Positron flight in human tissues and its influence on PET image spatial resolution

Abstract

The influence of the positron distance of flight in various human tissues on the spatial resolution in positron emission tomography (PET) was assessed for positrons from carbon-11, nitrogen-13, oxygen-15, fluorine-18, gallium-68 and rubidium-82. The investigation was performed using the Monte Carlo code PENELOPE to simulate the transport of positrons within human compact bone, adipose, soft and lung tissue. The simulations yielded 3D distributions of annihilation origins that were projected on the image plane in order to assess their impact on PET spatial resolution. The distributions obtained were cusp-shaped with long tails rather than Gaussian shaped, thus making conventional full width at half maximum (FWHM) measures uncertain. The full width at 20% of the maximum amplitude (FW20M) of the annihilation distributions yielded more appropriate values for root mean square addition of spatial resolution loss components. Large differences in spatial resolution losses due to the positron flight in various human tissues were found for the selected radionuclides. The contribution to image blur was found to be up to three times larger in lung tissue than in soft tissue or fat and five times larger than in bone tissue. For (18)F, the spatial resolution losses were 0.54 mm in soft tissue and 1.52 mm in lung tissue, compared with 4.10 and 10.5 mm, respectively, for (82)Rb. With lung tissue as a possible exception, the image blur due to the positron flight in all human tissues has a minor impact as long as PET cameras with a spatial resolution of 5-7 mm are used in combination with (18)F-labelled radiopharmaceuticals. However, when ultra-high spatial resolution PET cameras, with 3-4 mm spatial resolution, are applied, especially in combination with other radionuclides, the positron flight may enter as a limiting factor for the total PET spatial resolution--particularly in lung tissue.

Amyloid-β imaging with PET in Alzheimer’s disease: is it feasible with current radiotracers and technologies?

“…This effect is especially severe for structures that are less than 2.5 times the spatial resolution of the PET system, as measured by the full-width at half-maximum [35]. Since amyloid plaques are, on average, approximately 50 µm in diameter and the spatial resolution of PET is normally in the range of 2 to 3 mm for high-resolution systems and 5 to 7 mm for standard scans, the partial-volume effect should theoretically be factored heavily into amyloid imaging and should not be overlooked [36,37]. And yet Wong et al explicitly state that in a phase I clinical study of florbetapir partial-volume correction was not undertaken in the analysis of data [6].…”

Comparative Assessment of Energy-Mapping Approaches in CT-Based Attenuation Correction for PET

“…There are several parameters that affect the quality and quantitative accuracy of PET images, including positron range [1], the limited spatial resolution and resulting partial volume effect [2], contribution from scattered photons [3], photon attenuation [4], patient motion [5], and the image reconstruction algorithm [6]. Attenuation of photons in vivo degrades the visual quality and quantitative accuracy of PET images, thereby adversely affecting interpretation and quantitation of activity concentration.…”

Tumor Specific Imaging Using Tc-99m and Ga-68 Labeled Radiopharmaceuticals

“…Aspects of the detector design, physical properties and their influence on system spatial resolution have been extensively addressed by many authors, leading to a continuous optimization of hardware. Although the maximum positron energy of 68 [23,24]. It implies that with the spatial resolution at 5 to 7 mm of current clinical scanners, the imaging quality using 68 Ga-based tracers can be as good as that of 18 F-based agents and have stimulated others to investigate potential 68 Gabased imaging agents [25][26][27].…”