A radiologist’s guide to getting a CT scan

Dr. Ania Kielar, vice-president of the Canadian Association of Radiologists and vice-chair of the University of Toronto’s department of medical imaging, explains one of the most common and versatile tools they use to detect disease and injury

What is a CT scan?

Computed tomography (better known as a CT scan) is the “bread and butter of radiology,” according to Dr. Kielar, because of the clear, detailed images the procedure provides. Patients lie on a table that is slowly threaded through a doughnut-shaped machine that quietly rotates around them while taking a series of X-rays. A computer later combines these scans into detailed 3D images of the inner workings of the human body.

What is the difference between a CT scan vs. MRI?

While both of these scans provide clear images of the inner workings of the body, a CT scan achieves its results by using X-rays instead of a magnetic field and radio waves. CT scans are more commonly used because they are faster, cheaper and more readily available. MRI scans, on the other hand, are often ordered when doctors need to see more detail in hard-to-reach areas, such as those inside the brain.

What is a contrast agent?

Depending on the area being scanned, patients may be given a contrast agent, or dye, to help the machine differentiate between organs and improve the quality of the images produced. The dye, which can be administered orally or through an IV, is commonly used for scans of the abdomen or pelvis. “It helps the radiologist to be able to see all their organs and be able to identify what is happening inside of them,” Dr. Kielar says. “It doesn’t happen in all patients, it just depends on the reason we’re imaging the patient.”

What does a CT scan show?

“We use it for a lot of things,” Dr. Kielar says. “We use it to look at the head, the skull, the bones in the skull, sinuses, the brain. We use it in the neck to look at all of the different vascular structures — the vessels and the very soft tissues. We use it in the chest to look at the heart but also the lungs. We use it in the abdomen to look at all the different organs inside there.

“We also use it to look at the spine — so, if somebody’s had an injury to the spine or if they have degenerative disc disease — and we need to see if we need to operate. We look at the pelvis bone if somebody’s been in a car accident or if they have a tumour and then we look at the legs — not as often — but we do look at the legs for a complicated fracture of, say, the knee or the lower leg or ankle. We would also scan the lower leg to look at the vessels to see if they’ve got flow going in them or if they need bypass surgery.”

How long does a CT scan take?

When a patient undergoes a radiologist CT scan, the process typically takes about 30 minutes from check-in to check-out. The radiologist CT scan itself only takes a few seconds, once the patient is positioned correctly on the table. “From the time you check in to the time you leave it’s probably in the range of 30 minutes or so,” Dr. Kielar says. “If you’re wondering how long does it take to do a CT scan, the actual CT scan only takes a few seconds. Once they have you on the table in the right spot, it takes less than a minute. If you have to drink the oral contrast for a CT scan that would be an extra half an hour. It’s not like you just shoot it back, you have to drink it slowly over about half an hour and then they take you over to the CT scanner and image you.”

How can patients prepare for a CT scan?

Clothing/jewelry — Patients should wear comfortable clothes and leave jewelry and other unnecessary metal objects at home as they may interfere with the scan and the radiologist will likely ask you to remove them anyway.
Dietary requirements — Patients should follow the instructions they are given ahead of time as many — but not all — CT scans will require patients to arrive with an empty stomach. “It really depends on the body part,” Dr. Kielar says. “They will give you instructions when they call you. If you’re doing a CT scan of the abdomen, we don’t want the stomach to be full.”
Bloodwork — If a contrast agent will be used, Dr. Kielar says a blood test may be required to ensure a patient’s kidneys are functioning properly and will be able to clear the agent from their system following the scan. Their doctor will inform them of this ahead of time. “If we’re giving you intravenous contrast, we don’t want the stomach to be full because some people feel unwell after getting the contrast agent,” she says. “Not most, but sometimes.”
Medications — While regular medications can generally be taken, Dr. Kielar encourages people to come prepared anytime they enter a healthcare facility. “Whenever you go to the hospital, I strongly recommend that you have a list of your past medical history and your medications.”

Is there anything patients should do after a CT scan?

“Usually, we recommend for people to drink a reasonable amount of fluids after the scan because of the contrast agent, if we use it,” Dr. Kielar says. “Even if the kidneys are working very well, we don’t want that contrast to be inside of their veins for very long.”

How long does it take for results to come back?

In hospital, results may be available the same day. For out-patient, it may take in the range of three to five days for the radiologist to examine the scans and for the report of their findings to make it to a doctor.

Are there any other concerns or risks that accompany a CT scan?

Cancer — CT scans use low doses of ionizing radiation that can conceivably contribute to a future cancer diagnosis but the risks are small and generally outweighed by the information gained through the procedure, Dr. Kielar says. “We use ALARA — which is As Low As Reasonably Achievable — so the risk of getting cancer from one CT scan is very low. The risk of getting cancer from a few in a year is also very, very low. We obviously try not to image people if we don’t have to, but it’s for their best interest. We realize there is always a risk of radiation exposure and future cancer but that would be, like, 20 years down the road and if we don’t treat their cancer, there won’t be a 20 years down the road.”
Allergies — Less than one per cent of patients may feel a little flushed after being administered the dye, according to Dr. Kielar, and there is a small chance of having an allergic reaction that may require medical attention. “Any time we give a contrast agent there are risks but they are extremely low. If we need to find out what’s happening, it’s a risk that can be discussed with their doctor but it’s one that we’re generally willing to take because otherwise we can’t tell what’s happening.”
Claustrophobia — The fear of small, confined spaces isn’t generally an issue with CT scanners, which are shorter and make less noise than MRI machines, but patients should discuss any concerns with their doctor or radiologist ahead of time.
Pregnancy concerns — Radiologists and doctors exercise similar restraint when dealing with pregnant women, making it is vital to inform them of one’s status prior to the procedure. “We try not to scan people who are pregnant. We don’t want to radiate fetuses and developing fetuses are more sensitive to radiation damage. If we can avoid it until later, we will.”

How has the pandemic changed the way CT scans are performed?

Like all scans performed by a radiologist, wait times for a CT scan have generally gone from bad to worse, Dr. Kielar says. Radiologists have decreased the use of contrast agents, where possible, to limit the amount of time patients must spend in the waiting room. Any equipment used is cleaned even more frequently than it used to be. “We use various filters to try to keep the room clean, everyone has to wear a mask all the time — the technologist as well as the patient — and the technologist also wears a shield over their eyes.”