Low-dose CT makes gains in musculoskeletal imaging

A specific low-dose multislice CT protocol can provide clinically useful images of the wrists, according to a study in the September issue of the American Journal of Roentgenology. The findings take the trade-off between image quality and low radiation exposure up a notch.

MSCT has allowed imagers to scan sizable proportions of the anatomy while reducing both scanning times and section collimations. It has also allowed them to take advantage of 3D reconstruction and visualization tools. These attributes, however, have not brought notable gains to some aspects of musculoskeletal imaging. Small joints and other alignments of fine bone tissue and cartilage pose imaging challenges, including excessive noise.

Dr. Harald Bonel and colleagues at the University of Berne Hospital in Switzerland used 30 anatomic wrist specimens from cadavers to test several MSCT acquisition protocols. They found one that provided superior image resolution at the expense of bearable artifacts.

The best acquisition and reconstruction protocol featured:

• four-slice scanning

• 0.5-mm to 1-mm slice thickness

• 2-mm x 0.5-mm collimation

• 0.75-sec rotation time

• 80 kVp and 100 mAs

• 1.5 pitch (or lower)

Low-dose scanning using these acquisition parameters could become distinctly noisy, but it wouldn't degrade the visualization of the wrists' small bones. Even though 3D reconstructions provide a handy presentation format, most imaging workups of the wrists should rely on MRI for characterization of soft tissues, the researchers said.

Increasing tube voltages from 120 kVp to 140 kVp did not improve image quality. If radiologists need to compensate any attenuation of radiation due to fiberglass or plaster wrist castings, they should increase the tube current but not the voltage, the researchers said.

The study has some limitations. The imaging protocol in living patients requires them to lift their arms above the head. Scanning acquisition parameters, including dose, may vary. In addition, the study focused only on subtle anatomic details, not on actual abnormalities of the carpal bones. More studies of small and occult bone anomalies are needed.

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