Is It Better to Enter a Volume CT Dose Index Value before or after Scan Range Adjustment for Radiation Dose Optimization of Pediatric Cardiothoracic CT with Tube Current Modulation?

Objective: To determine whether the body size-adapted volume computed tomography (CT) dose index ($CTD_{vol}$) in pediatric cardiothoracic CT with tube current modulation is better to be entered before or after scan range adjustment for radiation dose optimization. Materials and Methods: In 83 patients, cardiothoracic CT with tube current modulation was performed with the body sizeadapted $CTDI_{vol}$ entered after (group 1, n = 42) or before (group 2, n = 41) scan range adjustment. Patient-related, radiation dose, and image quality parameters were compared and correlated between the two groups. Results: The $CTDI_{vol}$ after the CT scan in group 1 was significantly higher than that in group 2 ($1.7{\pm}0.1mGy$ vs. $1.4{\pm}0.3mGy$; p < 0.0001). Image noise ($4.6{\pm}0.5$ Hounsfield units [HU] vs. $4.5{\pm}0.7HU$) and image quality ($1.5{\pm}0.6$ vs. $1.5{\pm}0.6$) showed no significant differences between the two (p > 0.05). In both groups, all patient-related parameters, except body density, showed positive correlations (r = 0.49-0.94; p < 0.01) with the $CTDI_{vol}$ before and after the CT scan. The $CTDI_{vol}$ after CT scan showed modest positive correlation (r = 0.49; $p{\leq}0.001$) with image noise in group 1 but no significant correlation (p > 0.05) in group 2. Conclusion: In pediatric cardiothoracic CT with tube current modulation, the $CTDI_{vol}$ entered before scan range adjustment provides a significant dose reduction (18%) with comparable image quality compared with that entered after scan range adjustment.