LITMIR.BIZ

       Answers

       Comments:

      The four rectus muscles—superior and inferior, lateral and medial, are well demonstrated on a coronal MRI, and the lateral and medial rectus muscles are also common questions on axial imaging. The lateral rectus muscle is innervated by the abducens nerve (6th cranial nerve), whilst the other rectus muscles are innervated by the oculomotor nerve (3rd cranial nerve).

      The superior oblique muscle originates from the medial side of the orbit, superior to the medial rectus. It is innervated by the trochlear nerve (4th cranial nerve). The inferior oblique is a small muscle which, unlike the other extra-ocular muscles, does not have its origin at the orbital apex but instead originates from the maxillary bone in the anterior orbit. Like three of the rectus muscles, it is innervated by the oculomotor nerve.

      In the eye itself, the lens and the vitreous humour behind it are readily demonstrable on CT or MRI.

      The optic nerve travels posteriorly through the orbit from the retina. It travels through the optic canal into the suprasellar cistern, where it meets the contralateral nerve at the optic chiasm.

       Exam tip:

Q1.14 Parasagittal T1-weighted sequence from an MRI head

      1 Name the arrowed structure.

      2 Name the arrowed structure.

      3 Name the arrowed structure.

      4 Name the arrowed structure.

      5 Name the arrowed structure.

       Answers

      1 Cingulate sulcus.

      2 Parietal lobe.

      3 Parieto-occipital sulcus.

      4 Calcarine sulcus.

      5 Cingulate gyrus.

       Comments:

      There are four lobes within each brain hemisphere. The frontal lobe is the largest, and is separated from the parietal lobe by the central sulcus. The pre-central gyrus (motor strip) is part of the frontal lobe immediately anterior to the central sulcus, and is responsible for motor control. The post-central gyrus is immediately posterior to the central sulcus and contains the primary sensory cortex.

      The parieto-occipital sulcus lies between the parietal lobes and the occipital lobes. The visual cortex is situated on both sides of the calcarine sulcus, which sits in the middle of the occipital lobe, and joins with the parieto-occipital sulcus medially.

      The temporal lobes are located laterally—they are separated from the frontal lobes by the Sylvian fissure anteriorly and are continuous with the parietal and occipital lobes posteriorly.

      It is also important to recognise the paired cingulate gyri (which lie immediately above the corpus callosum) and the cingulate sulci superior to them.

       Exam tip:

       If possible, be specific when naming parts of the cerebral cortex, e.g. by giving the answer ‘superior frontal gyrus’ rather than ‘frontal lobe’ on an appropriate coronal image. However, sometimes it isn’t possible to be more specific, as with the parietal lobe question above—in that case, give the name of the lobe.

Q1.15 Axial section from a T2-weighted MRI of the brain

      1 Name the arrowed structure.

      2 Name the arrowed structure.

      3 Name the arrowed structure.

      4 Name the arrowed structure.

      5 Name the arrowed structure.

       Answers

      1 Left red nucleus.

      2 Left hippocampus.

      3 Quadrigeminal cistern.

      4 Temporal horn of the right lateral ventricle.

      5 Right substantia nigra.

       Comments:

      Anteriorly within the midbrain are the cerebral peduncles which connect the brainstem with the higher CNS structures. Just posterior to these on axial slices are bilateral strips containing dopaminergic cell bodies—the substantia nigra. The red nucleus is a spherical structure of cell bodies that is identifiable on MRI and is located posterior to the substantia nigra at certain axial levels.

      The cerebral aqueduct runs perpendicularly through the midline of the posterior part of the midbrain, between the third and fourth ventricles. This is surrounded by a ring of grey matter—the peri-aqueductal grey. Behind the coronal plane containing the cerebral aqueduct is the tectal (quadrigeminal) plate. On the posterior side of the tectal plate are two sets of rounded protrusions called the superior and inferior colliculi, beyond which lies the quadrigeminal cistern. The superior colliculi are at the same axial level as the red nucleus.

       Exam tip:

       The midbrain is recognisable on axial brain sections because of its’ ‘Mickey Mouse’ ears anteriorly, formed by the cerebral peduncles.

Q1.16 Angiographic study with contrast injection into the common carotid artery

      1 Name the arrowed structure.

      2 Name the arrowed structure.

      3 Which of the cranial foramina does this structure pass through?

      4 Name the arrowed structure.

      5 Name the arrowed structure.

       Answers

      1 Pericallosal artery.

      2 Anterior

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