Low-field MRI: An MR physics perspective

Abstract

Historically, clinical MRI started with main magnetic field strengths in the ∼0.05-0.35T range. In the past 40 years there have been considerable developments in MRI hardware, with one of the primary ones being the trend to higher magnetic fields. While resulting in large improvements in data quality and diagnostic value, such developments have meant that conventional systems at 1.5 and 3T remain relatively expensive pieces of medical imaging equipment, and are out of the financial reach for much of the world. In this review we describe the current state-of-the-art of low-field systems (defined as 0.25-1T), both with respect to its low cost, low foot-print, and subject accessibility. Furthermore, we discuss how low field could potentially benefit from many of the developments that have occurred in higher-field MRI. In the first section, the signal-to-noise ratio (SNR) dependence on the static magnetic field and its impact on the achievable contrast, resolution, and acquisition times are discussed from a theoretical perspective. In the second section, developments in hardware (eg, magnet, gradient, and RF coils) used both in experimental low-field scanners and also those that are currently in the market are reviewed. In the final section the potential roles of new acquisition readouts, motion tracking, and image reconstruction strategies, currently being developed primarily at higher fields, are presented. Level of Evidence: 5 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019.

Keywords: MRI; low-field systems; state-of-the-art.

Figure 1

Plot of the dependence of relaxation times as a function of magnetic field using for the (a) longitudinal relaxation the fit measured by Rooney et al11 and (b) for the apparent transverse relaxation the fits obtained by Pohmann et al.14

Figure 2

Plots of (a) T₂*, (b) T₁‐weighted, and (c) PD SNR for an optimized protocols at each given field strength. Dashed lines correspond to the fit of the relevant contrast with a function $c{B}_0^{\mathit{\text{powe}}{r}_{\mathit{eff},w}}$, where b is the power law reported in each plot.

Figure 3

Historical photographs and sketches showing one of the first MRI systems to produce human images, together with the RF coil and in vivo breast images.

Figure 4

Schematics of different types of permanent low‐field magnet. (a) An H‐shaped system, with two ferromagnetic yokes and two permanent magnets with shaped ferromagnetic pole‐pieces. (b) The most common C‐shaped geometry with one ferromagnetic yoke. (c) Examples of steps to improve the magnetic field inhomogeneity by changing the shape of the pole pieces (adapted from Tadic et al21). (d) For higher magnetic fields electromagnets can be incorporated, as well as a shielding coil. These can be either regular conductors or superconductors for field of 1T and above.

Figure 5

Wire patterns used to produce the (left and center) x‐ and y‐gradients and (right) the z‐gradient. The gradients form pairs with one of each set placed flat on the pole pieces of the magnet.

Figure 6

Examples of RF coils used on low‐field MRI systems. (a) Quadrature transmit/receive coil on the vertical 0.6T Fonar system, (b) Four‐element head array on the 0.25T Esaote. (c) Shoulder phased array for the Siemens 0.35T Magnetom. (d) One example of a research phased array designed for 0.25T with a loop/butterfly coil arrangement. (a–c courtesy of FONAR Corporation, Esaote and Siemens Healthineers, respectively).

Figure 7

Fast spin‐echo T₂‐weighted scans in the sagittal plane of the lumbar spine acquired at 0.25T. The left image is made in the supine position and the right image in the upright position. In the right image the disc protrusion becomes more evident. Adapted from Tarantino et al.45 Inserts within each figure demonstrate the functionality of the ESAOTE rotatable permanent magnet that could be used to acquire such images.

Figure 8

(a) Preoperative and (b) intraoperative MR scan (0.2T); speech‐relevant areas are denoted in the preoperative data with a white circle. Significant brain shift has occurred, explaining the need for interoperative imaging for target assessment. Adapted from Hastreiter et al.40

Figure 9

Plots of the (a) power law, power_eff,w, and (b) proportionality constant, c, dependence of SNR efficiency of different contrasts on the number of slices excited per TR when parameterizing it as $c{B}_0^{\mathit{\text{powe}}{r}_{\mathit{eff},w}}$.