Brain Lesion Detectability Studies with a High Resolution PET Operating in No-Septa and Partial-Septa Configurations

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 system (i.e., some detector rings lack septa) allowing "partial" 3-D acquisition may provide a better alternative to the 2-D acquisition and full 3-D acquisition. We investigated the effect of partial-septa configuration on the noise equivalent sensitivity and lesion detectability for a high-resolution PET camera. We used the MDAPET camera to detect small lesions in brain images obtained from scan of the Hoffman brain phantom. For this work, three small lesion phantoms with diameters of 3, 5, and 8.6 mm were embedded into the 3-D Hoffman brain phantom. The activity concentration ratio of the lesions to the brain gray matter ranged from 1.5 to 10. Data for the hot lesion phantoms and the normal 3-D Hoffman brain phantom were taken separately. Then, the two sets of sinogram data were selectively combined to generate the sinogram data for the desired standard uptake values (SUVs). In the reconstructed images, 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 mm lesions were observable at activity ratios of 2.2 and 5.4, respectively. We found that even though the use of septa increased the noise equivalent count rate and lowered the image noise, it did not necessarily improve lesion detectability. In partial-septa configurations, the white matter regions of the brain images had lower counts and the images visually looked better than no-septa images; however, images taken in the no-septa configuration had slightly higher contrast.