A Virtual-pinhole PET Device for Improving Contrast Recovery and Enhancing Lesion Detectability of a One-meter-long PET Scanner: A Simulation Study

This work presents a Monte Carlo simulation study to demonstrate that the contrast recovery coefficients (CRC) and detectability of small lesions of a one-meter-long PET scanner can be further enhanced by integrating high resolution Virtual-Pinhole (VP) PET devices to it. The scanner under investigation is a Siemens Biograph Quadra which has an axial field-of-view (FOV) of 106cm, equivalent to 4 Siemens Biograph Vision 600 PET scanner concatenated axially. The VP-PET devices under investigation contain high-resolution flat panel detectors, each composed of 2 (axial) × 8 (transverse) detector modules each of which consists of 64 (axial) × 32 (transverse) LSO crystals (1.0×1.0×10.0 mm3 each), spanning an active detector area of 128 × 256 mm for each panel. Two configurations were studied for the VP-PET devices: (a) one pair of VP-PET panel detectors (2 panels total) were placed below the patient bed at the center of the scanner’s axial FOV; (b) one VP-PET panel detector was placed below the patient bed at ¼ and ¾ of the scanner’s axial FOV. Sensitivity profiles of the scanner were measured by stepping a point 22Na source across the scanner’s FOV axially at different locations. To assess the improvement in CRC and lesion detectability by the VP-PET devices, an elliptical torso phantom (316 × 228 × 162 mm3) was first imaged by the native scanner and subsequently by two VP-PET geometry configurations. Spherical lesions (4 mm in diameter) having 5:1 lesion-to-background radioactivity concentration ratio were grouped and placed at different regions in the phantom so as to analyzed the dependence of the enhancement in plane. Average CRCs and their standard deviations of the 7 tumors in each group were computed and receiver operating characteristic (ROC) curves were drawn to evaluate the improvement in lesion detectability by the VP-PET device over the native PET scanner with 106cm long axial FOV. The VP-PET systems provide higher CRCs for lesions in all regions in the torso, with more significant enhancement at regions closer to the inserts, than the native scanner does. For any given false positive fraction, the VP-PET systems offer higher true positive fraction than the native scanner does. Configuration (a) provides maximal enhancement in lesion detectability at the CFOV of the scanner while configuration (b) improves the sensitivity and lesion detectability of the scanner at off-center axial locations to effectively reduce the nonuniformity of scanner performance near its axial edges. This work potentially provides a cost-effective solution to further enhance the image resolution and the uniformity of "effective sensitivity" of PET scanners with 1-meter long axial FOV.