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A portable MRI can be brought to the patient rather than bringing the patient to the MRI. Thus we define “portable” as a device that can be moved room-to-room by a single healthcare worker for use on a patient within minutes. The power and cooling infrastructure requirements must be met by a typical office or exam room infrastructure. Such portable MRI scanners equivalent to their counterparts in US, X-ray, and CT have traditionally not been available, although a commercial clinical portable MRI product has recently been introduced and tested in a clinical setting [36].

True portability seems to require both restriction to a limited body region (e.g. the head) and operation at low field (likely below 200 mT) and potentially employing new types of magnet architectures and encoding strategies. Here attention has turned to permanent magnets, which offer the ability to operate without external power or a cryogenic system. Modern rare-earth permanent magnets can produce head-sized B₀ field regions up to about 0.5 T with surprisingly small external field footprints. Many magnet and gradient configurations have emerged, with several prototypes being demonstrated. Notably the first fully portable clinical MRI product scanner is now Food and Drug Administration (FDA) approved for clinical care – the 64-mT Hyperfine scanner [36,37]. Figure 3.2 shows another such system under development, based on an 80-mT “Halbach bulb” rare-earth magnet configuration with a magnet weight of 24 kg [38].

Figure 3.2 A potential portable brain magnetic resonance imaging cart based on a “Halbach bulb” permanent magnet array for bedside use in the emergency department or intensive care unit. Prototype magnet, gradient, and radiofrequency (top) and envisioned mobile cart (bottom); system approaches bed from head end and utilizes a double sliding mechanism (radiofrequency coil slides followed by magnet sliding down).