# US 2013/0278262 A1 · Passive b1 field shimming

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- Number: US 2013/0278262 A1
- Document ID: US20130278262A1
- Status: Published application
- Kind: A1
- Application number: 13/993,689
- Filed: 2011-12-05
- Published: 2013-10-24
- Earliest claimed: 2010-12-16
- Assignees: Koninklijke Philips Electronics N.V.
- Inventors: Zhiyong Zhai; Michael Andrew Morich; Paul Royston Harvey; Miha Fuderer
- Web page: https://patentgrep.com/patent/US20130278262A1

## CPC

- G01R 33/3873: using ferromagnetic bodies {; Passive shimming}
- G01R 33/34: Constructional details, e.g. resonators {, specially adapted to MR}
- G01R 33/543: {Control of the operation of the MR system, e.g. setting of acquisition parameters prior to or during MR data acquisition, dynamic shimming, use of one or more scout images for scan plane prescription (G01R33/546 takes precedence)}
- G01R 33/5659: {caused by a distortion of the RF magnetic field, e.g. spatial inhomogeneities of the RF magnetic field (G01R33/56509, G01R33/56518, G01R33/56536 take precedence)}
- G01R 33/246: {Spatial mapping of the RF magnetic field B1}
- G01R 33/34076: {Birdcage coils}
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)

## Abstract

Coil elements (18) generate a B1 excitation field in an examination region (14), which B1 excitation field is distorted by patient loading (e.g., wavelength effects). Passive shimming elements (22, 24) are disposed between the coil elements and the subject in order to improve the B1 field uniformity. In one embodiment, passive shimming elements include one or more dielectric rods (55) disposed below the subject which generate no substantial MR proton signal and which have a permittivity of at least 100 and preferably greater than 500. In another embodiment, tubes (24) adjacent each coil element are supplied with a dielectric liquid, a thickness of the dielectric liquid between the coil element and the subject adjusting a phase of the B1 field generated by the coil element. Active B1 shimming may be combined with passive shimming elements (22, 24) to effect an improved RF field homogeneity result.

## Claims

### Claim 1

A magnetic resonance system, comprising:
radiofrequency (RF) coil elements adjacent to an examination region which generates a B1 excitation field in the examination region according to a generated excitation signal; and
at least one passive shimming device disposed in the examination region between the at least one RF coil and a subject which improve uniformity in the generated B1 excitation field, the at least one passive shimming device having a prearranged position, dimensions, and dielectric permittivity.

### Claim 2 (depends on 1)

The magnetic resonance system according to claim 1, further including:
a shimming processor which analyzes a B1 distribution of the generated excitation field and determines at least one of a position, dimensions, and dielectric permittivity for the at least one passive shimming device which improves a uniformity of the B1 distribution to optimize a B1 homogeneity of the excitation field.

### Claim 3 (depends on 1)

The magnetic resonance system according to claim 1, wherein the at least one passive shimming device is a rod composed of a solid dielectric material.

### Claim 4 (depends on 3)

The magnetic resonance system according to claim 3, further including:
an actuator which positions the dielectric rod in the examination region according to an uniformity distribution of the examination region.

### Claim 5 (depends on 3)

The magnetic resonance system according to claim 3, wherein the at least one dielectric rod is disposed on a lower side of the subject.

### Claim 6 (depends on 3)

The magnetic resonance system according to claim 3, wherein the dielectric permittivity of the dielectric rod is at least 100, preferably greater than 500.

### Claim 7 (depends on 3)

The magnetic resonance system according to claim 3, further including:
a tube or reservoir with a variable volume of dielectric fluid disposed adjacent to each of the coil elements.

### Claim 8 (depends on 1)

The magnetic resonance system according to claim 1, wherein the at least one passive shimming device includes one or more tubes with a volume of dielectric fluid disposed adjacent to the RF coil elements.

### Claim 9 (depends on 7)

The magnetic resonance system according to claim 7, further including:
a reservoir which supplies the dielectric fluid to each of the tubes; and
a fluid controller which controls at least one of the volume of dielectric fluid in each tube and the dielectric permittivity of the dielectric fluid supplied to each tube according to the analyzed uniformity distribution.

### Claim 10 (depends on 1)

The magnetic resonance system according to claim 1, further including:
two or more RF transmitters, each which generates an excitation signal for transmission via the RF coil elements to generate a passively shimmed excitation field.

### Claim 11 (depends on 1)

The magnetic resonance system according to claim 1, wherein the RF coil elements form a birdcage-type RF coil and the two or more RF transmitters each generate an excitation signal for transmission via the birdcage-type RF coil to generate a passively and actively shimmed excitation field.

### Claim 12 (depends on 10)

The magnetic resonance system according to claim 10, wherein the shimming processor controls each of the RF transmitters to generate a unique excitation signal according to the analyzed uniformity distribution to improve homogeneity of the passively shimmed excitation field.

### Claim 13 (depends on 1)

The magnetic resonance system according to claim 1, further including:
a main magnet which generates a static magnetic field of at least 3 Tesla in the examination region; and
at least one RF receiver which receive induced MR signals from examination region resulting from the excitation signal.

### Claim 14

A method for passively shimming a B1 excitation field, comprising:
disposing at least one passive shimming device in an examination region defined inside of coil elements of an RF coil to improve uniformity in the B1 excitation field generated by the RF coil, the at least one passive shimming device having a prearranged position and dielectric permittivity.

### Claim 15 (depends on 14)

The method according to claim 14, further including:
analyzing a B1 distribution of the generated excitation field; and
determining at least one of a position, dimensions, and dielectric permittivity for the at least one passive shimming device which improves a B1 uniformity of the generated excitation field.

### Claim 16 (depends on 14)

The method according to claim 14, wherein the at least one passive shimming device includes at least one rod composed of a solid dielectric material without a substantial MR proton signal and with a permittivity greater than 100 and preferably greater than 500.

### Claim 17 (depends on 14)

The method according to claim 14, wherein the at least one passive shimming device includes tubes each with a volume of dielectric fluid disposed adjacent to elements of the at least one RF coil.

### Claim 18 (depends on 16)

The method according to claim 16, further including:
with a controller, controlling at least one of the volume of dielectric fluid in each tube and a position of the at least one rod.

### Claim 19 (depends on 16)

The method according to claim 16, further including:
selecting one of a plurality of rods of different sizes and/or permittivity in accordance with a size of a subject to be examined; and
positioning the selected rod in the examination region.

### Claim 20

A magnetic resonance system, comprising:
an examination region for receiving a subject to be examined;
a plurality of radiofrequency (RF) coil elements adjacent to the examination region which generate an excitation field in the examination region according to a generated excitation signal;
two or more RF transmitters, each which generates an excitation signal for transmission via at least one of the plurality of RF coils to generate a B1 excitation field;
a passive shimming device including at least one of:
a rod composed of a solid dielectric material without a proton signal and with a permittivity greater than 100 disposed in the examination region at a prearranged position below the subject to passively shim the generated B1 excitation field;
a plurality of tubes configured to receive a variable volume of dielectric fluid disposed adjacent to the plurality of RF coil elements to passively shim the generated B1 excitation field; and
a shimming processor which analyzes uniformity of the B1 excitation field in the examination region and therefrom determines at least one of (a) at least one of a position, dimensions, and/or dielectric permittivity of the rod, (b) the volume of dielectric fluid in each tube and/or the dielectric permittivity of the dielectric fluid supplied to each tube, (c) amplitude and/or phase settings for the two or more RF transmitters.