PET is better than perfusion SPECT for early diagnosis of Alzheimer's disease -- against

Imaging blood-brain barrier dysfunction: A state-of-the-art review from a clinical perspective

The blood-brain barrier (BBB) consists of specialized cells that tightly regulate the in- and outflow of molecules from the blood to brain parenchyma, protecting the brain's microenvironment. If one of the BBB components starts to fail, its dysfunction can lead to a cascade of neuroinflammatory events leading to neuronal dysfunction and degeneration. Preliminary imaging findings suggest that BBB dysfunction could serve as an early diagnostic and prognostic biomarker for a number of neurological diseases. This review aims to provide clinicians with an overview of the emerging field of BBB imaging in humans by answering three key questions: (1. Disease) In which diseases could BBB imaging be useful? (2. Device) What are currently available imaging methods for evaluating BBB integrity? And (3. Distribution) what is the potential of BBB imaging in different environments, particularly in resource limited settings? We conclude that further advances are needed, such as the validation, standardization and implementation of readily available, low-cost and non-contrast BBB imaging techniques, for BBB imaging to be a useful clinical biomarker in both resource-limited and well-resourced settings.

A comparison of FDG-PET and blood flow SPECT in the diagnosis of neurodegenerative dementias: a systematic review

OBJECTIVE

Perfusion single photon emission computed tomography (SPECT) and 18F fluorodeoxyglucose positron emission tomography (FDG-PET) both have clinical utility for the differential diagnosis of dementia. Although PET is often viewed by some as more accurate and therefore preferential, the extent to which published evidence supports this is not clear. The aim of this review was to address the question by reviewing studies of SPECT and PET imaging in dementia diagnosis, with a particular focus on all published head-to-head studies.

DESIGN

A MEDLINE search was carried out using the following keywords: "PET" and "SPECT" and "dementia" or "Mild Cognitive Impairment," together with "alzheimers" or "DLB" or "lewy body" or "frontotemporal" or "FTD" or "Picks." Articles were included up to February 2013, limited to human studies and in English language.

RESULTS

Published studies of SPECT accuracy show that it is a useful tool for differential diagnosis, with sensitivities of 65-85% for diagnosing Alzheimer's disease (AD) and specificities (for other neurodegenerative dementias) of 72-87%. PET studies generally report higher accuracy, with sensitivities of 75-99% for AD and specificities of 71-93%. However, there have been few direct head-to-head comparisons, with some indicating SPECT and PET to be equally useful in dementia diagnosis and others favouring PET. Many of these studies are limited with respect to numbers and methodically with poorly matched control groups.

CONCLUSIONS

Overall, although studies suggest superiority of PET over SPECT, the evidence base for this is actually quite limited. We suggest that further direct comparative studies, including health economic and patient preference evaluations, are needed to help direct future service provision.

Neuroimaging in the clinical diagnosis of dementia: observations from a memory disorders clinic

OBJECTIVES

To determine how often neuroimaging confirms, clarifies, or contradicts initial diagnoses of late-life cognitive disorders.

DESIGN

Retrospective case review.

SETTING

Outpatient clinic specializing in memory disorders.

PARTICIPANTS

One hundred ninety-three consecutively referred cognitively impaired patients.

MEASUREMENTS

Diagnoses using research criteria were developed for each patient at the first visit and ranged from cognitive impairment without dementia to dementias of single, complex, or indeterminate etiology. Structural (noncontrast magnetic resonance imaging) and perfusion (technetium-99m ethyl cysteine dimer single photon emission computed tomography) images were categorized together as normal, suggestive of specific diseases, or abnormal/not diagnostic.

RESULTS

When a single neurodegenerative disease was suspected clinically (n=94), imaging confirmed the diagnosis in 50, contradicted the diagnosis in 32, and was abnormal/not diagnostic in 12. When more than one neurodegenerative etiology was clinically suspected (n=21), imaging assigned a single diagnosis in 13 and only cerebrovascular disease in one and was abnormal/not diagnostic in seven. In dementia not otherwise specified (NOS) (n=33), imaging suggested a specific etiology in 23 and was abnormal/not diagnostic in 10. Abnormal/not diagnostic images were more common in cognitive disorder NOS (n=25, 68%) than in other clinical groups (22%, chi-square=22.8 P<.001). Neuroimaging indicators of cerebrovascular disease were common (60% prevalence) but not predicted by the presence of vascular risk factors alone.

CONCLUSION

Overall, neuroimaging confirmed, clarified, or contradicted the initial clinical diagnosis in more than 80% of patients, whereas fewer than 20% had abnormal/not diagnostic patterns. Imaging suggested a complex dementia etiology in 21% of cases clinically thought to be caused by a single process, whereas 46% of complex clinical differential diagnoses appeared to reflect a single causal pattern. Further work is needed to determine whether refinement of clinical diagnoses using specialized neuroimaging improves clinical decision-making and patient outcomes.