To evaluate technical and patient dose indicators and to provide dosimetry approaches for high-resolution photon-counting spiral breast CT (pcBCT).
METHOD AND MATERIALSMeasurements were performed on a pcBCT (CT Imaging GmbH, Erlangen, Germany), using 60 kV, 3 mm Al filtration, 30 mm axial collimation and spiral scan mode offering 100 µm spatial resolution.As technical dose indicator, the weighted computed tomography dose index (CTDIw) was measured according to IEC 60601-2-44 using a PMMA phantom 160 mm in diameter and 150 mm in length and a calibrated 100 mm long pencil ionization chamber (type 30009, PTW, Freiburg, Germany). From this, volume CTDI (CTDIvol) and dose length product (DLP) were calculated. Additionally the CTDI free in air (CTDIair) and air kerma in the isocenter were assessed.As patient dose indicators, the average glandular dose (AGD) and the effective dose (E) according to ICRP publication 103 were determined. For this, conversion factors of AGD per air kerma and E per air kerma were calculated for different breast phantoms using Monte Carlo software (ImpactMC, CT Imaging GmbH, Erlangen, Germany) taking the system geometry, x‑ray spectrum, scan trajectory, breast size and patient body size into account. Using measured air kerma values, AGD and E for the scan protocol in question were calculated for the individual breast examined.
RESULTSMeasurements of technical dose indicators: CTDIw was 10.6 mGy per 100 mAs, and CTDIair was 20.5 mGy per 100 mAs. Simulations of patient dose indicators: AGD was 0.29 to 0.48 mGy and E was 0.039 to 0.059 mSv per 1 mGy air kerma, respectively, depending on breast size and composition. E.g., for a breast of 140 mm in diameter, 105 mm in length and 20% glandular tissue the investigation revealed: CTDIvol of 6.8 mGy, DLP of 71.4 mGy * cm, AGD of 4.8 mGy and E of 0.61 mSv for our protocols.
CONCLUSIONTechnical concepts established in clinical CT are suitable for dose assessment in pcBCT. Patient-specific dose can be estimated based on Monte Carlo simulations. AGD of about 5 mGy and E less than 1 mSv for bilateral examinations in pcBCT are low, acceptable and confirm photon-counting technology. Dose to all other organs not directly exposed appears negligible.
CLINICAL RELEVANCE/APPLICATIONDedicated high-resolution photon-counting spiral breast CT potentially offers higher sensitivity and specificity for breast cancer detection without increasing dose levels significantly.