Quantification of radiotracer uptake with a dedicated breast PET imaging system

Tomographic breast imaging techniques can be used to quantify radiotracer uptake in breast and tumor tissue. However, physical processes common to PET imaging can confound accurate quantification. In this investigation, we assessed the effects of these phenomena and tested correction schemes for our new positron emission mammography-tomography system (PEM-PET). The PEM-PET scanner utilizes two sets of rotating planar detector heads. Each unit consists of a 4 x 3 array of Hamamatsu H8500 flat panel position sensitive photomultipliers coupled to a 96 x 72 array of 2 x 2 x 15 mm3 LYSO detector elements (pitch = 2.1 mm). Image reconstruction is performed with a 3D-OSEM algorithm parallelized to run on a multiprocessor computer system. The reconstructed field-of-view is 15 x 15 x 15 cm3. Much of the testing procedures were based on NEMA-NU2/2001 protocols. Count rate losses due to pulse pile-up, image contamination due to acceptance of random coincidences and Compton scatter, and image artifacts produced by photon attenuation were measured. It was found that the system was susceptible to count rate losses when moderate levels of radiation were present in the scanner due to the current design of the event trigger electronics. Application of corrections for Compton scattering, photon attenuation and dead time resulted in improved estimations of 18F concentration in simplified phantom studies. Results from these preliminary studies indicate that the PEM-PET scanner will be useful for the quantification of radiotracer uptake in breast tumors, possibly facilitating early assessment of cancer treatments.