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

The recently introduced multislice data acquisition for computed tomography (CT) is based on multirow detector design, increased rotation speed, and a dvanced z-interpolation and z-filtering algorithms. We evaluated slice sensitivity profiles (SSPs) and noise of a clinical mult islice spiral CT (MSCT) scanner with M = 4 simultaneously acquired slices a nd adaptive axial interpolator (AAI) reconstruction software [11]. SSPs wer e measured with a small gold disk of 50 mum thickness and 2-mm diameter loc ated at the center of rotation (COR) and 100 mm off center. The standard de viation 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, we varied spi ral 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, we found the full-width at half maximum (FWH M) of the respective sensitivity profile at the COR equivalent to the selec ted nominal slice thickness. The profiles at 100 mm off center are broadene d less than 7% on the average compared with the FWHM at the COR, In additio n, variation of the full-width at tenth maximum (FWTM) at the COR was below 3.0% for p less than or equal to 1.75. Within this range, image noise vari ed 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 squ are root relationship. The MSCT system that we scrutinized provides reconstructed slice-widths and image noise, which can be regarded as constant within a wide range of tabl e 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 proto cols: the spiral pitch can be selected almost freely, and scan protocols ca n follow the diagnostic requirements without technical restrictions. In sum mary, MSCT offers constant image quality while scan times are reduced drast ically. Volume scans with three-dimensional (3-D) isotropic resolution are routinely feasible for complete anatomical regions.