Conventional and diffusion-weighted MRI of the hippocampus

The human hippocampus is a highly complex structure located on the medial surface of the cerebral hemispheres as a part of the intralimbic gyrus. For clinical purposes, in addition to routine transverse MRI slices, acquisitions parallel and perpendicular to the long axis of the hippocampus need to be performed to fully appreciate its curved anatomy. Clinicians should be acquainted with the normal appearance of the hippocampus regarding size, shape, symmetry, and signal as well as with the width and form of the cerebrospinal fluid spaces surrounding the hippocampus to be able to recognize abnormalities. The human hippocampus can be affected in a variety of very different acute or chronic neurological diseases, such as stroke and certain forms of encephalitis or epilepsy and dementia. Many of these pathologies are associated with distinct lesion patterns on conventional MRI. In hippocampal sclerosis, the typical imaging features - T2 hyperintensity, atrophy on T1-weighted images, and disturbed internal structures of the hippocampus - can be reliably diagnosed by visual analysis. Several visual rating scales exist for the evaluation of medial temporal lobe atrophy for the assessment of patients with cognitive disturbances; however, quantitative MRI-based volumetric analysis is increasingly being applied in research as well as clinical studies. In acute neurological disorders, diffusion-weighted imaging has the ability to demonstrate even minute and transient hyperintense hippocampal lesions. On the basis of distinct lesions patterns, diffusion-weighted MRI can provide additional diagnostic information that may facilitate and support a final diagnosis, especially in those cases in which clinical symptoms are inconclusive.