Attenuation values of fluid collections within the abdomen

In vivo attenuation values of 19 fluid collections in 17 patients were compared with in vitro values obtained on examination of the aspirated fluids within the calibration phantom of an EMI CT5005 body scanner. While the in vitro values showed a 5.2 Hounsfield unit (HU:1,000 scale). difference between cysts and abscesses, spectral errors of up to 28.6 HU were sufficiently large for this distinction to be lost in vivo. Good correlation was found between ascitic fluid protein concentration and in vitro attenuation values, but not with in vivo values. An experimental reconstruction program appeared to give more accurate in vivo results than the standard commercial algorithm. A water control showed errors of similar magnitude to those with the fluid collections and did not provide any simple correction for the errors in the fluid attenuation values. Similar results were found for urine examined in vitro and in vivo. The administration of intravenous contrast agents tended to elevate in vivo fluid attenuation values, but refrigeration for 6 months produced little change in in vitro values.