ABSTRACT
Absolute quantification of radiotracer distribution using SPECT/CT imaging is of great importance for dosimetry aimed at personalized radionuclide precision treatment. However, its accuracy depends on many factors. Using phantom measurements, this multi-vendor and multi-center study evaluates the quantitative accuracy and inter-system variability of various SPECT/CT systems as well as the effect of patient size, processing software and reconstruction algorithms on recovery coefficients (RC). METHODS: Five SPECT/CT systems were included: Discovery™ NM/CT 670 Pro (GE Healthcare), Precedence™ 6 (Philips Healthcare), Symbia Intevo™, and Symbia™ T16 (twice) (Siemens Healthineers). Three phantoms were used based on the NEMA IEC body phantom without lung insert simulating body mass indexes (BMI) of 25, 28, and 47 kg/m2. Six spheres (0.5-26.5 mL) and background were filled with 0.1 and 0.01 MBq/mL 99mTc-pertechnetate, respectively. Volumes of interest (VOI) of spheres were obtained by a region growing technique using a 50% threshold of the maximum voxel value corrected for background activity. RC, defined as imaged activity concentration divided by actual activity concentration, were determined for maximum (RCmax) and mean voxel value (RCmean) in the VOI for each sphere diameter. Inter-system variability was expressed as median absolute deviation (MAD) of RC. Acquisition settings were standardized. Images were reconstructed using vendor-specific 3D iterative reconstruction algorithms with institute-specific settings used in clinical practice and processed using a standardized, in-house developed processing tool based on the SimpleITK framework. Additionally, all data were reconstructed with a vendor-neutral reconstruction algorithm (Hybrid Recon™; Hermes Medical Solutions). RESULTS: RC decreased with decreasing sphere diameter for each system. Inter-system variability (MAD) was 16 and 17% for RCmean and RCmax, respectively. Standardized reconstruction decreased this variability to 4 and 5%. High BMI hampers quantification of small lesions (< 10 ml). CONCLUSION: Absolute SPECT quantification in a multi-center and multi-vendor setting is feasible, especially when reconstruction protocols are standardized, paving the way for a standard for absolute quantitative SPECT.
ABSTRACT
UNLABELLED: Poor and variable spatial resolution of the gamma camera, the movement of the heart and, above all, the inclusion of scattered photons in the acquisition data contribute to the deterioration of image contrast in 201Tl myocardium perfusion studies. Scatter correction algorithms may correct for the latter factor by removing (most of) the scattered photons from the acquisition data. METHODS: In this study we investigated the contrast changes induced by the Triple Energy Window scatter correction method (TEW) applied to clinical 201Tl myocardium perfusion studies and its influence on the reading of the images. Stress and rest studies of 30 consecutive patients were used for this study. Maximum image contrasts were measured between the myocardium and the left ventricular cavity in four mid-ventricular short axis slices, as well as between normally and abnormally perfused myocardium using bull's-eye displays of the activity within the myocardium. To assess image quality and perfusion abnormalities, an experienced nuclear medicine physician, blind to patient characteristics, visually reviewed all studies. RESULTS: In all individual measurements, the maximum contrast after scatter correction was higher than without correction (p < 0.001). The average increase in contrast between the myocardium and the left ventricular cavity was 43% and 48% for stress and rest studies respectively. The contrast within the myocardium increased by 25% and 32% respectively. After TEW, image quality was rated lower in almost half of the studies, while in only one study the quality was rated higher. In stress studies 11 additional perfusion defects were observed, with rest studies revealing 15 more defects after TEW, but this difference was not significant. Cohen's kappa indicated a moderate agreement of the image reading between studies with and without scatter correction. CONCLUSION: We conclude that image contrast improves significantly by scatter correction. However, image quality decreased as a result of an unfavorable signal-to-noise ratio. As an overall result, no significant change in the clinical outcome of the studies could be shown. Additional training of the readers may be required to obtain optimal results.
