Ramp test objects for slice sensitivity profile measurement in spiral CT

Spiral CT is increasingly becoming a routine imaging technique in the UK. The ability to scan whole organs within a single breath-hold often demands that the scan pitch (ratio of the patient translation distance per tube rotation (d) to the collimated slice thickness) be increased within technically acceptable limits. It is known that the full width at half maximum height (FWHM) of the slice sensitivity profile (SSP), degrades with increasing scan pitch and also varies with interpolation algorithm. Published literature frequently reports values for FWHM of the SSP for spiral acquisitions which have been measured with a ramp test tool. However, it has previously been shown that a ramp is inappropriate for SSP measurements in spiral CT. This is because of the physical basis of the premise for this measurement and the large artefacts generated by the application of the interpolation algorithm to the ramp spiral projection data. We present experimental evidence confirming the unsuitability of the ramp test tool in spiral CT. A cyclical variation (d/2) of the data is demonstrated; the magnitude of the measured SSP being dependent upon scan pitch and the interpolation algorithm used, as well as on the longitudinal position of the reconstruction plane.