A HIDA scan (Hepatobiliary Iminodiacetic Acid scan) is a nuclear medicine imaging procedure used to evaluate the function of the liver, gallbladder, and bile ducts. This diagnostic tool tracks the flow of bile from the liver, through the biliary system, and into the small intestine to identify potential blockages, leaks, or inflammation. Successful performance requires careful patient preparation, especially clarifying the instruction to be NPO.
The Critical Requirement of Fasting (NPO)
The preparation instructions for a HIDA scan universally include a period of fasting, which is medically referred to as NPO, an abbreviation for the Latin phrase nil per os, meaning “nothing by mouth.” This means the patient must not consume any food or beverages, except for plain water. Patients must strictly avoid gum, mints, and smoking, as these can trigger a digestive response.
The specific duration of the fast usually falls within four to twelve hours before the scheduled scan. Patients should confirm the exact time frame with their physician or the imaging facility, as the protocol varies depending on the reason for the exam. Adhering to this timeline directly influences the accuracy of the images obtained.
Why Fasting Ensures Accurate Results
Fasting is necessary because it relates directly to the natural physiology of the gallbladder and its response to food intake. When a person eats, the digestive system releases the hormone Cholecystokinin (CCK). CCK signals the gallbladder to contract and empty its stored bile into the small intestine to aid in fat digestion.
If the patient has recently eaten, the gallbladder will be contracted and largely empty due to CCK release. When the radioactive tracer is injected during the HIDA scan, it travels from the liver but cannot effectively enter an already contracted gallbladder. This inability to fill the organ can lead to a false-positive result, incorrectly suggesting chronic gallbladder inflammation.
Fasting for too long (typically exceeding 24 hours) can also compromise the scan’s accuracy. Extended fasting allows the bile within the gallbladder to become overly concentrated and thick, which can impede the entry of the radiotracer. In cases of prolonged fasting, the facility may administer a synthetic version of CCK, such as sincalide, before the tracer injection to empty the thickened bile and ensure a reliable result.
Other Essential Pre-Scan Preparations
Beyond strict fasting, several other preparatory steps ensure a successful HIDA scan. Patients must discuss all current medications with their healthcare provider, as certain drugs can interfere with the biliary system’s function. Opioid-based pain medications, for instance, must usually be withheld for a minimum of four to six hours, and sometimes up to 48 hours, before the test.
Opioids can cause a temporary spasm of the sphincter of Oddi, a muscle controlling the flow of bile and pancreatic juices into the small intestine. This spasm can mimic a true obstruction on the scan, leading to an inaccurate diagnosis. Patients should also confirm their pregnancy or breastfeeding status, as the radioactive tracer poses a risk; nursing mothers are instructed to pump and discard breast milk for a specified period after the scan.
Patients are advised to wear comfortable, loose-fitting clothing and remove all metal objects, such as jewelry, body piercings, and belts, from the trunk area. These metallic items can block the gamma camera’s view and create artifacts on the images, obscuring anatomical detail. Arriving at the imaging center 15 to 30 minutes before the appointment allows for administrative work and pre-scan checks.
What to Expect During the HIDA Scan Procedure
The HIDA scan is performed in a nuclear medicine department and begins with the placement of an intravenous (IV) line, usually in the arm. Through this IV, a small amount of a radioactive tracer, such as Technetium-99m Mebrofenin, is injected. The patient is then positioned lying flat on an examination table beneath a large gamma camera.
The gamma camera detects the gamma rays released by the tracer as it moves from the bloodstream into the liver, bile ducts, gallbladder, and small intestine. Initial imaging is captured over approximately sixty minutes to track the tracer’s path through the hepatobiliary system. The technologist may require the patient to remain still or change positions slightly to acquire the best views.
If the scan is measuring the gallbladder’s ability to contract and empty (Gallbladder Ejection Fraction or GBEF), a synthetic version of CCK is administered through the IV line. This infusion takes thirty to sixty minutes, and the camera continues to take images to measure the percentage of bile the gallbladder expels. The entire procedure usually lasts between 90 minutes and two hours, but it can extend longer if delayed images are necessary.