Why Is Fasting Required Before Imaging Procedures?


Why Is Fasting Required Before Imaging Procedures?

Imaging procedures are used for the diagnosis of the problem so that the root cause of the problem can be identified for effective treatment. The doc

Imaging procedures are used for the diagnosis of the problem so that the root cause of the problem can be identified for effective treatment. The doctor uses the images obtained from the scanning procedure to diagnose the patient and plan the best treatment plan according to the health condition. So, getting a clear and accurate picture of the inside of a specific area of the body is crucial. Because if the images won’t clear then it will be tough for the doctor to diagnose the problem accurately. 

In some cases, doctors ask for a fast for some specific period before undergoing the diagnostic and imaging procedure. A fast in medical terms is a period in which you are refrained from eating or drinking anything except water. The fasting time varies depending on the test. But in some cases, the doctor asks you to hold off on taking some specific medications which affect the result of the test otherwise you can take medications. 

In this article, we will discuss those imaging tests, you may need to fast before undergoing. In this article, you will get the answer to the questions that come into your mind when a doctor recommends fasting for some specific period.

Not all imaging procedures and laboratory tests require fasting. It is important to understand which imaging procedures require fasting and why fasting is necessary before undergoing the imaging procedure.

This Article Covers The Mentioned Below Points:

  • Which imaging procedure requires fasting?
  • Why is fasting necessary before undergoing the scan?
  • Final Conclusion

Here Are Some Specific Imaging Procedures That Require Fasting Such As:

  • Ultrasound
  • CT (Computed Tomography) scan 
  • CECT (Contrast Enhanced Computed Tomography) scan
  • CT Angiography
  • MRI (Magnetic resonance imaging) scan with contrast and MRI Angiography
  • PET-CT (Positron emission tomography /computed tomography) scan


Even not all kinds of ultrasound tests require fasting. Doctors recommend fasting usually in the case of abdominal ultrasound but ask to drink a lot of water to keep your bladder full. Before undergoing the abdominal ultrasound, every patient requires fasting be it adults or children. But the duration of fasting varies. 

For adults: – 6-8 hours of fasting 

For children: – 4-6 hours of fasting

Here we are going to discuss the reasons why abdominal ultrasound requires fasting.

An abdominal ultrasound is an imaging procedure that uses a small handheld device such as a transducer to produce the images of the inside of the abdomen such as structures and organs in the abdomen such as the liver, gallbladder, bile ducts, spleen, pancreas, and abdominal aorta. Once the sound waves are transmitted into the body through transducer and these soundwaves then bounce off the structure and organs, the transducer records the waves to create images.

If you eat anything before the procedure, the food remains undigested in the stomach and because of eating and drinking before the scan, the urine remains in the bladder. When the technician moves the transducer over the stomach area, the sound waves are transmitted into the body and sound waves are blocked by the undigested food and urine in the bladder. This way, it is not possible to get a clear picture of the inside of the body. To prevent food products or drinking products from introducing the air into the GI tract and make it easy for the radiologist to image your internal organs in the abdominal area such as the pancreas, bile ducts, and gallbladder clearly and properly. Throughout the day, the gallbladder fills with bile and expands but whenever you consume anything, the gallbladder starts releasing the bile into the stomach and shrinks after releasing it. In this way, it becomes tough for the radiologist to image the inside of the gallbladder so it is always recommended not to eat or drink anything except water before undergoing an abdominal ultrasound.

Now we come to another question that usually comes into mind when doctors instruct for fasting but allow for taking plain water. Why doctor didn’t ask to drink the plain water?

Here is an answer to your question.

By drinking plain water before undergoing abdominal ultrasound, it becomes easier for the radiologist to image the pancreas, bladder, prostate, uterus, and ovaries clearly and effectively.

Now we come to another imaging procedure that requires fasting.

Computed Tomography (CT) Scan

Computed Tomography (CT) Scan
Computed Tomography (CT) Scan

Not all CT scan procedure requires fasting but in the case of CT scan abdomen and CT scan intestine, you may require fasting because in the case of the small intestine, the fasting helps in emptying the small intestine and on the other hand, in the case of CT abdomen, you may be given a contrast agent to make it more highlighted.

Contrast Enhanced Computed Tomography (CECT) Scan

During the CECT scan, the technologist injects you with contrast dye that causes some side effects such as vomiting, allergy reaction, and nausea. If you eat or drink anything before the procedure, there is the possibility of vomiting which can pose blockage in the airway and leads to severe problem. The same happens in the case of CT angiography. This is the reason, the doctor may instruct you for the fasting of 6-8 hours before undergoing the procedure.

MRI (Magnetic Resonance Imaging) Scan With Contrast And MRI Angiography

Doctors recommend the imaging procedure of MRI with contrast or MRI angiography when he or she wants to diagnose the problem in depth. Involvement of the contrast dye in MRI scan with contrast or MRI angiography, even the smallest problems inside the body be they related to organs or blood vessels, can be imaged clearly. Consuming food or drinking anything can elevate the side effects of the contrast media. That’s why you are instructed to eat anything before undergoing the scanning procedure.

PET-CT (Positron Emission Tomography /Computed Tomography) Scan

For a PET-CT scan, the radioisotope that is given to the patient is a transporter of glucose. If you consume anything before the procedure, it can increase or decrease the level of glucose. So, it is always recommended not to eat anything for a minimum of 4-6 hours before the scan to normalize the glucose level. Apart from this, you may be instructed to follow a special diet for 24 hours before the scan.


The main aim of recommending an imaging procedure is to diagnose the problem. But in some cases, eating or drinking anything before undergoing the scanning procedure can cause problems. That’s why the doctor always recommends not consuming anything for a clear and accurate diagnosis of the problem with clear images.