Objectives: We have developed a second-generation virtual-pinhole (VP) PET insert that can enhance the image resolution and system sensitivity of a whole-body PET/CT scanner for an arbitrary organ-of-interest. This study validates the functionality of the system and evaluates its potential improvement in tumor-detectability.

Methods: The flat-panel insert comprises of 32 high-resolution detectors, each containing a 4-by-4 SiPM array and 16x16 LYSO crystals of 1.0x1.0x3.0 mm3 each. A robotic arm allows us to position the flat-panel at arbitrary locations. New firmware and software were developed to support the additional detector signals without compromising the scanner functions. We evaluated the positioning accuracy of the robot by (1) mounting a Na-22 point-source to the flat-panel and stepping the source across known scanner lines-of-response (LOR); (2) comparing centroids of the coincidence count profiles to their theoretical values. After system alignment, we imaged two Ge-68 line sources and a Ge-68 uniform cylinder. Images were reconstructed by a list-mode MLEM algorithm using coincidences from the scanner and the VP-PET device. We also used GATE to simulate a body phantom with 55 spherical tumors (5mm in diameter, 8-to-1 tumor-to-background-ratio) to evaluate tumor detectability with or without the VP-PET device.

Results: The positioning accuracy of the flat-panel by robot is better than 0.5-mm in all 3-directions. Line-source images show resolution improved from ~6 mm to ~3.2 mm (FWHM) along the directions orthogonal to the norm-vector of the flat-panel surface (not correcting for the line source dimension). Uniform cylinder image shows good uniformity with negligible artifacts. GATE simulation shows the peak-to-valley-ratio improves from 1.80±0.04 to 1.96±0.23 with the current 3-mm-thick crystals, or 2.41±0.12 with thicker crystals of 3-times detection efficiency.