Microtesla MRI with a superconducting quantum interference device.
MRI scanners enable fast, noninvasive, and high-resolution imaging of organs and soft tissue. The images are reconstructed from NMR signals generated by nuclear spins that precess in a static magnetic field B0 in the presence of magnetic field gradients. Most clinical MRI scanners operate at a magnetic field B0 = 1.5 T, corresponding to a proton resonance frequency of 64 MHz. Because these systems rely on large superconducting magnets, they are costly and demanding of infrastructure. On the other hand, low-field imagers have the potential to be less expensive, less confining, and more mobile. The major obstacle is the intrinsically low sensitivity of the low-field NMR experiment. Here, we show that prepolarization of the nuclear spins and detection with a superconducting quantum interference device (SQUID) yield a signal that is independent of B0, allowing acquisition of high-resolution MRIs in microtesla fields. Reduction of the strength of the measurement field eliminates inhomogeneous broadening of the NMR lines, resulting in enhanced signal-to-noise ratio and spatial resolution for a fixed strength of the magnetic field gradients used to encode the image. We present high-resolution images of phantoms and other samples and T1-weighted contrast images acquired in highly inhomogeneous magnetic fields of 132 microT; here, T1 is the spin-lattice relaxation time. These techniques could readily be adapted to existing multichannel SQUID systems used for magnetic source imaging of brain signals. Further potential applications include low-cost systems for tumor screening and imaging peripheral regions of the body.
McDermott
Robert
R
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. robertm@boulder.nist.gov
Lee
SeungKyun
S
ten Haken
Bennie
B
Trabesinger
Andreas H
AH
Pines
Alexander
A
Clarke
John
J
D002212:Capsicum; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008280:Magnetics; D009369:Neoplasms / Q000175:diagnosis; D011789:Quantum Theory