Performance of receive head arrays versus ultimate intrinsic SNR at 7 T and 10.5 T

Abstract

PurposeWe examined magnetic field dependent SNR gains and ability to capture them with multichannel receive arrays for human head imaging in going from 7 T, the most commonly used ultrahigh magnetic field (UHF) platform at the present, to 10.5 T, which represents the emerging new frontier of >10 T in UHFs.MethodsElectromagnetic (EM) models of 31‐channel and 63‐channel multichannel arrays built for 10.5 T were developed for 10.5 T and 7 T simulations. A 7 T version of the 63‐channel array with an identical coil layout was also built. Array performance was evaluated in the EM model using a phantom mimicking the size and electrical properties of the human head and a digital human head model. Experimental data was obtained at 7 T and 10.5 T with the 63‐channel array. Ultimate intrinsic SNR (uiSNR) was calculated for the two field strengths using a voxelized cloud of dipoles enclosing the phantom or the digital human head model as a reference to assess the performance of the two arrays and field depended SNR gains.ResultsuiSNR calculations in both the phantom and the digital human head model demonstrated SNR gains at 10.5 T relative to 7 T of 2.6 centrally, ˜2 at the location corresponding to the edge of the brain, ˜1.4 at the periphery. The EM models demonstrated that, centrally, both arrays captured ˜90% of the uiSNR at 7 T, but only ˜65% at 10.5 T, leading only to ˜2‐fold gain in array SNR in going from 7 to 10.5 T. This trend was also observed experimentally with the 63‐channel array capturing a larger fraction of the uiSNR at 7 T compared to 10.5 T, although the percentage of uiSNR captured were slightly lower at both field strengths compared to EM simulation results.ConclusionsMajor uiSNR gains are predicted for human head imaging in going from 7 T to 10.5 T, ranging from ˜2‐fold at locations corresponding to the edge of the brain to 2.6‐fold at the center, corresponding to approximately quadratic increase with the magnetic field. Realistic 31‐ and 63‐channel receive arrays, however, approach the central uiSNR at 7 T, but fail to do so at 10.5 T, suggesting that more coils and/or different type of coils will be needed at 10.5 T and higher magnetic fields.

RF coil design strategies for improving SNR at the ultrahigh magnetic field of 10.5 Tesla

“…We have previously reported that, in going from 7T to 10.5T, uiSNR calculated in a human head model increases 1.4-fold peripherally, 2-fold at the edge of the brain, and a 2.6-fold centrally (24), with similar increases calculated for the lightbulb phantom. In this paper, we demonstrate that we come close to capturing this predicted SNR gain with the 16Tx/80Rx configuration.…”

“…The identical 64Rx arrays captured a much larger fraction of the central uiSNR at 7T compared to 10.5T (Figure 7 and also see (24)); this implies that there remains room for improvement.…”

“…The 16 SD elements were tuned and matched at 447 MHz (10.5T 1 H frequency) using a 16-channel vector network analyzer while loaded with a lightbulb-shaped phantom approximating the size and shape of the head and neck. This phantom was filled with a polyvinylpyrrolidone (PVP) solution with measured conductivity and relative permittivity of 0.65 S/m and 47.2, respectively, at 447 MHz (24).…”

“…SNR was calculated following (25). uiSNR was calculated for the lightbulb phantom as described in (24).…”

“…Identical noise images were acquired without an RF excitation pulse and TR=1,000 ms. SNR was calculated following (25). uiSNR was calculated for the lightbulb phantom as described in (24).…”