Nuclear medicine

The researches find that the PET (positron emission tomography) biokinetic modelling relies on accurate quantitative data. One of the main corrections required in PET imaging to obtain high quantitative accuracy is tissue attenuation correction (AC). Incorrect non-uniform PET-AC may result in local bias in the emission images; and thus in relative activity distributions and time activity curves for different regions. MRI (magnetic resonance imaging)-based AC is an active area of research in PET/MRI neuroimaging, where several groups developed in the last few years different methods to calculate accurate attenuation (μ-)maps.

PET/MRI neuroimaging

Some AC methods have been evaluate for different PET radioisotopes and pathologies. However; AC in PET/MRI has scantly been investigated in dynamic PET studies where the aim is to get quantitative kinetic parameters; rather than semi-quantitative parameters from static PET studies. In this work, we investigated the impact of AC accuracy in PET image absolute quantification and, more importantly, in the slope of the Patlak analysis based on the simplified reference tissue model, from a dynamic [18F]-fluorodopa (FDOPA) PET/MRI study.

In the study, we considered the two AC methods provided by the vendor and an in-house AC method based on the dual ultrashort time echo MRI sequence; using as reference a multi-atlas-based AC method based on a T1-weighted MRI sequence. Non-uniform bias in absolute PET quantification across the brain, from 20% near the skull to  10% in the central region, was observed using the two vendor’s μ-maps. The AC method developed in-house showed a  5% and 1% bias, respectively.

The PET kinetic parameters

Our study resulted in a 5–9% overestimation of the PET kinetic parameters with the vendor-provided μ-maps, while our in-house-developed AC method showed < 2% overestimation compared to the atlas-based AC method, using the cerebellar cortex as reference region.  The overestimation obtained using the occipital pole as reference region resulted in a 7–10% with the vendor-provided μ-maps; while our in-house-developed AC method showed < 6% overestimation.

PET/MRI (positron emission tomography/magnetic resonance imaging); represents a comprehensive tool for neurology and neuro-oncology studies. Different works have illustrated the complementary information provided by PET/MRI imaging, allowing to better understand the underlying pathophysiological mechanisms of numerous pathologies. However, the understanding of the information provided by PET/MRI technology represents a challenge in some scenarios.

One of the technical challenges posed by PET/MRI is to extract an attenuation map (μ-map); for PET attenuation correction (AC), that accurately resembles the underlying electron density distribution of the subject under study. Since MRI provides proton density information; an intermediate step is require to convert proton density information to electron density, in the form of linear attenuation coefficients (LAC) at the PET energy of 511 keV.