Motor cortex

The motor cortex is the cortex on the gyrus anterior to the central sulcus, the precentral gyrus. It controls voluntary movement.

Premotor cortex

The premotor cortex is the anterior part of the precentral gyrus and adjoining gyri. This too is associated with the control of voluntary movement. The posteroinferior part of the premotor area on the dominant hemisphere deals with the motor aspects of speech and is called Broca’s speech area.

Prefrontal area

Anterior to the motor and premotor cortex, the frontal lobes are involved with intellectual, emotional and autonomic activity.

On the medial surface of the frontal lobe, above and parallel to the corpus callosum, is the callosal sulcus. Above this, the cingulate gyrus extends posteriorly from the frontal lobe into the parietal lobe. This, in turn, is separated superiorly from the remainder of the medial surface by the cingulate sulcus.

Sensory cortex

This is found on the gyrus posterior to the central sulcus and is known as the postcentral gyrus. This controls somatic sensations.

Parietal association cortex

This is posterior to the sensory cortex. This area is involved with recognition and integration of sensory stimuli.

Auditory cortex

Found on the superior temporal gyrus, the auditory cortex’s function is the reception of auditory stimuli.

Temporal association cortex

Situated around the auditory cortex, this area is involved with the recognition and integration of auditory stimuli.

Visual cortex

This surrounds the calcarine sulcus and receives visual stimuli from the opposite half field of sight.

Occipital association cortex

This lies anterior to the visual cortex and is involved with the recognition and integration of visual stimuli.

CT and MRI

Identification of lobes on CT slices depends on identification of their boundaries. The sylvian cistern and fissure separating the frontal and temporal lobes are easily identified on axial CT or MR slices (Fig. 2.3). The central sulcus that forms a boundary between the frontal and parietal lobes is less well seen, however. This lies at a transverse level just posterior to the anterior limit of the lateral ventricles. Because CT images are obtained parallel to the canthomeatal line, on upper images the central sulcus is quite posterior in position. On axial images it has been described as having an inverted omega shaped curve that marks the position of the hand motor cortex (Fig. 2.3D).

Figure 2.3 • MRI of brain: (A) level of the pons; (B) level of midbrain; (C) level of the lateral ventricles; (D) above lateral ventricles.

(A)

1 Ethmoid sinus

2 Globe

3 Left temporal lobe

4 Pons

5 Fourth ventricle

6 Right cerebellar hemisphere

7 Petrous part of temporal bone

8 Basilar artery

9 Cerebellar peduncle

10 Transverse sinus

11 Pinna of ear

12 Greater wing of sphenoid bone

(B)

1 Interhemispheric fissure

2 Left frontal lobe

3 Sylvian fissure (containing branches of the middle cerebral artery)

4 Cerebral peduncle of midbrain

5 Aqueduct of Sylvius

6 Great cerebral vein in quadrigeminal cistern

7 Cerebellar vermis

8 Optic tract

9 Left ambient cistern

10 Superior colliculus and quadrigeminal plate (tectum) of midbrain

11 Left occipital lobe

12 Superior sagittal sinus

13 Anterior cerebral arteries

(C)

1 Interhemispheric fissure

2 Tapetum

3 Corpus callosum

4 Head of caudate nucleus

5 Anterior horn of right lateral ventricle

6 Interventricular foramen

7 Sylvian fissure

8 External capsule

9 Anterior limb of internal capsule

10 Putamen of lentiform nucleus

11 Globus pallidus of lentiform nucleus

12 Posterior limb of internal capsule

13 Thalamus

14 Atrium and choroid plexus of lateral ventricle

15 Calcarine sulcus

16 Superior sagittal sinus

17 Septum pellucidum

(D)

1 Centrum semiovale

2 Central sulcus – note characteristic inverted omega shape

The parieto-occipital sulcus on the medial surface of the hemisphere can be seen on CT at the level of the lateral ventricles and on midline sagittal MR images (see Fig. 2.5). The parieto-occipital junction on the lateral surface has no anatomical landmark but lies at approximately the same transverse level as the sulcus.

Midline sagittal images also show the cingulate gyrus and callosal and cingulate sulci. The insula and the frontal, parietal and temporal opercula can be seen on axial CT and on MR imaging in all planes.

The corpus callosum (Figs 2.4, 2.5)

The corpus callosum is a large midline mass of commissural fibres, each of which connects corresponding areas of both hemispheres. It is approximately 10 cm long and becomes progressively thicker towards its posterior end. Named parts include the following:

Figure 2.4 • Corpus callosum and other commissures.

Figure 2.5 • MRI scan of brain: midline sagittal image.

1 Frontal lobe

2 Parietal lobe

3 Occipital lobe

4 Rostrum of corpus callosum

5 Genu of corpus callosum

6 Body of corpus callosum

7 Splenium of corpus callosum

8 Septum pellucidum

9 Foramen of Monro

10 Fornix

11 Massa intermedia of thalami

12 Third ventricle

13 Supraoptic recess of third ventricle

14 Suprapineal recess of third ventricle

15 Pineal gland

16 Optic chiasm

17 Midbrain

18 Interpeduncular cistern

19 Aqueduct of Sylvius

20 Quadrigeminal plate (superior and inferior colliculi)

21 Quadrigeminal plate cistern

22 Fourth ventricle

23 Vermis of cerebellum

24 Pons

25 Tonsil of cerebellum

26 Prepontine cistern

27 Medulla oblongata

28 Odontoid process

29 Cisterna magna

30 Clivus

31 Pituitary

32 Tentorium cerebelli

33 Spinal cord

In cross-section, fibres from the genu that arch forward to the frontal cortex on each side are called forceps minor, and fibres from the splenium passing posteriorly to each occipital cortex are called forceps major. Fibres extending laterally from the body of the corpus callosum are called the tapetum. These form part of the roof and lateral wall of the lateral ventricle.

Anterior commissure (Figs 2.4, 2.6)

This is a bundle of fibres in the lamina terminalis in the anterior wall of the third ventricle. The fibres pass laterally in an arc indenting the inferior surface of the globus pallidus. The anterior commissure is part of the olfactory system and connects the olfactory bulbs, the cortex of the anterior perforated substance and the piriform areas.

Figure 2.6 • Anterior commissure. (A) Axial T₂ MRI. (B) Midline sagittal MRI. (C) Cor MRI.

(B)

1 Corpus callosum

2 Head of caudate nucleus

3 Internal capsule

4 External capsule

5 Anterior commissure

(C)

1 Corpus callosum

2 Fornix

3 Anterior commissure

4 Mamillary body

Habenular commissure

This small commissure is situated above and anterior to the pineal body. It unites the habenular striae, which are fibres from the olfactory centre that pass posteriorly along the upper surface of each thalamus and unite in a ‘U’ configuration in this commissure.

Posterior commissure

This is situated anterior and inferior to the pineal body. It connects the superior colliculi, which are concerned with light reflexes (see brainstem).

Hippocampal commissure

This is the commissure of the fornix (see below).

Internal capsule (see Fig. 2.3C)

This contains sensory fibres from the thalamus to the sensory cortex and motor fibres from the motor cortex to motor nuclei in the brainstem, corticobulbar tracts and, in the spinal cord, the corticospinal (pyramidal) tracts.

In cross-section, the internal capsule has an anterior limb between the caudate and lentiform nuclei and a posterior limb between the lentiform nucleus and the thalamus. Both limbs meet at a right-angle called the genu.

The anterior limb is composed mainly of frontopontine fibres. The genu and the anterior two-thirds of the posterior limb contain motor fibres. The most anterior fibres at the genu are those of the head. Fibres to the arm, hand, trunk, leg and perineum lie progressively more posteriorly. Haemorrhage or thrombosis of thalamostriate arteries supplying this area leads to paralysis of these muscles.

Behind these fibres on the posterior limb and on the retrolentiform part of the internal capsule are parietopontine and occipitopontine fibres and the sensory fibres. More posteriorly are the visual fibres that extend towards the occipital pole as the optic radiation. Most posterior of all are the auditory fibres.

Plain films of the skull

Calcification of the habenular commissure is a common finding on skull radiographs. It is found anterior and superior to the pineal gland if this is also calcified. Typically the calcification is C-shaped, with the open part of the letter facing backwards. Some authors suggest that this calcification is in the choroid plexus of the third ventricles – the taenia habenulare – rather than in the commissure.

CT and MRI

The corpus callosum cannot be well seen on axial CT slices. The internal capsule is seen as a V-shaped low-attenuation or high T₁ signal structure (see Fig. 2.3C) between the caudate and lentiform nuclei anteriorly and the lentiform and thalamus posteriorly.

The rostrum, genu, body and splenium can be seen on sagittal MRI (Fig. 2.5). The anterior and posterior commissures can also be seen on this view. A line joining the anterior and posterior commissures, the AC–PC line, is used as a reference in image-guided procedures.

On coronal MRI scans (Fig. 2.7) the body of the corpus callosum and the tapetum can be seen superior to the lateral ventricles and the internal capsule can be seen lateral to the thalami. On this view the anterior commissure may also be visible inferior to the third ventricle, but this commissure is best seen as an arc of fibres on axial MRI (Fig. 2.6).

Figure 2.7 • Coronal MRI of brain. (A) Image through the third ventricle. (B) Subthalamic nuclei.

(A)

1 Superior sagittal sinus

2 Interhemispheric fissure

3 Tapetum

4 Body of corpus callosum

5 Septum pellucidum

6 Fornix

7 Third ventricle

8 Sphenoid sinus

9 Sylvian fissure

10 Insula

11 External capsule

12 Lentiform nucleus

13 Head of caudate nucleus

14 Internal capsule

(B)

1 Head of caudate nucleus

2 Thalamus

3 Putamen

4 Globus pallidus

5 Subthalamic nucleus

Radiology pearl

Unmyelinated immature white matter because of its higher water content is darker on T₁ sequences and brighter on T₂ sequences than mature myelinated white matter. The progression of myelination can therefore be seen on MRI of infants. This progresses in a predictable way from deep to superficial, from posterior to anterior and from inferior to superior (Fig. 2.8)

Figure 2.8 • Myelination of infant brain as seen on T₁ and T₂ MR images at level of the internal capsule. (A) T₁ newborn – only the white matter in the posterior limb of internal capsule is bright, indicating that only these fibres are myelinated. (B) T₁ at 1 year old; almost all white matter is bright now. Only the most superficial subcortical fibres have not achieved adult appearance so that grey–white differentiation remains a little blurred. (C) T₁ at 2 years old. All white matter is bright and myelinated now including the subcortical white matter into the depth of each gyrus. The grey–white differentiation is sharp now. (D) T₂ newborn – only the white matter in the posterior limb of internal capsule is dark indicating myelination. (E) T₂ at 1 year old. All of the internal capsule, the corpus callosum and the deep white matter (especially posteriorly) are dark. The superficial subcortical fibres have not achieved adult appearance and grey–white differentiation remains blurred. (F) T₂ at 2 years old. All white matter is dark and myelinated now including the subcortical white matter into the depth of each gyrus.

MR tractography is a technique that uses diffusion tensor imaging to obtain images of fibre tracts within the brain. These scans also give information about fibre direction (Fig. 2.9).

Figure 2.9 • MR tractography 3D image depicting fibres passing from brain stem to cerebral cortex. Reference axial slice shows fibres as they pass through the posterior limb of the internal capsule on both sides.

Ultrasound examination of the neonatal brain (Fig. 2.10)

The corpus callosum can be seen on midline sagittal scans as a thin band of tissue between the pericallosal artery in the pericallosal sulcus superiorly, and the fluid of the cavum septum pellucidum inferiorly. It is seen below the interhemispheric fissure on coronal scans, where both its upper and lower surfaces are perpendicular to the beam. On coronal scans the internal capsule can be seen lateral to the thalamus.

Figure 2.10 • Ultrasound of infant brain. (A) Coronal image through the frontal horns of the lateral ventricles; (B) coronal image through body of the lateral ventricle. (C) Midline sagittal image; (D) parasagittal image through lateral ventricle.

1 Interhemispheric fissure

2 Sulci

3 Frontal horn of lateral ventricle

4 Corpus callosum

5 Caudate above, lentiform nucleus below, separated by internal capsule (these three are not distinguished separately)

6 Sylvian fissure

7 Brainstem

8 Parahippocampal gyrus of temporal lobe

9 Choroid plexus in atrium of lateral ventricle

10 Calcarine sulcus

11 Genu of corpus callosum

12 Cavum septum pellucidum

13 Cingulate gyrus

14 Midbrain

15 Pons

16 Medulla

17 Fourth ventricle

18 Vermis of cerebellum

19 Third ventricle

20 Body of lateral ventricle

21 Hippocampus

22 Fornix

CT and MRI

On axial CT or MRI the head of the caudate nucleus can be seen projecting into the anterior horn of the lateral ventricle on slices taken at ventricular level (see Fig. 2.3C). The head of the caudate nucleus is usually more radiodense than the lentiform nucleus or the thalamus, especially in older subjects. The body of the caudate nucleus is seen as a thin, dense stripe on the superolateral margin of the lateral ventricle on higher cuts, where it should not be confused with heterotopic cortical tissue.

The claustrum can be seen on MRI (Fig. 2.12) as a high-attenuation stripe, separated from the putamen by the external capsule and from the insula by the extreme capsule.

The amygdala is an ovoid mass of grey matter at the anterior end of the tail of the caudate nucleus and can be seen on MRI anterior to the hippocampus in the medial temporal lobe (Fig. 2.11 and 2.16C).

The connecting fibres between the lentiform and caudate nuclei that cross the anterior limb of the internal capsule and are responsible for the name corpus striatum may also be seen on MR imaging.

Ultrasound examination of the neonatal brain (Fig. 2.10)

The thalami and caudate heads can be seen on coronal images. Parasagittal scans are angled to show most of the lateral ventricle on one image. These show the head of the caudate and the thalamus forming the floor of the lateral ventricle.

Skull radiographs

Pineal calcification is visible in approximately 50% of skull radiographs after the age of 10 years and in a greater percentage of CT scans (see section on the skull).

CT and MRI

The structures forming the hypothalamus can best be appreciated on midline sagittal MRI (Fig. 2.5). The optic chiasm, the tuber cinereum, the infundibular stalk and the interpeduncular cistern, containing the mamillary bodies and in the base of which lies the posterior perforated substance, can be identified. The thalami can be seen on axial images on each side of the third ventricle (Fig. 2.3C). Their relationship to the posterior limb of the internal capsule and to the lentiform nucleus can be appreciated on this slice. The pineal gland and its superior and inferior laminae can also be best seen on midline sagittal MRI. Subthalamic nuclei can best be seen on coronal MRI (Fig. 2.7B).

Ultrasound examination of the neonatal brain (Fig. 2.10)

In the parasagittal plane the thalamus can be seen in the floor of the lateral ventricle posterior to the head of the caudate nucleus. The relation of the thalami to the internal capsule is best seen on coronal ultrasound scans. On coronal views the interthalamic adhesion can be seen within the third ventricle, especially when this is dilated.

Skull radiographs

The appearance of the pituitary fossa is affected by disease processes in the gland. This is dealt with in the section on the skull.

MRI (Fig. 2.14)

Sagittal and coronal images are most useful. The size and shape of the normal pituitary gland varies. In adult males the gland ranges from 8 to 10 mm craniocaudad dimension in the sagittal plane. The dura above the sella should be horizontal, not convex. In females, the size of the gland varies with the menstrual cycle and pregnancy. In the post-partum period it may normally exceed 12 mm in height. It is also normal for the gland to have a convex upper border in menstruating or lactating females. During puberty, the gland may also have an convex upper surface in both males and females.

In normal adults the anterior pituitary occupies 70–80% of the total gland volume and is isointense to cerebral white matter on T₁ images. The posterior pituitary usually has high signal intensity on unenhanced T₁ images due to the effect of stored neurosecretory granules on the T₁ relaxation time. This so-called posterior pituitary bright spot is evident in up to 90% of children but is less consistently observed in adults. The pars intermedia, a vestigial remnant of Rathke’s pouch, is not normally visualized on imaging studies. In some cases, a small cyst may mark the location of the pars intermedia.

The infundibulum tapers from the floor of the third ventricle to the pituitary gland. The diameter of the infundibulum should be no bigger than 3 mm or the adjacent basilar artery. The sella itself is delineated by signal void of the bony cortex and by the high-intensity signal of marrow in the clivus. The optic nerves and chiasm and the intracranial carotid vessels above, and the sphenoid sinus below, are seen clearly on coronal sections.

CT

CT has a limited role other than for evaluation of the bony sella turcica or detection of calcification. Fine cut coronal post contrast images may be an alternative to MRI in patients who have a contraindication for MRI.