Atlas of Human Anatomy on MRI: Brain, Chest & Abdomen Hariqbal Singh, Parvez Sheik
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BrainCHAPTER 1

 
INTRODUCTION
The human brain is most complex organ in the body. It develops from the neural tube (Flow chart 1), undergoes complex divisions to form the forebrain (prosencephalon), hindbrain known as Rhombencephalon (Figs 1 to 5) and midbrain also known as mesencephalon (Figs 6 to 11 and Flow chart 1).
 
PROSENCEPHALON
The forebrain or prosencephalon separates into:
  1. Telencephalon
  2. Diencephalon
 
Telencephalon
It is part of prosencephalon and consists of: (A) Two cerebral hemispheres; (B) Basal ganglia and (C) White matter.
 
Cerebral Hemispheres
These are on either side of the cranial cavity above the tentorium cerebelli. On magnetic resonance imaging (MRI) the axial, coronal and sagittal sections provide excellent view of the various cerebral lobes and their pathology. The cerebral hemisphere is separated from each other in midline by the interhemispheric fissure. The interhemispheric fissure extends from the floor of anterior cranial fossa to the posterior cranial cavity. In the middle part of the interhemispheric fissure lies the corpus callosum which connects the two cerebral hemispheres in midline.
The gray matter and white matter are appreciated on axial, coronal and sagittal MR images. The gray matter on cerebral hemispheres is located on the outer aspect while the white matter also called as centrum semiovale is located deep in cerebral hemispheres.
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Flow chart 1: Brain structures and their divisions
2Some white matter extends from centrum semiovale into the cortical gyri; these are called subcortical white matter. The cortical gyri are separated from each other by cleft like involutions known as cortical sulci.
The cortical sulci help in subdividing the cerebral hemispheres for descriptive purposes into:
  1. Frontal lobe (Figs 8, 12 to 15 and 25 to 29) is the anterior part of cerebral hemisphere, it is limited posteriorly by the central sulcus (of Rolando) and inferiorly by the sylvian fissure. The frontal lobe has prominent gyri like the superior frontal gyrus, middle frontal gyrus and inferior lateral frontal gyrus appreciated on MR sagittal and coronal sections. On computed tomography (CT) scan image frontal lobe is seen posterior to the frontal sinuses and anterior to the middle cerebral arteries. Middle cerebral arteries which are the largest branches of internal carotid arteries are seen as hyperdense vascular structures just above the suprasellar cistern.
  2. Parietal lobe (Figs 12 to 16 and 21) lies posterior to the central sulcus of Rolando and extends up to the parieto-occipital sulcus beyond which the occipital lobe begins. The inferior border of parietal lobe is related to the sylvian fissure. On CT scan centrum semiovale forms the white matter of the internal portion of cerebral hemisphere. Cortical sulci are small involutions seen on convexity of cerebral cortex; the cortical gyrus is located between the sulci.
  3. Insular region (central lobe) is the fifth lobe of the brain. Hidden under the temporal, frontal and parietal opercula, and under dense arterial and venous vessels (Figs 12, 22 and 31), it has five gyri and is located deep within the lateral fissure between frontal and temporal lobes. It consists of an anterior and a posterior portion. It is concerned with homeostasis and regulation of emotions. The insular region is appreciated on axial, coronal and sagittal sections on MRI sections. The insular region is supplied by branches from the middle cerebral artery. Middle cerebral artery infarcts characteristically reveal loss of definition of the gray-white interface a hyperdense thrombus on axial CT images in insular region (known as the “Insular ribbon sign”). The insular region is appreciated on axial, coronal and sagittal sections on T1W and T2W images.
  4. Temporal lobe (Figs 5 to 9 and 30) is located below the sylvian fissure. It is visualized on T1W images in axial, coronal and sagittal sections. It has superior, middle and lower temporal gyri. The temporo-occipital gyrus is seen on posterior aspect of temporal lobe. The hippocampal gyrus is seen on sagittal T1W sections. Its medial aspect is better appreciated in coronal MR images. The upper part of the superior temporal gyrus contains the transverse temporal gyri of Heschl—the main auditory areas and Brodmann areas. Lesions in the auditory areas usually do not lead to complete deafness because each auditory area receives auditory impulses from both the ears. Each lateral lemniscus in the brainstem contains fibers derived from both the cochlear nuclei.
  5. Occipital lobe (Figs 6, 7, 13 and 15) is located at the posterior aspect of cerebral hemisphere; it is limited anteriorly by the parieto-occipital sulcus. It is well visualized on T1W images in axial, coronal and sagittal sections. The tentorium cerebelli on its inferior aspect separates the occipital lobe from cerebellar hemispheres.
 
Basal Ganglia
These are central gray matter nuclei in each cerebral hemisphere and consist of the caudate nucleus, lentiform nucleus, claustrum and amygdala. The basal ganglia are seen on T1W and T2W images in axial, sagittal and coronal sections. Corpus striatum is a collective term for the caudate nucleus and lentiform nucleus. The anterior and posterior limbs of internal capsule lie between the basal ganglia and thalamus.
The basal ganglia is subdivided for descriptive purposes into:
  1. Caudate nucleus (Figs 11, 21 to 23 and 31) has a head, body and tail. The head and body of caudate nucleus is seen on axial, coronal and sagittal MR sections. The head of caudate nucleus abuts the frontal horn of lateral ventricle. The body of caudate nucleus lies along the periventricular region of body of lateral ventricles. The tail of caudate nucleus is better seen on sagittal and coronal images. The tail of caudate nucleus is narrow and it is superolateral to the thalamus.
  2. Lentiform nucleus (Figs 11 and 23) appears as a wedge shaped nucleus. It is located lateral to the caudate nucleus and thalamus best seen in axial and coronal MRI sections. The lentiform nucleus is separated from the caudate nucleus by the anterior limb of internal capsule.
    The posterior limb of internal capsule separates the lentiform nucleus from thalamus. The lentiform nucleus has two components, an outer putamen and an inner globus pallidus. The sublentiform part of the internal capsule passes under the lentiform nucleus to the auditory cortex. The main afferent 3fibers come from the cortex, substantia nigra and thalamus. Fibers from the putamen mostly end in the globus pallidus. Efferent fibers from globus pallidus reach thalamus, subthalamic nuclei, hypothalamus, red nucleus, substantia nigra and reticular formation. The final outflow tracts reach the spinal cord via the rubrospinal and rubro-reticulospinal tracts. Lesions of the basal ganglia lead to extrapyramidal effects such as dyskinesias, rigidity and tremor.
  3. Claustrum (Fig. 33) is a thin strip of saucer-shaped nucleus which is located lateral to the putamen. It is easily seen on axial and coronal section in MR images.
  4. Amygdala is located in the roof of the temporal horn of lateral ventricle. It contains nuclei which are connected to the tail of caudate nucleus and the limbic system. Stria terminalis is the efferent connection of amygdala connecting the amygdala to the anterior hypothalamus.
  5. Internal capsule (Figs 12, 31 and 32) is best seen on axial and coronal sections on MRI. It is a compressed white band of projection fibers going to and from the brain. It lies medial to the lentiform nucleus and lateral to the caudate nucleus and thalamus. It has five parts: (a) Anterior limb; (b) Genu; (c) Posterior limb; (d) Retrolentiform part; and (e) Sublentiform part.
The anterior limb, genu and posterior limb contain frontopontine fibers.
The retrolentiform part contains the occipitopontine fibers and a few parietopontine fibers.
The sublentiform part contains the parietopontine and the temporopontine fibers. The corticonuclear fibers are located in the genu of the internal capsule. The posterior limb contains corticospinal and corticorubral fibers. Vascular lesions of the internal capsule can cause extensive damage. Lesions of the genu can cause paralysis of face of the opposite side due to involvement of corticonuclear fibers for head and neck. Lesions of the posterior limb can cause extensive contralateral hemiplegia and loss of sensations on the opposite side of the body. Lesions of the retrolentiform and sublentiform parts can cause visual and auditory defects. By far the most common cause of lesions is the rupture of one of the branches of the middle cerebral artery (Lenticulostriate arteries–Charcot's artery of cerebral hemorrhage).
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Fig. 1: MRI head axial section T2-weighted image
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Fig. 2: MRI head axial section T2-weighted image
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Fig. 3A: MRI head axial section T2-weighted image
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Fig. 3B: CT scan brain axial section
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Fig. 4: MRI head axial section T2-weighted image
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Fig. 5A: MRI head axial section T2-weighted image
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Fig. 5B: CT scan brain axial section
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Fig. 6: MRI head axial section T2-weighted image
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Fig. 7A: MRI head axial section T2-weighted image
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Fig. 7B: CT scan brain axial section
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Fig. 8A: MRI head axial section T2-weighted image
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Fig. 8B: CT scan brain axial section
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Fig. 9: MRI head axial section T2-weighted image
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Fig. 10: MRI head axial section T2-weighted image
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Fig. 11A: MRI head axial section T2-weighted image
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Fig. 11B: CT scan brain axial section
 
White Matter Tracts
These consist of corpus callosum, commissural fibers and corona radiata. The perivascular spaces (of Virchow Robin) surround the striatal vessels and appear as oval areas of bright signal intensity. The white matter tracts are subdivided for descriptive purposes into:
Corpus callosum (Figs 12 to 14, 17 and 31) is the largest commissural white matter tract of the brain. It connects the two cerebral hemispheres. It consists of rostrum, genu, trunk and splenium. The large rounded splenium forms the posterior end of corpus callosum. The corpus callosum develops in an anterior to posterior end, the genu forms first followed by the body and splenium, the rostrum is the last to develop. On MRI images the genu and rostrum show slightly lower signal intensity as compared to the body and splenium. The rostrum of corpus callosum continues inferiorly to join the lamina terminalis. The body of corpus callosum lies over the roof of lateral ventricle. The splenium of corpus callosum overlies the pineal body and colliculi. Fibers from the occipital lobes run forwards, cross to the opposite side in the splenium and reach the occipital lobe of the opposite side forming forceps major.
  1. Forceps major (Fig. 22) are commissural fibers that run from the occipital lobe and loop through the splenium of corpus callosum to the opposite occipital lobe.
  2. Forceps minor (Fig. 22) are commissural fibers that run from the frontal lobe and loop through the anterior part of corpus callosum to the opposite frontal lobe.
 
Diencephalon
Diencephalon consists of the thalamus, hypothalamus, subthalamus, epithalamus and metathalamus.
  1. Thalamus (Figs 12, 13 and 20) is an ovoid structure of brain seen on axial, sagittal and coronal MRI sections. The thalamus lies lateral to the third ventricle and medial to the posterior limb of internal capsule. The thalamus contains a large number of nuclei. The important nuclear groups are lateral, medial and anterior nuclei. The ventral posterior nucleus is a part of the lateral nuclear group and receives inputs from medial, spinal and trigeminal lemniscus. Fibers from here pass to the sensory part of the cerebral cortex. Some parts of lateral nucleus receive fibers from the dentate nucleus of the cerebellum and globus pallidum. The fibers project to the motor areas of the cerebral cortex. The medial group of thalamic nuclei receives fibers from the hypothalamus, corpus striatum and frontal lobes. The medial group of nuclei appears to be concerned with emotions and memory.12
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    Fig. 12A: MRI head axial section T2-weighted image
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    Fig. 12B: CT scan brain axial section
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    Fig. 13A: MRI head axial section T2-weighted image
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    Fig. 13B: CT scan brain axial section
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    Fig. 14A: MRI head axial section T2-weighted image
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    Fig. 14B: CT scan brain axial section
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    Fig. 15: MRI head axial section T2-weighted image
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    Fig. 16A: MRI head axial section T2-weighted image
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    Fig. 16B: CT scan brain axial section
    The anterior group of thalamic nuclei receives fibers mainly from the hypothalamus via the mamillothalamic fasciculus. Most ascending tracts pass through and relay in the thalamus as they proceed towards the cortex of the cerebral hemispheres. Fibers from anterior nuclei project to cingulate gyrus-part of the limbic system. The geniculate bodies, medial and lateral, are two rounded bodies forming the posterior aspect of the thalamus. The medial geniculate body is a relay station in the auditory pathway and the lateral geniculate body is a relay station in the visual pathway.
  2. Metathalamus is formed by the medial and lateral geniculate bodies. The geniculate bodies’ main function is to serve as relay stations for the visual and auditory pathways. The medial geniculate body is a part of the auditory pathway; the inferior brachium connects the medial geniculate body to the inferior colliculus. The medial geniculate body relays the information to the auditory cortex located on the superior temporal gyrus. The lateral geniculate body is a part of the visual pathway; the superior brachium connects the lateral geniculate body to superior colliculus. The lateral geniculate body relays the information to the visual cortex located in occipital lobe.
  3. Epithalamus is formed by the habenula, pineal body and posterior commissure seen on T1W and T2W images in axial, coronal and sagittal sections. The pineal stalk has a superior and inferior lamina. The Habenula forms the superior lamina. The posterior commissure forms the inferior lamina. The pineal recess is located between the superior and inferior lamina.
  4. Hypothalamus is a triangular brain structure inferior to the anterior part of the thalamus is seen on sagittal and axial MRI sections. It consists of the mammillary bodies, tuber cinereum, infundibulum, hypophysis and optic chiasm. In a sagittal MRI section, the hypothalamus is separated from the thalamus by an oblique hypothalamic sulcus which extends from the aqueduct of sylvius to the interventricular foramen of Monro. The anterior part of the hypothalamus is connected to the infundibular stalk of pituitary. The mammillary bodies form the most posterior part of the hypothalamus. The hypothalamic groove is seen on the lateral walls of the third ventricle and extends from the aqueduct of sylvius to the interventricular foramen. The hypothalamus has important roles in autonomic functions (modulation of visceral activities like thermoregulation, appetite, thirst, sexual 17responses and emotions), endocrine functions (regulates release of hormones from the pituitary gland) and pain response such as blunting of painful stimuli by releasing endorphins and encephalin.
  5. Subthalamus is best seen on sagittal and coronal MRI sections as a thin zone of transition between the thalamus and tegmentum of the midbrain.
 
Limbic System
The limbic system is concerned with emotional and personality aspects of each individual. The limbic system is made up of various nuclei and interconnecting fibers. The olfactory nerves, olfactory bulb and tract, hippocampal gyrus, fornix, fimbriae, dentate gyrus, mammillary body, uncus, cingulate and parahippocampal gyri, amygdaloid body, septal and piriform areas of cortex near lamina terminalis and anterior thalamic nuclei.
  1. Hippocampal gyrus (Figs 23, 24, 31 and 33) is seen on sagittal and coronal MRI sections. The hippocampal gyrus is located in temporal lobe and receives input from the mammillary body, the septal region, amygdala, superior and middle frontal gyri, superior temporal and cingulate gyri, precuneus, lateral occipital cortex, occipitotemporal gyri and subcallosal cortical areas.
  2. Mammillary body (Figs 9 and 17) is seen well on sagittal MRI sections. It is located as a small nodular area in the anterior aspect of interpeduncular cistern. The mammillary body is connected with the dorsal and ventral tegmental nuclei, anterior thalamic nucleus (via the mamillothalamic tract), septal nuclei, tegmental pontine and reticulotegmental nuclei.
  3. Amygdaloid complex is seen well on sagittal and coronal MRI sections at the roof of temporal horns of lateral ventricles. The amygdaloid nuclear complex receives input from and projects to the brainstem and forebrain centers via the stria terminalis and ventral amygdalofugal pathway. Corticoamygdaloid and amygdalocortical fibers interconnect the basal and lateral amygdaloid nuclei to cerebral cortex.
  4. Fornix (Figs 12 and 17) is an important part of the limbic system by providing efferent connections. The body of fornix is a bundle of white matter mass beneath the corpus callosum in midline and communicates with fibers from the other side of brain. The fornix has anterior and posterior fibers, the anterior fibers of fornix connect to the septal nuclei near lamina terminalis and the posterior fibers connect to anterior thalamic nuclei or to the mammillary bodies.
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Fig. 17 to 24: MRI head sagittal T2-weighted images
 
MESENCEPHALON AND RHOMBENCEPHALON
For convenience of description and understanding mesencephalon and rhombencephalon are dealt with together:
  1. Brainstem is formed by midbrain, pons and medulla oblongata. It is seen on axial, coronal and sagittal MRI sections.
    1. Midbrain (Figs 8, 9 and 35) is also called mesencephalon. It is seen on axial, coronal and sagittal MRI sections as a short upper segment of the brainstem at the level of tentorial inscisura. The midbrain on axial sections can be subdivided by an imaginary line passing horizontally through the central canal into cerebral peduncles anteriorly and tectum posteriorly. Between the cerebral peduncles and tectum is a central gray matter which surrounds aqueduct of sylvius passing through the substance of midbrain the aqueduct of Sylvius is best seen on midsagittal sections. The interpeduncular fossa containing the interpeduncular cistern is located anteriorly between the cerebral peduncles. The cerebral peduncles of midbrain have two parts, the ventral part is called the crus, the dorsal part is called the tegmentum.
      The tectum of midbrain has nodular prominences (corpora quadrigeminia). They are four in number and are called superior and inferior colliculi. Behind the corpora quadrigemina is the quadrigeminal cistern. The fibers of the superior colliculi are connected to the lateral geniculate body in the visual pathway. The fibers of the inferior colliculi are connected to the medial geniculate body in the auditory pathway. The midbrain contains the trochlear nerve nucleus at the level of the inferior colliculus and the occulomotor nucleus at the level of the superior colliculus. The mesencephalic part of the trigeminal nerve nucleus, responsible for receiving proprioceptive information from the regions of head and neck is located at all levels of midbrain. The pretectal nucleus is seen at the upper levels of the midbrain. It is a relay station in the pupillary light reflex arc. The anterior part is called as the cerebral peduncle and the posterior part is called the tectum. The tegmentum is further divided into the anterior crus cerebri, a middle layer called the substantia nigra and a posterior tegmentum. The posterior tegmentum contains the superior colliculus at the upper levels, a relay station in the visual pathway and the inferior colliculus at the lower levels of midbrain-a relay station in the auditory pathway.
    2. Pons (Figs 3 to 5 and 19) means “bridge” in latin language, because it is a part of the brainstem 22which connects the midbrain above to the medulla oblongata below. Pons is visualized as a large ventral protuberance in brainstem on T1W and T2W images in axial, sagittal and coronal sections. The superior pontine sulcus separates the pons from midbrain above. The inferior pontine sulcus separates the pons from medulla oblongata below. The pons consists of an anterior basilar part and a posterior tegmentum. The basilar part shows an anterior midline sulcus called the basilar sulcus which lodges the basilar artery. The basilar part contains pontine nuclei and the descending corticospinal and corticonuclear tracts. The posterior tegmentum contains the pontine reticular formation, the band of lemnisci (medial lemniscus, trigeminal lemniscus, spinal lemniscus, and lateral lemniscus) and cranial nerve nuclei. The anteriormost part of the tegmentum contains the trapezoid body-formed by the crossing over of fibers from the cochlear nuclei. The lower part of pons has nuclei of the abducent nerve, facial nerve and spinal nucleus of the trigeminal nerve. It also contains the vestibular nuclei and the dorsal and ventral cochlear nuclei. The upper part of pons contains the chief sensory nucleus and the motor nucleus of the trigeminal nerve. The pons is supplied by blood via the pontine branches of the basilar artery. Bilateral pontine hemorrhages result in pin point pupils, hyperpyrexia, coma, bilateral paralysis of face and limbs. Millard Gubler syndrome is due to a lesion in the anterior part of the lower pons which involves the pyramidal tracts, the emerging fibers of the abducent and facial nerves.
      Medulla oblongata of CT appears as structure in posterior fossa and lies anterior to the fourth ventricle. It is the lowest part of the brainstem which continues below as the spinal cord and seen on axial, coronal and sagittal MRI sections. The medulla oblongata is 3 cm in length, sagittal diameter is 1.25 cm and width is 2 cm. The medulla contains the vital cardio respiratory centers in its reticular formation. The medulla contains important cranial nerve nuclei, they are hypoglossal nucleus, dorsal nucleus of vagus, nucleus of tractus solitarius, the nucleus ambiguus, the spinal nucleus and tract of the trigeminal nerve, the vestibular nucleus and the cochlear nucleus. The upper part of the medulla contains the inferior olivary nucleus and the medial lemniscus. The posterolateral parts of the medulla are supplied by the posterior inferior cerebellar artery, the anteromedial parts are supplied by the anterior spinal artery, the part intervening being supplied by the anterior choroidal artery (Figs 18, 19 and 36).
 
Important Nuclei of Midbrain
  1. Substantia nigra (Fig. 9) is a layer of dark semilunar pigmented area located in the ventral part of midbrain, is identified by the hypointense signal on T1W images in axial MRI sections. It is composed of a dorsal pars compacta and a ventral pars reticularis. The pars reticularis is continuous above with the lentiform nucleus. The substantia nigra has important reciprocal connections with the corpus striatum, the pathways are dopaminergic. Parkinsonism is a disorder due to a depletion of dopamine in these pathways.
  2. Red nucleus (Fig. 10) is seen at the level of the superior colliculus on T2W images in axial sections as hypointense signals due to pigmentation from iron deposits. It is an oval shaped nucleus, varies from 0.5 to 0.6 cm in diameter. The main afferents of the red nucleus are the dentato-rubro-thalamo-cortical fibers and the cortico-striato-rubral fibers. Efferent fibers from the red nucleus go to various subcortical centers. The rubrospinal or the rubroreticulospinal tract is the main efferent pathway. The red nucleus is considered to be a part of the functional basal ganglia.
  1. Cerebellum (Figs 1 to 4, 41 and 42) is seen on axial, coronal and sagittal MRI sections. The cerebellum is located in the posterior cranial fossa below the tentorium cerebelli. The cerebellum is posterolateral to pons and medulla separated by fourth ventricle in the midline. The posterior surface of cerebellum is related to the inner table of occipital bone. The anterolateral surface of cerebellum is related to the inner table of temporal bone. The cerebellum is connected to the brainstem by the superior, middle and inferior cerebellar peduncles. The cerebellar tonsils are located posterolateral to the medulla and cerebellar hemisphere. Normal range for the cerebellar tonsils below foramen magnum is 2–3 mm in adults and 5 mm in infants. When the cerebellar tonsils are more than 4 mm below foramen magnum ectopic position of cerebellar tonsils is considered. The cerebellar hemispheres consist of an outer shell of gray matter and an inner core of white matter. The inner core of white matter contains a collection of gray matter called the cerebellar nuclei. These nuclei are named as nucleus fastigiis, nucleus globosus, nucleus emboliformis and dentate nucleus.
    Incoming afferent fibers are essentially of two types. Climbing fibers are those which climb around the dendrites of the Purkinje cells. Olivocerebellar fibers are the only examples of climbing fibers. All other afferent fibers constitute Mossy fibers. These fibers end 23as synaptic rosettes with whom dendrites of granule cells synapse. Generally, all afferent fibers terminate in the cortex while giving small collaterals to the cerebellar nuclei. Fibers from the cortex end in the cerebellar nuclei. Efferent fibers from the cerebellar nuclei pass through the superior or inferior cerebellar peduncles to reach their destinations in the brainstem, spinal cord and the cerebrum. Morphologically, the cerebellum is divided into archicerebellum-flocculonodular lobe, paleocerebellum-central lobule, culmen, pyramid, uvula and their corresponding hemispherical lobes- and neocerebellum - declive, folium, tuber and their corresponding cerebellar hemispheres. The archicerebellum corresponds to vestibulocerebellum, the paleocerebellum corresponds to spinocerebellum and the neocerebellum corresponds to corticocerebellum. The cerebellum is supplied by blood from superior cerebellar, anterior inferior and posterior inferior cerebellar arteries. The cerebellum is mainly concerned with maintenance of equilibrium, muscle tone and muscle coordination. Lesions of the cerebellum typically result in ataxia, atonia and motor coordination difficulties.
    Falx cerebri (Figs 37, 41 and 42) appears isointense to hypointense on T1W images and hypointense to hyperintense on T2W images due to calcifications. The falx cerebri is the largest fold of dura mater, is seen as a thin membrane between the two cerebral hemispheres in the interhemispheric fissure. It is a double fold of the inner meningeal layer of dura mater. It lies in the longitudinal cerebral fissure. It is attached anteriorly to crista galli and posteriorly to the tentorium cerebelli. The falx cerebri is more wide posteriorly. It contains the superior sagittal sinus in its upper margin and the inferior sagittal sinus in its lower free margin.
    Tentorium cerebelli (Fig. 40) is a dura mater partition which makes a strong membranous roof or superior surface over the cerebellum as an arched lamina, elevated in the middle and supports the occipital lobes. Its anterior border is free and concave, with a large oval opening called incisura tentorii, for the transmission of the cerebral peduncles. It is attached; behind, by its convex border, to the transverse ridges upon the inner surface of the occipital bone, and there encloses the transverse sinuses.
 
CEREBROSPINAL FLUID AND VENTRICULAR SYSTEM
  1. The cerebrospinal fluid (CSF) is hypointense on T1W images and hyperintense on T2W images. Its signal characteristics depend on image acquisition time (TR/TE), relaxation time of the CSF and its constituents, physiological flow factors. The CSF is produced by the choroid plexus of lateral ventricles, third and fourth ventricle. The CSF is in constant motion. In the ventricular system the CSF flow is cephalocaudal direction. In the subarachnoid cisterns the flow is in the caudal-cephalic direction.
  2. Ventricular system: The ventricular system (Fig. 43) comprise of four CSF filled, ependymal lined cavities that lie within the brain. The entire system consists of four ventricles, which are known as lateral ventricles, third ventricle and fourth ventricle.
    1. Lateral ventricles (Figs 10, 12, 13, 23, and 32) are large cavities located within the cerebral hemispheres. They contain the CSF and the choroid plexus. The lateral ventricles well seen on axial, coronal and sagittal MRI sections. The CSF within the lateral ventricles appears hypointense on T1W images and hyperintense on T2W images. The lateral ventricle has frontal horn, temporal horn and occipital horn. The trigone or atrium of lateral ventricles is triangular prominence of floor of lateral ventricle at the transition between occipital and temporal horns. The inner lining of the lateral ventricles is formed by ependymal cells. Near the midline on the floor of lateral ventricles lies the interventricular foramen of Monro which allows the flow of CSF to the third ventricle.
    2. Choroid plexus of lateral ventricle (Figs 14 and 35) is a cluster of vascular glomera with a spongy appearance and is responsible for producing CSF. The glomera measures between 3 and 22 mm in size normally. The glomera of the choroid plexus show hypointense signals on T1W images due to their vascularity or calcium deposits. On T2W images the glomerula of choroid plexus show variable heterogenous signal intensity due to flow void, cysts, lipid deposits and calcifications.
    3. Septum pellucidum (Figs 13, 14 and 19) appears as two thin leaves on the medial side of lateral ventricles in midline on axial and coronal MRI sections. They are isointense to hypointense on both T1W and T2W images. Occasionally, there may be a small cavity with CSF within the leaves of septum pellucidum; this is called cavum septum pellucidum.
    4. Interventricular foramen of Monro (Fig. 18) is seen at the floor of each lateral ventricle. This foramen connects the lateral ventricles to the third ventricle and allows the CSF to circulate.
    5. Third ventricle (Fig. 32) is a single midline cavity located between the two thalami on axial and coronal MRI sections. CSF within the third ventricle shows hypointense signal on T1W images and hyperintense signal on T2W images. The third ventricle communicates above with the lateral ventricles via the 24interventricular foramen of Monro. It communicates below with the fourth ventricle via the aqueduct of sylvius. Its anterior wall is formed by lamina terminalis. The floor of the third ventricle is related to optic chiasma, infundibulum, mammillary bodies and the midbrain tegmentum. The choroid plexus is located on the roof of the third ventricle. The lateral walls of the third ventricle are formed by the thalami.
    6. Aqueduct of Sylvius (Figs 17 and 34) is a thin slit like tubular hollow structure which connects the third ventricle to the fourth ventricle. It is closely related to the midbrain anteriorly and the colliculi posteriorly. It is well visualized on axial and sagittal T1W images.
    7. Fourth ventricle (Figs 3, 4, 17, 18 and 36) is seen on T1W axial and sagittal MRI sections. It appears as a diamond shaped cavity on sagittal sections located behind the pons and medulla and in front of the cerebellum. On T1W images it appears as hypointense signal and on T2W images it appears hyperintense. The fourth ventricle communicates above with the aqueduct of sylvius. The midsagittal section is useful in evaluating the shape and configuration of the aqueduct of sylvius and fourth ventricle. The fourth ventricle communicates below with the central canal of spinal cord.
    8. The foramina of Magendie and Luschka (Fig. 37) are openings in the posterior walls of fourth ventricle which allow the cerebrospinal fluid in the ventricular system to enter the subarachnoid space. The lateral openings are called the foramina of Luschka and the midline opening is called the foramina of Magendie. The floor of the fourth ventricle is formed by the pons in its upper part and the medulla in the lower half.
  3. Subarachnoid space is the space between the arachnoid and the pia matter. Large collections of CSF within the subarachnoid spaces are called cisternal spaces which are better seen on T2W images as hyperintense signals around the brain structures they surround. Blood vessels of the brain travel in the subarachnoid space for considerable distances before piercing the pia and entering the substance of the brain. The subarachnoid space contains the CSF. The subarachnoid space is continous with the subarachnoid space of the spinal cord. Subarachnoid hemorrhage is usually an arterial bleed while subdural hemorrhage is usually a venous bleed. Arterial bleeds could be a result of trauma to the bones of skull, rupture of aneurysms, etc. Venous bleeds are usually a result of tearing of the cerebral veins draining into the superior sagittal sinus due to a sudden anteroposterior movement of the brain as a result of severe injuries.
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    Fig. 39:
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    Fig. 40:
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    Fig. 41:
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    Figs 25 to 42: MRI head coronal T2-weighted images
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    Fig. 43: Ventricular system of brain
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  4. The cisternal spaces (Figs 18, 35 and 38) are large collections of CSF in subarachnoid space. These cisternal spaces are named according to their location. The cisternal spaces contain the CSF and appear hypointense on T1W images and hyperintense on T2W images. In addition to the CSF, the cisternal spaces contents contain nerves and vessels. The cisternal spaces are
    1. The olfactory sulcal cistern (Figs 18 and 31) contains olfactory bulb and tract.
    2. The suprasellar cistern contains the optic nerve, optic chiasm, optic chiasm tract, pituitary stalk, internal carotid arteries, origins of middle and anterior cerebral arteries and posterior communicating artery.
    3. The parasellar and sylvian cisterns (Figs 18 and 31) contain the middle cerebral artery, Meckel's cave and trigeminal nerve. The sylvian cistern is a lateral extension of the suprasellar cistern.
    4. The interpeduncular cistern (Figs 38 and 39) contains occulomotor nerve, basilar artery, posterior cerebral artery and superior cerebellar artery.
    5. Mesencephalic cistern includes ambient and quadrigeminal cistern, and contains choroidal arteries, basal vein of Rosenthal, trochlear nerve and abducens nerve.
    6. The cerebellopontine angle cistern: The facial and vestibulocochlear nerves traverse the cerebellopontine angle cistern (Fig. 4).
    7. The medullary cistern (Figs 1 and 2) contains the vertebral artery, the glossopharyngeal nerve, vagus nerve and hypoglossal nerve. The pontomedullary cistern is located in front of pons and medulla, it communicates laterally with the cerebellopontine cistern and posteriorly with the cisterna magna.
  5. Subdural space is a potential space between the dura mater and the underlying arachnoid matter. It acts like a bursa allowing movement between dura and the structures it encloses. Subdural hemorrhage usually occurs due to rupture of veins as they enter the dural venous sinuses and is identified on GRE recalled sequences.