How does a CT scan work?
A CT scan works similar to an x-ray. The body casts a “shadow” on film when it is exposed to the x-ray, much like when you hold a flashlight up to your hand and cast a shadow on a wall. All of the tissue that the x-ray passes through overlap on the image, making it hard to isolate different elements. A CT scan works around this limitation by capturing one narrow slice of your body at a time. Inside the CT machine, the x-ray tube circles around the patient taking pictures as it rotates. These slices can be viewed two-dimensionally or added back together to create a three-dimensional image of a body structure.
A dye (contrast agent) may be injected into your bloodstream to enhance certain body tissues. The dye contains iodine, a substance that x-rays cannot pass through. It circulates through the blood stream and is absorbed in certain tissues, which then stand out on the scan.
CT angiogram (CTA) can be used to view arteries and veins. Contrast dye injected into the bloodstream helps the computer “see” the vessels. CTA images can be 3-D reconstructed so that the cerebral vessels and accompanying pathology can be rotated and viewed from all angles.
What does a CT scans show?
CT scans are very good at showing bone, soft tissue, and blood vessels. While an MRI takes excellent pictures of soft tissue and blood vessels, a CT scan shows bone much better, so it’s often used to image the spine and skull. It’s also used to view the inner ear and sinuses because these areas are made of very fine bones.
Other detailed cross sections can be taken of the brain, vessels of the brain, neck, shoulders, spine, discs, spinal cord, and vessels of the spine.
A CT scan can help your doctor diagnose many conditions including:
- Brain damage after a head injury
- Brain tumors
- Ruptured or leaking aneurysms
- Hydrocephalus, or enlarged brain cavities
- Spinal stenosis, or narrowing of the spinal canal
- Herniated discs
- Blood clots or bleeding associated with stroke
Who performs the test?
A radiology technologist will perform the test in the CT suite of the Radiology department of the hospital or outpatient imaging center.
How should I prepare for the test?
You should wear loose clothing and remove all objects that would get in the way of the scan, such as hairpins. You may need to change into a hospital gown, depending on what part of your body is being imaged. Be sure to tell your doctor if you’ve ever had an allergic reaction to iodine contrast.
What happens during the test?
You will lie on a moveable table. If your head is being imaged, the technologist may position your head in a special head-holder that keeps it from moving. When you are comfortably positioned, the table will slowly move into the CT machine that looks like a large square doughnut with a hole in the middle. The technologist will stay in constant contact with you over an intercom. While each picture is taken, you will be asked to hold your breath and stay perfectly still for a few seconds. The technologist will move the table with a remove control after each picture is taken. You may be given an injection of contrast dye into your arm or through an IV to enhance the images.
The machine may quiet or noisy, depending on the brain. The noise you hear is the x-ray tube rotating around your body to produce the images. You will not feel any sensation from the scan. The scan can last from 5 to 15 minutes. After the test, the IV will be removed and you are free to go. You may be told to drink lots of fluids to help your kidneys remove the contrast dye from your body.
What are the risks?
There is a slight risk form x-ray radiation exposure, and some people are sensitive to the contrast agent. The most common side effects from the contrast agent are a brief metallic taste in your mouth and a feeling of warmth throughout your body. An allergic reaction to the contrast can cause severe hives and difficulty breathing. Medications such as antihistamines can reverse this action. If you have diabetes or kidney problems you may experience kidney failure, but this is very rare.
How I get the test results?
The radiologist will review your images promptly and provide the diagnosis to your referring physician, who will discuss the results with you.
If you have any questions regarding this test, the process or the results, please contact your referring provider.