How to Read a Head CT

by
Henry Z. Wang, MD, PhD
Department of Radiology
University of Rochester Medical Center

I. Introduction

The target audience of this article is the radiology resident during the first rotation in neuroradiology, the new neuroradiology fellow, and visiting medical students and residents from other departments. The majority of cases seen during the CT rotation in neuroradiology will be the head CT. This article is a practical guide and will not discuss anatomy or pathology in depth as there are many fine textbooks, atlases and references in these areas.

As with any imaging study, there are two ways of approaching a head CT: (1) the systematic approach, when the study is for screening purposes or when there is inadequate history and (2) the problem orientated approach, when the study has a specific clinical scenario or question. Experienced radiologists typically use a combination of the two approaches.

II. The systematic approach

1. Brain parenchyma.

Use the brain’s natural symmetry to help you. Is there a suspicious lesion in one half of the brain not in the other half? Is there a shift of the midline indicating mass effect. Account for the head being poorly positioned by using the eyes as reference points.

In neonates, the brain is usually diffusely hypodense due to lack of myelination.

In the elderly, small vessel disease commonly results in diffuse brain atrophy, in the periventricular white matter being hypodense due to gliosis, and in lacune infarcts within the basal ganglia.

The junction of gray matter and white matter adjacent to the cortex and the basal ganglia should be well defined. Poor delineation should raise suspicion of cerebral edema, if the finding is global, or acute infarction, if the finding is localized.

Hyperdensity within the parchenchyma is either due to hematoma (hemorrhage) or calcification. A hematoma will produce mass effect upon adjacent structures. Calcification will usually be punctate and have no mass effect. Focal calcification within the basal ganglia is common in the elderly and should not be confused with hemorrhage.

Linear segments of hyperdensity along the cerebral cortex are usually artifactual due to adjacent bone (beam hardening). The posterior fossa is commonly obscured by artifact and thus, is better evaluated by MRI. Artifact can be reduced by acquiring thinner sections, such as from multidetector scanners (e.g., GE LightSpeed).

2. Ventricles and subarachnoid spaces (sulci and cisterns)

The size of the ventricles and subarachnoid spaces depends on the age of the patient. Infants may have prominent sulci up to the age of 18 mos. Children and young adults usually have narrow ventricles and subarachnoid spaces. The elderly have proportionally large ventricles and subarachnoid spaces due to brain atrophy

Poor visualization of the basal cisterns should raise concern for increased intracranial pressure and possibly brain herniation.

Hyperdensity within the subarachnoid spaces and the dependent portions of the ventricles usually indicates hemorrhage. However, as mentioned above, beware of linear artifacts along the edges of the brain.

Asymmetry of cerebral sulci may indicate an acute infarct on the side where the sulci are smaller.

3. Dura and Subdural Space

Always check the subdural windows to check for subdural hemorrhage, especially along the edges of the intracranial cavity.

It is normal for the falx and dura to be denser than the brain parenchyma. Furthermore, the falx, and to a lesser the tentorium, is often calcified in the elderly. However, be suspicious of subdural hemorrhage if the density is symmetrically thicker on one side of the falx or tentorium than the other side.

4. Bone and air spaces.

Check for fractures, bone destruction and other bone lesions on the bone windows.

The sutures of the calvarium and skull base are symmetric. A linear lucency on one side of the skull not found on the other side is probably a fracture. Check the scout for any obvious fracture which may be in the plane of the axial sections.

Check to see if the paranasal sinuses, middle ear cavity and mastoid air cells are clear. In trauma cases, fluid within these areas always raises suspicion for fracture of the adjacent bone.

5. Skin and subcutaneous tissues.

Check for swelling of the extracranial soft tissues. Because we typically don't film soft tissue windows, the best window to use is the subdural window. In trauma cases, a focal region of scalp edema, hematoma or air requires a second look at the adjacent bone and brain.

III. The problem oriented approach

1. Trauma.

Check along the outer perimeter of the brain for contusions, which appear as irregular areas of mixed density involving the cortex and subcortical white matter. The majority of contusions occur at the base of the brain, especially the undersurface of the frontal and temporal lobes.

Check the sulci, basal cisterns and ventricles for subarachnoid hemorrhage.

Check the subdural windows for subtle subdural hematoma adjacent to the skull.

Check for fractures, especially along the skull base. Clear paranasal sinuses, middle ear cavities and mastoid air cells lowers the suspicion of skull base fracture whereas the presence of fluid within one of these raises the suspicion of adjacent bone fracture. Fracture of the lateral orbital wall may represent partial visualization of a zygomatic fracture and the need for a dedicated maxillofacial and orbit CT.

Check the scalp for swelling, hematoma, or laceration. The presence of a traumatic scalp lesion indicates the need to have a second look at the skull and brain beneath that area of the scalp.

A common pitfall in trauma cases is artifacts mistakened for contusions and subdural hematoma: 1) beam hardening artifact, which appears as linear streaks, and 2) volume averaging artifact through the skull base. Both of these artifacts are prevalent in the frontal and middle cranial fossae which, unfortunately, are also prevalent areas for contusions and subdural hemorrhages. A real abnormality should be seen on more than one contiguous image.

2. Infarct (i.e., R/O stroke or CVA)

Obtain and use the presenting symptom to focus your evaluation.

The majority of infarcts involve the middle cerebral artery territory. Hemiplegia of one side indicates a infarct in the cerebral hemisphere on the other side. Aphasia almost always involves the left cerebral hemisphere.

These are the three important early findings to check:

1. Dense MCA. The suspected MCA must be significantly denser than the contralateral MCA or basilar artery, which can be used for reference. Patients who have this sign are candidates for intraarterial thrombolysis.

2. Edema of the basal ganglia and/or insular cortex. The involved lentiform nucleus will appear hypodense with indistinct lateral border. The insular cortex will appear swollen compared to the contralateral side.

3. Sulcal effacement. The sulci along the cerebral convexity on the involved side will appear smaller than the other side.

The stroke neurologists will need to know the following which would exclude the patient from intravenous thrombolyics:

1. Infarction of more than 1/3 of the MCA territory.

2. A hematoma.

3. Dense MCA. (These patients are candidates for intraarterial thrombolysis.)

3, Headache

The majority of head CT’s for headache are negative.

Patients with subarachnoid hemorrhage typically have the “worst headache of life”. Look for SAH predominately in the basal cisterns, Sylvian fissures, and ventricles.

Check the paranasal sinuses , especially the sphenoid and ethmoid, for sinusitis as the cause of the headache.

4, “Mental status change” in the elderly brain.

Check for a subdural hematoma.

Check for a infarct.

Check for a mass or mass effect.

5, Suspected child abuse

Check for a skull fracture. Use the skull’s symmetry to differentiate a fracture from a suture.

Check for subdural hematoma. In the infant, a chronic subdural hematoma may be difficult to differentiate from “benign subarachnoid space of infancy”. Suspect a SDH if you see any flattening of the cerebral gyri. These patients will often go on to MRI for more definitive diagnosis of SDH’s and other brain injuries.

6, Birth related problems (“low APGAR”; seizures)

It is very difficult to detect acute brain lesions due to the natural high water content of the infant brain. An early positive finding is hypodensity of the basal ganglia. In the later stages of severe brain edema, the cerebellum will appear denser that the cerebrum.

Check for acute subdural hematoma.

7, Seizures

In first time seizures, check for a mass or mass effect.

Check for an infarct, which can occasionally present as a seizure.

Patients with chronic seizures will eventually get a MRI.