Neuroscience Clerkship

Computerized Tomography

Computerized Tomography (CT) is one of the most commonly used tests in neurology and neurosurgery. CT revolutionized neurology and neurosurgery in the 1970s enabling visualization of intracranial structures with excellent resolution. Although MRI has supplanted CT in the evaluation of many neurologic conditions, it stills plays an important role, especially in the acute evaluation of stroke and head trauma patients.

X-rays are applied in a circular motion with detectors on the opposite side of the body. Body tissue slices (typically 1 cm) are mathematically reconstructed and displayed on a gray scale matrix. The density of the tissue is in proportion to the attenuation of the x-ray which pass through. Tissues like air and water have little attenuation and are displayed as low densities (dark); whereas bone has high attenuation and is displayed as a high density (bright) on CT. Among pathologic conditions, high intensity lesions are often seen with freshly clotted blood, hyperemia and with the use of contrast. Low intensity lesions include edema and necrosis.

Similar to gadolinium in MRI, Iodinated Contrast agents are used in CT to demonstrate vascular structures and breakdown of the blood-brain barrier. The latter most often occurs in tumors, infection and inflammation.

NEUROLOGIC INDICATIONS FOR CRANIAL CT

CT is most commonly indicated in the emergency room evaluation of acute head trauma and acute neurologic dysfunction, primarily to look for suspected intracranial or subarachnoid hemorrhage. Its extremely rapid acquisition time, and sensitivity to detect hemorrhage makes it the modality of choice in the acute setting time.

CT is easily available at nearly all institutions. Claustrophobia is not a major issue, as it is in MRI.

In general, CT is useful in the following conditions:

Vascular

Ischemic stroke (> 2 days old)

Hemorrhagic stroke (acutely)

Tumor

Primary CNS and metastatic

Infection

Abscess

Hydrocephalus

Trauma

Epidural hematoma, subdural hematoma, contusion, and skull fractures

LIMITATIONS OF CT

• Subject to motion artifact

• Artifacts from bone may interfere with detection of disease at the skull base and in the posterior fossa

• Limited to axial views

• Unable to detect acute ischemic stroke (first 1-2 days)

• Unable to detect demyelination

• Poorer spatial resolution compared to MRI

• Often does not see lacunar strokes

• Herpes encephalitis poorly visualized compared to MRI

CONTRAINDICATIONS TO CT

There are few contraindications to CT. Although CT involves exposure to ionizing radiation, there are no associated biologic effects. Relative contraindications to CT include:

• Pregnancy

Potential harm of ionizing radiation to the fetus

• Use of intravenous contrast agents is associated with infrequent but substantial risks:

Rare cases of anaphylaxis

Kidney dysfunction (baseline creatinine required before contrast is given)