CT scan: what happens, contrast and radiation

What is a CT scan?

CT stands for computed tomography. A CT scanner rotates an X-ray source around the body while the person lies on a bed that moves through a ring shaped like a doughnut. Detectors on the opposite side pick up the X-rays, and a computer turns each rotation into a thin cross-sectional slice. The slices can be stacked to make a three-dimensional image. The NHS lists CT scans among the tests that use X-rays to give more detailed images than a standard X-ray.

How do people prepare for a CT scan?

Instructions differ between hospitals, and the appointment letter has the local details. Instructions about eating and drinking before a CT scan differ between NHS trusts, and the appointment letter says which apply. Some scans of the abdomen or pelvis involve drinking extra fluid or a drink containing contrast beforehand. Some people have a blood test first, mainly to check how well the kidneys are working. NHS trust leaflets ask people to attend in loose clothing with anything metallic removed. Staff ask about allergies, especially to iodine or to earlier contrast dye, and about medicines the person takes.

What happens during the scan?

The person lies on a flat, motorised table that slides into the scanner. They are asked to keep very still and may be asked to hold their breath for a few seconds. Leaflets quote different lengths for the scan itself, which is short, and the whole visit takes longer, especially when drinks are needed beforehand. After the scan the radiographer may ask the person to wait briefly while they check that no more images are needed, and the person is then free to leave.

What is the contrast dye?

Contrast is a substance that makes blood vessels, organs or other tissues easier to see. Depending on the area, it may be given by mouth, through a vein, or by enema. MedlinePlus says the most common type of contrast given into a vein contains iodine. MedlinePlus says contrast given through a vein can cause a slight burning feeling, a metallic taste and warm flushing, which usually fade quickly.

MedlinePlus says allergic reactions to the dye can happen, and that a life-threatening reaction is rare. MedlinePlus says contrast given through a vein can worsen kidney function, and that people with kidney disease or diabetes may be given extra fluids afterwards.

How much radiation does a CT scan use?

A CT scan uses more radiation than an ordinary X-ray, and MedlinePlus says most new scanners can reduce the dose. MedlinePlus says having many X-rays or CT scans over time may raise the risk of cancer, but the risk from any one scan is small. An NHS trust leaflet says children are more sensitive to radiation and that a CT is done only when it is essential for making a diagnosis. Trust leaflets ask people who might be pregnant to tell the imaging team before the scan. The NHS says the cancer risk from X-ray tests is thought to be very small. GOV.UK safety advice says that when imaging is properly justified and optimised, the risks are much less than the risks of the conditions being investigated, and that cancers linked to radiation can take decades to develop. An NHS trust leaflet says the benefit of a test should be greater than its radiation risk.

When do the results arrive?

A radiologist reviews the images and sends a report to the doctor who asked for the scan. That doctor then discusses the results with the person. Timing varies by hospital, and leaflets quote from about one or two weeks to about four weeks. Results are not given on the day.