Magnetic Resonance Imaging (MRI) uses a powerful magnetic field and radio waves to generate detailed pictures of the organs and tissues inside the body. The procedure is non-invasive and does not rely on ionizing radiation, making it a valuable diagnostic tool. Preparation often includes the instruction NPO (“nil per os”), meaning nothing by mouth. Fasting is imposed for specific safety and image quality reasons.
The Primary Safety Concern: Aspiration Risk
The primary reason for fasting before an MRI relates directly to patient safety, particularly when sedation is involved. Some patients, especially those with severe anxiety, claustrophobia, or young children, require sedation to remain perfectly still for the duration of the scan. When a patient is sedated or under anesthesia, the body’s protective reflexes, such as the cough and gag reflexes, are temporarily suppressed.
Anesthesia guidelines recommend fasting to minimize the risk of pulmonary aspiration. Aspiration occurs when the stomach contents, including food, liquid, and acidic digestive juices, are involuntarily inhaled into the lungs. This can lead to aspiration pneumonitis or pneumonia, which is a life-threatening complication.
Fasting guidelines (e.g., 6 to 8 hours for solid foods) ensure the stomach is empty, reducing the volume and acidity of contents available for aspiration. This precaution is the standard of care for any procedure that suppresses protective airway reflexes. A clear stomach is necessary even if sedation is not planned, should a patient require emergency intervention.
Ensuring Image Quality and Clarity
The presence of food and liquids within the digestive tract can compromise the diagnostic quality of an MRI scan, particularly when imaging the abdomen and pelvis. Digestion causes movement of the gastrointestinal system known as peristalsis. This rhythmic contracting and relaxing of the bowel muscles creates motion artifacts on the images, appearing as blurring or streaking.
Fasting limits digestive activity, minimizing peristalsis and allowing organs to remain still during image acquisition. An empty gastrointestinal tract also prevents gas accumulation, which can create signal voids or distortions that obscure nearby structures. Scans focused on the liver, pancreas, or bile ducts often require fasting to ensure internal structures are not hidden by bowel contents.
Fasting also reduces the natural secretions of the digestive system, which interfere with imaging signals. Some specialized procedures, such as MR Enterography, involve drinking a specific oral contrast agent to distend the bowel. An empty stomach ensures the patient can tolerate the required volume of this solution and that the solution is not diluted by residual food or liquid.
Fasting When Contrast Agents Are Used
Many MRI examinations require the intravenous injection of a contrast agent to enhance the visibility of specific tissues or blood vessels. This agent, often containing Gadolinium, alters the magnetic properties of tissues, helping to differentiate normal structures from abnormalities like inflammation or tumors. However, the injection is associated with side effects.
One common side effect following contrast injection is nausea, which can sometimes lead to vomiting. This reaction is generally mild, but it introduces a secondary risk of aspiration should the patient vomit while lying flat within the scanner.
Fasting before a contrast-enhanced MRI mitigates this emetic risk. Ensuring the stomach is relatively empty reduces the likelihood and severity of vomiting, protecting the patient from inhaling stomach contents. The typical fasting period for contrast administration alone is often two to four hours, focused on reducing gastric volume.
Specific Exceptions and Allowed Intake
Fasting is not required for all MRI procedures; it is determined by the body part being scanned and whether contrast or sedation will be used. Scans of the head, neck, spine, joints, or extremities rarely require dietary restriction unless sedation or contrast is planned. Since these areas are physically removed from the gastrointestinal system, food presence does not affect image quality.
When fasting is required, instructions typically involve abstaining from solid food and opaque liquids for four to six hours prior to the appointment. Clear liquids may be allowed up to two hours before the scan. Clear liquids include plain water, black coffee, or clear apple juice, but patients must always confirm with their imaging center.
Patients should continue to take prescribed medications with small sips of water, especially those for heart conditions or blood pressure, unless instructed otherwise. Chewing gum or hard candies is discouraged because they stimulate stomach acids and saliva. This increases fluid volume and defeats the fast’s purpose. Patients must follow the precise instructions provided by the health care facility.