Secretin-Enhanced MRCP: Review of Technique and Application ...

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Secretin-Enhanced MRCPTABLE 2: Description of Clinical Groups Used to Classify Chronic PancreatitisClinical Group Description of Group No. of PatientsNormalChronic abdominal pain and no risk factors for chronic pancreatitis. Risk factors include heavy31alcohol use, acute pancreatitis, pancreas divisum, very high triglycerides, and well-documentedsphincter of Oddi dysfunction. No early or late imaging findings of chronic pancreatitis. At least oneimaging test was performed (usually CT) with normal results.EquivocalChronic abdominal pain with at least one of the normal risk factors; endoscopic pancreatic function53test with normal results.Early chronic pancreatitis Pain, risk factors, and either mild imaging changes (e.g., ERCP Cambridge score of 2, endorectal32ultrasound ≥ 5), or abnormal endoscopic pancreatic function test (peak bicarbonate < 80 mEq/L).Established chronic pancreatitis Diagnostic imaging changes (CT with calcifications or ERCP Cambridge score 3–4). 18may show ductal communication better thanMRCP without secretin [24]. Intraductal papillarymucinous neoplasms (IPMNs) communicatewith the ductal system whereasovarian stroma-containing mucinous cysticneoplasms do not. If ductal communicationis seen, then these cystic neoplasms can beclassified as intraductal papillary mucinousneoplasms (Fig. 12). Sometimes direct communicationis not seen, but an increase insignal intensity of the cysts after secretin injectionsuggests ductal communication (Fig.13). Because the management of IPMNs andmucinous cystic neoplasms differs, visualizationof ductal communication plays an importantrole in both the confirmation of thediagnosis and the management algorithm.There is very little literature on the use of secretin-enhancedMRCP in the evaluation ofcystic neoplasms. The frequency with whichsecretin actually improves visualization ofductal communication is unknown.Status of Secretin-Enhanced MRCPThat secretin-enhanced MRCP holdsgreat promise in the evaluation of chronicpancreatitis is not disputed. However, despitethe availability of MRCP for about a decade,the medical community has yet to exploit itsfull potential. There are limitations to theother diagnostic tests used to evaluate chronicpancreatitis. ERCP has a high rate of complications,direct function test is cumbersomeand limited to few centers, and EUSis highly operator dependent, and none ofthese can match the unique combined functionaland morphologic perspective providedby secretin-enhanced MRCP. These limitationsaccentuate the advantages of secretinenhancedMRCP. Currently the use of secretin-enhancedMRCP is somewhat limited tolarge centers where it is often used in combinationwith other tests. Considering thehigh prevalence of chronic pancreatitis andthe extensive use of invasive investigations toestablish this diagnosis, secretin-enhancedMRCP has been significantly underutilized.Even patients who undergo MRCP for suspectedchronic pancreatitis often do not havesecretin administered despite the clear incrementalvalue of secretin to the test. Lackof awareness, cost, and paucity of large trialsproving its effectiveness are all partly responsiblefor this. The shortage of secretinfaced a few years ago has now resolved andsynthetic human secretin is easily available.Quantification of Duodenal SecretionA drawback of secretin-enhanced MRCPis the subjective nature of the reports [25].As a result, the reports do not harness thetrue potential of the test. The subjective assessmentof duodenal filling used in practiceintroduces large interobserver variations becauseradiologists may differ in their interpretationof adequate secretion. Because oursecretin-enhanced MRCP reports often donot include objective data, gastroenterologistssometimes perceive them to be ambiguousor inconclusive, creating a need for thedevelopment of an objective parameter to establishthe diagnosis of pancreatitis.Despite several prior attempts to quantifythe volume of duodenal fluid as measure ofexocrine function [12, 15, 26–28], a methodof quantification that can reliably differentiatebetween normal, early, and establishedchronic pancreatitis has been elusive. Matoset al. [12] suggested a grading system inwhich duodenal filling is defined as grade 0when no fluid is observed, grade 1 when fluidis limited to the duodenal bulb, grade 2 whenit partially fills the duodenum up to the genu,and grade 3, when it fills beyond the genu.A lower score on this grading scale was associatedwith a low bicarbonate concentrationand impaired exocrine function [10, 15].The main attraction of this grading system isits extreme simplicity. However, Cappeliez etal. [15] found that this grading system had alow sensitivity of 72% for impaired pancreaticfunction, whereas Hellerhoff et al. [14]reported it to have a low positive predictivevalue of 58% for chronic pancreatitis. Furthermore,this grading system was not able todifferentiate between patients with mild andestablished chronic pancreatitis [15].Gillams and Lees [29] measured changes insmall intestine water volume to calculate thepancreatic flow rate as an indicator of the volumeof secretion. This model was able to identifysevere pancreatitis from normal and moderatechronic pancreatitis but was unable to differentiatenormal from mild or moderate pancreatitis.Subjects and MaterialsIn view of the scant data available on secretin-enhancedMRCP quantification, weperformed a study to quantify the volume ofsecretions on secretin-enhanced MRCP inpatients with suspected chronic pancreatitisand compared it with the clinical diagnosisof chronic pancreatitis.PatientsBetween March 2005 and September 2008, 166patients underwent secretin-enhanced MRCP atour institution. After exclusion of 32 patients whohad either prior pancreatic surgery, a pancreaticneoplasm greater than 3 cm, or a technicallyinadequate study, 134 consecutive patients (45men, 89 women; mean age, 51 ± 14.1 years)with chronic abdominal pain were included inthis study. These patients underwent variousadditional investigations, including CT (n = 98),endoscopic pancreatic function tests (PFTs) (n =65), EUS (n = 84), and ERCP (n = 36), to diagnosechronic pancreatitis.Clinical GroupsAn experienced pancreatologist, who wasblinded to the secretin-enhanced MRCP results,classified the patients into four clinical groups(Table 2) on the basis of clinical risk factors andthe findings of the investigations mentioned earlier.AJR:198, January 2012 129
Sanyal et al.Fig. 14—35-year-old woman with normal pancreaticfunction. Image shows region of interest drawnaround fluid in duodenum to obtain area. Obtainingsimilar measurements on all slices allows volume ofsecretion to be calculated.Secretin-Enhanced MRCP QuantificationTechniqueVolume—In patients prepared with a negativeoral contrast agent, new high T2 signal areasin the duodenum after secretin administrationrepresent new pancreatic secretions. A region ofinterest (ROI) was drawn around the T2 brightduodenal and proximal jejunal fluid on each sliceof the postsecretin coronal HASTE sequence (Fig.14). The area (in cm 2 ) of the ROI in each slicewas added and the total multiplied by the slicethickness (0.4 cm) to obtain the total volume fluid(in mL) secreted in response to secretin.Rate of secretion—Diminished pancreaticexocrine function may not only manifest asa decreased volume of secretion but also as adecreased rate of secretion in response to secretin.To assess this function, the maximal rate ofsecretion was measured in each patient. An ROIwas drawn on each of the heavily T2-weightedimages obtained during the dynamic phase toinclude all the secreted fluid (Fig. 15). This ROI wasconstant for all 20 sequential images obtained overthe 10 minutes after secretin administration. Thetotal signal intensity within the ROI in each imagewas calculated by multiplying the mean intensityFig. 15—51-year-old woman evaluated for calculationof maximum pancreatic flow rate. Region of interestis drawn around duodenum in each of 20 images.by the pixel count. The total signal intensity withinthe ROI for each image was charted sequentially.The 2-minute segment (four sequential time points)showing the maximal change in signal intensity wasrecorded for each patient. This maximum changein signal intensity within any 2-minute segmentwas considered a measure of the maximum rate ofsecretion in response to secretin for each patient.Endoscopic pancreatic function test—Sixtyfivepatients underwent secretin-stimulatedendoscopic PFT for measurement of pancreaticexocrine function. After injection of 0.2 μg/kgof IV secretin, intermittent duodenal aspirateswere obtained at 15-minute intervals for 60minutes. The endoscopic fluid specimens wereanalyzed for bicarbonate concentration. Amaximum bicarbonate concentration < 80mEq/Lis considered abnormal [3].Statistical AnalysisA one-way analysis of variance was conducted tolook at the relationship between the clinical groupsand the two parameters measured on secretinenhancedMRCP—that is, volume of secretin andmaximum rate of secretion. When significancewas found, further exploration was conducted byexamining the mean differences between eachclinical group pairing. A Bonferroni correctionwas made to limit the probability of obtaining asignificant result by chance. A result that wouldhave been significant before the correction but notafter was referred to as marginally significant.The correlation between clinical groups andvolume and rate was determined by the Kendallrank coefficient (Kendal tau). The Kendall tau wasalso used to calculate the correlation between thevolume measurements and maximum bicarbonatelevels on endoscopic PFT, which were availablefor 65 patients.ResultsVolume QuantificationAmong the 134 patients included in thestudy, a significant association was observedbetween the clinical groups and volume measurements(p = 0.0003). Mean volumes becameprogressively smaller with increasingdegree of pancreatitis (Table 3). Significantvolume differences were found between thenormal group and established pancreatitisgroup as well as the equivocal group and establishedpancreatitis group. Marginally significantdifferences were also found betweenthe normal group and early pancreatitis groupas well as the early and established pancreatitisgroups (Table 4). Significant correlationwas found between the clinical groups and thevolume measurements on secretin-enhancedMRCP (τ = −0.324, p < 0.0001).Rate of SecretionThe mean values for the maximum rateof secretion in any 2-minute period tendedto decrease with higher degree or suspicionof pancreatitis (normal group mean, 10.7;equivocal group, 8.1; early chronic pancreatitis,8.1; established chronic pancreatitis,5.5). However, these values were not foundto be statistically significant using the analysisof variance test. There was a weak butTABLE 4: Differences in Mean Volumes Between Various Clinical Groups andp Values Using Analysis of Variance TestTABLE 3: Mean Secretory Volumesof Various Clinical GroupsClinical GroupMean Volume (mL)Normal 57.3Equivocal 49.7Early pancreatitis 39.8Established pancreatitis 22.1Groups Mean Volume Difference (mL) pNormal and equivocal 7.6 0.2367Normal and early pancreatitis 17.5 0.0150 aNormal and established pancreatitis 35.2 < 0.0001 bEquivocal and early pancreatitis 9.9 0.1177Equivocal and established pancreatitis 27.6 0.0005 bEarly and established pancreatitis 17.7 0.0351 aa Marginally significant at the 95% CI level.b Significant at the 95% CI level.130 AJR:198, January 2012
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