Magnetic Resonance Microscopy. Группа авторов
for materials characterization with MR [39,40] but also considered for biological applications [41–43] and using remarkably inexpensive components [44]. Single-sided spectrometers have also been introduced for assessing breast tissue [45,46] and muscle hydration [47,48]. Single-sided full-imaging systems are less common, but have been demonstrated [49] and applied to burn depth [50].
Figure 3.3 shows a single-sided scanner designed for brain imaging [51]. An ultra-lightweight (<10 kg) single-sided brain scanner will have extreme inhomogeneity by medical MRI standards and require unconventional encoding strategies. But these changes are likely necessary to make the leap from a scanner where the patient’s head still goes inside the device, to a more handheld scanner simply placed adjacent to the head. This step will not be without image quality reductions. For monitoring, the device must “reach” into the bed, operate adjacent to the patient (versus placing the anatomy inside the magnet), and be light and cheap enough for sustained operation as a monitoring device. An ED or ICU might benefit from such an MRI device to continuously image the brain watching for intracranial hemorrhage or changes in cerebral mass effect though monitoring a ventricular/cerebrospinal fluid left–right hemisphere asymmetry. An intracranial MR monitor could provide an early warning sign of impending herniation, particularly in patients where clinical examination is difficult (e.g. sedated patients).
Figure 3.3 Single-sided brain magnetic resonance imaging under development for monitoring applications in the emergency department or intensive care unit. The <7 kg Halbach sphere-section magnet generates ~80 mT in a subregion of the brain.
3.4 Clinical Use Scenarios of “Easy-to-Site” POC, and Monitoring MR Devices
Although engineers and researchers are often content with the “build it and they will come” philosophy, clinicians, funding agencies, and hospital administrators prefer a need-driven approach. Furthermore, knowledge of the clinical use is critical to many design decisions. Here we frame the technical needs by outlining some of the clinical use scenarios envisioned for POC neurological MRI.
3.4.1 ED and ICU
Neuroimaging in emergency medicine often focuses on immediately ruling out or identifying a source of increased ICP from hemorrhage, stroke, hydrocephalus (enlargement of the ventricles), or cerebral mass effect (displacement of brain tissue by blood or tumor or following trauma). This is currently done in the ED using CT scanners, which are fast, relatively cheap, and easy to site compared with MRI. The addition of POC MRI would be helpful for assessment of neurological emergencies for which CT provides a poor depiction or for patient populations sensitive to the ionizing radiation used in CT.
3.4.2 Acute Stroke Care
The use of low-field MRI in the stroke setting has recently been reviewed [52] including for resource-limited settings [53]. Portable low-field MRI has been contemplated for ruling out a hemorrhagic stroke to allow immediate reperfusion with the injection of recombinant tissue plasminogen activator (rtPA), which would exacerbate any existing cerebral hemorrhage. However, CT currently performs this job well, including portable units in EDs and even specialized CT-equipped ambulances [54–58] dispatched when stroke is suspected. Although a portable MRI would also be capable of this “rule out a bleed” role, it is worth looking to the next step of stroke care to avoid simply replacing one capable mobile imaging modality for another. Successful new intravenous thrombolytic therapies as well as widening windows for treatment have led to catheter-based thrombectomy treatments following the initial rtPA treatment [59,60]. A “hub-and-spokes” model for treatment sites has emerged for this treatment. First, patients with suspected stroke are rushed to the nearest “spoke hospital” where they receive a CT to verify stroke and rule out hemorrhage for immediate administration of rtPA. In the few cities with CT-equipped ambulances (mobile stroke units), this treatment is received in the field. Following rtPA administration, the patient is then evaluated for transfer to a “hub” hospital for catheter-based thrombectomy [59,60]. This assessment uses CT angiography (CTA) to identify a large vessel occlusion (LVO) target in the anterior circulation and perfusion (CTP) to estimate infarct size. An infarct core size >70 cc both reduces the likely benefit of revascularization and establishes a risk for hemorrhage during the procedure. Unfortunately, the infarct size determination is difficult and imprecisely measured on CTP [61]. In contrast, diffusion MRI is the established gold standard for early infarct core volume assessment [61] but is not commonly available in EDs [62,63]. The time-critical nature of acute stroke care and the cost of transport underscore the need for improved POC MR imaging in the spoke hospital ED to determine the eligibility of the patient for transport to receive catheter-based thrombectomy.
3.4.3 Assessing Pediatric Hydrocephalus in the Developing World
A low-cost and readily accessible POC MRI could be invaluable to guide and assess surgical treatment of pediatric hydrocephalus. In hydrocephalus, the brain’s ventricular system is blocked, which in turn causes swelling of the head to an abnormal size and increased cranial pressure. In Africa this is usually caused by a bacterial infection during the early months of life [64]. Untreated hydrocephalus can hamper brain development and incur a significant disease burden, including brain damage, blindness, and death [65]. Treatments include introducing a shunt to reestablish the cerebrospinal fluid (CSF) flow; however, this must be monitored and possibly replaced if clogged. More minimally invasive shuntless approaches [66] are becoming increasingly common in Africa, where follow-up assessment is difficult.
Imaging of the brain’s ventricular system is needed at all stages, from diagnosis to surgical follow-ups. Fortunately, the ventricular system is a large structure that dominates routine T1- or T2-weighted MR images and is perhaps one of the easiest intracranial structures to image with MRI and does not require high spatial resolution or sensitivity. It is also well depicted on CT, but the focus on a pediatric population and the need for repeated follow-up imaging creates a concern about the effects of CT’s ionizing radiation. This places MRI as the first choice for assessing and following this disorder. The combination of disease severity and prevalence, the available treatment, the limited spatial resolution needed, and remote distribution of the patients makes low-cost, portable MRI an ideal tool for pediatric hydrocephalus assessment [67–69]. Figure 3.4 shows a low-cost 50-mT permanent magnet-based system under development for this purpose [68,70].
Figure 3.4 A low-cost, lightweight scanner under development for pediatric hydrocephalus monitoring. The system uses a rare-earth permanent magnet “Halbach” arrangement producing a 50-mT transverse field. Magnet bore is 27 cm diameter × 50 cm length.
3.4.3 Mass-effect Monitor in ED or ICU Setting
Similar to the hydrocephalus application, knowledge of a mass effect (compression or displacement of the brain by a pathological source) is critical in acute care because it alerts the care team to the development of significant ICP (which is life-threatening). Mass effects can arise in a wide spectrum of conditions associated with hemorrhage (e.g. traumatic brain injury, rupture of an aneurysm or vascular malformation, hemorrhagic conversion of acute stroke, and hemorrhage resulting from a postsurgical event). Other sources include alterations of CSF dynamics (e.g. obstructive and nonobstructive hydrocephalus, intracranial hypertension, CSF leaks, and other disorders of CSF flow), and cerebral edema (e.g. from demyelinating disease, cerebral infections, or cytotoxic edema due to acute stroke). Finally ICP can be induced by tumors (primary brain tumors, metastases, and extra-axial masses). The presence of ICP is frequently detected by looking for a shift in mid-line structures of the brain, ventricular effacement (one ventricle bigger than the other), or