Quantitative CT measurements: the effect of scatter acceptance and filter characteristics on the EMI 7070.

Non-linearities in projection values on computed tomography (CT) scanners cause corresponding errors in derived Hounsfield unit attenuation measurements. Existing commercial machines have been refined for clinical usefulness but not necessarily for quantitative accuracy. In this report the results of measurements on the EMI 7070 fourth-generation scanner are presented and compared with theory with particular attention being paid to the scatter artefact and fan-beam energy profile. It is concluded that, irrespective of any quality assurance protocol, interpatient and interslice errors can be expected to range from 3 to 10% for water-equivalent materials and the intraslice positional dependence of the CT number can vary up to 5% for dense bone-like materials in a uniform phantom. It seems as though the principal cause of this variation is an increase in the effective x-ray energy in the fan beam with increasing distance from the central ray.