Phase-contrast cine MR gives musculoskeletal imaging dynamic twist

A new space shuttle mission reminds musculoskeletal specialists that astronauts share with bedridden patients a susceptibility to muscle and tendon atrophy. Phase-contrast cine MR at 3T provides a useful, accurate way to guide management of this atrophy, according to researchers at the University of California, Los Angeles.

Chronic unloading - during space flight, after severe fractures, or following prolonged whole-body immobilization - leaves muscles prone to fast atrophy and protein loss. It also has adverse effects on the mechanical properties of the tendons, leading to deterioration of overall musculoskeletal function.

Measuring the stiffness of the Achilles tendon could help predict and monitor recovery after sustained unloading. The microscopic detail of tendon fibers and movement artifacts inherent to these MSK structures, however, pose considerable challenges for conventional MR. A new technique that combines phase-contrast 3T MRI and an MR-compatible dynamometer allows noninvasive stiffness measurements in vivo, said principal investigator Dongsuk Shin of UCLA's biomedical engineering department.

Shin and colleagues recruited seven healthy subjects who underwent chronic unloading of one leg induced by 30 days of unilateral lower limb suspension (ULLS) followed by six weeks of physical rehabilitation. The investigators performed phase-contrast cine 3T MR scanning using a multichannel phased-array coil. They also used a strain-measuring dynamometer placed inside the cast covering the affected leg to monitor changes in Achilles tendon stiffness. Scanning took place before and immediately after ULLS and at weeks one, two, four, and six of physical rehabilitation.

The investigators found the phase-contrast cine technique successfully quantified the Achilles tendon changes after 30 days of chronic unloading. Data provided insight into the genesis and implications of strain injuries leading to or resulting from chronic unloading. Shin presented his group's findings at the 2006 International Society for Magnetic Resonance in Medicine meeting.

The fiberglass cast used to immobilize the leg induced 86 isometric tendon contractions that were captured in 22 phases using a FLASH 2D phase-contrast sequence with a temporal resolution of 80 msec. The phase-contrast cine technique showed the Achilles tendon stiffness decreased from 62.27 Newtons per millimeter (N/mm) to 42.07 N/mm, representing a 32.4% overall reduction. Stiffness gradually recovered to normal levels during the six weeks of physical therapy.

The study underscores the increased interest in 3T MR applications in clinical practice as well as its usefulness for imaging of tiny anatomic structures. The technique may lead MSK specialists to rethink the conventional wisdom regarding MR imaging as a "static" imaging modality. Findings suggest phase-contrast cine could open a new front in the assessment and management of MSK injuries using motion-based studies.

"Phase-contrast imaging at 3T for tracking muscle movement offers the distinctive advantage of high spatial resolution and large field-of-view," Shin said.

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