In this iconographic essay we illustrate the key imaging features of sclerosing cholangitis.

1)   Magnetic resonance cholangiopancreatography (MRCP):
MRCP is a non invasive imaging technique and remain the reference for detection, characterization of intra and extra hepatic bile ducts abnormalities and complications.
Types of used sequences :
1.SSFSE : shows the very high contrast between the hyperintense signal of the bile and hypointense signal of surrounding structures,hepatic dysmorphism and heterogenicity of signal intensity.
2.Fat saturated T1 weighted imaging :looks for intra or extra hepatic bile duct
lithiasis
3.Diffusion :particulary useful for the detection cholangiocarcinoma.
4.2D and 3D RARE :overview of biliary tract anatomy for identifying obstructions and strictures.
5.Fat saturated T1 weighted imaging dynamic gadolinium injection: inhomogeneous parenchmal enhancement, ductal wall enhancement, periportal enhancement, focal mass enhancement.

2)  Primary sclerosing cholangitis:

Primary sclerosing cholangitis is a chronic, progressive liver disease with inflammation and fibrosis of the bile ducts of unidentified etiology, which finally progresses to biliary cirrhosis and portal hypertension (1,2).
PSC affects men twice as often as women, and generally young patients with an age of onset of 30–40 years. PSC has a strong association with inflammatory bowel diseases. The clinical presentation can vary, including cholestatic laboratory findings and nonspecific symptoms including right upper quadrant pain or jaundice. Although the pathogenesis remains to be elucidated, many experts accept an autoimmune component (3,4).
Diagnosis of PSC can be made by typical cholangiographic findings and the exclusion of secondary causes. The magnetic resonance (MR) MRCP is preferred as the initial imaging investigation as it is non-invasive with no radiation exposure. The typical MR cholangiographic features include diffuse, multifocal short segmental strictures and mild dilatation in the intrahepatic and extrahepatic bile ducts alternating with normal ducts, which sometimes produce “beaded” appearance. As the fibrosis progresses and strictures worsen, the peripheral bile ducts are obliterated and become poorly visualized on MR cholangiography showing a “pruned tree” appearance. Diverticular outpouching of bile ducts is another characteristic finding that occurs in up to 27% of the patients with PSC. PSC commonly involves both intrahepatic and extrahepatic ducts in 75% of patients, whereas involvement of only the extrahepatic bile duct is uncommon (10% of patients) and isolated involvement of the intrahepatic bile ducts is reported in 15% of patients (2). Bile duct stones are detected in 8% of these patients.

Majoie's Classification
Type 1: irregularities, ulcerations with or without strictures, but without dilatation (or only slight) (fig. 2).
Type 2: short tight annular strictures alternating with normal or slightly dilated segments.
Type 3: pruned tree' appearance: rarefaction or non-opacification of peripheral ducts;
long stenosis (fig. 1).
Type 4: irregular nodular thickening with diverticulum-like outpouchings.

Hepatolithiasis, cholelithiasis, and choledocholithiasis are common (usually pigmented stones) and appear as signal voids on all pulse sequences.

T1W C+ images (particularly on arterial phase images) demonstrates heterogeneous, patchy hyperenhancement often in periductal distribution.

Thickening and hyperenhancement of bile ducts on T1WI C+ suggests acute inflammation.

3)  Secondary sclerosing cholangitis:

a) ischemic cholangitis

Ischemic cholangitis is defined as ischemia-induced bile duct injuries due to various causes. The bile ducts are vulnerable to ischemic injuries as the blood supply to the bile ducts depends completely on the arterial supply in contrast to the hepatic parenchyma, which has a dual blood supply from the hepatic artery and the portal vein.
Among the various conditions that compromise the arterial supply and can cause ischemic cholangitis, iatrogenic causes including liver transplantation, hepatic arterial infusion of chemotherapeutic agents, and vessel injury during biliary or pancreatic surgery constitute the most common etiologies (5,6).

Ischemic cholangitis can exist in patients with hereditary hemorrhagic telangiectasia or polyarteritis nodosa (6).
Pathological features of ischemic cholangitis vary depending on the degree and extent of hepatic arterial insufficiency and the stages of ischemic cholangitis (6).
In the acute stage of ischemic cholangitis, desquamated ischemic biliary epithelium with other bile components forms biliary casts. The radiologic findings of biliary casts include intraductal filling defects in the dilated bile duct showing high signal intensity on non enhanced T1-weighed MR images. At this stage, biliary casts appear similar to intraductal stones. However, they usually differ in their shapes, as biliary casts appear linear or have a branching pattern, whereas stones are usually oval or round.

In severe ischemic injury, bile duct necrosis presents as tubular low density or intensity structures along the portal tracts on enhanced MR images.
Cholangiography is the Gold standard for diagnosis in ischemic cholangitis and shows dilated bile ducts with irregular margins and intraluminal filling defects. Bilomas can appear as an intrahepatic fluid collection usually located in the vicinity of the injured bile ducts (7). At the chronic stage of ischemic cholangitis, cholangiographic findings include multifocal biliary strictures and dilatation, both of which can mimic the findings of PSC. The clinical context and predominant location of ischemic strictures, the middle third of the CBD and the hilar bile duct, are suggestive of ischemic cholangitis rather than PSC.

(b) obstructive cholestasis

Long stranding obstruction of the biliary tree by any form of obstructive lesions including: stones (choledocholithiasis), gall bladder stones (cystic duct stones with Mirrizi syndrome), neoplastic masses (HCC, cholangiocarcinoma, pancreatic masses, metastasis) and biliary strictures (iatrogenic, post-traumatic), all can result in development of inflammatory strictures, biliary stasis and secondary sclerosing cholangitis, the development of secondary sclerosing cholangitis in this clinical setting depends mainly on the degree of obstruction and its duration. The pathogenesis involves bile regurgitation from the obstructed duct into the perisunsoidal and periportal spaces with associated inflammation and reactive periportal fibroplastic changes. The relief of biliary obstruction early on the disease process can results in reversal of parenchymal changes, however, if fibrosis and biliary cirrhosis develops, the damage is irreversible.
MRCP is the best modality to identify choledocholithiasis, appearing as filling defects with low signal surrounded by the high signal of bile; However, other causes of filling defects on MRCP may mimic bile duct stones, including air bubbles and blood clots or flow voids from vessels, careful evaluation of 3D MRCP images and axial T2W images is mandatory for differentiation. Additional findings related to long standing choldecholithiasis are well evaluated with MRI. These include the development of biliary strictures and dilatation; biliary casts (can appear as high signal structures in T1WIs) as well as secondary biliary cirrhosis with parenchymal atrophy predominantly involving the lateral segment of the left liver lobe.

(c) infectious cholangitis (fig. 4)

• Ascending cholangitis

It’s a pyogenic infection of biliary tree as a result of obstruction.

The MRI shows:

1. Dilation of intrahepatic ± extrahepatic ducts with abrupt cut off at site of obstruction.
2. Bile duct wall thickening with hyperenhancement.
3. Intraductal purulent bile or pus: intermediate to low signal on T1 and T2WI MR.
4. Heterogeneous liver enhancement, which can be wedge- shaped, peribiliary, patchy, or diffuse. 

Can be associated with liver abscesses (1/4 of cases) or portal vein thrombosis.

• Recurrent pyogenic cholangitis   

Recurrent pyogenic cholangitis (RPC) is a disease characterized by recurrent episodes of bacterial cholangitis that occur in association with biliary obstruction from strictures and pigmented stones.
Although the exact pathogenesis of RPC is unknown, strong associations with parasitic infestation such as Clonorchis sinensis or Ascaris lumbriocoides, portal bacteremia, and malnutrition are suggested. Chronic recurrent infection of the bile ducts is thought to induce biliary stricture, bile stasis, and pigmented biliary stones. Approximately 30 to 80% of the stones in RPC are calcium bilirubinate stones commonly found in the intrahepatic bile duct. The hepatic segments containing stones demonstrateparenchymal atrophy and scarring. The diagnosis of RPC is based on the demographic background, as well as the clinical and imaging features.
For evaluation of patients with RPC, MR cholangiography in RPC demonstrates dilatation of the central bile ducts and rapid tapering in the peripheral bile ducts, known as the arrowhead appearance. Strictures usually occur in the intrahepatic bile ducts. Diffuse extrahepatic ductal strictures are a rare occurrence in infectious cholangitis. MR is more sensitive than CT for detecting radiolucent stones, which can appear hypointense on T2-weighted images and isointense to hyperintense on T1-weighted images. On the other hand, pneumobilia, which is commonly seen in RPC, can often mimic intrahepatic stones on MR or US, while it dilated bile duct.
(d) IGg 4 related cholangitis (fig. 3)

Immunoglobulin G4-SC is bile duct involvement of IgG4- related systemic disease (IgG4-RD). After the pancreas, bile ducts are the second most common organ, involved with IgG4-RD. Patients with IgG4-SC are predominantly males in their 60s. Patients with IgG4-SC commonly present with obstructive jaundice. Like other IgG4-related diseases, the serum IgG4 is frequently elevated. In cases of IgG4-SC, MR imaging demonstrates long-segmental, symmetrical, circumferential wall thickening and delayed contrast enhancement of the involved bile ducts. In approximately half of these patients, the involved bile duct lumens are narrowed, although visible. The most commonly involved location is the intrapancreatic segment of the common bile duct (CBD) followed by the hilar bile ducts in IgG4-SC. Multifocal involvement of both intrahepatic and extrahepatic bile ducts can occur, similar to PSC, while isolated stricture of the distal CBD can also be observed in IgG4-SC. The cholangiographic findings of IgG4-SC include long and continuous stricture of the bile ducts with prestenotic dilatation. Multifocal intrahepatic duct involvement with short segmental strictures as well as a beaded, pruned-tree, and diverticulum-like appearance are imaging features suggestive of PSC.

(e) eosinophilic cholangitis
Eosinophilic cholangitis is defined as eosinophilic infiltration of the biliary system. Peripheral eosinophilia is the most helpful laboratory finding, even though it is reported in only approximately half of the patients with eosinophilic cholangitis. Histopathologically, eosinophilic cholangitis is characterized by dense transmural eosinophilic infiltration associated with other inflammatory cell infiltration in the bile ducts.
Imaging findings in eosinophilic cholangitis mentioned in previous reports include: wall thickening of the CBD, cystic duct, and gallbladder; diffuse bile duct stricture from the hepatic hilum to the intrahepatic duct; focal stricture of the common hepatic duct at the cystic duct insertion level; and mild smooth narrowing of the proximal common duct. Although a preoperative diagnosis of eosinophilic cholangitis is challenging due to its nonspecific imaging features, it should be considered when bile duct wall thickening is present with involvement of the cystic duct, the gallbladder, and peripheral eosinophilia. Patients with eosinophilic cholangitis show a favorable response to corticosteroids and often show a self-limited disease course without treatment.

4)   Risk for cholangiocarcinoma:       

The risk of Cholangiocarcinoma in patients with PSC is approximately 5-15%. It is crucial to diagnose cholangiocarcioma in patients with sclerosing cholangitis prior to liver transplantation because they tend to have a poor prognosis post-transplantation.
The diagnosis of cholangiocarcinoma by MRI/MRCP can be difficult because of the co-existent benign biliary strictures in sclerosing cholangitis. The diagnosis is usually achieved by a combination of clinical, laboratory and imaging features. Carbohydrate antigen 19-9 (CA 19-9) is the most sensitive and specific tumor marker (78.6% and 98.5% respectively) with a cut-off value of 129 U/mL.
A dominant biliary stricture can develop in 45-58% of PSC patients during follow-up MRCP studies, which raises the suspicion for Cholangiocarcinoma.

A dominant stricture is defined as stenosis with a diameter of less of 1.5mm in the CBD or less than 1mm in the CHD. Benign strictures are far more common than malignant strictures .
The initial MRI should be performed with contrast-enhanced sequences to evaluate for possible masses and to determine its relation to the vessels.
Cholangiocarcinoma can be detected in various morphological forms; the mass-forming type, the periductal infitrative form and the intraductal form. The intraductal type of cholangiocarcinoma can present with marked ductectasia with or without papillary mass or can present with focal biliary stricture-like lesion with proximal dilatation. In MRI, the mass forming type of cholangiocarcinoma presents a mass lesion with high signal in
T2WIs and low signal in T2WIs. In the post-contrast sequences, the mass typically shows peripheral faint enhancement with gradual increase in enhancement being most avid in the delayed phase. It may be associated with capsular retraction or surrounded by satellite nodules.