Comparison of two different measurement techniques of hippocampal resection length in temporal lobe epilepsy: results of a prospective study.

BACKGROUND: Magnetic resonance imaging (MRI) volumetry has evolved to a highly sensitive method for presurgical detection of hippocampal sclerosis in temporal lobe epilepsy (TLE). Seizure resolution and neuropsychological sequelae are believed to correlate with extent of resection. Therefore an easy volumetric method to determine extent of resection is desirable. The purpose of this work is to evaluate and compare two different measurement techniques for hippocampal resection length.

METHODS: Sixty-one patients with a mean seizure history of 25.1 years and medically intractable TLE were included. They underwent MRI with sagittal acquired 3D T1-weighted spoiled gradient recalled echo sequence in 1 mm(3) isotropic voxel. Hippocampal resection length was calculated with two different methods. In the slice counting method (SCM) the number of consecutive 1-mm-thick slices containing resected hippocampus formation was counted. In the vector method (VM) the sum of the oblique and thus longer distances between the centre points of segmented hippocampal areas on each MRI slice were calculated.

RESULTS: Since the hippocampus is a curved body, the resection lengths measured with VM were always larger than measured with SCM. The comparison of resection length expressed in "percent of total length" showed good agreement between the two methods, because unlike the absolute values of resection length, the percentage values are unaffected by the three-dimensional shape of the hippocampus.

CONCLUSION: The easier and quicker method of "slice counting" may be used to determine resection length expressed in "percent of total length", giving reliable values for resection length but causing less volumetric work.