Depth of interaction resolution measurements for a high resolution PET detector using position sensitive avalanche photodiodes

Abstract

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.

A Prototype PET Scanner with DOI-Encoding Detectors

“…A stable temperature is preferred for these dual-ended readout detector modules. Although our results showed that the PSAPDs work well at room temperature (32), the temperature of the detector box becomes much higher than room temperature when several PSAPDs, preamplifiers, and other electronics parts are enclosed. To ensure low and stable temperature, the detector boxes were cooled, and the temperature variation of the detector boxes was controlled to be less than 61°C during imaging.…”

“…As shown previously (32), light loss along the crystal length is essential for high-resolution DOI measurement with dual-ended readout. In this work, unpolished LSO arrays were used to provide a strong depth dependence in the scintillation light reaching the 2 ends of the array.…”

“…The FWHM DOI resolution was calculated by using a gaussian fitting of the DOI histograms. The FWHM DOI resolution was then calibrated to units of millimeters by using the relationship between the peak of the measured DOI ratio and the known irradiation depth information (32).…”

“…Our previous results on individual detector modules have shown that with such a detector at room temperature, crystals arrays as small as 1 mm could be resolved and that an energy resolution of about 15% full width at half maximum (FWHM) and a uniform DOI resolution of 3-to 4-mm FWHM (including the radiation beam width of more than 2 mm) could be achieved (32). In this work, the imaging performance of a prototype PET scanner consisting of 8 detector modules arranged in 2 detector plates is evaluated.…”

A combined time‐of‐flight and depth‐of‐interaction detector for total‐body positron emission tomography

“…4. 20 The electronically collimated beam width at the phosphor-coated crystal was estimated to be 2-3 mm. All PMTs were biased at −1400 V. Detector signals were digitized at 5 GS/s using a DRS-based digitizer and each digitized pulse spanned 200 ns.…”