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Adams & VictorPositron Emission TomographyThis technique, commonly known as PET, measures the regional cerebralconcentration of systemically administered radioactive tracers. Positron-emittingisotopes (usually 11C, 18F, 13N, and 15O) are produced in a cyclotron or linearaccelerator and incorporated into biologically active compounds in the body. The

concentration of the tracers in various parts of the brain is determinednoninvasively, by detectors outside the body, and tomographic images areconstructed by techniques similar to those used in CT and MRI. Local patterns ofcerebral blood flow, oxygen uptake, and glucose utilization can be measured by PETscanning, and the procedure has proved to be of value in grading primary braintumors, distinguishing tumor tissue from radiation necrosis, localizing epileptic foci,and differentiating types of dementing diseases. The ability of the technique toquantitate neurotransmitters and their receptors promises to be of importance inthe study of Parkinson disease and other degenerative diseases. This technology isfound in relatively few medical centers and requires costly facilities and supportstaff;it is therefore not utilized for routine diagnosis.

Single-Photon Emission Computed Tomography (SPECT)This technique, which has evolved from PET, utilizes isotopes that do not require acyclotron for their production. Here also radioligands (usually containing iodine) areincorporated into biologically active compounds, which, on decay, emit only a singlephoton. This procedure allows the study of regional cerebral blood flow underconditions of cerebral ischemia or during intense tissue metabolism. The restrictedanatomic resolution provided by SPECT has limited its clinical usefulness, but itswider availability makes it appealing for clinical use. This has proved particularlytrue in helping to distinguish between Alzheimer dementia and a number of focalcerebral (lobar) atrophies and in the localization of epileptic foci in patients who arecandidates for cortical resection. Once injected, the isotope localizes rapidly in thebrain, with regional absorption proportional to blood flow, and is then stable for anhour or more. It is thus possible to inject the isotope at the time of the seizure

onset, while the patient is undergoing video and electroencephalographicmonitoring, and to scan the patient later. As with PET, the clinical potential of thistechnique has yet to be fully realized.

The widespread use of CT and MRI represents an important surrogate approach tothe pathologic study of epilepsy. More than 25 years ago, Gastaut and Gastautreported that in primary grand mal and absence epilepsies, CT abnormalities werefound in approximately 10 percent of cases, whereas in the Lennox-Gastautsyndrome, the West syndrome, and partial complex epilepsies it was found in 52,77, and 63 percent, respectively. Atrophy, calcification, and malformations were themost frequent changes. MRI and particularly the FLAIR images have proved to be aparticularly sensitive means of detecting epileptogenic lesions of the medialbasalportion of the temporal lobes (mesial temporal sclerosis; Fig. 16-2). Repeatedly,

patients are observed in whom MRI disclosed a cortical or subcorticaldevelopmental malformation such as a cortical heterotopia or another surgicallytreatable lesion of the temporal lobe, even after CT scanning had failed to do so.More subtle epileptogenic foci may be demonstrated by positron emissiontomography (PET) or by interictal single-photon emission computed tomography(SPECT). Ictal SPECT, which shows hyperperfusion of the seizure focus, is a moredemanding but also more sensitive and specific procedure.

Diagnostic Studies The most useful, but not definitive, ancillary tests in use areCT scanning and MRI (Fig. 39-3). In patients with advanced Alzheimer disease, thelateral and third ventricles are enlarged to about twice normal size and the cerebralsulci are widened. As stated previously, fine-section coronal MRI of the medialtemporal lobes reveals a disproportionate atrophy of the hippocampi and a

corresponding enlargement of the temporal horns of the lateral ventricles. Early inthe disease, however, the changes do not exceed those found in many mentallyintact old persons. For this reason, one cannot rely on imaging procedures alone for

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diagnosis. CT and MRI scans are most valuable in excluding brain tumor, subduralhematoma, cerebral infarction, and hydrocephalus. The EEG undergoes a diffuseslowing, but only late in the course of the illness. The CSF is also normal, thoughoccasionally the total protein is slightly elevated. Using the constellation of clinicaldata, CT scanning, and MRIalong with the age of the patient and time course ofthe diseasethe diagnosis of senile dementia of Alzheimer type is being made

correctly in 85 to 90 percent of cases. Of some value in our experience have beenstudies of cerebral blood flow (single-photon emission tomography, or SPECT) andmetabolism (positron emission tomography, or PET), which show diminished activityin the medial temporal lobes, sometimes early in the disease. Nevertheless, in mostcases, when such changes were evident, the diagnosis was already obvious onclinical grounds alone. Our recent experience has been that the pattern ofAlzheimer change on these blood flow and metabolic studies is too often appliedindiscriminately.