Usefulness of dual-head coincidence gamma camera with thick NaI crystals for nuclear oncology: comparison with dedicated PET camera and conventional gamma camera with thin NaI crystals

Monte Carlo Optical Simulations of a Small FoV Gamma Camera. Effect of Scintillator Thicknesses and Septa Materials

Optical Monte Carlo simulations have been extensively used for the accurate modeling of light transport in scintillators for the improvement of detector designs. In the present work, a GATE Monte Carlo toolkit was used to study the effect of scintillator thicknesses and septa materials in the performance parameters evaluation of a commercially available small animal gamma-optical camera, named “γ-eye”. Firstly, the simulated γ-eye system was validated against experimental data. Then, part of the validated camera was modeled defining all of the optical properties by means of the UNIFIED model of GATE. Different CsI:Na scintillator crystals with varying thicknesses (from 4 mm up to 6 mm) and different reflector (septa) materials were simulated and compared in terms of sensitivity, light output and spatial resolution. Results have demonstrated the reliability of the model and indicate that the thicker crystal array presents higher sensitivity values, but degraded spatial resolution properties. Moreover, the use of black tape around crystals leads to an improvement in spatial resolution values compared to a standard white reflector material.

An improved method for measurement of target-to-background ratio in assessing mediastinal lesions on 18F-FDG coincidence SPECT/CT imaging

OBJECTIVES

Target-to-background ratio (T/B) is a commonly used semiquantitative index in F-FDG dual-head coincidence single-photon emission computed tomography (c-SPECT) imaging. However, because of different methods of measurement, T/B varies. This study used F-FDG c-SPECT/CT imaging of mediastinal lesions to analyze the impact of different backgrounds for determining T/Bs.

METHODS

Thirty-five patients with mediastinal lesions underwent thoracic F-FDG c-SPECT/CT with 1-inch crystals. According to integrated CT, five types of backgrounds were precisely positioned as airway, fat, mediastinal space, vascular blood, and heart areas. Corresponding T/Bs (T/BA, T/BF, T/BM, T/BV, and T/BH) of benign and malignant lesions were calculated. Two-way analysis of variance, receiver-operating characteristic curves, and coefficient of variation were performed for statistical analyses.

RESULTS

Seventeen benign lesions and 23 malignant lesions were identified in patients with histological confirmation. Only T/BA was significantly different from other T/Bs in both the benign and malignant groups; yet, there were no significant differences in the remaining T/Bs (P<0.01). On the basis of receiver-operating characteristic curves, cutoff values, sensitivity, specificity, positive predictive value and negative predictive value, the accuracy of diagnosis followed the order of T/BV>T/BM> T/BA>T/BF>T/BH, and T/BV showed optimal sensitivity (87.0%), specificity (76.5%), and accuracy (82.5%) of diagnosis. Vascular blood and mediastinal space areas, which were in the same homogeneous subset, possessed the significantly lowest coefficient of variation, indicating good homogeneity in the regions.

CONCLUSION

Choosing different backgrounds has a great impact on the diagnostic accuracies of F-FDG c-SPECT/CT imaging of mediastinal lesions. The large blood vessel area is the best background choice for optimization of T/B calculation.