Abstract
BackgroundMRI does not offer a direct method to obtain attenuation correction maps as its predecessors (stand-alone PET and PET/CT), and bone visualisation is particularly challenging. Recently, zero-echo-time (ZTE) was suggested for MR-based attenuation correction (AC). The aim of this work was to evaluate ZTE- and atlas-AC by comparison to 68Ge-transmission scan-based AC.Nine patients underwent brain PET/MR and stand-alone PET scanning using the dopamine transporter ligand 11C-PE2I. For each of them, two AC maps were obtained from the MR images: an atlas-based, obtained from T1-weighted LAVA-FLEX imaging with cortical bone inserted using a CT-based atlas, and an AC map generated from proton-density-weighted ZTE images. Stand-alone PET 68Ge-transmission AC map was used as gold standard. PET images were reconstructed using the three AC methods and standardised uptake value (SUV) values for the striatal, limbic and cortical regions, as well as the cerebellum (VOIs) were compared. SUV ratio (SUVR) values normalised for the cerebellum were also assessed. Bias, precision and agreement were calculated; statistical significance was evaluated using Wilcoxon matched-pairs signed-rank test.ResultsBoth ZTE- and atlas-AC showed a similar bias of 6–8% in SUV values across the regions. Correlation coefficients with 68Ge-AC were consistently high for ZTE-AC (r 0.99 for all regions), whereas they were lower for atlas-AC, varying from 0.99 in the striatum to 0.88 in the posterior cortical regions. SUVR showed an overall bias of 2.9 and 0.5% for atlas-AC and ZTE-AC, respectively. Correlations with 68Ge-AC were higher for ZTE-AC, varying from 0.99 in the striatum to 0.96 in the limbic regions, compared to atlas-AC (0.99 striatum to 0.77 posterior cortex).ConclusionsAbsolute SUV values showed less variability for ZTE-AC than for atlas-AC when compared to 68Ge-AC, but bias was similar for both methods. This bias is largely caused by higher linear attenuation coefficients in atlas- and ZTE-AC image compared to 68Ge-images. For SUVR, bias was lower when using ZTE-AC than for atlas-AC. ZTE-AC shows to be a more robust technique than atlas-AC in terms of both intra- and inter-patient variability.
Highlights
Magnetic resonance imaging (MRI) does not offer a direct method to obtain attenuation correction maps as its predecessors, and bone visualisation is challenging
Mean parametric bias images for ZTE- and atlas-attenuation correction (AC) compared to 68Ge-AC indicated a mostly positive bias in standardised uptake value (SUV) values across the brain for both methods (Fig. 2)
Both ZTE- and atlas-AC resulted in an overall overestimation of SUV values
Summary
MRI does not offer a direct method to obtain attenuation correction maps as its predecessors (stand-alone PET and PET/CT), and bone visualisation is challenging. Stand-alone PET 68Ge-transmission AC map was used as gold standard. PET/MR scanners do not offer a direct method to obtain attenuation maps as the stand-alone PET and PET/CT scanners [2, 7, 9], since MR measures proton density which does not correlate directly to electron density [3, 10]. Linear attenuation coefficients (cm−1) at 511 keV vary between 0 in air, 0.151 in bone and 0.96 in water (6), posing a challenge for MR-based AC. Since ignoring bone may lead to a regional bias, as demonstrated for example by Andersen et al [11], new methods for MR-derived attenuation maps have been developed and incorporated into the commercially available systems [2, 7]
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