The hippocampus is traced in the oblique coronal viewing plane. Images are first reoriented to the long axis of the hippocampus by selecting the most anterior and posterior limits of the hippocampus and reorienting the images such that the anterior and posterior limits of the hippocampus are in parallel along the long axis. This has been previously reported to promote the clear and easy identification of hippocampal anatomy.
Once images are reformatted, the hippocampus is subdivided and traced as anterior and posterior segments in successive oblique coronal slices proceeding in a posterior to anterior manner.
The posterior hippocampus is first traced on the slice in which the crus of the fornix can be delineated (Fig. 1, A). As the fornix is the major efferent pathway of the hippocampus this has proved to be a reliable landmark that is based on hippocampal anatomy rather than structures unrelated to the hippocampus.
Following the identification of the crus of the fornix, the hippocampus is traced using the alveus as the superior boundary and the white matter of the parahippocampal gyrus as the inferior boundary. The inferior temporal horn of the lateral ventricle is used as the lateral boundary and the ambient cistern the medial boundary.
The hippocampus tracings include the head of the hippocampus (CA1, CA2, CA3 fields) and the subiculum. The shape of the hippocampus in posterior sections is one of a ball of gray matter that comprises the head and a straight segment of gray matter that is the subiculum (B, C). As you move in an anterior direction, the hippocampus begins to fold over on to itself and is the level at which you see the beginning of the hippocampal sulcus (D).
The last section in which to trace the posterior hippocampus is at the level that the crus cerebri is still connected to the pons.
The next slice is the first slice to start tracing the anterior hippocampus. The anterior hippocampus is traced using the same boundaries as the posterior hippocampus. The difference is that the amygdala, the other mesial temporal lobe anatomic structure, generally is observable at this level and is situated superior or above the hippocampus (D).
In beginning tracings of the anterior hippocampus the inferior temporal horn of the lateral ventricle clearly separates the hippocampus from the amygdala (D, E). However, as you move more anteriorly, the inferior temporal horn moves from a superior position to a more lateral position.
The alveus is then used to separate hippocampus from amygdala (F). In some cases the amygdala and hippocampus cannot be separated from each other using the temporal horn of the lateral ventricle and in these circumstances the alveus is also used as the boundary. If the alveus cannot be delineated then a straight line is traced from the middle of the inferior temporal horn of the ventricle to the ambient cistern.
In the most anterior sections, the anterior hippocampus is termed the pes hippocampi and generally does not have the folded shape that is characteristic in earlier slices. The hippocampus also becomes incrementally smaller in size and tends to move medially along with the temporal horn of the lateral ventricle.
The last section in which to trace the anterior hippocampus is the slice that the inferior temporal horn has moved completely from a lateral position to a medial one and has also moved completely beneath or under the amygdala (G).