Evaluation of a high resolution positron emission tomography (PET) scanner for positron emission mammography (PEM)

A small animal scanner (VUB-PET), built at the Vrije Universiteit, Brussels has been used to test the feasibility of building a dedicated positron emission mammography (PEM) scanner. VUB-PET is not ideal for breast scanning due to a small field of view (FOV) and an inactive region before the active detector. It does however have a resolution of 3-4 mm and high sensitivity, properties needed in a PEM scanner to detect small tumours when using /sup 18/F fluorodeoxyglucose (FDG). A cylindrical, water-filled phantom (69mm diameter by 65mm long) was used to simulate a small breast and small spherical inserts simulated tumours. Various parameters were investigated including 'tumour' size and position in phantom, counts acquired, activity in the phantom and tumour to background ratio (TBR). Background activity was added in some studies, to simulate the uptake of FDG in the heart by attaching /sup 22/Na sources to the back of the phantom. Region of interest (ROI) analysis was used to determine the mean counts in the 'tumour' and the cylinder and the ratio of the means (ROM), was determined The larger the ROM of an image, the more likely the 'tumour' is to be detected. The main conclusions of these studies are that the 'tumour' size and the TBR are the main contributors to the visibility of the 'tumour'. The level of activity in the breast seems to have little effect on the detection of the 'tumour', due to the small randoms rate of the scanner. This data will be used to help in the development of a clinical PEM scanner. The results indicate the activity in the heart should not be a problem if the detectors resolving time is kept short. Additionally, the activity injected may be less than for a normal PET scan reducing the dose to the patient. However it seems unlikely that small 'tumours' (<5-6mm) will be visible unless the TBR is very high (>10).