Abstract
BackgroundPET image reconstruction methods include modeling of resolution degrading phenomena, often referred to as point-spread function (PSF) reconstruction. The aim of this study was to develop a clinically relevant phantom and characterize the reproducibility and accuracy of high-resolution PSF reconstructed images of small lesions, which is a prerequisite for using PET in the prediction and evaluation of responses to treatment.Sets of small homogeneous 18F-spheres (range 3–12 mm diameter, relevant for small lesions and lymph nodes) were suspended and covered by a 11C-silicone, which provided a scattering medium and a varying sphere-to-background ratio. Repeated measurements were made on PET/CT scanners from two vendors using a wide range of reconstruction parameters. Recovery coefficients (RCs) were measured for clinically used volume-of-interest definitions.ResultsFor non-PSF images, RCs were reproducible and fell monotonically as the sphere diameter decreased, which is the expected behavior. PSF images converged slower and had artifacts: RCs did not fall monotonically as sphere diameters decreased but had a maximum RC for sphere sizes around 8 mm, RCs could be greater than 1, and RCs were less reproducible. To some degree, post-reconstruction filters could suppress PSF artifacts.ConclusionsHigh-resolution PSF images of small lesions showed artifacts that could lead to serious misinterpretations when used for monitoring treatment response. Thus, it could be safer to use non-PSF reconstruction for quantitative purposes unless PSF reconstruction parameters are optimized for the specific task.
Highlights
PET image reconstruction methods include modeling of resolution degrading phenomena, often referred to as point-spread function (PSF) reconstruction
The experiment was reproduced with a new set of spheres suspended in free air that was scanned on three different Siemens Biograph TrueV PET/CT systems with the same results: a monotonic relation between Recovery coefficients (RCs) and sphere size on images reconstructed without PSF, and a non-monotonic relation with a maximum value at 8 mm on the PSF images
For the images reconstructed without PSF (Fig. 2a), the monotonic relation between RC and sphere size developed as the true sphere-to-background ratio increased, and spill-out from spheres to the background became more important
Summary
PET image reconstruction methods include modeling of resolution degrading phenomena, often referred to as point-spread function (PSF) reconstruction. The spatial resolution and signal-to-noise ratio of PET images have improved significantly due to image reconstruction methods that include modeling of resolution degrading phenomena, detector effects such as crystal size, inter-crystal scattering, and crystal penetration, and positron range and angle deviation from 180° during electron-positron annihilation This more accurate system matrix is used in statistical reconstruction algorithms, referred to as resolution recovery, resolution modeling, or point-spread function (PSF) reconstruction [1, 2]. PSF reconstruction produces images with improved isotropic spatial resolution, reduced spill-in/spill-out, and increased activity concentration (Bq/mL) or standardized uptake value (SUV) in small lesions that are more detected and characterized These benefits have been demonstrated as higher recovery coefficients (RCs) in NEMA phantom studies [3] and improved lesion detectability in patient studies [4]. The fillable acrylic glass spheres in the phantoms separate the hot spheres from the background activity by a non-radioactive layer, which does not mimic the physiologic reality and cause quantitative errors [14], for small diameters
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