Abstract
In simultaneous PET-MR, attenuation maps are not directly available. Essential for absolute radioactivity quantification, they need to be derived from MR or PET data to correct for gamma photon attenuation by the imaged object. We evaluate a multi-atlas attenuation correction method for brain imaging (MaxProb) on static [18F]FDG PET and, for the first time, on dynamic PET, using the serotoninergic tracer [18F]MPPF.A database of 40 MR/CT image pairs (atlases) was used. The MaxProb method synthesises subject-specific pseudo-CTs by registering each atlas to the target subject space. Atlas CT intensities are then fused via label propagation and majority voting. Here, we compared these pseudo-CTs with the real CTs in a leave-one-out design, contrasting the MaxProb approach with a simplified single-atlas method (SingleAtlas). We evaluated the impact of pseudo-CT accuracy on reconstructed PET images, compared to PET data reconstructed with real CT, at the regional and voxel levels for the following: radioactivity images; time-activity curves; and kinetic parameters (non-displaceable binding potential, BPND).On static [18F]FDG, the mean bias for MaxProb ranged between 0 and 1% for 73 out of 84 regions assessed, and exceptionally peaked at 2.5% for only one region. Statistical parametric map analysis of MaxProb-corrected PET data showed significant differences in less than 0.02% of the brain volume, whereas SingleAtlas-corrected data showed significant differences in 20% of the brain volume. On dynamic [18F]MPPF, most regional errors on BPND ranged from -1 to +3% (maximum bias 5%) for the MaxProb method. With SingleAtlas, errors were larger and had higher variability in most regions. PET quantification bias increased over the duration of the dynamic scan for SingleAtlas, but not for MaxProb. We show that this effect is due to the interaction of the spatial tracer-distribution heterogeneity variation over time with the degree of accuracy of the attenuation maps.This work demonstrates that inaccuracies in attenuation maps can induce bias in dynamic brain PET studies. Multi-atlas attenuation correction with MaxProb enables quantification on hybrid PET-MR scanners, eschewing the need for CT.
Highlights
Accurate attenuation correction (AC) is a crucial step toward absolute quantification of radionuclide uptake
Results revealed that the performance discrimination between SingleAtlas and MaxProb is less obvious using metrics computed from the reconstructed images than when they are directly computed from the generated pseudo-CT
This section describes the impact of the attenuation correction approach on the quantification of dynamic [18F]MPPF PET data and the subsequent kinetic modelling
Summary
Accurate attenuation correction (AC) is a crucial step toward absolute quantification of radionuclide uptake. Implemented solutions, based on the segmentation of Dixon (Martinez-Möller et al 2009) and ultrashort-echo-time (UTE) MR sequences (Keereman et al 2010), are usually not accurate enough for reliable quantification (Dickson et al 2014). More than five years after the introduction of the first commercial PET-MR system (Delso et al 2011), attenuation map generation remains an area of active research. In the context of brain imaging, these methods can be grouped into three main families: joint emission and attenuation map estimation during the reconstruction process; MR-based segmentation; and methods that create a subject-specific pseudo-CT from a database of images; further described in the following paragraphs
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