Magnesium Diboride Superconducting Magnets Used in MRI

As a result of the growing concern over helium shortage and the need for direct conduction cooling, Hyper Tech’s magnesium diboride (MgB2) superconductors have been making substantial inroads into applications previously dominated by niobium titanium (NbTi). Working with Brookhaven National Laboratory, NASA, the National Institutes of Health (NIH) and others to solve the problems arising from this technological shift, Hyper Tech Research has taken a step toward making this technology a reality with their development of MgB2 based conduction cooled MRI magnets specifically targeted for image-guided radiation therapy (IGRT). This has taken the combined resources of Hyper Tech, Case Western Reserve University (CWRU), the Ohio State University Center for Superconducting and Magnetic Materials (OSU CSMM) and Eden Cryogenics (CSA CSM). Under a generous grant from the NIH, the magnet design had to be created from the stated specifications used for the original NbTi based version, now in clinical operation and being marketed worldwide.

Assembly of 1-meter-diameter Hyper Tech conduction cooled coil into test cryostat at OSU CSMM. Image: Hyper Tech

Assembly of 1-meter-diameter Hyper Tech conduction cooled coil into test cryostat at OSU CSMM. Image: Hyper Tech

Besides avoiding the effects of the helium shortage, there are several benefits from using a conduction cooled MgB2 magnet for MRI. Eliminating the need for liquid helium bath cooling prevents losses due to cooldown, initial fill, air transport, quenches occurring in the field and lost time. Due to its temperature margin, MgB2 greatly reduces the potential for the magnet to quench and prevents the hospital from performing imaging. It also eliminates the need for an ASME liquid helium pressure vessel, which is now required for current NbTi MRI systems, and eliminates the need for a venting system at the hospital to vent helium if the magnet quenches. Presently, most MRI systems are shipped by air to preserve helium filled in at the factory; without the onboard helium, the magnets can be shipped at a lower cost by conventional transportation. In addition, Eden Cryogenics and Hyper Tech are working together to engineer and design a cryostat that will not only be thermally efficient at helium temperatures but also will not affect the operation of the high magnetic field.

Early on, several challenges were identified that complicated the design, the first being existing hardware constraints and site installation requirements. The positioning of the coils to create a 50 cm DSV located precisely in the center of the gamma treatment zone required specialized software that CWRU developed. Once this was done, the mechanical, electrical and thermal designs could proceed. Mechanically, the MRI portion had to be designed with two independent coil sets mirrored on either side of the rotating gantry producing the treatment zone. Compressive forces across this gantry were reacted through three azimuthally symmetrical cold links joining the opposing coil sets. A persistent switch and coils had to be joined together into a continuous electrical loop with interconnecting persistent joints. These are not commercially available and are being developed by Hyper Tech. As with all superconducting magnets, the coil sets reside within a shroud cooled by the first stage of a cryocooler and are then cooled to 10K or below with the second stage. Unlike conventional LHe bath cooled NbTi-based MRI magnets, the coil sets, persistent switch and persistent joints had to be interconnected with a network of thermal buses terminating at the cryo­cooler’s second stage, with MLI minimizing radiation heat loss to the shroud and coil sets.

MRI safety requires that the magnetic field has to be reduced to safe levels within a prescribed time period via an emergency kill switch, the proverbial “big red button.” Due to the intrinsically high energy required to quench the coils, large quench heaters connected to a quickly deployable outside-stored energy source were necessary to heat the coils sufficiently to propagate quenches that dissipate the stored energy ohmically within each coil.

Hyper Tech has fabricated and tested 1-meter-diameter conduction cooled coils. Testing is being carried out at OSU CSMM in a special cryostat designed specifically for testing conduction cooled coils. Since the full-scale IGRT MRI coils are too large for this existing cryostat, a larger 1.7-meter-diameter cryostat had to be designed and is being built by Eden Cryogenics. This cryostat is nearing completion and will be installed before the end of 2015.

Early success with a persistent joint design gave cause to proceed with an overall MRI design requiring persistent joints in lieu of a continuously operating filtered power supply. The persistent joint development is currently underway using special low resistance field decay test equipment designed and built by Hyper Tech and OSU CSMM. A persistent switch has been designed and prototype fabrication is scheduled for early 2016.