Nuclear magnetic resonance imaging apparatus and RF shimming method

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, and

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.

2. The nuclear magnetic resonance imaging apparatus according to claim 1 ,

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.

3. The nuclear magnetic resonance imaging apparatus according to claim 2 ,

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.

4. The nuclear magnetic resonance imaging apparatus according to claim 2 ,

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.

5. The nuclear magnetic resonance imaging apparatus according to claim 3 ,

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.

6. The nuclear magnetic resonance imaging apparatus according to claim 3 ,

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.

7. The nuclear magnetic resonance imaging apparatus according to claim 1 ,

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.

8. The nuclear magnetic resonance imaging apparatus according to claim 4 , 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.

9. 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,

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.

10. The method for RF shimming according to claim 9 , 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.