Contemporary approaches to high-field magnetic resonance imaging with large field inhomogeneity

• Published in: Progress in nuclear magnetic resonance spectroscopy 2020-10, Vol.120-121, p.95-108
• Main Authors: Mullen, Michael, Garwood, Michael
• Format: Article
• Language: English
• Subjects: Brain - diagnostic imaging; Field inhomogeneity; Frequency-swept; Humans; Image Processing, Computer-Assisted - methods; Magnetic Resonance Imaging - methods; Phantoms, Imaging; Portable MRI; Signal-To-Noise Ratio
• ISSN: 0079-6565; 1873-3301
• DOI: 10.1016/j.pnmrs.2020.07.003

[Display omitted] •Low-cost, portable MRI promises to improve access to high quality imaging worldwide.•Small magnets have several thousand ppm field inhomogeneity to overcome.•Imaging in high-field portable systems is achievable with proper sequence choice.•Practical aspects of imaging with large field inhomogeneity are reviewed. Despite its importance as a clinical imaging modality, magnetic resonance imaging remains inaccessible to most of the world’s population due to its high cost and infrastructure requirements. Substantial effort is underway to develop portable, low-cost systems able to address MRI access inequality and to enable new uses of MRI such as bedside imaging. A key barrier to development of portable MRI systems is increased magnetic field inhomogeneity when using small polarizing magnets, which degrades image quality through distortions and signal dropout. Many approaches address field inhomogeneity by using a low polarizing field, approximately ten to hundreds of milli-Tesla. At low-field, even a large relative field inhomogeneity of several thousand parts-per-million (ppm) results in resonance frequency dispersion of only 1–2 kHz. Under these conditions, with necessarily wide pulse bandwidths, fast spin-echo sequences may be used at low field with negligible subject heating, and a broad range of other available imaging sequences can be implemented. However, high-field MRI, 1.5 T or greater, can provide substantially improved signal-to-noise ratio and image contrast, so that higher spatial resolution, clinical quality images may be acquired in significantly less time than is necessary at low-field. The challenge posed by small, high-field systems is that the relative field inhomogeneity, still thousands of ppm, becomes tens of kilohertz over the imaging volume. This article describes the physical consequences of field inhomogeneity on established gradient- and spin-echo MRI sequences, and suggests ways to reduce signal dropout and image distortion from field inhomogeneity. Finally, the practicality of currently available image contrasts is reviewed when imaging with a high magnetic field with large inhomogeneity.