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

Abstract

Background: Conventional calibration of the gamma camera consists of the calculation of calibration factors (CFs) (ratio of counts/cc and true concentration activity) as the function of the volume of interest (VOI). However, such method shows inconsistent results when the background activity varies. The aim of the present study was to propose a new calibration method by considering the sphere-to-background counts/voxel ratio (SBVR) in addition to the VOI for CFs calculation. A PET cylindrical flood phantom, a NEMA IQ body phantom, a Data spectrum Torso Phantom (ECT/TOR/P) and a LK-S Kyoto Liver/Kidney phantom were used. The NEMA IQ phantom was used to calibrate the camera and to produce CFs for the different spheres volumes and for varying sphere-to-background activity ratios. The spheres were filled with a uniform activity concentration of ¹⁷⁷Lu, while the background was first filled with cold water and activity was added between each SPECT scan. SPECT imaging was performed for 30-s, 20-s, and 10-s exposure per view. The calculated CFs were expressed as function of the sphere volume and SBVR. The obtained CFs were validated for an additional NEMA IQ acquisition with different activities in spheres and background and for the Torso and Liver/Kidney phantoms with inserted NEMA IQ spheres. The quantification accuracy was compared with the conventional method not taking SBVR into consideration.

Results: The relative errors in quantification using the NEMA IQ phantom with the new calibration method were 0.16%, 5.77%, 9.34% for the large, medium and small sphere, respectively, for a time per view of 30-s. The conventional calibration method gave errors of 3.65%, 6.65%, 30.28% for 30-s. The LK-S Kyoto Liver/Kidney Phantom resulted in quantification errors of 3.40%, 2.14%, 11.18% for the large, medium and small spheres, respectively, for 30-s; compared to 11.31%, 17.54%, 14.43% for 30-s, respectively, for the conventional method. Similar results were obtained for shorter acquisitions times with 20-s and 10-s time per view.

Conclusion: These results suggest that SBVR allows to improve quantification accuracy. The shorter time-per-view acquisitions had similar relative differences compared to the full-time acquisition which allows shorter imaging times with ¹⁷⁷Lu and improved patient comfort. The SBVR method is simple to set up and can be proposed for standardization.

Keywords: 177Lu activity quantification; Calibration factors; Gamma camera calibration; SPECT; Sphere-to-background counts/voxel ratio.

Fig. 1

a NEMA IQ phantom and b PET cylindrical flood phantom used for calibration. Anthropomorphic c Torso (ECT/TOR/P, (Data Spectrum, Hillsborough, NC)) and d LK-S Kyoto Liver/Kidney phantoms used for ¹⁷⁷Lu activity quantification validation in spheres and large organs

Fig. 2

a, b Spheres and c, d background volumes of interest (VOIs) drawn on the NEMA IQ Cal. phantom using the General Electric Dosimetry toolkit (GE DTK) software

Fig. 3

Delineation methods for anthropomorphic phantoms. In methods a 1 and b 2, the inner VOI (green or blue) represents the sphere VOI, while the outer thick ring delineates the surrounding background VOI. In method 2 (b), the thin middle ring is used to separate both VOIs to prevent spill in/out from sphere to background. With method 3 (c), random background spherical VOIs are placed around the sphere VOI

Fig. 4

CF values in cps/MBq versus SBVR calibration curves for the 26.52 (red curve), 11.49 (blue curve) and 5.71 cc (purple curve) spheres. The CFs data obtained from several NEMA Cal. 2 acquisitions (dots) were interpolated using a simple a linear regression fit. The “cold background” points were obtained from the point of intersection between the linear curve and the cold background CF value for each of the three spheres. The cold background CF value represents the maximum value of the CF for every sphere. The horizontal gray dashed line with a value of 4.87 represents the background CF for large volumes

Fig. 5

CF values in cps/MBq versus sphere/lesion volume curves for SBVR values of 6 (blue curve), 14 (green curve) and 20 (red curve). The gray dashed line represents the maximum CF (4.87). The circle, square, triangle symbols show the know CF values for the 5.71, 11.49 and 26.52 cc spheres, respectively, from Fig. 4. The star symbol represents the CF value calculated from the transition factor TF (Eq. 3) for a sphere/lesion volume of 110 cc

Fig. 6

Calibration factor map [cps/MBq] as function of sphere-to-background counts/voxel ratio (SBVR) and VOI volume [cc]

Fig. 7

Delineation methods for the large organs in the anthropomorphic phantoms. The top images (a–c) show the delineation of the kidneys in the liver/kidney phantom, and the bottom images (d–f) show the delineation of the liver cavity in the torso phantom