Evaluation of whole-body bone scans performed using a gamma camera with Cadmium Zinc Telluride detector

Sammanfattning: The bone scan is a common nuclear medicine imaging procedure that is mainly used for evaluating several oncological patient groups. A radioactive tracer, 99mTc -labelled diphosphonate, is injected intravenously and accumulates in the skeleton. The distribution of the tracer in the body is measured with a gamma camera 3 hours post-injection. Conventional gamma cameras used in nuclear medicine are based on scintillation detectors made of thallium-doped sodium iodine (NaI(Tl)). During recent years, progress has been made regarding the production of direct conversion room temperature semiconductor-detectors. One such detector is made of cadmium zinc telluride (CZT), and in comparison to NaI(Tl), CZT has better energy resolution. In the spring of 2020, a NM/CT 870 CZT gamma camera manufactured by GE Healthcare was installed at Sahlgrenska university hospital. The primary aim of this study was to evaluate planar whole-body bone scans on a NM/CT 870 CZT gamma camera by comparing the image quality with a conventional gamma camera. Additional aims were to, for the NM/CT 870 CZT gamma camera, evaluate the effect that reduced scan time/activity level and the use of the post-processing Clarity 2D has on the image quality and to determine the effect that different Clarity 2D blending has on the spatial resolution. Two different patient groups were used for the retrospective visual grading study. The first group consisted of 16 patients with advanced disease who had been scanned on both the NM/CT 870 CZT and a conventional gamma camera at separate occasions. The second group consisted of 32 patients who underwent their first bone scan, where 16 patients were scanned on the NM/CT 870 CZT, while the other 16 were scanned on a conventional gamma camera. For all examinations, the administered activity and scan time were approximately the same. For the examinations performed on the NM/CT 870 CZT, the original images were used to simulate 3 different scan times/activity levels (25 %, 50 % and 75 % of the original 100 %). Additionally, 5 different Clarity 2D blendings were used for each level. The evaluation was accomplished by performing a visual grading characteristics (VGC) analysis in a beta-version of the software ViewDEX 3.0, with two physicians participating as observers. Seven image criteria were rated with a five-step ordinal scale, where the observers stated their confidence on the fulfilment of each criterion. The software VGC Analyzer 1.0.2 was used to perform statistical analysis of the observer ratings. The effect of Clarity 2D on the spatial resolution was evaluated using a Triple Line Insert-phantom, model ECT/TRI/I. The images were also evaluated quantitatively, the bone-to-soft tissue relationship was calculated by dividing the average counts in the sinister sacroiliac joint with the standard deviation of counts in adjacent soft tissue The results showed that the original CZT images with no post-processing were graded equal to or significantly higher than the images from the conventional gamma cameras for all image quality criteria in both patient groups. For the CZT images with a simulated scan time/activity level of 75 % of the original settings, no significant differences were found. The CZT images with a simulated scan time/activity level of 50 % of the original settings were graded equal to or significantly lower than the images from the conventional gamma cameras. The CZT images with a simulated scan time/activity level of 25 % were graded significantly lower for IV most of the criteria. This indicated that the scan time/activity level for whole-body bone scans on the NM/CT 870 CZT could be lowered to 75 % of the original setting without decreasing the image quality compared to a conventional gamma camera. The results also showed that the positive effect of Clarity 2D post-processing was more prominent in the simulated images with reduced scan time/activity level. The quantitative measurements showed that the spatial resolution as well as the bone-to-soft tissue relationship improved with increasing Clarity 2D blendings.