Recognizing common PET patterns in neurodegenerative dementia

Neurodegeneration, primarily due to Alzheimer’s disease (AD), is the major cause of dementia.^1,2 Age is a major risk factor for AD and the prevalence of dementia is predicted to increase dramatically with the growing elderly population.¹ After AD, the most common neurodegenerative causes of dementia are dementia with Lewy bodies (DLB) and frontotemporal lobar degeneration (FTLD). Vascular dementia is the only non-neurodegenerative cause of dementia with a similar prevalence.³ Classically, neurodegenerative diseases tend to most strongly affect specific cognitive domains related to the specific pattern of neurodegeneration – memory for AD, for example – but there can be syndromic overlap especially early in the course of the disease.

This article reviews the appearances of the common neurodegenerative dementia on positron emission tomography (PET), specifically ¹⁸F-fluorodeoxyglucose (FDG) PET and amyloid PET, with the goal of increasing diagnostic accuracy and confidence of practicing radiologists in the use of PET for differentiating types of dementia.

Neurodegenerative disease and the role of ¹⁸F-FDG PET

The region of the brain affected varies with each neurodegenerative disease, but the underlying pathophysiology is the same: progressive neuronal dysfunction and loss, ultimately resulting in cognitive decline. Each cause has characteristic clinical symptoms and affects particular cognitive domains. Alzheimer’s disease, for example, shows decline in memory. The similarities in clinical presentation among these diseases, moreover, frequently lead to diagnostic uncertainty, especially early on, when symptoms are milder and typical cognitive findings are only partially expressed. In fact, when patients begin to show the earliest objective signs of cognitive decline, the term Mild Cognitive Impairment (MCI) is often used until a more certain diagnosis can be achieved.

During the past several decades – the last 15 years, in particular – advances in imaging have provided biomarkers to assist in diagnosis and provide prognostic information in neurodegenerative dementia. ¹⁸F-FDG uptake by the brain is one such biomarker of integrated metabolic activity, reflecting a combination of blood flow, gray matter volume, and synaptic function,⁴ all of which are affected by neurodegeneration. As such, ¹⁸F-FDG as a biomarker is not specific to a particular cause of dementia, but rather an identifier of regional neurodegeneration.

Although there is some variability and overlap, each disease has a typical regional pattern of hypometabolism caused by the typical regional pattern of neurodegeneration.⁵ These patterns match those previously identified in studies using perfusional single photon emission tomography (SPECT) with ^99mTc-ethyl cysteinate dimer (ECD) and ^99mTc-hexamethylpropyleneamine oxime (HMPAO), which measure the patterns of regional hypoperfusion associated with neurodegeneration. Due to improved spatial resolution and better diagnostic accuracy,^{6-8 18}F-FDG PET has largely replaced perfusion SPECT in evaluating cognitive decline and dementia in clinical practice. As a topographic marker of neurodegeneration, the hypometabolism pattern on ¹⁸F-FDG PET is affected by individual variability in neurodegeneration pattern.^9,10 This variability is best seen in several atypical but well characterized presentations of neurodegenerative diseases that may show patterns that differ from the archetype.

Under the current Centers for Medicare & Medicaid Services (CMS) coverage guidelines, ¹⁸F-FDG is approved clinically for differential diagnosis of AD versus FTLD in patients with cognitive decline of at least six months’ duration and who meet diagnostic criteria for both.^11-13 The use of ¹⁸F-FDG for this indication is based upon strong evidence that typical presentations of these diseases show clearly different patterns of hypometabolism.^5,9,14 However, multiple studies have also shown efficacy in differential diagnosis of other neurodegenerative diseases.^5,9,14

Typical patterns of ¹⁸F-FDG PET in neurodegenerative diseases

Classically in AD, ¹⁸F-FDG PET shows most prominently decreased metabolic activity in the lateral temporoparietal cortices, precuneus, posterior cingulate cortices, and medial temporal lobes, including the hippocampi. The posterior cingulate cortex is one of the most specific regions of abnormality in AD on ¹⁸F-FDG PET,^8,10 and it must be specifically evaluated in each case. The frontal lobes are also frequently involved later in the disease course, usually to a lesser degree than the temporal/parietal lobes (Figure 1A/B). These changes are typically bilateral and symmetric, but they can show some asymmetry. When this pattern of hypometabolism, albeit usually less severe, is seen in MCI patients, it suggests underlying AD pathology and future progression to AD. In typical AD, metabolism in the occipital and somatosensory cortex, as well as the deep cerebral nuclei, is preserved.^8,9

In DLB, where cognitive decline may be accompanied with hallucinations, sleep disorder, and parkinsonism, decreased metabolic activity in the occipital lobes may allow discrimination from AD. Otherwise, DLB also shows hypometabolism in many regions involved in AD, including the temporoparietal and frontal cortex (Figure 1C). Also suggestive of the diagnosis of DLB is preservation of posterior cingulate cortex uptake with hypometabolism in the adjacent precuneus and cuneus, termed the “cingulate island sign”.^15-17 In cases of DLB without AD pathology, there is also sparing of medial temporal metabolism. However, in about 70% of cases of DLB, there is coexistent AD pathology,¹⁸ likely accounting for a part of the overlap in metabolic pattern between DLB and AD.

FTLD consists of a group of clinically and pathologically heterogeneous neurodegenerative conditions with varied clinical features but overlapping underlying pathologic and genetic etiologies; as a result, FTLD can show one of several patterns on ¹⁸F-FDG PET. In the behavioral variant of FTLD, characterized by personality and emotional changes and originally called Pick’s disease, cerebral hypometabolism is seen in the frontal lobes, commonly with involvement of the anterior cingulate cortex and variable involvement of the anterior temporal lobes (Figure 1D).^8,9,19 In the semantic dementia variant of FTLD, associated with loss of ability to understand word meaning, there is asymmetric left-predominant anterior temporal lobe hypometabolism (Figure 1E).^9,20 The progressive nonfluent aphasia variant of FTLD, clinically associated with inability to produce language, is characterized by asymmetric left anterior perisylvian hypometabolism. Rarely, FTLD can show a pattern of hypometabolism similar to classic AD, although frontal hypometabolism is usually also seen in these cases.^19,21-23

Overall, although ¹⁸F-FDG is not a biomarker specific to a single dementia entity, there is a strong body of evidence on its high sensitivity and specificity in differentiating dementia types.^9,10,14 Perhaps as important, a normal ¹⁸F-FDG PET scan dramatically reduces the probability of an underlying neurodegenerative cause of a patient’s clinical symptoms and portends a favorable prognosis.

Atypical patterns on ¹⁸F-FDG PET

Several factors affect the sensitivity and specificity of ¹⁸F-FDG PET in neurodegenerative dementia. These include variants of AD with unusual patterns of neurodegeneration, non-neurodegenerative diseases, of which vascular dementia is the most common, and the effects of medications.^24-27

Several atypical, non-amnestic presentations of AD comprise about one-third of young-onset AD cases and about 5% of late-onset AD cases.²⁸ These presentations exhibit different patterns of cerebral hypometabolism with regional involvement that matches the specific patterns of cognitive decline.^23,29 The logopenic variant of primary progressive aphasia is usually due to AD but with primary impairment in language, specifically word repetition, and shows asymmetric left temporoparietal hypometabolism.²³ This pattern can be similar to one of the language variants of FTLD. The posterior cortical atrophy variant of AD, with a characteristic visuospatial impairment, tends to have symmetric parietal-occipital hypometabolism, a pattern overlapping with DLB.³⁰

Hypometabolism in the deep gray nuclei is atypical for AD, FTLD, and DLB in the absence of severe cortical atrophy. Involvement of these structures should raise consideration of other etiologies, ranging from vascular dementia to rare neurodegenerative conditions such as corticobasal degeneration or secondary neurodegenerative diseases like Creutzfeld-Jacob disease. Vascular dementia is relatively common, accounting for 10-20% of dementia.^31,32 It is caused by microvascular ischemia and/or multiple infarctions and is highly linked with cardiovascular risk factors. Vascular dementia can result in atypical patterns of hypometabolism on ¹⁸F-FDG PET commonly affecting the deep gray structures and frontal lobes, but with variability depending on the vascular territories affected in an individual case.³³ As vascular abnormality is frequently seen along with AD, this may account for some atypical metabolic patterns in AD patients.

Centrally active medications can have effects on cerebral metabolism, particularly during periods of medication changes as the brain adjusts to the new equilibrium. Barbiturates and sedatives have the strongest effects; however, other medications, including narcotics, antipsychotics, and corticosteroids, can have an effect.^34,35 Medication-induced hypometabolism can mask an underlying neurodegenerative condition.

Amyloid PET in neurodegeneration

Two hallmark pathologic protein aggregates are present in the brain of AD patients: ß-amyloid neuritic plaques and tau-containing neurofibrillary tangles. The development of ¹¹C Pittsburgh Compound B (¹¹C-PiB) PET just over one decade ago allowed in vivo imaging of ß-amyloid plaques.^36,37 Due to the short, 20.3-minute half-life of the ¹¹C radioisotope, it is not possible to produce ¹¹C-PiB for widespread use. Subsequently, three fluorinated amyloid PET tracers (¹⁸F-florbetaben, ¹⁸F-florbetapir, and ¹⁸F-flutemetamol) were developed and approved by the U.S. Food and Drug Administration (FDA) for clinical use. ^38-40 Due to more prominent, nonspecific white matter tracer binding, as part of the FDA approval process, all commercial vendors have been required to develop reader-training programs to ensure that readers appropriately detect the pathological cortical radiotracer binding. Imagers must complete this training and follow the specific guidelines described for each radiotracer in order to interpret amyloid PET studies. Clinically, amyloid PET is interpreted as either positive or negative for the presence of cortical ß-amyloid plaque (Figure 2). High inter-reader agreement can be achieved for amyloid PET.⁴¹

The three FDA-approved amyloid PET radiotracers have high sensitivity for beta-amyloid deposition, with intensity of binding correlated with burden of amyloid plaque on histopathology.^42-46 Specificity for amyloid plaque is also high. Patients with AD are nearly uniformly positive on amyloid PET. In addition, studies have shown that detectable cortical amyloid is present in approximately 50% of MCI patients,⁴⁷ and that these amyloid-positive MCI patients are more likely to progress to AD.^44,48,49 However, the detection of cerebral cortical amyloid deposition does not indicate that an individual has or will develop AD. Deposition of cortical amyloid plaque is commonly seen with aging, even in people without cognitive symptoms, with 10-15% at age 60 years old to approximately 50% at 80 years showing detectable cortical amyloid.⁵⁰ As a result, the specificity and positive predictive value of amyloid PET for AD declines with increasing age.^47,51

On the other hand, a negative amyloid PET scan indicates that there is no significant cerebral ß-amyloid plaque, which is incompatible with a diagnosis of AD and should direct clinicians to search for alternative etiologies. To help narrow the use of amyloid PET to a population that is most likely to benefit, the Alzheimer’s Association and Society for Nuclear Medicine and Molecular Imaging published a set of appropriateness criteria, summarized in the Table.⁵² Note that a growing body of evidence shows that cerebral amyloid may start accumulating decades before onset of dementia and may even have detrimental effects even in cognitively normal patients;^47,51 however, due to a lack of efficacious preventive therapies, these groups are not likely to benefit from knowledge of amyloid status.

The various subtypes of FTLD are associated with several different pathologic protein aggregates, most commonly containing either tau protein or TDP-43 (transactive response DNA binding protein, 43 kDa).⁵³ Due to the absence of amyloid pathology and typically younger age of presentation of FTLD, studies have shown high negative amyloid PET rates in FTLD as well as strong ability to distinguish AD from FTLD.^54-56 Positive amyloid scans in FTLD groups may represent clinical misdiagnosis or presence of multiple pathologies.

In contrast, about 70% of DLB patients have cerebral amyloid deposition,¹⁸ most of which meet pathologic criteria for mixed DLB-AD.^47,57 Although cognitive features are initially most prominent, DLB patients ultimately develop parkinsonism. Clinically, DLB is distinguished from Parkinson disease (PD) with dementia by dementia preceding or manifesting within one year after the onset of a Parkinsonian movement disorder.⁵⁸ Like Parkinson disease, the hallmark protein aggregates in DLB are α-synuclein-containing Lewy bodies, for which there is currently no clinically available radiotracer. However, as with PD, DLB shows decreased striatal uptake on molecular imaging of the dopaminergic system,^59,60 with ¹²³I-ioflupane SPECT being the most widely used tracer in the US.

Recent developments

Historically, medical images have been interpreted visually, with quantification largely restricted to the research arena. As demonstrated with measurements of hippocampal volume and cortical thickness decreases in AD on structural MRI, quantification can provide information that is difficult or impossible to see with the human eye with high accuracy and precision. Furthermore, quantification can reliably demonstrate subtle changes over time that carry important diagnostic and prognostic information. PET is intrinsically quantifiable, as intensity directly reflects radioligand binding. ¹⁸F-FDG binding is correlated with cerebral metabolism and amyloid PET radiotracers correlate with cerebral amyloid burden.^42,61 Techniques for quantification are starting to gain acceptance in clinical image interpretation.

For brain ¹⁸F-FDG PET, many commercial software packages are available to compare a patient’s scan to a database of controls, often matched for age and gender. These analyses are most often performed on a voxel-by-voxel basis to quantify metabolic changes across the whole brain. For example, Figure 3 depicts the results from voxel-based analysis for an AD patient (Figure 3A) and a FTLD patient (Figure 3B), with areas of decreased metabolic activity compared to the normal database highlighted in color. These analytic software packages aid the imaging specialist in visualizing the overall pattern of decreased cerebral metabolic activity. The required complex computational steps of co-registration and voxel-based comparisons can generate errors, particularly if there is asymmetric/focal atrophy, so correlation with direct evaluation of the patient’s images remains critical.

Although unassisted visual interpretation of amyloid PET is mandated by the reader training programs, there are several factors to suggest that value of amyloid PET will be improved with quantification. First, studies have shown that quantification improves reader agreement.^41,43 Amyloid PET, while sensitive, shows a low increase in signal between negative and positive⁴² and high non-target white matter binding that can make detection of cortical binding challenging for the human eye, particularly in borderline cases. Amyloid PET is typically quantified as a standardized uptake value ratio,⁶² where individual voxel uptake is normalized to a reference region. Historically, cerebellar uptake has been used as reference, as the cerebellum is known to have little amyloid deposition even in advanced AD, although recent studies suggest that whole white matter uptake may be more reliable, especially in longitudinal follow up.^63,64 This simple SUVR quantification method allows comparison between individuals and to normal values, and evaluation of change over time.⁶⁵ The precise role of quantification for clinical amyloid PET requires further investigation.

Future directions

The advancements in ¹⁸F-FDG PET and amyloid PET have contributed tremendously toward understanding the underlying pathophysiology of neurodegenerative dementia, and the future continues to look promising. While both ¹⁸F-FDG PET and amyloid PET have demonstrated robust clinical performance,⁶⁶ their role in the clinic in the setting of other imaging and laboratory biomarkers that have been developed as AD continues to evolve. The Imaging Dementia – Evidence for Amyloid Scanning (IDEAS) Study, which aims to evaluate the clinical effect of amyloid PET on patient management and outcomes, has been enrolling subjects since early 2016, provides temporary Medicare reimbursement for clinical amyloid PET scans that meet appropriate use criteria for the first time (Table). Ultimately, the clinical fate of amyloid PET most likely depends on future development of disease-modifying therapies.

Beyond these established radiotracers, novel tracers in various stages of development aim to target other critical protein aggregates in neurodegeneration such as tau^67,68 and alpha-synuclein.⁶⁹ In the near future, it may be possible to evaluate and quantify the presence of several pathologic protein aggregates to facilitate improved and earlier diagnosis and hopefully provide improved estimation of a patient’s future clinical course.

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