What to Expect During Your First CAT Scan (And Yes, You Keep Your Clothes On)

What is a CAT scan?

A CAT scan, also known as a CT (computed tomography) scan, is a medical imaging technique that uses x-rays and computers to create detailed cross-sectional images of the inside of the body. The term “CAT” stands for “computed axial tomography.”

CAT scans were invented in the 1970s by British engineer Sir Godfrey Hounsfield and South African physicist Allan Cormack. Hounsfield built the first CT scanner at EMI Central Research Laboratories in 1972 after years of research, with the first scan performed on a human patient in 1971 [1]. Cormack had developed the mathematical theory behind CT scanning earlier in the 1960s [2]. Their revolutionary work earned them a joint Nobel Prize in Physiology or Medicine in 1979.

During a CAT scan, the patient lies still on a table that moves through a large donut-shaped machine. X-rays from multiple angles pass through the body and are detected by the machine. A computer then processes this information to generate cross-sectional images “slices” of the internal organs and tissues. This provides more detail than conventional x-rays.

Why CAT scans are performed

CAT scans, also known as CT scans or computed tomography scans, are commonly performed for a variety of medical reasons. Some of the main reasons doctors order CAT scans include:

Detect abnormalities and diagnose medical conditions – One of the most common uses of CAT scans is to detect abnormalities in the body that may indicate medical conditions. CAT scans provide detailed images that can reveal tumors, cysts, infections, bone fractures, blood clots, and other issues that often cannot be seen on regular X-rays or physical exams. This allows doctors to make accurate diagnoses.

Identify causes of symptoms – CAT scans are often used to pinpoint the underlying cause of a patient’s symptoms when the source is unknown. For example, severe or persistent abdominal pain may warrant a CAT scan of the abdomen to determine if there is an obstruction, infection, or other issue that standard tests cannot detect. The high level of detail from a CAT scan can reveal the cause of vague symptoms.

Guide medical procedures – CAT scans are critical for guiding certain minimally invasive procedures such as biopsies, injections, catheter placements, and more. Surgeons can use real-time CAT scan imaging to see inside the body and safely maneuver medical instruments to exactly the right spot without large incisions.[1]

The CAT Scan Process

A CAT scan, also known as computed tomography or CT scan, uses a specialized X-ray machine to take pictures of the inside of the body. During the scan, the patient lies still on a table that slides into a large, ring-shaped scanner. The scanner consists of an X-ray tube that rotates around the body and takes images from different angles. The images are processed by a computer to create cross-sectional pictures, or “slices” of the body part being imaged.

As the Mayo Clinic explains, “The computer stacks the slices together to create a three-dimensional image of the scanned area that shows it in fine detail” (Source). The scan itself typically only takes 5-10 minutes, during which time the patient must lie very still. Movement can cause blurring in the images. The entire appointment takes about 30 minutes including preparation.

Do you need to remove clothing?

You do not need to completely undress for a CT scan. The important thing is to wear comfortable clothing without metal. Here are some recommendations:

  • Wear loose, comfortable clothing like sweatpants and a t-shirt or hospital gown.
  • Avoid clothing with zippers, snaps, buttons or other metal.
  • Remove jewelry, eyeglasses, hair clips, body piercings, and other metal objects.
  • You may keep your clothes on if they do not have metal that could interfere with the images.
  • The technologist will advise you if you need to change into a hospital gown.
  • Let the technologist know if you cannot remove an item for medical or religious reasons.

The key is avoiding metal that could cause issues with the magnetic fields used in a CT scan. While you do not need to be completely undressed, removing or avoiding metallic items can prevent image artifacts or distortions. Let the scanning facility know ahead of time if you have any concerns about clothing or metal items.

Are there alternatives to CAT scans?

In some cases, other imaging modalities may be used instead of a CAT scan, such as:


An MRI (magnetic resonance imaging) scan uses magnets and radio waves to produce detailed images of the body. It does not involve radiation like a CAT scan, so it may be preferred for pregnant patients or children (Hopkins Medicine). MRI scans take longer than CAT scans and cannot be done on patients with metal implants.


Ultrasound uses sound waves to image the body and is most commonly used to look at abdominal organs, muscles, tendons and unborn babies. It may be used instead of a CAT scan to assess pelvic masses (ImageWisely).


A standard x-ray produces basic images of bone and some body tissues. It does not give the detailed 3D views that a CAT scan provides. However, x-rays use less radiation and may be appropriate for examining bones or dental issues.

Overall, MRIs and ultrasound avoid radiation exposure but cannot match the detail provided by CAT scans for many conditions. X-rays are relatively basic images. The doctor will determine the most suitable scan based on the area and condition being investigated.

Risks and side effects

CAT scans expose patients to relatively high amounts of radiation compared to other imaging tests. According to the FDA, a single chest CT scan exposes patients to as much radiation as 100-200 x-rays (FDA, 2017). Frequent or accumulated exposure to radiation may increase cancer risks later in life. However, the risks from any single scan are generally low.

Some patients can have allergic reactions to the contrast dye used in CAT scans. This non-iodine based dye helps provide clearer images, but may cause itchiness, sneezing, hives, swelling or other symptoms in patients with allergies. In rare cases, a life-threatening reaction known as anaphylaxis may occur (Mayo Clinic, 2022).

The contrast dye used in CAT scans is filtered through the kidneys. In patients with pre-existing kidney dysfunction, the dye may further damage kidney function. Signs of kidney problems may include decreased urine output, swelling, or shortness of breath after receiving the dye.

Who should avoid CAT scans

Certain groups of people should avoid CAT scans when possible, including pregnant women and people with kidney problems or allergies.

Pregnant women should use caution with CAT scans because the radiation may potentially harm the developing fetus. According to the Mayo Clinic, CAT scans expose the fetus to less than 10 rad of radiation, which is generally considered safe. However, unnecessary radiation exposure should still be avoided. Ultrasounds and MRI scans are usually preferable for pregnant women if the same diagnostic information can be obtained.

People with kidney problems, including kidney failure or chronic kidney disease, should avoid iodinated contrast materials used for some CAT scans. The contrast dye can further impair kidney function in those who already have compromised kidneys. However, some CAT scan facilities offer non-contrast CT scans as an alternative.

Those with allergies or sensitivities to contrast dye used in CAT scans should also avoid this procedure when possible. According to the Mayo Clinic, reactions can range from mild itching to life-threatening anaphylactic shock. Antihistamines may be given prior to the test to reduce the risk of allergic reactions in susceptible individuals.

How to prepare for a CAT scan

Proper preparation is important for getting good results from your CAT scan. Here are some tips:

Fast for 4-6 hours beforehand. Don’t eat any food in the hours leading up to your appointment. You may drink water to stay hydrated. Fasting helps prevent nausea and vomiting during the exam [1].

Drink water to stay hydrated. Although you shouldn’t eat solid foods, be sure to drink plenty of water leading up to your scan. Dehydration can affect the results [2].

Take medications as normal. Don’t skip any prescribed medications unless your doctor instructs you to. Let your medical team know of any medications you are taking.

Follow any other instructions provided by your healthcare provider. You may need to drink a special contrast solution before the scan to help enhance the images.

Arriving prepared and on time will help ensure you get the most accurate results from your CAT scan.

What to expect during the scan

During a CAT scan, you will need to lie perfectly still on a table that slowly moves through a large scanner. The scanner is shaped like a large doughnut or ring that surrounds your body as you move through it. Inside this scanner are X-ray machines that will rotate around you and take images from different angles.

As the scanner buzzes, whirs, and rotates around your body, it is important not to move at all so the images come out clear. You may hear various loud mechanical noises from the scanner as it moves and takes images. Typically, you will need to lie still for up to 30 minutes depending on the area being scanned.

According to the Mayo Clinic, the scanner table moves very slowly through the scanning ring at a speed of about half an inch per second 1. So while the test may seem long, the table is slowly progressing at a pace of a few feet per minute. Remaining still allows the X-ray beam to follow the same path through your body with each rotation.

Some facilities also provide headphones with music to help patients relax and distract from the loud buzzing and whirring noises during the scan. However, you will need to remain motionless despite any discomfort from lying on a hard table or loud scanner noises.

Understanding your results

After the CAT scan is complete, a radiologist trained to interpret the images will analyze the scan. The radiologist will look for abnormalities or areas of concern, and write up a detailed report describing the findings [1]. This report will then be sent to the doctor who ordered the CAT scan.

Your doctor will review the radiology report and discuss the results with you at a follow-up appointment. They can explain if the findings are normal or show any problems that need further evaluation or treatment. It’s important to go through your results with your doctor, as they can interpret the radiologist’s report and explain what it means for your health in clear terms you understand [2].

Some key things your doctor will cover about your CAT scan results include:

  • Whether the results are normal or abnormal
  • If there are any areas of concern that need more testing
  • If the scan found any suspected tumors, infections, injuries or other issues
  • If treatment is recommended based on the findings
  • If any follow-up scans may be needed

Don’t hesitate to ask questions if you don’t understand something about your CAT scan results. Your doctor can explain the findings and what they mean for your health in a way you can understand.

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