Brain lesion detectability studies with a high resolution PET operating in no-septa and partial-septa configurations

We investigated the effect of partial-septa on the noise equivalent sensitivity and lesion detectability for a high resolution PET camera. For this purpose we used the MDAPET camera to detect small lesions in brain images obtained from the scan of the Hoffman brain phantom. The three-dimensional (3-D) positron emission tomography (PET) acquisition in comparison to two-dimensional (2-D) PET acquisition improves the sensitivity of the system at the cost of higher scatter and accidental coincidence contributions. A partial-septa allowing 3D-acquisition may provide a better alternative. For this work, three small lesion phantoms with diameters of 3, 5 and 8.6 mm were embedded into the Hoffman brain phantom. The activity concentration ratio of the lesions to the surrounding brain gray matter was ranging from 1.5 to 10. For this study, the eight detectors modules of the prototype MDAPET scanner were modified axially to extend from 38.5 mm to 131 mm in order to simulate more closely the performance of a clinical size camera. Data for the hot lesion phantoms and the normal Hoffman brain phantom were taken separately. Then, the two sets of the sinograms data were selectively combined to generate the sinograms data for the desired SUVs. Visual inspection of the lesion images show that we could clearly see the 8.6 mm lesion, with or without septa, at even the lowest activity ratio that we measured. The 5 mm and 3 min lesions were observable at activity ratio of 2.2 and 5.4, respectively. We found that even though the use of septa could increase the noise equivalent count rate and lower the image noise, it does not necessary translate into improvement of the lesion detectability. For partial-septa configurations the white matter regions of brain have less count and the brain images visually looked better; however, images from no-septa data had slightly higher contrast.