What Is a CT Scan? What Is a CAT Scan?

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Main Category: Radiology / Nuclear Medicine
Also Included In: MRI / PET / Ultrasound
Article Date: 10 Jun 2009 - 3:00 PDT

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A CT scan stands for Computed Tomography scan. It is also known as a CAT (Computer Axial Tomography) scan. It is a medical imaging method that employs tomography. Tomography is the process of generating a two-dimensional image of a slice or section through a 3-dimensional object (a tomogram). The medical device (the machine) is called a CTG scanner; it is a large machine and uses X-rays. It used to be called an EMI scan, because it was developed by the company EMI. Undergoing a CT scan is a painless procedure.

The CT scanner uses digital geometry processing to generate a 3-dimensional (3-D) image of the inside of an object. The 3-D image is made after many 2-dimensional (2-D) X-ray images are taken around a single axis of rotation - in other words, many pictures of the same area are taken from many angles and then placed together to produce a 3-D image. The Greek word tomos means "slice", and the Greek word graphein means "write".

How does a CT scan work?

A CT scanner emits a series of narrow beams through the human body as it moves through an arc, unlike an X-ray machine which sends just one radiation beam. The final picture is far more detailed than an X-ray one.

Inside the CT scanner there is an X-ray detector which can see hundreds of different levels of density. It can see tissues inside a solid organ. This data is transmitted to a computer, which builds up a 3-D cross-sectional picture of the part of the body and displays it on the screen.

Sometimes a contrast dye is used because it shows up much more clearly on the screen. If a 3-D image of the abdomen is required the patient may have to drink a barium meal. The barium appears white on the scan as it travels through the digestive system. If images lower down the body are required, such as the rectum, the patient may be given a barium enema. If blood vessels are the target images the barium will be injected.

The accuracy and speed of CT scans may be improved with the application of spiral CT. The X-ray beam takes a spiral path during the scanning - it gathers continuous data with no gaps between images. For a spiral scan of the chest, for example, the patient will be asked to hold his/her breath for a few seconds.

What is a CT scan like for the patient?

Most places will provide the patient with a gown. He/she will need to undress, usually down to their underwear, and put the gown on. If the place does not provide a gown the patient should wear loose-fitting clothes.

Any woman who suspects she may be pregnant should tell her doctor beforehand.

Doctors may ask the patient to fast (eat nothing) and even refrain from consuming liquids for a specific period before the scan.

The patient will be asked to lie down on a motorized examination table, which then goes into the giant doughnut-like machine. The couch with the patient goes into the doughnut hole.

Some patients may be given a contrast dye or substance which is either swallowed, given as an enema, or injected. This improves the picture of some blood vessels or tissues. If a patient is allergic to contrast material he/she should tell the doctor beforehand. There are some medications that reduce allergic reactions to contrast materials.

As metal interferes with the workings of the CT scanner the patient will need to remove all jewelry and metal fastenings. In the majority of cases the patient will lie on his/her back, facing up. But sometimes it may be necessary to lie face-down or sideways.

After the machine has taken one X-ray picture, the couch will move slightly, and then another picture is taken, etc. The patient needs to lie very still for best results.

During the scan everybody except for the patient will leave the room. The radiographer will still be able to communicate with the patient, and vice-versa, through an intercom. If the patient is a child, a parent or adult might be allowed to stand or sit nearby - that person will have to wear a lead apron to prevent radiation exposure.

Although the scan is painless, some people experience discomfort for having to be completely still for what for them is a long time. If you find the whole experience is going to be stressful you can ask your doctor for a mild sedative.

When is a CT scan used?

CT scanning is useful to get a very detailed 3-D image of certain parts of the body, such as soft tissues, the pelvis, blood vessels, the lungs, the brain, abdomen, and bones.

It is often the preferred method of diagnosing many cancers, such as liver, lung, and pancreatic cancers. The image allows a doctor to confirm the presence of a tumor. The tumor's size can be measured, plus its exact location, as well as to determine how much the tumor has affected nearby tissue.

A scan of the head can provide the doctor with important information about the brain - he/she may want to know whether there is any bleeding, swelling of the arteries, or tumors.

A CT scan will tell the doctor whether the patient has a tumor in his/her abdomen, and whether any internal organs in that area are swollen or inflamed. It will reveal whether there are lacerations of the spleen, kidneys or liver.

As a CT scan can detect abnormal tissue it is a useful device for planning areas for radiotherapy and biopsies.

A CT scan can also provide valuable data on the patient's vascular condition. Vascular refers to blood flow. Many vascular conditions can lead to stroke, kidney failure, and even death. It can help a doctor asses bone diseases, bone density, and the state of the patient's spine.

A CT scan can reveal vital data about injuries to the patient's hands, feet and other skeletal structures - even small bones can be seen clearly, as well as their surrounding tissue.

Who analyzes the image?

A radiologist who is trained in supervising and interpreting radiology examinations will analyze the images and send his/her report to the patient's doctor. A radiologist is a doctor.

What is the difference between a radiologist and a radiographer (or radiologic technician)?

Radiology is a branch of medicine. A radiologist is a fully qualified doctor who specializes in radiology - MRI, CT scans, radiographs, nuclear medicine scans, mammograms and sonograms.

A radiologic technologist or radiographer is the X-ray technician. The person who takes the x-rays.

The radiologist is a doctor; the radiographer is not a doctor.

What is the difference between MRI and CT scans?

• A CT scan uses X-rays. An MRI does not use X-rays; it uses magnets and radio waves.


• A CT scan does not show tendons and ligaments, an MRI does.


• MRI is better for looking at the spinal cord.


• A CT scan is better for looking at cancer, pneumonia, abnormal chest x-rays, bleeding in the brain (especially from injury).


• A brain tumor is better seen on MRI.


• A CT scan shows organ tear and organ injury more quickly - so it may be the best choice for accident victims.


• Broken bones and vertebrae are better seen on CT scan.


• CT scans are better at visualizing the lungs and organs in the chest cavity between the lungs.

Written by Christian Nordqvist
Copyright: Medical News Today
Not to be reproduced without permission of Medical News Today