Neuroimaging

Advances in neuroimaging have revolutionized the approach to patients with partial epilepsy. MRI has replaced CT as the study of choice, although CT remains superior in the detection of calcified lesions. SPECT and PET scans measure functional changes produced by seizures. All patients with partial epilepsy should undergo an MRI during the course of their illness to rule out structural lesions. The use of special imaging sequences and thin cuts increases the detection of small neoplasms, developmental abnormalities, and mesial temporal sclerosis. In most cases, mesial temporal sclerosis is associated with unilateral hippocampal

Figure 52-7 '13] FDG PET demonstrating significant hypometabolism involving the left temporal lobe in a patient with left mesial temporal lobe epilepsy. The darkness scale is divided into 10 equal intervals each representing a 10-percent change in hypometabolism. The level of darkness at the top of the scale represents an increase in metabolism as compared to the normal state with glucose metabolism decreasing as one progresses down the darkness scheme. (See CD-ROM for colorfPhofoin.) courtesy of C. Oliver Wong, MD, PhD, FACP.)

atrophy on T1-weighted images and increased signal in the mesial temporal structures on T2-weighted sequences (.Fig 52.-6.). These findings are highly predictive of seizure remission and lack of material-specific memory deficits following mesial temporal resection. Magnetic resonance spectroscopy is a new tool that demonstrates regional metabolic alterations in epileptogenic tissue.

Cerebral glucose metabolism measured with y FDG PET is reduced in some patients with partial epilepsy. Temporal lobe hypometabolism is seen in 60 to 90 percent of patients with intractable TLE (.Fig 52-7 ). '45' The extent of abnormality is usually larger than the epileptogenic zone, frequently extending into the ipsilateral basal ganglia, thalamus, and

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fronto-parietal cortices. The yield of PET in nonlesional extratemporal epilepsy is significantly lower. The short half-life of the isotope precludes its use for ictal studies.

SPECT is more readily available than PET because it utilizes commercially available isotopes and does not require an on-site cyclotron. Technetium 99m-hexamethylene-propylene-amine-oxime SPECT reveals hypoperfusion in the epileptogenic temporal lobe during the interictal state in over 50 percent of patients with TLE.'45' False lateralization has been reported in 15 to 20 percent of cases. The resolution of SPECT is inferior to that of PET, and quantitative analysis is not available. The long half-lives of the isotopes make ictal and immediate postictal SPECT a useful noninvasive localizing tool. The results of ictal SPECT depend on the time of injection of the isotope from seizure onset. Under optimal circumstances, 65 to 90 percent of studies demonstrate hyperperfusion ipsilateral to seizure origin ( Fig 52-8 ).y Delayed injections produce widespread hyperperfusion and a higher incidence of false lateralization.

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