Correlation of early-phase 18F-florbetapir (AV-45/Amyvid) PET images to FDG images: preliminary studies

The Early Perfusion Image Is Useful to Support the Visual Interpretation of Brain Amyloid-PET With 18F-Flutemetamol in Borderline Cases

PURPOSE

Visual interpretation of brain amyloid-β (Aβ) PET can be difficult in individuals with borderline Aβ burden. Coregistration with individual MRI is recommended in these cases, which, however, is not always available. This study evaluated coregistration with the early perfusion frames acquired immediately after tracer injection to support the visual interpretation of the late Aβ-frames in PET with 18F-flutemetamol (FMM).

PATIENTS AND METHODS

Fifty dual-time-window FMM-PET scans of cognitively normal subjects with 0 to 60 Centiloids were included retrospectively (70.1 ± 6.9 years, 56% female, MMSE score 28.9 ± 1.3, 42% APOE ɛ4 carrier). Regional Aβ load was scored with respect to a 6-point Likert scale by 3 independent raters in the 10 regions of interest recommended for FMM reading using 3 different settings: Aβ image only, Aβ image coregistered with MRI, and Aβ image coregistered with the perfusion image. The impact of setting, within- and between-readers variability, region of interest, and Aβ-status was tested by repeated-measure analysis of variance of the Likert score.

RESULTS

The Centiloid scale ranged between 2 and 52 (interquartile range, 7-19). Support of visual scoring by the perfusion image resulted in the best discrimination between Aβ-positive and Aβ-negative cases, mainly by improved certainty of excluding Aβ plaques in Aβ-negative cases (P = 0.030). It also resulted in significantly higher between-rater agreement. The setting effect was most pronounced in the frontal lobe and in the posterior cingulate cortex/precuneus area (P = 0.005).

CONCLUSIONS

The early perfusion image is a suitable alternative to T1-weighted MRI to support the visual interpretation of the late Aβ image in FMM-PET.

A deep learning model for generating [18F]FDG PET Images from early-phase [18F]Florbetapir and [18F]Flutemetamol PET images

INTRODUCTION

Amyloid-β (Aβ) plaques is a significant hallmark of Alzheimer's disease (AD), detectable via amyloid-PET imaging. The Fluorine-18-Fluorodeoxyglucose ([18F]FDG) PET scan tracks cerebral glucose metabolism, correlated with synaptic dysfunction and disease progression and is complementary for AD diagnosis. Dual-scan acquisitions of amyloid PET allows the possibility to use early-phase amyloid-PET as a biomarker for neurodegeneration, proven to have a good correlation to [18F]FDG PET. The aim of this study was to evaluate the added value of synthesizing the later from the former through deep learning (DL), aiming at reducing the number of PET scans, radiation dose, and discomfort to patients.

METHODS

A total of 166 subjects including cognitively unimpaired individuals (N = 72), subjects with mild cognitive impairment (N = 73) and dementia (N = 21) were included in this study. All underwent T1-weighted MRI, dual-phase amyloid PET scans using either Fluorine-18 Florbetapir ([18F]FBP) or Fluorine-18 Flutemetamol ([18F]FMM), and an [18F]FDG PET scan. Two transformer-based DL models called SwinUNETR were trained separately to synthesize the [18F]FDG from early phase [18F]FBP and [18F]FMM (eFBP/eFMM). A clinical similarity score (1: no similarity to 3: similar) was assessed to compare the imaging information obtained by synthesized [18F]FDG as well as eFBP/eFMM to actual [18F]FDG. Quantitative evaluations include region wise correlation and single-subject voxel-wise analyses in comparison with a reference [18F]FDG PET healthy control database. Dice coefficients were calculated to quantify the whole-brain spatial overlap between hypometabolic ([18F]FDG PET) and hypoperfused (eFBP/eFMM) binary maps at the single-subject level as well as between [18F]FDG PET and synthetic [18F]FDG PET hypometabolic binary maps.

RESULTS

The clinical evaluation showed that, in comparison to eFBP/eFMM (average of clinical similarity score (CSS) = 1.53), the synthetic [18F]FDG images are quite similar to the actual [18F]FDG images (average of CSS = 2.7) in terms of preserving clinically relevant uptake patterns. The single-subject voxel-wise analyses showed that at the group level, the Dice scores improved by around 13% and 5% when using the DL approach for eFBP and eFMM, respectively. The correlation analysis results indicated a relatively strong correlation between eFBP/eFMM and [18F]FDG (eFBP: slope = 0.77, R2 = 0.61, P-value < 0.0001); eFMM: slope = 0.77, R2 = 0.61, P-value < 0.0001). This correlation improved for synthetic [18F]FDG (synthetic [18F]FDG generated from eFBP (slope = 1.00, R2 = 0.68, P-value < 0.0001), eFMM (slope = 0.93, R2 = 0.72, P-value < 0.0001)).

CONCLUSION

We proposed a DL model for generating the [18F]FDG from eFBP/eFMM PET images. This method may be used as an alternative for multiple radiotracer scanning in research and clinical settings allowing to adopt the currently validated [18F]FDG PET normal reference databases for data analysis.

Amyloid PET Imaging: Standard Procedures and Semiquantification

Amyloid plaques are a neuropathologic hallmark of Alzheimer's disease (AD), which can be imaged through positron emission tomography (PET) technology using radiopharmaceuticals that selectively bind to the fibrillar aggregates of amyloid-β plaques (Amy-PET). Several radiotracers for amyloid PET have been validated (11C-Pittsburgh compound B and the 18F-labeled compounds such as 18F-florbetaben, 18F-florbetapir, and 18F-flutemetamol). Images can be interpreted by means of visual/qualitative, semiquantitative, and quantitative criteria. Here, we summarize the main differences between the available radiotracers for Amy-PET, the proposed interpretation criteria, and main proposed quantification methods.

Generation of Conventional 18F-FDG PET Images from 18F-Florbetaben PET Images Using Generative Adversarial Network: A Preliminary Study Using ADNI Dataset

Background and Objectives: ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography (PET) (PET_FDG) image can visualize neuronal injury of the brain in Alzheimer's disease. Early-phase amyloid PET image is reported to be similar to PET_FDG image. This study aimed to generate PET_FDG images from ¹⁸F-florbetaben PET (PET_FBB) images using a generative adversarial network (GAN) and compare the generated PET_FDG (PET_GE-FDG) with real PET_FDG (PET_RE-FDG) images using the structural similarity index measure (SSIM) and the peak signal-to-noise ratio (PSNR). Materials and Methods: Using the Alzheimer's Disease Neuroimaging Initiative (ADNI) database, 110 participants with both PET_FDG and PET_FBB images at baseline were included. The paired PET_FDG and PET_FBB images included six and four subset images, respectively. Each subset image had a 5 min acquisition time. These subsets were randomly sampled and divided into 249 paired PET_FDG and PET_FBB subset images for the training datasets and 95 paired subset images for the validation datasets during the deep-learning process. The deep learning model used in this study is composed of a GAN with a U-Net. The differences in the SSIM and PSNR values between the PET_GE-FDG and PET_RE-FDG images in the cycleGAN and pix2pix models were evaluated using the independent Student's t-test. Statistical significance was set at p ≤ 0.05. Results: The participant demographics (age, sex, or diagnosis) showed no statistically significant differences between the training (82 participants) and validation (28 participants) groups. The mean SSIM between the PET_GE-FDG and PET_RE-FDG images was 0.768 ± 0.135 for the cycleGAN model and 0.745 ± 0.143 for the pix2pix model. The mean PSNR was 32.4 ± 9.5 and 30.7 ± 8.0. The PET_GE-FDG images of the cycleGAN model showed statistically higher mean SSIM than those of the pix2pix model (p < 0.001). The mean PSNR was also higher in the PET_GE-FDG images of the cycleGAN model than those of pix2pix model (p < 0.001). Conclusions: We generated PET_FDG images from PET_FBB images using deep learning. The cycleGAN model generated PET_GE-FDG images with a higher SSIM and PSNR values than the pix2pix model. Image-to-image translation using deep learning may be useful for generating PET_FDG images. These may provide additional information for the management of Alzheimer's disease without extra image acquisition and the consequent increase in radiation exposure, inconvenience, or expenses.

Static first-minute-frame (FMF) PET imaging after 18F-labeled amyloid tracer injection is correlated to [18F]FDG PET in patients with primary progressive aphasia

OBJECTIVE

To study the correlation between a static PET image of the first-minute-frame (FMF) acquired with ¹⁸F-labeled amyloid-binding radiotracers and brain [¹⁸F]FDG PET in patients with primary progressive aphasia (PPA).

MATERIAL AND METHODS

The study cohort includes 17 patients diagnosed with PPA with the following distribution: 9 nonfluent variant PPA, 4 logopenic variant PPA, 1 semantic variant PPA, 3 unclassifiable PPA. Regional SUVRs are extracted from FMFs and their corresponding [¹⁸F]FDG PET images and Pearson's correlation coefficients are calculated.

RESULTS

SUVRs of both images show similar patterns of regional cerebral alterations. Intrapatient correlation analyses result in a mean coefficient of r=0.94±0.06. Regional interpatient correlation coefficients of the study cohort are greater than 0.81. Radiotracer-specific and variant-specific subcohorts show no difference in the similarity between the images.

CONCLUSIONS

The static FMF could be a valid alternative to dynamic early-phase amyloid PET proposed in the literature, and a neurodegeneration biomarker for the diagnosis and classification of PPA in amyloid PET studies.

Dual-phase 18 F-FP-CIT positron emission tomography and cardiac 123 I-MIBG scintigraphy of Parkinson's disease patients with GBA mutations: evidence of the body-first type?

BACKGROUND AND PURPOSE

Parkinson's disease (PD) with glucocerebrosidase (GBA) gene mutation (GBA-PD) is known to show more rapid clinical progression than sporadic PD without GBA mutation (sPD). This study was performed to delineate the specific patterns of cortical hypoperfusion, dopamine transporter uptake and cardiac meta-iodobenzylguanidine (MIBG) uptake of GBA-PD in comparison to sPD.

METHODS

Through next-generation sequencing analysis targeting 41 genes, a total of 16 GBA-PD and 24 sPD patients (sex, age matched) were enrolled in the study, and the clinical, dual-phase [¹⁸ F]-N-(3-fluoropropyl)-2β-carboxymethoxy-3β-(4-iodophenyl) nortropane (^{1 8} F-FP-CIT) positron emission tomography (PET) and cardiac ¹²³ I-MIBG scintigraphy results were compared between the two groups.

RESULTS

The GBA-PD group had higher rates of rapid eye movement sleep behavior disorder, orthostatic hypotension and neuropsychiatric symptoms than the sPD group. Early-phase ¹⁸ F-FP-CIT PET showed significantly lower standard uptake value ratio on bilateral posterior parietal cortex (0.94 ± 0.05 vs. 1.02 ± 0.04, p = 0.011) and part of the occipital cortex (p < 0.05) in the GBA-PD group than the sPD group. In striatal dopamine transporter uptake, the regional standard uptake value ratio, asymmetry index and caudate-to-putamen ratio were similar between the two groups. The GBA-PD group had a lower heart-to-mediastinum uptake ratio in ¹²³ I-MIBG scintigraphy than the sPD group.

CONCLUSIONS

The GBA-PD patients showed decreased regional perfusion in the bilateral posterior parietal and occipital cortex. Cardiac sympathetic denervation and non-motor symptoms (orthostatic hypotension, rapid eye movement sleep behavior disorder) were more common in GBA-PD than sPD. These findings suggest that GBA-PD patients have more widespread peripheral (extranigral) α-synuclein accumulation, representing a body-first PD subtype.

The RSNA QIBA Profile for Amyloid PET as an Imaging Biomarker for Cerebral Amyloid Quantification

A standardized approach to acquiring amyloid PET images increases their value as disease and drug response biomarkers. Most 18F PET amyloid brain scans often are assessed only visually (per regulatory labels), with a binary decision indicating the presence or absence of Alzheimer disease amyloid pathology. Minimizing technical variance allows precise, quantitative SUV ratios (SUVRs) for early detection of β-amyloid plaques and allows the effectiveness of antiamyloid treatments to be assessed with serial studies. Methods: The Quantitative Imaging Biomarkers Alliance amyloid PET biomarker committee developed and validated a profile to characterize and reduce the variability of SUVRs, increasing statistical power for these assessments. Results: On achieving conformance, sites can justify a claim that brain amyloid burden reflected by the SUVR is measurable to a within-subject coefficient of variation of no more than 1.94% when the same radiopharmaceutical, scanner, acquisition, and analysis protocols are used. Conclusion: This overview explains the claim, requirements, barriers, and potential future developments of the profile to achieve precision in clinical and research amyloid PET imaging.

Early-phase 18F-Florbetapir and 18F-Flutemetamol images as proxies of brain metabolism in a memory clinic setting

Background: Alzheimer's disease (AD) neuropathologic changes are β-amyloid (Aβ) deposition, pathologic tau, and neurodegeneration. Dual-phase amyloid-PET might be able to evaluate Aβ deposition and neurodegeneration with a single tracer injection. Early-phase amyloid-PET scans provide a proxy for cerebral perfusion, which has shown good correlations with neural dysfunction measured through metabolic consumption, while the late frames depict amyloid distribution. Our study aims to assess the comparability between early-phase amyloid-PET scans and 18F-fluorodeoxyglucose (18F-FDG)-PET brain topography at the individual level, and their ability to discriminate patients. Methods: 166 subjects evaluated at the Geneva Memory Center, ranging from cognitively unimpaired to Mild Cognitive Impairment (MCI) and dementia, underwent early-phase amyloid-PET - using either 18F-florbetapir (eFBP) (n = 94) or 18F-flutemetamol (eFMM) (n = 72) - and 18F-FDG-PET. Aβ status was assessed. Standardized uptake value ratios (SUVR) were extracted to evaluate the correlation of eFBP/eFMM and their respective 18F-FDG-PET scans. The single-subject procedure was applied to investigate hypometabolism and hypoperfusion maps and their spatial overlap by Dice coefficient. Receiver operating characteristic analyses were performed to compare the discriminative power of eFBP/eFMM, and 18F-FDG-PET SUVR in AD-related metaROI between Aβ-negative healthy controls and cases in the AD continuum. Results: Positive correlations were found between eFBP/eFMM and 18F-FDG-PET SUVR independently of Aβ status and Aβ radiotracer (R>0.72, p<0.001). eFBP/eFMM single-subject analysis revealed clusters of significant hypoperfusion with good correspondence to hypometabolism topographies, independently of the underlying neurodegenerative patterns. Both eFBP/eFMM and 18F-FDG-PET SUVR significantly discriminated AD patients from controls in the AD-related metaROIs (AUCFBP = 0.888; AUCFMM=0.801), with 18F-FDG-PET performing slightly better, however not significantly (all p-value higher than 0.05), than others (AUCFDG=0.915 and 0.832 for subjects evaluated with 18F-FBP and 18F-FMM, respectively). Conclusion: The distribution of perfusion was comparable to that of metabolism at the single-subject level by parametric analysis, particularly in the presence of a high neurodegeneration burden. Our findings indicate that eFBP/eFMM imaging can replace 18F-FDG-PET imaging, as they reveal typical neurodegenerative patterns, or allow to exclude the presence of neurodegeneration. The finding shows cost-saving capacities of amyloid-PET and supports the routine use of the modality for individual classification in clinical practice.

[11C]Martinostat PET analysis reveals reduced HDAC I availability in Alzheimer's disease

Alzheimer’s disease (AD) is characterized by the brain accumulation of amyloid-β and tau proteins. A growing body of literature suggests that epigenetic dysregulations play a role in the interplay of hallmark proteinopathies with neurodegeneration and cognitive impairment. Here, we aim to characterize an epigenetic dysregulation associated with the brain deposition of amyloid-β and tau proteins. Using positron emission tomography (PET) tracers selective for amyloid-β, tau, and class I histone deacetylase (HDAC I isoforms 1–3), we find that HDAC I levels are reduced in patients with AD. HDAC I PET reduction is associated with elevated amyloid-β PET and tau PET concentrations. Notably, HDAC I reduction mediates the deleterious effects of amyloid-β and tau on brain atrophy and cognitive impairment. HDAC I PET reduction is associated with 2-year longitudinal neurodegeneration and cognitive decline. We also find HDAC I reduction in the postmortem brain tissue of patients with AD and in a transgenic rat model expressing human amyloid-β plus tau pathology in the same brain regions identified in vivo using PET. These observations highlight HDAC I reduction as an element associated with AD pathophysiology.

The link between amyloid and tau proteins with Alzheimer’s disease progression remains unclear. Here, the authors propose HDACs I downregulation as an element linking the deleterious effects of brain proteinopathies with disease progression.

Measurement of neurodegeneration using a multivariate early frame amyloid PET classifier

Introduction

Amyloid measurement provides important confirmation of pathology for Alzheimer's disease (AD) clinical trials. However, many amyloid positive (Am+) early-stage subjects do not worsen clinically during a clinical trial, and a neurodegenerative measure predictive of decline could provide critical information. Studies have shown correspondence between perfusion measured by early amyloid frames post-tracer injection and fluorodeoxyglucose (FDG) positron emission tomography (PET), but with limitations in sensitivity. Multivariate machine learning approaches may offer a more sensitive means for detection of disease related changes as we have demonstrated with FDG.

Methods

Using summed dynamic florbetapir image frames acquired during the first 6 minutes post-injection for 107 Alzheimer's Disease Neuroimaging Initiative subjects, we applied optimized machine learning to develop and test image classifiers aimed at measuring AD progression. Early frame amyloid (EFA) classification was compared to that of an independently developed FDG PET AD progression classifier by scoring the FDG scans of the same subjects at the same time point. Score distributions and correlation with clinical endpoints were compared to those obtained from FDG. Region of interest measures were compared between EFA and FDG to further understand discrimination performance.

Results

The EFA classifier produced a primary pattern similar to that of the FDG classifier whose expression correlated highly with the FDG pattern (R-squared 0.71), discriminated cognitively normal (NL) amyloid negative (Am-) subjects from all Am+ groups, and that correlated in Am+ subjects with Mini-Mental State Examination, Clinical Dementia Rating Sum of Boxes, and Alzheimer's Disease Assessment Scale-13-item Cognitive subscale (R = 0.59, 0.63, 0.73) and with subsequent 24-month changes in these measures (R = 0.67, 0.73, 0.50).

Discussion

Our results support the ability to use EFA with a multivariate machine learning-derived classifier to obtain a sensitive measure of AD-related loss in neuronal function that correlates with FDG PET in preclinical and early prodromal stages as well as in late mild cognitive impairment and dementia.

Highlights

The summed initial post-injection minutes of florbetapir positron emission tomography  correlate with fluorodeoxyglucose.A machine learning classifier enabled sensitive detection of early prodromal Alzheimer's disease.Early frame amyloid (EFA) classifier scores correlate with subsequent change in Mini-Mental State Examination, Clinical Dementia Rating Sum of Boxes, and Alzheimer's Disease Assessment Scale-13-item Cognitive subscale.EFA classifier effect sizes and clinical prediction outperformed region of interest standardized uptake value ratio.EFA classification may aid in stratifying patients to assess treatment effect.

Dynamic Amyloid PET: Relationships to 18F-Flortaucipir Tau PET Measures

Measuring amyloid and predicting tau status using a single amyloid PET study would be valuable for assessing brain AD pathophysiology. We hypothesized that early-frame amyloid PET (efAP) correlates with the presence of tau pathology because the initial regional brain concentrations of radioactivity are determined primarily by blood flow, which is expected to be decreased in the setting of tau pathology. Methods: The study included 120 participants (63 amyloid-positive and 57 amyloid-negative) with dynamic 18F-florbetapir PET and static 18F-flortaucipir PET scans obtained within 6 mo of each other. These subjects were predominantly cognitively intact in both the amyloid-positive (63%) and the amyloid-negative (93%) groups. Parameters for efAP quantification were optimized for stratification of tau PET positivity, assessed by either a tauopathy score or Braak regions. The ability of efAP to stratify tau positivity was measured using receiver-operating-characteristic analysis of area under the curve (AUC). Pearson r and Spearman ρ were used for parametric and nonparametric comparisons between efAP and tau PET, respectively. Standardized net benefit was used to evaluate improvement in using efAP as an additional copredictor over hippocampal volume in predicting tau PET positivity. Results: Measuring efAP within the hippocampus and summing the first 3 min of brain activity after injection showed the strongest discriminative ability to stratify for tau positivity (AUC, 0.67-0.89 across tau PET Braak regions) in amyloid-positive individuals. Hippocampal efAP correlated significantly with a global tau PET tauopathy score in amyloid-positive participants (r = -0.57, P < 0.0001). Compared with hippocampal volume, hippocampal efAP showed a stronger association with tau PET Braak stage (ρ = -0.58 vs. -0.37) and superior stratification of tau PET tauopathy score (AUC, 0.86 vs. 0.66; P = 0.002). Conclusion: Hippocampal efAP can provide additional information to conventional amyloid PET, including estimation of the likelihood of tau positivity in amyloid-positive individuals.

Regional uptakes from early-frame amyloid PET and 18F-FDG PET scans are comparable independent of disease state

Purpose

Positron emission tomography (PET) imaging with amyloid-beta (Aβ) tracers and 2-[18F] fluoro-2-Deoxy-d-glucose (¹⁸F-FDG) is extensively employed in Alzheimer’s disease (AD) studies as biomarkers of AD pathology and neurodegeneration. To reduce cost and additional burdens to the patient, early-frame uptake during Aβ PET scanning has been proposed as a surrogate measure of regional glucose metabolism. Considering the disease state specific impact of AD on neurovascular coupling, we investigated to what extent the information captured in the early frames of an Aβ-PET (¹⁸F-florbetapir or ¹⁸F-florbetaben) scan is comparable to that of a ¹⁸F-FDG PET scan, independent of disease state.

Method

A partial correlation was performed on early-frame ¹⁸F-florbetapir and ¹⁸F-FDG regional data from 100 participants. In a secondary analysis, we compared 92 ¹⁸F-florbetapir and 21 ¹⁸F-florbetaben early-frame Aβ scans from cognitively unimpaired and mild cognitive impairment participants to ascertain if regional early-frame information was similar across different Aβ-PET radioligands.

Results

The partial correlation of early-frame ¹⁸F-florbetapir with ¹⁸F-FDG was significant in all 84 brain ROIs, with correlation values ranging from 0.61 to 0.94. There were no significant differences between early-frame ¹⁸F-florbetapir and ¹⁸F-florbetaben images.

Conclusion

Overall, we find that the regional uptake measurements from early-frame ¹⁸F-florbetapir are strongly correlated with regional glucose metabolism as measured in ground-truth ¹⁸F-FDG PET scans, regardless of disease state. Future studies should focus on longitudinal early-frame amyloid PET imaging studies to further assess the value of early-frame imaging as a marker of brain metabolic decline.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41824-021-00123-0.

Recent Updates on PET Imaging in Neurodegenerative Diseases

양전자방출단층촬영(PET)을 이용한 단백질병리의 생체영상기술은 퇴행성 치매의 질병 기전을 이해하는데 필요한 정보를 제공할 뿐 아니라, 질병의 조기 발견과 치료법 개발에서 중요한 역할을 수행하고 있다. 베타아밀로이드와 타우 PET 영상은 인체 뇌병리에 기반한 알츠하이머병 연속체에 대한 진단 바이오마커로 확립되어 조기진단과 감별진단을 용이하게 하고, 질병 예후를 예측하고 있다. 또한, 치매치료제 개발에서 예후 및 대리 바이오마커로의 역할이 커지고 있다. 이 종설에서는 치매를 유발하는 알츠하이머병 및 기타 퇴행성 뇌질환에서 베타아밀로이드와 타우 단백질의 뇌축적을 영상화하는 PET의 최근 임상적 적용과 최근 동향을 살펴보고, 잠재적 유용성을 소개하고자 한다.

High Correlation of Static First-Minute-Frame (FMF) PET Imaging after 18F-Labeled Amyloid Tracer Injection with [18F]FDG PET Imaging

Dynamic early-phase PET images acquired with radiotracers binding to fibrillar amyloid-beta (Aβ) have shown to correlate with [18F]fluorodeoxyglucose (FDG) PET images and provide perfusion-like information. Perfusion information of static PET scans acquired during the first minute after radiotracer injection (FMF, first-minute-frame) is compared to [18F]FDG PET images. FMFs of 60 patients acquired with [18F]florbetapir (FBP), [18F]flutemetamol (FMM), and [18F]florbetaben (FBB) are compared to [18F]FDG PET images. Regional standardized uptake value ratios (SUVR) are directly compared and intrapatient Pearson's correlation coefficients are calculated to evaluate the correlation of FMFs to their corresponding [18F]FDG PET images. Additionally, regional interpatient correlations are calculated. The intensity profiles of mean SUVRs among the study cohort (r = 0.98, p < 0.001) and intrapatient analyses show strong correlations between FMFs and [18F]FDG PET images (r = 0.93 ± 0.05). Regional VOI-based analyses also result in high correlation coefficients. The FMF shows similar information to the cerebral metabolic patterns obtained by [18F]FDG PET imaging. Therefore, it could be an alternative to the dynamic imaging of early phase amyloid PET and be used as an additional neurodegeneration biomarker in amyloid PET studies in routine clinical practice while being acquired at the same time as amyloid PET images.

Dual-phase 18F-florbetaben PET provides cerebral perfusion proxy along with beta-amyloid burden in Alzheimer's disease

BACKGROUND

This study investigated changes in brain perfusion and Aβ burden according to the progression of Alzheimer's disease (AD) by using a dual-phase ¹⁸F-florbetaben (FBB) PET protocol.

METHODS

Sixty subjects, including 12 with Aβ-negative normal cognition (Aβ^-NC), 32 with Aβ-positive mild cognitive impairment (Aβ⁺MCI), and 16 with Aβ-positive AD (Aβ⁺AD), were enrolled. A dynamic PET scan was obtained in the early phase (0-10 min, eFBB) and delayed phase (90-110 min, dFBB), which were then averaged into a single frame, respectively. In addition to the averaged eFBB, an R1 parametric map was calculated from the eFBB scan based on a simplified reference tissue model (SRTM). Between-group regional and voxel-wise analyses of the images were performed. The associations between cognitive profiles and PET-derived parameters were investigated.

RESULTS

Both the R1 and eFBB perfusion reductions in the cortical regions were not significantly different between the Aβ^-NC and Aβ⁺MCI groups, while they were significantly reduced from the Aβ⁺MCI to Aβ⁺AD groups in regional and voxel-wise analyses. However, cortical Aβ depositions on dFBB were not significantly different between the Aβ⁺MCI and Aβ⁺AD groups. There were strong positive correlations between the R1 and eFBB images in regional and voxel-wise analyses. Both perfusion components showed significant correlations with general and specific cognitive profiles.

CONCLUSION

The results of this study demonstrated the feasibility of dual-phase ¹⁸F-FBB PET to evaluate different trajectories of dual biomarkers for neurodegeneration and Aβ burden over the course of AD. In addition, both eFBB and SRTM-based R1 can provide robust indices of brain perfusion.

Diagnostic accuracy of dual-phase 18F-FP-CIT PET imaging for detection and differential diagnosis of Parkinsonism

Delayed phase 18F-FP-CIT PET (dCIT) can assess the striatal dopamine transporter binding to detect degenerative parkinsonism (DP). Early phase 18F-FP-CIT (eCIT) can assess the regional brain activity for differential diagnosis among parkinsonism similar with 18F-FDG PET. We evaluated the diagnostic performance of dual phase 18F-FP-CIT PET (dual CIT) and 18F-FDG PET compared with clinical diagnosis in 141 subjects [36 with idiopathic Parkinson's disease (IPD), 77 with multiple system atrophy (MSA), 18 with progressive supranuclear palsy (PSP), and 10 with non-DP)]. Visual assessment of eCIT, dCIT, dual CIT, 18F-FDG and 18F-FDG PET with dCIT was in agreement with the clinical diagnosis in 61.7%, 69.5%, 95.7%, 81.6%, and 97.2% of cases, respectively. ECIT showed about 90% concordance with non-DP and MSA, and 8.3% and 27.8% with IPD and PSP, respectively. DCIT showed ≥ 88% concordance with non-DP, IPD, and PSP, and 49.4% concordance with MSA. Dual CIT showed ≥ 90% concordance in all groups. 18F-FDG PET showed ≥ 90% concordance with non-DP, MSA, and PSP, but only 33.3% concordance with IPD. The combination of 18F-FDG and dCIT yielded ≥ 90% concordance in all groups. Dual CIT may represent a powerful alternative to the combination of 18F-FDG PET and dCIT for differential diagnosis of parkinsonian disorders.

Evaluation of the early-phase [18F]AV45 PET as an optimal surrogate of [18F]FDG PET in ageing and Alzheimer's clinical syndrome

Dual-phase [18F]AV45 positron emission tomography (PET) is highly promising in the assessment of neurodegenerative diseases, allowing to obtain information on both neurodegeneration (early-phase; eAV45) and amyloid deposition (late-phase; lAV45) which are highly complementary; yet eAV45 needs further evaluation. This study aims at validating eAV45 as an optimal proxy of [18F]FDG PET in a large mixed-population of healthy ageing and Alzheimer's clinical syndrome participants (n = 191) who had [18F]FDG PET, eAV45 and lAV45 scans. We found early time frame 0-4 min to give maximal correlation with [18F]FDG PET and minimal correlation with lAV45. Moreover, maximal overlap of [18F]FDG PET versus eAV45 associations with clinical diagnosis and cognition was obtained with pons scaling. Across reference regions, classification performance between clinical subgroups was similar for both eAV45 and [18F]FDG PET. These findings highlight the optimal use of eAV45 to assess neurodegeneration as a validated proxy of [18F]FDG PET. On top of this purpose, this study showed that combined [18F]AV45 PET dual-biomarker even outperformed [18F]FDG PET or lAV45 alone.

Early-phase 18F-FP-CIT and 18F-flutemetamol PET were significantly correlated

Little is known about whether early-phase PET images of ¹⁸F-FP-CIT match those of amyloid PET. Here, we compared early-phase ¹⁸F-FP-CIT and ¹⁸F-flutemetamol PET images in patients who underwent both within a 1-month interval. The SUVR on early-phase ¹⁸F-FP-CIT PET (median, 0.86) was significantly lower than that of ¹⁸F-flutemetamol PET (median, 0.91, p < 0.001) for total brain regions including all cerebral lobes and central structures. This significant difference persisted for each brain region except central structures (p = 0.232). The SUVR of total brain regions obtained from early ¹⁸F-FP-CIT PET showed a very strong correlation with that of ¹⁸F-flutemetamol PET (rho = 0.80, p < 0.001). Among the kinetic parameters, only R1 showed a statistically significant correlation between the two techniques for all brain regions (rho = 0.89, p < 0.001). R1 from ¹⁸F-FP-CIT (median, 0.77) was significantly lower in all areas of the brain compared to R1 from ¹⁸F-flutemetamol PET (median, 0.81, p < 0.001).¹⁸F-FP-CIT demonstrated lower uptake in cortical brain regions than ¹⁸F-flutemetamol on early-phase PET. However, both early-phase PETs demonstrated significant correlation of uptake.

Dual-Phase β-Amyloid PET Captures Neuronal Injury and Amyloidosis in Corticobasal Syndrome

Objectives: In recent years several ¹⁸F-labeled amyloid PET (Aβ-PET) tracers have been developed and have obtained clinical approval. There is evidence that Aβ-PET perfusion can provide surrogate information about neuronal injury in neurodegenerative diseases when compared to conventional blood flow and glucose metabolism assessment. However, this paradigm has not yet been tested in neurodegenerative disorders with cortical and subcortical affection. Therefore, we investigated the performance of early acquisition ¹⁸F-flutemetamol Aβ-PET in comparison to ¹⁸F-fluorodeoxyglucose (FDG)-PET in corticobasal syndrome (CBS). Methods: Subjects with clinically possible or probable CBS were recruited within the prospective Activity of Cerebral Networks, Amyloid and Microglia in Aging and Alzheimer's Disease (ActiGliA) observational study and all CBS cases with an available FDG-PET prior to Aβ-PET were selected. Aβ-PET was acquired 0-10 min p.i. (early-phase) and 90-110 min p.i. (late-phase) whereas FDG-PET was recorded statically from 30 to 50 min p.i. Semiquantitative regional values and asymmetry indices (AI) were compared between early-phase Aβ-PET and FDG-PET. Visual assessments of hypoperfusion and hypometabolism were compared between both methods. Late-phase Aβ-PET was evaluated visually for assessment of Aβ-positivity. Results: Among 20 evaluated patients with CBS, 5 were Aβ-positive. Early-phase Aβ-PET and FDG-PET SUVr correlated highly in cortical (mean R = 0.86, range 0.77-0.92) and subcortical brain regions (mean R = 0.84, range 0.79-0.90). Strong asymmetry was observed in FDG-PET for the motor cortex (mean |AI| = 2.9%), the parietal cortex (mean |AI| = 2.9%), and the thalamus (mean |AI| = 5.5%), correlating well with AI of early-phase Aβ-PET (mean R = 0.87, range 0.62-0.98). Visual assessments of hypoperfusion and hypometabolism were highly congruent. Conclusion: Early-phase Aβ-PET facilitates assessment of neuronal injury in CBS for cortical and subcortical areas. Known asymmetries in CBS are captured by this method, enabling assessment of Aβ-status and neuronal injury with a single radiation exposure at a single visit.

Phase II (NAVIGATE-AD study) Results of LY3202626 Effects on Patients with Mild Alzheimer's Disease Dementia

Background:

LY3202626 is a small molecule inhibitor of β-site amyloid precursor protein cleaving enzyme (BACE)1 shown to reduce amyloid-β (Aβ)_1–40 and Aβ_1–42 concentrations in plasma and cerebrospinal fluid developed for the treatment of Alzheimer’s disease (AD).

Objective:

To assess the change from baseline in flortaucipir positron emission tomography (PET) after treatment with LY3202626 compared with placebo in patients with mild AD dementia.

Methods:

Patients received daily 3 mg or 12 mg doses of LY3202626 or placebo for 52 weeks. The primary outcome was assessment of cerebral neurofibrillary tangle load by flortaucipir PET. The study was terminated early following an interim analysis due to a low probability of identifying a statistically significant slowing of cognitive and/or functional decline.

Results:

A total of 316 patients were randomized and 47 completed the study. There was no statistically significant difference between placebo and either dose of LY3202626 from baseline to 52 weeks, or in annualized change for flortaucipir PET. There was no clinically meaningful difference between placebo and LY3202626 doses on efficacy measures of cognition and function. No deaths or serious adverse events considered related to LY3202626 were reported. A statistically significant increase in treatment-emergent adverse events in the psychiatric disorders system organ class was reported for both LY3202626 doses compared to placebo.

Conclusion:

LY3202626 tested at doses generating 70–90% BACE inhibition was generally well tolerated in this study. LY3202626 treatment did not result in a clinically significant change in cerebral tau burden as measured by flortaucipir nor in change of functional or cognitive decline compared to placebo.

Uncertainty analysis of MR-PET image registration for precision neuro-PET imaging

Accurate regional brain quantitative PET measurements, particularly when using partial volume correction, rely on robust image registration between PET and MR images. We argue here that the precision, and hence the uncertainty, of MR-PET image registration is mainly driven by the registration implementation and the quality of PET images due to their lower resolution and higher noise compared to the structural MR images. We propose a dedicated uncertainty analysis for quantifying the precision of MR-PET registration, centred around the bootstrap resampling of PET list-mode events to generate multiple PET image realisations with different noise (count) levels. The effects of PET image reconstruction parameters, such as the use of attenuation and scatter corrections and different number of iterations, on the precision and accuracy of MR-PET registration were investigated. In addition, the performance of four software packages with their default settings for rigid inter-modality image registration were considered: NiftyReg, Vinci, FSL and SPM. Four distinct PET image distributions made of two early time frames (similar to cortical FDG) and two late frames using two amyloid PET dynamic acquisitions of one amyloid positive and one amyloid negative participants were investigated. For the investigated four PET frames, the biggest impact on the uncertainty was observed between registration software packages (up to 10-fold difference in precision) followed by the reconstruction parameters. On average, the lowest uncertainty for different PET frames and brain regions was observed with SPM and two iterations of fully quantitative image reconstruction. The observed uncertainty for the varying PET count-level (from 5% to 60%) was slightly lower than for the reconstruction parameters. We also observed that the registration uncertainty in quantitative PET analysis depends on amyloid status of the considered PET frames, with increased uncertainty (up to three times) when using post-reconstruction partial volume correction. This analysis is applicable for PET data obtained from either PET/MR or PET/CT scanners.

Multimodality Imaging of Dementia: Clinical Importance and Role of Integrated Anatomic and Molecular Imaging

Neurodegenerative diseases are a devastating group of disorders that can be difficult to accurately diagnose. Although these disorders are difficult to manage owing to relatively limited treatment options, an early and correct diagnosis can help with managing symptoms and coping with the later stages of these disease processes. Both anatomic structural imaging and physiologic molecular imaging have evolved to a state in which these neurodegenerative processes can be identified relatively early with high accuracy. To determine the underlying disease, the radiologist should understand the different distributions and pathophysiologic processes involved. High-spatial-resolution MRI allows detection of subtle morphologic changes, as well as potential complications and alternate diagnoses, while molecular imaging allows visualization of altered function or abnormal increased or decreased concentration of disease-specific markers. These methodologies are complementary. Appropriate workup and interpretation of diagnostic studies require an integrated, multimodality, multidisciplinary approach. This article reviews the protocols and findings at MRI and nuclear medicine imaging, including with the use of flurodeoxyglucose, amyloid tracers, and dopaminergic transporter imaging (ioflupane). The pathophysiology of some of the major neurodegenerative processes and their clinical presentations are also reviewed; this information is critical to understand how these imaging modalities work, and it aids in the integration of clinical data to help synthesize a final diagnosis. Radiologists and nuclear medicine physicians aiming to include the evaluation of neurodegenerative diseases in their practice should be aware of and familiar with the multiple imaging modalities available and how using these modalities is essential in the multidisciplinary management of patients with neurodegenerative diseases.^©RSNA, 2020.

Association of quality of life with structural, functional and molecular brain imaging in community-dwelling older adults

BACKGROUND

As the population ages, maintaining mental health and well-being of older adults is a public health priority. Beyond objective measures of health, self-perceived quality of life (QoL) is a good indicator of successful aging. In older adults, it has been shown that QoL is related to structural brain changes. However, QoL is a multi-faceted concept and little is known about the specific relationship of each QoL domain to brain structure, nor about the links with other aspects of brain integrity, including white matter microstructure, brain perfusion and amyloid deposition, which are particularly relevant in aging. Therefore, we aimed to better characterize the brain biomarkers associated with each QoL domain using a comprehensive multimodal neuroimaging approach in older adults.

METHODS

One hundred and thirty-five cognitively unimpaired older adults (mean age ± SD: 69.4 ± 3.8 y) underwent structural and diffusion magnetic resonance imaging, together with early and late florbetapir positron emission tomography scans. QoL was assessed using the brief version of the World Health Organization's QoL instrument, which allows measuring four distinct domains of QoL: self-perceived physical health, psychological health, social relationships and environment. Multiple regression analyses were carried out to identify the independent global neuroimaging predictor(s) of each QoL domain, and voxel-wise analyses were then conducted with the significant predictor(s) to highlight the brain regions involved. Age, sex, education and the other QoL domains were entered as covariates in these analyses. Finally, forward stepwise multiple regressions were conducted to determine the specific items of the relevant QoL domain(s) that contributed the most to these brain associations.

RESULTS

Only physical health QoL was associated with global neuroimaging values, specifically gray matter volume and white matter mean kurtosis, with higher physical health QoL being associated with greater brain integrity. These relationships were still significant after correction for objective physical health and physical activity measures. No association was found with global brain perfusion or global amyloid deposition. Voxel-wise analyses revealed that the relationships with physical health QoL concerned the anterior insula and ventrolateral prefrontal cortex, and the corpus callosum, corona radiata, inferior frontal white matter and cingulum. Self-perceived daily living activities and self-perceived pain and discomfort were the items that contributed the most to these associations with gray matter volume and white matter mean kurtosis, respectively.

CONCLUSIONS

Better self-perceived physical health, encompassing daily living activities and pain and discomfort, was the only QoL domain related to brain structural integrity including higher global gray matter volume and global white matter microstructural integrity in cognitively unimpaired older adults. The relationships involved brain structures belonging to the salience network, the pain pathway and the empathy network. While previous studies showed a link between objective measures of physical health, our findings specifically highlight the relevance of monitoring and promoting self-perceived physical health in the older population. Longitudinal studies are needed to assess the direction and causality of the relationships between QoL and brain integrity.

Validation of the Alzheimer Disease Dementia Conversion-Related Pattern as an ATN Biomarker of Neurodegeneration

OBJECTIVE

To determine whether the Alzheimer disease (AD) dementia conversion-related pattern (ADCRP) on [¹⁸F]FDG PET can serve as a valid predictor for the development of AD dementia, the individual expression of the ADCRP (subject score) and its prognostic value were examined in patients with mild cognitive impairment (MCI) and biologically defined AD.

METHODS

A total of 269 patients with available [¹⁸F]FDG PET, [¹⁸F]AV-45 PET, phosphorylated and total tau in CSF, and neurofilament light chain in plasma were included. Following the AT(N) classification scheme, where AD is defined biologically by in vivo biomarkers of β-amyloid (Aβ) deposition ("A") and pathologic tau ("T"), patients were categorized to the A-T-, A+T-, A+T+ (AD), and A-T+ groups.

RESULTS

The mean subject score of the ADCRP was significantly higher in the A+T+ group compared to each of the other group (all p < 0.05) but was similar among the latter (all p > 0.1). Within the A+T+ group, the subject score of ADCRP was a significant predictor of conversion to dementia (hazard ratio, 2.02 per z score increase; p < 0.001), with higher predictive value than of alternative biomarkers of neurodegeneration (total tau and neurofilament light chain). Stratification of A+T+ patients by the subject score of ADCRP yielded well-separated groups of high, medium, and low conversion risks.

CONCLUSIONS

The ADCRP is a valuable biomarker of neurodegeneration in patients with MCI and biologically defined AD. It shows great potential for stratifying the risk and estimating the time to conversion to dementia in patients with MCI and underlying AD (A+T+).

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that [¹⁸F]FDG PET predicts the development of AD dementia in individuals with MCI and underlying AD as defined by the AT(N) framework.

Imaging of Tau Pathology in Neurodegenerative Diseases: An Update

Pathological accumulated misfolded tau underlies various neurodegenerative diseases and associated clinical syndromes. To diagnose those diseases reliable before death or even at early stages, many different tau-specific radiotracers have been developed in the last decade to be used with positron-emission-tomography. In contrast to amyloid-β imaging, different isoforms of tau exist further complicating radiotracer development. First-generation radiotracers like [¹¹C]PBB3, [¹⁸F]AV1451 and [¹⁸F]THK5351 have been extensively investigated in vitro and in vivo. In Alzheimer's disease (AD), high specific binding could be detected, and evidence of clinical applicability recently led to clinical approval of [¹⁸F]flortaucipir ([¹⁸F]AV1451) by the FDA. Nevertheless, absent or minor binding to non-AD tau isoforms and high off-target binding to non-tau brain structures limit the diagnostic applicability especially in non-AD tauopathies demanding further tracer development. In vitro assays and autoradiography results of next-generation radiotracers [¹⁸F]MK-6240, [¹⁸F]RO-948, [¹⁸F]PM-PBB3, [¹⁸F]GTP-1 and [¹⁸F]PI-2620 clearly indicate less off-target binding and high specific binding to tau neurofibrils. First in human studies have been conducted with promising results for all tracers in AD patients, and also some positive experience in non-AD tauopathies. Overall, larger scaled autoradiography and human studies are needed to further evaluate the most promising candidates and support future clinical approval.

Spatial Normalization Using Early-Phase [18F]FP-CIT PET for Quantification of Striatal Dopamine Transporter Binding

Purpose

The precise quantification of dopamine transporter (DAT) density on N-(3-[¹⁸F]Fluoropropyl)-2β-carbomethoxy-3β-(4-iodophenyl) nortropane positron emission tomography ([¹⁸F]FP-CIT PET) imaging is crucial to measure the degree of striatal DAT loss in patients with parkinsonism. The quantitative analysis requires a spatial normalization process based on a template brain. Since the spatial normalization method based on a delayed-phase PET has limited performance, we suggest an early-phase PET-based method and compared its accuracy, referring to the MRI-based approach as a gold standard.

Methods

A total of 39 referred patients from the movement disorder clinic who underwent dual-phase [¹⁸F]FP-CIT PET and took MRI within 1 year were retrospectively analyzed. The three spatial normalization methods were applied for quantification of [¹⁸F]FP-CIT PET-MRI-based anatomical normalization, PET template-based method based on delayed PET, and that based on early PET. The striatal binding ratios (BRs) were compared, and voxelwise paired t tests were implemented between different methods.

Results

The early image-based normalization showed concordant patterns of putaminal [¹⁸F]FP-CIT binding with an MRI-based method. The BRs of the putamen from the MRI-based approach showed higher agreement with early image- than delayed image-based method as presented by Bland-Altman plots and intraclass correlation coefficients (early image-based, 0.980; delayed image-based, 0.895). The voxelwise test exhibited a smaller volume of significantly different counts in putamen between brains processed by early image and MRI compared to that between delayed image and MRI.

Conclusion

The early-phase [¹⁸F]FP-CIT PET can be utilized for spatial normalization of delayed PET image when the MRI image is unavailable and presents better performance than the delayed template-based method in quantitation of putaminal binding ratio.

Pharmacokinetic modelling for the simultaneous assessment of perfusion and 18F-flutemetamol uptake in cerebral amyloid angiopathy using a reduced PET-MR acquisition time: Proof of concept

PURPOSE

Cerebral amyloid angiopathy (CAA) is a cerebral small vessel disease associated with perivascular β-amyloid deposition. CAA is also associated with strokes due to lobar intracerebral haemorrhage (ICH). ¹⁸F-flutemetamol amyloid ligand PET may improve the early detection of CAA. We performed pharmacokinetic modelling using both full (0-30, 90-120 min) and reduced (30 min) ¹⁸F-flutemetamol PET-MR acquisitions, to investigate regional cerebral perfusion and amyloid deposition in ICH patients.

METHODS

Dynamic¹⁸F-flutemetamol PET-MR was performed in a pilot cohort of sixteen ICH participants; eight lobar ICH cases with probable CAA and eight deep ICH patients. A model-based input function (mIF) method was developed for compartmental modelling. mIF 1-tissue (1-TC) and 2-tissue (2-TC) compartmental modelling, reference tissue models and standardized uptake value ratios were assessed in the setting of probable CAA detection.

RESULTS

The mIF 1-TC model detected perfusion deficits and ¹⁸F-flutemetamol uptake in cases with probable CAA versus deep ICH patients, in both full and reduced PET acquisition time (all P < 0.05). In the reduced PET acquisition, mIF 1-TC modelling reached the highest sensitivity and specificity in detecting perfusion deficits (0.87, 0.77) and ¹⁸F-flutemetamol uptake (0.83, 0.71) in cases with probable CAA. Overall, 52 and 48 out of the 64 brain areas with ¹⁸F-flutemetamol-determined amyloid deposition showed reduced perfusion for 1-TC and 2-TC models, respectively.

CONCLUSION

Pharmacokinetic (1-TC) modelling using a 30 min PET-MR time frame detected impaired haemodynamics and increased amyloid load in probable CAA. Perfusion deficits and amyloid burden co-existed within cases with CAA, demonstrating a distinct imaging pattern which may have merit in elucidating the pathophysiological process of CAA.

PET Parametric Imaging: Past, Present, and Future

Positron emission tomography (PET) is actively used in a diverse range of applications in oncology, cardiology, and neurology. The use of PET in the clinical setting focuses on static (single time frame) imaging at a specific time-point post radiotracer injection and is typically considered as semi-quantitative; e.g. standardized uptake value (SUV) measures. In contrast, dynamic PET imaging requires increased acquisition times but has the advantage that it measures the full spatiotemporal distribution of a radiotracer and, in combination with tracer kinetic modeling, enables the generation of multiparametric images that more directly quantify underlying biological parameters of interest, such as blood flow, glucose metabolism, and receptor binding. Parametric images have the potential for improved detection and for more accurate and earlier therapeutic response assessment. Parametric imaging with dynamic PET has witnessed extensive research in the past four decades. In this paper, we provide an overview of past and present activities and discuss emerging opportunities in the field of parametric imaging for the future.

Dopamine transporter imaging in neurodegenerative movement disorders: PET vs. SPECT

Purpose

The dopamine transporter (DAT) serves as biomarker for parkinsonian syndromes. DAT can be measured in vivo with single-photon emission computed tomography (SPECT) and positron emission tomography (PET). DAT-SPECT is the current clinical molecular imaging standard. However, PET has advantages over SPECT measurements, and PET radioligands with the necessary properties for clinical applications are on the rise. Therefore, it is time to review the role of DAT imaging with SPECT compared to PET.

Methods

PubMed and Web of Science were searched for relevant literature of the previous 10 years. Four topics for comparison were used: diagnostic accuracy, quantitative accuracy, logistics, and flexibility.

Results

There are a few studies directly comparing DAT-PET and DAT-SPECT. PET and SPECT both perform well in discriminating neurodegenerative from non-neurodegenerative parkinsonism. Clinical DAT-PET imaging seems feasible only recently, thanks to simplified DAT assessments and better availability of PET radioligands and systems. The higher resolution of PET makes more comprehensive assessments of disease progression in the basal ganglia possible. Additionally, it has the possibility of multimodal target assessment.

Conclusion

DAT-SPECT is established for differentiating degenerative from non-degenerative parkinsonism. For further differentiation within neurodegenerative Parkinsonian syndromes, DAT-PET has essential benefits. Nowadays, because of wider availability of PET systems and radioligand production centers, and the possibility to use simplified quantification methods, DAT-PET imaging is feasible for clinical use. Therefore, DAT-PET needs to be considered for a more active role in the clinic to take a step forward to a more comprehensive understanding and assessment of Parkinson’s disease.

Early-phase [18F]PI-2620 tau-PET imaging as a surrogate marker of neuronal injury

Purpose

Second-generation tau radiotracers for use with positron emission tomography (PET) have been developed for visualization of tau deposits in vivo. For several β-amyloid and first-generation tau-PET radiotracers, it has been shown that early-phase images can be used as a surrogate of neuronal injury. Therefore, we investigated the performance of early acquisitions of the novel tau-PET radiotracer [¹⁸F]PI-2620 as a potential substitute for [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG).

Methods

Twenty-six subjects were referred with suspected tauopathies or overlapping parkinsonian syndromes (Alzheimer’s disease, progressive supranuclear palsy, corticobasal syndrome, multi-system atrophy, Parkinson’s disease, multi-system atrophy, Parkinson's disease, frontotemporal dementia) and received a dynamic [¹⁸F]PI-2620 tau-PET (0–60 min p.i.) and static [¹⁸F]FDG-PET (30–50 min p.i.). Regional standardized uptake value ratios of early-phase images (single frame SUVr) and the blood flow estimate (R₁) of [¹⁸F]PI-2620-PET were correlated with corresponding quantification of [¹⁸F]FDG-PET (global mean/cerebellar normalization). Reduced tracer uptake in cortical target regions was also interpreted visually using 3-dimensional stereotactic surface projections by three more and three less experienced readers. Spearman rank correlation coefficients were calculated between early-phase [¹⁸F]PI-2620 tau-PET and [¹⁸F]FDG-PET images for all cortical regions and frequencies of disagreement between images were compared for both more and less experienced readers.

Results

Highest agreement with [¹⁸F]FDG-PET quantification was reached for [¹⁸F]PI-2620-PET acquisition from 0.5 to 2.5 min p.i. for global mean (lowest R = 0.69) and cerebellar scaling (lowest R = 0.63). Correlation coefficients (summed 0.5–2.5 min SUVr & R₁) displayed strong agreement in all cortical target regions for global mean (R_SUVr 0.76, R_R1 = 0.77) and cerebellar normalization (R_SUVr 0.68, R_R1 = 0.68). Visual interpretation revealed high regional correlations between early-phase tau-PET and [¹⁸F]FDG-PET. There were no relevant differences between more and less experienced readers.

Conclusion

Early-phase imaging of [¹⁸F]PI-2620 can serve as a surrogate biomarker for neuronal injury. Dynamic imaging or a dual time-point protocol for tau-PET imaging could supersede additional [¹⁸F]FDG-PET imaging by indexing both the distribution of tau and the extent of neuronal injury.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04788-w) contains supplementary material, which is available to authorized users.

Longitudinal evaluation of surrogates of regional cerebral blood flow computed from dynamic amyloid PET imaging

Surrogates of neuronal activity, typically measured by regional cerebral blood flow (rCBF) or glucose metabolism, can be estimated from dynamic amyloid PET imaging. Using data for 149 participants (345 visits) from the Baltimore Longitudinal Study of Aging, we assessed whether the average of early amyloid frames (EA) and R1 computed from dynamic 11C-Pittsburgh compound B (PiB) PET can serve as surrogates of rCBF computed from 15O-H2O-PET. R1 had the highest longitudinal test-retest reliability. Interquartile range (IQR) of cross-sectional Pearson correlations with rCBF was 0.60-0.72 for EA and 0.63-0.72 for R1. Correlations between rates of change were lower (IQR 0.22-0.50 for EA, 0.25-0.55 for R1). Values in the Alzheimer's metabolic signature meta-ROI were negatively associated with age and exhibited longitudinal declines for each PET measure. In age-adjusted analyses, meta-ROI rCBF and R1 were lower among amyloid+ individuals; EA and R1 were lower among males. Regional PiB-based measures, in particular R1, can be suitable surrogates of rCBF. Dynamic PiB-PET may obviate the need for a separate scan to measure neuronal activity, thereby reducing patient burden, radioactivity exposure, and cost.

Reduced acquisition time PET pharmacokinetic modelling using simultaneous ASL-MRI: proof of concept

Pharmacokinetic modelling on dynamic positron emission tomography (PET) data is a quantitative technique. However, the long acquisition time is prohibitive for routine clinical use. Instead, the semi-quantitative standardised uptake value ratio (SUVR) from a shorter static acquisition is used, despite its sensitivity to blood flow confounding longitudinal analysis. A method has been proposed to reduce the dynamic acquisition time for quantification by incorporating cerebral blood flow (CBF) information from arterial spin labelling (ASL) magnetic resonance imaging (MRI) into the pharmacokinetic modelling. In this work, we optimise and validate this framework for a study of ageing and preclinical Alzheimer's disease. This methodology adapts the simplified reference tissue model (SRTM) for a reduced acquisition time (RT-SRTM) and is applied to [¹⁸F]-florbetapir PET data for amyloid-β quantification. Evaluation shows that the optimised RT-SRTM can achieve amyloid burden estimation from a 30-min PET/MR acquisition which is comparable with the gold standard SRTM applied to 60 min of PET data. Conversely, SUVR showed a significantly higher error and bias, and a statistically significant correlation with tracer delivery due to the influence of blood flow. The optimised RT-SRTM produced amyloid burden estimates which were uncorrelated with tracer delivery indicating its suitability for longitudinal studies.

Usefulness of Dual-Point Amyloid PET Scans in Appropriate Use Criteria: A Multicenter Study

BACKGROUND

Biomarkers of neurodegeneration play a major role in the diagnosis of Alzheimer's disease (AD). Information on both amyloid-β accumulation, e.g., from amyloid positron emission tomography (PET), and downstream neuronal injury, e.g., from 18F-fluorodeoxyglucose (FDG) PET, would ideally be obtained in a single procedure.

OBJECTIVE

On the basis that the parallelism between brain perfusion and glucose metabolism is well documented, the objective of this work is to evaluate whether brain perfusion estimated in a dual-point protocol of 18F-florbetaben (FBB) PET can be a surrogate of FDG PET in appropriate use criteria (AUC) for amyloid PET.

METHODS

This study included 47 patients fulfilling international AUC for amyloid PET. FDG PET, early FBB (pFBB) PET (0-10 min post injection), and standard FBB (sFBB) PET (90-110 min post injection) scans were acquired. Results of clinical subjective reports and of quantitative region of interest (ROI)-based analyses were compared between procedures using statistical techniques such as Pearson's correlation coefficients and t-tests.

RESULTS

pFBB and FDG visual reports on the 47 patients showed good agreement (k  >  0.74); ROI quantitative analysis indicated that both data modalities are highly correlated; and the t-test analysis does not reject the null hypothesis that data from pFBB and FDG examinations comes from independent random samples from normal distributions with equal means and variances.

CONCLUSIONS

A good agreement was found between pFBB and FDG data as obtained by subjective visual and quantitative analyses. Dual-point FBB PET scans could offer complementary information (similar to that from FDG PET and FBB PET) in a single procedure, considering pFBB as a surrogate of FDG.

18F-FDG PET, the early phases and the delivery rate of 18F-AV45 PET as proxies of cerebral blood flow in Alzheimer's disease: Validation against 15O-H2O PET

INTRODUCTION

Dual-biomarker positron emission tomography (PET), providing complementary information on cerebral blood flow and amyloid-β deposition, is of clinical interest for Alzheimer's disease (AD). The purpose of this study was to validate the perfusion components of early-phase ¹⁸F-florbetapir (eAV45), the ¹⁸F-AV45 delivery rate (R1), and ¹⁸F-FDG against ¹⁵O-H₂O PET and assess how they change with disease severity.

METHODS

This study included ten controls, 19 amnestic mild cognitive impairment, and 10 AD dementia subjects. Within-subject regional correlations between modalities, between-group regional and voxel-wise analyses of covariance per modality, and receiver operating characteristic analyses for discrimination between groups were performed.

RESULTS

FDG standardized uptake value ratio, eAV45 (0-2 min) standardized uptake value ratio, and AV45-R1 were significantly associated with H₂O PET (regional Pearson r = 0.54-0.82, 0.70-0.94, and 0.65-0.92, respectively; P < .001). All modalities confirmed reduced cerebral blood flow in the posterior cingulate of patients with amnestic mild cognitive impairment and AD dementia, which was associated with lower cognition (r = 0.36-0.65, P < .025) and could discriminate between patient and control groups (area under the curve > 0.80). However, eAV45 was less sensitive to reflect the disease severity than AV45-R1 or FDG.

DISCUSSION

R1 is preferable over eAV45 for accurate representation of brain perfusion in dual-biomarker PET for AD.

Spatial distributions of cholinergic impairment and neuronal hypometabolism differ in MCI due to AD

Elucidating the relationship between neuronal metabolism and the integrity of the cholinergic system is prerequisite for a profound understanding of cholinergic dysfunction in Alzheimer's disease. The cholinergic system can be investigated specifically using positron emission tomography (PET) with [¹¹C]N-methyl-4-piperidyl-acetate (MP4A), while neuronal metabolism is often assessed with 2-deoxy-2-[¹⁸F]fluoro-d-glucose-(FDG) PET. We hypothesised a close correlation between MP4A-perfusion and FDG-uptake, permitting inferences about metabolism from MP4A-perfusion, and investigated the patterns of neuronal hypometabolism and cholinergic impairment in non-demented AD patients. MP4A-PET was performed in 18 cognitively normal adults and 19 patients with mild cognitive impairment (MCI) and positive AD biomarkers. In nine patients with additional FDG-PET, the sum images of every combination of consecutive early MP4A-frames were correlated with FDG-scans to determine the optimal time window for assessing MP4A-perfusion. Acetylcholinesterase (AChE) activity was estimated using a 3-compartmental model. Group comparisons of MP4A-perfusion and AChE-activity were performed using the entire sample. The highest correlation between MP4A-perfusion and FDG-uptake across the cerebral cortex was observed 60-450 s after injection (r = 0.867). The patterns of hypometabolism (FDG-PET) and hypoperfusion (MP4A-PET) in MCI covered areas known to be hypometabolic early in AD, while AChE activity was mainly reduced in the lateral temporal cortex and the occipital lobe, sparing posterior midline structures. Data indicate that patterns of cholinergic impairment and neuronal hypometabolism differ significantly at the stage of MCI in AD, implying distinct underlying pathologies, and suggesting potential predictors of the response to cholinergic pharmacotherapy.

Synaptic vesicle protein 2A as a potential biomarker in synaptopathies

Measuring synaptic density in vivo using positron emission tomography (PET) imaging-based biomarkers targeting the synaptic vesicle protein 2A (SV2A) has received much attention recently due to its potential research and clinical applications in synaptopathies, including neurodegenerative and psychiatric diseases. Fluid-based biomarkers in proteinopathies have previously been suggested to provide information on pathology and disease status that is complementary to PET-based measures, and the same can be hypothesized with respect to SV2A. This review provides an overview of the current state of SV2A PET imaging as a biomarker of synaptic density, the potential role of fluid-based biomarkers for SV2A, and related future perspectives.

Hybrid PET-MRI in Alzheimer's Disease Research

Multiple factors, namely amyloid, tau, inflammation, metabolic, and perfusion changes, contribute to the cascade of neurodegeneration and functional decline occurring in Alzheimer's disease (AD). These molecular and cellular processes and related functional and morphological changes can be visualized in vivo by two imaging modalities, namely positron emission tomography (PET) and magnetic resonance imaging (MRI). These imaging biomarkers are now part of the diagnostic algorithm and of particular interest for patient stratification and targeted drug development.In this field the availability of hybrid PET/MR systems not only offers a comprehensive evaluation in a single imaging session, but also opens new possibilities for the integration of the two imaging information. Here, we cover the clinical protocols and practical details of FDG, amyloid, and tau PET/MR imaging as applied in our institutions.

Dual-phase [18F]florbetapir in frontotemporal dementia

PURPOSE

The PET tracer [18F]florbetapir is a specific fibrillar amyloid-beta (Aβ) biomarker. During the late scan phase (> 40 min), it provides pathological information about Aβ status. Early scan phase (0-10 min) can provide FDG-'like' information. The current investigation tested the feasibility of using florbetapir as a dual-phase biomarker in behavioural variant frontotemporal dementia (bvFTD).

METHODS

Eight bvFTD patients underwent [18F]florbetapir and [18]FDG-PET scans. Additionally, ten healthy controls and ten AD patients underwent florbetapir-PET only. PET data were acquired dynamically for 60-min post-injection. The bvFTD PET data were used to define an optimal time window, representing blood flow-related pseudo-metabolism ('pseudo-FDG'), of florbetapir data that maximally correlated with the corresponding real FDG SUVR (40-60 min) in a composite neocortical FTD region.

RESULTS

A 2 to 5-min time window post-injection of the florbetapir-PET data provided the largest correlation (Pearson's r = 0.79, p = 0.02) to the FDG data. The pseudo-FDG images demonstrated strong internal consistency with actual FDG data and were also visually consistent with the bvFTD patients' hypometabolic profiles. The ability to identify bvFTD from blind visual rating of pseudo-FDG images was consistent with previous reports using FDG data (sensitivity = 75%, specificity = 85%).

CONCLUSIONS

This investigation demonstrates that early phase florbetapir uptake shows a reduction of frontal lobe perfusion in bvFTD, similar to metabolic findings with FDG. Thus, dynamic florbetapir scans can serve as a dual-phase biomarker in dementia patients to distinguish FTD from AD and cognitively normal elderly, removing the need for a separate FDG-PET scan in challenging dementia cases.

Improved beta-amyloid PET reproducibility using two-phase acquisition and grey matter delineation

PURPOSE

We investigated whether the reproducibility of standard visual reporting (STD method) in flutemetamol (FMM) PET can be improved using a newly introduced method that uses grey matter edges derived from the perfusion phase (GM-EDGE method).

METHODS

Two-phase FMM PET was performed in 121 patients with mild cognitive impairment. Five nuclear medicine physicians blindly and independently evaluated all late-phase scans, initially employing the STD method and later the GM-EDGE method. A five-point scale was used to express the degree of amyloid positivity, and a binary classification (positive/negative) was used in combination with subjective confidence (five-point scale). Multirater Fleiss' kappa, intraclass correlation coefficient (ICC) and inter-rater reliability (Cohen's kappa) were determined for the STD and GM-EDGE methods.

RESULTS

The weighted Cohen's kappa values for the five-point measure of amyloid positivity ranged from 0.63 to 0.73 (median 0.70) for the STD method and from 0.76 to 0.89 (median 0.80) for the GM-EDGE method (ICC 0.84, 95% CI 0.79-0.88, for the STD method; 0.91, 95% CI 0.89-0.94, for the GM-EDGE method). The nonweighted Cohen's kappa value for the binary classification ranged from 0.73 to 0.93 (median 0.82) for the STD method and 0.90 to 0.97 (median 0.93) for the GM-EDGE method (Fleiss' kappa 0.82, 95% CI 0.77-0.88, for the STD method; 0.93, 95% CI 0.87-0.99, for the GM-EDGE method). The GM-EDGE method resulted in significantly greater subjective confidence in the readings of four physicians (p < 0.010). The binary classification was concordant among all five physicians in 80.8% of the scans using the STD method and in 91.6% of the scans using the GM-EDGE method (p = 0.016).

CONCLUSION

The newly introduced GM-EDGE method was associated with significantly higher inter-rater agreement among physicians and higher subjective confidence in the reading. The method is easy to implement in clinical practice, especially when the perfusion phase is utilized clinically.

Relative cerebral flow from dynamic PIB scans as an alternative for FDG scans in Alzheimer's disease PET studies

In Alzheimer's Disease (AD) dual-tracer positron emission tomography (PET) studies with 2-[18F]-fluoro-2-deoxy-D-glucose (FDG) and 11C-labelled Pittsburgh Compound B (PIB) are used to assess metabolism and cerebral amyloid-β deposition, respectively. Regional cerebral metabolism and blood flow (rCBF) are closely coupled, both providing an index for neuronal function. The present study compared PIB-derived rCBF, estimated by the ratio of tracer influx in target regions relative to reference region (R1) and early-stage PIB uptake (ePIB), to FDG scans. Fifteen PIB positive (+) patients and fifteen PIB negative (-) subjects underwent both FDG and PIB PET scans to assess the use of R1 and ePIB as a surrogate for FDG. First, subjects were classified based on visual inspection of the PIB PET images. Then, discriminative performance (PIB+ versus PIB-) of rCBF methods were compared to normalized regional FDG uptake. Strong positive correlations were found between analyses, suggesting that PIB-derived rCBF provides information that is closely related to what can be seen on FDG scans. Yet group related differences between method's distributions were seen as well. Also, a better correlation with FDG was found for R1 than for ePIB. Further studies are needed to validate the use of R1 as an alternative for FDG studies in clinical applications.

Secondary prevention of Alzheimer's dementia: neuroimaging contributions

BACKGROUND

In Alzheimer's disease (AD), pathological changes may arise up to 20 years before the onset of dementia. This pre-dementia window provides a unique opportunity for secondary prevention. However, exposing non-demented subjects to putative therapies requires reliable biomarkers for subject selection, stratification, and monitoring of treatment. Neuroimaging allows the detection of early pathological changes, and longitudinal imaging can assess the effect of interventions on markers of molecular pathology and rates of neurodegeneration. This is of particular importance in pre-dementia AD trials, where clinical outcomes have a limited ability to detect treatment effects within the typical time frame of a clinical trial. We review available evidence for the use of neuroimaging in clinical trials in pre-dementia AD. We appraise currently available imaging markers for subject selection, stratification, outcome measures, and safety in the context of such populations.

MAIN BODY

Amyloid positron emission tomography (PET) is a validated in-vivo marker of fibrillar amyloid plaques. It is appropriate for inclusion in trials targeting the amyloid pathway, as well as to monitor treatment target engagement. Amyloid PET, however, has limited ability to stage the disease and does not perform well as a prognostic marker within the time frame of a pre-dementia AD trial. Structural magnetic resonance imaging (MRI), providing markers of neurodegeneration, can improve the identification of subjects at risk of imminent decline and hence play a role in subject inclusion. Atrophy rates (either hippocampal or whole brain), which can be reliably derived from structural MRI, are useful in tracking disease progression and have the potential to serve as outcome measures. MRI can also be used to assess comorbid vascular pathology and define homogeneous groups for inclusion or for subject stratification. Finally, MRI also plays an important role in trial safety monitoring, particularly the identification of amyloid-related imaging abnormalities (ARIA). Tau PET to measure neurofibrillary tangle burden is currently under development. Evidence to support the use of advanced MRI markers such as resting-state functional MRI, arterial spin labelling, and diffusion tensor imaging in pre-dementia AD is preliminary and requires further validation.

CONCLUSION

We propose a strategy for longitudinal imaging to track early signs of AD including quantitative amyloid PET and yearly multiparametric MRI.

Quantitative study of 18F-(+)DTBZ image: comparison of PET template-based and MRI based image analysis

[¹⁸F]9-fluoropropyl-(+)-dihydrotetrabenazine (¹⁸F-(+)DTBZ) is a recently developed PET tracer to investigate the vesicular monoamine transporter type 2 (VMAT2) activity in measuring dopaminergic degeneration in vivo and monitoring the severity of Parkinson’s disease (PD). However, manual drawing of the striatal regions is time consuming and prone to human bias. In the current study, we developed an automated method to quantify the signals of the striatum on ¹⁸F-(+)DTBZ images. 39 patients with PD and 26 controls were enrolled. Traditional brain magnetic resonance imaging (MRI) and ¹⁸F-(+)DTBZ PET were acquired. Both indirect normalization of native PET images to the standard space through individual brain MRI and directly coregistration of native images to the transporter-specific PET template in standard space were performed. Specific uptake ratios (SURs) in 10 predefined regions were used as indicators of VMAT2 activities to correlate with motor severity. Our results showed patients with PD had significant lower SURs in the bilateral putamina, caudates and globus pallidi than controls. SURs in the caudate and putamen were significantly correlated with motor severity. The contralateral putaminal region performed best in discriminating between PD patients and controls. Finally, the results from the application of the ¹⁸F-(+)DTBZ PET template were comparable to those derived from the traditional MRI based method. Thus, ¹⁸F-(+)DTBZ PET imaging holds the potential to effectively differentiate PD patients from controls. The ¹⁸F-(+)DTBZ PET template-based method for automated quantification of presynaptic VMAT2 transporter density is easier to implement and may facilitate efficient, robust and user-independent image analysis.

MR approaches in neurodegenerative disorders

Neurodegenerative disease is the umbrella term which refers to a range of clinical conditions causing degeneration of neurons within the central nervous system leading to loss of brain function and eventual death. The most prevalent of these is Alzheimer's disease (AD), which affects approximately 50 million people worldwide and is predicted to reach 75 million by 2030. Neurodegenerative diseases can only be fully diagnosed at post mortem by neuropathological assessment of the type and distribution of protein deposits which characterise each different condition, but there is a clear role for imaging technologies in aiding patient diagnoses in life. Magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques have been applied to study these conditions for many years. In this review, we consider the range of MR-based measurements and describe the findings in AD, but also contrast these with the second most common dementia, dementia with Lewy bodies (DLB). The most definitive observation is the major structural brain changes seen in AD using conventional T1-weighted (T1w) MRI, where medial temporal lobe structures are notably atrophied in most symptomatic patients with AD, but often preserved in DLB. Indeed these findings are sufficiently robust to have been incorporated into clinical diagnostic criteria. Diffusion tensor imaging (DTI) reveals widespread changes in tissue microstructure, with increased mean diffusivity and decreased fractional anisotropy reflecting the degeneration of the white matter structures. There are suggestions that there are subtle differences between AD and DLB populations. At the metabolic level, atrophy-corrected MRS demonstrates reduced density of healthy neurons in brain areas with altered perfusion and in regions known to show higher deposits of pathogenic proteins. As studies have moved from patients with advanced disease and clear dysfunction to patients with earlier presentation such as with mild cognitive impairment (MCI), which in some represents the first signs of their ensuing dementia, the ability of MRI to detect differences has been weaker and further work is still required, ideally in much larger cohorts than previously studied. The vast majority of imaging research in dementia populations has been univariate with respect to the MR-derived parameters considered. To date, none of these measurements has uniquely replicated the patterns of tissue involvement seen by neuropathology, and the ability of MR techniques to deliver a non-invasive diagnosis eludes us. Future opportunities may lie in combining MR and nuclear medicine approaches (position emission tomography, PET) to provide a more complete view of structural and metabolic changes. Such developments will require multi-variate analyses, possibly combined with artificial intelligence or deep learning algorithms, to enhance our ability to combine the array of image-derived information, genetic, gender and lifestyle factors.

Utility of perfusion PET measures to assess neuronal injury in Alzheimer's disease

INTRODUCTION

18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is commonly used to estimate neuronal injury in Alzheimer's disease (AD). Here, we evaluate the utility of dynamic PET measures of perfusion using 11C-Pittsburgh compound B (PiB) to estimate neuronal injury in comparison to FDG PET.

METHODS

FDG, early frames of PiB images, and relative PiB delivery rate constants (PiB-R1) were obtained from 110 participants from the Dominantly Inherited Alzheimer Network. Voxelwise, regional cross-sectional, and longitudinal analyses were done to evaluate the correlation between images and estimate the relationship of the imaging biomarkers with estimated time to disease progression based on family history.

RESULTS

Metabolism and perfusion images were spatially correlated. Regional PiB-R1 values and FDG, but not early frames of PiB images, significantly decreased in the mutation carriers with estimated year to onset and with increasing dementia severity.

DISCUSSION

Hypometabolism estimated by PiB-R1 may provide a measure of brain perfusion without increasing radiation exposure.