AbstractID: 7885 Title: Effects of Singles Count Rate in Quantification using DHCI
This work presents some results of a study of the influence of singles count rate on the evaluation of contrast, calibration factor and SUV,
for a DHCI system. Two experimental setups with hot background were considered: 1- cylinders with same activity concentration (10:1)
and different diameters and 2- cylinders with different activity concentrations (10:7:5:3:1) and same diameter. It was observed that the
contrast increases with the object size and tends to be constant for sources with diameter > 4FWHM. It also increases linearly with
concentration. As dead time correction is not performed by the system, different calibration factors (CF) were obtained for different
singles rates. For setup 1, good agreement was obtained between SUVs calculated considering 1 pixel and square ROIs with ~2FWHM
side; large discrepencies (>35%) were found for objects with diameter > 3FWHM, when real object size ROIs were used. For the setup 2,
the results showed that SUV increases with concentration, as expected; it was also found that there is a maximum contrast value above
which the standard deviations for SUVs become very large, >0.6 for an average of 0.03. This result indicates that CFs calculated for
extended sources are not adequate for correcting localized hot lesions (obj:bkg>7:1). Due to this limitation, it is very difficult to find
recovery coefficients independent of singles rate. In order to be an useful SUV, it is mandatory to perform all the corrections and local
activity contrast needs to be accounted for.