5 Imaging Anatomy and Pathology of the Brainstem, Thalamus, and Pineal Region
Abstract
This chapter introduces the reader to the common imaging modalities used to investigate the brainstem, thalamus, and pineal region. It also presents an overview of the pertinent imaging anatomy of these regions. Here, we provide a framework for the unique imaging anatomy of these regions that will allow accurate interpretation of imaging studies in patients with brainstem pathology. This anatomy includes that which can be directly viewed using clinical magnetic resonance imaging (MRI) and that which can be inferred by reproducible anatomical landmarks. In addition, we describe the imaging features of the common pathology of the brainstem, thalamus, and pineal region.
Imaging Modalities Used to Investigate the Brainstem, Thalamus, and Pineal Region
Magnetic Resonance Imaging
Routine clinical magnetic resonance imaging (MRI) sequences used to evaluate the brainstem include T1-weighted (short TR, short TE), T2-weighted (long TR, long TE), fluid-attenuated inversion recovery (FLAIR), T2*-weighted (gradient-recalled echo [GRE]), and diffusion-weighted imaging. Three-dimensional (3D) T2-weighted turbo spin-echo (TSE) and steady-state free precession imaging are also commonly used in the assessment of the posterior fossa, including the brainstem. These techniques provide an excellent contrast of the cisternal segments of dark cranial nerves (CNs) as they course through bright cerebrospinal fluid (CSF); however, they do not allow for the assessment of brainstem nuclei. 1 , 2 Steady-state free precession imaging is the preferred technique in patients without skull base region susceptibility-induced field distortions, and it is commonly referred to by trade names (i.e., CISS [constructive interference into steady state, Siemens] and FIESTA [fast imaging employing steady-state acquisition, General Electric]). Three-dimensional TSE imaging yields less sharply defined cisternal CNs, but it is less adversely affected by the susceptibility-induced field distortions of dental amalgam. Proton density sequences (long TR, short TE) have been shown to improve the visualization of some thalamic nuclei. 3
Advanced MRI techniques include diffusion tensor imaging, associated 3D fiber tractography, and functional MRI. Diffusion tensor imaging provides the benefit of assessing ascending and descending projection fibers, as well as commissural and association fibers. 4 In contrast, functional MRI is useful in localizing nuclei of the brainstem on the basis of the performance of their respective tasks; however, in the brainstem, the technique is not routinely performed in clinical practice. 5 Experimental high-field-strength (7T) imaging of magnetic susceptibility maps and quantitative maps of relaxation rates have shown improved visualization of brainstem nuclei. 6
Computed Tomography
Compared to MRI, computed tomography (CT) captures the exquisite anatomy of the brainstem, thalamus, and pineal region in broad strokes, given the relatively decreased contrast resolution of CT. In addition, CT assessment of the brainstem and posterior fossa is often made more difficult by beam-hardening artifact from the calvarium and osseous skull base. Nonetheless, CT remains the mainstay of first-line imaging in the emergency, inpatient, and outpatient settings. Despite these challenges, a great deal of information can be obtained by CT. The primary role of this modality in the clinical context of patients with brainstem lesion signs is the exclusion of hemorrhage, edema, mass lesion, and ischemia. CT cisternography may be used to assess the cisternal segments of CNs in patients who have a contraindication to MRI.
Imaging Anatomy of the Brainstem, Thalamus, and Pineal Region
The brainstem, thalamus, and pineal region can be embryologically subdivided into the myelencephalon, metencephalon, mesencephalon, and diencephalon. 7 Nomenclature more practical to clinical imaging would be the anatomical divisions of the medulla, pons, midbrain, and separately, the thalamus and pineal regions.
A detailed description of the anatomy of this region can be found in Chapter 2, Anatomy of the Brainstem, Thalamus, Pineal Region, and Cranial Nerves. Although MRI reveals this anatomy in exquisite detail, there are limitations to which structures can be discerned by imaging. In this chapter, the reader will be introduced to the imaging anatomy of these complex regions. Certainly, some anatomical structures can be visualized by routine clinical MRI sequences. For example, the red nucleus can be easily identified within the mid-brain. In addition to the anatomy that can be directly visualized, there are reproducible anatomical landmarks which, when appropriately identified and understood, allow for an accurate estimation of the location of adjacent nuclei and white matter tracts. These landmarks are the external contours of the brainstem and the location of exiting CNs and their proximity and relationship to the anatomy that can be directly visualized by imaging. Knowledge of this anatomy is essential to interpreting images, to correctly localizing lesions, and to accounting for, or predicting, a patient’s clinical presentation.
Key white matter tracts and the boundaries they demarcate provide a framework for the study of the imaging anatomy of the brainstem. The brainstem can be divided at all levels into anterior, middle, and posterior compartments. These compartments are the basis, the tegmentum, and the tectum, respectively. The basis largely consists of corticospinal and corticobulbar tracts. These tracts descend from the precentral gyrus, with corticobulbar fibers terminating at various levels throughout the brainstem. The corticospinal tracts continue to descend, and the majority of the fibers decussate at the level of the lower medulla within the pyramids. The middle compartment of the brainstem is referred to as the tegmentum, which contains white matter tracts and CN nuclei that will be detailed in this chapter. The posterior compartment of the brainstem is the tectum. At the level of the midbrain, the tectum comprises the superior and inferior colliculi. At the level of the pons, the tectum comprises the superior medullary velum, which forms the roof of the fourth ventricle, and the upper part of the inferior medullary velum. At the level of the medulla, the tectum consists of the inferior part of the inferior medullary velum. The tectum of the brainstem contains no ascending or descending tracts and no CN nuclei.
The medial lemniscus is a white matter tract that can be viewed as an approximate boundary between the anterior compartment (the basis) and the middle compartment (the tegmentum) of the midbrain and pons. Importantly, the medial lemniscus is not observed in the same location at all levels of the brainstem. Rather, the course is oblique, as will be detailed. Functionally, the medial lemniscus is the second-order neuron system of the dorsal column pathway that transmits fibers from the nucleus gracilis and the nucleus cuneatus to the ventroposterolateral nucleus of the thalamus. These nuclei receive afferent fibers from the dorsal column system of the spinal cord, the fasciculus gracilis and the fasciculus cuneatus. This system relays information related to fine touch and proprioception from the upper and lower extremities, respectively. With this framework in mind, we will further explore the imaging anatomy of the brainstem from the top down, beginning with the midbrain.
Midbrain
In the axial imaging plane, the midbrain is roughly heart-shaped. The CSF spaces that border the midbrain include the interpeduncular cistern anteriorly, the ambient cisterns anterolaterally, and the quadrigeminal plate cistern posteriorly ( Fig. 5.1 ). These three cisterns can be collectively referred to as the perimesencephalic cisterns. The interpeduncular and ambient cisterns are subunits of the greater suprasellar cistern.
An easily identifiable landmark within the midbrain is the cerebral aqueduct. The cerebral aqueduct courses through the midline posterior aspect of the midbrain and connects the third ventricle above and the fourth ventricle below. The cerebral aqueduct is surrounded by the periaqueductal gray matter, which can be seen on clinical MRI ( Fig. 5.2 ).
At the level of the midbrain, the medial lemniscus is situated approximately along the posterolateral margin of the substantia nigra. The anterior compartment of the brain-stem contains the substantia nigra and the somatotopically organized corticospinal, corticobulbar, and corticocerebellar fibers. 8 Immediately posterior lies the midbrain tegmentum, which extends to the cerebral aqueduct. Posterior to the cerebral aqueduct lies the tectal plate. The midbrain can be further divided into superior and inferior segments at the level of the superior and inferior colliculi, respectively.
Level of the Superior Colliculi
Anteriorly and superiorly, the midbrain is contiguous with the cerebral peduncles. The red nuclei lie at the level of the superior colliculi and are clearly seen on clinical MRI ( Fig. 5.2 ). The paired red nuclei are located anteriorly and near the midline within the tegmentum. The substantia nigra lies between the red nuclei and the cerebral peduncles and is divided into the pars reticulata and the pars compacta. These subcomponents can only be roughly delineated by clinical MRI. The ventral tegmental area is associated with the substantia nigra and is part of the dopaminergic system. The ventral tegmental area is bounded posteriorly by the red nucleus, anterolaterally by the substantia nigra, and anteromedially by the interpeduncular cistern. It extends approximately 4 mm laterally from the midline. 9 High-field-strength (7T) MRI can separately identify the pars compacta and the pars reticulata of the substantia nigra, as well as the ventral tegmental area and the vascularized and non-vascularized components of the red nucleus 10 ; however, this level of detailed anatomy cannot be detected by current clinical magnets.
The oculomotor nuclear complex (ONC) is composed of somatic and visceral nuclei. The somatic nuclei control several extraocular muscles: the superior, inferior, and medial recti; the inferior oblique muscles; and the levator palpebrae. 8 , 11 The Edinger-Westphal nucleus lies posteromedial. The Edinger-Westphal nucleus provides parasympathetic preganglionic motor input to the ciliary ganglion to control pupillary constriction and accommodation. The ONC is situated between the periaqueductal gray matter and the red nuclei within the paramedian midbrain, at the level of the superior colliculi. 8 The fasciculi of the oculomotor nerve (CN III) exit the ONC and course ventrally, traversing the medial aspect of the red nuclei to exit the brainstem at the level of the lateral wall of the interpeduncular fossa ( Fig. 5.2 ). The ONC and the fasciculi of the oculomotor nerve cannot be directly visualized on clinical imaging; however, they can be inferred by the location of the periaqueductal gray matter, the red nuclei, and the cisternal course of the oculomotor nerve.
Level of the Inferior Colliculi
At the level of the inferior colliculi, the decussation of the superior cerebellar peduncles occupies the central midbrain anterior to the periaqueductal gray matter 4 ( Fig. 5.3 ) directly inferior to the red nuclei. The trochlear nuclei are located near the midline, inferior to the level of the ONC. That is, they are located posterior to the decussation of the superior cerebellar peduncles and anterior to the cerebral aqueduct and the periaqueductal gray at the level of the inferior colliculi. From the trochlear nuclei, the trochlear nerve (CN IV) wraps around the periaqueductal gray matter to decussate within the superior medullary velum and exit the brainstem posteriorly ( Fig. 5.3 ). The trochlear nerve is unique in that it exits the brainstem posteriorly. The trochlear nerve is also the CN with the longest cisternal course.
Pons
The transition from the midbrain to the pons is demarcated by the pontomesencephalic sulcus. The bulbous basilar pons constitutes the majority of the pons and primarily houses corticospinal, corticobulbar, and corticopontine tracts; pontine nuclei; and transverse pontine fibers. The transverse pontine fibers can be divided into superficial and deep, as well as superior and inferior fibers. The superficial and deep transverse pontine fibers are demarcated by their position relative to the corticospinal tracts. 12 The superior and inferior trans verse pontine fibers are separated by their orientation relative to the root entry zone of the trigeminal nerve (CN V), a landmark easily identified on clinical MRI. These transverse pontine fibers course posteriorly and laterally to form the middle cerebellar peduncles.
Within the pons, the approximate location of the medial lemniscus may be inferred as a region of subtle change in signal intensity on 3D TSE imaging ( Fig. 5.4 ). The pontine tegmentum contains the nuclei of the trigeminal nerve, the abducens nerve (CN VI), the facial nerve (CN VII), and the vestibulocochlear nerve (CN VIII). These nuclei cannot be directly visualized by clinical imaging. The pontine tegmentum is much smaller than the pontine basis, and it is directly contiguous with the midbrain above and the medulla below.
Posteriorly and superiorly within the pons, the cerebral aqueduct transitions into the fourth ventricle. The roof of the fourth ventricle consists of the superior medullary velum, a structure easily seen in the sagittal plane ( Fig. 5.5 ). Anatomical landmarks within the pons, which can be identified on clinical MRI and therefore serve as useful sections for further study, include the root entry zone of the trigeminal nerves and the facial colliculus.
Level of the Root Entry Zone of the Trigeminal Nerve
The root entry zone of the trigeminal nerve is easily identified on MRI at the lateral margin of the pons, where the nerve courses anteriorly along the cisternal segment toward the gasserian ganglion within Meckel’s cave ( Fig. 5.4 ). The trigeminal nerve has both motor and sensory components. The smaller motor fibers of the cisternal segment of the trigeminal nerve and the larger sensory fibers are not distinguishable by imaging.
The four nuclei of the trigeminal nerve consist of a single motor nucleus, which is the main motor nucleus of the trigeminal nerve, and three sensory nuclei: the primary sensory nucleus, the mesencephalic nucleus, and the trigeminal spinal nucleus. These nuclei are not located at a single level within the brainstem but rather course variably in the craniocaudal direction. The sensory nuclei are roughly organized in a craniocaudal fashion such that the main sensory nucleus is the most centrally located, with the mesencephalic nucleus above and the trigeminal spinal nucleus below.
At approximately the level of the root entry zone, the complex of the main motor nucleus and the primary sensory nucleus of the trigeminal nerve is situated at the lateral aspect of the pontine tegmentum, just anterior to the superior cerebellar peduncle ( Fig. 5.4 ). The motor nucleus is located medially and the sensory nucleus laterally. From this complex, the sensory and motor fibers of the trigeminal nerve course anterolaterally through the posterior aspect of the basilar pons to exit the brain-stem at the root entry zone.
The mesencephalic nucleus is located slightly posterior and medial to the complex of the main motor nucleus and the primary sensory nucleus of the trigeminal nerve at the level of the root entry zone ( Fig. 5.4 ). The mesencephalic nucleus and tract extend superiorly to the midbrain. As the tract ascends, it assumes a position just along the lateral margin of the central aspect of the periaqueductal gray.
The spinal trigeminal nucleus and tract originate just inferior to the complex of the main motor nucleus and the primary sensory nucleus, approximately at the level of an axial slice just inferior to the root entry zone ( Fig. 5.4 ). As they descend, the spinal trigeminal nucleus and tract course lateral to the facial nerve. These fibers continue to the level of the upper cervical spine.
Level of the Facial Colliculus
The facial colliculus appears as a bump along the floor of the fourth ventricle ( Fig. 5.4 ). Deep to the facial colliculus lie the abducens nuclei and fibers of the facial nerve. These structures cannot be directly visualized by clinical MRI; however, direct visualization of the facial colliculus bump and an understanding of the course of the facial nerves allow for an accurate estimation of the location of the facial and abducens nuclei and their central fasciculi. As part of this discussion, it is important to introduce the medial longitudinal fasciculus (MLF) and its relationship to adjacent anatomy.
The MLF is a network of heavily myelinated fibers responsible for the coordination of horizontal eye movements connecting the nuclei of the oculomotor nerve, the trochlear nerve, the abducens nerve, and the vestibulocochlear nerve, among others. 13 , 14 The MLF consists of paramedian tegmental tracts that run from the midbrain inferiorly to the level of the upper cervical spine. 14 A lesion of the MLF results in internuclear ophthalmoplegia, which clinically presents as impaired adduction of the ipsilateral eye and normal abduction of the contralateral eye with gaze away from the lesion.
The MLF, which is approximately 1 mm wide, occupies the paramedian pontine tegmentum at the level of the facial colliculus ( Fig. 5.4 ). 12 Just lateral to the MLF, at the level of the facial colliculus, lie the abducens nuclei. The abducens fasciculi arise from the anterior margin of their corresponding nuclei and course anterolaterally to the corticospinal tracts within the basilar pons to exit the brainstem anteriorly.
The facial nerve nuclei are situated anterior and lateral to the abducens nuclei, roughly bounded by the medial lemniscus anteriorly and the inferior cerebellar peduncle posterolaterally ( Fig. 5.4 ). They are situated approximately 5.5 mm from the roof of the fourth ventricle in an anterolateral direction. 12 The facial nerve fasciculi originate at the posterior aspect of their nuclei and course posteriorly and medially. The facial nerve courses between the abducens nuclei and the MLF to wrap around the abducens nuclei and then courses anterolaterally to exit the brainstem at the pontocerebellar junction.
Both the vestibular and cochlear nerves have bipolar neurons. The fibers of the superior and inferior vestibular nerves converge near the level of the porus acusticus of the internal auditory canal to form the vestibular nerve. Just medial to this point, the vestibular nerve merges with fibers from the cochlear nerve. At this point, the nerve is referred to as the vestibulocochlear nerve, and from there it enters the brainstem at the lateral pontomedullary junction. This entrance into the brainstem can be seen on high-resolution MRI ( Fig. 5.4 ).
The cochlear nerve has two nuclei: the ventral and dorsal cochlear nuclei. These nuclei are situated laterally within the brainstem and extend in the craniocaudal direction through the medulla and lower pons. Within the medulla, these nuclei lie lateral to the restiform body, which is a component of the inferior cerebellar peduncle.
The vestibular nerve has four nuclei: the superior, inferior, medial, and lateral nuclei. These nuclei form a complex that is located at the level of the lower aspect of the pons, roughly along the lateral aspect of the fourth ventricle.
Medulla
The medulla is the inferior extent of the brainstem and is contiguous with the upper cervical spinal cord. The nuclei of the glossopharyngeal nerve (CN IX), vagus nerve (CN X), spinal accessory nerve (CN XI), and hypoglossal nerve (CN XII) are located within the medulla. As mentioned previously, the dorsal column tracts of the spinal cord extend superiorly to their corresponding nuclei gracilis and cuneatus at the level of the medulla. The nucleus gracilis and its associated tract are located at the paramedian posterior-most aspect of the lower medulla. The nucleus cuneatus and its associated tract are located just lateral to the nucleus gracilis. From these nuclei, fibers of the medial lemniscus first decussate at the mid-medulla and then ascend superiorly to the thalamus.
The decussation of the corticospinal tracts also occurs at the level of the medulla within the pyramids, which are located anteriorly. The pyramids form characteristic contours of the anterior medulla, which can be identified by clinical imaging ( Fig. 5.6 ). The midline ventral sulcus of the medulla separates the pyramids. Lateral to the pyramids, an additional highly reproducible contour can be identified: the olives. Deep to the olives lie the inferior olivary nuclei. The preolivary sulcus lies between the pyramids and the olives, and the postolivary sulcus lies farther posterolateral to the olives ( Fig. 5.6 ). The anterolateral system, including the spinothalamic tract, lies deep to the postolivary sulcus.
The nucleus ambiguous is the shared nucleus of the bulbar motor nerve fibers of the glossopharyngeal nerve, the vagus nerve, and the spinal accessory nerve. This nucleus lies in the mid-medullary tegmentum, just medial to the descending spinal trigeminal nucleus and tract, and its location can only be roughly estimated with imaging. The glossopharyngeal nerve has additional sensory and parasympathetic fibers, which terminate in the solitary tract nucleus, the spinal trigeminal nucleus, and the inferior salivatory nucleus. In addition, the vagus nerve has sensory and parasympathetic fibers that project to the dorsal vagal nucleus and the solitary tract nucleus. The spinal accessory nerve has an additional nucleus located within the upper spinal cord, the spinal nucleus of the spinal accessory nerve. These nuclei cannot be seen on clinical MRI. The glossopharyngeal, vagus, and spinal accessory nerves collectively exit the brainstem at the postolivary sulcus.
The hypoglossal nucleus sits at the paramedian posterior aspect of the medulla and forms a subtle contour into the floor of the fourth ventricle, the hypoglossal eminence (hypoglossal trigone) ( Fig. 5.6 ). The hypoglossal eminence lies superior to the nucleus gracilis and its tract. The central course of the hypoglossal nerve arcs from its nucleus anterolateral to the root exit zone at the preolivary sulcus ( Fig. 5.6 ). The hypoglossal nerve at this level consists of multiple nerve fibers that span approximately 12.5 mm in the craniocaudal direction. 15 These nerve rootlets converge to form the cisternal segment of the hypoglossal nerve and eventually pierce the dura at the hypoglossal canal.
Thalamus
The thalamus is the principal site for the relay of sensory input. The dorsal thalamus (commonly referred to as just the thalamus) is part of the diencephalon, along with the subthalamus, which includes the subthalamic nucleus; the epithalamus, which includes the pineal gland and habenular nuclei; and the hypothalamus. Neuroanatomy atlases reveal a rich network of nuclei and tracts throughout the thalami that are not discernible by routine clinical imaging. However, experimental high-field-strength 7T imaging has been shown to be able to identify many subcomponents of the thalami. 16
The thalamus is bound anterolaterally by the posterior limb of the internal capsule and medially, continuing posteromedially, by the third and lateral ventricles ( Fig. 5.7 ). From a clinical imaging perspective, the thalamus is homogeneous in signal intensity, which does not allow for discrimination of specific thalamic nuclei. Pathology in the dorsomedial thalamus, the pulvinar, can be identified in various disease states and is a classic description of variant Creutzfeldt-Jakob disease. In the sagittal plane, the connection between the two thalami is referred to as the massa intermedia.
Pineal Region
The pineal gland is responsible for the rhythmic nighttime production and secretion of melatonin and functions to regulate circadian rhythm and the sleep-wake cycle. 17 The pineal gland is an ellipsoid pinecone-shaped structure, measuring approximately 79±30.2 mm3, that is located centrally deep within the brain. 18 Complex pineal region anatomy can be imaged with routine clinical MRI. The sagittal plane offers the best appreciation of this anatomy. The imaging boundaries of the pineal region are as follows: superiorly—the splenium of the corpus callosum; inferiorly—the tectal plate of the midbrain; anteriorly—the third ventricle and its posterior recesses; posteriorly—the posterior aspect of the quadrigeminal plate cistern; and laterally—the pulvinar of the thalamus ( Fig. 5.8 ).
The pineal gland lies centrally within this region and is oriented with its base anterior and superior, and its apex poste rior and inferior ( Fig. 5.8 ). The pineal gland is suspended within the quadrigeminal plate cistern. The base of the pineal gland is attached to superior and inferior laminae ( Fig. 5.8 ). The fibers of the habenular commissure course within the superior aspect of the superior lamina and communicate laterally with the habenular nuclei. The fibers of the posterior commissure course within the inferior lamina and communicate laterally with the thalamus. The apex of the superior and inferior lamina, at the level of the base of the pineal gland, forms the pineal recess of the third ventricle ( Fig. 5.8 ). The inferior lamina continues inferiorly to merge with the tectal plate. Superior to the habenular commissure is another posterior third ventricular recess, the suprapineal recess ( Fig. 5.8 ).
As noted previously, the splenium of the corpus callosum forms the superior border of the pineal region ( Fig. 5.8 ). The f ornices course along the inferior aspect of the corpus callosum. The velum interpositum is situated beneath the fornices, and the internal cerebral veins (ICVs) are enclosed within the velum interpositum. The velum interpositum and ICVs lie superior to the pineal gland as well as the third ventricle. This relationship provides a useful frame of reference to localize a mass within or outside the pineal gland. That is, in the presence of a large pineal region mass, if the ICVs are displaced superiorly, the mass may arise from the pineal gland or other cells of origin inferior to the ICVs. If the ICVs are displaced inferiorly, the mass is not pineal parenchymal in origin. Anterior to the pineal gland, the third ventricle lies inferior to the velum interpositum. The choroid plexus wraps along the roof of the third ventricle and forms an imaging demarcation between the third ventricle below and the velum interpositum above.
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