. 12  shows a diagram for a turbo spin echo pulse sequence which can be used for both T2W imaging. The bar labeled  1200  shows the RF timing. The bar labeled  1202  shows the timing for the G slice parameter.  1204  shows the value for G phase.  1206  shows the value for G read. The bar labeled  1208  represents the inter-echo spacing or IES. The bar labled  1112  indicates that the reading of the echoes is performed N times. Corresponding echoes are labeled E1 through En. 
         [0097]    As was mentioned before,  FIG. 12  shows a timing diagram of turbo spin echo (TSE) sequence which can be used both for T2w imaging. Effective Echo Time (TE) (essentially referring to the echo where the center of the k-space is acquired) defines the sensitivity of the sequence for the purpose. The sequence can be either single shot, when all the phase encodings are collected after a single excitation pulse, or multishot, when part of the encodings are collected and the sequence is then repeated with different phase encodings. Inter Echo spacing (IES) affects on the sensitivity of the sequence to J-coupling effect. There is a modification of the sequence where two (or, in principle, even more) images with different effective echo times are produced. This would allow quantitative estimation of T2. 
         [0098]    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. 
         [0099]    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 
       [0100]      200  first item 
         [0101]      300  apparatus 
         [0102]      302  magnetic resonance imaging system 
         [0103]      304  high intensity focused ultrasound system 
         [0104]      306  magnet 
         [0105]      308  imaging volume 
         [0106]      310  subject 
         [0107]      312  subject support 
         [0108]      314  magnetic field gradient coil 
         [0109]      316  magnetic field gradient coil power supply 
         [0110]      318  radio frequency coil 
         [0111]      320  radio-frequency transceiver 
         [0112]      322  ultrasound transducer 
         [0113]      324  fluid filled chamber 
         [0114]      325  ultrasound window 
         [0115]      326  path of ultrasound 
         [0116]      327  gel pad 
         [0117]      328  sonication zone 
         [0118]      330  target zone 
         [0119]      332  temperature measurement volume 
         [0120]      334  hardware interface 
         [0121]      336  computer system 
         [0122]      338  processor 
         [0123]      340  user interface 
         [0124]      342  computer storage 
         [0125]      344  computer memory 
         [0126]      346  magnetic resonance data 
         [0127]      348  magnetic resonance imaging image 
         [0128]      350  treatment plan 
         [0129]      352  pulse sequence 
         [0130]      354  magnetic resonance imaging control module 
         [0131]      356  tissue heating system control module 
         [0132]      358  temperature calculation module 
         [0133]      360  treatment plan modification module 
         [0134]      362  image reconstruction module 
         [0135]      400  apparatus 
         [0136]      402  radio-frequency coil 
         [0137]      404  radio-frequency generator 
         [0138]      500  apparatus 
         [0139]      502  laser 
         [0140]      504  fiber optic 
         [0141]      506  laser catheter 
         [0142]      700  5 ms 
         [0143]      702  13 ms 
         [0144]      704  20 ms 
         [0145]      800  PRF pulse sequence performed 
         [0146]      802  T2W pulse sequence performed 
         [0147]      900  PRF pulse sequence performed 
         [0148]      902  T2W pulse sequence performed 
         [0149]      1000  PRF pulse sequence performed 
         [0150]      1002  T2W pulse sequence performed 
         [0151]      1004  reference measurement 
         [0152]      1006  heating performed