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/m 2. Six spheres (0.5-26.5 mL) and background were filled with 0.1 and 0.01 MBq/mL 99m Tc-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 (RC max) and mean voxel value (RC mean) 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. Intersystem variability (MAD) was 16 and 17% for RC mean and RC max , 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.

Quantitative analysis can improve the sensitivity and specificity of single photon emission tomography (SPET) procedures, as well as reduce inter- and intraobserver variabilities. Quantification of the radioactivity distribution is the ultimate goal of SPET. In this review we consider the basic requirements for an optimum three-dimensional reconstruction of the radionuclide distribution to enable quantification. Attenuation and scatter correction as well as varying resolution are the major problems. In the older SPET systems quantification was hampered by the lack of system sensitivity and sufficient computer power. Therefore, the imaging system was often assumed to be shift invariant and linear and the attenuation throughout the object uniform. More sophisticated solutions have been proposed and with more or less success implemented, but not for application in daily practice. Knowledge (measurement) of the attenuation is often required. New generation SPET systems employing multi-detectors and super minicomputers will ease the implementation of these solutions.

Triple energy window (TEW) scatter correction estimates the contribution of scattered photons to the acquisition data by acquiring additional data through two narrow energy windows placed adjoined to the main (photopeak) energy window. The contribution is estimated by linear interpolation and then subtracted. Noise amplification is reduced by filtering both the photopeak scintigram and the scatter estimate. We have studied the filter settings of each filter using a physical phantom filled with a 201Tl-solution resulting in count densities comparable to clinical studies. The performance of order-8 Butterworth filters at different cut-off frequencies (CoFs) were compared based on signal to noise ratios (SNRs). The highest SNRs were obtained when the noisy scatter information was strongly filtered with the CoF less than or equal to 0.07 cycles/pixel (cpp). The best CoF for the filter of the photopeak image is object size dependent; smaller objects require a higher CoF. For objects with a size near the SPECT spatial resolution (approximately 15 mm) the optimal CoF is equal to 0.18 cpp. For larger objects (31.8 mm) the highest SNR was obtained with a CoF equal to 0.13 cpp. A CoF equal to 0.16 cpp is a good compromise for all objects with a diameter equal to the spatial resolution or larger. These results depend on the initial signal to noise ratio of the acquisition data and so on the count density.

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