Specific absorption rate benefits of including measured electric field interactions in parallel excitation pulse design

Abstract

Specific absorption rate management and excitation fidelity are key aspects of radiofrequency pulse design for parallel transmission at ultra-high magnetic field strength. The design of radiofrequency pulses for multiple channels is often based on the solution of regularized least-squares optimization problems for which a regularization term is typically selected to control the integrated or peak pulse waveform amplitude. Unlike single-channel transmission, the specific absorption rate of parallel transmission is significantly influenced by interferences between the electric fields associated with the individual transmission elements, which a conventional regularization term does not take into account. This work explores the effects upon specific absorption rate of incorporating experimentally measurable electric field interactions into parallel transmission pulse design. Results of numerical simulations and phantom experiments show that the global specific absorption rate during parallel transmission decreases when electric field interactions are incorporated into pulse design optimization. The results also show that knowledge of electric field interactions enables robust prediction of the net power delivered to the sample or subject by parallel radiofrequency pulses before they are played out on a scanner.

Figure 1

Simulation setup. Setup A, B and C represent different coil configurations which are used on a water block phantom (a, b, c) and a human mesh (d). e and f show the phase and the magnitude of the B₁₊ map for setup C in the water phantom.

Figure 2

a: Constant density spiral-in excitation k-space trajectory used in simulations (acceleration factor R = 1). b: Desired excitation profile. c, d: Bloch equation simulation results for RF pulses designed with conventional (c) and proposed (d) methods for setup C.

Figure 3

Phantom experiment setup. a: 8 channel transmit-receive coil array. b: Cylindrical water phantom. c: B₁₊ amplitude map for each element of the array. d: B₁₊ phase map for each element of the array.

Figure 4

Additional inputs used in pulse design for the phantom experiments. a: Desired excitation profile. b: Measured off-resonance map. c: Calibrated power correlation matrix of the phantom-coil setup. d: Variable density spiral-in excitation k-space trajectory used in experiments.

Figure 5

Global SAR when the power correlation matrix is incorporated into parallel RF pulse design. Values are reported as the percent improvement in global SAR with respect to using a conventional regularization term for the same flip angle and excitation fidelity. Results of different RF pulse design methods and acceleration factors for three different transmit array setups are shown for the water phantom (a) and the human model (b). Calculated power correlation matrices for three different array setups are shown for the water phantom (c) and the human model (d).

Figure 6

a, b Bloch equation simulation results for NRMSE-aligned RF pulses calculated with conventional (a) and proposed (b) regularization terms. c: Shimmed reference image used for transmit sensitivity mapping. d, e: MR images obtained using parallel RF pulses designed with conventional (d) and proposed (e) approaches as saturation pulses. f,g,h,i: Flip angle (f,g) and phase (h,i) maps of the conventional RF design with transmit voltage 135V (f,h) and the proposed RF design with transmit voltage 130V (g,i).

Figure 7

a: Amplitude of the designed RF pulse waveforms in one of the transmit channels. b: Linearity of the system and RF pulse design process with respect to transmit voltage. c,d: Measured and predicted net power (in kW) of the transmit array with a 7 ms LCLTA RF pulse. c: Conventional RF design method with transmit voltage 135V. d: Proposed RF design method with transmit voltage 130V.