Quantification in positron emission mammography (PEM) with planar detectors: contrast resolution measurements using a custom breast phantom and novel spherical hot-spots

The authors have previously demonstrated that their Positron Emission Mammography-1 (PEM-1) system can successfully defect small (<2 cm diameter) tumors. Images in the current system are judged qualitatively for the presence of focal uptake of ¹⁸F-FDG. The authors are presently taking steps toward quantifying the emission images. To this end, they have constructed a breast phantom and developed a novel technique for fabricating very small wall-less radioactive hot-spots. The hot-spots are made by adding ¹⁸F-FDG to a 25 mg/cc solution of Agarose in water. The heated solution is poured into spherical molds which are separated upon congealing to yield robust wall-less radioactive hot-spots. The hot-spots were found to be uniform to within 1-5 parts in 100. Less than 0.1% of the total hot-spot activity leaked into the background in 30 minutes. Contrast resolution experiments have been performed with 12 mm and 16 mm diameter hot-spots in the breast phantom containing water with various amounts of background activity. In both cases, the observed contrast values agree well with the ideal values. In the case of the 12 mm hot-spot with a 350-650 keV energy window, mean image contrast was 86±4% of ideal contrast. The image contrast for 12 mm hot-spot improved by 40% and the number of detected events decreases by 35% when the low energy threshold was increased from 300 keV to 450 keV. Activity uptake in the heart was found to have negligible effect on the contrast resolution