Cranial to caudal, the brainstem consists of the midbrain, pons, and medulla. In turn, anterior to posterior the brainstem can be divided into the basis, tegmentum, and tectum. Throughout the craniocaudal axis, the outer contour has characteristic appearance in the axial plane at various levels. This appearance can be used to identify expected internal structures at a given location. This is useful since the internal structures are not well visualized on typical routine clinical pulse sequences. These internal structures include: 1. white matter tracts; 2. nuclei; 3. the more disperse reticular activating system (RAS) in the mid to cranial tegmentum. While it is possible to simply memorize the location of every structure at each level, it is far more effective to learn some general patterns and concepts, learn the common clinically-relevant pathologic derangements and syndromes, and have a convenient reference available when recall of further detail is needed.

The main white matter tracts to recognize are the descending corticospinal (CST), corticobulbar, and corticopontine tracts which course through the mid-posterior cerebral peduncles and the brainstem basis to the decussation of the CSTs in the very inferior medulla. The corticopontine tracts decussate in the mid-pons en route to the cerebellum via the middle cerebellar peduncle (MCP) and can be seen in certain conditions such as some cases of multiple-system atrophy.

The major ascending white matter tracts are generally named according to location in the spinal cord [dorsal cerebellospinal tract (DCST), anterolateral system (ALS), dorsal columns]. These maintain these general locations in the caudal medulla, although the dorsal columns decussate to become the medial lemniscus (ML). As these tracts ascend, the DSCT extends into the cerebellum via the inferior cerebellar peduncle (ICP) while the ALS and ML become closely associated with additional sensory tracts of the trigeminothalamic tract (TTT) to ascend in the ventral tegmentum of the pons and midbrain. Another important ascending tract in the brainstem is the medial longitudinal fasciculus (MLF) which is located near midline along the ventral margin of the 4th ventricle.

The general location of the cranial nerve (and other) key nuclei can be approximated by level (external contour) and knowledge of general expected location within that level.

The RAS is not discretely visualized on imaging, but bilateral lesions of the tegmentum in the mid to rostral brainstem is one potential cause of coma due to effect on the RAS.

The external contour of the brainstem varies by level along the cranial-caudal axis. The key internal contents at each level can then be approximated by the expected location (noting that these are 3D structures that can actually span multiple levels). While it is not practical to memorize the locations of each internal structure at each level even though all are important, there are three levels that are worth committing to memory: The rostral midbrain (mickey mouse face), facial colliculus level, and inferior olive (butterfly) levels.

The superior midbrain 'mickey mouse face' level has numerous key internal structures. The cerebral peduncles (mouse ears) contain the CST, CBT, and CPTs. Other key structures include the substantia nigra pars reticulata and pars compacta (dopamine center, relevant to Parkinson's disease), red nucleus (eyes of the face, part of triangle of Mollaret, nearby crossing cerebellothalamic tracts), ventral tegmental area (forehead region, another dopamine center), and the cranial nerve III nuclei (nostrils, and target of the MLF). The ascending ALS, TTT, and ML remain in close proximity but deviate laterally to form cheeks. The periaquaductal gray forms an open mouth. The superior colliculus forms the chin.

Axial image at the level of the upper midbrain. At this level, the outline of the midbrain will typically have a Mickey Mouse Face appearance, with the cerebral peduncles representing the ears. The general outline on axial images can be used to predict expected internal structures at a given level (although an approximation as internal nuclei and tracts are 3D structures).

The ‘swallow tail sign’ is demonstrated 45-year-old male without Parkinson’s disease. Axial 7T SWI MRI demonstrates the normal hypointense medial lemniscus (white arrow) and hypointense pars reticulata, with the intervening nigrosome of the pars compacta (*). The overall appearance has been described as the swallow tail sign; with Parkinson’s disease the nigrosome atrophies, and the sign is frequently absent. Although described on SWI, the sign can be faintly seen in the same patient on this high resolution T2 weighted image performed at 7T.

The caudal midbrain butterfly level is marked by the decussation of the superior cerebellar peduncles (SCPs), the nuclei for cranial nerve IV, and the inferior colliculi. The CSTs...The ML, ALS, and TTT remain closely associated.

Not to be confused with the molar tooth appearance of Joubert's syndrome.

The crab level is defined by the relatively prominent cranial nerve V cisternal segments and internally by key related nuclei-the motor V nucleus which is medial to the sensory V nucleus. The ML, ALS, and TTT are closely associated within the ventral portion of the tegmentum.

The butterfly level is marked by a complex external contour that signifies key related structures including the pyramids, inferior olives, and the ICPs.

Axial FGATIR of the upper midbrain at the inferior olive level. FGATIR or similar sequences allow delineation of some internal brainstem features. In this case, the inferior olives are well-delineated.

The gumdrop level corresponds to the decussation of the medial lemniscus. This wraps around central gray matter, which is present here and caudally within the spinal cord. The anterior shape corresponds to the pyramids containing the corticospinal tracts.

The most caudal level is nearly round and resembles the contour of the spinal cord. The key points are that this level contains the decussation of the pyramids where the CSTs cross from the pyramids to the location within then ventral gray matter of the spinal cord. The locations of the ascending tracts correspond to their respective names. The spinal tract.....