Passive B1 field shimming

Patent metadata

Field	Value
Publication number	US-9689941-B2
Application number	US-201113993689-A
Country	US
Kind code	B2
Filing date	Dec 5, 2011
Priority date	Dec 16, 2010
Publication date	Jun 27, 2017
Grant date	Jun 27, 2017

Abstract

Coil elements ( 18 ) generate a B 1 excitation field in an examination region ( 14 ), which B 1 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 B 1 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 B 1 field generated by the coil element. Active B 1 shimming may be combined with passive shimming elements ( 22, 24 ) to effect an improved RF field homogeneity result.

First claim

Having thus described the preferred embodiments, the invention is now claimed to be:

1. A magnetic resonance system, comprising:
radiofrequency (RF) coil elements disposed adjacent to an examination region and configured to generate a B 1 excitation field in the examination region according to a generated excitation signal; and
a plurality of solid dielectric rods disposed in the examination region between the at least one RF coil and a subject disposed in the examination region which improve uniformity in the generated B 1 excitation field, the rods having prearranged dimensions and dielectric permittivity;
at least one actuator configured to position the dielectric rods in the examination region between the RF coil elements and the subject and/or replace one or more of the dielectric rods with one or more dielectric rods with different dimensions and/or different permittivity;
a shimming processor configured to analyze a B 1 field distribution of the generated excitation field with the subject disposed in the examination region and determine at least one of position, dimensions, and dielectric permittivity for the at least one dielectric rods and control the at least one actuator to reposition and/or replace at least one of the dielectric rods to improve a uniformity of the B 1 field distribution to optimize a B 1 field homogeneity of the excitation field in the examination region.

2. The magnetic resonance system according to claim 1 , wherein the at least one dielectric rods are disposed on a lower side of the subject.

3. The magnetic resonance system according to claim 1 , wherein the dielectric permittivity of the dielectric rods is at least 100, preferably greater than 500.

4. The magnetic resonance system according to claim 1 , further including:
a tube or reservoir with a variable volume of dielectric fluid disposed adjacent to each of the RF coil elements and wherein the shimming processor is further configured to control a volume of dielectric fluid in the tube or reservoir.

5. 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.

6. The magnetic resonance system according to claim 5 , 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.

7. The magnetic resonance system according to claim 5 , 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.

8. 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.

9. A magnetic resonance system comprising:
radiofrequency (RF) coil elements adjacent to an examination region configured to generate a B 1 excitation field in the examination region according to a generated excitation signal;
one or more tubes configured to hold a dielectric fluid disposed in the examination region between the at least one RF coil and a subject;
a reservoir configured to supply the dielectric fluid to the at least one tube; and
a fluid controller configured to control at least one of a volume of dielectric fluid in the at least one tube and a dielectric permittivity of the dielectric fluid supplied to the one or more tubes to improve uniformity in the generated B 1 excitation field in the examination region.

10. The magnetic resonance system according to claim 9 , further including:
a shimming processor configured to analyze a B 1 distribution of the generated excitation field and control the fluid controller to optimize B 1 homogeneity of the examination field.

11. The magnetic resonance system according to claim 9 , further including:
a main magnet configured to generate a static magnetic field of at least 3 Tesla in the examination region; and
at least one RF receiver configured to receive induced MR signals from the examination region resulting from the excitation signal.

12. A method for passively shimming a B 1 excitation field comprising:
disposing at least one passive shimming device in an examination region defined inside coil elements of an RF coil to improve uniformity in the B 1 excitation field generated by the RF coil, the at least one passive shimming device including at least one of:
tubes with a volume of dielectric fluid disposed adjacent to the coil elements of the at least one RF coil, and
at least one rod composed of a solid dielectric material without a substantial MR proton signal and with a permittivity greater than 100;

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.

13. The method according to claim 12 , further including:
analyzing a B 1 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 B 1 uniformity of the generated excitation field.

14. The method according to claim 12 , 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.

15. 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 the excitation signal for transmission via at least one of the plurality of RF coils to generate a B 1 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 below the subject to passively shim the generated B 1 excitation field and an actuator for repositioning or replacing the rod;
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 B 1 excitation field and a controller for controlling the volume of dielectric fluid in the tubes; and

a shimming processor which analyzes uniformity of the B 1 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, the shimming processor being connected with at least one of the actuator and the controller to control repositioning or replacing the rod and/or controlling the volume of dielectric fluid in the tubes based on the uniformity analysis.

Assignees

• Zhai Zhiyong
• Morich Michael Andrew
• Harvey Paul Royston
• Fuderer Miha
• Koninl Philips Electronics Nv

Inventors

• Zhai Zhiyong
• Morich Michael Andrew
• Harvey Paul Royston
• Fuderer Miha

Classifications

• G01R33/543

  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) · CPC title

• G01R33/5659

  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) · CPC title

• G01R33/246

  Spatial mapping of the RF magnetic field B1 · CPC title

• G01R33/34 Primary

  Constructional details, e.g. resonators {, specially adapted to MR} · CPC title

• G01R33/34076

  Birdcage coils · CPC title