Feasibility of strong diffusion encoding and fast readout using a plug-and-play head gradient insert at 7 T

Diffusion weighting is achieved by the application of external field gradients typically for tens of milliseconds, during which the signal also substantially decays due to inherent T₂ relaxation. Employing strong gradients will shorten the echo time (TE) and increase the signal-to-noise ratio. This study demonstrates a proof-of-principle of using an ultrastrong head gradient insert for diffusion MRI at 7 T.

A lightweight (45 kg) single-axis gradient coil operating in the z-direction was interfaced with a 7 T system and operated as an additional fourth-gradient axis to the whole-body gradient setup. Several diffusion MRI experiments were conducted to compare acquisitions taken with a gradient insert operating at full capacity (200 mT/m and 1300 T/m/s) to those using conventional gradient strengths (40-80 mT/m and 200 T/m/s).

A shorter TE of (28.8, 32.4, 38.5, 48.7) ms was achieved using the insert gradient at full capability for readout and diffusion encoding, compared to (49.7, 54.5, 65.9, 85.5) ms (80 mT/m) or (59.9, 68.8, 88.3, 120.9) ms (40 mT/m) for b = (0, 500, 1000, 3000, 10 000) s/mm² using conventional gradient strength. A SNR increase for all b-value acquisitions was observed. When using strong gradients for readout, TE was shortened by 20 ms, and a lower degree of geometrical distortions was observed.

The results illustrate a successful proof-of-concept for performing diffusion MRI using a plug-and-play head gradient insert at 7 T. Use of low-cost gradient inserts could make advanced diffusion MRI experiments more widely available.