Princeton's New MRI Unit Improves Visualization of Joint Implants

Sep 08, 2014 at 04:20 pm by steve

Sherry Pair with the MAVRIC SL MRI.

They are two of clinical medicine's most influential advances of the past generation: joint replacement surgery that restores normal function for patients with arthritic or dysfunctional hips, knees, and shoulders; and magnetic resonance imaging (MRI), a special scanner that gives physicians unprecedented diagnostic views of soft-tissue structures within the body that don't show up well on traditional X-rays.

Until recently, the two breakthroughs have not coexisted very well. That's because the metal portions of the implants can interfere with the MRI's electromagnetic signal, causing "blurs," "voids," or other artifacts in surrounding areas.

But a new software system known as Slice Encoding for Metal Artifact Correction, or SEMAC, offers enhanced clarity in visualizing the soft tissues and bones around implants, according to Sherry Pair, manager of outpatient radiology and hospital CT at Baptist Medical Center/Princeton.

"Up until now, the metal has been a real problem for doctors," Pair says, "and they've often had to go back to routine fluoroscopic X-rays to get some idea of what's in and around the joint. But this new suppression technique gives much better results in being able to see the joint and the surrounding tissue within the joint space."

BMC/Princeton is the first hospital in central Alabama to use the new technology; the device, developed by GE Healthcare, is known as the MAVRIC SL. It works, according to the manufacturer, by "creating an excellent signal-to-noise ratio and image integrity near MR-Conditional metallic devices."

The traditional distortion caused by medical devices on MRI scans shows up as large blurred areas of black, fanning out from the structure of the implant.

"In the past," Pair says, "without the new software, the metal created a signal void that blocked the visualization of the area above a hip implant, for example. And with a knee implant, that image can be blurred out above and below the knee.

"But now, it's possible to see the soft tissue where those signal voids used to be, and to determine whether there's fluid there that's causing a problem. We can find out what's wrong, and what to do about it."

Side-by-side comparisons of traditional implant MRIs and the new suppression technique show a dramatic difference. The new technology, which received FDA clearance in December, 2012, has two parts. First it acquires multiple overlapping images at a variety of frequencies and combines them. The process is somewhat similar to the way conventional color film works, with components that separate the frequencies of light into blue, green, and red images. Then, the software uses a de-blurring algorithm to reduce the signal void and distortion.

The new development comes at an opportune time for the field; with the aging of the American population, total joint replacement is increasingly common for patients. Some implants develop infections or other problems and require "revision surgery," and the improved diagnostic images are vital for detailed diagnoses and surgical planning.

The MAVRIC device is incorporated into the MRI unit itself and requires no additional training on the part of technicians.

"We have many physicians who are ordering the process specifically," Pair says, "and we're trying to get the word out that we have this new breakthrough capability."

The new MAVRIC is the third MRI device for the Princeton campus. It's a wide-bore unit, reducing anxiety for patients with claustrophobia issues--it doesn't fit as tightly around the arms as traditional units. It can also accommodate patients with weights up to 500 pounds. In the past, such "open MRIs" have shown less resolution than traditional machines, but Pair says the new unit bumps up field strength and resolution so that it competes with conventional, or short-bore, MRIs.

One of the current MRI units has a Tesla rating (a measure of the magnet's strength) of 3T, the others are 1.5T.

In brief, Pair says, the new MAVRIC software "presents a phenomenal opportunity for giving patients the best care, and we're excited about making it available."

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