Computed tomography of the brain: what a CT scan shows

The hum of the machine, a ring that slowly moves around the head, and a request not to move — the procedure itself takes from 30 seconds to several minutes. But questions arise afterwards: what exactly is visible on the images, why contrast is needed, and how computed tomography of the head differs from MRI. Let's break it down step by step.

Computed tomography of the brain: what kind of study is this

Computed tomography of the head is a method in which multiple X-ray images taken from different points around the body are converted by a computer into slice images: the skull, brain, orbits, and paranasal sinuses are visible on individual slices, and the slices can be combined into a three-dimensional model. The method was proposed in 1972 by Godfrey Hounsfield and Allan Cormack, who received the Nobel Prize for its development; the first clinical scanner, tested in 1971, was designed only for scanning the head. The device assesses tissue density using the Hounsfield scale — from −1024 to +3071 units: zero corresponds to the density of water, negative values to air and fat tissue, positive values to soft tissues, bones, and metal.

The procedure is performed by a radiologic technologist. You lie on a flat table, the ring of the machine moves around you, and the specialist controls it from the adjacent room and hears you through an intercom. You need to lie still — movement blurs the images.

What a CT scan of the head and brain shows

CT builds an image based on tissue density (the Hounsfield scale); the images show the skull, brain, orbits, and sinuses. Therefore, it is prescribed to identify or monitor congenital defects of the head and brain, infections, tumors, hydrocephalus, head and facial trauma, stroke, and hemorrhages in or around the brain. On the images, the doctor can see an aneurysm, subdural and epidural hematoma, abscess, brain tissue edema, brain atrophy, problems with the auditory nerve, stroke, or transient ischemic attack.

Sometimes the task is different — to find the cause of a symptom:

• changes in thinking or behavior;
• fainting;
• headache, if certain other signs are present;
• hearing loss in some patients;
• symptoms of damage to a brain area — visual disturbances, muscle weakness, numbness and tingling, difficulty with speech or swallowing.

CT of the head and neck: what is special

Head tomography covers the skull, brain, orbits, and sinuses. The method is considered one of the safest ways to study the head and neck and can reduce the need for invasive procedures.

CT or MRI of the brain: what is the difference

The difference lies in the basis of the method: CT works on X-ray radiation and builds images based on tissue density. MRI of the head and PET examination of the head are named as alternatives that can be performed instead of computed tomography — the choice is made by the doctor according to the task.

Indications and limitations of the study

Before the procedure, you will be asked about pregnancy and breastfeeding, medication use (including metformin for diabetes), asthma, kidney and thyroid problems, and allergy to contrast. For a contrast-enhanced study, you may be asked not to eat or drink for 4–6 hours, to remove jewelry and clothing with metal fasteners; if you weigh more than 136 kg, check in advance whether the machine can withstand such a load. Contrast is injected into a vein, given to drink, or administered into the rectum — it is excreted in urine, and the heat, metallic taste, and sensation as if you are urinating pass within seconds. After contrast, you will be asked to wait at the clinic for up to 30 minutes.

Risks: radiation exposure, allergic reaction to contrast, and kidney damage from contrast. The dose from a single scan is small, but multiple studies over a lifetime increase the cumulative load — the doctor weighs the benefit of an accurate diagnosis against the risk. Results are usually ready in 1–2 weeks, sometimes the wait extends to 4 weeks: a radiologist describes the images and sends the report to the doctor who referred you.

A CT referral itself is a planned study, and it is not a reason to call an ambulance. 103/112 are needed if weakness, sweating, and difficulty breathing appear after contrast administration, as well as for signs of stroke — such conditions are life-threatening.

This material is for reference: it explains what computed tomography shows, but does not replace a doctor's consultation — the decision on the study is made by your doctor after examination.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer