Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 60

Recent advances have been made in cryogenic technology that facilitate maintenance and siting of superconducting high- and mid-field MRI systems in diverse locations. Modern cryocoolers can now remove more heat than previous generations allowing reduction and even elimination of the “bath” of liquid helium (LHe) that has typically surrounded the superconducting wire in the magnet to maintain it at 4.2 K. These cryogenic technologies and those leading up to them have been extensively reviewed [97]. In the earliest superconducting magnets, a two-cryogen system was used; LHe on the inner vessel providing a liquid bath in direct contact with the niobium titanium (NbTi) superconducting wire, and liquid nitrogen (LN) in an outer vessel to reduce heat flow to the environment. The residual heat entering the LHe vessel was countered only by the latent heat of evaporation of helium, about 20.5 kJ/kg. Thus, boiling off ~8 l of Lhe per day (~1 kg per day) compensates for a heat leak of only 0.24 W. The next step was to replace the use of nitrogen with a mechanical refrigeration system (“cold-head”). This is considerably safer, since nitrogen gas poses increased asphyxiation risk compared with helium (which floats up rather than sinking down). In addition to being more convenient, holding a radiation shields outside of the LHe vessel to 20 K and 70 K using cold-heads also decreases the heat flow compared with a similar 77 K LN cooled surface.

Superconducting MRI systems typically employ Gifford–McMahon (GM) cryocoolers, a variant of the older Stirling cycle. GM cryocoolers have a moving rotary valve in the subsystem attached to the magnet, which makes the familiar steady “washing machine” sound of a nonscanning MRI, and is also a potential source of failure and mechanical vibrations. Current cold-heads can achieve more than 1 W of cooling at 4.2 K and tens of Watts at 12 K. This is enough to not only lessen the entering heat flux, but to fully overcome it, allowing GM cryocooler technology to provide sufficient heat removal at 4 K that helium is liquefied inside the cryostat. The result is a “zero-boiloff” magnet, now the norm for new clinical MRI systems. While not a fully “dry magnet,” current zero-boiloff designs eliminate the need for regular cryogen fills.

The next step is to eliminate the LHe completely, or at least reduce LHe volume to where the system can be safely operated without an emergency quench vent system (<10 l of LHe). This can save significantly on siting costs, especially in a complex hospital setting such as the ED. Elimination of most of the LHe bath requires direct “conduction cooling” of the superconductor by the cold-head (as opposed to bathing the windings in LHe). Although first envisioned as needing more exotic higher temperature superconductors such MgB₂ [98,99], “dry magnets” have recently been introduced by the major clinical MRI manufacturers using standard NbTi wire. The use of high-temperature superconductors that operate well above 4.2 K is attractive because they have an easier cooling target but is currently held back by their increased cost.