SU‐F‐I‐31: Reproducibility of An Automatic Exposure Control Technique in the Low‐Dose CT Scan of Cardiac PET/CT Exams

Abstract

Purpose:Two separate low‐dose CT scans are usually performed for attenuation correction of rest and stress N‐13 ammonia PET/CT myocardial perfusion imaging (PET/CT). We utilize an automatic exposure control (AEC) technique to reduce CT radiation dose while maintaining perfusion image quality. Our goal is to assess the reproducibility of displayed CT dose index (CTDI) on same‐day repeat CT scans (CT1 and CT2).Methods:Retrospectively, we reviewed CT images of PET/CT studies performed on the same day. Low‐dose CT utilized AEC technique based on tube current modulation called Smart‐mA. The scan parameters were 64 × 0.625mm collimation, 5mm slice thickness, 0.984 pitch, 1‐sec rotation time, 120 kVp, and noise index 50 with a range of 10–200 mA. The scan length matched with PET field of view (FOV) with the heart near the middle of axial FOV. We identified the reference slice number (RS) for an anatomical landmark (carina) and used it to estimate axial shift between two CTs. For patient size, we measured an effective diameter on the reference slice. The effect of patient positioning to CTDI was evaluated using the table height. We calculated the absolute percent difference of the CTDI (%diff) for estimation of the reproducibility.Results:The study included 168 adults with an average body‐mass index of 31.72 ± 9.10 (kg/m2) and effective diameter was 32.72 ± 4.60 cm. The average CTDI was 1.95 ± 1.40 mGy for CT1 and 1.97 ± 1.42mGy for CT2. The mean %diff was 7.8 ± 6.8%. Linear regression analysis showed a significant correlation between the table height and %diff CTDI. (r=0.82, p<0.001)Conclusion:We have shown for the first time in human subjects, using two same‐day CT images, that the AEC technique in low‐dose CT is reproducible within 10% and significantly depends on the patient centering.