We retrospectively reviewed records from the Department of Nuclear Medicine, University of Friborg Medical Center (January 2018 to April 2021) for patients who had undergone [18F]FDG PET on a fully digital PET/CT system for the differential diagnosis of NP. Two movement disorder specialists (NS, MR) made a consensus diagnosis based on all available anamnestic, clinical, and diagnostic information. One hundred seventy-one patients met the criteria for probable PD (not= 115), MSA (not= 35), or PSP (not= 21) and were therefore included in this study2,3,4. We excluded patients with corticobasal degeneration due to small sample size (not= 5). The consensus diagnosis was not blinded to clinical reports of PET imaging (but not yet considering nigra metabolism; see Discussion) and incorporated the results of [123I]FP-CIT SPECT in 75 patients (PD, not= 62; MSA, not= 7; PSP, not= 6). The latter consistently showed pathologically reduced availability of the dopamine transporter (DAT) in all patients. Thirteen HC subjects (54% male, 68.4 ± 6.8 years) recruited by local advertising combined with 35 control patients (51% male, 65.3 ± 9.0 years) from a prior study28 served as a control cohort. HCs were healthy by medical history (no neurological or psychiatric condition or any other relevant comorbidity) and intact on neuropsychological evaluation, had no neurological deficits on clinical examination, and had normal MRI findings of the brain. For patient demographics and characteristics, see Table 1.
PET emission data were acquired 50 min after intravenous injection of 209 ± 12 MBq [18F]FDG for 10 min on a fully digital PET/CT system (Vereos, Philips Healthcare). Fully corrected emission datasets were reconstructed with low-dose CT-based attenuation correction and the vendor-specific response line time-of-flight ordered subsets. 3D iterative reconstruction algorithm using spherically symmetric basis functions (BLOB-OS-TF reconstruction; number of iterations = 5, number of subsets = 11, 2 mm Gaussian post-filtering, resulting voxel size = 1 .0 × 1.0 × 1.0mm3). This gave a reconstructed isotropic image resolution of approximately 4.5–5 mm full-width at half-height. It should be noted that we did not use resolution recovery to avoid Gibb artifacts.
Image processing and analysis
All processing steps were implemented with an internal pipeline in MATLAB (The MathWorks, Inc., Natick, MA, USA) and Statistical Parametric Mapping Software (SPM12) (www.fil.ion.ac. uk/spm).
[18F]FDG PET scans were stereotaxically normalized to an internal test [18F]FDG PET template in the space of the Montreal Neurological Institute (MNI). For validation of PET-based stereotaxic normalization, we also performed MRI-based stereotaxic normalization of [18F]FDG PET scans for people who had a T1 MPRAGE MRI sequence available (not= 96 DP; not= 33 MSA; not= 21 PSP; not= 13 HC). For this, individual MRIs were segmented and stereotaxically normalized to MNI space using CAT12 software in SPM12. [18F]FDG PET scans were co-registered with MRI and then transformed into MNI space by applying an MRI-defined transformation matrix. Mean [18F]FDG uptake in the NS from normalized MRI-based stereotactic data was correlated with uptake derived from normalized PET-based stereotactic data using Spearman’s r. In addition, differences between groups were assessed. Absolute mean counts in the substantia nigra from stereotaxically normalized scans based on PET and MRI showed a strong correlation (r= 0.96, p
To assess metabolism in the substantia nigra, we normalized PET data to uptake in a cohort-specific composite reference region (i.e., the individual’s average uptake value in the substantia nigra was divided by the absorption value of the individual in the reference region). The cohort-specific reference region was defined by excluding the brainstem but including the substantia nigra, as well as regions with PD-, MSA-, and PSP-specific hypometabolism (top 20% of voxels with most pronounced hypometabolism within MSA-, PSP-, and PD-related patterns9.29) from a brain parenchyma mask (Supplementary Fig. 3). The anatomical definition of the substantia nigra was adopted from the Atlas of the Human Motor Thalamus30 and binarized to substantia nigra probability of >50% (Fig. 3). Normalized mean, lateral mean [18F]FDG uptake was calculated for each patient and control. To assess the effect of the reference region used for intensity scaling, we assessed individual white matter (for all groups) and pons (for PD and controls, but not for MSA and PSP due to midbrain atrophy in these diseases) as an alternative reference Regions. We observed significant differences in the bilateral mean [18F]FDG uptake in the substantia nigra normalized to individual white matter uptake (ANCOVA, factor group: pF= 17.57, see Supplementary Table and Fig. additional 2b) and to the pons (p
Side-split nigra metabolism
When evaluating [18F]FDG uptake for each side separately, we observed significantly different values between groups (ANCOVA, factor group: pF= 16.68, pF= 15.08, for the left and right sides, respectively). Mean [18F]FDG uptake by the substantia nigra was significantly reduced in NP groups compared to controls (at least p18F]FDG uptake on both sides showed similar significant results (ANCOVA, factor group: pF= 17.57; NP groups vs controls all p
For stratification of patients with PD, we assessed well-defined PDRP expression using ScAnVP software (SSM/PCA, The Feinstein Institute for Medical Research) in each patient with PD and CH. This pattern is characterized by relatively increased glucose metabolism in the pallidothalamic, pontine, and cerebellar clusters as well as decreased glucose metabolism in the premotor and posterior parietal areas.9. PD patients with PDRP expression 2 standard deviations above the normal HC value were considered to have high PDRP expression (i.e. clear PD cases based on PET scan) . In turn, those without elevated PDRP expression constitute the clinically difficult subgroup of patients with PD who are usually advised to undergo additional confirmatory imaging (eg, DAT SPECT).
Statistical analysis was performed with R software (version 4.1.0, http://www.R-project.org/). We compared the demographic and clinical characteristics of the patient and control groups with analysis of variance (ANOVA) followed by Tukey’s honest significance test. Differences between the groups in terms of [18F]Absorption of FDG by the substantia nigra (NP and subgroups [PD, MSA, and PSP] vs controls) were assessed using age-adjusted analysis of covariance (ANCOVA) followed by Tukey’s honest significance test. We estimated effect sizes for pairwise comparisons with Cohen’s d. Receiver operating characteristic (ROC) analysis was used to assess the diagnostic performance of nigra [18F]FDG uptake for differentiation between NP and its subgroups and controls by means of area under the ROC curve (AUC). In addition, we determined threshold values for the normalized mean [18F]FDG binding of the substantia nigra with a required minimum specificity of 90%, consistent with the clinical need for a confirmatory test (reduction [18F]FDG fixation of the substantia nigra defined as a positive or pathological case). We also performed ROC analyzes for PD subgroups with and without elevated PDRP expression. Additionally, we assessed group differences between controls and PD patients with early (Hoehn & Yahr ≤ 2.5) and late (Hoehn & Yahr > 2.5) stages and short disease duration ( 5 years). For Parkinsonian patients, the relationship between the normalized mean [18F]Absorption of FDG, [123I]FP-CIT specific binding potential (equal to binding ratio -131) in the putamen ipsilateral to the clinically most affected side (i.e., the most appropriate region to detect changes in striatal DAT density with progression of PD32), disease duration, and Unified Parkinson’s Disease Rating Scale (UPDRS) motor part 3 (excluding medication) were assessed using the age-adjusted Pearson correlation coefficient. The significance level was set at Bonferroni corrected p
Ethical approval and patient consent
All patients and controls gave written informed consent to [18F]FDG PET imaging. The local Institutional Review Board of University Hospital Freiburg (22/20) approved the retrospective analysis.