A Tabulated Summary of the FDG PET Literature

Dementia Work-Up

Disease Background.
Dementia is defined as loss of memory and at least one other area of complex behavior sufficient to interfere with day-to-day function. The magnitude of the problem is increasing, and it is estimated that 5% of the population older than 65 y and up to 25% of the population older than 80 y has some form of dementia. Causes of dementia include degenerative changes (e.g., Alzheimer´s disease, Pick´s disease, Parkinson´s disease, Huntington´s disease), vascular insufficiency, trauma, endocrine changes, and other causes. Metabolic changes in the brain have been shown to precede structural changes by at least 5 y. Treatment for the degenerative forms of dementia, such as Alzheimer´s, is improving with the use of cholinesterase inhibitors and treatment options continue to grow. The diagnosis of early Alzheimer´s disease and its differential diagnosis from other organic dementias or the benign effects of aging remain clinically difficult today. PET with FDG has been shown to provide an accurate and positive differential diagnosis of Alzheimer´s and of other forms of organic dementias. In some ways, the diagnosis of dementia is similar to that for cancer in the separation of benign from malignant disease. In the case of dementia, it is the separation of benign from organic degenerative disease.

Case Example.
A 67-y-old man presented with a 3-y history of progressive loss of memory and day-to-day function and a clinical diagnosis of possible Alzheimer´s. A brain MR image showed no anatomic indications of disease. An FDG PET scan was ordered to evaluate for possible Alzheimer´s disease. Shown in the left column in Figure 21 is a normal FDG PET scan from a 64-y-old man. In the right column is the FDG PET scan from the patient in this case. Two representative slices are shown from each individual. The right column clearly shows low FDG metabolism in the back portion of the brain (arrows) in the parietal and temporal regions. This hypometabolism pattern is consistent with Alzheimer´s disease.

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FIGURE 21. Case example, dementia.

Why Did FDG PET Help?
FDG PET established with a high degree of accuracy that the patient´s symptoms were the result of Alzheimer´s disease and not other causes of dementia. The diagnosis of Alzheimer´s disease was confirmed 6 y later at autopsy.

Key Management Issues.

Early diagnosis of dementia versus benign memory loss

Differential diagnosis of dementia from frontotemporal disease, diffuse Lewy bodies, or cerebrovascular diseases

Differentiation from pseudodementia/depression (This is a dementia-like state that is caused by depression and not Alzheimer´s disease.)

Summary of Evidence for FDG PET in Dementia Work-Up.
Primary neurodegeneration is the most common process underlying dementia, and Alzheimer´s disease alone accounts for approximately two-thirds of cases. Regional cerebral metabolic patterns reflect pathophysiologic changes in brain that will lead to Alzheimer´s disease, even before they give rise to symptoms. In addition to the diagnostic value FDG PET may have in evaluation of dementia, it can also serve as a prognostic tool to determine the likelihood of deterioration of mental status during the years after scanning, thereby facilitating planning by the patient and family members. Although results have varied, depending in part on the severity and diagnostic mix of patients, nearly all studies designed to assess the accuracy of FDG PET in the diagnosis of dementia have found sensitivity for Alzheimer´s disease to be >90%, with specificity typically approximating 75% (range, 67% - 97%). Meeting the challenge of accurately identifying minimally affected patients to allow them to reap the greatest potential therapeutic benefits requires making the diagnosis with a high degree of sensitivity and overall accuracy at the earliest possible stage of disease. The consistently high sensitivity of FDG PET in patients with even mild impairment makes it well suited for assisting with that task (Table 22).

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TABLE 22 FDG PET in Dementia Work-Up: Results of Literature Search

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Seizure Work-Up

Disease Background.
Epilepsy is a common condition, with a prevalence in the population of about 1 in 200 people. Several abnormalities within the brain can lead to abnormal "synchronous firing" of neurons, causing a seizure. Depending on which part of the brain is epileptogenic, seizures will have different outward appearances. In a grand mal seizure, all extremities move as a result of abnormal neuronal firing, which spreads within the brain to cause a diffuse motor seizure. Imaging of all types helps to locate abnormalities within the brain, and, when coupled with electroencephalography (EEG, scalp electrical signal monitoring), can help to manage epilepsy patients. Many patients can be controlled well on medications. Patients who have seizures despite having tried several medications are referred to as patients with intractable seizures. In these patients, identifying the source of the seizure within the brain often can lead to surgery that can stop or reduce the seizures. Imaging, including FDG PET, can play an important role in determining whether a patient is a candidate to be operated on for seizure control. The alternative (invasive electronic monitoring) requires putting electrodes into the brain parenchyma or meninges, with attending morbidity and mortality.

Case Example.
An 11-y-old boy, diagnosed with epilepsy at age 7, had been treated with medications for 4 y. During the last year, he had continued to have seizures, even with a change in antiseizure medications. An FDG PET scan was ordered to evaluate for the possible source of the seizure. MRI showed no structural abnormality. The FDG PET scan (Fig. 22) showed moderate-to-severe hypometabolism (lower than normal glucose utilization) in the right parietal, posterior, frontal, occipital, and temporal lobes (arrows) in the interictal period (i.e., between seizures).

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FIGURE 22. Case example, seizure.

Why Did FDG PET Help?
PET showed eleptogenic tissue in the localized brain. Surgery was performed to resect the dysfunctional tissue. The child, after surgery, was seizure free.

Key Management Issues.

Diagnosis of partial epilepsy (MRI negative)

Localization of seizure focus

Prediction of surgical outcome (prognosis)

Summary of Evidence for FDG PET in Seizure Work-Up.
In patients who have medically intractable epilepsy, neurosurgery to resect epileptogenic foci can decrease or eliminate seizure episodes and reduce neurologic impairment resulting from recurrent seizures and/or high doses of anticonvulsants. Patients with complex partial seizures, particularly those who have EEG evidence of a temporal lobe focus but inconclusive findings on MRI, often are referred for functional brain imaging to assess interictal metabolism. PET with FDG can identify epileptogenic zones through localization of hypometabolic brain tissue interictally. Interictal FDG PET has been demonstrated to be as useful for presurgical planning in most patients with temporal lobe epilepsy as the more logistically cumbersome ictal SPECT or more invasive EEG monitoring with depth electrodes. Patients with unilateral foci of hypometabolism identified by PET have been found in numerous studies to have a high likelihood of benefiting from neurosurgery, regardless of whether invasive electrode monitoring is also undertaken. Patients can thus be saved risks and costs otherwise incurred with invasive monitoring. Further study is needed to define more specifically the role of depth electrodes and surgical therapy in patients with findings of bilateral hypometabolism (Table 23).

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TABLE 23 FDG PET in Seizure Work-Up: Results of Literature Search

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