Purpose: The combination of single photon emission computed tomography (SPECT) and computer tomography (CT) that incorporates iterative reconstruction algorithms with attenuation and scatter correction should facilitate accurate non-invasive quantitative imaging. Quantitative SPECT (QSPECT) may improve diagnostic ability and could be useful for many applications including dosimetry assessment. Using 177Lu, we developed a QSPECT method using a commercially available SPECT/CT system. Methods: Serial SPECT of 177Lu sources (89–12,400 MBq) were acquired with multiple contiguous energy windows along with a co-registered CT, and were reconstructed using an iterative algorithm with attenuation and scatter correction. Camera sensitivity (based on reconstructed SPECT count rate) and dead-time (based on wide-energy spectrum count rate) were resolved by non-linear curve fit. Utilizing these parameters, a SPECT dataset can be converted to a QSPECT dataset allowing quantitation in Becquerels per cubic centimetre or standardized uptake value (SUV). Validation QSPECT/CT studies were performed on a 177Lu cylindrical phantom (7 studies) and on 5 patients (6 studies) who were administered a therapeutic dose of [177Lu]octreotate. Results: The QSPECT sensitivity was 1.08 × 10−5 ± 0.02 × 10−5 s−1 Bq−1. The paralyzing dead-time constant was 0.78 ± 0.03 µs. The measured total activity with QSPECT deviated from the calibrated activity by 5.6 ± 1.9% and 2.6 ± 1.8%, respectively, in phantom and patients. Dead-time count loss up to 11.7% was observed in patient studies. Conclusion: QSPECT has high accuracy both in our phantom model and in clinical practice following [177Lu]octreotate therapy. This has the potential to yield more accurate dosimetry estimates than planar imaging and facilitate therapeutic response assessment. Validating this method with other radionuclides could open the way for many other research and clinical applications.

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...