Attenuation correction for phantom tests. An alternative to maximum-likelihood attenuation correction factor-based correction for clinical studies in time-of-flight PET

Objectives

This study evaluates the phantom attenuation correction (PAC) method as an alternative to maximum-likelihood attenuation correction factor (ML-ACF) correction in time-of-flight (TOF) brain positron emission tomography (PET) studies.

Methods

In the PAC algorithm, a template emission image ${\lambda }_{Ref}$ and a template attenuation coefficient image ${\mu }_{Ref}$ are prepared as a data set based on phantom geometry. Position-aligned attenuation coefficient image ${\mu }_{Acq}$ is derived by aligning ${\mu }_{Ref}$ using parameters that match the template emission image ${\lambda }_{Ref}$ to measured emission image ${\lambda }_{Acq}$. Then, attenuation coefficient image ${\mu }_{Acq}$ combined with a headrest image is used for scatter and attenuation correction in the image reconstruction. To evaluate the PAC algorithm as an alternative to ML-ACF, Hoffman 3D brain and cylindrical phantoms were measured to obtain the image quality indexes of contrast and uniformity. These phantoms were also wrapped with a radioactive sheet to obtain attenuation coefficient images using ML-ACF. Emission images were reconstructed with attenuation correction by PAC and ML-ACF, and the results were compared using contrast and uniformity as well as visual assessment. CT attenuation correction (CT-AC) was also applied as a reference.

Results

The contrast obtained by ML-ACF was slightly overestimated due to its unique experimental condition for applying ML-ACF in Hoffman 3D brain phantom but the uniformity was almost equivalent among ML-ACF, CT-AC, and PAC. PAC showed reasonable result without overestimation compared to ML-ACF and CT-AC.

Conclusions

PAC is an attenuation correction method that can ensure the performance in phantom test, and is considered to be a reasonable alternative to clinically used ML-ACF-based attenuation correction.