Performance evaluation of the MOLECUBES β-CUBE—a high spatial resolution and high sensitivity small animal PET scanner utilizing monolithic LYSO scintillation detectors

Abstract

The MOLECUBES β-CUBE scanner is the newest amongst commercially available preclinical PET scanners for dedicated small animal imaging. The scanner is compact, lightweight and utilizes a small footprint to facilitate bench-top imaging. It can be used individually, or in combination with the X-CUBE CT scanner, which provides the ability to perform all necessary PET data corrections and provide fully quantitative PET images. The PET detector comprises of an 8 mm thick monolithic LYSO scintillator read-out by an array of 3 mm × 3 mm Hamamatsu silicon photomultipliers. The monolithic scintillator provides the ability to measure depth-of-interaction which aids in the development of such a compact scanner. With a scanner diameter of 7.6 cm and axial length of 13 cm it is suitable for imaging both whole-body mice and rats. This paper presents the design and imaging performance of the β-CUBE scanner. NEMA NU4-2008 characterization and a variety of phantom and animal imaging studies to demonstrate the quantitative imaging performance of the PET scanner are presented. Spatial resolution of 1 mm is measured with a filtered-back projection reconstruction algorithm at the center of the scanner and DOI measurement helps maintain the excellent spatial resolution over the entire imaging FOV. An absolute peak sensitivity of 12.4% is measured with a 255-765 keV energy window. The scanner demonstrates good count-rate performance, with a peak NEC of 300 kcps and 160 kcps measured with ~900 µCi in the NEMA mouse and rat phantoms, respectively. Imaging data with the NEMA image quality phantom and Micro Derenzo phantoms demonstrate the ability to achieve good image quality and accurate quantitative data. Image uniformity of 7.4% and spill-over ratio of 8% were measured. The superior spatial resolution, excellent energy resolution and sensitivity also provide superior contrast recovery, with ~70% recovery for the 2 mm rods. While current commercial preclinical PET scanners have spatial resolution in the 1-2 mm range, the 1 mm volumetric resolution presents significant improvement over current commercially available preclinical PET scanners. In combination with the X-CUBE scanner it provides the ability to perform fully quantitative imaging with spatially co-registered high-resolution 3D PET-CT images.

Comments on the NEMA NU 4-2008 Standard on Performance Measurement of Small Animal Positron Emission Tomographs

“…To evaluate the spatial resolution, the NEMA standard mandates the use of point source scans which are reconstructed using filtered backprojection. However, basically all modern PET scanners instead use an iterative maximum likelihood expectation maximization (MLEM) algorithm for reconstruction [5,6,7,8,9,10,4,11,12,13,14,15], so a scanner's spatial resolution using filtered backprojection is not necessarily indicative of its spatial resolution for applications. While the mandated filtered backprojection is intended to benchmark the detector performance alone, we will demonstrate in the following that it disadvantages certain scanner geometries.…”

“…The Derenzo phantom is already well-established in the community as a benchmark to evaluate the spatial resolution. For instance, several NEMA performance publications already include such a measurement as a benchmark of spatial resolution [5,7,12,15]. However, these results are not easily comparable, as there currently isn't a standardized quantitative analysis method to determine the spatial resolution from a measurement of a Derenzo phantom.…”

“…However, most modern scanners store their data in listmode format and often only implement sinogram support to conduct the NEMA measurements. To our knowledge, all NEMA NU-8 measurements published in in the last 5 years had to convert their native listmode files to sinograms after the measurements [5,6,7,8,9,10,4,11,12,13,14,15]. Spinks et al [8] even mentions that the calculation of scatter fractions were omitted due to missing sinogram support, so this performance evaluation did apparently only use listmode data for the data analysis.…”

Exploring TOF capabilities of PET detector blocks based on large monolithic crystals and analog SiPMs

“…Detector block configurations which make use of monolithic crystals provide some advantages when compared to pixelated crystals and, therefore, make them good candidates for PET applications [18][19][20][21]. The crystal thickness and geometry, as well as treatments on the walls vary depending on the application.…”

A Novel DOI Positioning Algorithm for Monolithic Scintillator Crystals in PET Based on Gradient Tree Boosting

“…Digital Object Identifier 10.1109/TRPMS.2018.2884320 the brain or female breast [4]- [6]. Recently, the advantages of DOI information have also been experimentally validated for a clinical case with a 70 cm diameter tomographic setup [7].…”