Dynamic volume propels CT into fourth dimension

The ability to control time-or, more specifically, temporal resolution-for volumetric acquisitions, is helping to create new applications and boost diagnostic confidence, particularly in musculoskeletal examinations.

During a special international session held at the Journées Françaises de Radiologie in Paris in late October, one of Europe’s leading musculoskeletal experts showed that rapid, continuous acquisitions, made possible with a 320-detector CT scanner, can produce clinically valuable, video-like studies.

Prof. Alain Blum, chief of radiology at Centre Hospitalier Universitaire in Nancy, France, displayed a detailed examination of thoracic outlet syndrome that helps patients avoid a procedure for rib dissection. He also demonstrated how intermittent timing allowed him to vary the delays between acquisitions to lower radiation exposure while still capturing volumes sufficient for a 3D reconstruction. In one case, this led to the detection of a schwannoma lesion in the neck within the full anatomical context of the bone structure. It also offers the ability to overlay blood vessels, including the delicate microvessels that feed a tumor.

“There is always a balancing act between the number of acquisitions, the spatial resolution, the temporal resolution, and exposure for the patient,” he said. “The ability to better manage temporal resolution changes that formula and opens up new possibilities.”

Unexpectedly vivid explorations of the interplay of vascular and osteoarticular structures presented by Blum were enhanced by the addition of the fourth dimension in the videolike reconstructions. Large-volume dynamic CT acquisition has dramatically modified the study of entrapment syndrome, the compression of arteries and veins by deformities, muscles, and tendons that results in claudication and chronic leg ischemia, he explained. Nancy is now the reference center in France for these types of conditions.

Assessing the cause of compression of the popliteal artery in the knee or a subclavian artery or vein was difficult on a 64-slice unit because of the combination of contrast enhancement, a higher radiation exposure, and a stop-start shuttering acquisition of the patient movement. However, set for continuous acquisition during the patient head movement prescribed for the study of thoracic outlet syndrome, the Aquilion One scanner from Toshiba can capture 20 motions in the arterial phase.

“It is very fast for acquiring the entire volume in a single shot,” Blum said. “One take and the patient can leave. And instead of reading sequential slices, we watch a video where we can see the compression is not in the costoclavicular ligament, but occurs in the scalene triangle.”

Most institutions do not distinguish between these two potential causes of compression and routinely send such patients for a procedure to resect the first rib. This can work, but if the compression is in the scalene triangle, only a single muscle needs to be cut, and the rib need not be removed, thus avoiding a severe procedure with many complications, Blum said.

The take-home message for orthopedic specialists is that CT technology enhancements have opened a new chapter in the evolution of the modality.

“Since I was a fellow in radiology, I have been hearing that CT is finished and MRI is going to take over,” he said. “Yet today as much is being spent on CT research as MRI because it is fast and yields rich data.”

Manufacturers over the past two years have been working to advance technologies for dual energy, dual-layer detectors, spectral energy, larger detector arrays, and reconstruction algorithms, all while continually reducing radiation dose levels.

“I am sure we will see a convergence of technologies and a consensus on clinical value, but for the moment it is exciting that so many possibilities have been created,” Blum said.

He will host the Fifth Annual Symposium on Volumetric Scanners in Nancy on Jan. 24-25, 2011.