Image Quantification for Radiation Dose Calculations—limitations and Uncertainties

Abstract

Purpose-Radiation dose calculations in nuclear medicine depend on quantification of activity via planar and/or tomographic imaging methods. However, both methods have inherent limitations, and the accuracy of activity estimates varies with object size, background levels, and other variables. The goal of this study was to evaluate the limitations of quantitative imaging with planar and SPECT approaches, with a focus on activity quantification for use in calculating absorbed dose estimates for normal organs and tumors. To do this we studied a series of phantoms of varying complexity of geometry, with three radionuclides whose decay schemes varied from simple to complex. Tc, 131 I and 111 In (74, 185, 370 and 740 kBq/ml) were placed in spheres of four different sizes in a water-filled phantom, with three different levels of activity in the surrounding water. Planar and SPECT images of the phantoms were obtained on a modern SPECT/CT system. These radionuclides and concentration/background studies were repeated using a cardiac phantom and a modified torso phantom with liver and 'tumor' regions containing the radionuclide concentrations and with the same varying background levels. Planar quantification was performed using the geometric mean approach, with attenuation correction (AC), and with and without scatter corrections (SC and NSC). SPECT images were reconstructed using attenuation maps (AM) for AC; scatter windows were used to perform SC during image reconstruction. Methods-Four aqueous concentrations of 99mResults-For spherical sources with corrected data, good accuracy was observed (generally within ± 10% of known values) for the largest sphere (11.5 ml) and for both planar and SPECT methods with 99m Tc and 131 I, but were poorest and deviated from known values for smaller objects, most notably for 111 In. SPECT quantification was affected by the partial volume effect in smaller objects and generally showed larger errors than the planar results in these cases for all radionuclides. For the cardiac phantom, results were the most accurate of all of the experiments, for all radionuclides. Background subtraction was an important factor influencing these results. The contribution of scattered photons was important in quantification with 131 I; if scatter was not accounted for, activity tended to be overestimated using planar quantification methods. For the torso phantom experiments, results show a clear understimation of activity when compared to previous experiment with spherical sources, for all radionuclides. Despite some variations that were observed as the level of background increased, the SPECT results were more consistent across different activity concentrations. Conclusion-Planar or SPECT quantification on state-of-the-art gamma cameras with appropriate quantitative processing can provide accuracies of better than 10% for large objects and modest targetto-background concentrations; however when smaller objects are used, in the presence of higher background, and for nuclides with more complex decay...

An Evidence-Based Review of Quantitative SPECT Imaging and Potential Clinical Applications

“…These include 111 In-labeled antibodies and tumor-seeking radiopharmaceuticals (37)(38)(39), 131 I uptake in tumors (40,41), 177 Lu for dosimetry calculations in therapeutic applications (42,43), 186 Re in palliative bone cancer therapy (44), and even quantitative bremsstrahlung imaging of 90 Y (45).…”

Chemoradionuclide Therapy with186Re-Labeled Liposomal Doxorubicin: Toxicity, Dosimetry, and Therapeutic Response

“…Planar imaging method has the limitation that it represents a 3D distribution of radioactivity in a 2D display, causing difficulty in accurate localization and errors in radiotracer quantification. 68,69 The high radiation absorbed dose calculated for the different organs in the present study could be due to the low resolution of the planar images leading to overestimation of the radiotracer activity in the target organ. Also, the high radiation absorbed dose calculated in heart and lungs may be due to the blood pool in these organs for which no correction was applied during image quantification.…”

Image-Guided Personalized Predictive Dosimetry by Artery-Specific SPECT/CT Partition Modeling for Safe and Effective 90Y Radioembolization

“…Hepatic intraarterial 99m Tc-MAA scintigraphy is a validated means of simulating the 90 Y radioembolization therapy field (13,14). SPECT/CT-based dosimetry is superior to planar scintigraphy by tomographically resolving overlapping radiotracer activity, evaluating heterogeneous radiotracer uptake, and detecting activity in small lesions (12,14,17). Phantom studies have shown that 99m Tc-MAA SPECT/CT volume measurements are accurate and reproducible (13).…”

“…SPECT/CT is superior to both planar scintigraphy and SPECT for assessing intrahepatic biodistribution of 99m Tc-macroaggregated albumin ( 99m Tc-MAA), and for estimating the tumor-to-normal liver (T/ N) ratio (12)(13)(14). The partition model is a validated dosimetric method for 90 Y resin microspheres, scientifically superior to the body surface area (BSA) method (5,10).…”