Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 48

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.