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3 Portable Brain Scanner Technology for Use in Emergency Medicine

       Lawrence L. Wald1,2,3 and Clarissa Z. Cooley1,2

       ¹ Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA

       ² Harvard Medical School, Boston, MA, USA

       ³ Division of Health Sciences and Technology, Harvard – Massachusetts Institute of Technology, Cambridge, MA, USA

      3.1 Where Would You Use a Portable or Small Footprint Magnetic Resonance Imager?

      Portable magnetic resonance imaging (MRI) technology could expand the benefits of MRI to patients who traditionally do not have access. These recipients fall roughly into two segments: patients who live in regions with abundant MRI access but who are difficult to transport into a conventional MRI suite, and those in the large fraction of the world underserved by clinical MRI due to equipment and operational costs.

      For the first group, the goal is to influence patient outcomes by bringing the scanner to vulnerable patient populations difficult to bring to a conventional MRI suite. Brain imaging is a prime target due to the supremacy of MR contrast in this important organ, and the time-sensitive need for brain evaluation in many conditions, including stroke, elevated intracranial pressure (ICP) from hemorrhage, edema, hydrocephalus, or neonatal injuries such as hypoxic-ischemic encephalopathy (HIE). Furthermore, MR is intrinsically well suited for repeated scanning scenarios (such as monitoring) if a sufficiently nonobtrusive device is available.

The vast majority of emergency departments (EDs) and intensive care units (ICUs) currently lack MRI scanners. Although these facilities are located in hospitals equipped with MRI suites, it is also not always easy to transport the patient, even within the same building. EDs rely almost exclusively on ultrasound (US) and computed tomography (CT) for tomographic imaging. While the unique imaging capabilities of MRI (e.g. for evaluation of acute stroke) have led some hospitals to install scanners within their ED, this is a rarity, existing in only a small number of large tertiary care centers [1–3]. Even fewer hospitals have an MRI scanner within their ICUs. Additionally, the COVID-19 pandemic has taught us the importance of portable imaging modalities [4], with portable chest X-ray systems by far the most utilized [5], followed by CT [6,7] and US [8]. MR has shown the potential for assessing unique neurological manifestations of COVID-19 not sufficiently evaluated by CT [9] but is little used due to the barriers to transporting these acutely ill and infectious patients. The first-choice modality for patients in such a setting would ideally be brought into the patient’s room, utilized for a point-of-care (POC) scan, wheeled out, and disinfected.

The second group (access in low-income areas) is addressed mainly through cost reduction. Geographical MRI accessibility gaps have been recently reviewed in West Africa [10] and worldwide [11]. The degree to which some regions lag behind worldwide averages for the number of scanners per capita is shocking. For example, 11 African countries have no scanner. However, there is also some indication that low-income countries overspend on high-end medical equipment relative to other infrastructure [12]. Together, this underscores the need to reduce the cost of the MRI equipment if we hope to serve this community. Additionally, economic accessibility gaps are not limited to low-income countries, but also rural areas of middle- to high-income countries where the density of patients is insufficient to support the cost and operation of the scanner [13].

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].

      3.2 Rethinking System-level Approaches

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.

      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 СКАЧАТЬ