MRI volumetry of the hippocampus: the effect of slice thickness on volume formation.

The hippocampus is a mesial temporal lobe structure of complex shape and the main locus of temporal lobe epilepsy and also affected early in Alzheimer's disease. Magnetic resonance imaging of the hippocampus has been found to be a helpful diagnostic tool for these conditions. Measurement of hippocampal volumes has been found to be not only the most accurate approach to evaluation of the hippocampal pathology, but the most demanding and time consuming as well. One of the factors contributing to this is the slice thickness-the thinner the slice, the more time is required. On the other hand, it has been hypothesized, that the use of thick slice would result in volume aberrations due to, for example, the volume averaging effect. We therefore measured the volumes of the hippocampus on coronal slices in 10 healthy, young control subjects using slice thicknesses of 1, 3 and 5 mm, and evaluated the effect of the slice thickness on the volume formation. No significant difference between the volumes of the hippocampus using different thicknesses was found. Therefore, it appears that use of the thick slices would not necessarily introduce systematic bias to the volumetric measurement. Yet, we do not feel compelled to advise the use of "thick" slices either, since the use of "thin" slices seems the most authoritative and less vulnerable to being affected by a single false estimate.