Contrast--detail--dose analysis of six different computed tomographic scanners.

Low-contrast visibility of various rotate-translate, rotating detector, and stationary detector computed tomogrphy (CT) scanners is measured at contrasts ranging from 0.3 to 3% using a phantom based on the partial volume of water and polystyrene. The results are plotted on log-log curves, as a function of entrance dose, to provide a family of contrast (C)--detail (d)--dose (D) visibility curves. The transition region, approximately 1 to 10% contrast, depends on modulation transfer function (MTF), contrast, and the dose (noise), while the noise region, less than or equal to 1% contrast, is independent of MTF. The slopes of the curves in the noise region are consistent with a value of -1, as opposed to a theoretically expected value of -3/2 for ideal CT noise. Relative dose efficiency of various scanners is examined by plotting CdD(1/2) versus contrast for both peak dose (Dp) and summation dose (D sigma). The rotating detector systems are shown to be, on the average (at 0.5% contrast), 3.4 (Dp) and 4 (D sigma) times more dose efficient than the stationary array system examined, and 2.4 (Dp) and 3.2 (D sigma) times more efficient than the rotate-translate units examined.