Characterization of Noise and Resolution for Quantitative ¹⁷⁷Lu SPECT/CT with xSPECT Quant

Abstract

Quantitative SPECT/CT imaging forms the basis for internal dosimetry in molecular radiotherapies. While the conversion from counts to activity is typically performed based on conversion factors individually measured by each site, a recently introduced commercially available reconstruction (xSPECT Quant) offers a standardized and traceable calibration of SPECT/CT systems. The aim of this work was to assess the characteristics of xSPECT Quant in combination with Lu as one of the most important radionuclides used in molecular radiotherapies and to compare it to a widely used ordered subset expectation maximization reconstruction (Flash3D). In a series of Lu-filled phantom measurements, several important features were investigated for xSPECT Quant and Flash3D: Noise behavior and accuracy of the activity determination were evaluated in a large cylinder. Recovery coefficients were assessed in a hot-sphere phantom with and without background. Additionally, the resolutions were determined in a line source phantom as well as in a matched-filter resolution analysis of the hot-sphere-cold-background phantom. Both reconstruction algorithms improve the spatial resolution at the cost of noise build-up. Despite its slower convergence, Flash3D features a more efficient recovery. Although resolution recovery methods are applied within both reconstructions, partial volume errors - namely activity overestimation in the object center and spill-out of counts from the object edges - remain of relevance. In contrast to Flash3D where only the total number of updates (iterations×subsets) plays a role, the exact subdivision into iterations and subsets affected all characteristics of xSPECT Quant (optimum: 1 subset). The optimal trade-off between noise build-up and resolution improvement was found for 48 iterations and 1 subset, resulting in a quantitative accuracy of 1.2% in the Jaszczak cylinder (xSPECT Quant cross-calibrated to the dose calibrator). If the reconstruction parameters are chosen with care, both examined reconstructions can provide absolute quantitative SPECT images with high image quality (sub-centimeter resolution at an acceptable noise build-up) as well as high quantitative accuracy (given a well-calibrated Flash3D conversion from counts to activity concentration). With its standardized (and traceable) activity determination, xSPECT Quant dispenses with site-specific calibration protocols, enabling a reliable activity determination comparable across sites, which is especially useful for multi-centric molecular radiotherapy studies.

The Relevance of Dosimetry in Precision Medicine

“…An overview of the most recently applied methods for SPECT/ CT quantification with therapeutically used radionuclides, different phantom geometries, volumes, camera vendors, reconstruction methods, and the related accuracies has been provided by Tran-Gia et al (24). Table 1 shows only publications with reported accuracies of less than 10% (25)(26)(27)(28)(29)(30)(31)(32).…”

“…de/molecular-imaging/xspect/xspect-technology/features#). Using this calibration method, Tran-Gia et al (26) showed that a quantification accuracy of less than 2% could be achieved.…”

Phantom and clinical evaluation of bone SPECT/CT image reconstruction with xSPECT algorithm

“…However, the xQ applies a CT-derived reconstruction mask to reduce background noise [26]. When comparing the two reconstruction methods, researchers found an unexpected behavior in xQ, where the subset >2 decreased image quality [27,28]. For both…”

Technical Note: Optimal sampling schedules for kidney dosimetry based on the hybrid planar/SPECT method in 177Lu‐PSMA therapy

“…Noise was also added to the activity data assumed to be provided by the SPECT/CT measurement. For SPECT/CT measurements, activity accuracies below 5% are reported in literature for physiological activity concentrations and phantom sizes similar to kidney size . Thus, a noise with FSD SPECT = 5% was used for the simulations.…”

“…For SPECT/CT measurements, activity accuracies below 5% are reported in literature for physiological activity concentrations and phantom sizes similar to kidney size. [20][21][22][23] Thus, a noise with FSD SPECT = 5% was used for the simulations. The noise was assumed to be independent of the systematic error of the planar images.…”