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Single-Photon Emission Computed Tomography in Epilepsy Surgery Evaluation
This chapter includes an accompanying lecture presentation that has been prepared by the authors: .
Key Concepts
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Single-photon emission computed tomography (SPECT) is a reliable and important imaging technique facilitating hemispheric lateralization and lobar or sublobar localization in many nonlesional epilepsy cases.
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Ictal SPECT, unlike other functional imaging modalities of epilepsy evaluation such as positron emission tomography (PET) and magnetoencephalography (MEG), is an attempt to image the seizure itself, rather than an epileptogenic brain between seizures.
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SPECT alone is not a sufficient imaging modality, but when there is concordance with semiology it provides adjunctive data to guide intracranial electrode implantation. It is critical that the surgeon have a firm understanding of factors that would make a SPECT or subtraction ictal SPECT coregistered to MRI (SISCOM) study unreliable or misleading, such as a late or potentially postictal injection.
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SPECT imaging remains an important tool in developing an implantation hypothesis and, if used correctly, can improve surgical success.
Focal epilepsy, the most common of seizure disorders, may be amenable to surgery if the seizure onset zone can be identified and if its ablation is expected to result in low morbidity. Localization, therefore, is the key to surgically remediable epilepsy syndromes. In lesional epilepsy cases, this can be relatively straightforward with standard MRI. In nonlesional cases, however, and those with discordant features, alternative techniques such as single-photon emission computed tomography (SPECT) are sometimes used to localize seizure onset. The most frequent use of SPECT would be in nonlesional epilepsy patients in whom the semiology appears focal as a way of localizing a potential target for implantation of intracranial electrodes. No epilepsy center would resect a SPECT focus simply based on its presence and correlation with semiology, but rather epilepsy centers would use it as an implantation target for intracranial electrodes to prove seizure onset before resective surgery.
SPECT is an imaging technique that assesses cerebral blood flow by deposition of a metabolized radiopharmaceutical in the brain. Technetium 99m ( 99m Tc) (ethyl cysteinate diethylester (ECD; Neurolite) or hexamethylpropyleneamine oxime (HMPAO; Ceretec) is distributed to brain tissue in proportion to cerebral perfusion, is deposited, and remains stable for up to 4 hours after injection. This deposition is used to indicate areas that are either hyperperfused during a seizure (called ictal SPECT ) or hypoperfused after or between seizures (called postictal or interictal SPECT ) ( Fig. 85.1 ). The technique is time and resource intensive but can be extremely valuable in hemispheric lateralization and seizure onset localization and can help achieve the ultimate goal of epilepsy surgery: rendering patients seizure free, with as little comorbidity as possible.
(A) Interictal, or baseline single-photon emission computed tomography (SPECT) image. (B) Ictal SPECT image. (C) The ictal-interictal subtraction image (masked to exclude extracerebral activity) from a 39-year-old woman with recurring left temporal seizures after a selective neocortical resection at age 19 years. The tracer was injected 21 seconds following onset in a seizure lasting 5 minutes. The subtraction image shows increased tracer deposition, and hence blood flow, in the left temporal lobe along the margin of the prior resection. The assumption is that seizures continue to arise in the left temporal lobe.
Other techniques such as positron emission tomography (PET) and magnetoencephalography (MEG) are available to aid in seizure localization. , Each technique has its own limitations: PET relies on the detection of metabolic abnormality in the seizure focus, and MEG on the detection of interictal discharges. Both of these studies are essentially interictal. The perceived advantage of subtraction ictal SPECT coregistered to MRI (SISCOM) and SPECT is that these investigations are an attempt to image the seizure, not a patient with epilepsy between seizures. These investigations should be seen as complementary in the evaluation of epilepsy.
ICTAL Single-Photon Emission Computed Tomography
Two tracers are commonly used in this technique: 99m Tc-HMPAO (Ceretec) and 99m Tc-bicisate (Neurolite). Notably, there appears to be no difference in imaging outcomes, sensitivity, or specificity between these tracers. These agents have high first-pass brain extraction, with nearly complete deposition in target tissues within 30 to 60 seconds of intravenous injection. Thus, this technique allows an assessment of cerebral blood flow at a single point in time as the tracer first passes through the brain.
As a practical example, we use the following hypothetical situation: a patient undergoing video electroencephalography (EEG) monitoring alerts unit staff after experiencing the first signs of the aura that most commonly accompanies a “typical” seizure. Fifteen seconds into that seizure, a technician is able to inject the tracer agent; the seizure continues for an additional 40 seconds (a seizure lasting 55 seconds, in total). The patient is then taken to the nuclear medicine department, typically within 4 hours, and is scanned with a tomographic gamma camera capable of detecting the gamma ray emissions of the deposited radiopharmaceutical. It is assumed that areas of high isotope deposition (indicating increased cerebral blood flow) correlate with the general localization of the seizure.
With this technique, both the timing of the injection and whether the semiology was representative of the patient’s seizures are critical to understand whether the ictal SPECT image gives a true picture of the patient’s epilepsy. Initially, tracer injection within 100 seconds of ictal onset was thought to be critical to identify the seizure onset zone. However, further studies with larger patient numbers suggest that an injection less than 45 seconds from ictal onset is critical. Longer seizures do not allow for a later injection time. In cases in which the tracer has been injected too late, the imaging may be falsely localizing, or the seizure focus may actually be hypoperfusing rather than the expected hyperperfusion owing to seizure onset zone metabolic burnout. Ictal SPECT has been shown to be most valuable in lobar localization and hemispheric lateralization (see Fig. 85.1A ).
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