Magnetic Resonance Imaging (MRI) is a diagnostic tool that uses strong magnetic fields and radio waves to create detailed images of organs and tissues within the body. The term NPO, which stands for nil per os (Latin for “nothing by mouth”), signifies a requirement to fast before a medical procedure. While many MRI scans do not require dietary restrictions, the necessity for NPO is entirely conditional upon the specific type of scan being performed. This article clarifies the specific conditions under which fasting is necessary and provides actionable guidelines for compliance.
The Necessity of Fasting for Specific Scans
The primary reasons for requiring a patient to fast before an MRI relate to minimizing internal motion, reducing risks associated with contrast agents, and ensuring patient safety during sedation. A full stomach increases gastrointestinal activity, causing motion artifacts that blur images, especially in the abdomen or pelvis. The constant movement of the bowel, known as peristalsis, can obscure fine anatomical details, potentially leading to misdiagnosis. Fasting helps to quiet the digestive system, thereby producing sharper, more reliable diagnostic images.
Fasting is also a precaution when the procedure involves injecting a contrast agent, such as gadolinium, used to enhance the visibility of tissues or blood vessels. The contrast material can sometimes induce temporary nausea or vomiting in patients. If a patient vomits while lying flat on the scanner table, there is a risk of aspiration, where stomach contents enter the lungs, which can lead to serious complications. An empty stomach significantly mitigates this safety risk.
A third reason for an NPO order is preparation for sedation or general anesthesia, often necessary for pediatric patients or adults with claustrophobia. Anesthesia relaxes the protective reflexes that prevent aspiration. The American Society of Anesthesiologists (ASA) guidelines recommend specific fasting periods to ensure the stomach is empty before administering sedating medications. Adhering to these guidelines is a standard safety protocol to safeguard the patient’s airway during the procedure.
Common MRI Procedures Requiring NPO
Fasting is mandatory for procedures targeting the digestive system or those requiring a high degree of image stillness in the torso. Abdominal and Pelvic MRIs often require NPO, especially when focusing on organs adjacent to the gastrointestinal tract, such as the liver, pancreas, or biliary system. For instance, Magnetic Resonance Cholangiopancreatography (MRCP), which images the bile ducts and pancreatic duct, requires fasting. This ensures the gallbladder is distended and minimizes peristaltic movement that could interfere with visualizing these small structures.
Another procedure requiring fasting is MR Enterography (MRE), a dedicated study of the small bowel used to evaluate inflammatory bowel conditions. Although MRE involves drinking oral contrast, the patient must fast from solid food for several hours beforehand. This clears the bowel of undigested material that would otherwise obstruct clear imaging of the intestinal walls. Additionally, any MRI scan necessitating intravenous sedation or general anesthesia mandates NPO status, regardless of the body part being scanned.
Practical Guidelines for NPO Compliance
The fasting duration must be followed precisely as instructed by the imaging facility. The standard NPO protocol, based on ASA guidelines, suggests avoiding solid foods and non-clear liquids for a minimum of six to eight hours before the procedure. This timeframe ensures the stomach is empty of solid matter.
Clear liquids are typically permitted up to two hours prior to the scheduled scan time. A clear liquid is defined as any liquid through which print can be read. The two-hour window applies to most procedures, including those involving sedation, as these fluids pass quickly through the stomach. Examples of clear liquids include:
- Plain water
- Black coffee or tea without milk or cream
- Clear apple juice without pulp
- Clear broth
Patients taking essential medications, such as those for blood pressure or cardiac conditions, should consult their physician about whether to take them. Generally, these medications can be taken with a small sip of water, but non-essential medications should be held until after the scan. Diabetic patients require individualized instructions, as prolonged fasting can lead to hypoglycemia. They must discuss their insulin or oral medication schedule with their doctor and the imaging center to safely manage blood sugar levels during the NPO period. Failing to adhere to the NPO instructions, particularly for procedures involving sedation or contrast, will likely result in the cancellation and rescheduling of the appointment to protect the patient’s safety.