Specific absorption rate reduction using nonlinear gradient fields

Abstract

The specific absorption rate is used as one of the main safety parameters in magnetic resonance imaging. The performance of imaging sequences is frequently hampered by the limitations imposed on the specific absorption rate that increase in severity at higher field strengths. The most well-known approach to reducing the specific absorption rate is presumably the variable rate selective excitation technique, which modifies the gradient waveforms in time. In this article, an alternative approach is introduced that uses gradient fields with nonlinear variations in space to reduce the specific absorption rate. The effect of such gradient fields on the relationship between the desired excitation profile and the corresponding radiofrequency pulse is shown. The feasibility of the method is demonstrated using three examples of radiofrequency pulse design. Finally, the proposed method is compared with and combined with the variable rate selective excitation technique. Magn Reson Med 70:537-546, 2013.

Experimental O‐space turbo spin echo imaging

“…More recently, scan time has been reduced by spatial encoding with nonlinear magnetic fields, such as in Pat-Loc imaging (14), O-space imaging (15,16), null space imaging (17), four-dimensional radial in/out(4D-RIO) (18), echo planar imaging (EPI)-PatLoc (19)(20)(21), and others (22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). Here we focus on O-space imaging, which encodes spatial information through a radially varying magnetic gradient field B i ¼ z 2 -1/2((x -x i ) 2 þ (y À y i ) 2 ) centered at different center placements (x i , y i ), which form a ring about the center of the field of view (15).…”

Simultaneous use of linear and nonlinear gradients for B1+ inhomogeneity correction

“…Kopanoglu et al have also introduced the excitation k ‐space variables for N‐SEMs . These authors used a coordinate transformation from linear spatial variables to nonlinear spatial variables by assuming that encoding fields are bijective, which is generally not the case for higher order spherical harmonics.…”

“…Some studies that include the phase accumulation due to N‐SEM channels in the RF pulse design problem already exist . Some other studies explicitly define the excitation k ‐space variables for N‐SEMs, which satisfies the Fourier transform relationship between the excitation k ‐space and transverse magnetization distribution under special conditions . In our study, the independent excitation k ‐space dimension for each N‐SEM is defined when they are simultaneously used with L‐SEMs.…”

Mitigation of B1+ inhomogeneity using spatially selective excitation with jointly designed quadratic spatial encoding magnetic fields and RF shimming

“…pTx methods (13,(21)(22)(23)(24)(25) provide more degrees of freedom for RF pulse design because they enable different RF waveforms being transmitted through each independent transmit coil element. Nonlinear spatial encoding magnetic fields (SEMs) have been used for selective magnetization excitation to reduce RF pulse length if the target excitation profile is a function of a linear combination of SEMs (26)(27)(28)(29)(30)(31). Driving linear and quadratic SEMs between two excitation pulses can generate a spatially dependent transverse magnetization phase distribution that counteracts B þ 1 inhomogeneities to achieve a homogeneous flip angle distribution (32).…”