Head Computed Tomography Scanning

Computed tomography (CT) scanning of the head is an x-ray based imaging procedure generating images of the intracranial contents as a result of specific x-ray absorption by the tissues examined (see the image below). The CT scanner generates multiple sequential cross-sectional images of the brain, and 3-dimensional images of cranium or blood vessels can be created on special software on dedicated workstations, if needed. Depending on the clinical indication, the scan can be performed with or without intravenous contrast injection. It can also be used to guide biopsies.

Axial CT scan of the head showing intra cranial hemorrhage.

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Indications for head CT examination are numerous, but the most frequent include the following ^{[1, 2, 3, 4, 5, 6, 7, 8]} :

• Head trauma

• Stroke

• Headaches

• Initial evaluation for space-occupying lesions

• Unexplained change in mental status

• Seizures

• Suspected hydrocephalus

• Suspected intracranial hematoma

• Psychiatric disorders

• Dizziness

• Vascular occlusive disease and aneurysm evaluation

CT scanning has no absolute contraindications. Caution is indicated in pregnant women, particularly during the first trimester to avoid fetal abnormalities. Risk-to-benefit considerations should be evaluated. Repeated x-ray exposure may increase the patient's risk for cancer. However, the risk from any one scan is small, with the benefit of an accurate diagnosis outweighing the risks. In certain clinical situations, intravenous contrast administration is indicated. Contrast media is safe in most patients; however, adverse reactions may range from mild to severe. It is recommended that nursing mothers should wait 24 hours after contrast administration to resume breastfeeding.

CT scanning of the head can be performed with a sequential single-slice technique, multislice helical (spiral) protocol, or multidetector multislice algorithm. For CT scans of the brain, the typical slice thickness should not exceed 5 mm. In most circumstances, soft tissue and bony window algorithms should be applied. Window settings should be adjusted manually, when appropriate.

According to American College of Radiology (ACR) recommendations, a CT scanner should meet or exceed the following specifications to achieve acceptable clinical brain CT scans ^[9] :

• Scan times - No more than 2 seconds per slice or image.

• Slice thickness - Minimum slice thickness of 2 mm or less; reconstructed slice thickness should be 5 mm or less.

• Interscan delay - Not to exceed 4 seconds (although may be longer if intravascular contrast media is not used).

• Limiting spatial resolution - Must be measured to confirm that it meets the specifications of the unit manufacturer; limiting spatial resolution should exceed 10 lp/cm for a display field of view (DFOV) of less than 24 cm

• Table pitch: Not to exceed 2:1 for most CT scanners; pitch may be increased for dual-energy scanners for sole evaluation of bone anatomy (craniofacial)

• Cine-capable scanners are preferred for advanced applications (eg, perfusion imaging or computed tomography angiography [CTA]), with tube rotation of 1 second or less and continuous cine imaging of 60 seconds or more

The referring physician should obtain CT scanning for only appropriate indications, for answering a specific clinical question, and when no other alternative imaging study is available. Physicians should be familiar with alternative and complementary imaging and diagnostic procedures.

All radiologists, medical physicists, radiologic technologists, and supervising physicians should make every effort possible to minimize radiation dose to patients, staff, and society as a whole while maintaining the necessary diagnostic image quality. Imaging facilities should have in place, and should adhere to, policies and procedures that specify protocols tailoring the CT scan to the patient’s body habitus in accordance with ACR guidelines.

Safety screens should be in place, with more than one verification, to identify and counsel patients who are or may be pregnant.

Similar measures must be in place in order to identify patients at risk for allergic reactions.

In most cases, contrast media is typically avoided in patients with medial renal disease.

Contrast media must be stored under secure conditions within the temperature range recommended by the manufacturer and administered under sterile conditions.

Appropriate emergency equipment and medications must be immediately available to treat adverse contrast reactions, and these should be monitored for inventory and drug expiration dates regularly.

If a patient has experienced a prior contrast reaction or has an allergic predisposition or asthma, his or her physician should, when appropriate, consider prescribing medications to reduce the risk of such reactions.

The physician should inquire about any history of heart disease, asthma, diabetes, or kidney or thyroid disease.

Typically, patients are asked not to eat or drink for a few hours before the procedure, especially if contrast media is to be administered.

Any metallic object worn or carried by the patient should be stored away, as it can result in image artifact, degrading the study.

Patients should wear comfortable clothing, although they might be given a gown to wear during the test.

Female patients should be queried not only at the time of scheduling but also again by the CT technologist just prior to the scan as to whether they are or think they could be pregnant.

It also very important for the patient to bring with them any previous radiographs, CT scans, MRIs or other medical images, with reports, for comparison.

Imaging protocols and radiations doses should be revisited regularly.

A radiation-control quality-control program should be in place at every imaging facility.

Every effort should be made to identify and counsel patient who are or may be pregnant.

Every effort should be made to identify patients at risk for allergic reactions or have medical renal disease.

If the patient has diabetes and is taking metformin, the health care provider should be alerted, as the drug needs to be stopped for a period of time.

Patients should be screened for medical renal disease.