FIELD OF THE INVENTION

The invention pertains to a magnetic resonance imaging system for guiding an energy depositing therapy system and to a method for operating the magnetic resonance imaging system.

BACKGROUND OF THE INVENTION

The stability and homogeneity of the main magnetic field of clinical magnetic resonance (MR) scanners is known to vary in time, as the scan progresses. The variations are known to be both temporally and spatially dependent. It has been shown, for instance in the article "Monitoring and correcting spatial-temporal variations of the MR scanner 's static magnetic field' by A. M. El-Sharkawy et al. (Magnetic Resonance Materials in Physics, Biology and Medicine, (2006), 19: 223-236) that the main magnetic field drifts are directly correlated to hardware stress of the MR system. Gradient heavy scans such as echo planar imaging (EPI) are known to cause particularly severe magnetic field drifts.

It is further known that a magnetic field probe system can be employed to assess the actual spatial field distribution of the main magnetic field during an MR acquisition sequence with a time period in the range of an order of magnitude of 500 μβ. An example of this method is, for instance, described in the article "Spatiotemporal Magnetic Field Monitoring for MR' by C. Barmet et al. (Magnetic Resonance in Medicine 60: 187-197 (2008)).

SUMMARY OF THE INVENTION

Proton resonance frequency (PRF) shift thermometry is particularly sensitive to main magnetic field variations. In PRF shift thermometry, a conventional reference method is to acquire a reference magnetic resonance (MR) image before the start of heating a subject of interest or parts thereof for therapy and to subtract the reference MR image from current dynamically acquired phase images to obtain a corrected phase difference image. When performing PRF shift thermometry to monitor large region hyperthermia for radiation sensitization, chemo sensitization, drug delivery or gene expression, it is generally accepted that an elevated temperature of the subject of interest or parts thereof needs to be kept for a duration of thirty to sixty minutes.

Because PRF shift thermometry uses temporal changes in phase images to reconstruct a temperature estimate, any non-temperature caused magnetic field changes such as magnetic field drifts will translate into temperature artifacts.

It is therefore an object of the invention to provide a magnetic resonance imaging system for guiding an energy depositing therapy system with an improved performance regarding temperature artifacts due to magnetic field drift. Energy depositing therapy systems, provided for locally depositing energy into a therapy zone of the subject of interest for therapy purposes, that are considered are, for instance, high-intensity focused ultrasound (HIFU) therapy systems, radio frequency heating devices, microwave heating devices, microwave ablation devices, laser heating devices, or Shockwave generating devices. However, it will be obvious to the one skilled in the art that this list of systems and devices is not exhaustive and that the disclosed magnetic resonance imaging systems may as well be applied to other energy depositing therapy systems.

In one aspect of the present invention, the object is achieved by a magnetic resonance imaging system, comprising:

an examination space provided to position a subject of interest within;

a main magnet provided for generating a substantially static magnetic field in the examination space, wherein the substantially static magnetic field is directed substantially parallel to a center axis of the examination space;

a magnetic gradient coil system provided for generating gradient magnetic fields superimposed to the static magnetic field;

an image processing unit that is provided to subtract a reference magnetic resonance image acquired at a reference point in time from at least a second magnetic resonance image acquired at a second point in time;

a magnetic field probe system provided for determining an actual magnetic field pattern within the examination space, the magnetic field probe system comprising

a plurality of magnetic field probes that are configured to be arranged spaced from each other within the examination space, and

a magnetic field evaluation unit that is configured to acquire signals from the plurality of magnetic field probes to determine an actual magnetic field pattern within the examination space as a magnetic reference field pattern at a third point in time in proximity to the reference point in time;

to acquire signals from the plurality of magnetic field probes at a fourth point in time in proximity to the second point in time to determine at least one coefficient of a first-order or higher-order spatial variation of the magnetic field within the examination space in at least one spatial direction, and

to correct a change of the magnetic reference field pattern on the basis of the at least one determined coefficient,

wherein the third point in time of determining the magnetic reference field pattern and the fourth point in time of determining the at least one coefficient have a difference in time of at least 0.5 minutes.

The phrase "order of a spatial variation", as used in this application, shall be understood particularly as the number of coordinate variables in a mathematical product term used to describe the spatial variation of the magnetic field.

The phrase "a first point in time in proximity to a second point in time", as used in this application, shall be understood particularly such that a difference in time between the second point in time and the first point in time is less than 10%, preferably less than 5%, and, most preferably, less than 2% of the time difference between the second point in time and the reference point in time.

By that, contributions to a phase of the acquired magnetic resonance signal caused by a spatial and/or temporal magnetic field drift developing over time can be corrected for in the described time frame. As a beneficial result, more accurate thermographic image data and PRF temperature estimates can be obtained.

The phrase "magnetic field probe", as used in this application, shall be understood particularly as a device intended to dynamically measure the actual magnetic field strength during an MR measurement sequence. Having a plurality of such devices arranged within a bore of an MR scanner of an MR system allows for characterizing the precise magnetic field pattern generated by a switching of the field-gradients, in addition to any other intended or unintended causes of magnetic field variations at a given measurement point in time. The temporal evolution of said field can then be monitored, and this temporal evolution can then be subtracted from the phase data used for thermometry. Alternatively, the background B ₀ field as estimated from the field probe samples can directly be used as an "reference image" and can be subtracted from the acquired phase image to provide a temperature estimate. With the knowledge of the actual magnetic field pattern, i.e. magnetic field strength and direction, that existed at the time of acquiring MR signals, MR images or spectra may be reconstructible with fewer artifacts and less distortion. Various embodiments of magnet field probes for MR applications have been reported, for instance, in document EP 1 847 845 Al .

The magnetic field probes may comprise a field probe body containing an amount of a magnetic resonance-active species of nuclei. The magnetic field probe may further comprise a radio frequency (RF) transmit antenna that is arranged in close proximity to the amount of the magnetic resonance-active species of nuclei and that is provided to apply an RF magnetic field to the amount of the resonance-active species of nuclei for resonant excitation. The magnetic field probe may further comprise a radio frequency (RF) receive antenna (RF) antenna that is arranged in close proximity to the amount of the magnetic resonance-active species of nuclei and that is provided for receiving magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei. The RF transmit antenna and the RF receive antenna may be the same antenna, wherein the operation of the RF antenna is controlled via a transmit/receive switch, as is commonly known in the art.

The difference in time between the third point in time and the fourth point in time may be more than ten minutes long. For special treatments, it may even be longer than thirty minutes, and may be up to or even beyond sixty minutes for any of the treatments described above.

In a preferred embodiment, the plurality of magnetic field probes comprises a number of magnetic field probes that are arranged in the at least one spatial direction that is equal to or greater than k +2, with k denoting the order of the spatial variation. This allows to determine the at least one coefficient of the spatial variation of the magnetic field of order k with an improved accuracy.

According to yet another aspect, a method for operating a magnetic resonance imaging system that is configured for acquiring magnetic resonance signals and for guiding an energy depositing therapy system is provided, wherein the magnetic resonance imaging system comprises

an examination space provided to position a subject of interest within;

a main magnet for generating a substantially static magnetic field in the examination space, wherein the substantially static magnetic field is directed substantially parallel to a center axis of the examination space; a magnetic gradient coil system provided for generating gradient magnetic fields superimposed to the static magnetic field;

an image processing unit that is provided to subtract a reference magnetic resonance image acquired at a reference point in time from at least a second magnetic resonance image acquired at a second point in time that is later than the reference point in time; and

a magnetic field probe system for determining an actual magnetic field pattern within the examination space, the magnetic field probe system comprising a plurality of magnetic field probes that are configured to be arranged spaced from each other within the examination space.

The method comprises the following steps:

(a) acquiring signals from the plurality of magnetic field probes at a third point in time in proximity to the reference point in time as magnetic field reference signals;

(b) acquiring signals from the plurality of magnetic field probes at a fourth point in time in proximity to the second point in time;

(c) based on the signals acquired at the fourth point in time relative to the magnetic field reference signals and on the spacing between the magnetic field probes, determining at least one coefficient of a first-order or higher-order spatial variation of the magnetic field within the examination space;

(d) adjusting the reference magnetic resonance image, based on the at least one determined coefficient, for correcting at least the second magnetic resonance signal acquired from the subject of interest for a contribution to the change in a phase caused by the at least first-order or higher-order spatial variation of the magnetic field within the examination space;

wherein the third point in time and the fourth point in time have a difference in time of at least 0.5 minutes. By that, more accurate thermographic image data and PRF temperature estimates can be provided for guiding the energy depositing therapy system.

The steps (c) and (d) may be repeatedly carried out during a session of acquiring magnetic resonance signals from the subject of interest, so as to provide a consecutive row of data for adjusting the reference magnetic resonance image.

In a suitable embodiment, the step of acquiring signals from the plurality of magnetic field probes at a fourth point in time in proximity to the second point in time may be carried out simultaneously to the acquiring of magnetic resonance signals. By that, any interleaving of acquiring signals from the plurality of magnetic field probes and acquiring magnetic resonance signals from the subject of interest can be omitted. A suitable embodiment of the magnetic field probes may include the use of fluorine ( ¹⁹ F) as a magnetic resonance-active species of nuclei. Preferably, the fluorine nuclei ¹⁹ F may be chemically bound in molecules of hexafluorobenzene C ₆ F ₆ , so that an amount of a magnetic resonance- active species of nuclei in which the nuclei are chemically bound in an identical way can readily be provided. With the magnetic resonance frequency (Larmor frequency) of ¹⁹ F nuclei being about 40 MHz/T and the Larmor frequency of protons 1H being about

42.6 MHz/T, the requirement for the electronic data acquisition equipment regarding bandwidth can be kept moderate.

In another embodiment, the difference between the third point in time and fourth point in time may be larger than thirty minutes. This allows for correcting

contributions to the phase of the acquired magnetic resonance signals caused by the magnetic field drift developing with time and for providing more accurate PRF temperature estimates in long and large region hyperthermia applications.

According to still another object, a high-intensity focused ultrasound HIFU therapy system is provided, comprising an embodiment of the disclosed magnetic resonance imaging systems, at least for guidance with regard to a control of a temperature change generated in the subject of interest by applying high-intensity focused ultrasound waves.

In a preferred embodiment of the high-intensity focused ultrasound HIFU therapy system, the magnetic resonance imaging system is, in at least one operational mode, operated according to one of the disclosed methods or combinations thereof, providing the advantages described before.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

In the drawings:

Fig. 1 is a schematic illustration of a part of an embodiment of an MR imaging system in accordance with the invention,

Fig. 2 illustrates an exemplary embodiment of an arrangement of magnetic field probes,

Fig. 3 shows a flow diagram of a method in accordance with the invention, and

Fig. 4 schematically shows a timeline of acquiring signals in accordance with the method of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a schematic illustration of a part of an embodiment of a magnetic resonance (MR) imaging system 10 for guiding an energy depositing therapy system that is designed as a high-intensity focused ultrasound (HIFU) therapy system comprising a HIFU array 60, via proton resonance frequency (PRF) thermometry. Although the energy depositing therapy system is designed as a HIFU therapy system, it is obvious to the one skilled in the art that the energy depositing therapy system may as well be designed as a radio frequency heating device, a microwave heating device, a microwave ablation device, a laser heating device, a Shockwave generating device or any other energy depositing therapy device that creates a change in temperature in at least of portion of the subject of interest 20, usually a patient, for therapy purposes.

The MR imaging system 10 includes an MR scanner 12 comprising a main magnet 14 with a center bore that defines an examination space 16 for the subject of interest 20, to be positioned within. A patient table has been omitted in Fig. 1 for reasons of clarity. The main magnet 14 is provided for generating a substantially static magnetic field in the examination space 16, wherein the substantially static magnetic field is directed substantially parallel to a center axis 18 of the examination space 16. Further, the MR imaging system 10 comprises a magnetic gradient coil system 22 for generating gradient magnetic fields superimposed to the static magnetic field. The magnetic gradient coil system 22 is concentrically arranged within the bore of the main magnet 14, as is well known in the art.

The magnetic field shows a temporal drift due to various reasons, among those being unavoidable losses, heating up of the gradient coils during operation, and varying environmental conditions. Furthermore, there are also spatial variations in the magnetic field that are induced by hardware stress of the MR imaging system 10 such as eddy currents caused by switching gradients that heat up resistive parts of the MR scanner 12. The spatial variations of the magnetic field are therefore also time-dependent.

The MR imaging system 10 includes an MR imaging system control unit 24 with a monitoring unit to control functions of the MR scanner 12, as is commonly known in the art. The MR imaging system 10 further comprises an image processing unit 26 that is, amongst other things, provided to subtract a reference magnetic resonance image I _re f acquired at a reference point in time t _re f from at least a second magnetic resonance image I ₂ acquired at a second point in time t ₂ to obtain a phase difference image (Fig. 4). The reference magnetic resonance image I _re f is acquired before an application of the high-intensity focused ultrasound commences, and the second point in time t ₂ is later than the reference point in time t _re f, and usually lies within a duration of the application of the high-intensity focused ultrasound, as will be explained in more detail in the following.

Going back to Fig. 1, the MR imaging system 10 comprises a magnetic field probe system 30 provided for determining an actual magnetic field pattern within the examination space 16. The magnetic field probe system 30 comprises a plurality of sixteen magnetic field probes 32, which in an operational state of the MR imaging system 10 are configured to be arranged spaced from each other within the examination space 16. The magnetic field probes 32 are arranged on a support that is not shown in Fig. 1 for reasons of clarity. The support enables to know a relative position of the magnetic field probes 32 within a precision of better than one millimeter. The one skilled in the art is familiar with suitable supports which therefore do not have to be described in more detail herein.

An exemplary embodiment of an arrangement of the sixteen magnetic field probes 32 is illustrated in Fig. 2. The sixteen magnetic field probes 32 are arranged in four groups of four magnetic field probes 32 each. Each group of four magnetic field probes 32 is aligned in a spatial direction that is parallel to the center axis 18 of the examination space 16, usually denoted as the z-direction of a coordinate system used with the MR scanner 12. The four groups are equally spaced from the center axis 18 in such a way that there exists at least one coordinate system in which all the sixteen magnetic field probes 32 have the same absolute x-coordinates and the same absolute y-coordinates. One of the possible coordinate systems is shown in the lower part of Fig. 2.

Each of the magnetic field probes 32 contains an amount of a magnetic resonance-active species of nuclei which is ¹⁹ F fluorine. The fluorine nuclei are chemically bound in an identical way in molecules of hexafluorobenzene C ₆ F ₆ , and thus show a low chemical shift, resulting in a narrow-banded, well-defined magnetic resonance frequency for a given magnetic field strength. The magnetic resonance frequency (Larmor frequency) of ¹⁹ F is about 40 MHz/T. The MR scanner 12 is furnished with a birdcage coil 56 (Fig. 1, placed within an RF screen 58 as is known in the art) that is configured to simultaneously transmit RF energy at the resonance frequency of fluorine and also at the resonance frequency of hydrogen 1H at the magnetic field strength present in the examination space 16, and electronic equipment (not shown) with a frequency bandwidth that is sufficient to process the acquired magnetic resonance signals, so that an acquiring of magnetic resonance signals from nuclei within the subject of interest 20 and an acquiring of signals from the plurality of magnetic field probes 32 can be carried out simultaneously without any need for interleaving.

Moreover, the MR imaging system 10 comprises a magnetic field evaluation unit 34 that is configured to acquire signals from the plurality of magnetic field probes 32 to determine an actual magnetic field pattern within the examination space 16 as a magnetic reference field pattern at a third point in time t ₃ in proximity to the reference point in time t _re f.

Further, the magnetic field evaluation unit 34 is configured to acquire signals from the plurality of sixteen magnetic field probes 32 at a fourth point in time t ₄ in proximity to the second point in time t ₂ to determine at least one coefficient of a first-order or higher- order spatial variation of the magnetic field within the examination space 16 in at least one spatial direction.

Determining a spatial variation of the magnetic field of order k in the z- direction requires simultaneously acquiring signals of at least k+1 magnetic field probes 32 arranged in this direction. As the arrangement of magnetic field probes 32 shown in Fig. 2 has four magnetic field probes 32 in each group, a number of magnetic field probes 32 that are arranged in the at least one spatial direction is equal to or greater than k +2, with k = 2 denoting a second order spatial variation. One signal of the four signals of each group can be used to improve the accuracy of determining at least one coefficient of a second-order spatial variation in the z-direction, and for determining a first-order spatial variation in the z- direction, two signals of the four signals of each group of magnetic field probes 32 are redundant and can be employed to reduce an error in determining a coefficient of the first- order spatial variation.

The arrangement of sixteen magnetic field probes 32 of Fig. 2 also allows for determining coefficients of zero-order and first-order spatial variations in the x- and in the y- direction.

The magnetic field evaluation unit 34 is configured to correct the change of the magnetic reference field pattern on the basis of the determined coefficients. The corrected change of the magnetic reference field pattern can then be used for obtaining more accurate PRF thermometry results. Steps of the method for operating the magnetic resonance imaging system 10 for guiding the high-intensity focused ultrasound therapy system are illustrated in Fig. 3.

It shall be understood that the high-intensity focused ultrasound therapy system is in an operational state including a device for generating the high-intensity focused ultrasound waves, and wherein the magnetic resonance imaging system 10 is operable and ready for acquiring magnetic resonance signals, the magnetic field probe system 30 is in an operational state with the magnetic field evaluation unit 34 ready to acquire signals from the installed plurality of magnetic field probes 32, and the subject of interest 20, the patient, is positioned within the examination space 16 of the MR scanner 12.

In a first step 38, the reference magnetic resonance image I _re f is acquired with the MR imaging system 10 at the reference point in time t _re f, and then stored in a memory 28 of the image processing unit 26. At the third point in time t ₃ , which is in proximity to the reference point in time t _re f, signals are acquired from the plurality of magnetic field probes 32 in another step 40 as magnetic field reference signals B _re f, which are then stored in a memory 36 of the magnetic field evaluation unit 34.

In a next step 42, the hyperthermia therapy is started at the start point in time ts by applying high-intensity focused ultrasound waves to the subject of interest 20.

Sometime after the start point in time ts of the hyperthermia therapy and within its duration, another magnetic resonance image I ₂ is acquired in another step 44 at the second point in time t ₂ with the MR imaging system 10. The second point in time t ₂ is therefore later than the reference point in time t _re f.

As another step 46, at the fourth point in time U which is in proximity to the second point in time t ₂ , signals B _re f' are acquired from the plurality of magnetic field probes 32.

In the next step 48 then, the magnetic field evaluation unit 34 determines, based on the signals B _re f' acquired from the plurality of magnetic field probes 32 at the fourth point in time U relative to the magnetic field reference signals B _re f, and also on the known spacing between the magnetic field probes 32, coefficients of first-order and second-order spatial variations of the magnetic field within the examination space 16 in various directions. As is well known to the one skilled in the art, a coordinate basis may be employed that ensures that the coefficients determined for the first-order and second-order spatial variations in the same direction are independent from each other, which allows for an easy evaluation of a contribution of the various orders of spatial variation to the total magnetic field variation in this direction. Based on the determined coefficients, the magnetic field evaluation unit 34 in a next step 50 corrects a change of the magnetic reference field pattern and stores the corrected magnetic reference field pattern as a new, dynamically acquired magnetic reference field pattern in the memory 36 of the magnetic field evaluation unit 34 (Fig. 1).

In another step 52, the reference magnetic resonance image I _re f that has been acquired at the reference point in time t _re f is adjusted by the image processing unit 26, based on the corrected magnetic reference field pattern from the memory 36 of the magnetic field evaluation unit 34.

In a final step 54 then, the image processing unit 26 subtracts the adjusted reference magnetic resonance image I _re f' from the magnetic resonance image acquired at the second point in time t ₂ . By this, a contribution to the change of a phase of the magnetic resonance image acquired a second point in time t ₂ caused by the spatial variations of the magnetic field within the examination space 16 is mainly corrected for, so that a subsequent proton resonance frequency thermometry can provide more accurate thermographic image data with reduced temperature artifacts.

The steps 44-54 described above are repeatedly carried out during the hyperthermia therapy session, guided by further magnetic resonance signals acquired during this period. The hyperthermia therapy session may last as long as about sixty minutes or more, so that a difference in time Δ between the third point in time t ₃ , at which the magnetic reference field B _re f was acquired, and in proximity to which the first reference magnetic resonance image I _re f was determined, and the fourth point in time t ₄ , at which the dynamically acquired reference field pattern B _re f' was acquired, is as long as sixty minutes or more (Fig. 4).

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. REFERENCE SYMBOL LIST

10 MR imaging system Iref reference magnetic resonance image

12 MR scanner Iref' reference magnetic resonance image (adjusted)

14 main magnet h second magnetic resonance image

16 examination space Bref reference signals from 32

18 center axis Bref' signals from 32

20 subject of interest tref reference point in time

22 magnetic gradient coil system t ₂ second point in time

24 MR imaging system control unit t ₃ third point in time

26 image processing unit U fourth point in time

28 memory of IPU ts start point in time of

hyperthermia therapy

30 magnetic field probe system Δ difference in time (between t ₃ and t ₄ )

32 magnetic field probe

34 magnetic field evaluation unit

36 memory of 34

38 step of acquiring a reference MR

image

40 step of acquiring signals from 32

(at t ₃ )

42 step of starting hyperthermia

therapy

44 step of acquiring another MR

image

46 step of acquiring signals from 32

(at t ₄ )

48 step of determining coefficients

50 step of correcting the MRFP and

storing corrected MRFP in