Spiral interpolation algorithms for multislice spiral CT. II. Measurement and evaluation of slice sensitivity profiles and noise at a clinical multislice system

For pt. I see ibid., vol. 19, no. 9, p. 822-34 (2000). The recently introduced multislice data acquisition for computed tomography (CT) is based on multirow detector design, increased rotation speed, and advanced z-interpolation and z-filtering algorithms. The authors evaluated slice sensitivity profiles (SSPs) and noise of a clinical multislice spiral CT (MSCT) scanner with M=4 simultaneously acquired slices and adaptive axial interpolator (AAI) reconstruction software. SSPs were measured with a small gold disk of 50 μm thickness and 2-mm diameter located at the center of rotation (COR) and 100 mm off center. The standard deviation of CT values within a 20-cm water phantom was used as a measure of image noise. With a detector slice collimation of S=1.0 mm, the authors varied spiral pitch p from 0.25 to 2.0 in steps of 0.025. Nominal reconstructed slice thicknesses were 1.25, 1.5, and 2.0 mm. For all possible pitch values, the authors found the full-width at half maximum (FWHM) of the respective sensitivity profile at the COR equivalent to the selected nominal slice thickness. The profiles at 100 mm off center are broadened less than 7% on the average compared with the FWHM at the COR. In addition, variation of the full-width at tenth maximum (FWTM) at the COR was below 10% for p≤1.75. Within this range, image noise varied less than 10% with respect to the mean noise level. The slight increase in measured slice-width above p=1.75 for nominal slice-widths of 1.25 and 1.50 mm is accompanied by a decrease of noise according to the inverse square root relationship. The MSCT system that the authors scrutinized provides reconstructed slice-widths and image noise, which can be regarded as constant within a wide range of table speeds. With respect to this, MSCT is superior to single-slice spiral CT. These facts can be made use of when defining and optimizing clinical protocols: the spiral pitch can be selected almost freely, and scan protocols can follow the diagnostic requirements without technical restrictions. In summary, MSCT offers constant image quality while scan times are reduced drastically. Volume scans with three-dimensional (3-D) isotropic resolution are routinely feasible for complete anatomical regions.