Static and dynamic non-localized efficiency radio frequency shimming for parallel transmission in magnetic resonance imaging
A non-localized efficiency shimming technique is used to generate radio frequency (RF) shimming values for imaging with a multi-channel transmit RF coil that minimizes subject-specific imperfections in the transmit magnetic field (B1+) and reduces or eliminates signal dropout in the acquired images, while keeping the coil working in an optimal mode with a high transmit efficiency. The non-localized efficiency shimming can be used for both small and large fields-of-view where a specific ROI does not need to be specified. The static non-localized efficiency shim is advantageous for turbo spin echo (TSE) imaging of smaller anatomical targets, whereas the dynamic non-localized efficiency shim is advantageous for larger fields-of-view, such as in human torsos.
1 . A method for generating radio frequency (RF) shimming values for use with a magnetic resonance imaging system, the method comprising:
(a) accessing RF transmit sensitivity profile data with a computer system;
(b) generating RF shimming values with the computer system by:
inputting the RF transmit sensitivity profile data to a non-localized efficiency shimming cost function that penalizes under-flipping based on a minimum flip angle tuning parameter; and
optimizing the non-localized efficiency shimming cost function, generating an output as the RF shimming values, wherein the RF shimming values minimize destructive B1+ interferences within an entire imaging field-of-view; and
(c) storing the RF shimming values for use with an MRI system.
2 . The method of claim 1 , wherein the non-localized efficiency shimming cost function further optimizes for maximum flip angle across modes.
3 . The method of claim 1 , wherein optimizing the non-localized efficiency shimming cost function includes setting at least one constraint on the non-localized efficiency shimming cost function while the non-localized efficiency shimming cost function is being optimized.
4 . The method of claim 1 , wherein the non-localized efficiency shimming cost function penalizes under-flipping without explicitly constraining over-flipping.
5 . The method of claim 1 , further comprising:
accessing the RF shimming values with an MRI system;
controlling the MRI system to acquire k-space data using a pulse sequence that implements the RF shimming values; and
reconstructing an image from the k-space data.
6 . The method of claim 5 , wherein the pulse sequence includes a gradient-recalled echo (GRE) acquisition.
7 . The method of claim 1 , wherein the RF transmit sensitivity profile data comprises RF transmit sensitivity profiles for a plurality of subjects and the non-localized efficiency shimming cost function incorporates the RF transmit sensitivity profiles for a plurality of subjects to generate universal modes.
8 . The method of claim 2 , wherein the non-localized efficiency shimming cost function further optimizes for maximum flip angle across modes based in part on a maximum flip angle tuning parameter.
9 . The method of claim 3 , wherein the at least one constraint comprises a constraint for a desired efficiency in a local region-of-interest using a Rayleigh quotient.
10 . The method of claim 3 , wherein the at least one constraint comprises a constraint for local specific absorption rate (SAR) based on virtual observation points.
11 . The method of claim 5 , wherein the pulse sequence includes a spin echo acquisition.
12 . The method of claim 7 , wherein the non-localized efficiency shimming cost function is optimized across the plurality of subjects.
13 . The method of claim 9 , wherein the Rayleigh quotient is used as a starting point for performing a phase-only RF shim.
14 . The method of claim 11 , wherein the spin echo acquisition is a turbo spin echo (TSE) acquisition.
15 . The method of claim 11 , wherein the spin echo acquisition is a fast spin echo (FSE) acquisition.
16 . The method of claim 12 , wherein optimizing the non-localized efficiency shimming cost function across the plurality of subjects includes constructing a plurality of non-localized efficiency shimming cost functions comprises a different non-localized efficiency shimming cost function for each of the plurality of subjects, and minimizing a Euclidian norm of the plurality of non-localized efficiency shimming cost functions.