Most of PET scanners worldwide use 4 mm voxel size reconstruction in clinical practice, which reflects in limited image spatial resolution. Relatively poor spatial resolution represents an aggravated localization and quantification of small lesions. In present investigation we scanned and analyzed NEMA body phantom with various sphere vs. background radioactivity concentration ratios to demonstrate that detection and quantification of small lesions with PET/CT TOF scanning can be improved if reconstruction with 2 mm voxel size is used.
We compared image quality in reconstruction with 2 mm and 4 mm voxel sizes, by calculating SUV, CRC and CNR values for NEMA body phantom spheres over all sphere vs. background radioactivity ratios. We found significant increase of mean and maximum CRC, SUV and CNR values of spheres with diameter 10 mm and 13 mm in images reconstructed with 2 mm voxels size, in comparison to 4 mm. In larger spheres and spheres of all sizes, no statistical differences in SUV and CRC were found between different voxel sizes. Nevertheless, CNR was found significantly higher for spheres of all sizes, but significantly lower in large spheres, in images with 2 mm voxel sizes compared to 4 mm.
These findings clearly demonstrate that 2 mm voxel size PET reconstruction allows more precise semi-quantitative reconstruction and assessment of PET images, especially in lesions with diameter smaller than 13 mm which are most challenging in clinical diagnostics.
We also confirmed the findings in the previous publications that despite the increase in noise using the 2 mm voxel size, the image quality is improved. [13], [14], [15]. Our results are in line with Koopman et al [14], who demonstrated on NEMA body phantom that the use 2 mm voxel size significantly increases SUV (max and mean) and significantly improves CRC and the SNR of small spheres with diameter less than ≤ 13 mm. They also analyzed 66 18F-FDG PET positive lesions in the chest and upper abdominal region. For all 18F-FDG PET positive lesions, the average SUVmean and SUVmax increase using the 2 mm voxel size (17 % and 32 %). At lesions with volume less than 0.75 mL, the average increase was 21% and 44%. Moreover, averaged over all lesions, the mean and maximum SNR increased by 20% and 27%. For lesions less than 0.75 mL, these values increased up to 23% and 46%. The same group later extended their research included 61 benign and 169 malignant lymph nodes and concluded that small-voxel PET/CT improves the sensitivity of visual lymph node characterization and provides a higher detection rate of malignant lymph nodes [15].
The study by Li et al. [13], who similarly demonstrated on 39 patients in which 18FDG PET/CT was performed for assessment of lymph node metastases in head and neck squamous cell carcinoma, that matrix size is important factor influencing lesion detect ability. In their study the 2 mm reconstructions with PSF and TOF provided the best overall performance in terms of quantitation over the different lesion size and concluded that 2 mm voxel size detected more lymph nodes, had a higher sensitivity for lesion detection, and a better image-quality output in comparison with 4 mm voxel size.
Previous studies analyzed the impact of 2 mm voxel size reconstruction on image quality in one sphere vs. background ratio of NEMA body phantom. In our study we extended the research to systematically evaluate CRC, SNC and SUV for different sphere vs. background ratios to simulate the conditions in the patient lesions where the accumulation of RP is differently intense.
The reason for improvements in CRC, CNR and increased SUV using 2 mm voxel size are improved spatial resolution and PVE of PET scanner which reduces the apparent activity concentrations in imaging of smaller lesions [16]. However, the limited spatial resolution results in a spillover of activity distribution into neighboring voxels or regions in the image which in the case of small spheres decreases measured VOI activity in contrast to larger spheres with the same activity concentration. The measured lower concentration in small spheres (diameter < 13 mm) on 4 mm voxel size indicates that smaller volumes are indeed more affected by partial volume effects and when using the 2 mm voxel size this effect is reduced. On the contrary partial volume effect affects minimally the largest sphere (diameter 37 mm). Nevertheless, although the small 2 mm voxel size improves the resolution, it may produce high fluctuations of the signal that increase the noise because of lower number of counts per voxel. In contrast, large 4 mm voxel size produces homogeneous images, however, with low spatial resolution. The image noise in 2 mm voxel size images can be compensated by using an iterative True-X reconstruction algorithm which incorporates PSF and TOF correction [7], [8], [17]. The TOF information does not directly lead to a higher image spatial resolution, but provides images with higher signal to noise ratio (SNR), which improves the detections of small lesions with relatively low activity [5], [8], [18]. The resolution modeling of PSF leads to higher and more uniform spatial resolution over the transaxial field of view (FOV) [19], [20].