Portable cone-beam CT (CBCT) technology now allows for weight-bearing imaging of the lower extremities, offering a useful alternative to standard radiographs when imaging the knee, ankle and foot, according to an article in the January issue of the American Journal of Roentgenology.
Cross-sectional modalities like CT and MRI have been limited, until recently, to imaging patients in the supine position, explained authors Esa K. J. Tuominen, MD, of the Medical Imaging Centre of Southwest Finland, Turku University Hospital, Turku, Finland. To attain sectional imaging of weight-bearing extremities, pressure would have to be applied to patients feet as they lie on their back in the scanner.
The new scanner offers a more accurate picture of extremities under natural weight-bearing conditions. “CBCT technology enables a small gantry size customized for orthopedic imaging of extremities,” wrote the authors. “The scanner design allows a wide range of gantry movements, including tilting the gantry to the horizontal orientation and lowering it close to floor level, making weight-bearing imaging with the patient in a standing position possible.”
Tuominen and colleagues described the scanner in use at their institutions as weighing 350 kg, with dimensions of 185 x 76 x 169 cm. It can be powered by a regular power socket with a power requirement of 150 kVA, with no need for external cooling. The scanner, derived from clinically established dental cone-beam scanners, can be deployed in existing general x-ray rooms alongside permanently installed equipment.
Tube voltages of 84-96 kVp and tube currents of 1-10 mA can be used, according to the authors. Total exposure time for isotropic scanning is approximately six seconds.
“According to experiences in our institutions, the CBCT technology allows peripheral extremity scanning with equal or smaller effective radiation doses compared with MDCT without sacrificing the image quality, in keeping with the previously published reports for other CBCT scanners,” wrote the authors.
They added that radiation doses often can be reduced by moving the scan area further away from the patient’s trunk, and that dosed measured for typical extremity scans range from 0.01 to 0.03 mSv per scan.