For certain medical conditions, X-ray or ultrasound images don’t provide enough information. A CT scan provides a more detailed view of tissues, blood vessels, and bones.
Doctors and other medical professionals have years of training in their field, but there are still many things they can’t diagnose simply by looking at or listening to your body. That’s where imaging tests, like CT scans, are helpful.
This article reviews how a CT scan works, what health conditions it’s typically used for, and what the procedure is like.
A CT scan uses computers and rotating X-ray machines to create cross-sectional images of the body. CT is short for “computed tomography.”
The images provide more detailed information than typical X-ray images. They can show the soft tissues, blood vessels, and bones in various parts of the body.
A CT scan may be used to visualize the:
- head
- shoulders
- spine
- heart
- abdomen
- knee
- chest
During a CT scan, you lie in a tunnel-like machine while the inside rotates and takes a series of X-rays from different angles.
These pictures are then sent to a computer, where they’re combined to create slices, or cross-section images, of the body. They may also be combined to produce a 3-D image of a particular area of the body.
CT scan vs. CAT scan: What’s the difference?
A CT scan and a CAT scan are the same thing. CT stands for computerized tomography, and CAT stands for computerized axial tomography.
The original name for this scan was an EMI scan, named after the company that created the technology. That evolved to CAT scan, and CT is simply a more modern, simplified version of this name.
The image gallery below shows pictures of a CT scanning machine and examples of CT scans.
CT scans can provide detailed images of bones, tissues, and even blood vessels inside your body.
However, the images produced by these scans appear in shades of black and gray. Even for a trained eye, it can be difficult at times to differentiate one tissue type from another in certain situations.
Contrast dyes contain barium or iodine and can be given orally or intravenously (into a vein). These dyes increase the contrast level and resolution of the final images produced with the CT scan for a more exact diagnosis.
However, there are a few risks associated with using contrast dyes. For example, there’s a higher chance of allergic reactions to the dyes, and they’re also not good for your kidneys.
Still, every CT scan exposes you to a certain level of radiation, and a CT scan with contrast may produce better results than one without. It may also prevent the need for a repeated scan.
Below is a comparison of when CT scans may be used with or without a contrast dye.
| With contrast | Without contrast |
|---|---|
| acute appendicitis | acute stroke |
| staging cancer | closed head injuries |
| diverticulitis | lung disease |
| inflammatory bowel disease | tissue swelling or injury in your arms or legs |
| pancreatitis | kidney stones |
| pulmonary embolism | spinal trauma |
A CT scan has many uses, but it’s particularly well-suited for diagnosing diseases and evaluating injuries. This imaging technique can help a doctor:
- diagnose infections, muscle disorders, and bone fractures
- pinpoint the location of masses and tumors, including cancer
- study the blood vessels and other internal structures
- assess the extent of internal injuries and internal bleeding
- guide procedures, such as surgeries and biopsies
- monitor the effectiveness of treatments for certain medical conditions, including cancer and heart disease
The test is minimally invasive and can be conducted quickly.
A CT scan is painless, but it does take a few steps to get successful images.
How to prepare for a CT scan
CT scans don’t require much preparation. If needed, you can do a CT scan with or without contrast very quickly. In fact, this happens in most cases where a CT scan is needed to diagnose traumatic injuries or a stroke.
Emergency CT scans may also be performed to diagnose other life threatening illnesses where a quick diagnosis is critically important for a person’s health.
If you’re scheduled for a CT scan with contrast dye, it may help to refrain from eating solid foods for up to 4 hours before your test. This is especially true if your CT scan is being done to get images of your abdomen.
It’s worth noting that IV contrast can cause nausea and/or vomiting in some people. This is less likely if you haven’t recently eaten.
If your doctor is using oral contrast for your CT scan, you’ll probably be given the contrast before the day of your scan and instructed on how to prepare and drink it. Generally, you will want to start drinking the solution within 1 to 2 hours of your scan, drinking a portion of the solution every 15 minutes.
Your doctor or radiologist will give you specific instructions. If you’re having intravenous (IV) contrast, a catheter will be inserted into your vein when you arrive at the testing facility.
Otherwise, the only preparations you need to take before a CT scan are to remove metallic objects and medication devices from your body. This includes:
- jewelry and piercings
- watches
- glasses
- hairpins
- hearing aids
- dentures
- bras with underwire
- antimicrobial clothing with silver technology
- nicotine patches
- other medication patches
While these objects will not cause you harm if left on during a CT scan, they will lead to a poor-quality test result.
What to expect during a CT scan
When you arrive for your CT scan, you’ll be asked to change into a hospital gown. The technician doing your scan may insert an IV catheter in your arm or leg and ask whether you have removed any metal devices or medication patches prior to your arrival.
They may also review your reason for having the scan, any allergies you may have, and any other special instructions.
When it’s time to begin the scan, you’ll be positioned on a long, narrow table, and you may be secured in place with velcro straps or other safety devices. The table will slide in and out of the circular scanner depending on which parts of your body need to be visualized.
The technician will leave the room before operating the scanner and may give you instructions over an intercom.
As the table moves in and out of the scanner, the machine will rotate around you, making a loud noise. You may be asked to hold your breath or maintain certain positions. Otherwise, you should hold as still as possible to prevent the scanner from capturing blurry images.
The entire process should take between 20 minutes and 1 hour.
After your CT scan
Once the CT scan is over, the images are sent to a radiologist for examination. A radiologist is a doctor who specializes in diagnosing and treating conditions using imaging techniques, such as CT scans and X-rays.
Your doctor will follow up with you to explain the results of your scan.
There are very few risks associated with a CT scan. These include:
- exposure to radiation
- allergic reactions to contrast dyes
- increased cancer risk with multiple scans
If you have an allergy to contrast dye, your doctor may choose to do the scan without contrast. If using contrast is absolutely necessary, your doctor may treat you with steroids or other medications to help prevent an allergic reaction.
After the scan, the contrast dye you were given will be eliminated naturally from your body through your urine and stool.
Contrast dye can strain the kidneys, so you may be instructed to drink a lot of water after your exam.
In addition, if you take metformin for diabetes, you may be asked not to take this medication for up to 48 hours after you receive IV contrast.
Can I have a CT scan if I’m pregnant?
You might be nervous about the risks of imaging scans while pregnant, but the American College of Obstetricians and Gynecologists (ACOG) says the doses of radiation in a CT scan are far below what would cause harm to a developing fetus.
Ultrasounds and MRI scans are usually the first choices for pregnant people, but CT scans are safe and should not be avoided if medically necessary, according to ACOG.
A contrast dye can still be used, but its use should be limited unless it can significantly improve the accuracy of a diagnosis.
CT scan results are considered typical if the radiologist didn’t see any of the following in the images:
- tumors
- blood clots
- fractures
- other atypical characteristics
If any atypical characteristics are detected during the CT scan, you may need further tests or treatments depending on the type of atypicality found.
CT scans are an excellent tool for diagnosing problems with soft tissues, blood vessels, and other body parts that can’t be seen with X-ray or ultrasound imaging.
These painless scans don’t require much preparation and can be done quickly in emergency situations. A CT scan takes less than an hour to complete, but depending on who interprets the results, you may not get results right away.
Your doctor will let you know if a contrast dye is necessary for your scan and what action you need to take after the images are evaluated.