Characterization of radiation dose from tube current modulated CT examinations with considerations of both patient size and variable tube current

Purpose: The volume CT dose index (CTDI_vol ) and the size-specific dose estimate (SSDE) are widely used for monitoring patient dose from CT examinations. Both metrics may represent the average dose over the central scan plane of the CTDI phantom or the patient under constant tube current (mA), but they are not intended for the tube current modulation (TCM)-enabled CT examinations, in which the peak dose across the scanned range may not be at the scan range center. To overcome the limitation, this paper illustrates an alternative approach, its implementation, and the relationship between longitudinal dose distribution D_L (z) in water cylinder and mA line shape, scan length, as well as phantom diameter.

Methods: A dose calculation algorithm and the published data by Li et al. [Med. Phys. 40, 031903 (10pp.) (2013); 41, 111910 (5pp.) (2014)] were used to calculate D_L (z) for the central and peripheral axes of 10- to 50-cm diameter water phantoms undergoing CT scans of one constant and three variable mA distributions, each of which in three scan lengths of 10, 28.6, and 50 cm. All scans had an identical average tube current over the scan ranges. The results in the scanned ranges were used to assess the D_L (z) to mA(z) ratios, and their coefficients of variation (CV = stdev/mean) were used to compare the line shapes of D_L (z) and mA(z) for congruence: identical line shapes would result in CV = 0, but largely different line shapes would result in high CV.

Results: In 30-cm diameter water phantom, the line shape of D_L (z) was largely different from that of mA(z). CV was higher in a variable mA scan than in a constant mA scan. As the scan length of variable mA scan increased, CV mostly decreased, and the line shape of D_L (z) more closely resembled that of mA(z). When two phantom axes were compared, CV was smaller and the line shape of D_L (z) more closely resembled that of mA(z) on the peripheral axis than on the central axis. In 41 water phantoms included in this study, CV mostly increased with phantom diameter, and approached the limiting levels on the peripheral axes of large phantoms. In constant mA scans, CV ranged from 5.5% to 14.0% on the phantom central axes and from 4.6% to 6.4% on the phantom peripheral axes. However, in variable tube current scans, CV ranged from 7.4% to 70.0% on the phantom central axes and from 5.1% to 35.9% on the phantom peripheral axes.

Conclusion: D_L (z) (water) may be advantageous over current CT dose metrics in characterizing the dose dependences on both patient size and mA line shape from tube current modulated examinations. Evaluating D_L (z) (water) with the water equivalent diameter and tube current curve from clinical examinations has a potential to improve CT dose monitoring program.