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.

Weight-bearing MRI of the Lumbar Spine: Technical Aspects

“…Since the earliest conventional supine MRI systems were introduced operating at low-field strength, there has been a trend toward using higher field strengths in clinical practice driven by the desire to reduce acquisition time, increase signal-to-noise ratio, and increase image quality. 28 However, these conventional high-field MRI scanners, typically 1.5 to 3 T, are expensive to acquire and have higher maintenance costs. Therefore, there has been an increasing emphasis of late on the further development of low-field MRI systems (0.25-0.6 T) that are less expensive, with lower running costs, and low siting requirements, yet still produce images of comparable diagnostic capability for most clinical MRI studies that do not require ultra-high resolution.…”

“…Therefore, there has been an increasing emphasis of late on the further development of low-field MRI systems (0.25-0.6 T) that are less expensive, with lower running costs, and low siting requirements, yet still produce images of comparable diagnostic capability for most clinical MRI studies that do not require ultra-high resolution. 28,29 The new open-bore configuration of low-field MRI systems can, depending on the particular machine design, allow for intervention MR guidance and scanning in various body positions including upright weight-bearing postures.…”

Assessment of cardiac function, blood flow and myocardial tissue relaxation parameters at 0.35 T

“…While increasing static magnetic field strength boosts the baseline SNR, B0 and B1 field inhomogeneities also scale with field strength 15 ; thus, artifacts may be reduced and techniques sensitive to field inhomogeneities may be more effective at lower field. Longitudinal relaxation time (T1) decreases, and transverse relaxation times (T2 and T2*) of biological tissues increase with decreasing B0, potentially offsetting some of the SNR lost due to reduced magnetization 16 . Metal and susceptibility artifacts, acoustic noise levels and SAR are all reduced at lower B0 17–19 .…”

“…Diagnostic quality may be improved and safety concerns lessened at lower B0 in these patients. The availability of a high‐performance, low‐field system could potentially broaden MR availability to these patient groups 16,22–25 …”

The MR Cap: A single‐sided MRI system designed for potential point‐of‐care limited field‐of‐view brain imaging

“…Recent reviews have examined the development of portable and low‐cost MRI systems for brain imaging . These include ultralow field systems that attempt to reduce cost and weight by reducing B 0 below 10 mT; prepolarized systems; low‐field systems employing resistive magnets or permanent magnet arrays, potentially employing built‐in encoding fields; and high‐ field systems with reduced cryogen use or new superconductor or cryostat technology . Portable MRI systems have also been developed for extremity imaging and have found applications in musculoskeletal imaging .…”