The Effect of Calibration Factors and Recovery Coefficients on 177Lu SPECT Activity Quantification Accuracy: A Monte Carlo Study

Abstract

Background: Different gamma camera calibration factor (CF) geometries have been proposed to convert SPECT data into units of activity concentration. However, no consensus has been reached on a standardized geometry. The CF is dependent on the selected geometry and affected by partial volume effects. This study investigated the effect of two CF geometries and their corresponding recovery coefficients (RCs) on the quantification accuracy of 177Lu SPECT images using Monte Carlo simulations.Methods: The CF geometries investigated were (i) a radioactive-sphere surrounded by non-radioactive water (sphere-CF) and (ii) a cylindrical phantom uniformly filled with radioactive water, (cylinder-CF). Recovery coefficients were obtained using the sphere-CF and cylinder-CF, yielding the sphere-RC and cylinder-RC values, respectively, for partial volume correction (PVC). The quantification accuracy was evaluated using four different sized spheres (15.6 ml – 65.4 ml) and a kidney model with known activity concentrations inside a cylindrical, torso and patient phantom. Images were reconstructed with the OS-EM algorithm incorporating attenuation, scatter and detector-response corrections. Segmentation was performed using the physical size and a small cylindrical volume inside the cylinder for the sphere-CF and cylinder-CF, respectively.Results: The sphere quantification error (without PVC) was better for the sphere-CF (≤ -5.54%) compared to the cylinder-CF (≤ -20.90%), attributed to the similar geometry of the quantified and CF spheres. Partial volume correction yielded comparable results for the sphere-CF (≤ 3.47%) and cylinder-CF (≤ 3.53%). The accuracy of the kidney quantification was poorer (≤ 22.34%) for the sphere-CF without PVC compared to the cylinder-CF (≤ 2.44%). With PVC, the kidney quantification results improved and compared well for the sphere-CF (≤ 3.50%) and the cylinder-CF (≤ 3.45%).Conclusion: The study demonstrated that upon careful selection of CF-RC combinations, comparable quantification errors (≤ 3.53%) were obtained between the sphere-CF and cylinder-CF, when all corrections were applied.

Towards accurate 177Lu SPECT activity quantification and standardization using lesion-to-background voxel ratio

“…Image reconstruction was performed using the General Electric (GE) Dosimetry toolkit (DTK) software [19] available for the Xeleris 3.0 Workstation (International General Electric, General Electric Medical Systems, Haifa, Israel). The ordered subsets expectation maximization (OSEM) algorithm with a subset and iteration product of 100 (10 iteration and 10 subsets) was used in order to obtain counts convergence in small VOIs [12]. Additionally, attenuation correction (from CT attenuation maps) and resolution recovery (for reducing image blurring) included in the Xeleris 3.0 workstation were used.…”

“…acquisitions. The conventional quantification method [12,21] uses the sphere CFs calculated from NEMA Cal. 1 acquisition and the background CF calculated from PET cylindrical phantom for quantification of the large organs, without taking background effects into consideration.…”

“…Conventional gamma camera calibration [11][12][13][14][15] is performed using a simple cylindrical phantom with spherical inserts filled with uniform activity concentration while the background is filled with cold or hot water, in order to simulate lesions and approximate the clinical conditions of a real patient. Using image processing software, phantom spheres are delineated as volumes of interest (VOIs), and a CF is calculated as the ratio between counts/cc in sphere VOIs and the true activity concentration for different sphere volumes.…”