Lightweight, compact, and high-performance 3T MR system for imaging the brain and extremities

Abstract

Purpose: To build and evaluate a small-footprint, lightweight, high-performance 3T MRI scanner for advanced brain imaging with image quality that is equal to or better than conventional whole-body clinical 3T MRI scanners, while achieving substantial reductions in installation costs.

Methods: A conduction-cooled magnet was developed that uses less than 12 liters of liquid helium in a gas-charged sealed system, and standard NbTi wire, and weighs approximately 2000 kg. A 42-cm inner-diameter gradient coil with asymmetric transverse axes was developed to provide patient access for head and extremity exams, while minimizing magnet-gradient interactions that adversely affect image quality. The gradient coil was designed to achieve simultaneous operation of 80-mT/m peak gradient amplitude at a slew rate of 700 T/m/s on each gradient axis using readily available 1-MVA gradient drivers.

Results: In a comparison of anatomical imaging in 16 patients using T₂ -weighted 3D fluid-attenuated inversion recovery (FLAIR) between the compact 3T and whole-body 3T, image quality was assessed as equivalent to or better across several metrics. The ability to fully use a high slew rate of 700 T/m/s simultaneously with 80-mT/m maximum gradient amplitude resulted in improvements in image quality across EPI, DWI, and anatomical imaging of the brain.

Conclusions: The compact 3T MRI system has been in continuous operation at the Mayo Clinic since March 2016. To date, over 200 patient studies have been completed, including 96 comparison studies with a clinical 3T whole-body MRI. The increased gradient performance has reliably resulted in consistently improved image quality.

Figure 1

Illustration of the key components of the compact 3.0T MRI system showing the dimensions relative to a patient, the locations of the magnet coils, gradient coil, and the transmit/receive birdcage RF coil, together with the cold-head/cryo-cooler.

Figure 2

Loading (a) and unloading (b) of the compact 3.0T MRI scanner for transport using standard commercial 53-foot semi-trailers, and standard fork-lifts. (c) The completed and installed compact 3.0T MRI system at Mayo Clinic showing the detachable/dockable table and the 32-channel NOVA head coil mounted on the custom head-holder. Note the absence of a cryo-vent.

Figure 3

Magnet cool-down profile. Within 6-days after installation of the compact 3.0T MRI system, the magnet coil temperature reached 4.15K, which was sufficient to ramp the magnet to field.

Figure 4

(a) B_z-field map after conventional passive shimming showing the field homogeneity of the compact 3T magnet in the coronal plane. (b) Measured field homogeneity at different diameter spherical volumes.

Figure 5

T₂-weighted Fast Spin Echo (FSE) images acquired with an 8-channel brain coil (22-cm FOV; 320×320 matrix; TE/TR = 98.7/4686 ms; 4-mm slices; 36 locations in 2:16) with (a) no fat suppression, and (b) with fat suppression. The uniformity of the fat suppression over the 22-cm FOV is an indication of the field homogeneity realized with the compact 3.0T magnet design.

Figure 6

Stray field map outside of the compact 3.0T magnet showing the |B|-field lines from 5 to 200 Gauss (0.5–20 mT).

Figure 7

(a) Iso-contour curves of minimum TE times for a diffusion weighted EPI pulse sequence (with b=2,000 s/mm²; 24-cm FOV; 1.5 mm in-plane resolution; R=1; partial Fourier fraction of 75%) at a fixed receiver bandwidth, and ramp sampling enabled. (b) Iso-contours of the EPI readout echo spacing for the same acquisition. Overlaid are the PNS thresholds for bipolar pulses for the head-only compact 3T gradients (solid line) and whole-body gradients (dashed line). The circles indicate gradient specification for current clinical and research whole-body MR scanners, while the square indicates the performance of the compact 3T MRI.

Figure 8

(a) Sagittal 3D MP-RAGE acquisition with 0.7 × 0.7 × 0.7 mm³ spatial resolution covering the entire brain in 8:25 with the 32-channel brain coil. Other acquisition parameters were: FOV = 22 cm, TE/TR = 3.1/7.3 ms; TI=1000 ms. Image detail of the brain and cerebellum is exquisite. Signal remains uniform and robust down to the level of the C2/C3 cervical interspace. There are only minimal susceptibility artifacts at the interface of the brain and paranasal sinuses (b) Maximum intensity projection of the 3D MP-RAGE acquisition showing high signal within the internal and external carotid arteries, and the proximal vertebral arteries. This bright blood extends more superiorly on the C3T MP-RAGE images, as the IR pulse does not extend into the lower neck and body as it would with conventional whole-body coil excitation.

Figure 9

(a) Results of the right-side Wilcoxon Sign-Rank test from the assessment of 3D CUBE T₂-weighted FLAIR image quality by 2 blinded neuroradiologists. (b) Comparison images in a patient with mild cognitive impairment showing white-matter lesions (arrows). Improved SNR and lesion conspicuity with the C3T system is evident from the shorter echo-spacing, resulting in reduced T₂ blurring of the FSE echo train and overall improvement in image sharpness. The 8-channel brain coil was used in all instances with TR=7600 ms; 256 × 256 acquisition matrix in a 24-cm FOV; 152 1.2-mm partitions (0.94 mm × 0.94mm × 1.2 mm voxels); TE/TI=93.0 ms/2025 ms for the whole-body system, and TE/TI = 91.3 ms/2060 ms for the C3T system; echo-train length (ETL) = 200; in-plane acceleration R=2. The echo-spacing for the whole-body system was 4824 μs, and 3544 μs for the C3T scanner.