FIELD OF THE INVENTION

The invention relates to the field of magnetic resonance imaging (MRI) and more specifically to MRI guided treatment. BACKGROUND OF THE INVENTION

Current MR-HIFU systems are able to start and stop single dynamic MR imaging sequences (e.g. temperature mapping or T2-mapping for adipose tissue). Interleaved scanning is a technology which enables to switch between different sequences at different acquisition levels during runtime. The advantage is that multiple parameters can be measured within the same time frame having their optimal parameter space. This technology does not speed up the acquisition, but creates flexibility in the temporal resolution of the individual interleaved scans.

MR Thermometry data can be used to guide a high intensity focused ultrasound (HIFU) treatment. Currently, the proton resonance frequency shift sequence (PRFS) is the most used sequence for temperature mapping which is only able to determine the temperature in non-adipose tissue. EP2423700A1 describes a method for calculating temperature from MRI T2 relaxometry data for adipose tissue. To determine the temperature of non-adipose and adipose tissue during MR-HIFU therapy, the PRFS measurements and T2 mapping sequence are interleaved at a predetermined time interval.

Keupp et al, Simultaneous T2 mapping in near- field subcutaneous fat layer and PRFS temperature mapping in the target region using fast interleaved sequences to monitor MR-HIFU sonication, Proc. Intl, Soc. Mag. Reson. Med 23 (2015))

describes simultaneous T2 mapping in near- field subcutaneous fat layer and PRFS temperature mapping in the target region using fast interleaved sequences to monitor MR- HIFU sonication. As part of the study it has been investigated if a time delay of 200ms before the T2 mapping segment would improve the T2 image quality (results and discussion section). The insertion of such delay showed to be beneficial.

SUMMARY OF THE INVENTION It is an object of the invention to provide for a MRI guided therapy system with a higher flexibility. This object is achieved by a MRI guided therapy system according to claim 1.

It is an insight of the inventors that, while interleaving of sequences with a fixed pattern may be used to constantly monitor different variables simultaneously, distinct phases of an MRI guided therapy have different requirements with respect to temporal and spatial resolutions and required contrast types. Therefore, a flexible adaptation of the interleaved MRI acquisition scheme is needed.

According to embodiments of the invention, the interleaved sequences are defined on the MRI system, including and preparing all possible MRI sequences required during the treatment procedure. This information is stored in the MRI sequence storage. A combination of interleaved sequences is called a package. A software module on the MRI guided treatment system starts the (interleaved sequence) package when starting the treatment procedure. Before and during treatment delivery (e.g. sonication,

(RF/microwave/laser) heating, irradiation) updates made by the package alteration module on the MRI contrasts / sequences in the package, their timing (e.g. duty-cycle, temporal resolution) and their stack positions (geometry update) can be send to the MRI system. Also the required order ('rhythm') of interleaved sequence parts may be updated (e.g. ABCABC... or ABACAB...), as well as their respective individual repetitions per switch (e.g. kxA, lxB, mxC with k,l,m e {0, 1 ,2, ... } ), with A, B and C being different MRI sequences (e.g. T 1 , T2, PRFS, DWI, perfusion, etc) and the acquisition level can be selected at which the data of the sequence is acquired (e.g. k-line, part of k-space, slice, stack, dynamic level, etc.). For all the sequences their stack positions (a single or multiple slices) can be changed with a geometry update. Changing the switching times between sequences or stack positions, or in other words altering a package can be performed any possible moment and is executed on the upcoming switching moment. In this way, the information acquired can be adapted to a specific therapeutic situation on demand. For example, one may want to change the package as a result of a certain MRI readout (e.g. temperature gradient or sudden motion in the treatment area). In order to follow SAR regulations, prescan SAR calculations are based on the sequence with the highest SAR level within the patient's treatment area. If full-time scanning with this sequence would not lead to exceeding predefined SAR limits, any deviation from that, resulting from interleaving with lower SAR sequences, would not cause any SAR related safety issues either. Sound level calculations and determine the maximum slew rates of gradients are preferably done for the individual sequences. According to embodiments of the invention, the MRI sequence storage comprises for at least for some of the MRI sequences multiple potential orders and / or timings. In addition to this, the package alteration module is configured for changing the order and / or timing of one or more of the multiple MRI sequences and / or their respective interleaves in the package based on the available acquisition orders and / or timings in the MRI sequence storage.

According to further embodiments, one may want to use an altered package during actual treatment delivery compared to during a cool-down period in between different deliveries. According to further embodiments, one may want to alter the package based on information received from the patient (e.g. information related to patient discomfort) or an oberservation of the physician. This allows for an increased flexibility and in addition it allows for using the total MRI scan time in a more efficient way, as sequence and package switching can be performed almost without delay. In this way, a temporal resolution by which certain information (e.g. T2 mapping for adipose tissue temperature mapping) is acquired can be varied. In certain situations one could even choose not to acquire certain information at all and use the time to acquire a different kind of MR parameter/contrast. For example, in MRI guided radiotherapy, a time that is not needed for geometric tracking could be used for acquisition of more functional contrast like, e.g. DWI or BOLD. However, if it is detected that a patient moves more than expected, the temporal resolution by which DWI or BOLD images are acquired can be decreased, or the sequence(s) could even be skipped completely.

According to further embodiments, the package could be altered such that the geometries of the scans in the package can be altered in real time in order to deal with different situation e.g. for compensating for organ motion and/or moving the treatment area. The order of the interleaved sequences is for example sent by the package alteration module. This special update message is detected by software module running on the MRI system and the contained information on required MR sequences in the package, geometry, rhythm, repetitions per sequence and acquisition level per sequence is decoded for subsequent execution in real-time (no latency sequence switching). The order, geometry and timing can be adapted by the software module on the MRI guided treatment system console based on the actual image data and/or phase of the therapy delivery by sending new specialized update messages if necessary. These update messages can also be created within the MRI console software itself (e.g. descisions on motion tracking, image quality, etc.). According to further embodiments of the invention, the MRI guided therapy system comprises a user interface configured for receiving a user input for the order and / or timing of one or more of the multiple MRI sequences and / or their respective interleaves and the package alteration module is configured to change the package according to the user input. The user input could for example be based on patient's feedback or a treating physician's decision. This embodiment is advantageous, because it provides a lot of flexibility to react on unforeseen situations.

According to further embodiments, the package alteration module is configured to receive information from the MRI system and / or from the therapy system and wherein the package alteration module is configured to automatically change the order and / or timing of one or more of the MRI sequences and / or their respective interleaves based on MRI measurements and / or information provided by the therapy system. This embodiment is advantageous, because in this way package alteration can be performed very fast.

According to further embodiments, the MRI guided therapy system as claimed the package alteration module is configured to automatically change the order and / or timing of one or more MRI sequences and / or their respective interleaves in the package based on at least one out of whether the therapy system is switched on or off, a dose delivered by the therapy system, a position of the therapy system relative to the subject or the orientation of the system relative to the subject. This is advantageous, because specific parts of a treatment procedure may benefit from different contrast types.

According to further embodiments of the invention, the MRI guided therapy system is a high intensity focused ultrasound (HIFU) system. According to further embodiments, the MRI measurements are temperature measurements, and the order and / or timing of one or more MRI sequences and / or their respective interleaves is at least partly determined based on a measured temperature.

According to further embodiments, at least one of the MRI sequences in the package is configured to detect motion and the order and / or timing of one or more MRI sequences and / or their respective interleaves is at least partly determined based on the detected motion. This embodiment is advantageous as it provides for more flexibility when (unexpected) motion is detected or one would like to compensate the motion of the body, body part or organ.

According to further embodiments, the MRI guided therapy system comprises a storage configured to store predefined parameters related to the acquisition orders and / or timings for the multiple MRI sequences which are configured to be used in one or more specific situations that are expected to potentially happen (e.g. therapy system being switched on or off, dose delivered by the therapy system, a predefined temperature change, motion exceeding a predetermined threshold) during the treatment procedure. The MRI guided therapy system is configured to automatically change the order and / or timing of one or more of the MRI sequences and / or their respective interleaves in case one of the specific situations does happen during the treatment procedure, wherein the automatic change is based on predefined parameters corresponding to the specific situation. This embodiment is advantageous as it allows for quick adaptation to a situation.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 diagrammatically shows an MRI guided therapy system according to embodiments of the invention and

Figure 2 diagrammatically shows variations of a package according to embodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 diagrammatically shows an MRI guided therapy system 100 according to embodiments of the invention. The system comprises an MRI system 101 and a high intensity focused ultrasound (HIFU) system 110. The MRI system is configured to guide the HIFU treatment by performing multiple MRI sequences (Fig. 2 A, B, C) in an interleaved manner on a subject 102 treated with the MRI guided therapy system 100. The MRI system 101 comprises a main magnet 104. The subject 102 can be positioned on a subject support 103. MRI images can be acquired from the part of the subject that is positioned in an examination zone 14 within the MRI system. The MRI system also comprises gradient coils 105, which are configured for spatial encoding. Further, the MRI system is provided with RF coils 107, which are configured to transmit and receive RF signals to and from the subject.

The HIFU system 110 is positioned in a fluid filled chamber configured for acoustic coupling of ultrasound energy to the subject. The subject support 103 comprises an opening, wherein a gel pad 114 is placed. The gel pad provides for acoustic coupling between the fluid filled chamber and the subject, such that ultrasound energy can be deposited from the HIFU system via the fluid filled chamber and the gel pad into a sonication zone 116 in the subject. The MRI system is configured to perform temperature measuments in the sonication zone.

The MRI system 101 and the HIFU system are connected to a computer system 180 comprising a hardware interface 140. The computer system is configured to control the MRI system and HIFU system. The computer system further comprises an MRI sequence storage 160 comprising the multiple MRI sequences (Fig. 2, A, B, C), wherein the multiple MRI sequences and the acquisition order and timing of their respective interleaves are stored in a package (Fig. 2, 201, 202). The computer system further comprises a package alteration module 150 configured for changing the order and / or timing of one or more of the multiple MRI sequences in the package during a treatment procedure. The package (201, 202) can be altered automatically and / or based on user 170 input. Alteration can be achieved selecting a prestored parameter set describing a package. Alternatively, parameters describing the package (e.g. duty-cycle, temporal resolution) can be freely changed during the treatment procedure.

Figure 2 diagrammatically shows variations of a package according to embodiments of the invention. 201 is a first package and 202 is an altered package (i.e. the same package as 201, but with different settings, e.g. timing an order), which has been altered because circumstances have changed. 'A' could for example be a T2 sequence, 'B' a PRFS sequence, and 'C could be a DWI sequence. The different sequences are divided into multiple interleaves. Sequence "A" has been divided into interleaves 203 and 204. The variable k, 1 and m are repetition numbers at a certain acquisition level (k-line, slice, stack, dynamic) before switching to the next sequence at a predefined acquisition level (k-line, slice, stack, dynamic). Therefore k, 1, m relate to the timing of the interleaves. 201 could be a package that could be used during HIFU sonication, whereas 202 could be a package that could be used during a cool-down period between two HIFU sonications. During cool-down one may want to spend more time on monitoring temperature in fat compared to during an actual sonication, since the end of the cool-down period may be determined by the temperature in fat. Therefore, in package 202, k will be larger than in package 201.

Alternatively or additionally, one could also use a T2 with higher resolution, measure perfusion or acquire a baseline temperature sequence to improve the phase drift correction for the next PRFS acquisition.

Packages 201 and 202, or all parameters relevant for the packages could be stored in the MRI guided therapy system prior to treatment as packages to be automatically used during the treatment procedure. These packages or all parameters relevant for the packages could be linked to the occurrence of specific events, in this case switching the HIFU system on (package 201) and off (package 202). Also, for example more time may be spend on sequences related to safety (e.g. temperature, motion tracking) in more critical phases of the treatment procedure (e.g. when more dose is delivered or when organs at risk are closer to a treatment area).

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