Chapter 25 Recognizing Some Common Causes of Intracranial Pathology

Advances in neuroimaging have had a remarkable impact on the diagnosis and treatment of neurologic diseases ranging from earlier detection and treatment of stroke to a more timely diagnosis of dementia, from the rapid detection and treatment of cerebral aneurysms to the ability to diagnose multiple sclerosis after a single attack.

Both CT and MRI are utilized for studying the brain and spinal cord, but MRI is the study of first choice for most clinical scenarios (Table 25-1). Conventional radiography has no significant role in imaging intracranial abnormalities.

TABLE 25-1 IMAGING STUDIES OF THE BRAIN FOR SELECTED ABNORMALITIES

Abnormality	Study of First Choice	Other Studies
Acute stroke	Diffusion-weighted MR imaging for acute or small strokes, if available	Noncontrast CT can differentiate hemorrhagic from ischemic infarct
Headache, acute and severe	Noncontrast CT to detect subarachnoid hemorrhage	MR-angiography (MRA) or CT-angiography (CTA) if subarachnoid hemorrhage is found
Headaches, chronic	MRI without and with contrast	CT without and with contrast can be substituted
Seizures	MRI without and with contrast	CT without and with contrast can be substituted if MRI not available
Blood	Noncontrast CT	Ultrasound for infants
Head trauma	Nonenhanced CT is readily available and the study of first choice in head trauma	MRI is better at detecting diffuse axonal injury but requires more time and is not always available
Extracranial carotid disease	Doppler ultrasonography	MRA excellent study
Hydrocephalus	MRI as initial study	CT for follow-up
Vertigo and dizziness	Contrast-enhanced MRI	MRA if needed
Masses	Contrast-enhanced MRI	Contrast-enhanced CT if MRI not available
Change in mental status	MRI without or with contrast	CT without contrast is equivalent

Normal Anatomy

The normal anatomy of the brain is somewhat easier to understand with CT than MRI (Fig. 25-1).

In the posterior fossa, the 4^th ventricle appears as an inverted U-shaped structure. Like all cerebrospinal fluid (CSF) containing structures, it normally appears black. Superior to the 4^th ventricle are the cerebellar hemispheres, inferiorly lie the pons and medulla oblongata. The tentorium cerebelli separates the infratentorial components of the posterior fossa (medulla, pons, cerebellum, and 4^th ventricle) from the supratentorial compartment.

The interpeduncular cistern lies in the midbrain and separates the paired cerebral peduncles (which emerge from the superior surface of the pons). The suprasellar cistern is anterior to the interpeduncular cistern and usually has a five-point or six-point starlike appearance.

The sylvian fissures are bilaterally symmetrical and contain CSF. They separate the temporal from the frontal and parietal lobes.

The lentiform nucleus is composed of the putamen (laterally) and globus pallidus (medially). The 3^rd ventricle is slitlike and midline. At the posterior aspect of the 3^rd ventricle is the pineal gland. Farther posterior is the quadrigeminal plate cistern.

The corpus callosum connects the right and left cerebral hemispheres and forms the roof of the lateral ventricle. The anterior end is called the genu and the posterior end is called the splenium.

The basal ganglia are represented by the subthalamic nucleus, substantia nigra, globus pallidus, putamen, and caudate nucleus. The putamen and caudate nucleus are called the striatum.

The frontal horns of the lateral ventricles hug the head of the caudate nucleus. The two frontal horns are separated by the midline septum pellucidum. The temporal horns, which are normally very small, are more inferior and contained in the temporal lobes. The posterior horns of the lateral ventricle (occipital horns) lie in the occipital lobes. The most superior portion of the ventricular system is formed by the bodies of the lateral ventricles.

The falx cerebri lies in the interhemispheric fissure, which separates the two cerebral hemispheres, and is frequently calcified in adults.

The surface or cortex of the brain is made up of gray matter convolutions, which in turn are composed of sulci (grooves) and gyri (elevations). The medullary white matter lies below the cortex.

Figure 25-1 Normal unenhanced CT scans of the head.

A, Frontal lobes (F); temporal lobes (T); temporal horns (solid white arrows); fourth ventricle (4); cerebellum (C); pons (P). B, Suprasellar cistern (S); cerebral peduncles (P); interpeduncular cistern (solid white arrow). C, Sylvian fissures (S); 3^rd ventricle (3); interpeduncular cistern (solid black arrow); quadrigeminal plate cistern (solid white arrow). D, Anterior horns of the lateral ventricles (solid white arrows); caudate nuclei (C); 3^rd ventricle (3); occipital lobes (O). E, Caudate nuclei (C); lentiform nuclei (L); calcified pineal gland (solid white arrow). F, Genu of corpus callosum (dotted white arrow); lateral ventricles (L); septum pellucidum (dashed white arrow); parietal lobes (P); occipital horn (solid black arrow); calcified choroid plexus (solid white arrows); occipital lobes (O).

On an unenhanced CT scan of the brain, anything that appears “white” will generally either be bone (calcium) density or blood in the absence of a metallic foreign body (Table 25-2).

Calcifications that may be seen on CT of the brain which are nonpathologic:

• Pineal gland (Fig. 25-2A)

• Basal ganglia (see Fig. 25-2A)

• Choroid plexus (see Fig. 25-1F)

• Falx and tentorium (Fig. 25-2B)

Normal structures that can enhance after administration of iodinated intravenous contrast:

• Venous sinuses

• Choroid plexus

• Pituitary gland and stalk

Metallic densities in the head can cause artifacts on CT scans. Dental fillings, aneurysm clips, and bullets can all cause streak artifacts.

TABLE 25-2 CT DENSITIES

Hypodense (Dark) (AKA Hypointense)	Isodense	Hyperdense (Bright) (AKA Hyperintense)
Fat (not usually present in the head) Air (e.g., sinuses) Water (e.g., CSF)	Normal brain Some forms of protein (e.g., subacute subdural hematomas)	Metal (e.g., aneurysm clips or bullets) Iodine (after contrast administration) Calcium Hemorrhage (high protein)

Figure 25-2 Physiologic calcifications.

A, There are small, punctate calcifications in the basal ganglia (white circles) and calcification of the pineal gland (solid white arrow). B, Calcification of the falx cerebri is present (dotted white arrow). Anything that appears white on a noncontrast enhanced CT of the brain is either calcium or blood. Physiologic calcifications tend to increase in incidence with advancing age.

Mri and the Brain

In general, MRI is the study of choice for detecting and staging intracranial and spinal cord abnormalities. It is usually more sensitive than CT because of its superior contrast and soft tissue resolution. It is, however, less sensitive than CT in detecting calcification in lesions and cortical bone, which appear as signal voids with MR. It cannot be used in patients with pacemakers.

MRI is more difficult to interpret because the same structure or abnormality will appear differently depending on the pulse sequence, the scan parameters, and the fact that MRI is more variable in its depiction of differences which occur over the time course of some abnormalities (e.g., hemorrhage) than is CT.

Initial evaluation of an MRI of the brain might start with the T1-weighted sagittal view of the brain. On this view the brain looks more like the anatomic specimens or diagrams you are accustomed to looking at (Fig. 25-3). We are fortunate that many structures in the brain are paired, so don’t forget to compare one side to the other on axial scans of the brain (Fig. 25-4).

Table 25-3 summarizes the signal characteristics of various tissues seen on MRI.

Figure 25-3 Normal midline MRI.

Sagittal T1-weighted close-up image demonstrates the midline structures of the brain. For orientation purposes, anterior is to your left (A). The corpus callosum (CC) is located superiorly. The pituitary gland (P) sits in the sella turcica and connects to the hypothalamus via the pituitary stalk or infundibulum (dotted white arrow). The mamillary bodies (M) are located anterior to the brain stem. The cerebral aqueduct (solid white arrow) is superior to the midbrain. The brain stem is composed of the midbrain (Mi), the pons (Po), and the medulla (Me). The 4^th ventricle (4) communicates with the cerebral aqueduct and lies between the cerebellum (Ce) and the brain stem.

Figure 25-4 Normal MRI of the brain, T1 and T2.

Axial T1-weighted (A) and T2-weighted (B) images of the brain demonstrate that CSF in the lateral ventricles is dark on T1 and bright on T2 (solid black arrows). Gray matter, which contains the neuronal cell bodies, is actually gray on T1-weighted images (open white arrow), and white matter, which contains myelinated axon tracts, is whiter (white circle). The caudate nuclei (solid white arrows) and lentiform nucleus (dotted white arrows) together form the basal ganglia. The thalamus (dotted black arrows) is located posterior to the basal ganglia.

TABLE 25-3 SIGNAL CHARACTERISTICS OF VARIOUS TISSUES SEEN ON T1-WEIGHTED AND T2-WEIGHTED MRI SCANS

Head Trauma

Traumatic brain injuries extract a huge cost to the patient and society, not only as a result of the acute injury but also for the long-term disability they produce. In the United States, motor vehicle accidents account for nearly half of traumatic brain injuries.

Unenhanced CT is the study of choice in acute head trauma. The primary goal in obtaining the scan is to determine if there is a life-threatening, but treatable, lesion.

Initial CT evaluation of the brain in the emergency setting focuses on whether there is mass effect or blood.

To determine if there is mass effect, look for a displacement or compression of key structures from their normal positions by analyzing the location and appearance of the ventricles, basal cisterns and the sulci.

Blood will usually be hyperintense (bright) and might collect in the basal cisterns, sylvian and interhemispheric fissures, ventricles, the subdural or epidural spaces, or in the brain parenchyma (intracerebral).

Skull Fractures

Skull fractures are usually produced by direct impact to the skull and they most often occur at the point of impact. They are important primarily because their presence implies a force substantial enough to cause intracranial injury.

In order to visualize skull fractures, you must view the CT scan using the “bone window” settings that optimize visualization of the osseous structures (Fig. 25-5).

Skull fractures can be described as linear, depressed, or basilar.

• Linear skull fractures are the most common and have little importance other than for the intracranial abnormalities that may have occurred at the time of the fracture, such as an epidural hematoma. Fractures of the cranial vault are most likely to occur in the temporal and parietal bones (see Fig. 25-5B).

• Depressed skull fractures are more likely to be associated with underlying brain injury. They result from a high-energy blow to a small area of the skull (e.g., from a baseball bat), most often in the frontoparietal region and are usually comminuted. They may require surgical elevation of the depressed fragment when the fragment lies deeper than the inner table adjacent to the fracture (Fig. 25-6A).

Figure 25-5 CT, “brain” and “bone” windows.

In order to visualize skull fractures, you must view the CT scan using the “bone” windows. A, Using the “brain window,” a lenticular-shaped, hyperintense lesion is seen in the left frontal region displaying the typical appearance of an epidural hematoma (solid black arrows). B, Viewing the same scan at the “bone window” setting shows a fracture (solid white arrow) of the left frontal bone at the site of the epidural hematoma.

Figure 25-6 Skull fractures.

A, There is a depressed fracture of the right parietal bone with the fragment lying deeper than the inner table of the adjacent bone (solid white arrow). Depressed skull fractures occur more often in the frontoparietal region. B, On this close-up axial CT of the mastoids, there is a comminuted fracture of the right temporal bone (solid white arrows), fluid in the mastoid air cells (white circle), and air in the brain (pneumocephalus) (dotted white arrow). Basilar skull fractures can be associated with tears in the dura mater with subsequent CSF leak. (A = Anterior.)

Basilar skull fractures are the most serious and consist of a linear fracture at the base of the skull. They can be associated with tears in the dura mater with subsequent CSF leak, which can lead to CSF rhinorrhea and otorrhea. They can be suspected if there is air seen in the brain (traumatic pneumocephalus), fluid in the mastoid air cells, or an air-fluid level in the sphenoid sinus (Fig. 25-6B).

Facial Fractures

CT is the imaging study of choice for evaluating facial fractures. Multislice scanners allow for reconstruction in the coronal plane so the patient does not have to be repositioned in the scanner.

Care must be taken in diagnosing facial fractures based upon viewing what appears to be a fracture on only one image since CT scans, by their nature, produce sections so thin, they may not demonstrate the entire contour of the bone in question. Look for a fracture to appear on several contiguous images.

The most common orbital fracture is the blow-out fracture, which is produced by a direct impact on the orbit (e.g., a baseball strikes the eye) that causes a sudden increase in intraorbital pressure leading to a fracture of the inferior orbital floor (into the maxillary sinus) or the medial wall of the orbit (into the ethmoid sinus). Sometimes, the inferior rectus muscle can be trapped in the fracture leading to restriction of upward gaze and diplopia.

Recognizing a blow-out fracture of the orbit (Fig. 25-7A).

• Orbital emphysema. Air in the orbit from communication with one of the adjacent air-containing sinuses, either the ethmoid or maxillary sinus.

• Fracture through either the medial wall or floor of the orbit.

• Entrapment of fat and/or extraocular muscle, which projects downward as a soft tissue mass into the top of the maxillary sinus.

• Fluid (blood) in the maxillary sinus.

A tripod fracture, usually a result of blunt force to the cheek, is another relatively common facial fracture. This fracture involves separation of the zygoma from the remainder of the face by separation of the frontozygomatic suture, fracture of the floor of the orbit, and fracture of the lateral wall of the ipsilateral maxillary sinus (Fig. 25-7B).

Figure 25-7 Facial bone fractures.

A, Blow-out fracture. Air is demonstrated in the left orbit representing orbital emphysema (black circle). A fracture of the floor of the orbit is seen, and soft tissue (in this case, fat) extends inferiorly into the top of the maxillary sinus (solid white arrow). B, Tripod fracture. There is diastasis of the frontozygomatic suture on the left (dotted white arrow), a fracture of the floor of the orbit with orbital emphysema (dashed white arrow), and a fracture (solid white arrow) through the lateral wall of the maxillary sinus (M), which is filled with blood.

Intracranial Hemorrhage

Skull fractures may be associated with intracranial hemorrhage and/or diffuse axonal injury.

There are four types of intracranial hemorrhages that may be associated with head trauma:

• Epidural hematoma

• Subdural hematoma

• Intracerebral hemorrhage

• Subarachnoid hemorrhage (discussed with aneurysms)

Epidural Hematoma (Extradural Hematoma)

Epidural hematomas represent hemorrhage into the potential space between the dura mater and the inner table of the skull (Table 25-4).

Most cases are due to injury to the middle meningeal artery or vein from blunt head trauma, typically from a motor vehicle accident.

TABLE 25-4 THE MENINGES

Layer	Comments
Dura mater	Composed of two layers, an outer periosteal layer which cannot be separated from the skull and an inner meningeal layer; the inner meningeal layer enfolds to form the tentorium and falx.
Arachnoid	The avascular middle layer, it is separated from the dura by a potential space known as the subdural space.
Pia mater	Closely applied to the brain and spinal cord, the pia mater carries blood vessels that supply both; separating the arachnoid from the pia is the subarachnoid space; together the pia and arachnoid are called the leptomeninges.