B.SUB.0 .shimming device for MRI

Abstract

A magnetic resonance (MR) apparatus comprises magnet means for generating a main magnetic field in a sample region, encoding means for generating encoding magnetic fields superimposed to the main magnetic field, RF transmitter means for generating MR radiofrequency fields, driver means for operating said encoding means and RF transmitter means to generate superimposed time dependent encoding fields and radiofrequency fields according to an MR sequence for forming images or spectra; and acquisition means for acquiring an MR signal from said object. The magnet means comprise a primary magnetic field source providing a static magnetic field B.sub.0 and at least one secondary magnetic field source providing an adjustable magnetic field B′. To provide improved shimming, the secondary magnetic field source comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m1 and said second magnetic material having a second magnetic moment density m2, and means for independently adjusting said second magnetic moment density m2 by variation of an external control parameter.

Claims

1. A magnetic resonance (MR) apparatus comprising: a) magnet for generating a main magnetic field in a sample region adapted for introduction of a sample or subject from which a MR signal is to be acquired; b) wherein the apparatus is configured to superimpose encoding magnetic fields onto the main magnetic field; c) RF transmitter configured to generate MR radiofrequency fields; d) driver configured to operate said RF transmitter which is configured to generate radiofrequency fields according to a MR sequence for forming images or spectra and wherein superimposed time dependent encoding fields are generated as a result of operating the driver; and e) wherein said magnet comprises a primary magnetic field source providing a static magnetic field B.sub.0 and at least one secondary magnetic field source providing an adjustable magnetic field B′; wherein said secondary magnetic field source comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m1 and said second magnetic material having a second magnetic moment density m2, and means for independently adjusting said second magnetic moment density m2 by variation of an external control parameter, the apparatus further comprising at least three magnetic field probes configured to allow determination of an instant spatial distribution of the static magnetic field resulting from said static magnetic field B.sub.0 and said adjustable magnetic field B′.

2. The MR apparatus according to claim 1, wherein said second magnetic moment density m2 is adjustable in a range extending from values that are smaller than the first magnetic moment density m1 to values that are larger than the first magnetic moment density m1.

3. The MR apparatus according to claim 1, wherein said external control parameter is selected from the group consisting of temperature, pressure, shear and light illumination prevailing at the secondary magnetic field source and electric current flowing therethrough.

4. The MR apparatus according to claim 3, wherein said external control parameter is the local temperature of said portion of second magnetic material, and wherein said adjusting means are configured to adjust said local temperature in a control temperature range extending from a lowest control temperature T.sub.L to a highest control temperature T.sub.H, and wherein said second magnetic material has a Curie temperature T.sub.C lying within said control temperature range.

5. The MR apparatus according to claim 3, wherein said Curie temperature T.sub.C is in the range from 300K to 450K.

6. The MR apparatus according to claim 1, wherein said secondary magnetic field source is formed as an elongated body having a longitudinal axis, and wherein a plurality of spatially distinct portions of said second magnetic material are arranged at distinct positions along said longitudinal axis.

7. The MR apparatus according to claim 6, comprising a plurality of secondary magnetic field sources arranged circumferentially around the sample region.

8. The MR apparatus according to claim 1, wherein each secondary magnetic field source comprises means for determining an instant value of the external control parameter.

9. The MR apparatus according to claim 1, wherein said second magnetic material is formed of an alloy containing Fe, Ni, Ga, Mn, Gd, Sm, Nd, Dy, Eu or Co.

10. The MR apparatus according to claim 1, wherein the adjustable magnetic field B′ of each secondary magnetic field source in an axial direction z of the static magnetic field B.sub.0 can be switched between positive and negative values as a function of the external control parameter.

11. A method of operating a MR apparatus including a sample region adapted for introduction of a sample or subject, the method comprising: a) providing a static magnetic field B.sub.0 via a primary magnetic field source and at least one adjustable magnetic field B′ via at least one secondary magnetic field source and generating, via said static magnetic field B.sub.0 and said adjustable magnetic field B′, a main magnetic field; b) generating superimposed time dependent encoding fields and MR radiofrequency fields according to an MR sequence for forming images or spectra; and c) acquiring an MR signal from said sample or subject; wherein said secondary magnetic field source in a) comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m1 and said second magnetic material having a second magnetic moment density m2, wherein said second magnetic moment density m2 is independently adjustable by variation of an external control parameter, and determining an instant spatial distribution of the static magnetic field resulting from said B.sub.0 and said B′ via a plurality of at least three magnetic field probes wherein, after said sample or subject has been introduced into the sample region, said external control parameter is adjusted to obtain a predetermined main magnetic field B resulting from the superposition of the static magnetic field B.sub.0 and the adjustable magnetic field B′ of each secondary magnetic field source, wherein the adjustment is carried out at a beginning of a MR sequence for forming images or spectra.

12. The method according to claim 11, wherein said second magnetic moment density m2 is adjusted via the external control parameter between values of m2<m1 and values of m2>m1.

13. The MR apparatus according to claim 3, wherein said secondary magnetic field source is formed as an elongated body having a longitudinal axis, and wherein a plurality of spatially distinct portions of said second magnetic material are arranged at distinct positions along said longitudinal axis.

14. The MR apparatus according to claim 13, wherein said external control parameter is the local temperature of said portion of second magnetic material, and wherein said adjusting means are configured to adjust said local temperature in a control temperature range extending from a lowest control temperature T.sub.L to a highest control temperature T.sub.H, and wherein said second magnetic material has a Curie temperature T.sub.C lying within said control temperature range.

15. The MR apparatus according to claim 13, wherein said Curie temperature T.sub.C is in the range from 300K to 450K.

16. The MR apparatus according to claim 13, comprising a plurality of secondary magnetic field sources arranged circumferentially around the sample region.

17. A magnetic resonance (MR) apparatus with a sample region adapted for introduction of a sample or subject from which a MR signal is to be acquired comprising: a) a primary magnetic field source providing a static magnetic field B.sub.0 and at least one secondary magnetic field source providing an adjustable magnetic field B′ configured to generate a main magnetic field in the sample region, wherein said secondary magnetic field source comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m1 and said second magnetic material having a second magnetic moment density m2, wherein said second magnetic moment density m2 is configured to be independently adjustable by variation of an external control parameter; b) wherein the apparatus is configured to superimpose encoding magnetic fields onto the main magnetic field, c) RF transmitter configured to generate MR RF fields, wherein the RF coil and the RF transmitter are operationally connected to: a driver configured to operate the RF coil and the RF transmitter and which are configured to generate superimposed time dependent encoding fields and RF fields according to an MR sequence configured to form images or spectra; and at least three magnetic field probes configured to determine an instant spatial distribution of the static magnetic field resulting from said static magnetic field B.sub.0 and said adjustable magnetic field B′.

18. The MR apparatus according to claim 17, wherein said secondary magnetic field source is formed as an elongated body having a longitudinal axis, and wherein a plurality of spatially distinct portions of said second magnetic material are arranged at distinct positions along said longitudinal axis.

19. The MR apparatus according to claim 18, comprising a plurality of secondary magnetic field sources arranged circumferentially around the sample region.

20. The MR apparatus according to claim 17, wherein said second magnetic material is formed of an alloy containing Fe, Ni, Ga, Mn, Gd, Sm, Nd, Dy, Eu or Co.

21. The method according to claim 11, wherein the adjustable magnetic field B′ of each secondary magnetic field source in an axial direction z of the static magnetic field B.sub.0 can be switched between positive and negative values as a function of the external control parameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better understood by reference to the following description of embodiments of this invention taken in conjunction with the accompanying drawings, wherein are shown:

(2) FIGS. 1A to 1D: A schematic representation of one embodiment of a secondary magnetic field source providing an adjustable magnetic field. FIG. 1A: device comprising a rod-like object having a central section formed of a second material with a magnetic moment density m2(T) that is temperature dependent. FIG. 1B: here the temperature of the central section has been set to establish a local magnetic moment density that is higher than that of the surrounding parts of the rod, i.e. m2(T)>m. FIG. 1C: here the temperature of the central section has been set to establish a local magnetic moment density that is lower than that of the surrounding parts of the rod, i.e. m2(T)<m. 1D: situation when In the case of a long rod-like object the two terminal magnetic dipoles shown in FIG. 1C would be longitudinally stretched and thus the resulting field surrounding the object would be substantially zero.

(3) FIG. 2 A schematic representation of a further embodiment of a secondary magnetic field source providing an adjustable magnetic field.

(4) FIG. 3 An exemplary arrangement of an MRI apparatus including a plurality of adjustable magnetic field elements.

(5) FIGS. 4A and 4B: FIG. 4A: Typical temperature dependence of the magnetic moment density (m) of a ferromagnetic particle in a strong external field. Towards the Curie temperature (T.sub.C) the strong ferromagnetic moment diminishes. Above T.sub.C the paramagnetism scales still down with temperature. FIG. 4B: T.sub.C of cupric alloys can be adjusted by the admixture of copper. Thereby the temperature range in which a magnetic particle has to be controlled can be adjusted.

(6) FIG. 5 Spatial distribution of magnetic material. The distribution of the static material (m) and the controllable units (m.sub.c(T)) settled at half of their maximum moment provides zero magnetic field outside the cylindrical structure given it is much longer than the volume of interest. Lowering one unit's temperature provides then a net field of a paramagnet and increasing the temperature that of a net strong diamagnet. The shim units can thereby adjust the fields with both signs (bipolarly).

(7) FIGS. 6A to 6C: Setup. FIG. 6A shows an embodiment in which controllable units were made of a (3 mm).sup.3 Ni75Cu25 shot thermally coupled to SMD resistors. A Pt500 measures its temperature. 50 μm diameter wires are used for connecting. A wrapper of foam is applied for isolation and aluminum foil for RF shielding. FIG. 6B: Photograph of a unit. FIG. 6C: setup for the measurements with 3 controllable units. The static material distribution has been discretized into shots of half the volume than the units because no wire material was available.

(8) FIGS. 7A to 7C: FIG. 7A: Field as recorded by the field probes heating m1-m3 sequentially. FIG. 7B: B.sub.0 field maps in s1-s4 acquired as marked in a). Isolines at 0±15±30±50±100±200 Hz. The fields show a distinct dipolar pattern in the range of ±300 Hz. Close range ripples (solid arrow) result from the discretization of the magnetization distribution into shots. Casting the material into cylinders should remove this problem. FIG. 7C shows off-resonance line profiles 4 cm distant from the shim units.

DETAILED DESCRIPTION OF THE INVENTION

(9) In the following we will use temperature T as a representative example of an external control parameter that influences the magnetic moment density of a material of interest. As explained earlier, there are other quantities that could be used as control parameter instead of temperature, so the following examples shall not be construed as a limitation to the use of temperature as the control parameter.

(10) The basic principle of an adjustable secondary magnetic field source is illustrated in FIGS. 1A-D. The device shown in FIG. 1A comprises a rod-like object formed predominantly of a first material with a substantially uniform magnetic moment density m1=m and having a central section formed of a second material with a magnetic moment density m2(T) that is temperature dependent. The device is placed in an external magnetic field B.sub.0 oriented in z-direction.

(11) FIG. 1B shows the situation where the temperature of the central section has been set to establish a local magnetic moment density that is higher than that of the surrounding parts of the rod, i.e. m2(T)>m. This situation corresponds to an arrangement wherein one magnetic dipole is located in the central part of the rod-like object. In contrast, FIG. 1C shows the situation where the temperature of the central section has been set to establish a local magnetic moment density that is lower than that of the surrounding parts of the rod, i.e. m2(T)<m. This situation corresponds to having a pair of magnetic dipoles located at the two ends of the rod-like object with a gap in the central part having a magnetic field distribution corresponding to an arrangement with one centrally arranged magnetic dipole now pointing in the opposite direction as compared to FIG. 1B. It should be noted that in the case of a long rod-like object the two terminal magnetic dipoles shown in FIG. 1C would be longitudinally stretched and thus the resulting field surrounding the object would be substantially zero. This would effectively lead to a situation as shown in FIG. 1D.

(12) If the second material exhibits ferromagnetic behavior, the high magnetic moment situation shown in FIG. 1B can be established by keeping the local temperature T below the Curie temperature T.sub.c whereas the low magnetic moment situation shown in FIG. 1C can be established by keeping the local temperature T above the Curie temperature T.sub.c.

(13) From a comparison of FIG. 1B on the one hand and FIGS. 1C and 1D on the other hand, it is seen that the secondary magnetic field in the region surrounding the central section has opposite directions in the two cases.

(14) A more complex situation is shown schematically in FIG. 2. Here a rod-like device comprises an elongated element formed of a first magnetic material having a first material M1 with a substantially uniform magnetic moment density m1=m and two portions of a second magnetic material M2 arranged axially displaced from each other at axial positions z.sub.a and z.sub.b, respectively. In the situation shown in FIG. 2, the portions shown at the left and right, respectively, are set to different temperatures T.sub.low and T.sub.high, respectively. In particular, T.sub.low and T.sub.high can be adjusted to be below and above the Curie temperature T.sub.c, respectively. With such an arrangement it is possible to generate secondary magnetic fields with disparate local patterns. In the example shown, it is indicated schematically how the secondary magnetic field in the neighborhood of the portion at z.sub.a has a component B.sub.z(z.sub.a) directed in positive z-direction, whereas in the neighborhood of the portion at z.sub.a the secondary magnetic field has a component B.sub.z(z.sub.b) directed in negative z-direction.

(15) An exemplary arrangement of an MRI apparatus including a plurality of adjustable magnetic field elements acting as secondary magnetic field sources for providing an adjustable magnetic field B′ is shown in FIG. 3. The arrangement comprises a radiofrequency (RF) coil 1 and a cylindrically formed element 2 made of a first magnetic material M1 and provided with a plurality of distinct shimming portions 3 of a second magnetic material M2. The element 2 is arranged surrounding a schematically shown sample or object zone 4. As generally known for such arrangements, the RF coil 1 is provided with a plurality of capacitor elements 5.

Example 1

Magnetic Pebbles—Materials with Controllable Magnetism for Compact, Low-Power Shim Units

(16) Introduction

(17) Susceptibility induced off-resonances challenge many cutting-edge applications using single shot read-outs, balanced acquisitions or high-resolution spectroscopy, in particular at ultra-high fields. B.sub.0 shimming with increased number of channels yields critical improvements [8], but significantly reduces free bore diameter. Coil conductors in close proximity to the subject [9] or even on the RF coils [4] reduce space and power requirements. However, unwanted interactions with RF and gradient operation was reported as a main issue and the handling is aggravated by the amount of conductors in the unit and the large number of high-current wires routed through the bore. Furthermore highly stabilized current supplies need to be fitted in the technical room.

(18) As an alternative we present the integration of ferromagnetic materials whose magnetization can be accurately controlled in-situ as opposed to traditional passive shims [1]. Thereby the secondary field produced by this material is used to shim highly localized. The proposed geometric arrangement allows producing fields with both polarities.

(19) Methods

(20) The magnetic moment density (m) of a ferromagnetic particle in a strong external magnetic field can be controlled by its temperature ([10], FIG. 4A). At Curie temperature (T.sub.C) the ferromagnetism vanishes and renders the material paramagnetic. By heating its magnetic moment is gradually reduced by more than an order of magnitude, correspondingly the secondary fields nearly vanishes. Hence by controlling the material temperature a shim field can be tailored.

(21) For obtaining a reasonable temperature range, Nickel and Copper were alloyed to shots in a Ni75Cu25 stoichiometry [7] resulting in a T.sub.C of 350° K (FIG. 4B). Each shot of ˜(3 mm).sup.3 is restively heated, its temperature is measured by a Pt500 thermistor and it is thermally isolated and shielded (FIGS. 6A and 6B).

(22) To produce fields of both polarities using materials with only positive susceptibilities, the magnetic material is arranged in a matrix such that its net secondary field is uniform when the magnetization of the heated particles is roughly halved (e.g. FIG. 5). In a first example, 35 particles were arranged on the axis of the main magnetic field by press fitting in a wooden bar with 16 mm distance (FIG. 6C). Three controllable magnetic particles (having 2 m at low temperatures) were mounted in the center-slots, obtaining from each a net field outside the cylinder of approximately a dipole with a magnetic moment of −m to +m.

(23) The secondary field of the shim unit was measured by B.sub.0 mapping (Philips 3T Achieva, Best Netherlands) with 3 ms echo-spacing in a phantom bottle placed directly on top of the unit. 3 magnetic field probes (Skope MRT, Zurich, Switzerland) were each placed about 1 cm from each unit and 2 on top of the bottle.

(24) Results

(25) FIGS. 7A and 7B show the net B.sub.0 fields induced by the three shim units temporally and spatially. Voltages from 0-10 V were applied in steps resulting in a temperature range from 293-420° K. The surface of the unit did warm up hardly noticeably. The slew rate was about 1 μT/s. FIG. 7B shows field profiles in about 4 cm distance from the units. The ripples in the shim field next to the unit in the sagittal images result from the discretization of the magnetic moment distribution from a continuous cylinder into individual shots. They are expected to be drastically reduced once the NiCu is cast into a cylindrical geometry.

(26) Discussion

(27) Particles with controllable magnetism produce shim-field patterns with high spatial degrees of freedom. Since the source of the field is not an electric current but the magnetism of the material, smaller form factors and lower current consumptions are achieved and the particles are well decoupled from gradient and shim as well as from RF coils. Opposed to passive shims, rearranging the material is not required to fit subject specific susceptibility distributions.

(28) The heat required to control the units can be administered by DC and AC currents as well as optically tunable materials can be employed [11]. Furthermore the power delivery for the heating can be efficiently modulated by switched mode schemes such as by PWM current sources similarly as used for LED lightings where tens of channels can be housed in a single IC package. This allows placing the required electronics in the bore which dramatically reduces the involved cabling efforts.

(29) Very large numbers of independent shim channels can be integrated in RF coils with low additional weight and space requirements. Thereby high degrees of freedom can be obtained therefore the approach is expected to be well suited for shimming of susceptibility induced off-resonances i.e. in the prefrontal cortex, ear channels or the spine.

Example 2

Adaptive Shimming Procedure

(30) Preparations

(31) The dependence and spatial pattern of the fields induced by the individual shim units in the volume of interest in the subject have to be known in advance. There are various methods for obtaining this information. Typically this information will be gathered during the design or installation/maintenance procedure of the device.

(32) The field patterns can be either obtained by magnetostatic field simulations/calculations using the knowledge of the geometrical distribution of the employed materials and its magnetic properties. Alternatively, the fields can be measured using a field camera, a scanning magnetic field probe or an MRI based tomographic procedure (B0 mapping sequence, [12, 13]) performed on a phantom or in-vivo/in-situ. For isolating the fields induced by the shimming units from the field induced by other structures and magnets, the field can be measured with different control parameter values or with and without shimming unit present and the measurements can subsequently be compared.

(33) Similarly, the dependence of the induced shimming field on the control parameter can be directly measured in an MRI scanner using one of the method mentioned above. Thereby it might be sufficient in many cases to acquire a calibration curve for the employed material and using this information together with the spatial distribution of the material to calculate an estimate of the induced fields.

(34) Furthermore, in most cases the spatial distribution of the induced fields is linearly dependent on the magnetic moment of the employed material at the given control variable. Therefore the acquisition/estimate of the field distribution can be separated from the estimation of the magnetic moment of the material. Consequently, only few field maps covering the entire volume are required.

(35) Finally, since the induced correction fields are in most cases very small compared to the background fields, the individual shim units will only interact very weakly. Consequently, the induced total fields of an array of such shim units will be linearly dependent on the induced field of each unit alone. This linear relation significantly simplifies the calibration procedure in that each unit can be measured separately and accounted according to its geometric position relative to the volume of interest. Furthermore, the calculation of the induced total field is greatly simplified too, which makes the subsequently described optimization procedures much simpler.

(36) Procedure

(37) Before starting the intended MR signal acquisition, the B0 field in the volume of interest is measured in-situ. I.e. the subject/sample is positioned in the scanner as suitable for the subsequent scanning procedure. Well known measurement procedures for active shimming can be employed [13-16] by which the field is obtained on full grid or on projections to appropriate basis functions.

(38) If the shimming units are mounted on movable parts such as a local RF coil or the patient support, the position of the shim units or at least the entire array has to be determined. This can be achieved either by optical markers for positioning or referencing to light visors, direct mechanical or optical measurements or by acquiring and evaluating the signal from NMR active fiducial markers or field probes during positioning scans.

(39) Using the knowledge of the field induced by each shim unit in the sample, the control value for each shim unit can be calculated such that the field distribution in the volume of interest will approximate the given target distribution (typically a uniform field) best. For this purpose, well-known optimization techniques can be employed. In most cases it will be beneficial to employ constrained optimization algorithms incorporating the maximum fields that can be induced by each shimming unit.

(40) The calculated control values are then applied to the shim units.

(41) The procedure described above can then be iterated in order to optimize the accuracy of the field correction. In other words, once the calculated control values have been applied to the shim units, one can repeat the in situ measurement of B0 in the volume of interest and if the deviation from the target distribution is larger than a pre-defined threshold, one can calculate and apply a refined set of control values.

(42) Once the B0 field in the volume of interest is deemed acceptable, one can proceed to carry out the intended MRI signal acquisition.

REFERENCES

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