CT scan: what to expect, contrast dye and radiation risks explained

1. Introduction

A CT scan, short for computed tomography and also called a CAT scan, is a test that makes detailed pictures of the inside of the body. It uses specialised X-ray equipment and a computer to build cross-sectional pictures, which are like slices through the body. The slices can be combined into a three-dimensional view ^{[2, 4]}. The NHS says CT scans are mainly used to diagnose conditions or to check whether treatment is working ^[1]. This article explains why CT scans are done, what happens, what contrast dye is and what to know about radiation. Your doctor decides whether a scan is right for you.

2. How a CT scanner works

In a CT scanner, an X-ray source rotates around you while you lie on a table that moves through a ring-shaped opening. Detectors on the opposite side pick up the X-rays, and a computer builds each slice, usually 1 to 10 millimetres thick ^[4]. This gives more detail than a standard X-ray ^[4].

3. Why a CT scan is done

Doctors may use CT to ^{[1, 2, 4]}:

• Check bones or internal organs after an injury, including looking for bleeding ^{[1, 2]}.
• Find the cause of symptoms, for example when a tumour or blood clot is suspected.
• Look at the head, chest, abdomen (belly), spine or heart.
• Check how a treatment is working, such as the size of a tumour during and after cancer treatment.
• Guide a procedure such as taking a tissue sample, called a biopsy ^{[1, 2]}.

4. Getting ready

The hospital will give you instructions. These may include changes to your diet, adjusting medicines, or fasting for a few hours ^{[1, 2]}. You will be asked to remove metal items such as jewellery, hair clips and clothes with zips or snaps, and you may change into a gown ^{[1, 2]}.

The NHS asks you to tell the hospital if you are or might be pregnant, are breastfeeding, take medicines, have asthma, kidney or thyroid problems or diabetes, or have allergies, including any past reaction to contrast dye ^[1]. If you feel anxious or are afraid of small spaces, tell a doctor beforehand. Help may be available, and you can ask to bring someone with you ^[1].

5. What happens during the scan

A radiographer does the scan ^[1]. You lie on a flat bed while the scanner moves around the part of your body being imaged. The radiographer works the scanner from another room but can hear and speak to you. You need to keep still and may be asked to breathe in, breathe out or hold your breath at certain points ^{[1, 2]}. You may hear whirring sounds ^[2]. Most scans take about 10 to 20 minutes ^[1].

Contrast dye

Contrast is a substance that makes certain areas show up more clearly. It may be given as a drink, through a vein or rectally, and it leaves the body in urine ^{[1, 2]}. Dye given through a vein can cause a feeling of warmth, a metallic taste or a feeling that you need to pass urine ^[1]. Drinking fluids afterwards helps the body clear it ^[2]. Some contrast contains iodine, and others are barium-based for imaging the digestive system ^[4].

An allergic reaction to contrast dye is possible, and it can cause weakness, sweating and difficulty breathing, so tell the radiographer at once if you feel unwell ^[1]. You may be asked to wait up to 30 minutes afterwards so staff can check ^[1]. Rarely, contrast can cause temporary kidney failure, and contrast given through a vein should generally be avoided in people whose kidneys do not work normally ^[4].

6. Results

A radiologist reviews the scan and sends a report to the doctor who referred you ^{[1, 2]}. The NHS says results usually take 1 to 2 weeks and may take up to 4 weeks, depending on urgency and location. Your doctor will talk through the results, and you may need further tests ^[1]. If you have not heard after 4 weeks, contact the referring doctor ^[1].

7. Radiation, children and pregnancy

CT uses ionizing radiation, a form of radiation that can damage tissue ^[4]. The dose is higher than for a standard X-ray, though it is kept as low as possible ^[2]. The radiation adds up over a lifetime and carries a small cancer risk ^[4]. The NHS says having many CT scans carries a very small risk of cancer, and doctors weigh this against the benefit ^[1]. The FDA also notes that scans can show unexpected findings that lead to extra tests, which can have their own risks ^[3].

Children are more sensitive to radiation, so parents can ask whether the settings are adjusted for children ^{[3, 4]}. In pregnancy, NIBIB says CT of areas other than the belly or pelvis has no known risk to the baby, and that for the belly or pelvis, MRI or ultrasound is usually preferred unless CT is urgently needed ^[4]. Always tell the hospital if you are or might be pregnant ^{[1, 2]}.

8. When to see a doctor

Talk to a doctor if you have an injury, new or ongoing pain, a lump or other symptoms that might need imaging. After a scan, contact the referring doctor if you have not heard about your results within the time you were given ^[1]. If you feel weak, start sweating or have difficulty breathing after contrast dye, tell staff or seek help at once ^[1].

In an emergency, call your local emergency number (112 in India, 911 in the United States).

9. Conclusion

A CT scan is a quick, painless test that gives detailed pictures and can answer important questions about injuries, symptoms and treatment. It uses more radiation than a standard X-ray, so it should be done when it is clearly helpful. Tell staff about pregnancy, allergies, kidney problems and your medicines, and ask your doctor to explain why the scan is recommended and what the result means.