The Influence of Minimal Misalignment on the Repeatability of PET Images Examined by the Repositioning of Point Sources.

We aimed to evaluate the influence of minimal misalignment of a hot spot on the repeatability of PET images using repositioning of point sources. Point sources with an inner diameter of 1 mm were made with 1 μL of F solution. Seven point sources were placed on the -axis in the field of view. For fixed-position imaging, PET data were acquired for 10 min 5 times serially. For variable-position imaging, PET data were acquired for 10 min each with the point sources placed at 0, ±0.5, and ±1.0 mm in the -axis direction. The data were reconstructed using ordered-subsets expectation maximization (OSEM) and OSEM plus point-spread function (PSF). The image matrix was 128 × 128, 200 × 200, 256 × 256, 400 × 400, and 512 × 512 pixels. The normalized maximum count (rMax), the coefficient of variance (CV), and the full width at half maximum were analyzed. The hot spots on OSEM images far from the center became faint and broad, whereas those on OSEM+PSF images became small and dense. Although rMax was overestimated at the 5-cm position on OSEM images, rMax at other positions was overestimated on OSEM+PSF images with a matrix of at least 256 × 256. rMax showed a similar pattern in fixed- and variable-position images. CV in fixed-position OSEM images was less than 2%, irrespective of matrix size. In contrast, CV in variable-position images was higher than in fixed-position images. CV was higher for OSEM+PSF images than for OSEM images. The full width at half maximum increased at positions far from the center on OSEM images but was stable at all positions on OSEM+PSF images. The repeatability of the small hot spot was affected by the minimal misalignment, especially on OSEM+PSF images. Precise positioning is necessary if PET is to be used as a biomarker. Professionals should recognize that PSF correction worsens the repeatability of the small hot spot although improving the spatial resolution of PET images.