Comparison of simulated parallel transmit body arrays at 3 T using excitation uniformity, global SAR, local SAR, and power efficiency metrics

Abstract

Purpose We compare the performance of 8 parallel transmit (pTx) body arrays with up to 32 channels and a standard birdcage design. Excitation uniformity, local SAR, global SAR and power metrics are analyzed in the torso at 3 T for RF-shimming and 2-spoke excitations. Methods We used a fast co-simulation strategy for field calculation in the presence of coupling between transmit channels. We designed spoke pulses using magnitude least squares (MLS) optimization with explicit constraint of SAR and power and compared the performance of the different pTx coils using the L-curve method. Results PTx arrays outperformed the conventional birdcage coil in all metrics except peak and average power efficiency. The presence of coupling exacerbated this power efficiency problem. At constant excitation fidelity, the pTx array with 24 channels arranged in 3 z-rows could decrease local SAR more than 4-fold (2-fold) for RF-shimming (2-spoke) compared to the birdcage coil for pulses of equal duration. Multi-row pTx coils had a marked performance advantage compared to single row designs, especially for coronal imaging. Conclusion PTx coils can simultaneously improve the excitation uniformity and reduce SAR compared to a birdcage coil when SAR metrics are explicitly constrained in the pulse design.

Analysis of an Integrated 8-Channel Tx/Rx Body Array for Use as a Body Coil in 7-Tesla MRI

“…To cope with the lower efficiency, the power for each element has to be increased. Alternatively, a larger number of elements, including elements placed along the longitudinal axis of the magnet, should be beneficial to lower the power per element [50] and, furthermore, increase the degrees of freedom for transmit field control, leading to better capability to achieve excitation field uniformity. Since there is quite a bit of bore liner surface area that can be covered, a higher number of channels could easily be integrated, while the cabling and the necessary electronics can also be placed in the gap between the gradient coil and the bore liner so as not to take up valuable space in the bore.…”

“…50,000 random shims and for a MLS shim for both the local array and the integrated array including kernel density plots is shown. The center of mass for each set of shims is indicated with a white cross.…”

Realistic modeling of deep brain stimulation implants for electromagnetic MRI safety studies

“…We used a co-simulation approach based on HFSS (ANSYS, Canonsburg PA) for the electromagnetic (EM) field calculation and ADS (Agilent, Santa Clara CA) for the circuit simulation (Kozlov and Turner, 2009; Guérin et al , 2014; Guérin et al , 2015). …”

“…A key component of the method is a topology correction algorithm that automatically removes self-intersections and curvature violations in the path estimated from the CT data (code available at http://ptx.martinos.org/index.php/Main_Page). We model the RF interactions between the DBS implant and the MRI coil using a simulation strategy based on the field solver HFSS (Kozlov and Turner, 2009; Guérin et al , 2014; Guérin et al , 2015), which allows efficient modeling of the smallest implant details (simulation time ~6 hours). We find that detailed modeling of the DBS implant path and internal components has a large impact on the SAR induced at the electrode-tissue interface tip by the MRI RF coil.…”

Pregnant women models analyzed for RF exposure and temperature increase in 3TRF shimmed birdcages

“…Emerging technologies including pTx excitations (more than two channels) and dynamic RF shimming with varying waveforms per channel (25) are beyond the scope of this study, but may be part of future research activities.…”