Attenuation Correction of Cerebellum in PET/MR Data

Most approaches of modelling neuroreceptor PET studies apply the cerebellum as reference area. Therefore, it is mandatory that the respective attenuation correction (AC) method for reconstructing the emission data is most appropriate regarding cerebellar areas. PET data from PET/MR scanners require alternative AC methods. These ought to be tested with respect to the performance within the cerebellum. This study aimed to compare various AC methods for PET/MR data focussing on the cerebellum. Data of 16 subjects undergoing
¹⁸
FDG imaging in the Siemens 3TMR-BrainPET scanner and a whole head CT scan at the same day were used. The latter were transformed to CT-based attenuation maps (AM
_CT
). The MR images were used to obtain AMs using the Boston-MGH method (AM
_MGH
), the London-UCL method (AM
_UCL
), the CT-template-based (AM
_CT-Juel
) and Tx-template-based (AM
_Tx-Juel
) Juelich methods. BrainPET emission data were reconstructed with the five AM
_s
. Using the SUIT tool the cerebellum was extracted from the MR images and normalized to a cerebellum VOI atlas. The cerebellum of the PET data was extracted by applying the same parameters. Correlation plots with regression equations, coefficients of determination R
²
, normalized and absolute normalized errors (NErr) between AM
_CT
and the other four AM
_s
were calculated. The values of cerebellar NErr varied to a high extent between the four AC methods. In one subject AM
_MGH
showed the lowest NErr (3.03±1.60%), in three participants the AMUCL performed best (NErr from -0.13±2.33% to 0.91±2.93%), in three others the AM
_Tx-Juel
had the best results (NErr from -1.24±3.01% to 0.86±2.07%), while for the remaining seven subjects the AM
_CT-Juel
performed best (NErr from -2.08±0.94% to 7.98±3.43%). Our results demonstrate that the quantitation of radiotracer uptake in the cerebellum is very susceptible to the respective attenuation correction applied to the PET data. This, in turn, has to be considered in neuroreceptor modelling studies which rely on the cerebellum as reference.