Low-cost and portable MRI

Abstract

Research in MRI technology has traditionally expanded diagnostic benefit by developing acquisition techniques and instrumentation to enable MRI scanners to "see more." This typically focuses on improving MRI's sensitivity and spatiotemporal resolution, or expanding its range of biological contrasts and targets. In complement to the clear benefits achieved in this direction, extending the reach of MRI by reducing its cost, siting, and operational burdens also directly benefits healthcare by increasing the number of patients with access to MRI examinations and tilting its cost-benefit equation to allow more frequent and varied use. The introduction of low-cost, and/or truly portable scanners, could also enable new point-of-care and monitoring applications not feasible for today's scanners in centralized settings. While cost and accessibility have always been considered, we have seen tremendous advances in the speed and spatial-temporal capabilities of general-purpose MRI scanners and quantum leaps in patient comfort (such as magnet length and bore diameter), but only modest success in the reduction of cost and siting constraints. The introduction of specialty scanners (eg, extremity, brain-only, or breast-only scanners) have not been commercially successful enough to tilt the balance away from the prevailing model: a general-purpose scanner in a centralized healthcare location. Portable MRI scanners equivalent to their counterparts in ultrasound or even computed tomography have not emerged and MR monitoring devices exist only in research laboratories. Nonetheless, recent advances in hardware and computational technology as well as burgeoning markets for MRI in the developing world has created a resurgence of interest in the topic of low-cost and accessible MRI. This review examines the technical forces and trade-offs that might facilitate a large step forward in the push to "jail-break" MRI from its centralized location in healthcare and allow it to reach larger patient populations and achieve new uses. Level of Evidence: 5 Technical Efficacy Stage: 6 J. Magn. Reson. Imaging 2019. J. Magn. Reson. Imaging 2020;52:686-696.

Keywords: MRI value; accessible MRI; low-cost MRI; point-of-care MRI; portable MRI.

FIGURE 1:

Marie Curie circa 1915 shown with one of her mobile x-ray units used during WWI. Source: http://www.nobelprize.org/nobel_prizes/physics/articles/curie/images/c_truck.jpg.

FIGURE 2:

Estimate of the relative component costs within a low-end conventional 1.5T scanner. Costs are estimated for high-volume commodity production (runs of ~1000 per year) and would be considerably higher for one-off, or laboratory instrumentation. Note that a low-end system with a market price of about $800,000 USD and a 4× markup over components cost suggests a total components cost of $200,000 USD leaving a cost of about $75,000 USD for the magnet and cryostat. Costs, of course, depend on the detailed specifications, and are treated as proprietary figures in the industry. Thus, these estimates are based on the authors’ intuition and not specific data.

FIGURE 3:

Magnet wire cost as a function of magnet length L and bore diameter D (both expressed as a fraction of the diameter of the homogeneous region, DSV) for superconducting solenoid designs. This design analysis shows the rapid rise in cost for short magnets and constructing a homogeneous magnet shorter than L_opt quickly becomes expensive (a 1 ppm homogeneity in the DSV was used in this analysis). For all the designs studied, the optimal length followed the relationship L_opt = 1.18 DSV + 0.77 D. From Xu et al. In: Proc 7th Annual Meeting ISMRM, Philadelphia; 1999. p 475.

FIGURE 4:

Halbach cylinder designs of potential interest for low-field MRI. In the Halbach cylinder, a nearly uniform transverse field is produced inside the cylinder if the magnetic moment of the magnetized material is phased from 0 to 4π azimuthally. Note that this is similar to the phase relationship for a birdcage coil where a 0 to 2π azimuthal phase relationship is used. Top row shows an ideal cylinder with continuous magnetization and a more practical approximation comprising keystone-shaped sections. Far top right shows a simpler to construct configuration using only identical rectangular blocks and with all the magnetization vectors normal to a face. The phase relationship comes only from rotations of the blocks. The bottom row shows further optimizations allowing degrees of freedom to be adjusted to achieve a target field pattern (typically either a uniform field or a monotonic gradient) despite the imperfections of the array (eg, finite cylinder and sparse population). One option is to maintain linear rungs of material but vary the material. A second approach is to maintain rings of material but allow varying diameters.

FIGURE 5:

A prototype portable brain MRI scanner based on the Halbach permanent magnet described in Cooley et al (IEEE Trans Magn 2018;54) and configured for rotational encoding as in Cooley et al (Magn Reson Med 2015;73:872–883). The magnet weighs ~125 kg and achieves an 80 mT B₀ field.