Magnetic Resonance Microscopy. Группа авторов

or permanent magnets where the magnetic energy density can be stored without the use of cryogens. Other departures examined include nonswitched readout gradients built into the static magnet design (saving power, cooling, and reducing acoustic noise) [20], encoding by rotation of a built-in gradient (further reducing encoding electronics) [21–27], and shrinking the imaging field of view (FOV) even further to a subset of the organ and perhaps not fully encoding all spatial dimensions.

      3.3 Three Levels of POC Use

We arbitrarily divide POC MRI use cases into three levels based on their deviation from a conventional high-field scanner suite. The closest level employs modest deviations from a standard 1.5-T scanner approach and attempts to improve siting (and perhaps cost) to facilitate siting within a tight ED or ICU space. This “easy-to-site suite” scanner could utilize a standard superconducting solenoid magnet architecture, perhaps at reduced field strength, but with modifications to decrease its cost, size, and stray-field footprint. For example, the system might use a short-bore, conduction-cooled superconducting magnet to eliminate cryogens and the quench-pipe. The footprint, size, and cost can be reduced compared with conventional whole-body scanners if the magnet is sized for brain imaging and operated at mid-field (between 0.5 T and 1.0 T). This intermediate field strength can provide sensitivity and imaging contrasts similar to conventional 1.5-T scanners while increasing accessibility – a topic recently reviewed and put into historical perspective [28]. Addition of active electromagnetic interference (EMI) mitigation could further ease siting by eliminating the standard radiofrequency shielded room. Although the system retains many aspects of conventional suites including high-power electrical hookups (for conventional gradients), maintenance-prone cryogenic equipment, water cooling, and a safety exclusion zone, the potential siting benefits have motivated several commercial MRI manufacturers to initiate development of this type of device (see Figure 3.1).

      Figure 3.1 Superconducting MRI systems recently introduced to the market to provide high-quality imaging with reduced siting needs. All employ conduction-cooled magnets to eliminate the need for “quench pipes” to vent cryogenic gases. From left to right: GE (Waukesha WI) 3-T “compact head scanner,” the Synaptive Medical (Toronto, ON, Canada) 0.5-T “Envry” compact scanner, and the Siemens Healthineers (Erlangen, Germany) 0.55-T “Free Max” compact 80-cm diameter patient-bore scanner (Siemens Healthcare GmbH).

      The second level would be a truly portable scanner that could be pushed down the hallways of the hospital by a single staff member who brings it into the ward or even to the bedside and powers it up for POC use. This device must operate using a standard electrical outlet or perhaps battery power, the latter allowing it to be embedded in an ambulance or mobile setting. The mobile POC scanner would likely operate at low field (50–200 mT), need unconventional EMI mitigation, and must operate with substantially reduced electrical power compared with conventional systems and without water cooling or cryogenics. This class of POC devices is being actively pursued by several companies and academic groups as discussed in Section 3.3.2.

      Finally, the third class is a more speculative device that extends MRI to a near “handheld” level, likely with a greatly reduced imaging capabilities, but inexpensive and small enough to be considered an MR detector or monitoring device more than a diagnostic imaging device. Such a lightweight device could reach into the bed and monitor an organ, perhaps using 1D imaging or just the MR signal itself. This rethinks the role of MRI as a tomographic imager and, as such, is the most distant from conventional MRI scanner architectures. Nonetheless, examples of this more speculative device are starting to emerge in the literature as outlined in Section 3.3.3.

      3.3.1 Brain MRI in an “Easy-to-Site Suite”

A head–neck focus can still utilize conventional but “shrunk-down” superconducting magnet architectures (perhaps with asymmetric gradients) to provide the “easy-to-site suite” and possibly reduce cost. High-field superconducting head-only scanners include the Siemens Allegra 3-T clinical scanner [29] introduced in the early 2000s but no longer produced, and the more recent GE high-performance 3-T head scanner employing a conduction-cooled magnet with no cryogen vent-pipe [30]. While the magnet and gradients of these high-field head-only systems are more compact, the focus of these two systems was on performance rather than siting alone, which is only modestly simplified. There is renewed interest in making additional changes to provide easier siting of superconducting systems within an ED, ICU, or interventional suite. These approaches all leverage intermediate-field (B₀ near 0.5 T) superconducting systems with cryogen-free refrigeration systems such as a conduction-cooled 0.55-T [31] or 0.5-T scanner [32–34], both employing modern, high-performance gradient systems in a standard architecture. Other efforts are underway with an even smaller head-focused 1.5-T high-temperature superconducting magnet [35]. Figure 3.1 shows three “easy-to-site” superconducting approaches recently introduced by manufacturers. Extremely small bore size superconducting magnets have also been introduced as needed for extremity or neonatal imaging.

      3.3.2 Brain MRI with a Portable Device

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).

      3.3.3 Brain MRI as a Monitoring Device

In addition to employing permanent magnets and reducing the magnet field strength and size, we also consider more extreme changes to the scanner architecture to further reduce the scanner size toward a handheld device. This includes dropping the goal of whole-organ imaging, for example probing only a section of the body with limited image encoding (2D, 1D, or even no image encoding). But, even if detailed anatomy is not visualized, the MR data can be collected and monitored for changes that might accompany, for example, important intracranial pathology. This type of MRI is thus a patient-monitoring device in the way a pulse-oximeter or electrocardiogram (ECG) device is used at the bedside in the ED or ICU. It is new territory for MRI and pushes the technology into a more radical configuration. This extreme approach does not attempt to have the system fully encircle a body part, but is limited to a “single-sided”