Nuclear magnetic resonance imaging apparatus and rf shimming method

Patent metadata

Field	Value
Publication number	US-2017108564-A1
Application number	US-201515127866-A
Country	US
Kind code	A1
Filing date	Mar 25, 2015
Priority date	Apr 16, 2014
Publication date	Apr 20, 2017
Grant date	—

Abstract

The present invention provides an RF shimming method for a nuclear magnetic resonance imaging apparatus comprising: a transmission coil having a plurality of channels that respectively transmit high frequencies to an object and a calculation unit performing RF shimming calculation that determines at least one of amplitudes and phases of the high frequencies to be transmitted respectively to a plurality of the channels so as to improve homogeneity of a high-frequency magnetic field distribution generated by the transmission coil and reduce a specific absorption ratio of the object. Objective function parameters for setting the objective function are determined according to contribution to the SAR for each of the channels during the RF shimming calculation based on the objective function and the restriction condition.

First claim

1 . A nuclear magnetic resonance imaging apparatus comprising:
a transmission coil having a plurality of channels that respectively transmit high frequencies (RF) to an object; an RF shimming section determining at least one of an amplitude and a phase of the high frequencies to be transmitted respectively to a plurality of the channels as a high frequency magnetic field condition based on an objective function and a restriction condition so as to improve homogeneity of a high-frequency magnetic field (B 1 ) distribution that the transmission coil generates and to reduce a specific absorption ratio (SAR) of the object; and a high-frequency signal generator generating the high frequencies for each of the channels based on the high frequency magnetic field condition, wherein the RF shimming section determines objective function parameter values for each of the channels for setting the objective function according to contribution to the specific absorption ratio.

2 . The nuclear magnetic resonance imaging apparatus according to claim 1 ,
wherein the objective function parameters includes a distribution weight (w), a weight (w amp ) of each of the channels, and an exponent (k),
the objective function is a linear combination in which a term showing reduction of the specific absorption ratio and a term showing improvement of the homogeneity are weighted with the distribution weight (w),
the term showing the reduction of the specific absorption ratio is formed by performing weighted addition for the exponential (the k-th power) of the amplitude of the high frequencies for each channel with the weight (w amp ), and
the RF shimming section determines the high frequency magnetic field condition as a solution that optimizes the objective function.

3 . The nuclear magnetic resonance imaging apparatus according to claim 2 ,
wherein the RF shimming section determines a value of the weight (w amp ) of each of the channels according to contribution to the specific absorption ratio.

4 . The nuclear magnetic resonance imaging apparatus according to claim 3 ,
wherein the RF shimming section sets a weight (w amp ) of a channel whose contribution to the specific absorption ratio is relatively large so as to be greater than a weight (w amp ) of a channel whose contribution to the specific absorption ratio is relatively small.

5 . The nuclear magnetic resonance imaging apparatus according to claim 3 ,
wherein the RF shimming section sets a weight (w amp ) of a channel disposed in a position relatively closer to the object so as to be greater than a weight (w amp ) of a channel disposed in a position relatively farther from the object.

6 . The nuclear magnetic resonance imaging apparatus according to claim 4 ,
wherein the transmission coil has upper channels disposed on the upper side and lower channels disposed on the lower side in a magnetic field space, and the RF shimming section sets weights (w amp ) of the lower channels so as to be relatively greater than weights (w amp ) of the upper channels.

7 . The nuclear magnetic resonance imaging apparatus according to claim 4 ,
wherein the RF shimming section has a condition storage part that stores the objective function parameter values for each of the imaging sites and extracts a weight (w amp ) of each of the channels from the condition storage part according to an imaging site set by an operator and sets the weight.

8 . The nuclear magnetic resonance imaging apparatus according to claim 2 ,
wherein the RF shimming section receives input of the objective function parameter values by an operator in order to set the objective function based on the input objective function parameter values.

9 . The nuclear magnetic resonance imaging apparatus according to claim 5 , comprising:
an imaging position setting section that acquires a positioning image in order to determine a position for imaging the object and determines a disposition position of the object in a magnetic field space based on the positioning image, wherein the RF shimming section previously stores positions in which the respective channels of the transmission coil are arranged in the magnetic field space, evaluates distances between the object and each of the channels based on the object disposition position in the magnetic field space and the disposition positions of each of the channels in the magnetic field space, and then sets a weight (w amp ) of a channel disposed in a position relatively closer to the object so as to be greater than a weight (w amp ) of a channel disposed in a position relatively farther from the object.

10 . The nuclear magnetic resonance imaging apparatus according to claim 1 ,
wherein the RF shimming section acquires the specific absorption ratio of the respective channels of the transmission coil and sets a weight (w amp ) of a channel whose specific absorption ratio is relatively large so as to be greater than a weight (w amp ) of a channel whose specific absorption ratio is relatively small.

11 . A method for RF shimming in a nuclear magnetic resonance imaging apparatus that includes a transmission coil having a plurality of channels for respectively transmitting high frequencies (RF) to an object and a calculation unit performing RF shimming calculation that determines at least one of amplitudes and phases of the high frequencies to be transmitted respectively to a plurality of the channels so as to improve homogeneity of a high-frequency magnetic field distribution (B 1 ) generated by the transmission coil and to reduce a specific absorption ratio (SAR) of the object, the method comprising:
determining objective function parameter values for setting an objective function according to contribution to the specific absorption ratio for each of the channels during the RF shimming calculation based on the objective function and a restriction condition.

12 . The method for RF shimming according to claim 11 ,
wherein the objective function parameter values includes a distribution weight (w), a weight (w amp ) of each of the channels, and an exponent (k), the objective function is a linear combination in which a term showing reduction of the specific absorption ratio and a term showing improvement of the homogeneity are weighted with the distribution weight (w), the term showing the reduction of the specific absorption ratio is formed by performing weighted addition for the exponential (the k-th power) of the amplitude of the high frequencies for each of the channels with the weight (w amp ), and a weight (w amp ) of a channel whose contribution to the specific absorption ratio is relatively large is set so as to be greater than a weight (w amp ) of a channel whose contribution to the specific absorption ratio is relatively small.

13 . The method for RF shimming according to claim 12 ,
wherein a weight (w amp ) of a channel disposed in a position relatively closer to the object is set so as to be greater than a weight (w amp ) of a channel disposed in a position relatively farther from the object.

Assignees

• Hitachi Ltd

Inventors

• Ito Kosuke

Classifications

• A61B5/055

  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging · CPC title

• G01R33/288

  Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room · CPC title

• G01R33/3607 Primary

  RF waveform generators, e.g. frequency generators, amplitude-, frequency- or phase modulators or shifters, pulse programmers, digital to analog converters for the RF signal, means for filtering or attenuating of the RF signal · 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

• A61B5/0033

  Features or image-related aspects of imaging apparatus, e.g. for MRI, optical tomography or impedance tomography apparatus; Arrangements of imaging apparatus in a room · CPC title