**3. Recurrent acute pancreatitis**

Recurrent acute pancreatitis (RAP) is a common clinical problem, among the first causes of emergency and expense for gastroenterological pathologies in US [11, 12]. Common complications such as the evolution to episodes of acute pancreatitis, the onset of diabetes or the progression to chronic pancreatitis represent an eventuality with a serious impact on the patient's quality of life and on healthcare costs [13]. About one-third of cases of acute pancreatitis have a recurrence, resulting in the onset of chronic pancreatitis over time [14]. In these cases, it is important since the first episode of pancreatitis to study its causes in order to be able to prevent the onset of new ones, thus avoiding progression to chronic forms. In this sense radiology plays an important role, allowing to identify the causes early and treat them promptly, improving the patient's outcome. In fact, the idiopathic forms of acute pancreatitis, those without an apparent underlying cause that can be treated, have a significantly worse outcome [15].

Clinically it is defined as RAP when two or more episodes of pancreatitis are documented three months apart [14]. The underlying causes of RAP are primarily biliary and alcoholic. There are also rarer causes, such as hypertriglyceridemia, about 5%, autoimmune pancreatitis (AIP) and genetic causes. For this reason, it is always recommended to measure blood triglycerides, search for possible autoimmune etiologies (especially type II AIP), and search for mutations affecting at least four genes, especially in young patients with early onset of acute pancreatitis, in which a genetic mutation exists very frequently [11, 16].

However, radiology is crucial in identifying only some of these causes of recurrent pancreatic inflammation, in particular those of biliary origin, those related to an anomaly of the pancreatic ductal system, a sphincter of Oddi dysfunction (SOD) or other causes of intrapancreatic obstruction.

Although CT and MRI are first-level methods for studying the pancreatic and biliary ductal system, currently the gold standard to identify small calculi or even small tumors that hinder the outflow of pancreatic juice is endoscopic ultrasonography (EUS) [17]. However, this is a highly operator-dependent method, which requires high

### **Figure 7.**

*Male, 66 yo with RAP (3 episodes). Dilatation of the main pancreatic duct (arrows) is appreciable in the tail of the pancreas, both at T2w (a) and MRCP (b), compatible with MD-IPMN.*

expertise of the physician and which is not available in all centers. For this reason, CT and MRI are more commonly used. CT is useful for detecting ductal obstructions due to calcific stones. MRI with cholangiopancreatography (MRCP), on the other hand, allows evaluation of the anatomy and possible anomalies of the pancreatic duct and its branches. Both methods allow us to study the pancreatic parenchyma, in order to identify those causes of RAP that originate from parenchymal disease, such as AIP, which will be discussed later. Radiology also has the role of identifying and characterizing any intraductal papillary neoplasms (IPMN), which can be the cause of RAP. Both main duct and branch ducts IPMNs produce mucus, which is viscous and can temporarily obstruct the outflow into the main pancreatic duct, causing small painful colic that can also lead to real episodes of pancreatitis that recur over time. In this case, MRI is essential to characterize pancreatic cystic lesions and above all to clarify their communication with the ductal system in IPMNs (**Figure 7**). Other causes of the obstacle to the outflow of pancreatic juice that can be effectively studied with radiology are congenital variants of pancreatic ducts. The main anatomical anomaly of the ducts is the pancreas divisum (**Figure 8**), in which the pancreatic duct remains divided into its two embryonic components: the ventral duct and the dorsal duct. However, only about 5% of the population has this anatomical variant and among them, only 5% will develop symptoms of RAP [18]. Pancreas divisum is classically classified into three types: type 1 is when there is an incomplete fusion between the ventral and dorsal ducts (more common), type 2 is when there is the total dominance of the dorsal duct with complete absence of the Wirsung duct, while type 3 is when there is a thin communication between the two ducts (incomplete divisum) [19].

MRCP is also important for detecting the presence of a santorinicele with high sensitivity, which can be one of the causes of pancreatic outflow impairment (**Figure 8**).

S-MRCP is a morpho-functional method that requires the administration of secretin to the patient and the serial acquisition of MRCP images to evaluate the various phases of pancreatic secretion in the duodenum [20]. In a healthy pancreas, 1 minute after administration, a filling with minimal dilation of the main pancreatic duct and all the side branches is expected, starting to glimpse a duodenal filling. After 5 minutes the ductal system is completely relaxed and the duodenum filled with liquid. In case

### **Figure 8.**

*M, 41 yo. History of recurrent epigastric pain. At MRCP a pancreas divisum is visible with a small ectasia at the level of the minor papilla (santorinicele).*

### **Figure 9.**

*M, 42 yo. Recurrent abdominal pain with amylase elevation. S-MRCP during (a) and 5 minutes after (b) administration of secretin. The main duct persists dilatated even after 5 minutes, the diagnosis is compatible with SOD. The physiological duodenal filling is delayed but partially conserved.*

of impaired functional alterations, such as SOD, which is not detectable with morphological imaging, this physiological flow in the duodenum is not observed and the ductal system remains dilated for a longer time, without emptying regularly (**Figure 9**).

S-MRCP is useful for identifying cases of santorinicele, as well as for evaluating the effects of sphincterotomy [21]. Finally, it is useful for highlighting any anatomical anomalies of the ducts not clearly visible to the common MRCP which can lead to obstructive disease, as previously described.
