Hand-wrist, knee, and foot-ankle dosimetry and image quality measurements of a novel extremity imaging unit providing CBCT and 2D imaging options.

PURPOSE

Radiation dose is a general concern in diagnostic imaging and a special concern for children who are at greater risk from radiation effects. This study evaluates effective doses (E) produced during 2D and volume imaging with a novel cone beam computed tomography (CBCT) based extremity imaging device. The device's compact size and protocol options offer image choices that enhance the potential for reduced dose and improved diagnostics when evaluating sports injuries.

METHODS

Hand-wrist, foot-ankle, and knee phantoms were developed for use with optically stimulated luminescent dosimeters (OSL). Dosimetry of transmission radiographs (2D) and CBCT volumes (3D) was assessed for Standard and lowered dose (Lite) exposure protocols. Effective dose was calculated for child and adult age groups. Image quality was assessed with contrast-to-noise ratio (CNR) and modulation transfer function (MTF). A figure of merit was calculated as the square of CNR divided by E.

RESULTS

Standard 2D doses ranged from 0.001-0.06 μSv for adults, 0.001-0.05 μSv for 15-yr olds, 0.003-0.13 μSv for 10-yr olds, and 0.005-0.20 for 5-yr olds. Dose reductions with Lite protocols ranged from 26 to 51%. Standard CBCT doses ranged from 0.3-4.6 μSv for adults, 0.3-4.4 μSv for 15-yr olds, 0.6-9.8 μSv for 10-yr olds, and 1-22.6 μSv for 5-yr olds. For both 2D and 3D a trend of increasing imaging dose with reduced age was present and was statistically significant for children below the age of 10 (P = 0.0009). Lite dose reductions averaged 47%. CNR was statistically reduced in Lite dose scans (P = 0.0384) but was not statistically different using FOM analysis (P = 0.3089). MTF was not significantly affected by the two dose protocols (P = 0.8104).

CONCLUSION

CBCT effective doses calculated from anthropomorphic phantom exposures following manufacturer suggested protocols appear to be substantially less than previously reported doses for similar MDCT extremity examinations. In this study, effective dose from 2D radiographic imaging was approximately two orders of magnitude less than CBCT imaging. Doses were on the order of a few minutes to hours of ubiquitous per-capita background dose for 2D imaging and a few hours to days for 3D imaging. Dose significantly increased for children younger than age 10. Lite protocols resulted in substantial dose reductions and can be recommended for children and those examinations where reduced CNR will not affect diagnosis. Flexibility of 2D and 3D imaging options with low-dose protocols make this technology a promising option for radiographic evaluation of the extremities. Efficacy studies are needed to determine when MDCT, CBCT or Digital Radiography are best used for particular diagnostic tasks.