Magnetic Resonance Microscopy

Costs of MRI components are hard to pin down because the costs a system manufacturer faces for volume-sourced parts are proprietary and likely considerably cheaper than seen by academic researchers, who constitute a niche market. Also, in some locations import tariffs on imported equipment incurs a significant additional cost to the user, which could be avoided if the device could bypass tariffs through local manufacture [14]. Nonetheless, a recent review has attempted to outline system component costs [15]. Lowering costs by reducing component-level costs is only part of the equation. Operating expenses must be lowered, including service costs, cryogen and cryogenic equipment maintenance, and electrical consumption of both the equipment and the carefully controlled air-conditioned environment typically required [14].

For any of these three levels of POC devices, a new system-level approach is needed. Ample industrial effort has already been put into shortening the bore of conventional high-field (1.5 T and above) superconducting magnet-based systems. After nearly four decades of whole-body scanner design, the result is still a multi-ton, nontransportable system with high power and cooling needs and a relatively large magnetic field footprint. The reason for this partly stems from trying to achieve ever-higher imaging speed and resolution. But which foundations of the existing high-field system architecture could go if that goal were relaxed? The first that comes to mind is the conventional whole-body focus of current clinical scanners. If a specific body part must be chosen, the head is an ideal target due to the importance of brain injury and disease, and because the anatomy allows for smaller bore size. Here we omit discussion of small systems for extremity imaging (knee, wrist, etc.) since small versions of these scanners have been available for some time including a mini-van mounted device [16]. Other obvious departures include lowering the static magnetic field strength (at the expense of sensitivity) and/or its homogeneity. This has multiple positive and negative implications for the system and its performance [17–19], but allows for cheaper, smaller superconducting magnets 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.

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.

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 ( B 0near 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.