Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 59

The goal of reducing cost, siting needs, and even enabling portability has resurrected interest in low-field MRI technology, including new magnet and encoding methods. Low-field MRI, loosely defined as scanners operating between 10 mT and 500 mT, has a long history in MRI, starting with the first human imaging systems [80–83]. Although high-field superconducting magnet-equipped scanners have largely displaced them, thousands of low-field MRI systems (mainly in the 0.2–0.35-T range) are in daily operation and their clinical capabilities are well known. Additionally, commercial systems operating below 100 mT have an established clinical history; for example, the Toshiba Access 64-mT system introduced in 1991. These older generation low-field systems are often called “open MRIs” due to the two-pole-piece dipole magnet geometry used. They are still big and heavy and have most of the siting issues of their high-field superconducting cousins. This review focuses on the modern generation of low-field scanners specifically aimed at the three POC uses outlined earlier. The “low” vs. “ultralow” tradeoffs have been recently reviewed in depth [84].

In addition to low-field architectures, considerable research has also focused on ultralow field (ULF), with B₀ below 10 mT [85]. However, several barriers remain to using ULF approaches to achieve low-cost or POC and/or portable medical imaging. The sensitivity of MR as a function of field strength is well known [18], and the severe penalty at ULF requires either additional technology such as prepolarization [86], cryogenic detectors [87–90], hyperpolarized media [91], or efficient but nonstandard brain-imaging sequence schemes [92]. This additional technology can be a barrier to achieving reduced costs or portability. Additional issues arise for image encoding at ULF due to unwanted encoding by the gradient coil’s concomitant field terms [93,94].

Because of the signal-to-noise ratio limitations of ULF MRI, this review focuses mainly on compact magnets with B₀ of above 50 mT or compact superconducting magnets in the 0.5–1.0-T range. We focus on magnet and system architecture and component technology; the constraints of MR physics at low field are well understood and are discussed in several recent reviews [15,18,19,84,95,96].