Magnetic Resonance Imaging (MRI) has been shown to be an excellent imaging platform for understanding structural and functional connectivity in the brain as well as neuroanatomy. It has been proposed that MRI can potentially provide a better understanding of neurocognitive disorders, dementia, depression, traumatic brain injury, and stroke. The gold standard for neuroimaging has been 3.0T MRI. However, access to this imaging technology has been limited due to high installation costs and the space needed to site a conventional whole-body 3.0T MRI scanner. There is a need for a compact 3.0T that is easy to site and has sufficiently high performance to raise brain imaging to a new level.

Previous work using Nb₃Sn superconducting wire operating at 10K demonstrated that a 3,000kg 0.5T magnet could be built and operated routinely without using cryogen (1,2). This earlier work established the proof-of-principle that a magnet could be maintained at superconducting temperatures using conduction-cooling only (i.e., with a pair of Gifford-McMahon (GM) cryo-coolers). The objective of this work was to increase the field to 3.0T for a compact magnet to image the brain while using conventional NbTi superconducting wire. This entailed increasing the total stored energy by about a factor of 3 compared to the previous whole-body cryogen-free 0.5T system.

A novel conduction-cooled magnet for 3.0T brain imaging was designed to have a warm-bore inner diameter of 62 cm, and an imaging field-of-view (FOV) of 26 cm. The system design targets are listed in Figure 1. The use of NbTi wire facilitated operation at 4K using a single 1.5W GM cryo-cooler (Model RDK-415A3, Sumitomo Heavy Industries, Allentown, PA).

The gradient coil for this system was first validated in a whole-body 3.0T MRI system and achieved 85 mT/m peak field and 700 T/m/s slew rate (3,4). Due to the smaller size and asymmetric design of the head-gradient coil, it was able to operate close to 700 T/m/s with minimal peripheral nerve stimulation (5). The performance of the gradient coil exceeded that of any clinical whole-body system and utilized only 1 MVA of peak power per axis. For imaging, a 32-channel receive array was used (Nova Medical, Wilmington, MA). Imaging tests were conducted on the 3.0T compact system to determine image quality, eddy current effects and acoustic noise.

All in-vivo imaging tests were conducted under an Institutional Review Board approved protocol. Written informed consent was received from 4 healthy volunteers who were scanned multiple times.

The completed magnet (Figure 2) had an overall mass of <1,900 kg, compared to 5-7,000 kg for conventional whole-body 3.0T systems. In addition, the magnet demonstrated successful operation at temperatures <4.5K. Magnet drift was measured at <0.005 ppm/hour, substantially better than the industry standard of <0.1 ppm/hour. The magnet utilized passive shims and achieved a magnetic field homogeneity <1.9 ppm (p-p) over a 26-cm DSV before application of linear shim correction. Initial magnet tests indicated a stable imaging platform that met all design targets.

Imaging tests with gradient-recalled echo, fast spin echo and echo-planar imaging pulse sequences showed excellent image quality and minimal spatial distortion over a 26-cm FOV, specifically in the cerebellum and frontal lobe of the brain (Figure 3). The high-performance gradients allowed high-spatial resolution EPI images to be acquired without severe geometric distortion as usually encountered with whole-body MRI systems. EPI images with 1.5-mm isotropic resolution had high SNR with signal dropouts and severe geometric distortion typical of whole-body MRI systems largely absent.

The system successfully operated at 80 mT/m and 700 T/m/s using the standard system electronics of a 3.0T MR750 whole-body system. Preliminary measurements indicated sound pressure levels slightly above conventional 3.0T systems but within the prescribed safety limits.

A lightweight, low-cryogen 3.0T MRI system was successfully demonstrated. The performance of this system has met or exceeded expectations. The light weight and extremely low cryogen features of this platform permit this system to be installed in areas where space is constrained (as in emergency rooms, neurology or psychiatry offices), upper floors of buildings, and also in interior rooms where it is impractical to run helium cryo-vents. Initial imaging tests indicated that image quality was at least equivalent if not better than that encountered in whole-body 3.0T scanners. Clinical evaluation is scheduled for a patient population.