In patients with medically intractable focal epilepsy MRI is an essential tool for determining an underlying pathology. In these patients, resection of an epileptogenic zone can be considered. A structural MRI abnormality is a strong predictor of good outcome of surgery compared to so called MRI-negative cases¹. One important cause of focal refractory epilepsy and the most common pathology in MRI-negative patients is focal cortical dysplasia (FCD)^2,3.
The promise of 7T MRI is that structural abnormalities can be found where none were found at lower field strengths. The aim of this study was to determine what the diagnostic value of 7T MRI is in epilepsy patients evaluated for the possibility of surgical treatment, who already underwent 1.5 or 3T MRI.

Since November 2008 a selection of patients underwent 7T imaging in the University Medical Center Utrecht in the context of clinical evaluation of medically intractable epilepsy and surgical candidacy. During regular multidisciplinary epilepsy surgery meetings (ESM) all available clinical information and the results of all ancillary investigations are reviewed in order to decide on the indication for epilepsy surgery. For this series we assessed all 26 patients who underwent 7T MRI for pre-surgical evaluation, and whose scans (both 7T and lower field) were discussed during ESM. We compared the conclusions of the visual assessments of 1.5T or 3T, and 7T MRI as agreed upon by the ESM team, including an experienced epilepsy neuroradiologist.

Scans were performed on a Phillips Achieva 7T MRI system (Philips Healthcare) with a 16 channel receive coil or, after May 2011, a 32-channel receive head coil combined with dual channel transmit coil.

Scan protocols.
The current protocol consists of T1, T2, FLAIR, and T2*-weighted sequences, and a gradient echo white matter suppression (WMS) sequence, with 0.5 - 0.8 mm resolution; acquisition time ≈ 45 minutes. Since 2008 there have been some adjustments in the protocol, mainly the addition of the WMS sequence and modification of the T2* echo times (current parameters in figure 1).

23 patients underwent 7T MRI because absence of a lesion at 1.5/3T MRI (age 7-44, median 16). In 4 of 23 patients a lesion could be identified on 7T MRI. Two of these patients were operated, with histology confirming FCD type 2A in both (exemplary case in figure 3). The other two patients are planned for resective surgery with tailoring using electrocorticography, in both FCD is suspected (exemplary case in figure 4).
Three patients (age 14, 15 and 28) had 7T MRI because of clinical suspicion of additional lesions undetected at lower field strength. In one patient with Tuberous Sclerosis Complex additional subtle tubers and radial migration lines were identified. This patient underwent a temporal lobe resection and electrocorticography-guided lesionectomy of these right frontal tubers. In two other patients, one with hippocampal sclerosis and one with white matter abnormalities of unclear clinical relevance, no additional lesions were identified, nonetheless this information proved valuable for the clinical management.
Of note is that 7T MRI revealed additional lesions after these patients had previous good quality 3T MRI scans with dedicated epilepsy protocols including coronal FLAIR and 3D T1 sequences.
Figure 2 summarizes the imaging conclusions of the ESM based on lower-field MRI and those after 7T MRIs had been discussed and lists the number of patients who underwent surgery and their histological findings. In 5 of 26 (19.2%) patients with no lesion or the suspicion of additional lesions on previous imaging, assessment of 7T MRI was an important or even pivotal element of the clinical evaluation. These patients have been operated on, with confirmation of FCD (n=2), TSC (n=1) or were now planned for surgical treatment under suspicion of FCD (n=2).
The greatest benefit of 7T is the possibility to obtain high resolution 3D images in clinical settings with good SNR, thus eliminating partial volume effects while maintaining sufficient tissue contrast.
In 19 out of 23 (82.6%) patients with non-lesional lower-field MRI, 7T MRI did not show epilepsy-related lesions either. Possibly there is no structural substrate, but it is known that a portion of these patients do prove to have histological abnormalities eventually². In our series three patients had confirmed FCD or mild malformation of cortical development in the absence of any reported MR abnormalities at 7T, even after retrospective review.7T MRI holds a promise to improve identification of epileptogenic structural abnormalities in patients with intractable epilepsy. In our series of 26 patients with refractory focal epilepsy, multidisciplinary evaluation of 7T MRI identified additional lesions not seen on lower-field MRI in five patients (19.2%).