Quantitative analysis of phantom studies of 111In and 68Ga imaging of neuroendocrine tumours

Abstract

BackgroundNuclear medicine imaging of neuroendocrine tumours is performed either by SPECT/CT imaging, using 111In-octreotide or by PET/CT imaging using 68Ga-radiolabelled somatostatin analogs. These imaging techniques will give different image quality and different detection thresholds for tumours, depending on size and activity uptake. The aim was to evaluate the image quality for 111In-SPECT and 68Ga-PET imaging, i.e. the smallest volume possible to visualize for different source-to-background activity ratios. The accuracy of quantification of lesion volume and activity was also investigated to develop an objective evaluation for radionuclide therapy eligibility.The phantom study was performed using the NEMA IEC Body Phantom with six hot spheres having inner diameters of 10, 13, 17, 22, 28, and 37 mm, filled with either 68Ga or 111In with sphere-to-background ratios (SBRs) of no background activity, 5:1, 2.5:1, and 1.25:1. Activity ratios of 1.25:1 and 2.5:1 are clinically found for lesions close to the liver and spleen. Clinical acquisition and reconstruction protocols were applied. Line profiles were drawn to evaluate the smallest detectable volume within a given SBR. Recovery curves based on threshold-based VOIs, threshold-based VOIs adapted to the background and CT-based ROIs were obtained for all SBRs and sphere diameters, allowing for quantification.ResultsThe 10-mm sphere was not possible to detect in SPECT images. It was detectable in PET images for SBRs of 2.5:1 and higher. In a background corresponding to the activity uptake in the liver, spheres larger than 22–37 mm were detectable in the 111In-SPECT images and spheres larger than 13–22 mm were detectable in the 68Ga-PET images. The maximum activity concentration was accurately quantified for spheres larger than 22 mm in the PET images; however, the quantification was impaired by sphere size and background activity.ConclusionsIt was not possible to detect the 10-mm sphere in any of the SPECT images. In a background corresponding to the activity uptake in the liver, spheres larger than approximately 30 mm were visible in the 111In-SPECT images and spheres larger than approximately 17 mm were visible in the 68Ga-PET images. Sphere diameter and background activity strongly affect the possibility of a correct quantification.

Standardisation of conventional and advanced iterative reconstruction methods for Gallium-68 multi-centre PET-CT trials

“…Similarly, in the multi-centre study performed by Huizing et al [11], at least 5 out of 13 scanners did not meet the EARL criteria, even after the proposed correction for variances in the well counter, while valid results presented in other studies would also render the scanners ineligible from participating in multi-centre trials [33,35]. The proposed limits in this study are closer to previously published results [11,[33][34][35] and consistent with the observation by Huizing et al of higher variation between PET-CT systems [11]. Despite the fact that all scanners performed similarly in this study in terms of recovery, it needs to be taken into account that only six were used for the definition of the specification limits which is a limitation of this study, including scanners of different generations such as the Siemens Biograph TrueV and the GE MI DR.…”

“…The p value from the paired t test is also quoted on each figure with * indicating significant differences at the 5% significance level reconstruction methods in terms of SUVs were also in agreement with the results from clinical analysis. Consistent with the literature, the RCs for Gallium-68 were at the lower limits of the EARL Fluorine-18 standard 1 for conventional iterative reconstruction [11,[33][34][35]. No additional correction factors were needed in this study to account for well counter variability [11] as they all had traceability to the National Physics Laboratory.…”

Optimization of injected 68Ga-PSMA activity based on list-mode phantom data and clinical validation

“…In both graphs, the proposed acquisition time 140 s/bed is indicated (vertical dotted line) the proposed activity regimen is optimized for lesions ≥ 10 mm. However, since detectability of lesions indeed depends on the uptake and size of lesions, further research is required for different uptake ratios and lesion sizes [23].…”

Evaluation of the Spatial Resolution in Monte Carlo-Based Spect/Ct Reconstruction of 111in-Octreotide Images

“…The authors report the use of iterative reconstruction using the OSEM algorithm for the 111 In SPECT imaging and do not specify further, thus indicating that no corrections for PSF, scatter or even attenuation were made. Among the references, in this study, comparing 111 In-SPECT and 68 Ga-PET, only one ( 6 ) has reported the use of attenuation and PSF correction, remaining authors have not reported any corrections at all, indicating that many comparisons may not have been completely fair. Furthermore, the field of SPECT is developing with new camera designs ( 17 , 18 ) as well as improved reconstruction techniques.…”

“…The method utilises the overexpression of somatostatin receptors on the tumour cell surface. In recent years, scintigraphy with 111 In-octreotide has at many sites been replaced by PET imaging with somatostatin analogues labelled with Gallium-68 ( 68 Ga), which yields better image quality ( 6 ) and higher diagnostic sensitivity ( 7 ) . A considerable number of studies have concluded the superiority of 68 Ga PET/CT over 111 In-octreotide SPECT/CT ( 8–10 ) .…”