Patients with skeletal metastases from hormone-refractory prostate cancer have shown variable responses to high-activity therapy with 186Re-HEDP and peripheral stem cell support. In this paper, we report on the use of a novel technique to compare sequential planar images acquired post-186Re-HEDP therapy administration with pretherapy diagnostic 99mTc-MDP scans, to evaluate the turnover of the radiopharmaceutical in normal and abnormal bone. It was found that the activity in normal (i.e., disease-free) segments of the spine demonstrates a faster effective decay than that of the metastases, with the latter showing only physical decay. This study showed, for the first time, a detailed correlation in the behavior of the 99mTc-MDP and 186Re-HEDP images, encouraging the possibility of using the pretherapy 99mTc-MDP scan for estimations of absorbed doses to be delivered by prescribed activities of 186Re-HEDP.

Comparison of two medical images often requires image scaling as a preprocessing step. This is usually done with the scaling-to-the-mean or scaling-to-the-maximum techniques which, under certain circumstances, in quantitative applications may contribute a significant amount of bias. In this paper, we present a simple scaling method which assumes only that the most predominant values in the corresponding images belong to their background structure. The ratio of the two images to be compared is calculated and its frequency histogram is plotted. The scaling factor is given by the position of the peak in this histogram which belongs to the background structure. The method was tested against the traditional scaling-to-the-mean technique on simulated planar gamma-camera images which were compared using pixelwise statistical parametric tests. Both sensitivity and specificity for each condition were measured over a range of different contrasts and sizes of inhomogeneity for the two scaling techniques. The new method was found to preserve sensitivity in all cases while the traditional technique resulted in significant degradation of sensitivity in certain cases.

The PETRRA positron camera consists of two 60×40cm2 detectors mounted on a rotating gantry. The detectors each contain 1cm thick BaF2 crystals interfaced to a MWPC filled with tetrakis dimethylamino ethylene (TMAE) vapour. PETRRA acquires data in 3D only and images can contain high levels of scatter and random coincidences. Scatter comes from the patient and the detector support structure whereas randoms rates depend on the detector count rates and coincidence timing resolution. The camera has little energy resolution but low energy scattered photons produce a smaller range of pulses and can be discriminated against using signal thresholding. Scatter can also be reduced by shielding the camera from radioactivity outside of the field of view and by minimising the amount of scattering material in the camera itself. We conclude that the most effective way of reducing scatter and randoms count rates is to minimise the support structures in the detectors and shield the detectors from out-of-field activity. Energy thresholding does reduce scatter and randoms but severely reduces the numbers of true events.

The PETRRA camera has been simulated using Monte-Carlo techniques including effects associated to the detection process of scintillator crystals coupled to MWPCs (Multi wire proportional chambers). The code has been used for the investigation of the origin of the high scatter observed in experimental PETRRA data, simulation of the SRF (scatter response function) of a centered point source in air and in water, and for possible ways of improving its performance by reducing the SF (scatter fraction) and adding shield for out of FoV (field of view) activity.

The PETRRA positron camera consists of two 60 cm /spl times/ 40 cm detectors mounted on a rotating gantry. Each detector contains a layer of 1 cm thick barium fluoride (BaF/sub 2/) crystals interfaced to a multiwire proportional chamber (MWPC) filled with 4.2 mbar of the photosensitive vapor tetrakis dimethylamino ethylene (TMAE). The camera acquires data in list mode and produces 3D images which can contain high levels of scatter and random coincidences. Scatter comes from the patient and the detector support structure whereas randoms rates depend on the detector count rates and coincidence timing resolution. The camera has little energy resolution but low energy scattered photons produce a smaller range of pulse sizes and can be discriminated against using signal thresholding.