A Magnetic Resonance Imaging (MRI) scan is a non-invasive diagnostic tool that uses powerful magnets and radio waves to create detailed images of internal body structures. While many MRI scans do not require special preparation, patients are often instructed to fast before certain procedures. There are important medical and technical reasons behind this requirement.
Why Fasting is Necessary
Fasting before an MRI scan is primarily a safety precaution and a measure to improve image clarity, particularly for specific types of scans. One significant reason for fasting relates to the use of contrast agents, which are sometimes injected intravenously to enhance the visibility of tissues, organs, or blood vessels. These contrast agents can occasionally cause side effects like nausea or vomiting. If a patient vomits while lying flat inside the MRI scanner, there is a risk of aspiration, where vomit enters the lungs, potentially leading to serious complications such as aspiration pneumonia.
An empty stomach also contributes to clearer images, especially for MRI scans of the abdomen or pelvis. Peristalsis, the involuntary muscle contractions of digestion, can cause motion artifacts. Food and liquids in the stomach and intestines amplify this, appearing as blurring or ghosting on images. This can obscure the area of interest and make accurate interpretation difficult. Fasting minimizes gastrointestinal activity, reducing “noise” and providing sharper diagnostic images of organs like the liver or pancreas.
Fasting is required if sedation or anesthesia is administered for the MRI procedure. Sedation is used for children, claustrophobic individuals, or patients who may have difficulty remaining still. Anesthesia and sedatives can suppress protective airway reflexes, increasing the risk of aspiration if food or liquid is in the stomach. Therefore, an empty stomach is important for patient safety during these procedures.
What and How Long to Fast
Fasting instructions for an MRI vary depending on the scan type, body area, and facility policies. Patients must follow the precise instructions from their healthcare provider or imaging center. Abstaining from solid foods is required for 4 to 8 hours before the scan. For abdominal or pelvic MRIs, patients are advised to avoid eating or drinking for 4 to 6 hours prior.
While solid food is restricted, clear liquids might be permitted up to 2 hours before the procedure. Clear liquids include water, black coffee or tea without milk or sugar, and clear fruit juices without pulp. Avoid sugary drinks, milk, and pulpy juices. Patients should discuss regular medications with their doctor, as some might need to be taken with a small sip of water, while others may require adjustment. Chewing gum or smoking should also be avoided, as these can stimulate digestion and introduce air into the gastrointestinal tract, potentially affecting image quality.
What Happens If You Don’t Fast
Failure to adhere to fasting instructions before an MRI can have several negative consequences, primarily impacting patient safety and the quality of the diagnostic images. The most common outcome is that the MRI procedure may be delayed or rescheduled. Imaging centers prioritize patient safety, and if fasting protocols are not met, the risks of proceeding with the scan may be too high, particularly if contrast agents or sedation are involved.
If the scan proceeds without proper fasting, food and fluid in the digestive system can compromise image quality. This can lead to blurry images due to peristalsis, making accurate diagnosis difficult. A compromised scan may necessitate a repeat procedure, causing inconvenience and inefficient use of healthcare resources. For procedures involving contrast or sedation, not fasting increases the risk of nausea, vomiting, and dangerous aspiration of stomach contents into the lungs.