Purpose: To compare effective doses for hip imaging with a cone beam CT (CBCT) scanner permitting weight-bearing acquisitions and conventional multidetector CT (MDCT) scanner.
Methods: An anthropomorphic sectional adult hip phantom (ATOM 701-G, CIRS, Norfolk, VA; 14 sections; 25 mm thickness of section), composed of tissue equivalent material with 70 slots for optically stimulated luminescence (OSL) dosimeters, was used. The OSL dosimeters (screened nanoDots, Landauer Inc., Glenwood, IL) were used with calibrated microStar reader (Landauer Inc.) for organ dose measurements with free-in-air CT calibration protocol and energy-based depth-dependent correction factors (each dosimeter was read thrice with average values were used in analysis). The phantom was scanned with an MDCT (iCT 256, Philips Healthcare, Andover, MA) employing a clinical protocol (n=5; kV:140; mAs:122; CTDIvol:11.6 mGy; scan length:20 cm) and with a commercial CBCT (HiRise, CurveBeam, Hatfield, PA) designed for bilateral weight-bearing hip imaging using manufacturer’s default protocol (n=10; kV:130; mA:6.5; pulse duration: 35 ms; 720 frames; scan length: 20 cm). The resulting average tissue-absorbed doses were computed and combined with fraction of the organ/tissue irradiated and tissue-weighting factors (ICRP 2007) to compute effective doses. Effective doses were also estimated using published k-values (mSv/mGy.cm) and dose-length-product (DLP). CBCT scans were repeated to assess reproducibility and rotational variability.
Results: There was a statistically significant difference in the average tissue-absorbed dose between CBCT (3.6±2.1 mGy) and MDCT (11.7±5.1 mGy) acquisitions (p<0.001). The resulting measured effective dose values for CBCT and MDCT were 0.9 mSv and 3.5 mSv, respectively. Using published k-values and DLP, the estimated effective dose values were 1.5 mSv for CBCT and 4.3 mSv for MDCT acquisitions. There were no statistically significant differences when assessing reproducibility (p=0.89) and rotational dependency (p=0.26) on the CBCT.
Conclusion: Effective dose values for hip imaging with CBCT could potentially be substantially lower than with MDCT.
Funding Support, Disclosures, and Conflict of Interest: JD: Medical physicist consulting agreement with Rayscan Inc (NJ); Research consulting with CurveBeam; Research support from Philips Healthcare; Contrast agent support from Lantheus Medical Imaging and GE Healthcare; XJ: Research consulting with CurveBeam; TM: Employee of CurveBeam