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

Abstract

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 (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 impro...

Quantification, improvement, and harmonization of small lesion detection with state-of-the-art PET

“…In this section, an overview of several PET technological developments that took place during the last decade will be given, as well as a short description of their underlying principles. In particular, this review addresses TOF [ 5 ], PSF modeling [ 6 ], MAP-based reconstruction [ 7 ], smaller voxels [ 8 ], respiratory gating [ 9 ], metal artefact reduction [ 10 ], as well as hardware improvements like CBM [ 11 ], the development of solid-state photodetectors using digital photon counting technology [ 12 ] and the introduction of combined PET/MR imaging [ 13 ].…”

“…In current practice, the image voxel size for whole-body FDG-PET scans is typically around 4 × 4 × 4 mm 3 [ 18 , 38 , 39 ], which is in the order of the NEMA spatial resolution of the PET scanner [ 40 ], defined as the full width at half and tenth maximum (FWHM/FWTM) of a point source when reconstructed using filtered back-projection without any corrections. Recent studies demonstrated that the use of smaller voxels and corresponding larger matrices, in combination with TOF-PET/CT systems, improves the detection of small lesions [ 8 , 41 – 43 ]. Li et al [ 41 ] demonstrated that using a 400 × 400 matrix (2 × 2 mm 2 ) resulted in more detected lymph nodes and a better visual image quality, as compared to a 200 × 200 matrix (4.1 × 4.1 mm 2 ).…”

The Impact of Patient’s Body Habitus on PET Image Quality in Digital and Analogic PET/CT

“…On dPET/CT also a smaller 2mm voxel size diameter was used versus 4mm on aPET/CT. Smaller voxel size increases image noise, by dividing detected number of photons, but improves small lesion detectability [28].…”

Reply to the letter

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Dear Sir, We thank Koopman et al for their comments and interest in our recent article [1,2]. It is well known that various technical and physics issues, such as detector performance, voxel size and reconstruction parameters, influence SUV measurements [3,4].The aim of our study was not to evaluate the influence of such factors on SUV measurements, but to compare under true clinical conditions the values produced by a digital scanner with those produced by an analogue scanner. The patients were scanned on the same day following a single injection of radiotracer in one imaging session with a digital scanner and with an analogue scanner installed next to each other in our department.

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“…Dear Sir, We thank Koopman et al for their comments and interest in our recent article [1,2]. It is well known that various technical and physics issues, such as detector performance, voxel size and reconstruction parameters, influence SUV measurements [3,4].…”