Performance evaluation of a high-resolution brain PET scanner using four-layer MPPC DOI detectors

Abstract

A high-resolution positron emission tomography (PET) scanner, dedicated to brain studies, was developed and its performance was evaluated. A four-layer depth of interaction detector was designed containing five detector units axially lined up per layer board. Each of the detector units consists of a finely segmented (1.2 mm) LYSO scintillator array and an 8 × 8 array of multi-pixel photon counters. Each detector layer has independent front-end and signal processing circuits, and the four detector layers are assembled as a detector module. The new scanner was designed to form a detector ring of 430 mm diameter with 32 detector modules and 168 detector rings with a 1.2 mm pitch. The total crystal number is 655 360. The transaxial and axial field of views (FOVs) are 330 mm in diameter and 201.6 mm, respectively, which are sufficient to measure a whole human brain. The single-event data generated at each detector module were transferred to the data acquisition servers through optical fiber cables. The single-event data from all detector modules were merged and processed to create coincidence event data in on-the-fly software in the data acquisition servers. For image reconstruction, the high-resolution mode (HR-mode) used a 1.2 mm crystal segment size and the high-speed mode (HS-mode) used a 4.8 mm size by collecting 16 crystal segments of 1.2 mm each to reduce the computational cost. The performance of the brain PET scanner was evaluated. For the intrinsic spatial resolution of the detector module, coincidence response functions of the detector module pair, which faced each other at various angles, were measured by scanning a 0.25 mm diameter Na point source. The intrinsic resolutions were obtained with 1.08 mm full width at half-maximum (FWHM) and 1.25 mm FWHM on average at 0 and 22.5 degrees in the first layer pair, respectively. The system spatial resolutions were less than 1.0 mm FWHM throughout the whole FOV, using a list-mode dynamic RAMLA (LM-DRAMA). The system sensitivity was 21.4 cps kBq as measured using an F line source aligned with the center of the transaxial FOV. High count rate capability was evaluated using a cylindrical phantom (20 cm diameter × 70 cm length), resulting in 249 kcps in true and 27.9 kcps at 11.9 kBq ml at the peak count in a noise equivalent count rate (NECR_2R). Single-event data acquisition and on-the-fly software coincidence detection performed well, exceeding 25 Mcps and 2.3 Mcps for single and coincidence count rates, respectively. Using phantom studies, we also demonstrated its imaging capabilities by means of a 3D Hoffman brain phantom and an ultra-micro hot-spot phantom. The images obtained were of acceptable quality for high-resolution determination. As clinical and pre-clinical studies, we imaged brains of a human and of small animals.

Simulation study of a high‐performance brain PET system with dodecahedral geometry

“…Furthermore, the dodecahedral system provides images with less noise than the HRRT system and shows higher CNR values. Although a detailed comparison will be conducted in the future, based on our simulation, we notice that the dodecahedral system also outperforms the brain PET with four‐layer MPPC DOI detectors in terms of sensitivity, since the latter (2.14%) has lower sensitivity than the HRRT system (2.5%) …”

“…For a compact design such as the dodecahedral system, DOI information is critical to the image quality. The HRRT system used 10 mm LSO and 10 mm LYSO for DOI; the helmet‐chin system used four layers in the 20 mm crystal; and Gong et al simulated the helmet system with three layers of crystal, which made the DOI resolution 6.67 mm; the ring PET with four‐layer MPPC DOI detectors was designed with crystal lengths 3, 4, 5, and 8 mm for each layer. In our resolution comparison (Figs.…”

“…Thus, we chose this three‐layer DOI in other simulations. However, the ring PET with four‐layer MPPC DOI detectors shows even better resolution thanks to the smaller crystal sectional area (1.2 × 1.2 mm 2 ) and the shorter crystal lengths.…”

“…The ring detector PET might be a good choice for whole body imaging while not optimized for specific organs such as the brain, due to the low sensitivity. In order to increase image quality of the PET brain imaging, dedicated brain PET systems were designed with small diameter detector rings, such as the High‐Resolution Research Tomograph (HRRT) system and the one with four‐layer MPPC DOI detectors . However, the sensitivity of these ring detector systems degrades heavily with the increase of the distance from axis in the transaxial plane .…”

A depth encoding PET detector using four‐crystals‐to‐one‐SiPM coupling and light‐sharing window method

“…Many methods have been developed to design DOI detectors with the single‐ended readout. One method is to use multiple layers of scintillator crystals arranged with offsets . However, the DOI detectors with multiple layer of crystals are difficult to construct and costly.…”