Abstracts

Rationale: MRI-guided laser ablation of the hippocampus is a relatively new concept in the treatment of medically refractory temporal lobe epilepsy. Seizures of similar semiology after open temporal lobectomy is generally a predictor of poor seizure control. In order to examine whether this is true for patients after MRI-guided laser ablation, we examine MRI changes in two patients who had seizures of similar semiology immediately after ablation, but then went on to achieve seizure freedom.Methods: Two epilepsy patients were monitored at regular intervals (24-hour, 1-month, 3-months, 6-months) post-LITT via MRI (T1-w, T2-w, GRE, FLAIR, ADC) as a part of an ongoing study, after initial 3-Tesla pre-LITT MRI. 3D affine registration is first implemented to accurately align every post-LITT MRI sequence post-LITT (at every follow up time point) with reference to pre-LITT MRI. Post registration, a differences map is computed at every voxel by computing an absolute difference of MR intensity markers (T1-w, T2-w, FLAIR, GRE, ADC) between pre- and every post-LITT MRI sequence (Fig 1). A weighted MRI map is then obtained by leveraging difference maps for every MR protocol, by computing a weighted combination of relative contribution of individual imaging markers in quantifying treatment changes, at every time point. We develop a time dependent MRI map by plotting mean intensity changes within the ablation zone at every time point (Fig 1(j)). Results: Trends across different time-dependent profiles (normalized between 0-1) in Fig. 1 suggest that, (a) mean intensity differences consistently decrease over all protocols for patients with successful treatment, and are substantially reduced between 1, 3, and 6- months as compared to that within the first 1-month, and (b) ADC changes most dramatically over time compared to T2-w, T1-w, GRE, and FLAIR protocols. Based on clinical findings, the exaggerated changes in MP- MRI markers during the first one month may be attributed to edema and swelling caused due to the treatment. Fig. 2 illustrates the contributions of each protocol (normalized between 0-1), over the two studies for different time points. Fig. 2 reflects that, ADC is consistently identified as the most important protocol immediately post- LITT (24-hours), while a combination of T1-, T2-w, and ADC was identified as most contributory in identifying changes after 24-hours post-LITT.Conclusions: A novel quantitative framework for evaluating MP-MRI marker changes post-LITT on a per-voxel basis was developed to identify MRI markers that change most-dramatically over time post-LITT, and create an optimal MP-MRI profile to investigate MP-MRI markers relating to post-LITT changes. Our results suggest that changes within the hippocampus continue to occur within the first month after laser ablation. This may provide some rationale for early seizures after the procedure, and may not necessarily indicate worse seizure control as is often the case after open temporal lobectomy.