Brain CT scan

Overview

A computerized tomography (CT) scan of the brain also is called a brain CT scan or sometimes a brain CAT scan. It uses X-rays and a computer to create detailed images of the head. These images show cross-sections — or slices — of brain, bone, blood vessels and soft tissues. CT scans of the head show more-detailed images than plain X-rays do.

Brain CT scans allow for quick imaging of the brain and skull from different angles. This can help show issues such as bleeding, swelling or tumors. Brain CT scans are typically the first test done after head injuries or sudden signs of a stroke.

In nonemergency cases, a CT scan of the head may be used to monitor known conditions or plan radiation treatment for brain cancer. It also is used to follow up on certain symptoms, such as memory loss, confusion or headaches that don't go away.

Some brain CT scans are done with a special dye called contrast to highlight blood vessels or tissue. Others are done without contrast because the scan needs to be done quickly. Certain conditions also are better seen without contrast. Some people have health conditions that might make using contrast risky, such as kidney disease.

In general, a brain CT scan is considered safe. It uses a low amount of radiation. And the radiation is carefully controlled to avoid unnecessary exposure. For most people, the benefits of having a CT to find a serious brain problem outweigh any potential risks from radiation. Children are smaller and more sensitive to radiation than adults. So CT scans for children are used only when necessary and with lower doses of radiation.

Types of brain CT scans

Depending on why you're getting a CT scan of the brain, one or more of the following types may be used.

Contrast-enhanced CT scans

Contrast is a special dye that appears bright on images, helping make certain parts of the body show up clearer. For a CT scan of the brain, contrast is given through an artery or vein in the arm.

• CT angiography (CTA). This test uses contrast for highlighting the brain's blood vessels to look at blood flow. This can help show the presence of a blood clot in someone having a stroke and guide treatment. CTA also is used to find bulging blood vessels, called an aneurysm, or issues with blood vessel connections, which are called arteriovenous malformations.
• CT perfusion scan. This scan measures blood flow in the brain. It can help show brain tissue that may be saved after a stroke. This tissue is called the ischemic penumbra.
• CT venography. Contrast is used to show the network of veins that drain blood from the brain. CT venography (CTV) may be used to look for blood clots in the brain's large veins or sinuses, a condition called cerebral venous sinus thrombosis.

Noncontrast CT scans

Noncontrast CT scans are the fastest type of CT imaging. They typically are used to evaluate emergency situations such as head trauma, bleeding, stroke or fluid buildup in the brain. Depending on the findings, a contrast CT scan may be used for further evaluation.

Other specialized CT techniques

• Dual-energy CT (DECT). This type of CT scan uses two different energy levels. It gives clearer, more-detailed images of the brain. DECT can help show the difference between blood, calcium and contrast. This can help healthcare teams figure out what's going on in the brain and whether it's an emergency. DECT can be useful to look for bleeding and see tumors more clearly.

• Combined PET-CT scan. Sometimes, CT scans are combined with positron emission tomography (PET). This test is done when a structural image by itself can't find the cause of symptoms such as memory loss or seizures. PET-CT scans also can help show the difference between types of dementia and evaluate brain tumors.

  During a PET-CT scan, a small amount of a radioactive substance called a tracer is injected into the veins. Cells that use a lot of energy absorb the substance and "light up" on the scan. This can signal conditions such as cancer, infection or brain disease. The scans are done on a machine that takes separate CT and PET scans. A computer merges the scans into a 3D image that shows both the brain structure and the brain activity.

Why it's done

A brain CT can be used to detect:

• Tumors.
• Sinus-related bone issues.
• Skull fractures.
• Stroke.
• Structural brain issues.
• Causes of sudden symptoms such as severe headaches, loss of consciousness, seizures and weakness.
• Fluid buildup in the brain, also called hydrocephalus.
• Traumatic brain injuries. These include bruising, also called contusions. They include pooling blood, also called hematomas. And they include bleeding, also called hemorrhaging.

Brain CT scans can help healthcare professionals during procedures such as biopsies, where a small sample of brain tissue is taken for testing. Brain CT scans also are used to guide brain surgeries by showing the exact location of a tumor or an area of interest. In addition, CT scans may be used to plan and guide radiation therapy. This makes sure the treatment focuses on the right spot. In people being treated for brain cancer, repeated CT scans can show whether the tumor is shrinking, growing or staying the same. In some cases, such as stroke, more areas of the body may need scanning, including the neck.

There is some overlap in what CT scans and magnetic resonance imaging (MRI) scans evaluate. MRI scans can produce more-detailed images for certain conditions, but CT scans are faster.

Compared with MRIs, CT scans:

• Cost less.
• Are more widely available.
• Are safer for people who have pacemakers or implanted metal devices.
• May be more comfortable for those who feel nervous or uncomfortable in tight spaces.

Risks

During a CT scan, you're briefly exposed to a type of energy called ionizing radiation. The amount of radiation is greater than the amount from a plain X-ray. That's because the CT scan gathers more detailed information. One brain CT scan has about the same amount of radiation that you are exposed to over several months from the environment around you.

The low doses of radiation used in CT scans have not been shown to cause long-term harm. But for repeated scans, there may be a small increase in the lifetime risk of cancer. This can affect children more than adults.

Healthcare professionals use the lowest dose of radiation to get the medical information needed. And newer, faster machines and techniques use less radiation than older CT scans. Talk with your healthcare professional about the benefits and risks of a CT scan.

Harm during pregnancy

Radiation from a brain CT scan is not likely to harm a developing baby, called a fetus. But your health professional may suggest another type of exam so that the unborn baby isn't exposed to any radiation. Exams that don't use radiation include ultrasound and MRI.

Reactions to contrast

Although not common, medical problems or allergic reactions can happen with contrast. Most reactions are mild, such as a rash. But in rare cases, an allergic reaction can be serious or life-threatening. If you have a kidney condition, contrast might not be a choice for you because your body may not filter it out properly. Tell your healthcare professional if you've ever had a reaction to contrast or have issues with your kidneys.

How you prepare

Depending on which part of your body is being scanned, you may be asked to:

• Remove some or all your clothing and wear a hospital gown.
• Remove metal objects that might affect image results. These include belts, jewelry, dentures and eyeglasses.
• Not eat or drink for a few hours before your scan. This helps avoid potential complications, such as nausea.

Before your scan, you may be asked about any allergies or past reactions to contrast. You also may be asked about kidney conditions. Tell your healthcare professional if you are pregnant, breastfeeding or taking any medicines. In some cases, you may be asked to pause certain medicines before the scan. If you feel anxious or worried about being in a tight space, tell a member of the healthcare team. Medications called sedatives may be available to help you relax.

What you can expect

You can have a CT scan in a hospital or an outpatient facility. CT scans are painless and with newer machines, take only a few minutes. However, the length of time can vary.

Before the procedure

A member of the healthcare team will explain the process and answer any questions you may have. If you're receiving contrast, you'll have an IV line placed. This is a small needle that goes into a vein in your arm to deliver the contrast during the scan.

You'll be taken to a special CT room that has the scanner. The scanner looks like a large doughnut or ring with a table that moves through it. The room may be dimly lit and feel cool. A healthcare professional called a CT technologist will explain the process and help you get onto the table and position your head. The technologist may use foam pads or straps to keep you from moving, if needed.

If contrast is being used, it may be injected through an IV while you're on the table. You might feel a warm sensation, a brief flush or a metallic taste when the contrast enters your body. These feelings are common and usually pass quickly.

During the procedure

The table moves through the hole in the center of the CT scanner. You may see lines of light on your body. These lines help make sure you are in the correct position on the table. The CT technologist sits in another room and can see and hear you. The two of you can talk through a speaker.

For children having a brain CT scan, a parent or guardian may be able to stay in the room if that person wears a lead apron to lower radiation exposure.

As the table moves you into the scanner, the X-ray tube rotates around your head. Each time it rotates, it creates images of thin slices of your head. You may hear buzzing and whirring noises as the scanner works. To help you keep still during the scan, the technologist might ask you to hold your breath at certain points. Movement can blur the images.

CT scans are not painful. But some people may feel brief discomfort from lying still, having an IV placed or holding an awkward position for several minutes. This can be especially true for people with back or neck pain. If you're not able to lie flat or still because of pain or anxiety, tell the technologist. Adjustments often can be made to help you complete the scan comfortably.

The scan itself typically lasts only a few minutes. The full appointment may take longer depending on whether contrast is used or how quickly the machine processes images.

Once the images are taken, they are sent to a computer for review to make sure they're clear. In urgent situations, results may be sent to your healthcare professional within minutes.

After the procedure

After the exam, you can return to your regular routine.

If you were given contrast, the IV will be removed and the site covered with a small bandage. You may need to wait for a short time before leaving to make sure that you feel OK after the exam. You may be told to drink lots of fluids to help your kidneys remove the contrast from your body.

Results

CT images are stored as electronic data files. They're most often reviewed on a computer screen. A doctor who specializes in imaging, called a radiologist, looks at the images and creates a report. The report is kept in your medical records and sent to the healthcare professional who ordered the test. In urgent situations, the results can be ready in minutes. Your healthcare professional will talk with you about the results.

The color or shading that appears on a CT scan tells the radiologist what's going on. For example, new bleeding in the brain appears on a scan as a bright white area.

It's important to note that CT scans may not catch everything, especially if changes to the brain are subtle or difficult to capture, such as the early stages of stroke.

Some people will need a follow-up exam. Your healthcare professional will tell you why. In some cases, the CT scan may have found a potential issue, and more imaging is needed.