Why You Need to Fast Before a CT Scan: Essential Information
If you are planning on having a CT scan, your care team will advise you on how to prepare. This will include what to wear, when to arrive and what to bring with you.
Another key aspect of getting ready for a CT scan is whether or not you can consume anything beforehand. You might have read or been told about the importance of fasting for CT scans. So, if you're wondering why do you have to fast before a CT scan. It is because there is a risk of nausea and vomiting when a contrast medium is used.
A contrast medium is also commonly referred to as a contrast agent or contrast dye. Whether one is used will depend on the type of CT scan you are having.
Read on to better understand what a CT Scan is, what a contrast agent is, and why you may need to fast before your CT scan. Also, how to fast comfortably, as well as whether you can drink fluids before a CT scan.
Understanding CT Scans and How They Work
A CT Machine provides a computerised tomography scan (CT scan). It uses computer technology and X-rays to create detailed, 3D images of your internal body. The images created by a scan are cross-sectional and referred to as slices.
The CT machine comprises a scanner table, on which you will lie down, and a ring-shaped structure that looks like a doughnut. The CT scanner table moves through this ring during your scan, while the ring rotates around your body to create detailed images.
As the ring rotates around your body, it releases narrow beams of X-rays that pass through your body. The X-ray signals are then detected by sensors in the ring. These signals are processed by a computer into a detailed image of a single slice through your body.
The scanner table then moves along, so that the CT scanner can repeat the process to collect another image of a slice through the next part of your body.
At the end of your scan, all of the slices are stitched together by a computer to create a 3D image of the inside of your body.
CT Scans with or without a Contrast Agent
For certain CT scans, your doctor’s advice may be to have a contrast agent before your scan. A contrast agent is a special dye that helps create clearer images during a medical scan.
What Contrast Agent is Used for CT Scans?
For CT scans, the contrast agent used is an iodinated dye. This means it contains iodine. However, the liquid is colourless and it does not stain or dye your internal organs.
Instead, it works by making it harder for X-rays to pass through your body. Consequently, when the contrast agent is in your blood vessels and organs, it makes them appear brighter on the CT scan images.
How is the Contrast Agent Delivered?
Depending on which part of your body will be scanned, the contrast agent will be administered via an injectorion into a vein, as an enema (ie administered via your back passage) or taken orally as a drink.
Why Do You Have To Fast If You Have a Contrast Agent?
If you have a contrast agent, you will be requested to fast (ie avoid eating) for 2-4 hours before your scan.
As explained previously, this is because eating before a CT scan with a contrast agent may increase your risk of nausea and vomiting.
This advice may change in the future as new evidence from multiple studies [1, 2, 3] suggests that fasting may not reduce your risk of nausea and vomiting. More research is needed to confirm these findings and until then, you will be advised to fast.
However, although you will be asked to fast before your CT scan with a contrast agent, you will still be allowed to drink fluids. For certain CT scans, it is actively encouraged that you drink fluids immediately before your scan.
If you are having a scan without a contrast agent, you will be able to eat and drink as usual.
It is important to note that the contrast agent for a CT scan is different to the contrast agent used for an MRI scan.
So, if you have previously had an adverse reaction to a contrast agent used in an MRI scan, it does not mean you will have an adverse reaction to the iodine-based contrast agent used in a CT scan. However, you should still inform your care team.
Why Drink Water Before a CT Scan
If you are having a CT scan of your urinary system, which is also known as a CT urogram, you will be asked to drink a certain amount of fluid in the hour leading up to your scan.
This is so that your bladder will fill up and get bigger, which makes it easier to capture clear images of it.
You may also be asked to drink water before an abdominal CT scan for the same reason ie to make it easier to capture clear images of structures within your abdomen.
Foods and Drinks to Avoid Before Your CT Scan
As mentioned earlier, if you are having a CT scan with a contrast agent, you should not eat anything in the hours leading up to your CT scan. If you are not having a contrast agent, you can eat as normal.
Whether you are having a CT scan with or a CT scan without a contrast agent, you are allowed to drink most clear fluids, such as water, juice, black tea or black coffee.
However, if you are having a cardiac CT scan, whether that is a CT coronary angiogram or CT calcium score, you should avoid drinking caffeinated beverages, such as energy drinks, tea or coffee. This is because caffeine makes your heart beat faster, which in turn makes it harder to capture clear images of your heart.
Tips for Managing Fasting Comfortably
To help manage fasting before your CT scan, you can drink small but frequent amounts of fluids. This will help sate your appetite.
To help avoid the temptation of eating food, you can try to keep yourself busy, for example, by reading a book or watching a TV show. However, avoid exercising or anything that requires a lot of exertion as this may make you feel more hungry.
If you have diabetes, your care team will provide you with specific instructions on what to eat and when so you can avoid becoming hypoglycaemic. If you take insulin, you may be asked not to take it before your appointment but to bring it with you, along with some food to have immediately after your CT scan.
Who Can’t Have a CT scan?
As mentioned earlier, CT scans use X-rays. X-rays are a type of ionising radiation. Although only low doses of X-rays are used during a CT scan, unnecessary radiation exposure is not recommended.
This is because if you have a high number of CT scans, there is a very small risk of the ionising radiation causing changes in your cells that lead to cancer.
Consequently, if you are pregnant, a CT scan of your abdomen or pelvis will only be recommended when it’s absolutely necessary to reduce exposing your baby to unnecessary radiation. A CT scan of other parts of your body while pregnant will not expose your baby to ionising radiation.
For the same reason, these scans are not used for screening but only for diagnostics. Screening scans refer to scanning all or part of the body when you have no symptoms to assess your general health. Diagnostic scans refer to scanning a specific part of your body to investigate particular symptoms.
Preparing for a CT Scan Appointment
Make sure you follow the guidance provided by your care team on what to consume and drink before your CT scan.
Dress in comfortable clothing without any metal parts (eg zips, buckles, clasps, bra underwires) on the day of your scan. Also, make sure you remove any metal jewellery, piercings and hair accessories. This is because metal can interfere with the images captured during your scan by creating streaks that obscure the image.
If you are experiencing anxiety about your scan, you can bring a friend or family member with you. Also, make sure you tell your care team, so they can take steps to help you feel better. Calming techniques, such as mindfulness, visualisation and breathing exercises can also help ease your anxiety.
Next Steps after Your CT Scan
After your CT, you can return to your usual activities straight away. However, if you have had a contrast agent, you will be required to remain at the scanning facility for 30 minutes. This is to check that you do not develop an adverse reaction to the contrast agent.
It is important to note that the vast majority of individuals do not develop an adverse reaction to the contrast agent used in CT scans.
At Vista Health, we aim to deliver your CT scan results within 3 working days. Your results can also be sent to your NHS GP and will include a comprehensive report from one of our experienced radiologists – a doctor who specialises in diagnosing and treating conditions using imaging technologies.
Are You Ready to Book?
If you have unexplained symptoms and would like to have them investigated with a CT scan, you can book your private CT scan today at one of our conveniently located nationwide clinics.