Phantom and clinical evaluation of the effect of full Monte Carlo collimator modelling in post-SIRT yttrium-90 Bremsstrahlung SPECT imaging.

BACKGROUND: Post-therapy SPECT/CT imaging of 90Y microspheres delivered to hepatic malignancies is difficult, owing to the continuous, high-energy Bremsstrahlung spectrum emitted by 90Y. This study aimed to evaluate the utility of a commercially available software package (HybridRecon, Hermes Medical Solutions AB) which incorporates full Monte Carlo collimator modelling. Analysis of image quality was performed on both phantom and clinical images in order to ultimately provide a recommendation of an optimum reconstruction for post-therapy 90Y microsphere SPECT/CT imaging. A 3D-printed anthropomorphic liver phantom was filled with 90Y with a sphere-to-background ratio of 4:1 and imaged on a GE Discovery 670 SPECT/CT camera. Datasets were reconstructed using ordered-subsets expectation maximization (OSEM) 1-7 iterations in order to identify the optimal OSEM reconstruction (5 iterations, 15 subsets). Quantitative analysis was subsequently carried out on phantom datasets obtained using four reconstruction algorithms: the default OSEM protocol (2 iterations, 10 subsets) and the optimised OSEM protocol, both with and without full Monte Carlo collimator modelling. The quantitative metrics contrast recovery (CR) and background variability (BV) were calculated. The four algorithms were then used to retrospectively reconstruct 10 selective internal radiation therapy (SIRT) patient datasets which were subsequently blind scored for image quality by a consultant radiologist. RESULTS: The optimised OSEM reconstruction (5 iterations, 15 subsets with full MC collimator modelling) increased the CR by 42% (p < 0.001) compared to the default OSEM protocol (2 iterations, 10 subsets). The use of full Monte Carlo collimator modelling was shown to further improve CR by 14% (30 mm sphere, CR = 90%, p < 0.05). The consultant radiologist had a significant preference for the optimised OSEM over the default OSEM protocol (p < 0.001), with the optimised OSEM being the favoured reconstruction in every one of the 10 clinical cases presented. CONCLUSIONS: OSEM (5 iterations, 15 subsets) with full Monte Carlo collimator modelling is quantitatively the optimal image reconstruction for post-SIRT 90Y Bremsstrahlung SPECT/CT imaging. The use of full Monte Carlo collimator modelling for correction of image-degrading effects significantly increases contrast recovery without degrading clinical image quality.

Heterogeneity of myocardial 2-[18F]fluoro-2-deoxy-D-glucose uptake is a typical feature in cardiac sarcoidosis: a study of 231 patients.

Aims: The goal of the investigation was to evaluate whether a semi-quantitative method reflecting myocardial 2-[18F]fluoro-2-deoxy-D-glucose (FDG) uptake heterogeneity has added value in addition to visual analysis in the diagnosis of cardiac sarcoidosis (CS). Methods and results: This retrospective analysis included 271 consecutive patients suspected of CS attending cardiac positron emission tomography combined with computed tomography (PET-CT) at our institution between 2007 and 2013. Visual analysis of PET-CT and semi-quantitative analysis of heterogeneity [coefficient of variation (CoV)] of myocardial FDG uptake were performed. The presence of CS and initial symptoms were verified from patient data. The criteria for CS included histological verification from the myocardium or from an extracardiac site. Thirty cancer patients without cardiac disease were included as controls. CS was diagnosed in 48/231 (20.8%) of analysed patients. Of these, 13 (27.1%) had no extracardial signs of the disease and 30 (62.5%) had FDG positive mediastinal lymph nodes. Visual analysis of PET-CT identified 48.9% of the CS patients. We found a cut-off value of 0.184 for CoV to have the best accuracy to detect CS from a patient population with suspected CS (75.0% sensitivity and 51.4% specificity). Compared to controls, CoV identified CS patients with a good accuracy (68.8% sensitivity and 93.3% specificity). CS patients with FDG positive mediastinal lymph nodes had higher CoV than CS patients without lymph node involvement (0.282 vs. 0.208, P = 0.016). CS patients with more severe initial symptoms had a higher CoV than patients with more benign symptoms (0.283 vs. 0.195, P = 0.01). Conclusion: CoV provides a good addition to visual analysis of cardiac FDG PET-CT in diagnosis of CS. As a semi-quantitative measure, it reduces intra-observer variability. It also seems to indicate more severe disease, but to confirm this, prospective studies are needed.

Dosimetry software Hermes Internal Radiation Dosimetry: from quantitative image reconstruction to voxel-level absorbed dose distribution.

OBJECTIVE: The aim of this work is to validate a software package called Hermes Internal Radiation Dosimetry (HIRD) for internal dose assessment tailored for clinical practice. The software includes all the necessary steps to perform voxel-level absorbed dose calculations including quantitative reconstruction, image coregistration and volume of interest tools. METHODS: The basics of voxel-level dosimetry methods and implementations to HIRD software are reviewed. Then, HIRD is validated using simulated SPECT/CT data and data from Lu-DOTATATE-treated patients by comparing absorbed kidney doses with OLINDA/EXM-based dosimetry. In addition, electron and photon dose components are studied separately in an example patient case. RESULTS: The simulation study showed that HIRD can reproduce time-activity curves accurately and produce absorbed doses with less than 10% error for the kidneys, liver and spleen. From the patient data, the absorbed kidney doses calculated using HIRD and using OLINDA/EXM were highly correlated (Pearson's correlation coefficient, r=0.98). From Bland-Altman plot analysis, an average absorbed dose difference of -2% was found between the methods. In addition, we found that in Lu-DOTATATE-treated patients, photons can contribute over 10% of the kidney's total dose and is partly because of cross-irradiation from high-uptake lesions close to the kidneys. CONCLUSION: HIRD is a straightforward voxel-level internal dosimetry software. Its clinical utility was verified with simulated and clinical Lu-DOTATATE-treated patient data. Patient studies also showed that photon contribution towards the total dose can be relatively high and voxel-level dose calculations can be valuable in cases where the target organ is in close proximity to high-uptake organs.