We proposed and tested a novel geometry for PET system design analogous to pinhole SPECT called the virtual-pinhole PET (VP-PET) geometry to determine whether it could provide high-resolution images.
Methods: We analyzed the effects of photon acolinearity and detector sizes on system resolution and extended the empiric formula for reconstructed image resolution of conventional PET proposed earlier to predict the resolutions of VP-PET. To measure the system resolution of VP-PET, we recorded coincidence events as a (22)Na point source was stepped across the coincidence line of response between 2 detectors made from identical arrays of 12 x 12 lutetium oxyorthosilicate crystals (each measuring 1.51 x 1.51 x 10 mm(3)) separated by 565 mm. To measure reconstructed image resolution, we built 4 VP-PET systems using 4 types of detectors (width, 1.51-6.4 mm) and imaged 4 point sources of (64)Cu (half-life = 12.7 h to allow a long acquisition time). Tangential and radial resolutions were measured and averaged for each source and each system. We then imaged a polystyrene plastic phantom representing a 2.5-cm-thick cross-section of isolated breast volume. The phantom was filled with an aqueous solution of (64)Cu (713 kBq/mL) in which the following were imbedded: 4 spheric tumors ranging from 1.8 to 12.6 mm in inner diameter (ID), 6 micropipettes (0.7- or 1.1-mm ID filled with (64)Cu at 5x, 20x, or 50x background), and a 10.0-mm outer-diameter cold lesion.
Results: The shape and measured full width at half maximum of the line spread functions agree well with the predicted values. Measured reconstructed image resolution (2.40-3.24 mm) was +/-6% of the predicted value for 3 of the 4 systems. In one case, the difference was 12.6%, possibly due to underestimation of the block effect from the low-resolution detector. In phantom experiments, all spheric tumors were detected. Small line sources were detected if the activity concentration is at least 20x background.
Conclusion: We have developed and characterized a novel geometry for PET. A PET system following the VP-PET geometry provides high-resolution images for objects near the system's high-resolution detectors. This geometry may lead to the development of special-purpose PET systems or resolution-enhancing insert devices for conventional PET scanners.