Quantitative evaluation of PSMA PET imaging using a realistic anthropomorphic phantom and shell-less radioactive epoxy lesions

Abstract

Background Positron emission tomography (PET) with prostate specific membrane antigen (PSMA) have shown superior performance in detecting metastatic prostate cancers. Relative to [18F]fluorodeoxyglucose ([18F]FDG) PET images, PSMA PET images tend to visualize significantly higher-contrast focal lesions. We aim to evaluate segmentation and reconstruction algorithms in this emerging context. Specifically, Bayesian or maximum a posteriori (MAP) image reconstruction, compared to standard ordered subsets expectation maximization (OSEM) reconstruction, has received significant interest for its potential to reach convergence with minimal noise amplifications. However, few phantom studies have evaluated the quantitative accuracy of such reconstructions for high contrast, small lesions (sub-10 mm) that are typically observed in PSMA images. In this study, we cast 3 mm–16-mm spheres using epoxy resin infused with a long half-life positron emitter (sodium-22; 22Na) to simulate prostate cancer metastasis. The anthropomorphic Probe-IQ phantom, which features a liver, bladder, lungs, and ureters, was used to model relevant anatomy. Dynamic PET acquisitions were acquired and images were reconstructed with OSEM (varying subsets and iterations) and BSREM (varying β parameters), and the effects on lesion quantitation were evaluated. Results The 22Na lesions were scanned against an aqueous solution containing fluorine-18 (18F) as the background. Regions-of-interest were drawn with MIM Software using 40% fixed threshold (40% FT) and a gradient segmentation algorithm (MIM’s PET Edge+). Recovery coefficients (RCs) (max, mean, peak, and newly defined “apex”), metabolic tumour volume (MTV), and total tumour uptake (TTU) were calculated for each sphere. SUVpeak and SUVapex had the most consistent RCs for different lesion-to-background ratios and reconstruction parameters. The gradient-based segmentation algorithm was more accurate than 40% FT for determining MTV and TTU, particularly for lesions $$\le$$ ≤ 6 mm in diameter (R2 = 0.979–0.996 vs. R2 = 0.115–0.527, respectively). Conclusion An anthropomorphic phantom was used to evaluate quantitation for PSMA PET imaging of metastatic prostate cancer lesions. BSREM with β = 200–400 and OSEM with 2–5 iterations resulted in the most accurate and robust measurements of SUVmean, MTV, and TTU for imaging conditions in 18F-PSMA PET/CT images. SUVapex, a hybrid metric of SUVmax and SUVpeak, was proposed for robust, accurate, and segmentation-free quantitation of lesions for PSMA PET.

Neural blind deconvolution for deblurring and supersampling PSMA PET

“…The deblurring methodology was further tested using PET images acquired using the Quantitative PET Prostate Phantom (Q3P) (Fedrigo et al 2024). The phantom had been developed for a previous study (Fedrigo et al 2022) and included a set of 'shell-less' spheres (3-16 mm diameters) of 22 NaCl infused epoxy resin. The positron energy via 22 Na decay is similar to that from 18 F (220.3 keV and 252 keV respectively) (Jødal et al 2014(Jødal et al , 2012, making 22 Na suitable for approximating the decay seen in [18F]DCFPyL PET.…”

“…The phantom contains eight spheres with diameters of 16mm, 14 mm, 10 mm, 8 mm, 7 mm, 6 mm, 5 mm, and 3 mm arranged evenly in a coaxial ring and 50 mm from the radial centre of the phantom. The spheres were prepared with a 22 Na concentration of 57.6 kBq ml −1 to represent lesion concentrations from an analysis of patients imaged with [18F]DCFPyL PET (Fedrigo et al 2022). By the time of acquisition, activity concentrations had decayed to approximately 31 kBq ml −1 .…”

“…where a h,true is the ground truth activity concentration for each sphere determined in a previous study (Fedrigo et al 2022), and stat = {mean, median, max}. The mean inner product between combinations of phantom kernels was calculated.…”

Reliability of gradient-based segmentation for measuring metabolic parameters influenced by uptake time on 18F-PSMA-1007 PET/CT for prostate cancer

“…This further suggests the limitation of using threshold-based method on 68Ga-PSMA PSMA PET/CT as it relies on direct computation on SUVmax. Furthermore, the accuracy of gradient-based method for volumetric measurement at high lesion contrast commonly seen on PSMA PET/CT has been validated recently ( 32 ). In this regard, gradient-based method is a better option with smaller variation.…”

Development of the quantitative PET prostate phantom (Q3P) for improved quality assurance of 18F‐PSMA PET imaging in metastatic prostate cancer

“…44,45 The spheres were prepared with a 22 Na concentration of 57.6 kBq/mL (as of January 2020), to match lesion concentrations from an analysis of 33 mPCa patients imaged with 18 F-PSMA PET-based radiopharmaceuticals. 46 Sphere diameters of 3, 5, 7, 8, 10, 12, 14, 16 mm were cast using an aluminum mold. 46 The techniques and radiation safety procedures for the casting process are described in further detail by Fedrigo et al 46 To accurately determine the ground truth radioactivity, a 6-h PET acquisition was performed using a 5-ring Discovery MI PET/CT scanner.…”