Attenuation-based estimation of patient size for the purpose of size specific dose estimation in CT. Part I. Development and validation of methods using the CT image.

PURPOSE: For the purpose of size-specific dose estimation, information regarding patient attenuation is required. The purpose of this work is to describe a method for measuring patient attenuation and expressing the results in terms of a water cylinder, with cross sectional area A(w), which would absorb the same average dose as the irradiated patient. The ability to calculate A(w) directly from the CT image was validated with Monte Carlo simulations and an analytical model.
METHODS: A series of virtual cylinders were created with diameters ranging from 10 to 40 cm and lengths of 40 cm. The cylinders were given an atomic number equal to that of water; the density of the cylinders was varied from 0.26 to 1.2 g∕cm(3). The average dose to the cylinders from an axial scan at the longitudinal center position was calculated using Monte Carlo simulation and an analytical model. The relationship between phantom cross sectional area and calculated dose was determined for each density value to determine the dependence of A(w) on object attenuation. In addition, A(w) was estimated from the virtual CT images based on two derived models expressing the potential dependence of A(w) on object attenuation, one model assuming a linear dependence and the other assuming a quadratic dependence. Model results were compared with those from the Monte Carlo simulation and the analytical dose calculation approach. Virtual thorax and abdomen phantoms of adult and pediatric sizes were created, and A(w) was estimated using geometrical size parameters or the derived models. The accuracy of each approach for estimating A(w) was determined by comparing the average dose to the virtual phantom calculated using Monte Carlo simulation to the average dose to a water equivalent phantom of cross sectional area A(w).
RESULTS: In the absence of a bowtie filter, both the Monte Carlo simulation and analytical model showed that (A(w)∕A) had a quadratic dependence on (μ∕μ(w)). However, including a bowtie filter in the Monte Carlo simulation altered the relationship, such that A(w)∕A was linearly dependent on μ∕μ(w). Using this relationship, the dose absorbed by a water cylinder of area A(w) agreed with the dose absorbed by adult and pediatric, thorax and abdomen phantoms to within 6% (mean difference = 0.5 ± 4.8%). Estimates of A(w) (or the water equivalent diameter D(w)) using only anterior-posterior and lateral phantom dimensions led to dose estimates that agreed with Monte Carlo-derived dose values within 3% and 6% for the abdomen adult and pediatric phantoms, respectively. However, because of density differences between lung and tissue, larger differences in dose relative to Monte Carlo-derived values were observed in the thorax adult and pediatric phantoms (15% and 11%, respectively) when only geometrical parameters were used to estimate D(w).
CONCLUSIONS: Patient attenuation can be quantified in terms of the diameter of a water cylinder that absorbs same average dose as the irradiated cross section of the patient. The linear dependence of A(w) on object attenuation makes it straightforward to calculate A(w) from a CT image on most operator consoles or clinical workstations.