The examination of small animals using high resolution in-vivo imaging methods is a crucial tool in pharmaceutical research projects. These type of studies frequently require the measurement of very low activity lesions in a background region or high activity body parts for instance in case of dynamic investigations. To suffice these demands, two independent qualities of the PET scanner need to be appropriate, the low count detectability and the count rate performance. In our study we investigated the detectability of the low level activity area using our LYSO based small animal PET camera. The minimum detectable activity (MDA) was evaluated by the Currie equation and by taking into account the scanner recovery coefficient for different source diameters. We used two different water fillable cylindrical phantoms, the radius and the length of the cylinders are 15 mm and 50 mm, respectively. The phantom I includes five teflon (water density) rods with different diameter (0.5, 1.5, 2.5, 4, 6.5 mm). The phantom II includes tree glass rods with diameter of 0.5 mm. Filling the same F18 activity concentration to the rods of phantom I., the background (cylinder) region we injected C11 activity. This way we could examine different target to background ratios (T/B) simply. We found that the MDA depends on the actual background level and the size of the object. We also found that only the RC corrected MDA values and related points correspond to the real detectability (visibility). The smallest rod (diameter 0.5 mm) could not be imaged by our mini PET scanner with the activity range we used in this study.