Low-cost MR scanner opens door to wide-scale diabetes, breast imaging

A novel low-field electromagnetic MR scanner has the potential to deliver high-quality images, even in the presence of metal implants, and can do so for radically less than the cost of conventional superconducting magnets.

The prototype, called a prepolarized MRI knee scanner (PMRI), operates at low field strengths — between 0.06T and 0.4T. But the system, which cost only about $25,000 to build, might be scaled up to a 1T system for only a few thousand dollars more, according to Steven Conolly, an associate professor of bioengineering at the University of California, Berkeley.

Its low cost and ability to deliver high-quality images make the technology especially promising for budget-constrained applications, such as imaging patients with diabetes or performing breast MRI.

"A low-cost MRI scanner could play a role in diagnosing vascular changes and infections of the diabetic foot," said Conolly, who will present the clinical results Friday. "Breast MRI is another possibility. Traditional x-ray mammography is nearly 10 times less expensive than a conventional MRI study."

The key to keeping costs low and image quality high is the use of a strong inhomogeneous magnetic field pulsed inside a separate, homogenous low magnetic field. This configuration, produced by the PMRI scanner, is the outgrowth of more than a decade of work, which began at Stanford University and migrated to Berkeley two years ago, when Conolly took a faculty position there.

The system includes a copper four-coil homogeneous water-cooled readout electromagnet capable of generating a field up to 0.2T with a polarizing magnet inserted in the bore. This second magnet would be wide enough to fit an adult knee. Total polarizing field strength is 0.4T; readout field strength is 0.06T.

"If you do the engineering carefully, you get the same image quality as a conventional scanner," he said.

Tests on normal subjects revealed excellent detail of the cartilage, although signal to noise was about one-fifth of the SNR obtained when comparison scans were performed on a 1.5T Signa system. Striking differences were seen in comparison images, however, between the two systems, as the prepolarized image showed only minor artifacts caused by stainless steel screws in two of the subjects' knees, which substantially impaired the images obtained at 1.5T.

The trick behind the image quality is to continuously vary the polarizing and readout fields. Conolly and colleagues simultaneously turn the polarizing magnet to 0.2T and the readout magnet to 0.2T to create 0.4T. They then turn off the polarizing magnet and turn the readout magnet down to 0.06T for a low-field readout that minimizes artifacts caused by metal implants.

"Metal artifacts are reduced from a few centimeters to a fraction of a millimeter shift," Conolly said. "We have imaged patients with total knee replacements. The improvement near metal is impressive."