Doctors and other healthcare 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.

Certain medical conditions require a deeper look, usually at the tissues, blood vessels, and bones inside your body. X-rays and ultrasounds can provide some information, but when a more detailed view is required, a computed tomography (CT) scan is usually the next step.

In this article, we take a closer look at how a CT scan works, what 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. These 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 of the machine 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 images of slices, or cross-sections, 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.

CT scans can provide detailed images of bones, tissues, and even blood vessels inside your body.

However, the images that are produced by these scans appear in shades of blacks and grays. It can be difficult at times even for a trained eye to differentiate one tissue type from another in certain situations.

Contrast dyes contain barium or iodine and can be given in a number of ways, including orally and intravenously (in your 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 contrastWithout contrast
acute appendicitisacute stroke
staging cancerclosed head injuries
diverticulitislung disease
inflammatory bowel diseasetissue swelling or injury in your arms or legs
pancreatitiskidney stones
pulmonary embolismspinal trauma

A CT scan has many uses, but it’s particularly well-suited for diagnosing diseases and evaluating injuries. The imaging technique can help your 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.

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.

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 an hour or two 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

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 why you’re having the scan, any allergies you may have, and 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.

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 cause some strain to the kidneys, so you may be instructed to drink a lot of water after your exam.

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 do, but you may not get results right away depending on who is interpreting the results.

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.