Improving the Detection of Small Lesions Using a State-of-the-Art Time-of-Flight PET/CT System and Small-Voxel Reconstructions

D. Koopman, J. A. van Dalen, M. C. M. Lagerweij, H. Arkies, J. de Boer, A. H. J. Oostdijk, C. H. Slump, P. L. Jager
2015 Journal of Nuclear Medicine Technology  
A major disadvantage of 18 F-FDG PET involves poor detection of small lesions and lesions with low metabolism, caused by limited spatial resolution and relatively large image voxel size. As spatial resolution and sensitivity are better in new PET systems, it is expected that small-lesion detection could be improved using smaller voxels. The aim of this study was to test this hypothesis using a state-of-the-art time-of-flight PET/CT device. Methods: 18 F-FDG PET scans of 2 image-quality phantoms
more » ... (sphere sizes, 4-37 mm) and 39 consecutive patients with lung cancer were analyzed on a time-of-flight PET/CT system. Images were iteratively reconstructed with standard 4 · 4 · 4 mm voxels and smaller 2 · 2 · 2 mm voxels. For the phantom study, we determined contrast-recovery coefficients and signal-to-noise ratios (SNRs). For the patient study, 18 F-FDG PET-positive lesions in the chest and upper abdomen with a volume less than 3.0 mL (diameter, ,18 mm) were included. Lesion mean and maximum standardized uptake values (SUV mean and SUV max , respectively) were determined in both image sets. SNRs were determined by comparing SUV max and SUV mean with background noise levels. A subanalysis was performed for lesions less than 0.75 mL (diameter, ,11 mm). For qualitative analysis of patient data, 3 experienced nuclear medicine physicians gave their preference after visual side-by-side analysis. Results: For phantom spheres 13 mm or less, we found higher contrast-recovery coefficients and SNRs using small-voxel reconstructions. For 66 included 18 F-FDG PET-positive lesions, the average increase in SUV mean and SUV max using the small-voxel images was 17% and 32%, respectively (P , 0.01). For lesions less than 0.75 mL (21 in total), the average increase was 21% and 44%, respectively. Moreover, averaged over all lesions, the mean and maximum SNR increased by 20% and 27%, respectively (P , 0.01). For lesions less than 0.75 mL, these values increased up to 23% and 46%, respectively. The physicians preferred the small-voxel reconstructions in 76% of cases. Conclusion: Supported by a phantom study, there was a visual preference toward 18 F-FDG PET images reconstructed with 2 · 2 · 2 mm voxels and a profound increase in standardized uptake value and SNR for small lesions. Hence, it is expected that small-lesion detection improves using smallvoxel reconstructions. Whole-body 18 F-FDG PET integrated with CT is widely used for primary tumor analysis and mediastinal lymph node staging in patients with non-small cell and small cell lung cancer (1,2). For these patients, accurate lymph node staging is of high importance because both prognosis and therapeutic management depend on the tumor stage (3). A major disadvantage of 18 F-FDG PET involves poor detection of small lesions and lesions with low metabolism. With a sensitivity of 76% and specificity of 88% (4), PET/ CT is less sensitive for nodal involvement of small nodes (diameter, ,10 mm) (3,5). This poor detection is caused by limited spatial resolution and sensitivity (3,6). To improve work flow and patient comfort, relatively large image voxel sizes are generally preferred in clinical practice to effectively obtain more counts per voxel and subsequently a reduction in image noise (7). However, the low spatial resolution introduces the partial-volume effect, which affects images both visually and quantitatively because small lesions may appear larger and their 18 F-FDG uptake may seem lower than is actually the case (8). Consequently, the detection of small lesions is limited by the partial-volume effect. In the literature, several ways to improve lesion detectability on PET/CT are described. With the introduction of scintillators such as lutetium orthosilicate and lutetium yttrium orthosilicate, time-of-flight (TOF) PET became the new standard technology for PET manufacturers. With the implementation of TOF PET, the increased signal-to-noise ratio (SNR) appeared to improve the detectability of small lesions (9-12). Furthermore, the detection of small lesions can be improved by changing reconstruction settings, for
doi:10.2967/jnmt.114.147215 pmid:25613334 fatcat:cv64tdvxlveqjnaz2n767ihhu4