Single-shot echo-planar imaging (ss-EPI) is routinely utilized for diffusion imaging and fMRI due to its fast readout. Improvements to spatial resolution are however limited by SNR and EPI distortion. In-plane parallel imaging is frequently utilized to reduce EPI distortion at the cost of reduced SNR. As compared to conventional whole-body systems, a compact head 3T system¹ with a dedicated, high-performance head-gradient system² has superior gradient slew rate (SR) performance by 3.5 times (Gmax=80 mT/m, SR=700 T/m/s), allowing echo spacing (ESP) to be reduced by up to a factor of two without encountering peripheral nerve stimulation limits^3-4. The purpose of this work is to compare spin-echo (SE) and gradient-recalled-echo (GRE) EPI performance between the compact 3T and whole-body scanners, and to evaluate the high performance gradient of the compact scanner in advanced diffusion imaging.

The compact 3T system is built around a conduction-cooled, low-croygen magnet that has a warm-bore inner diameter of 62 cm, a magnet weight less than 2,000 kg, a 5 Gauss fringe field area of 24m², and an imaging FOV (DSV) of 26 cm. Healthy human subjects (under an IRB-approved protocol) were imaged using the compact 3T system, and on a conventional whole-body 3T MRI system (GE MR750). 2D SE-EPI and GRE-EPI axial acquisitions were performed at 1.5-3.4 mm-isotropic sampling (FOV=20.8 cm, TR/TE=1500/40 ms) without in-plane parallel imaging. The ss-EPI readout amplitude was set to 51mT/m on the compact 3T, which allowed full-sampling of the phase-encoding k-space while keeping under TE=40 ms in GRE-EPI. GRE-EPI with the whole-body 3T was performed with partial-ky=0.75, as were all SE-EPI scans on both systems.

Whole-brain diffusion-imaging was also tested, using the high performance head-gradient inserted in a whole-body 3T magnet (GE MR750w), to evaluate the feasibility of acquisition at 1.5 mm-isotropic spatial resolution. Simultaneous multi-slice acquisition⁵ with no in-plane parallel acceleration (R=3×1) and with slice-select gradient reversal⁶ was used to shorten acquisition time without residual fat artifacts from the fat saturation pulses. A 32-channel brain coil (Nova Medical) was used in all experiments. The acquisitions tested were: compressed-sensing diffusion spectrum imaging (CS-DSI)⁷ (bmax=8000 s/mm², TR/TE=3800/75.1 ms, CS-acceleration R=4, 6 T2 + 127 acquired gradient directions, scan time=8.5 minutes) and multi-shell HARDI⁸ (bmax=3000 s/mm², TR/TE=3800/71.4 ms, two b-values with total of 6 T2 + 98 acquired gradient directions, 6.6 minutes). No EPI-distortion correction was applied in the image reconstruction. The readout gradient was reduced to 40 mT/m to increase image SNR, at the expense of slightly increased ESP=456 µs (vs 388 µs at 51mT/m).

For all spatial resolutions tested (1.5-3.4 mm), the ESP with the compact 3T EPI scans was approximately half that with the whole-body scanner (Table 1). The ESP for 1.5 mm EPI in the compact scanner was shorter than that of 3.4 mm EPI with the whole-body scanner. At 1.5 mm, there was visibly reduced susceptibility-induced distortion and signal dropout in both SE-EPI and GRE-EPI with the compact scanner (Figure 1), especially in the vicinity of air-tissue-bone interfaces. However, there was increased residual fat ghosting. The compact scanner produced images with low distortion at all spatial resolutions (Figure 2). Moreover, the extent of signal dropout in the GRE-EPI due to dephasing was reduced with thinner slices.

Diffusion imaging at 1.5 mm-isotropic with multi-shell and DSI provided adequate SNR, sufficient for visualizing colorized FA maps and diffusion kurtosis⁹ (Figure 3), as well as the major fiber bundles as processed using orientation distribution functions that resolve fiber-crossings (Figure 4).

Preliminary imaging experience using 2D ssEPI on the compact 3T system equipped with the high performance head-only gradient suggests the feasibility of routine acquisition of 1.5 mm in-plane resolution with both spin-echo and gradient-recalled-echo. In particular for GRE, the short echo spacing allows for full-echo sampling at TE≤40 ms with low signal dropout, which makes high-resolution fMRI feasible at 1.5 mm. The combined advantages of reduced ESP and TE in diffusion imaging may enable high-resolution and high b-value acquisitions. Susceptibility artifacts are reduced primarily by the decreased ESP, and secondarily by the smaller voxel volumes, which are better supported by the increased gradient performance of the compact system. The EPI ghosting may be reduced with further optimizations of the gradient driver. The low image distortion provided by the compact 3T system provides future avenues for researching rapid ssEPI-based sequences to replace conventional workhorse pulse sequences such as T1-FLAIR and T2-FLAIR.