11C-PiB PET studies in typical sporadic Creutzfeldt-Jakob disease

Multimodality imaging of neurodegenerative disorders with a focus on multiparametric magnetic resonance and molecular imaging

Neurodegenerative diseases afflict a large number of persons worldwide, with the prevalence and incidence of dementia rapidly increasing. Despite their prevalence, clinical diagnosis of dementia syndromes remains imperfect with limited specificity. Conventional structural-based imaging techniques also lack the accuracy necessary for confident diagnosis. Multiparametric magnetic resonance imaging and molecular imaging provide the promise of improving specificity and sensitivity in the diagnosis of neurodegenerative disease as well as therapeutic monitoring of monoclonal antibody therapy. This educational review will briefly focus on the epidemiology, clinical presentation, and pathologic findings of common and uncommon neurodegenerative diseases. Imaging features of each disease spanning from conventional magnetic resonance sequences to advanced multiparametric methods such as resting-state functional magnetic resonance imaging and arterial spin labeling imaging will be described in detail. Additionally, the review will explore the findings of each diagnosis on molecular imaging including single-photon emission computed tomography and positron emission tomography with a variety of clinically used and experimental radiotracers. The literature and clinical cases provided demonstrate the power of advanced magnetic resonance imaging and molecular techniques in the diagnosis of neurodegenerative diseases and areas of future and ongoing research. With the advent of combined positron emission tomography/magnetic resonance imaging scanners, hybrid protocols utilizing both techniques are an attractive option for improving the evaluation of neurodegenerative diseases.

Molecular Imaging Approaches in Dementia

The increasing prevalence of dementia worldwide places a high demand on healthcare providers to perform a diagnostic work-up in relatively early stages of the disease, given that the pathologic process usually begins decades before symptoms are evident. Structural imaging is recommended to rule out other disorders and can only provide diagnosis in a late stage with limited specificity. Where PET imaging previously focused on the spatial pattern of hypometabolism, the past decade has seen the development of novel tracers to demonstrate characteristic protein abnormalities. Molecular imaging using PET/SPECT is able to show amyloid and tau deposition in Alzheimer disease and dopamine depletion in parkinsonian disorders starting decades before symptom onset. Novel tracers for neuroinflammation and synaptic density are being developed to further unravel the molecular pathologic characteristics of dementia disorders. In this article, the authors review the current status of established and emerging PET tracers in a diagnostic setting and also their value as prognostic markers in research studies and outcome measures for clinical trials in Alzheimer disease.

Advances in Brain Amyloid Imaging

Amyloid-β (Aβ) PET imaging has now been available for over 15 years. The ability to detect Aβ in vivo has greatly improved the clinical and research landscape of Alzheimer's disease (AD) and other neurodegenerative conditions. Aβ imaging provides very reliable, accurate, and reproducible measurements of regional and global Aβ burden in the brain. It has proved invaluable in anti-Aβ therapy trials, and is now recognized as a powerful diagnostic tool. The appropriate use of Aβ PET, when combined with comprehensive clinical evaluation by a dementia-trained specialist, can improve the accuracy of a clinical diagnosis of AD and substantially alter management. It can assist in differentiating AD from other neurodegenerative conditions, often by its ability to rule out the presence of Aβ. When combined with tau imaging, further increase in specificity for the diagnosis of AD can be achieved. The integration of Aβ PET, in conjunction with biomarkers of tau, neurodegeneration and neuroinflammation, into large, longitudinal, observational cohort studies continues to increase our understanding of the development of AD. Its incorporation into clinical trials has been pivotal in defining the most effective anti-Aβ biological therapies and optimal dosing so that effective disease modifying therapy now appears imminent. Aβ deposition is a gradual and protracted process, permitting a wide treatment window for anti-Aβ therapies and Aβ PET has made trials in this preclinical AD period feasible. Continuing improvement in Aβ tracer target to background ratio is allowing trials in earlier AD that tailor drug dosage to Aβ level. The quest to standardize quantification and define universally applicable thresholds for all Aβ tracers has produced the Centiloid method. Centiloid values that correlate well with neuropathologic findings and prognosis have been identified. Rapid cloud-based automated individual scan analysis is now possible and does not require MRI. Challenges remain, particularly around cross camera standardized uptake value ratio variation that need to be addressed. This review will compare available Aβ radiotracers, discuss approaches to quantification, as well as the clinical and research applications of Aβ PET.

A new perspective for advanced positron emission tomography-based molecular imaging in neurodegenerative proteinopathies

Recent studies in neurodegenerative conditions have increasingly highlighted that the same neuropathology can trigger different clinical phenotypes or, vice-versa, that similar phenotypes can be triggered by different neuropathologies. This evidence has called for the adoption of a pathology spectrum-based approach to study neurodegenerative proteinopathies. These conditions share brain deposition of abnormal protein aggregates, leading to aberrant biochemical, metabolic, functional, and structural changes. Positron emission tomography (PET) is a well-recognized and unique tool for the in vivo assessment of brain neuropathology, and novel PET techniques are emerging for the study of specific protein species. Today, key applications of PET range from early research and clinical diagnostic tools to their use in clinical trials for both participants screening and outcome evaluation. This position article critically reviews the role of distinct PET molecular tracers for different neurodegenerative proteinopathies, highlighting their strengths, weaknesses, and opportunities, with special emphasis on methodological challenges and future applications.

Amyloid- and FDG-PET in sporadic Creutzfeldt-Jakob disease: Correlation with pathological prion protein in neuropathology

INTRODUCTION

The role of positron emission tomography (PET) in Creutzfeldt-Jakob disease is less defined than in other neurodegenerative diseases. We studied the correlation between the uptake of ¹⁸F-florbetaben and ¹⁸F-fluorodeoxyglucose with pathological prion protein deposition in histopathology in a case.

METHODS

A patient with 80 y old with a rapid neurological deterioration with a confirmed diagnosis of CJD was studied. PET and MRI studies were performed between 13-20 d before the death. A region of interest analysis was performed using Statistical Parametric Mapping.

RESULTS

MRI showed atrophy with no other alterations. FDG-PET showed extensive areas of hypometabolism including left frontoparietal lobes as well as bilateral thalamus. Correlation between uptake of ¹⁸F-florbetaben and pathological prion protein deposition was r = 0.786 (p < 0.05). Otherwise, correlation between uptake of ¹⁸F-FDG and pathological prion protein was r = 0.357 (p = 0.385). Immunohistochemistry with β-amyloid did not show amyloid deposition or neuritic plaques.

CONCLUSIONS

Our study supports the use of FDG-PET in the assessment of CJD. FDG-PET may be especially useful in cases of suspected CJD and negative MRI. Furthermore, this case report provides more evidence about the behavioral of amyloid tracers, and the possibility of a low-affinity binding to other non-amyloid proteins, such as the pathological prion protein, is discussed.

Post-mortem histopathology underlying β-amyloid PET imaging following flutemetamol F 18 injection

In vivo imaging of fibrillar β-amyloid deposits may assist clinical diagnosis of Alzheimer's disease (AD), aid treatment selection for patients, assist clinical trials of therapeutic drugs through subject selection, and be used as an outcome measure. A recent phase III trial of [¹⁸F]flutemetamol positron emission tomography (PET) imaging in 106 end-of-life subjects demonstrated the ability to identify fibrillar β-amyloid by comparing in vivo PET to post-mortem histopathology. Post-mortem analyses demonstrated a broad and continuous spectrum of β-amyloid pathology in AD and other dementing and non-dementing disease groups. The GE067-026 trial demonstrated 91% sensitivity and 90% specificity of [¹⁸F]flutemetamol PET by majority read for the presence of moderate or frequent plaques. The probability of an abnormal [¹⁸F]flutemetamol scan increased with neocortical plaque density and AD diagnosis. All dementia cases with non-AD neurodegenerative diseases and those without histopathological features of β-amyloid deposits were [¹⁸F]flutemetamol negative. Majority PET assessments accurately reflected the amyloid plaque burden in 90% of cases. However, ten cases demonstrated a mismatch between PET image interpretations and post-mortem findings. Although tracer retention was best associated with amyloid in neuritic plaques, amyloid in diffuse plaques and cerebral amyloid angiopathy best explain three [¹⁸F]flutemetamol positive cases with mismatched (sparse) neuritic plaque burden. Advanced cortical atrophy was associated with the seven false negative [¹⁸F]flutemetamol images. The interpretation of images from pathologically equivocal cases was associated with low reader confidence and inter-reader agreement. Our results support that amyloid in neuritic plaque burden is the primary form of β-amyloid pathology detectable with [¹⁸F]flutemetamol PET imaging. ClinicalTrials.gov NCT01165554. Registered June 21, 2010; NCT02090855. Registered March 11, 2014.

Clinical Amyloid Imaging

Amyloid plaques, along with neurofibrillary tangles, are a neuropathologic hallmark of Alzheimer disease (AD). Recently, amyloid PET radiotracers have been developed and approved for clinical use in the evaluation of suspected neurodegenerative disorders. In both research and clinical settings, amyloid PET imaging has provided important diagnostic and prognostic information for the management of patients with possible AD, mild cognitive impairment (MCI), and other challenging diagnostic presentations. Although the overall impact of amyloid imaging is still being evaluated, the Society of Nuclear Medicine and Molecular Imaging and Alzheimer's Association Amyloid Imaging Task Force have created appropriate use criteria for the standard clinical use of amyloid PET imaging. By the appropriate use criteria, amyloid imaging is appropriate for patients with (1) persistent or unexplained MCI, (2) AD as a possible but still uncertain diagnosis after expert evaluation and (3) atypically early-age-onset progressive dementia. To better understand the clinical and economic effect of amyloid imaging, the Imaging Dementia-Evidence for Amyloid Scanning (IDEAS) study is an ongoing large multicenter study in the United States, which is evaluating how amyloid imaging affects diagnosis, management, and outcomes for cognitively impaired patients who cannot be completely evaluated by clinical assessment alone. Multiple other large-scale studies are evaluating the prognostic role of amyloid PET imaging for predicting MCI progression to AD in general and high-risk populations. At the same time, amyloid imaging is an important tool for evaluating potential disease-modifying therapies for AD. Overall, the increased use of amyloid PET imaging has led to a better understanding of the strengths and limitations of this imaging modality and how it may best be used with other clinical, molecular, and imaging assessment techniques for the diagnosis and management of neurodegenerative disorders.

Amyloid imaging: Past, present and future perspectives

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by the gradual onset of dementia. The pathological hallmarks of the disease are Aβ amyloid plaques, and tau neurofibrillary tangles, along dendritic and synaptic loss and reactive gliosis. Functional and molecular neuroimaging techniques such as positron emission tomography (PET) using functional and molecular tracers, in conjuction with other Aβ and tau biomarkers in CSF, are proving valuable in the differential diagnosis of AD, as well as in establishing disease prognosis. With the advent of new therapeutic strategies, there has been an increasing application of these techniques for the determination of Aβ burden in vivo in the patient selection, evaluation of target engagement and assessment of the efficacy of therapeutic approaches aimed at reducing Aβ in the brain.

Neuroimaging biomarkers in Alzheimer's disease and other dementias

In vivo imaging of β-amyloid (Aβ) has transformed the assessment of Aβ pathology and its changes over time, extending our insight into Aβ deposition in the brain by providing highly accurate, reliable, and reproducible quantitative statements of regional or global Aβ burden in the brain. This knowledge is essential for therapeutic trial recruitment and for the evaluation of anti-Aβ treatments. Although cross sectional evaluation of Aβ burden does not strongly correlate with cognitive impairment, it does correlate with cognitive (especially memory) decline and with a higher risk for conversion to AD in the aging population and MCI subjects. This suggests that Aβ deposition is a protracted pathological process starting well before the onset of symptoms. Longitudinal observations, coupled with different disease-specific biomarkers to assess potential downstream effects of Aβ are required to confirm this hypothesis and further elucidate the role of Aβ deposition in the course of Alzheimer's disease.

Multimodal imaging in Alzheimer's disease: validity and usefulness for early detection

Alzheimer's disease is a progressive neurodegenerative disease that typically manifests clinically as an isolated amnestic deficit that progresses to a characteristic dementia syndrome. Advances in neuroimaging research have enabled mapping of diverse molecular, functional, and structural aspects of Alzheimer's disease pathology in ever increasing temporal and regional detail. Accumulating evidence suggests that distinct types of imaging abnormalities related to Alzheimer's disease follow a consistent trajectory during pathogenesis of the disease, and that the first changes can be detected years before the disease manifests clinically. These findings have fuelled clinical interest in the use of specific imaging markers for Alzheimer's disease to predict future development of dementia in patients who are at risk. The potential clinical usefulness of single or multimodal imaging markers is being investigated in selected patient samples from clinical expert centres, but additional research is needed before these promising imaging markers can be successfully translated from research into clinical practice in routine care.

Clinical, FDG and amyloid PET imaging in posterior cortical atrophy

The purpose of this study was to identify the clinical, [¹⁸F]-fluorodeoxyglucose positron emission tomography (FDG-PET) and amyloid-PET findings in a large cohort of posterior cortical atrophy (PCA) patients, to examine the neural correlates of the classic features of PCA, and to better understand the features associated with early PCA. We prospectively recruited 25 patients who presented to the Mayo Clinic between March 2013 and August 2014 and met diagnostic criteria for PCA. All patients underwent a standardized set of tests and amyloid imaging with [¹¹C] Pittsburg compound B (PiB). Seventeen (68 %) underwent FDG-PET scanning. We divided the cohort at the median disease duration of 4 years in order to assess clinical and FDG-PET correlates of early PCA (n = 13). The most common clinical features were simultanagnosia (92 %), dysgraphia (68 %), poly-mini-myoclonus (64 %) and oculomotor apraxia (56.5 %). On FDG-PET, hypometabolism was observed bilaterally in the lateral and medial parietal and occipital lobes. Simultanagnosia was associated with hypometabolism in the right occipital lobe and posterior cingulum, optic ataxia with hypometabolism in left occipital lobe, and oculomotor apraxia with hypometabolism in the left parietal lobe and posterior cingulate gyrus. All 25 PCA patients were amyloid positive. Simultanagnosia was the only feature present in 85 % of early PCA patients. The syndrome of PCA is associated with posterior hemisphere hypometabolism and with amyloid deposition. Many of the classic features of PCA show associated focal, but not widespread, areas of involvement of these posterior hemispheric regions. Simultanagnosia appears to be the most common and hence sensitive feature of early PCA.

Amyloid biomarkers in Alzheimer's disease

Aggregation of amyloid-β (Aβ) into oligomers, fibrils, and plaques is central in the molecular pathogenesis of Alzheimer's disease (AD), and is the main focus of AD drug development. Biomarkers to monitor Aβ metabolism and aggregation directly in patients are important for further detailed study of the involvement of Aβ in disease pathogenesis and to monitor the biochemical effect of drugs targeting Aβ in clinical trials. Furthermore, if anti-Aβ disease-modifying drugs prove to be effective clinically, amyloid biomarkers will be of special value in the clinic to identify patients with brain amyloid deposition at risk for progression to AD dementia, to enable initiation of treatment before neurodegeneration is too severe, and to monitor drug effects on Aβ metabolism or pathology to guide dosage. Two types of amyloid biomarker have been developed: Aβ-binding ligands for use in positron emission tomography (PET) and assays to measure Aβ42 in cerebrospinal fluid (CSF). In this review, we present the rationales behind these biomarkers and compare their ability to measure Aβ plaque load in the brain. We also review possible shortcomings and the need of standardization of both biomarkers, as well as their implementation in the clinic.

Amyloid-β positron emission tomography imaging probes: a critical review

The rapidly rising prevalence and cost of Alzheimer's disease in recent decades has made the imaging of amyloid-β deposits the focus of intense research. Several amyloid imaging probes with purported specificity for amyloid-β plaques are currently at various stages of FDA approval. However, a number of factors appear to preclude these probes from clinical utilization. As the available "amyloid specific" positron emission tomography imaging probes have failed to demonstrate diagnostic value and have shown limited utility for monitoring therapeutic interventions in humans, a debate on their significance has emerged. The aim of this review is to identify and discuss critically the scientific issues contributing to the extensive inconsistencies reported in the literature on their purported in vivo amyloid specificity and potential utilization in patients.

Cerebral amyloid PET imaging in Alzheimer's disease

The devastating effects of the still incurable Alzheimer's disease (AD) project an ever increasing shadow of burden on the health care system and society in general. In this ominous context, amyloid (Aβ) imaging is considered by many of utmost importance for progress towards earlier AD diagnosis and for potential development of effective therapeutic interventions. Amyloid imaging positron emission tomography procedures offer the opportunity for accurate mapping and quantification of amyloid-Aβ neuroaggregate deposition in the living brain of AD patients. This review analyzes the perceived value of current Aβ imaging probes and their clinical utilization and, based on amyloid imaging results, offers a hypothesis on the effects of amyloid deposition on the biology of AD and its progression. It also analyzes lingering questions permeating the field of amyloid imaging on the apparent contradictions between imaging results and known neuropathology brain regional deposition of Aβ aggregates. As a result, the review also discusses literature evidence as to whether brain Aβ deposition is truly visualized and measured with these amyloid imaging agents, which would have significant implications in the understanding of the biological AD cascade and in the monitoring of therapeutic interventions with these surrogate Aβ markers.

Amyloid PET Imaging: MCI and AD

PET with "β-amyloid-specific" molecular imaging probes is proposed for the measurement of brain β-amyloid protein amyloidosis in the new guidelines for diagnosis of Alzheimer disease (AD) at different levels of disease progression. This article discusses limitations of this proposed use pointing to unresolved issues and inconsistencies between PET scan results and correlation with other biomarkers, and with postmortem histopathological studies. These unresolved issues do not warrant the conclusion that PET imaging with "β-amyloid-specific" molecular imaging probes can be used as a biomarker in AD or in the various stages of disease progression.

Appropriate use criteria for amyloid PET: a report of the Amyloid Imaging Task Force, the Society of Nuclear Medicine and Molecular Imaging, and the Alzheimer's Association

Positron emission tomography (PET) of brain amyloid b is a technology that is becoming more available, but its clinical utility in medical practice requires careful definition. To provide guidance to dementia care practitioners, patients, and caregivers, the Alzheimer's Association and the Society of Nuclear Medicine and Molecular Imaging convened the Amyloid Imaging Taskforce (AIT). The AIT considered a broad range of specific clinical scenarios in which amyloid PET could potentially be used appropriately. Peer-reviewed, published literature was searched to ascertain available evidence relevant to these scenarios, and the AIT developed a consensus of expert opinion. Although empirical evidence of impact on clinical outcomes is not yet available, a set of specific appropriate use criteria (AUC) were agreed on that define the types of patients and clinical circumstances in which amyloid PET could be used. Both appropriate and inappropriate uses were considered and formulated,and are reported and discussed here. Because both dementia care and amyloid PET technology are in active development, these AUC will require periodic reassessment. Future research directions are also outlined, including diagnostic utility and patient-centered outcomes.

Synthesis of an (11) C-labeled antiprion GN8 derivative and evaluation of its brain uptake by positron emission tomography

A radiolabeled PET! A (11) C-labeled derivative of N,N'-(methylenedi-4,1-phenylene)bis[2-(1-pyrrolidinyl) acetamide] (GN8), an antiprion agent currently under development, was synthesized by palladium-catalyzed rapid methylation of aryltributylstannane and assessed for brain penetration and organ distribution in rats by positron emission tomography (PET).

Amyloid imaging with PET in early Alzheimer disease diagnosis

In vivo imaging of amyloid-β (Aβ) with positron emission tomography has moved from the research arena into clinical practice. Clinicians working with cognitive decline and dementia must become familiar with its benefits and limitations. Amyloid imaging allows earlier diagnosis of Alzheimer disease and better differential diagnosis of dementia and provides prognostic information for mild cognitive impairment. It also has an increasingly important role in therapeutic trial recruitment and for evaluation of anti-Aβ treatments. Longitudinal observations are required to elucidate the role of Aβ deposition in the course of Alzheimer disease and provide information needed to fully use the prognostic power of this investigation.

Brain imaging in Alzheimer disease

Imaging has played a variety of roles in the study of Alzheimer disease (AD) over the past four decades. Initially, computed tomography (CT) and then magnetic resonance imaging (MRI) were used diagnostically to rule out other causes of dementia. More recently, a variety of imaging modalities including structural and functional MRI and positron emission tomography (PET) studies of cerebral metabolism with fluoro-deoxy-d-glucose (FDG) and amyloid tracers such as Pittsburgh Compound-B (PiB) have shown characteristic changes in the brains of patients with AD, and in prodromal and even presymptomatic states that can help rule-in the AD pathophysiological process. No one imaging modality can serve all purposes as each have unique strengths and weaknesses. These modalities and their particular utilities are discussed in this article. The challenge for the future will be to combine imaging biomarkers to most efficiently facilitate diagnosis, disease staging, and, most importantly, development of effective disease-modifying therapies.

Noninvasive Detection of Misfolded Proteins in the Brain Using [11C]BF-227 PET

Alzheimer’s disease (AD) and many other neurodegenerative disorders belong to the family of protein misfolding diseases. These diseases are characterized by the deposition of insoluble protein aggregates containing an enriched ß-sheet structure. To evaluate PET amyloid-imaging tracer [11C]BF-227 as an agent for in vivo detection of various kinds of misfolded protein, a [11C]BF-227 PET study was performed in patients with various protein misfolding diseases, including AD, frontotemporal dementia (FTD), dementia with Lewy bodies (DLB), sporadic Creutzfeldt-Jakob disease (sCJD) and Gerstmann-Sträussler-Scheinker disease (GSS). BF-227 binds to ß-amyloid fibrils with high affinity. Most of the AD patients showed prominent retention of [11C]BF-227 in the neocortex. In addition, neocortical retention of BF-227 was observed in the subjects with mild cognitive impairment who converted to AD during follow-up. DLB patients had elevated [11C]BF-227 uptake in the neocortex. However, FTD and sCJD patients showed no cortical retention of [11C]BF-227. Patients with multiple system atrophy had elevated BF-227 binding in the putamen. Finally, GSS patients had elevated BF-227 uptake in the cerebellum and other brain regions. This chapter confirms that BF-227 can selectively bind to a-synuclein and prion protein deposits using postmortem brain samples. Based on these findings, [11C]BF-227 is not necessarily specific for ß-amyloid in AD patients. However, this tracer could be used to detect various types of protein aggregates in the brain.

Using Pittsburgh Compound B for in vivo PET imaging of fibrillar amyloid-beta

The development of Aβ-PET imaging agents has allowed for detection of fibrillar Aβ deposition in vivo and marks a major advancement in understanding the role of Aβ in Alzheimer's disease (AD). Imaging Aβ thus has many potential clinical benefits: early or perhaps preclinical detection of disease and accurately distinguishing AD from dementias of other non-Aβ causes in patients presenting with mild or atypical symptoms or confounding comorbidities (in which the distinction is difficult to make clinically). From a research perspective, imaging Aβ allows us to study relationships between amyloid pathology and changes in cognition, brain structure, and function across the continuum from normal aging to mild cognitive impairment (MCI) to AD; and to monitor the effectiveness of anti-Aβ drugs and relate them to neurodegeneration and clinical symptoms. Here, we will discuss the application of one of the most broadly studied and widely used Aβ imaging agents, Pittsburgh Compound-B (PiB).

Disclosure of amyloid imaging results to research participants: has the time come?

Amyloid imaging is becoming an increasingly popular tool in clinical research on Alzheimer's disease. In early studies, questions of whether to disclose amyloid imaging results were largely mooted by the immature state of the science. Lack of clarity as to what constituted a positive scan and what a positive scan meant from a prognostic or diagnostic perspective required investigators to advise research participants that, because their scans could not be meaningfully interpreted in a clinical sense, individual research scan results would not be disclosed. With a focus on the most widely used research tracer, Pittsburgh Compound B (PiB), we consider how advances in amyloid imaging are raising new questions about the appropriateness of withholding research results from study participants. We conclude that, although it remains advisable to withhold amyloid scan results from cognitively normal participants, it is no longer reasonable to uphold policies that unilaterally advise research participants with documented cognitive impairment that their PiB scans are uninterpretable. We outline circumstances that we believe compel investigators to provide research participants with the option of receiving their PiB scan results in a carefully managed fashion. Our findings can potentially be generalized to research involving all validated amyloid tracers.

Correspondence between in vivo (11)C-PiB-PET amyloid imaging and postmortem, region-matched assessment of plaques

The definitive Alzheimer's disease (AD) diagnosis requires postmortem confirmation of neuropathological hallmarks-amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs). The advent of radiotracers for amyloid imaging presents an opportunity to investigate amyloid deposition in vivo. The (11)C-Pittsburgh compound-B (PiB)-PET ligand remains the most widely studied to date; however, regional variations in (11)C-PiB binding and the extent of agreement with neuropathological assessment have not been thoroughly investigated. Sojkova and colleagues [35] reported variable agreement between CERAD-based neuropathologic diagnosis of AD lesions and mean cortical PiB, suggesting the need for a more direct quantification of regional Aβ in relation to in vivo imaging. In the present study, we extend these findings by examining the correspondence among regional (11)C-PiB load, region-matched quantitative immunohistological assessments of Aβ and NFTs, and brain atrophy (MRI) in six older Baltimore Longitudinal Study of Aging participants who came to autopsy (imaging-autopsy interval range 0.2-2.4 years). The total number of Aβ plaques (6E10) and NFTs (PHF1) in paraffin sections from hippocampus, orbito-frontal cortex, anterior and posterior cingulate gyrus, precuneus and cerebellum was quantified using a technique guided by unbiased stereological principles. We report a general agreement between the regional measures of amyloid obtained via stereological assessment and imaging, with significant relationships evident for the anterior (r = 0.83; p = 0.04) and posterior (r = 0.94; p = 0.005) cingulate gyri, and the precuneus (r = 0.94; p = 0.005). No associations were observed between (11)C-PiB load and NFT count for any of the regions examined (p > 0.2 in all regions), or between regional Aβ or NFT counts and corresponding brain volumes. The strong associations of PiB retention with region-matched, quantitative analyses of Aβ in postmortem tissue offer support for the validity of (11)C-PiB-PET imaging as a method for evaluation of plaque burden in vivo.

Imaging of prion diseases

Prion diseases are caused by self-replicating proteins that induce lethal neurodegenerative disorders. In the last decade, the understanding of the different clinical, pathological, and neuroimaging phenotypes of this group of disorders has evolved paralleling the advances in prion molecular biology. From an imaging standpoint, the implementation of diffusion-weighted imaging in routine practice has markedly facilitated the detection of prion diseases, especially Creutzfeldt-Jakob. Less frequent prion-related disorders, including genetic diseases, may also benefit from progresses in the field of quantitative diffusion-weighted imaging, MR spectroscopy or molecular imaging. Herein, we present a review of the neuroimaging features of the prion disorders known to affect humans emphasizing the important contribution of MRI in the diagnosis of this group of disorders.

Early AD pathology in a [C-11]PiB-negative case: a PiB-amyloid imaging, biochemical, and immunohistochemical study

Amyloid-β (Aβ) deposits are detectable in the brain in vivo using positron emission tomography (PET) and [C-11]-labeled Pittsburgh Compound B ([C-11]PiB); however, the sensitivity of this technique is not well understood. In this study, we examined Aβ pathology in an individual who had clinical diagnoses of probable dementia with Lewy bodies and possible Alzheimer's disease (AD) but with no detectable [C-11]PiB PET retention ([C-11]PiB(-)) when imaged 17 months prior to death. Brain samples were processed in parallel with region-matched samples from an individual with a clinical diagnosis of probable AD and a positive [C-11]PiB PET scan ([C-11]PiB(+)) when imaged 10 months prior to death. In the [C-11]PiB(-) case, Aβ plaques were sparse, occupying less than 2% cortical area, and were weakly labeled with 6-CN-PiB, a highly fluorescent derivative of PiB. In contrast, Aβ plaques occupied up to 12% cortical area in the [C-11]PiB(+) case, and were intensely labeled with 6-CN-PIB. The [C-11]PiB(-) case had low levels of [H-3]PiB binding (< 100 pmol/g) and Aβ1-42 (< 500 pmol/g) concentration except in the frontal cortex where Aβ1-42 values (788 pmol/g) approached cortical values in the [C-11]PiB(+) case (800-1, 700 pmol/g). In several cortical regions of the [C-11]PiB(-) case, Aβ1-40 levels were within the range of cortical Aβ1-40 values in the [C-11]PiB(+) case. Antemortem [C-11]PiB DVR values correlated well with region-matched postmortem measures of Aβ1-42 and Aβ1-40 in the [C-11]PiB(+), and with Aβ1-42 only in the [C-11]PiB(-) case. The low ratios of [H-3]PiB binding levels to Aβ concentrations and 6-CN-PiB to Aβ plaque loads in the [C-11]PiB(-) case indicate that Aβ pathology in the brain may be associated with low or undetectable levels of [C-11]PiB retention. Studies in greater numbers of [C-11]PiB PET autopsy cases are needed to define the Aβ concentration and [H-3]PiB binding levels required to produce a positive [C-11]PiB PET signal.

Amyloid imaging as a biomarker for cerebral β-amyloidosis and risk prediction for Alzheimer dementia

Since the introduction of amyloid imaging nearly 10 years ago, this technique has gained widespread use and acceptance. More recently, published reports have begun to appear in which amyloid imaging is used to detect the effects of antiamyloid therapies. This review will consider the issues involved in the use of amyloid imaging in the development and evaluation of drugs for the treatment of Alzheimer's disease. Current evidence regarding the postmortem correlates of in vivo amyloid imaging data are considered. The application of amyloid imaging to screening subjects for trials and use as an outcome measure is discussed in light of longitudinal changes in the in vivo amyloid signal. While the bulk of this review is directed at symptomatic patients with dementia, consideration is given to the use of amyloid imaging in nondemented subjects as well. Similarities and differences of cerebral amyloid assessment by amyloid imaging and cerebrospinal fluid (CSF) measurements are delineated and an agenda for further research to improve the applicability of amyloid positron emission tomography (PET) to clinical trials is proposed.

Amyloid imaging

Molecular neuroimaging techniques such as PET are proving valuable in the early and differential diagnosis of Alzheimer's disease (AD).With the advent of new therapeutic strategies aimed at reducing β-amyloid (Aβ) burden in the brain to potentially prevent or delay functional and irreversible cognitive loss, there is increased interest in developing agents that allow assessment of Aβ burden in vivo. Amyloid imaging with PET has proven useful in the discrimination of dementias, showing significantly higher Aβ burden in the gray matter of AD patients when compared with healthy controls or patients with frontotemporal dementia. ApoE ε4 carriers, independent of diagnosis or disease severity, present with higher Aβ burden than non-ε4 carriers. Amyloid imaging matches histopathological reports in aging and dementia, reflecting the true regional density of Aβ plaques in cortical areas. It also appears to be more sensitive than FDG-PET for the diagnosis of AD. In healthy older people there is an increasing prevalence of amyloid positive scans with age, rising from 20% in the seventh decade to 60% in the ninth decade. Of people with mild cognitive impairment (MCI), 40-60% present with detectable cortical Aβ deposition. In both groups, Aβ deposition is associated with a higher risk for cognitive decline and dementia due to AD. These observations suggest that Aβ deposition is not part of normal aging, supporting the hypothesis that it occurs well before the onset of symptoms and is likely to represent preclinical AD in asymptomatic persons and prodromal AD in MCI. Further longitudinal observations, coupled with different disease-specific tracers and biomarkers, are required to confirm this hypothesis and further elucidate the precise role of Aβ deposition in the course of AD.

From high-throughput cell culture screening to mouse model: identification of new inhibitor classes against prion disease

Transmissible spongiform encephalopathies (TSE) or prion diseases belong to a category of fatal and so far untreatable neurodegenerative conditions. All prion diseases are characterized by both degeneration in the central nervous system (CNS) in humans and animals and the deposition and accumulation of Proteinase K-resistant prion protein (PrP(res)). Until now, no pharmaceutical product has been available to cure these diseases or to alleviate their associated symptoms. Here, a cell-culture screening system is described that allows for the large-scale analysis of the PrP(res) inhibitory potential of a library of compounds and the identification of structural motifs leading potent compounds able to cause PrP(res) clearance at the cellular level. Based on different scrapie-infected cell lines, 10,000 substances were tested, out of which 530 potential inhibitors were identified. After re-screening and validation using a series of dilutions, 14 compounds were identified as the most effective. These 14 compounds were then used for therapeutic studies in a mouse bioassay to test and verify their in vivo potency. Two compounds exhibited therapeutic potential in the mouse model by significantly extending the survival time of intracerebrally infected mice, when treated 90 days after infection with scrapie.

Association between in vivo fluorine 18-labeled flutemetamol amyloid positron emission tomography imaging and in vivo cerebral cortical histopathology

OBJECTIVE

To determine the correspondence of in vivo quantitative estimates of brain uptake of fluorine 18-labeled flutemetamol with immunohistochemical estimates of amyloid levels in patients who underwent previous biopsy.

DESIGN

Cross-sectional study of ¹⁸F-flutemetamol positron emission tomography (PET) findings in patients with prior cortical biopsy specimen stained for the presence or absence of amyloid plaques.

SETTING

University hospital. Patients Seven patients who previously had a prior right frontal cortical biopsy at the site of ventriculoperitoneal placement for presumed normal pressure hydrocephalus were recruited. Inclusion criteria included an adequate biopsy specimen for detection and quantification of β-amyloid pathology and age older than 50 years. Intervention All patients underwent an ¹⁸F-flutemetamol PET scan.

MAIN OUTCOME MEASURES

Quantitative measures of ¹⁸F-flutemetamol uptake (standardized uptake value ratio, a ratio of mean target cortex activity divided by that in a cerebellar reference region) were made at a location contralateral to the biopsy site and compared with estimates of amyloid load based on immunohistochemical and histological staining.

RESULTS

There was complete agreement between visual reads of ¹⁸F-flutemetamol PET scans (3 blinded readers with majority rule) and histology. A regression model, including time from biopsy as a covariate, demonstrated a significant relationship (P = .01) between ¹⁸F-flutemetamol uptake and percentage of area of amyloid measured by a monoclonal antibody raised against amyloid (NAB228). Similar results were found with the amyloid-specific monoclonal antibody 4G8 and Thioflavin S.

CONCLUSION

To our knowledge, these data are the first to demonstrate the concordance of ¹⁸F-flutemetamol PET imaging with histopathology, supporting its sensitivity to detect amyloid and potential use in the study and detection of Alzheimer disease.

Amyloid imaging in aging and dementia: testing the amyloid hypothesis in vivo

Amyloid imaging represents a major advance in neuroscience, enabling the detection and quantification of pathologic protein aggregations in the brain. In this review we survey current amyloid imaging techniques, focusing on positron emission tomography (PET) with (11)carbon-labelled Pittsburgh Compound-B ((11)C-PIB), the most extensively studied and best validated tracer. PIB binds specifically to fibrillar beta-amyloid (Abeta) deposits, and is a sensitive marker for Abeta pathology in cognitively normal older individuals and patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD). PIB-PET provides us with a powerful tool to examine in vivo the relationship between amyloid deposition, clinical symptoms, and structural and functional brain changes in the continuum between normal aging and AD. Amyloid imaging studies support a model in which amyloid deposition is an early event on the path to dementia, beginning insidiously in cognitively normal individuals, and accompanied by subtle cognitive decline and functional and structural brain changes suggestive of incipient AD. As patients progress to dementia, clinical decline and neurodegeneration accelerate and proceed independently of amyloid accumulation. In the future, amyloid imaging is likely to supplement clinical evaluation in selecting patients for anti-amyloid therapies, while MRI and FDG-PET may be more appropriate markers of clinical progression.