Most presently installed full-body MRI systems are based on 1.5 T liquid-He- cooled NbTi magnets, but the rate of installation of 3.0 T systems is rapidly increasing. However, the increasing cost of helium is well documented, and this leads to increases in the life cycle cost of MRI systems, which could limit ready access for traditional patient groups, and limit the ability to expand availability to traditionally less well served patint groups. The root cause of the problem is the growing shortage of helium worldwide. The major MRI magnet manufacturers (Siemens, GE, and Phillips) each have development programs focused on converting the 1.5T solenoid coils from liquid helium bath cooling to dry (cryogen free) conduction cooling. Indeed, freedom from the need for liquid-helium cooling (i.e. cryogen free, conduction-cooled, operation) is becoming more and more important. Presently, MRI systems (2000-3000 liters) are filled with liquid He helium at the factory prior to shipping at a cost in the U.S. of about 20-30 K. Given that from 2006 to 2011 the cost of LHe has tripled, and it is expected to continue to increase, corresponding increases in the cost of liquid-He MRI units can be expected. In some parts of the world liquid He costs about $9-13 per liter and in others about $25 to $70 per liter leading to re-filling costs of $18,000-$210,000 depending on the MRI's size and location. Thus a growing demand is predicted for MRI magnets that do not depend on liquid He for cooling. Even beyond the liquid cryogen issues for MRI, the development of cryocooled MRI magnets could allow for new geometries and MRI applications, opening up new treatment possibilities. However, we must focus where the industry can transition this new technology commercially. Accordingly, the proposed program focuses on developing the technology to enable cryogen free 3 T systems using a cryocooled mode. We proposed to do this by; (1) Increases practical MgB2 engineering current density, Je, by one order of magnitude over presently developed 1 G wire, in the fields and temperature ranges appropriate to 3 T MRI (4-10 K, 0-4 T), (2) Developing n-values sufficient for persistent mode magnet operation (30 or more in the relevant field range) in the multifilament 2 G wires of (1), (3) Developing a magnet model with sufficient detail to develop the conductor specification/targets, and the tradeoffs between cooling, temperature gradients across a coil segment, and conductor performance, (4) Determining the proper Cu/SC ratio for wire-in-channel MgB2/Cu, and demonstrate its long-length manufacture, (5) Demonstrating one segment of the total coil, using the WIC of item (4), with the needed current, n-value, temperature gradient, and quench performance, and (6) Developing a robust persistent joint for MgB2 conductors in a magnet engineering friendly react & wind mode.
Present day MRI systems, whether 1.5 T or 3 T, use NbTi superconductors and operate in liquid helium. However, the cost of liquid helium has been rising rapidly, and is expected to continue to do so, helium is a finite resource. Thus, costs for systems based on liquid helium are expected to increase, with potential negative impact on MRI availability, or machine and thus cost-per scan. This proposed program aims to develop essential technology to permit systems which can operate with much lower amounts of helium using a cryocooling method. This will also enable new MRI applications not constrained by the designs requiring liquid cryogen cooling, thus opening up new treatment possibilities. In order to do this, improvements in conductor, magnet design, and wire joints are needed. However, program success would lead to reduced costs and improved health care for the public.
