Computed Tomography

Also known as

CT, CT scan, CAT (computerized axial tomography) scan.


What is CT scan?

Computed tomography is an examination that uses X-Rays to obtain cross-sectional images of the human body. When X-Rays are irradiated onto the human body, some of them are absorbed and some pass through the body to produce an image.

In plain X-Ray imaging, the film directly absorbs penetrated X-Rays; whereas, with a CT scan, an electronic device called a detector array absorbs the penetrated X-Rays, measures their amount, then transmits this data to a computer system.

A sophisticated computer system calculates and analyzes data from each detector and reconstructs multiple two-dimensional cross-sectional images. An X-Ray source and a CT detector rotate around the patient to obtain each cross-sectional image.

CT images represent density and the atomic number of human tissue just like a general X-Ray image. On a CT scan, the denser the tissue and the higher the atomic number, the whiter the CT image: bone and calcium appear white; air in lungs appears black; water, blood and internal organs, such as liver, kidneys, and brain appear gray; and fat tissue appears dark gray.


Advantages

* Provides detailed two-dimensional images with great clarity.
* Better images of many types of tissues, including the lungs; bones, soft tissue, and blood vessels.
* In trauma cases, a CT scan can reveal internal bleeding and other life-threatening injuries quickly.
* Simple, painless and noninvasive.
* Sensitive in detecting traumatic injuries, cancer and many other diseases.
* Cost-effective imaging tool for a wide range of clinical problems (when compare to MRI).
* Distinguishes normal and abnormal structures accurately: Useful in guiding biopsy, fluid removal/aspiration, drainage of abscesses, other minimally invasive procedures, and radiotherapy.
* Can provide three-dimensional images.

Types of CT scans

Head CT Scans

* Brain
* Facial
* Paranasal sinus
* Orbit
* Sellar (to view pituitary gland)
* Temporal bone

Body CT Scans

* Chest
* Abdominal
* Pelvis

Spine CT Scans

* Cervical spine
* Thoracic spine
* Lumbosacral spine

Extremity CT Scans


How is the procedure performed?

You will be asked to lie down on the CT table. For Head CT Scans, a pillows will be placed to support your head. For Body CT Scans, your arms will be raised above your head.

After placing you in the proper position, a technologist will move into the console room and begin exposing you to X-Rays. Once the X-Ray tube rotates around your body 360 degrees to take one cross-sectional image, the table will be slightly moved in order to get the next plane. You will be asked to hold your breath during the scanning process.

Though you will be alone in the examination room during X-Ray exposure, you will be able to communicate with your technologist by using an intercom or raising your hands. The technologist will watch you during the scanning through a glass window, as well as a video camera.

If a contrast medium is required to make organs and blood vessels more visible, it will be injected into a vein during the exam.

The exam usually takes from 10-30 minutes.

Preparing for a CT scan

You will be asked to put on a hospital gown, and to remove all metal objects from your body, such as zippers, snaps, and other accessories, as they can interfere with the imaging quality. You will be required to lie still on the exam table until the technologist tells you otherwise.

For contrast-enhanced CT scans, you should not eat or drink for one or more hours before the exam. If you know that you have allergies to contrast mediums, medications, iodine, specific foods, or if you have a history of kidney problems, asthma, thyroid problems, diabetes or a heart condition, inform your technologist or doctor beforehand. Contrast agents will be injected into your bloodstream before X-Ray exposure.

For abdominal/pelvic CT scans, you may need to avoid food or drink for several hours. You may be asked to swallow a liquid contrast agent to allow better viewing of the stomach, small bowel, and colon. Sometimes the contrast agent needs to be administrated by enema to study the colon, or by vaginal tampon to evaluate the uterine cervix.

For a Head CT Scan, you may be asked to remove jewelry, eyeglasses, dentures, hairpins, hearing aids, etc. If you are pregnant or suspect pregnancy, you should inform your doctor, technologist, or radiologist prior to the scan.


Obtaining results

A radiologist will review the CT scan and report the results to your personal doctor, who will inform you of the results.


Risks of CT scan

Radiation exposure during a CT examination is equal to a year's worth of natural background radiation (i.e., radon gas in a home, soil, or cosmic rays). It is much more than the radiation exposure from a general X-Ray exam, such as a chest X-Ray or a skull X-Ray, but it is less than that from barium enema, upper GI series, or cardiac catheterization.

The benefits of CT study outweighs the risk of radiation exposure. Special care should be taken, however, to expose children only when absolutely necessary, because growing children are more sensitive the hazards of radiation.
Adverse reactions to contrast agent given intravenously, include:

Allergic reactions
Rashes, hives, itching, etc. Usually self-limiting, antihistamine can be administered.

Anaphylactic reaction
This very rare reaction can cause breathing difficulty or swelling in the throat or other parts of the body. Potentially life-threatening, it can be treated with epinephrine, corticosteroids, or antihistamines. Radiology departments are well trained to deal with it.

Kidney failure
Particularly among patients taking Glucophage (Metformin) for diabetes.

Mothers nursing their babies may resume breast-feeding 24 hours after the contrast medium is given intravenously.


Limitations

Bone, air, fat, and other tissues are easy to distinguish with a CT scan because they distinctly appear with different densities; however, it is difficult to distinguish between body organs, tumors, cancer, blood vessels, muscles, and fluids. Also, CT scans have limited ability to image knee joints, shoulder joints, intervertebral discs and related structures. MRI is a better imaging modality for the evaluation of soft tissue.

CT scans only provide axial cross-sections. If longitudinal cross-sections are necessary to evaluate a problem, such as in a meniscus tear of knee joint, ligament tear of rotator cuff, or a three-dimensional evaluation of cancer before planning surgical removal, an MRI is recommended instead.