We explore dual-ended read out of LSO arrays with two position sensitive avalanche photodiodes (PSAPDs) as a high resolution, high efficiency depth-encoding detector for PET applications. Flood histograms, energy resolution and depth of interaction (DOI) resolution were measured for unpolished LSO arrays with individual crystal sizes of 1.0, 1.3 and 1.5 mm, and for a polished LSO array with 1.3 mm pixels. The thickness of the crystal arrays was 20 mm. Good flood histograms were obtained for all four arrays, and crystals in all four arrays can be clearly resolved. Although the amplitude of each PSAPD signal decreases as the interaction depth moves further from the PSAPD, the sum of the two PSAPD signals is essentially constant with irradiation depth for all four arrays. The energy resolutions were similar for all four arrays, ranging from 14.7% to 15.4%. A DOI resolution of 3-4 mm (including the width of the irradiation band which is approximately 2 mm) was obtained for all the unpolished arrays. The best DOI resolution was achieved with the unpolished 1 mm array (average 3.5 mm). The DOI resolution for the 1.3 mm and 1.5 mm unpolished arrays was 3.7 and 4.0 mm respectively. For the polished array, the DOI resolution was only 16.5 mm. Summing the DOI profiles across all crystals for the 1 mm array only degraded the DOI resolution from 3.5 mm to 3.9 mm, indicating that it may not be necessary to calibrate the DOI response separately for each crystal within an array. The DOI response of individual crystals in the array confirms this finding. These results provide a detailed characterization of the DOI response of these PSAPD-based PET detectors which will be important in the design and calibration of a PET scanner making use of this detector approach.

Detectors with depth-encoding allow a PET scanner to simultaneously achieve high sensitivity and high spatial resolution. Methods: A prototype PET scanner, consisting of depth-encoding detectors constructed by dual-ended readout of lutetium oxyorthosilicate (LSO) arrays with 2 position-sensitive avalanche photodiodes (PSAPDs), was developed. The scanner comprised 2 detector plates, each with 4 detector modules, and the LSO arrays consisted of 7 · 7 elements, with a crystal size of 0.9225 · 0.9225 · 20 mm and a pitch of 1.0 mm. The active area of the PSAPDs was 8 · 8 mm. The performance of individual detector modules was characterized. A line-source phantom and a hot-rod phantom were imaged on the prototype scanner in 2 different scanner configurations. The images were reconstructed using 20, 10, 5, 2, and 1 depth-of-interaction (DOI) bins to demonstrate the effects of DOI resolution on reconstructed image resolution and visual image quality. Results: The flood histograms measured from the sum of both PSAPD signals were only weakly depth-dependent, and excellent crystal identification was obtained at all depths. The flood histograms improved as the detector temperature decreased. DOI resolution and energy resolution improved significantly as the temperature decreased from 20°C to 10°C but improved only slightly with a subsequent temperature decrease to 0°C. A full width at half maximum (FWHM) DOI resolution of 2 mm and an FWHM energy resolution of 15% were obtained at a temperature of 10°C. Phantom studies showed that DOI measurements significantly improved the reconstructed image resolution. In the first scanner configuration (parallel detector planes), the image resolution at the center of the field of view was 0.9-mm FWHM with 20 DOI bins and 1.6-mm FWHM with 1 DOI bin. In the second scanner configuration (detector planes at a 40°angle), the image resolution at the center of the field of view was 1.0-mm FWHM with 20 DOI bins and was not measurable when using only 1 bin. Conclusion: PET scanners based on this detector design offer the prospect of high and uniform spatial resolution (crystal size, ;1 mm; DOI resolution, ;2 mm), high sensitivity (20-mm-thick detectors), and compact size (DOI encoding permits detectors to be tightly packed around the subject and minimizes number of detectors needed). Over the past decade, many small-animal PET scanners have been developed (1-12), and this technology has played a very important role in the rapidly growing field of molecular imaging. High sensitivity is needed to increase signal-to-noise ratio of the images to reliably detect lower levels of radiotracer uptake and to reduce the injected dose (reducing radiation dose to the subject) (13) and scan time (increasing temporal resolution for dynamic studies). High spatial resolution is required to detect small structures and lesions and to improve quantification by reducing the partialvolume effect. A compromise between sensitivity and spatial resolution due to depth-of-interaction (DOI) effects always exists in small-ani...

A prototype small-animal PET scanner was developed based on depth-encoding detectors using dual-ended readout of very small scintillator elements to produce high and uniform spatial resolution suitable for imaging the mouse brain. Methods The scanner consists of 16 tapered dual-ended readout detectors arranged in a ring of diameter 61 mm. The axial field of view is 7 mm and the transaxial field of view is 30 mm. The scintillator arrays consist of 14×14 lutetium oxyorthosilicate (LSO) elements, with a crystal size of 0.43×0.43 mm2 at the front end and 0.80×0.43 mm2 at the back end, and the crystal elements are 13 mm long. The arrays are read out by 8×8 mm2 and a 13×8 mm2 position-sensitive avalanche photodiodes (PSAPDs) placed at opposite ends of the array. Standard nuclear instrumentation module (NIM) electronics and a custom designed multiplexer are used for signal processing. Results The detector performance was measured and all except the very edge crystals could be clearly resolved. The average detector intrinsic spatial resolution in the axial direction was 0.61 mm. A depth of interaction resolution of 1.7 mm was achieved. The sensitivity of the scanner at center of the field of view was 1.02% for a lower energy threshold of 150 keV and 0.68% for a lower energy threshold of 250 keV. The spatial resolution within a field of view that can accommodate the entire mouse brain was ~0.6 mm using a 3D Maximum Likelihood-Expectation Maximization (ML-EM) reconstruction algorithm. Images of a micro hot-rod phantom showed that rods with diameter down to 0.5 mm could be resolved. First in vivo studies were obtained using 18F-fluoride and confirmed that 0.6 mm resolution can be achieved in the mouse head in vivo. Brain imaging studies with 18F-fluorodeoxyglucose were also acquired. Conclusion A prototype PET scanner achieving a spatial resolution approaching the physical limits for a small-bore PET scanner set by positron range and acolinearity was developed. Future plans are to add more detector rings to extend the axial field of view of the scanner and increase sensitivity.

Small animal PET scanners may be improved by increasing the sensitivity, improving the spatial resolution and improving the uniformity of the spatial resolution across the field of view. This may be achieved by using PET detectors based on crystal elements that are thin in the axial and transaxial directions and long in the radial direction, and by employing depth of interaction (DOI) encoding to minimize the parallax error. With DOI detectors, the diameter of the ring of the PET scanner may also be decreased. This minimizes the number of detectors required to achieve the same solid angle coverage as a scanner with a larger ring diameter and minimizes errors due to non-collinearity of the annihilation photons. In this study, we characterize prototype PET detectors that are finely pixelated with individual LSO crystal element sizes of 0.5 mm × 0.5 mm × 20 mm and 0.7 mm × 0.7 mm × 20 mm, read out at both ends by position sensitive avalanche photodiodes (PSAPDs). Both a specular reflector and a diffuse reflector were evaluated. The detectors were characterized based on the ability to clearly resolve the individual crystal elements, the DOI resolution and the energy resolution. Our results indicate that a scanner based on any of the four detector designs would offer improved spatial resolution and more uniform spatial resolution compared to present day small animal PET scanners. The greatest improvements to spatial resolution will be achieved when the detectors employing the 0.5 mm × 0.5 mm × 20 mm crystals are used. Monte Carlo simulations were performed to demonstrate that 2 mm DOI resolution is adequate to ensure uniform spatial resolution for a small animal PET scanner geometry using these detectors. The sensitivity of such a scanner was also simulated using Monte Carlo simulations and was shown to be greater than 10 % for a four ring scanner with an inner diameter of 6 cm, employing 20 detectors per scanner ring.

Many laboratories are developing depth-encoding detectors to improve the trade-off between spatial resolution and sensitivity in positron emission tomography (PET) scanners. One challenge in implementing these detectors is the need to calibrate the depth of interaction (DOI) response for the large numbers of detector elements in a scanner. In this work, we evaluate two different methods, a linear detector calibration and a linear crystal calibration, for determining DOI calibration parameters. Both methods can use measurements from any source distribution and location, or even the intrinsic LSO background activity, and are therefore well-suited for use in a depth-encoding PET scanner. The methods were evaluated by measuring detector and crystal DOI responses for all eight detectors in a prototype depth-encoding PET scanner. The detectors utilize dual-ended read out of lutetium oxyorthosilicate (LSO) scintillator arrays with position-sensitive avalanche photodiodes (PSAPDs). The LSO arrays have 7×7 elements, with a crystal size of 0.92×0.92×20 mm 3 and pitch of 1.0 mm. The arrays are read out by two 8×8 mm 2 area PSAPDs placed at opposite ends of the arrays. DOI is measured by the ratio of the amplitude of the total energy signals measured by the two PSAPDs. Small variations were observed in the DOI responses of different crystals within an array as well as DOI responses for different arrays. A slightly nonlinear dependence of the DOI ratio on depth was observed and the nonlinearity was larger for the corner and edge crystals. The DOI calibration parameters were obtained from the DOI responses measured in singles mode. The average error between the calibrated DOI and the known DOI was 0.8 mm if a linear detector DOI calibration was used, and 0.5 mm if a linear crystal DOI calibration was used. A line source phantom and a hot rod phantom were scanned on the prototype PET scanner. DOI measurement significantly improved the image spatial resolution no matter which DOI calibration method was used. A linear crystal DOI calibration provided slightly better image spatial resolution compared with a linear detector DOI calibration.