Calibration Tools Used to Enhance Ultralow Field MRI Technology

The NIST prototypes are the first standard calibration tools for ULF-MRI, offering a quantitative means to assess performance, validate the technique, and directly compare different experimental and clinical MRI scanners. “Tissues that may look the same in clinical MRI can look very different in ULF-MRI, which provides new contrast mechanisms,” NIST physicist Dr. Michael Boss stated. “Our hope is that we can move this technique along to attract more interest from [industry] vendors.”

MRI noninvasively images soft tissues based on measurements of how hydrogen nuclei respond to magnetic fields. ULF-MRI enhances tissue contrast in specific types of MRI scans. Prostate tumors, for example, can be difficult to see with conventional MRI but show up clearly under ULF-MRI. ULF-MRI has also been used in research approaches to image the brain, and assessed in at least one nonmedical application, inspection of liquids at airports.

ULF-MRI also offers practical advantages: the systems are more streamlined, lighter in weight, and less costly than conventional MRI scanners. That is because ULF-MRI operates at much lower magnetic field strengths, measured in micro-Teslas, thousands of times lower than conventional MRI, which operates at up to 3 Teslas and requires large magnets. The low magnetic field strength means ULF-MRI requires the most sensitive magnetometers available: superconducting quantum interference devices (SQUIDs). This is convenient in that it makes ULF-MRI suitable for combining with other SQUID-based imaging techniques such as magnetoencephalography.

NIST scientists earlier designed phantoms for standard MRI systems and also have extensive experience both making and using SQUIDs. NIST’s new ULF-MRI phantoms are short plastic cylinders, shaped like hockey pucks but a bit smaller, containing six or 10 plastic jars filled with various salt solutions that become magnetized in a magnetic field. Each phantom measures a different aspect of scanner performance such as spatial resolution. NIST researchers tested the new phantoms on both a conventional MRI system at the University of Colorado Health Sciences Center (Denver, CO, USA) and a research ULF-MRI scanner at the University of California (UC) at Berkeley, where the technique was first demonstrated about than 10 years ago.

Tests findings revealed that the prototype phantoms are well matched to ULF-MRI applications and they allow direct comparison of ULF and clinical MRI system performance. NIST researchers now plan to integrate design improvements based on lessons gleaned from the prototypes, with the aim of improving phantom stability and providing traceability to standard measurement units. NIST and the University of California (UC) Berkeley (USA) researchers also plan to work together to develop ULF-MRI technology for the detection of prostate and breast cancers.

NIST’s phantoms for conventional MRI systems are currently being tested by hospitals and MRI manufacturers, and Sigma-K Corp. (Durham, NC, USA) is devising ways for making copies for more widespread distribution under a NIST Small Business Innovation Research Award (SBIR).

The study’s findings were published online November 26, 2013, in the journal Magnetic Resonance in Medicine.

Related Links:
National Institute of Standards and Technology
University of California, Berkeley
Sigma-K