Abstract
BackgroundSpatial normalization is a prerequisite step for analyzing positron emission tomography (PET) images both by using volume-of-interest (VOI) template and voxel-based analysis. Magnetic resonance (MR) or ligand-specific PET templates are currently used for spatial normalization of PET images. We used computed tomography (CT) images acquired with PET/CT scanner for the spatial normalization for [18F]-N-3-fluoropropyl-2-betacarboxymethoxy-3-beta-(4-iodophenyl) nortropane (FP-CIT) PET images and compared target-to-cerebellar standardized uptake value ratio (SUVR) values with those obtained from MR- or PET-guided spatial normalization method in healthy controls and patients with Parkinson’s disease (PD).MethodsWe included 71 healthy controls and 56 patients with PD who underwent [18F]-FP-CIT PET scans with a PET/CT scanner and T1-weighted MR scans. Spatial normalization of MR images was done with a conventional spatial normalization tool (cvMR) and with DARTEL toolbox (dtMR) in statistical parametric mapping software. The CT images were modified in two ways, skull-stripping (ssCT) and intensity transformation (itCT). We normalized PET images with cvMR-, dtMR-, ssCT-, itCT-, and PET-guided methods by using specific templates for each modality and measured striatal SUVR with a VOI template. The SUVR values measured with FreeSurfer-generated VOIs (FSVOI) overlaid on original PET images were also used as a gold standard for comparison.ResultsThe SUVR values derived from all four structure-guided spatial normalization methods were highly correlated with those measured with FSVOI (P < 0.0001). Putaminal SUVR values were highly effective for discriminating PD patients from controls. However, the PET-guided method excessively overestimated striatal SUVR values in the PD patients by more than 30% in caudate and putamen, and thereby spoiled the linearity between the striatal SUVR values in all subjects and showed lower disease discrimination ability. Two CT-guided methods showed comparable capability with the MR-guided methods in separating PD patients from controls and showed better correlation between putaminal SUVR values and the parkinsonian motor severity than the PET-guided method.ConclusionCT-guided spatial normalization methods provided reliable striatal SUVR values comparable to those obtained with MR-guided methods. CT-guided methods can be useful for analyzing dopamine transporter PET images when MR images are unavailable.
Highlights
Dopamine transporter positron emission tomography (PET) is popularly used in clinics to visualize the integrity of presynaptic dopaminergic nerve terminals, and a quantitation of striatal binding in PET images is important for detecting preclinical stage, monitoring disease progression, and evaluating responses to disease modifying treatment in Parkinson’s disease (PD) [1]
Compared to the intensity-transformed CT (itCT)-guided method, the striatal standardized uptake value ratio (SUVR) values derived from the skull-stripped CT (ssCT)-guided method showed slightly better correlation with those measured by FreeSurfer-generated VOIs (FSVOI) and with clinical severity measured by unified Parkinson’s disease rating scale (UPDRS) motor score
computed tomography (CT)-guided spatial normalization methods provide reliable striatal SUVR values almost comparable to those obtained with Magnetic resonance (MR)-guided methods and can be useful for analyzing dopamine transporter PET images when structural MR images are unavailable
Summary
Dopamine transporter positron emission tomography (PET) is popularly used in clinics to visualize the integrity of presynaptic dopaminergic nerve terminals, and a quantitation of striatal binding in PET images is important for detecting preclinical stage, monitoring disease progression, and evaluating responses to disease modifying treatment in Parkinson’s disease (PD) [1]. A striatal volume-of-interest (VOI) template overlaid on spatially normalized PET images is required to achieve objective, user-independent quantification of striatal binding. While the spatial normalization of dopamine transporter PET images using ligand-specific PET template can be readily performed, it frequently causes errors in PD patients due to markedly reduced uptake in the posterior putamen. The MR-guided spatial normalization method is generally more accurate than the PET-guided method and works independently from receptor density [7, 8] Another approach based on structural image for spatial normalization is the computed tomography (CT)-guided method [9, 10]. The PET/CT scanners are widely used, and structural CT images always accompany PET images for the attenuation correction These CT images can be used for spatial normalization. We used computed tomography (CT) images acquired with PET/ CT scanner for the spatial normalization for [18F]-N-3-fluoropropyl-2-betacarboxymethoxy-3beta-(4-iodophenyl) nortropane (FP-CIT) PET images and compared target-to-cerebellar standardized uptake value ratio (SUVR) values with those obtained from MR- or PET-guided spatial normalization method in healthy controls and patients with Parkinson’s disease (PD)
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