Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 52

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” approach following that originally introduced 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.