Abstract
The purpose of this study was to investigate whether a correction for annihilation photon attenuation in small objects such as mice is necessary. The attenuation recovery for specific organs and subcutaneous tumors was investigated. A comparison between different attenuation correction methods was performed. Methods: Ten NMRI nude mice with subcutaneous implantation of human breast cancer cells (MCF-7) were scanned consecutively in small animal PET and CT scanners (MicroPETTM Focus 120 and ImTek’s MicroCATTM II). CT-based AC, PET-based AC and uniform AC methods were compared. Results: The activity concentration in the same organ with and without AC revealed an overall attenuation recovery of 9–21% for MAP reconstructed images, i.e., SUV without AC could underestimate the true activity at this level. For subcutaneous tumors, the attenuation was 13 ± 4% (9–17%), for kidneys 20 ± 1% (19–21%), and for bladder 18 ± 3% (15–21%). The FBP reconstructed images showed almost the same attenuation levels as the MAP reconstructed images for all organs. Conclusions: The annihilation photons are suffering attenuation even in small subjects. Both PET-based and CT-based are adequate as AC methods. The amplitude of the AC recovery could be overestimated using the uniform map. Therefore, application of a global attenuation factor on PET data might not be accurate for attenuation correction.
Highlights
Small animal positron emission tomography (PET) imaging is frequently used in biomedical research and plays a key role in the studies of biodistribution and pharmacokinetics of new tracers
The accuracy of small animal PET imaging can suffer from degradation factors that are related to the photon interactions in matter
The comparison revealed that the estimation of the activity concentration in the small animal could be underestimated if no attenuation compensation was applied (Figure 1)
Summary
Small animal PET imaging is frequently used in biomedical research and plays a key role in the studies of biodistribution and pharmacokinetics of new tracers. Small animal PET imaging is a powerful tool for studying the response of new therapy methods. Murine models in everyday practice are ideal subjects due to rapid breeding with low cost, well developed transgenic models and the ability of modelling different human diseases. The accuracy of small animal PET imaging can suffer from degradation factors that are related to the photon interactions in matter. Annihilation photon will interact with tissue and other materials as they travel through the body. Quantitative positron emission tomography (PET) requires different types of correction methods where attenuation correction (AC) is an important one [2]. The magnitude of attenuation can mathematically be expressed by the exponential equation:
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