The Joule–Thomson micro-refrigerator and rf coil assembly. (top) Photograph: pumping port; for temperature sensor; pressure gas input line; pressure gas output line; capacitors; -ring seal between JT module and glass vacuum chamber; coil; cable. (bottom) Schematic for the assembly, drawn approximately to scale.
Cool-down curve demonstrating the JT cryo-coil system performance (closed in K, open ). Dashed vertical lines at 28 and at indicate, respectively: (1) the time at which input nitrogen gas pressure was increased from , initiating the cool-down; and (2) the time at which the vacuum-assist pump was turned on.
2D gradient echo MR images of a bottle of NaCl in water: (a) using the room-temperature rf coil ; (b) under same conditions as for (a), except using the cooled rf coil , showing a SNR gain of 2.3 within the drawn circles. The same image display settings were used for both (a) and (b). Scale .
SNR profiles from spin echo images of a sheep intervertebral disc, for the room-temperature coil (thin line) and for the JT-cooled coil (thick line). Each SNR profile represents the average of 20 adjacent intensity profiles through the disc (inset) divided by the average background intensity. The narrow peak at corresponds to the anterior ligament of the motion segment, small peaks at correspond to lamellae of the annulus fibrosus, and the broad peak at corresponds to the nucleus pulposus of the disc.
MR microscopy of a sheep intervertebral disc in which are visible collagen lamellae of the annulus fibrosus (arrows). The rf surface coil (located from the left edge of the specimen) was cooled to using the JT micro-refrigerator in a clinical MRI scanner. The entire 3D data set was acquired in . Scale . (top) Sagittal slice, acquired with in-plane , interpolated to , and slice . (bottom) Reformatted axial slice through the disc, showing microstructure of the annulus lamellae, with in-plane interpolated to , and slice .
Expected cryogenic SNR gain (squares), at Larmor frequencies relevant to clinical and research MRI, simulated using the experimental conditions of Fig. 4. Also shown at each frequency is the simulated SNR for a room-temperature coil (circles). The solid lines are least-squares fits to the lowest two and highest two SNR values, respectively, of functions representing the limiting regimes of coil noise dominance and sample noise dominance.
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