Abstracts

Rationale: Hyperperfusion on ictal SPECT is well established technique for the localisation of the epileptogenic zone (EZ) in patients with focal epilepsy.  The accuracy of ictal SPECT can be vastly improved by subtraction of an inter-ictal image, a technique known as subtraction ictal SPECT coregistered to MRI (SISCOM).1 However, the routine application of ictal SPECT is limited by the necessity for rapid injection of the radiotracer, with poor localisation observed for later injection times (>45 seconds). 1 Conversely secondary generalisation of the seizure has not been shown to affect the likelihood of achieving an accurate ictal scan. The purpose of this study is to investigate the optimal tracer injection time from seizure onset and secondary generalisation in ictal-SPECT for localising the EZ using SISCOM.  Methods: Retrospective data on patients who had an ictal SPECT scan whilst within the video-EEG monitoring unit at the Royal Melbourne Hospital was collected (2009-2017). Patient data was evaluated and patients with focal epilepsy with complete imaging (ictal, inter-ictal SPECT and MRI) were included in subsequent analysis. The information collected included age, gender, seizure type (focal seizure with/without secondary generalisation), epilepsy diagnosis (TLE or extra-TLE), and injection time of the radiotracer from seizure onset and from generalisation.  SISCOM images were created within Analyze (v10.0). Thresholding for hyperperfusion was set at 1.0SDfor each patient. SISCOM images were then reviewed by two blinded reviewers naive to patient information to identify the EZ and extent of abnormality in each image. The results were stratified according to injection time  and presence or absence of secondary generalization Results: A total of 136 patients had an ictal SPECT. 17 where excluded due to non-focal epilepsy (5 generalised epilepsy, 12 non-epileptic seizures). 50 patients were excluded due to incomplete imaging. This resulted in an evaluable patient group of 69 patients for whom SISCOM images have been generated. An interim analysis was conducted on 24 patients, demonstrating an overall sensitivity of 62.5% (15/24) and specificity of 83.3% (15/18), excluding non-epileptic seizures resulted in an overall sensitivity of 55% (11/20) and specificity of 78.6% (11/14). Positive predictive value was 69.2% (9/13) and negative predictive value was 28.6% (2/7), Cohen’s Kappa score of agreement between the two reviewers was low at 0.38. Earlier injection times were associated with higher localisation rates (60s, sensitivity 25%, specificity 66.7%). Localisation rates were higher for TLE than extra-TLE (66.7% vs 37.5%), and secondary generalisation appeared to increase the chances of correctly localizing the seizure origin (focal only 38.5%, secondary generalised 85.7%).  Conclusions: Consistent with previous studies, earlier injection times of the radiotracer (       Funding: no funding