Advanced SPECT image processing in MRI-negative refractory focal epilepsy

The phenomenon of hyperemia at the region of the seizure focus has been known for decades. Perfusion SPECT is utilized to image patterns of altered regional cerebral blood flow (rCBF) for localizing the seizure onset zone. Thus far, perfusion SPECT is the only test that could image the altered physiology of all peri-ictal states (i.e., interictal, ictal, and postictal states). The alteration during the interictal state is typically one of baseline hypoperfusion at the focus of seizure activity. In contrast, hyperperfusion transiently characterizes ictal activity, at the longest probably for 2 to 3 minutes beyond seizure termination. Ictal hyperperfusion switches to hypoperfusion during the early postictal state, typically and transiently to a degree markedly less than that of interictal perfusion (Figure 5.1). The most frequently used SPECT radiotracers for peri-ictal perfusion imaging are ^99mTc-hexamethylpropylene amine oxime (Tc-HMPAO), and ^99mTc-Bicisate (Tc-ECD). Following injection of these ^99mTc-linked radiotracers during a seizure for ictal studies, they are quickly fixed to brain receptors because of their high first-pass brain extraction rate. The SPECT images can be acquired up to 3 to 4 hours after the injection, because of the relatively slow radioactivity decay of these ^99mTc-linked radiotracers. Compared to ictal SPECT, interictal SPECT used alone has been shown to have low sensitivity in detecting the seizure focus. A major advancement in the use of SPECT in epilepsy evaluation has been the development of computerized subtraction techniques for detecting differences in perfusion patterns between the ictal state and the baseline interictal state in the same patient, or between the patient’s ictal perfusion pattern and the pattern in normal subjects.