Hepatobiliary/GI Application Tips: Secretin Magnetic Resonance Cholangiopancreatography (MRCP)

Assessment of pancreatic exocrine function is important in the diagnosis, management and pre-operative planning of patients with chronic pancreatitis. There are two principal kinds of pancreatic function tests: the secretin test and the intraductal secretin test. The secretin test involves collection of duodenal juice after secretin stimulation of the pancreas. The intraductal secretin test involves collection of pure pancreatic juice after endoscopic retrograde cannulation of the main pancreatic duct during an endoscopic retrograde pancreatography (ERP). Both of these tests are invasive, time-consuming, operator dependent, and expensive. In addition, acute pancreatitis still remains the most severe complication of ERP, with an incidence of 1-4%. Many studies have demonstrated the high diagnostic accuracy of magnetic resonance cholangiopancreatography (MRCP) for patients with advanced pancreatic disease and marked pancreatic ductal alterations. However, owing to a lower spatial resolution than ERP, the use the evaluation of MRCP for the evaluation of patients with ductal abnormalities in cases of mild pancreatitis still presents a diagnostic challenge. The side ductal branches usually are depicted by MRCP only when dilated. Secretin MRCP has been developed as a refinement of MRCP and is a non-invasive, fast, low risk method for quantifying pancreatic exocrine function. Secretin administration helps improve the delineation of ductal morphology in both healthy individuals and in patients suspected of having chronic pancreatitis. Secretin MRCP is also demonstrates the pancreatic ductal system differently than is demonstrated by ERP. In ERP retrograde injection of contrast medium creates enlargement of the ducts, whereas in MRCP, the physiologic or physiopathologic ductal liquid content and dilation is demonstrated.

IV administration of secretin induces the secretion of fluid and bicarbonate by the exocrine pancreas. Thus, ductal filling is increased, and visualization of the pancreatic tract is improved. In addition, the extent of positive duodenal contrast induced by the drainage of pancreatic fluid via the ampulla can be assessed semi-quantitatively and may be used for the noninvasive evaluation of the exocrine function of the pancreas. The extent of pancreatic fluid secretion, over 10 minutes of imaging after secretin administration, is classified using the duodenal filling score as follows:

Pancreatic exocrine function is considered to be reduced when the duodenal filling volume grade is less than grade 3.

Coronal T2-weighted fast, thick-section section, spin-echo MR sequences with no post-processing are used allowing for repeated breath-holds for a dynamic series. These images allow for direct visualization of the pancreatic duct in a few seconds. A negative oral contrast agent, such as superparamagnetic nanoparticles suspensions (ferumoxil oral suspension, e.g Lumirem, GastroMARK) is given 10 minutes before dynamic imaging. The negative oral contrast suppresses gastrointestinal overlap and prevents confusion between pancreatic fluid outflow and possible gastric emptying, i.e. nulls the fluid signal in the stomach and duodenum. Our institution has been using pineapple juice as a negative oral contrast agent, given at a dose of 3 cups (approximately 500-600 cc) 5-10 minutes before the imaging study. Pineapple juice is very inexpensive, easily available, and well accepted by most patients. Compared to certain commercially available superparamagnetic oral contrast agents, pineapple juice offers a more subtle shortening of T2 by our observation, therefore avoiding the susceptibility artifacts associated with these standard superparamagnetic contrast agents. The mechanism of pineapple juice to shorten T2 is due to its relatively high manganese concentration. There have been reports to administer blueberry juice as oral contrast in the past for the same mechanism, however, we use pineapple juice because of its easy availability. Our institution does not give an antiperistaltic drug to eliminate motion artifact, since each image acquistion takes only 3 seconds and motion artifact is not a big issue.

Next, the patient is placed in supine position for imaging. A set of images is acquired before secretin stimulation to allow for optimal positioning of the imaged section. After administration of IV secretin (dose of 1 clinical unit per kilogram body weight or dose stated on the specific secretin brand insert), image acquisition of the optimal section is repeated every 30 seconds. This dynamic procedure is conducted for 10 minutes. The delay between ingestion of the oral contrast and the acquisition of the secretin dynamic images is less than 5 minutes. The overall study time is approximately 15 minutes. It is assumed that once the pancreatic fluid reaches the duodenum its signal is not dramatically suppressed by the negative oral contrast agent. In fact, the density of the negative oral contrast agents is different from the density of the pancreatic fluid, thus, the two liquid phases may coexist without mixing. The pancreatic outflow after secretin stimulation in healthy subjects is in the range of 2-5 mL per minute, presumably low enough to allow the two liquid phases to coexist without mixing. The extent of pancreatic fluid secretion is then classified using the above duodenal filling scoring system.