Precision in quantitative CT: impact of x-ray dose and matrix size

A study was performed to determine whether recommended technique factors for postprocessing dual-energy (DE) quantitative computed tomography are optimum in terms of precision and x-ray dose. In particular, possible dose reduction as a result of an upgrade of the CT scanner to more efficient detectors was explored. Series of images of an anthropomorphic phantom containing a human vertebra, a tissue-simulating lumbar simulator with various marrow inserts, and a polyethylene cylinder were generated. Recommended DE x-ray technique factors as well as factors resulting in about two times, one-half, and one-fourth the x-ray dose were employed. The effects of reconstruction with different matrix sizes was studied. Standard deviations of the CT numbers within regions of interest in individual images (noise) and standard deviations of mean CT numbers, single-energy (SE), and DE measurements for series of images (reproducibilities) were computed. It was found that the low-energy component of the DE technique was optimum, but the high-energy component could be reduced by a factor of 2 with negligible loss in precision. This translates into a dose reduction of 36% relative to the recommended DE technique. Vertebral inhomogeneities were responsible for more than 65% of the standard deviations in individual images of the vertebra even at the lowest doses. For all of the techniques, the noise in images of all objects decreased as the x-ray dose increased and as the matrix size decreased. Reproducibility of mean values, however, did not necessarily improve, and aberrant results such as improvement in reproducibility with a reduction in dose were sometimes observed. It is hypothesized that this may be due to variations in the actual kVp for each image in a series.