2003년도 대한쉐담도연구회 학술대회 口 Recent Hot Issues (1) 口

Can MRCP Replace ERCP?

성균관의대 삼성서울병원 영상의학과

임

Summary

Magnetic resonance cholangiopancreatography (M

RCP) has replaced direct cholangiography and pan­

creatography in many instances. lts complete non­

invasiveness and flexibility are less onerous for pa­

tients. For the use of screening as well as scrutiny,

MRCP has played an important role in diagnosing

various pathologies in this field. The usefulness of

MRCP is not limited to anatomical evaluations; it can

also yield physiologic and functional information

From a cost-performance basis, MRCP is undoubtedly

superior to direct methods. Coupled with a cutting­

edge MR system, MRCP has the potential to limit the

use 0 invasive transpapi1lary and percutaneous methods

merely to interventional purposes. ln the near future,

the emergence of interventional MR scanners wi1l

make MRCP even more competitive, and the replace­

ment wi1l be accelerated.

Introduction

In the field of abdominal imaging, magnetic resonance

cholangiopancreatography (MRCP) has been a great deal

of success in the past 5 years. Although 이rect

cholangiography and pancreatography are generally safe,

reliable, and widely available, these methods are

significantly operator-dependent, and they have the

potential to create significant complications, inc1uding

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67

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acute pancreatitis, infection, hemorrhage, gastrointe­

stinal perforation, and adverse reactions to contrast

media or premedications. The cost of endoscopic

retrograde cholanpancreatography (E RCP) is higher

than that of MRCP. MRCP has contributed to the

diminution of major and minor complications by

limiting the indications of direct cholangiography and

pancreatography. The implementations of many se­

quence options and hardware refinements have made

MRCP one of the most robust imaging methods

Consequent recent success rates of diagnostic image

acquisition are reported to be increasingly high. Aside

from issues of safety and flexibility , MRCP may yield

more information about the pathologic and physio­

logic conditions of the pancreatic duc t. Following is

a review of recent technologic achievements con­

cerning MRCP and a description of the potential and

limitations of MRCP in comparison with those of

direct cholangiography and pancreatography.

Technical Considerations

Several technologic innovations have supported the

success of this technique. MRCP uses stationary water in

biliary and pancreatic secretions as an inσinsic contrast

media, thus imaging all fluidfilled structures in the upper

abdomen. The imaging strategy involves the acquisition

of a series of heavily T2-weighted images. Because

fluid-filled st ructures, biliary and pancreatic ducts, and

du여enum 뼈ve long 10ngitudinaI and σansverse relaxation

68 2003년도 대한춰l담도연구회 학술대회

times, they have hi양 si맑al intensity in T2-weighted

images. In these “hyd.ro맹phic images", everything 1∞k

black and white. Observers can evaluate the pancreatic and

biliary pathologies in이rectly using these “ all-or-nothing

images". To enhance the clarity of reconstructed images,

several postpr'α:essing tec뼈ques are sometimes r여비red.

뼈처mum intensity projection (MlP) algorithm makes a

thre엉imensional projection overview of the biliary and

pancreatic duct possible from any arbitrary view.

Multiplanar reforrnatting α1PR) allows the 뻐alysis of

arbitrary sections through inteπ:>O!ation

Secretin Administration

Secretin increases the secretion of bicarbonate by the

duct cells of the pancreas and bili앙y tract. Because

secretin causes marked secretion of an alkaline pancreatic

juice via the MPD, it can improve its delineation. The

initial effect of secretin is to contract Oddi ’s sphincter

Because the output of pancreatic juice is increased, the

occlusion of the outflow σact consequent1y elevates the

intraductal pr，않sure of MPD and results in the dilation of

MPD. Several minutes after aφ띠피sσations， Oddi ’s

sphincter begins to relax, and then the pancreatic juice

empties into the duodenum, which results in the deflation

of the MPD. All the proc않ses occur within 10 minutes,

which means that the imaging window for the dilated

MPD is quite linùted. If one uses secretin to estimate the

MPD dilation, MRCP with g∞d temporal resolution is

essential.

Cases Involving Failure to Cannulate or Fill

the Pancreatic or Biliary Ducts

Generally, MRCP plays an important role as a

substitute for direct cholangiography or pancreato­

graphy when direct methods fai l. The widely rec­

ognized success rate of ERCP (approximately 90%)

appears to be high, but the rate does not include

patients declined for relative contraindications such as

acute pancreatitis, age or infirmity, allergy to the

contrast media or premedications, prior upper gas­

trointestinal surgery, gastroduodenal obstruction, or

pancreaticojejunostomy with Roux-en-Y modification.

In the cases with congenital anomalies or surgical

alterations of the upper gastrointestinal tract, the

success rate is lower. MRCP delineation of the

pancreaticobiliary system is not hampered by these

factors.

Biliary Obstructions

With transpapillary or percutaneous cholangio­

graphy, the contrast media cannot pass beyond the

completely obstructed segment; thus, the accurate

length of the stenotic segment is not identified by

either ERCP or percutaneous transhepatic ch이an­

giography (PTC) alone. To know the actual length of

the stenotic segment, contrast filling from both sides

(transpapillary and percutaneous) is sometimes need­

ed. MRCP, on the other hand, can depict the stenosed

portion of the biliary tract in a single image. This

feature of MRCP is beneficial when planning inter­

ventional procedures or radiotherapy for these patho­

logies. The reported accuracy in detecting stenosed

segments is comparable to direct cholangiography

To differentiate malignant from benign lesions of

the biliary tree, general guidelines for interpretation of

cholangiography can also be applied to MRCP. Most

malignant 1esions cause an abrupt narrowing of the

bile duct, often with shouldered margins and mucosal

irregularities. Benign lesions, on the other hand, are

often smooth, gradually tapered stenosis. Nevertheless,

it should be remembered that a significant amount of

overlap does exist between the appearance of

malignant and benign lesions. In addition, the ability

of visualization of both the pancreatic and biliary

ducts in MRCP can often assist in diagnosis,

임 재훈 Can MRCP Replace ERCP? 69

particularly when adjacent strictures in both pancreatic documented ‘

and biliary ducts are seen. This is more suggestive of

a pancreatic carcinoma than of a biliary duct cancer.

Biliary Stones

The sensitivity of MRCP in detecting choledo­

cholithiasis (81-95%) may be better than that of

direct cholangiography. Stones are shown as low­

intensity filling defects within the bi \iary tract irre­

spective of their calcifications. The finding is much

c\earer using MPR. These defects may not be apparent

on MIP images because of over1aying fluid. In our

own se디es， biliary stones as small as 3 mm were

correctly identified by MRCP. The detectability of the

bi \iary duct stones with MRCP is simply dependent

on their size. Hence, if compared with CT, MRCP

may be less sensitive to very small calculi with

calcification. As CT is so sensitive to calcified

materials, it can detect calcu\i as small as 1 mm, as

long as they are calcified.

Many investigators have found that, compared with

direct methods, spatial resolution of MRCP is poor.

However, MRCP enjoys superb contrast resolution. As

we see in the detectabi \ity of cho\edocholithiasis,

MRCP can often detect smaller calcu\i than direct

methods can. Further technologic achievements, may

improve the spatial resolution, but the practical and

reasonable spatial resolution required for accurate

diagnosis should be determined.

MRCP is reported to be able to detect hepato­

\ithiasis in the se!ected cases. However, there is an

ambiguity if eveη discontinuity of the nondilated

intrahepatic biliary duct seen on MRCP can be

correctly differentiated from small hepatoliths. Co­

existing pneumobilia often seen in patients with prior

biliojejunostomy may also be problematic in the MR

diagnosis of biliary duct stones. The role of MRCP

in diagnosing hepatolithaisis remains to be

Chronic Pancreatitis

In the initial workup of patients with chronic

pancreatitis, MRCP is useful in depicting dilated or

narrowed segments of the MPD, which are the

hallmarks of chronic pancreatitis. Pancreatic ca1cu\i

and plaques are detectable as round filling defects

within the MPD. Deviation and/or dilation of the

bi\iary duct is also seen when affected. Side branches

of the MPD, however, are not usually depicted unless

dilated. The reported sensitivity was 87% to 100% for

dilation, 75% for narrowing, and 100% for filling

defects. Because stenotic portions may be recon­

structed or resected surgically, detection of the

stenosis is an important role of MRCP.

Nonspecific ductal dilation may be found in elder1y

patients and should be differentiated from charac­

teristics of chronic pancreatitis. Because such patients

are often asymptomatic, correlation with laboratory

data and patients ’ symptoms and history is essential.

To rule out pathologic conditions, determination with

noninvasive MRCP is the first choice.

Desmoplastic ductal cancers and chronic fibrosing

focal pancreatitis are often difficult to differentiate. In

selected cases, ERCP has been said to be helpful in

differentiating masses created by focal pancreatitis

from adenocarcinoma. A dilated duct may be seen

within a mass produced by focal pancreatitis but not

within a ductal carcinoma. ERCP often fails to fill the

entire segment of the affected MPD and the side

branches because of occ\usion of ducts involved

within the mass. MRCP, on the other hand, can depict

both the narrowed MPD and isolated dilated side

branches within the mass. MRCP, .on the other hand,

can depict both the narrowed MPD and isolated

dilated side branches within the mass, which may

suggest the benign nature of the mass. This finding

70 2003년도 대한춰l담도연구회 학술대회

is characteristic of, but not specific to, chronic

pancreatitis. With the use of MRCP, duct penetration

is found within the mass in five of six cases (83%)

with mass-forming pancreatitis, but the imaging fea­

ture was also found in 2 of 20 (10%) cases with

malignant solid tumors.

Acute Pancreatitis

Severe acute pancreatitis is sometimes accom­

panied by complications, such as pancreatic fluid

collections, extrapancreatic fluid collections, or

pancreatic pseudocysts. The surgical treatment of

pancreatic pseudocysts inc1udes excision or intemal or

extemal drainage. The success of surgical treatment

depends on correct localization and depiction of the

morphology as well as the status of the pancreatic

ducts. MRCP demonstrates the anatomy of the MPD

and biliary tract in relation to the location of

pseudocysts. ERCP can readily show the site of

extravasation in pancreatic fistulas, localize the source

of pancreatic ascites, and demonstrate the continuity

of abscesses or pseudocysts with the ductal system

However, contrast media injection into a cyst may

cause infection that results in pancreatic or peri­

pancreatic abscesses. Our limited experience in acute

pancreatitis suggests that MRCP is safe and helpful

in providing information about 1) the presence or ab­

sence of the ductal distention, 2) disruption, 3)

leakage of the pancreatic duct, and 4) intraductal

lesions that might raise the intraductal pressure and

create a predisposition to pseudocyst formation. The

detection or pseudocysts using MRCP is straight­

forward, and localization, size, shape, and number are

also adequately shown through its use. The

information is important because it determines the

severity of acute pancreatitis and influences the choice

of treatment.

Pancreatic Cancer

ERCP has been used primarily as an adjunctive

method for investigating suspected pancreatic carci­

noma in patients with ambiguous CT findings

Because symptomatic pancreatic cancer usually in­

volves the MPD, obstruction of the MPD is the most

common finding in carcinoma. The ducts downstream

to the occ1usion are basically normal in carcinoma, whereas changes reflecting pancreatitis are usually

present in the ducts upstream to the obstruction. The

pattem of obstruction is usually irregular, nodular,

rat-tailed, or eccentric in carcinoma and smooth or

blunt in pancreatitis. MRCP can depict the narrowed

or amputated MPD as well as dilated upstream ducts

quite wel l. Ampullary cancer is easily detected by

diffuse and smooth dilation of the biliary duct and the

pancreatic duc t. With adequate fluid filling, the tumor

is often outlined within the duodenum around the

papilla of Vater. Detection of early ductal cancer,

however, is extremely difficult. As small ductal cancer

often causes very subtle ductal abnormalities in the

MPD or side branches, such lesions are diffic비t to

identify through the use of MRCP. Although not

frequently seen, ERCP could detect small peripheral

carcinomas that focally obliterate small lateral side

branches or cause a focal defect in parenchymal

acinarization. Such subtle findings, for example, might

not be seen by current MRCP. However, small cancer

detection might not be a role of MRCP. Because MR

examination can offer parenchymal contrast,

combination of high-resolution, fat-saturated Tl con­

trast images and dynamic contrast MR images can

detect smaller tumors within the pancreas. The

information is superior or equivalent to that derived

from ERCP with parenchymal acinarization.

Intraductal Papillary Mucinous Tumor

임재훈 Can MRCP Replace ERCP? 71

duodenal gas may migrate into the pancreatic duc t.

The presence of air bubbles within the dilated main

pancreatic duct may mislead observers to make a

Endoscopic inspection typically reveals an enlarged false-positive diagnosis of papillary tumor growth

papilla with mucus flowing from a patulous orifice

On ERCP, diffuse or segmental dilation of the MPD Cost-Effectiveness

or cystic dilations of the side branches, sometimes

grape-like, are common. Fungating lesions are One of the most notable features of MRI is that MR

sometimes detected as filling defects in ERCP after can offer several different tests as an allin-one

successful drainage of the mucin. Because ERCP can package. An MR examination includes MRCP, MR

prove the existence of communicating channels angiography, perfusion imaging, and of course, ordi-

btween the cystic lesions and the MPD, ERCP is said nary cross-sectional MRI based on Tl- or T2-

to be the most reliable method in diagnosing this weighted irnages with or without fat saturation.

neoplasm. However, it is also true that the mucin Interventional procedures using MR imagers are being

often prevents the dilated side branches from contrast implemented, as wel l. When all of these individual

filling. On the other hand, MRCP detection of tests are optimized, MRI may obviate the additional

communicating channels is not affected by mucin, use of other modalities such as direct cholan-

thus more accurate diagnosis is possible in cases with giography, pancreatography, angiography, or contrast-

dilated communicating channels. In cases with pat- enhanced CT

ulous orifice of the enlarged ampulla of Vater,