Standard Phantom Designed for Calibrating MRI Machines

The first phantom for calibrating magnetic resonance imaging (MRI) machines has been developed that is traceable to standardized values.

MRI, a widely used medical application that relies on magnetic fields and radiowaves to visualize the body's internal structures, particularly soft tissues, may soon become even more useful. The U.S. National Institute of Standards and Technology (NIST; Gaithersburg, MD, USA) has unveiled the prototype, named Phannie, which was developed in collaboration with the standards committee of the International Society for Magnetic Resonance in Medicine (ISMRM; Berkeley, CA, USA).

Traceable MRI calibrations are expected to enable accurate, quantitative measurements of tumors and other disease markers that can be reproduced across many different patients, scanners, and clinics over time and potentially reduce medical costs.

Displayed at the annual ISMRM meeting held in Stockholm, Sweden, in May 2010, the NIST phantom is a plastic sphere about the size of a person's head, filled with water-bathed grids of 100 small plastic spheres containing various salt solutions that become magnetized in a magnetic field. By making MRI scans of Phannie, users can evaluate the image contrast, resolution, and accuracy of distance and volume measurements. A machine's performance can be compared to standards, to other MRI machines, and to itself over time.

MRI scanner performance may drift, or different machines may produce different images of the same patient. The new phantom is intended to help generate more accurate and consistent images, validate disease mechanisms and treatments, and reduce medical costs by improving image quality and effectiveness. The phantom will assist multisite clinical trials that use quantitative MRI to test the efficacy of novel drugs.

A number of specialized MRI phantoms already exist; the need for new ones to support quantitative studies was recognized at a NIST workshop in 2006. NIST's is the first phantom designed to ensure that MRI system properties and image data are traceable to international system of units (SI) standards. The ISMRM Ad Hoc Committee on Standards for Quantitative MR defined the phantom requirements and values. NIST modeled and built the prototype device and assured the accuracy of measured quantities. NIST also developed and tested various systems used in the mini-spheres as contrast-enhancing agents and measurement reference markers. Phannie will now undergo testing at other institutions for about four months.

Dr. Stephen Russek, the physicist leading NIST's part of the project, reported that the phantom is intended to be not only accurate and traceable but also physically stable and cost-effective, so that it can become as widely used in MRI machines as seatbelts are in cars. He demonstrated the durability of the mini-spheres by bouncing one on the floor. Materials for Phannie cost US$10,000, but in mass production, the cost per phantom could be reduced to $2,000. "If it's accurate, reliable, and affordable, then you have a way to measure the accuracy of MRI scanners all across the country,” Dr. Russek stated. "If used routinely, it will allow us to get a complete snapshot of the quality and consistency of scanning.”

Related Links:
National Institute of Standards and Technology
International Society for Magnetic Resonance in Medicine