A PET/CT Phantom Study on the Potential Overcorrection of Small Lesion Activity in Cortical Bone like Structures

Ziel/Aim: In multimodal PET/CT systems, an attenuation map (μ-Map) is created by extrapolating LAC-values from CT images (photon energy below 140 keV) into LAC-values from 511 keV PET photons for attenuation correction. Since the CT photon is more prone to the photoelectric effect than the 511keV photon, it is more strongly attenuated in high-density tissues.

This could lead to overcorrection of lesion activity in high-density tissue, especially for small lesions.

Methodik/Methods: Two phantoms were used to simulate a high-density tissue environment. Phantom 1 was 3D printed using stone-like PLA based filament with CT mean value of 500HU a with-holes in the phantom wall to allow the lesions to be embedded to simulate cortex intrusion lesions; phantom 2 was made of 1.2L plaster with CT mean value as 700HU and has a hole on top that allows the simulated lesion to be inserted completely.

PCR tubes of different parameters (7mm, 10mm and 15mm) were filled with ¹⁸F-FDG of the same activity and fixed in the two phantoms to simulate bone lesions, as well as in water to simulate soft tissue lesions. The phantoms were placed in water, then separately scanned in a clinical PET/CT (Biograph Vision 600, Siemens).

Ergebnisse/Results: In both FBP-TOF and IT-TOF attenuation-corrected images, the SUV_mean and SUV_max of the reference tube are essentially the same. The SUV_mean of the 7mm-diameter bone lesions in Phantom 1 is 21% larger, the SUV_max is 27% larger than the soft tissue lesions for both reconstructions, while in phantom 2 the discrepancy is 28% and 24%. This difference gradually decreases as the lesion diameter increases. When the lesion diameter reaches 15 mm, the SUV_mean and SUV_max of bone lesions are under 3% higher than those of soft tissue lesions in both groups.

Schlussfolgerungen/Conclusions: In PET/CT scans, the overestimation of small bone lesions is not only active in a phantom, which simulates a more extreme situation of being surrounded by hard tissue, but also in a phantom, which simulates the cortex invasion bone lesion. This overestimation can be attributed to an overcorrection of the lesion activity in high-density tissue.

Publication History

Article published online:
12 March 2025