CTDI(vol) in PET-CT: Fixed kV/Fixed Quality Reference mAs vs Topogram-based kV/Topogram-based Quality Reference mAs

Objectives The purpose of this study is comparison of calculated CT dose index estimates [CTDI(vol)] that result from two different CT acquisition protocols in PET-CT. One protocol uses a fixed kV, fixed quality reference mAs, and data-driven mAs modulation; the other uses a topogram-based kV, topogram-based quality reference mAs, and data-driven mAs modulation.

Methods This was a retrospective quality-assessment study. Thirty-two patients were identified as having had sequential PET-CT studies using two similar, yet different, protocols (oral contrast was used in most patients, no intravenous contrast was used). Both studies were acquired on a single Biograph mCT 64 (Siemens, Knoxville, TN, USA). Patient weights (lbs) during their first study ranged from 116 to 275 (mean ± st dev = 181.53 ± 38.61) and ranged from 102 to 280 (mean ± st dev = 178.46 ± 42.19) for the second. The first study in each case was done using CARE Dose4D without CarekV; the second with CARE Dose4D with CarekV at setting 3. Study dates for each subject were separated by 2-12 months. Protocol A (Care kV off) used a fixed kV = 120 and a fixed quality reference mAs = 120 (BMI < 30) and mAs = 150 (BMI > 30). Protocol B (Care kV on, setting 3) used a kV determined from topogram data (kV = 100, 120, or 140) and a quality reference mAs (range = 104 to 251) determined from topogram data.

Results The differences in patient weights between protocols (range = -30 to +21 ) were not significant; the mean ± st dev of the weight differences was 3.06 ± 1.87 (p = 0.11). The differences in CTDI(vol)s between protocols (range = -3.71 to 4.51) were significant; the mean ± st dev of the differences in CTDI(vol)s was -1.21 ± 0.34 (p = 0.0013). The CDTI(vol)s obtained using Protocol B were significantly higher than those obtained using Protocol A.

Conclusions For the PET-CT studies performed on this population at this institution, use of CARE Dose 4D with CarekV off results in less CT radiation dose to most patients than does use of CARE Dose 4D with CarekV on at setting 3.