A computerized tomography (CT) or computerized axial tomography (CAT) scan combines data from several X-rays to produce a detailed image of structures inside the body.
CT scans produce 2-dimensional images of a "slice" or section of the body, but the data can also be used to to construct 3-dimensional images. A CT scan can be compared to looking at one slice of bread within a whole loaf.
CT scans are used in hospitals worldwide.
What is a CT scan?
A CT scan provides a more detailed image than an x-ray.
A CT scanner emits a series of narrow beams through the human body as it moves through an arc.
This is different from an X-ray machine, which sends just one radiation beam. The CT scan produces a more detailed final picture than an X-ray image.
The CT scanner's X-ray detector can see hundreds of different levels of density. It can see tissues within 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 can help show certain structures more clearly.
For instance, 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 vessel images are the target, the barium will be injected into the veins.
The accuracy and speed of CT scans may be improved with the application of spiral CT, a relatively new technology. The beam takes a spiral path during the scanning, so it gathers continuous data with no gaps between images.
CT is a useful tool for assisting diagnosis in medicine, but it is a source of ionizing radiation, and it can potentially cause cancer.
The National Cancer Institute advise patients to discuss the risks and benefits of CT scans with their doctors.
When is a CT scan used?
It is useful for obtaining images of:
- Soft tissues
- The pelvis
- Blood vessels
CT is often the preferred way of diagnosing many cancers, such as liver, lung, and pancreatic cancers.
The image allows a doctor to confirm the presence and location of a tumor, its size, and how much it has affected nearby tissue.
A CT scan can reveal tumors, abnormal tissue, and problems with arteries.
A scan of the head can provide important information about the brain, for instance, if there is any bleeding, swelling of the arteries, or a tumor.
A CT scan can reveal a tumor in the abdomen, and any swelling or inflammation in nearby internal organs. It can show any lacerations of the spleen, kidneys, or liver.
As a CT scan detects abnormal tissue, it is useful for planning areas for radiotherapy and biopsies, and it can provide valuable data on blood flow and other vascular conditions.
It can help a doctor assess bone diseases, bone density, and the state of the patient's spine.
It can also provide vital data about injuries to a patient's hands, feet, and other skeletal structures. Even small bones are clearly visible, as well as their surrounding tissue.
CT and MRI: What is the difference?
The main differences between CT and MRI are:
- A CT scan uses X-rays, but an MRI uses magnets and radio waves
- Unlike an MRI, a CT scan does not show tendons and ligaments
- MRI is better for examining the spinal cord
- A CT scan is better suited to cancer, pneumonia, abnormal chest x-rays, bleeding in the brain, especially after an injury
- A brain tumor is more clearly visible on MRI
- A CT scan shows organ tear and organ injury more quickly, so it may be more suitable for trauma cases
- Broken bones and vertebrae are more clearly visible on a CT scan
- CT scans provide a better image of the lungs and organs in the chest cavity between the lungs
What happens in a CT scan?
The patient may need to abstain from food, and possibly drink, for a specific period before the scan.
On the day
In most places, the patient will need to undress, usually down to their underwear, and put on a gown that the health center will provide. If the hospital does not provide a gown, the patient should wear loose-fitting clothes.
Some patients may have to drink a contrast dye, or the dye may be given as an enema, or injected.This improves the picture of some blood vessels or tissues.
Any patient who is allergic to contrast material should tell the doctor beforehand. Some medications can 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.
During the scan
The patient will need to lie down on a motorized examination table that slides into a doughnut-shaped CT scanner machine.
In most cases, the patient will lie on their back, facing up. But, sometimes, they may need to lie facedown or sideways.
After one x-ray picture, the couch will move slightly, and then the machine will take another image, and so on. The patient needs to lie very still for the best results.
During the scan, everybody except for the patient will leave the room. An intercom will enable two-way communication between the radiographer and the patient.
If the patient is a child, a parent or adult might be allowed to stand or sit nearby, but they will have to wear a lead apron to prevent radiation exposure.
Can I have a CT scan if I am pregnant?
Any woman who suspects she may be pregnant should tell her doctor beforehand, because there is a risk that the x-rays could harm the fetus.
Citing the American College of Radiography, the American Pregnancy Association (APA) point out that "No single diagnostic x-ray has a radiation dose significant enough to cause adverse affects in a developing embryo or fetus."
However, the APA notes that CT scans are not recommended for pregnant women, "Unless the benefits clearly outweigh the risk."
CT scans and breastfeeding
If a lactating, or breastfeeding, mother needs an iodinated intravenous dye for contrast, she should avoid breastfeeding for about 24 hours as may pass into the breast milk.