SAR reduction in 7T C‐spine imaging using a “dark modes” transmit array strategy

Abstract

Purpose Local specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a “dark mode”) to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional loop elements can lower the local SAR hotspot in a C-spine array at 7 T. Methods We model electromagnetic fields in a head/torso model to calculate SAR and excitation B1+ patterns generated by conventional loop arrays and loop arrays with added electric dipole elements. We utilize the dark modes that are generated by the intentional and inefficient orientation of dipole elements in order to reduce peak 10g local SAR while maintaining excitation fidelity. Results For B1+ shimming in the spine, the addition of dipole elements did not significantly alter the B1+ spatial pattern but reduced local SAR by 36%. Conclusion The dipole elements provide a sufficiently complimentary B1+ and electric field pattern to the loop array that can be exploited by the radiofrequency shimming algorithm to reduce local SAR.

Parallel transmission for ultrahigh‐field imaging

“…from ref. , where dipole antennas are employed in conjunction with loops. Although this design is unconventional, each element in this array is driven independently, and it may be used in exactly the same way as any other PTx array using any of the optimization methods outlined previously.…”

“…Although this design is unconventional, each element in this array is driven independently, and it may be used in exactly the same way as any other PTx array using any of the optimization methods outlined previously. It has been demonstrated that potentially large reductions in local SAR can be achieved. Although these approaches are still in their infancy, previous theoretical studies into optimal current distributions suggest that there are significant benefits yet to be obtained .…”

Partially orthogonal resonators for magnetic resonance imaging

“…On the other hand, all the elements of the partially orthogonal array have sensitivity to the signal, at least partially. This is also distinctive from previous approaches that employed entire elements virtually insensitive to the signal used to reduce E-fields 19 20 .…”

7 Tesla 22‐channel wrap‐around coil array for cervical spinal cord and brainstem imaging

“…An increased number of transmit elements, either ‐producing loops or loops combined with dipole elements , can potentially reduce peak local SAR involved in producing a given distribution. Compared with a simulated one‐panel transmit array, our two‐panel design indeed deposited a lower maximum 10‐gram averaged SAR while producing greater in the center of the neck and the same field in the center of the lower part of the head.…”