TY - JOUR T1 - Impact of body mass index on CT dose-index reproducibility with automatic tube current modulation JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 1954 LP - 1954 VL - 57 IS - supplement 2 AU - Dillenia Rosica AU - Vikram Agarwal AU - Hyewon Hyun AU - Jon Hainer AU - Stephen Moore AU - Ron Blankstein AU - Hicham Skali AU - Marcelo DiCarli AU - Sharmila Dorbala AU - Mi-Ae Park Y1 - 2016/05/01 UR - http://jnm.snmjournals.org/content/57/supplement_2/1954.abstract N2 - 1954Objectives To assess the reproducibility of an automated dose-modulation technique used in the CT portion of PET/CT exams performed on patients undergoing multiple scans on the same day and to determine whether body-mass index (BMI) has an impact on dose reproducibility. Dose modulation is a dose reduction technique that automatically estimates the thickness of the body part from scout views and applies different radiation exposure to each projection view. CTDIvol and DLP are commonly cited dose indices that are now routinely documented on CT scans. The reproducibility of this technique has been examined in phantoms but, to our knowledge, has not been reported in human subjects who underwent low-dose cardiac CT scans. Cardiac same day rest/stress PET/CT offers a unique opportunity to assess the reproducibility of the dose modulation technique as the same patient is scanned in one bed position with the same parameters on the same scanner within the same day.Methods A retrospective review of displayed CT indices for adult patients who underwent rest and stress cardiac PET/CT within 24 hours over a two year period was performed with a dose-modulation technique, Smart mA, turned on. All patients were scanned on the same scanner for both their studies. Scan parameters involved one bed position for the PET and the corresponding scan length for the CT portion to cover the heart. CT scan parameters were 64 x 0.625mm collimation, 5mm slice thickness, 0.984 pitch, 1sec rotation time, 120kVp tube voltage. Noise Index (NI) was set at 50 with minimum and maximum tube current of 10 and 200 mA. The displayed CTDIvol was estimated from measured exposure values using a 100mm pencil ionization chamber in a body32 phantom. The percent difference between stress and rest CTDIvol values was defined as the absolute value of the difference between two CTDIvol values, divided by the average of those two CTDIvol values, and then multiplied by 100. Patient BMI and time between scans were analyzed as well.Results 217 patients with same-day cardiac rest/stress PET/CT scans were included in the study: 101 patients had a BMI <30 kg/m2 and 116 patients had a BMI蠅30 kg/m2. Time between scans ranged from 31 to 117 minutes with a mean of 62 minutes. Patient BMI ranged from 18.6 to 64.8 kg/m2 with a mean of 32.4 kg/m2. The average CT scan length over all scans was 19.64 ± 0.04 cm. The percent difference of the CTDIvol between two scans was calculated. Table 1 demonstrates mean, minimum, maximum, standard deviation of CTDIvol for each group, and mean percent difference. The average CTDIvol was 2.78 ±1.46 mGy for the subjects with BMI蠅30 and 1.18 ± 0.30 mGy for the subjects with BMI<30. For the group with BMI<30, the minimum percent difference of CTDIvol between the two scans was 0%, the maximum was 34.66% with a mean of 6.26%. For the group with BMI蠅30, the minimum difference between the two scans was 0, the maximum was 31.39% with a mean of 8.59%. Both two-tailed t-test and a Mann-Whitney test yielded p value < 0.05 with a confidence level of 95%, indicating a statistically significant difference in reproducibility of dose between the two groups.Conclusions For patients scanned the same day on the same scanner using automatic dose modulation, the dose index, CTDIvol, was reproducible within 7.5% of average percent difference, and reproducibility was worse with higher BMI (蠅30). View this table:Table 1 ER -