Low-Cost High-Performance MRI

Abstract

Magnetic Resonance Imaging (MRI) is unparalleled in its ability to visualize anatomical structure and function non-invasively with high spatial and temporal resolution. Yet to overcome the low sensitivity inherent in inductive detection of weakly polarized nuclear spins, the vast majority of clinical MRI scanners employ superconducting magnets producing very high magnetic fields. Commonly found at 1.5-3 tesla (T), these powerful magnets are massive and have very strict infrastructure demands that preclude operation in many environments. MRI scanners are costly to purchase, site, and maintain, with the purchase price approaching $1 M per tesla (T) of magnetic field. We present here a remarkably simple, non-cryogenic approach to high-performance human MRI at ultra-low magnetic field, whereby modern under-sampling strategies are combined with fully-refocused dynamic spin control using steady-state free precession techniques. At 6.5 mT (more than 450 times lower than clinical MRI scanners) we demonstrate (2.5 × 3.5 × 8.5) mm(3) imaging resolution in the living human brain using a simple, open-geometry electromagnet, with 3D image acquisition over the entire brain in 6 minutes. We contend that these practical ultra-low magnetic field implementations of MRI (<10 mT) will complement traditional MRI, providing clinically relevant images and setting new standards for affordable (<$50,000) and robust portable devices.

Figure 1. Ultra-low field MRI system.

Custom built biplanar 6.5 mT electromagnet with biplanar gradients (Gx, Gy, and Gz). The diameter of the outermost B₀ coil is 220 cm. The subject lays supine in the scanner and a custom built single channel transmit/receive spiral head coil wound with litz wire for operation at 276 kHz is placed to cradle the head.

Figure 2. 3D renderings of the single channel form-fitting head coil.

(A) isometric, (B) back, and (C) side views are shown. The final design was 3D printed on a Fortus 360 mc printer (Stratasys, Eden Prairie, MN, USA) in polycarbonate using fused deposition modeling technology. The 30-turn spiral was wound with Type 1 40/38 Litz wire, parallel resonated to 276 kHz, and capacitively matched to 50 ohms.

Figure 3

3D images of the living brain acquired in 6 minutes at 6.5 mT in (a) axial, (b) coronal, and (c) sagittal orientation. The corresponding maximum SNRs are a. 15, b. 21, and c. 16. Acquisition matrix: 64 × 75 × 15, voxel size: a. (2.5 × 3.5 × 8.5) mm³, b. (2.5 × 3.5 × 11.5) mm³, and c. (2.5 × 3.5 × 14.4) mm³.

Figure 4. Comparison of single channel ULF MRI to 32-channel high magnetic field scans.

(a) b-SSFP at 6.5 mT. (b–d), PD, , and weighted contrast at 3 T, respectively. Most of the anatomic features seen at higher magnetic field can be identified on the ultra-low field scans. At low field, approaches , and the resulting image contrast in (a) is very similar to PD-weighting (b).

Figure 5

Comparison of b-SFFP images at (a) 3T and (b) 6.5 mT. Strong banding artifacts appear at high magnetic field (yellow arrows) in all orientations (coronal, sagittal, and axial) whereas no artifact is seen in the images acquired at ultra-low field.