Attenuation-based estimation of patient size for the purpose of size specific dose estimation in CT. Part II. Implementation on abdomen and thorax phantoms using cross sectional CT images and scanned projection radiograph images.

PURPOSE: To estimate attenuation using cross sectional CT images and scanned projection radiograph (SPR) images in a series of thorax and abdomen phantoms.
METHODS: Attenuation was quantified in terms of a water cylinder with cross sectional area of A(w) from both the CT and SPR images of abdomen and thorax phantoms, where A(w) is the area of a water cylinder that would absorb the same dose as the specified phantom. SPR and axial CT images were acquired using a dual-source CT scanner operated at 120 kV in single-source mode. To use the SPR image for estimating A(w), the pixel values of a SPR image were calibrated to physical water attenuation using a series of water phantoms. A(w) and the corresponding diameter D(w) were calculated using the derived attenuation-based methods (from either CT or SPR image). A(w) was also calculated using only geometrical dimensions of the phantoms (anterior-posterior and lateral dimensions or cross sectional area).
RESULTS: For abdomen phantoms, the geometry-based and attenuation-based methods gave similar results for D(w). Using only geometric parameters, an overestimation of D(w) ranging from 4.3% to 21.5% was found for thorax phantoms. Results for D(w) using the CT image and SPR based methods agreed with each other within 4% on average in both thorax and abdomen phantoms.
CONCLUSIONS: Either the cross sectional CT or SPR images can be used to estimate patient attenuation in CT. Both are more accurate than use of only geometrical information for the task of quantifying patient attenuation. The SPR based method requires calibration of SPR pixel values to physical water attenuation and this calibration would be best performed by the scanner manufacturer.