Does performing image registration and subtraction in ictal brain SPECT help localize neocortical seizures?

Ictal brain SPECT (IS) findings in neocortical epilepsy (patients without m esiotemporal sclerosis) can be subtle. This study is aimed at assessing how the seizure focus identification was improved by the inclusion of individu al IS and interictal brain SPECT (ITS)-MRI image registration as well as pe rforming IS ITS image subtraction. Methods: The study involved the posthoc analysis of 64 IS scans using Tc-99m-ethyl cysteinate dimer that were obtai ned in 38 patients without mesiotemporal sclerosis but with or without othe r abnormalities on MRI. Radiotracer injection occurred during video-electro encephalographic (EEG) monitoring. Patients were injected 2-80 s (median ti me, 13 a) after clinical or EEG seizure onset. All patients had sufficient follow-up to correlate findings with the SPECT results. All patients had IT S and MRI, including a coronal volume sequence used for registration. Image registration (IS and ITS to MRI) was performed using automated software. A fter normalization, IS - ITS subtraction was performed. The IS, ITS, and su btraction studies were read by 2 experienced observers who were unaware of the clinical data and who assessed the presence and localization of an iden tifiable seizure focus before and after image registration and subtraction. Correlation was made with video-EEG (surface and invasive) and clinical an d surgical follow-up. Results: Probable or definite foci were identified in 38 (59%) studies in 33 (87%) patients. In 52% of the studies, the image re gistration aided localization, and in 58% the subtraction images contribute d additional information. In 9%, the subtraction images confused the interp retation. In follow-up after surgery, intracranial EEG or video-EEG monitor ing (or both) has confirmed close or reasonable localization in 28 (74%) pa tients. In 6 (16%) patients, SPECT indicated false seizure localization. Co nclusion: Image registration and image subtraction improve the localization of neocortical seizure foci using IS, but close correlation with the origi nal images is required. False localizations occur in a minority of patients .