Purpose: The scanner-reported CTDIvol for automatic tube current modulation (TCM) has a different physical meaning from the traditional CTDIvol at constant mA, resulting in the dichotomy "CTDI vol of the first and second kinds" for which a physical interpretation is sought in hopes of establishing some commonality between the two. Methods: Rigorous equations are derived to describe the accumulated dose distributions for TCM. A comparison with formulae for scanner-reported CTDI vol clearly identifies the source of their differences. Graphical dose simulations are also provided for a variety of TCM tube current distributions (including constant mA), all having the same scanner-reported CTDIvol. Results: These convolution equations and simulations show that the local dose at z depends only weakly on the local tube current i(z) due to the strong influence of scatter from all other locations along z, and that the "local CTDIvol(z)" does not represent a local dose but rather only a relative i(z) ≡ mA(z). TCM is a shift-variant technique to which the CTDI-paradigm does not apply and its application to TCM leads to a CTDIvol of the second kind which lacks relevance. Conclusions: While the traditional CTDIvol at constant mA conveys useful information (the peak dose at the center of the scan length), CTDIvol of the second kind conveys no useful information about the associated TCM dose distribution it purportedly represents and its physical interpretation remains elusive. On the other hand, the total energy absorbed E ("integral dose") as well as its surrogate DLP remain robust between variable i(z) TCM and constant current i0 techniques, both depending only on the total mAs = it0 = i0 t0 during the beam-on time t0.
- Computed Tomography(CT)
- CT Dosimetry
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging