It is of great importance to accurately detect bile duct and pancreatic duct abnormalities in patients with obstructive jaundice as it helps both surgeons and endoscopists to take care of such patients and plan further line of management [12].
MRCP has become the standard noninvasive technique for the investigation of the biliary tree to visualize ductal dilatation, strictures, and intraluminal filling defects. It provides noninvasive images comparable to those obtained by direct retrograde or trans-hepatic cholangiography [8].
However, MRCP is a safer technique without the risks for complications associated with the ERCP such as pancreatitis, cholangitis, hemorrhage, and duodenal perforation, as well as the study, involves radiation, contrast material is necessary and there is a requirement for sedation [13].
MRCP can provide indirect evidence of malignant lesions, such as irregularity of the inner border, abrupt stenosis, and stenosis length of the biliary tree. Although these findings are highly sensitive, they are not specific to malignancy [13, 14].
In the current study, we evaluated the value of adding DWI to conventional MRCP in differentiating benign from malignant biliary obstructive pathologies, as DWI has begun to gain wide acceptance from many radiologists as a functional MRI technique that is a useful complementary imaging technique. It has the advantage of requiring a short acquisition-time, without the need for contrast administration.
Regarding the SI on high b value DWI (b 800) and ADC maps; a hyperintensity on DWI with a corresponding hypointensity on ADC maps was considered as positive diffusion restriction and was significantly higher among the malignant lesions compared to the benign lesions, 24 of the malignant cases showed restricted diffusion, and only two cases were missed by DWI, these two cases were peri-ductal infiltrative cholangiocarcimona, one was intrahepatic and the other one was extrahepatic mid-CBD cholangiocarcinoma, we believe that they were missed by DWI due to their small size; both were less than 4 mm, on the other hand, none of our benign cases showed diffusion restriction.
A previous study by Tsai et al. [8] included 42 patients with malignant obstructive biliary lesions also reported two false-negative malignant lesions missed by DWI, they were both peri-ductal cholangiocarcinomas located in the left hepatic lobe. Their respective sizes were 1.7 cm and 1.4 cm. They explained this misreading by the lesser diffusion restriction and motion artifact in the left hepatic lobe from the heartbeat. Also, they reported one false-positive malignant peri-ampullary lesion showing a high signal in DWI secondary to the T2 shine-through effect and low spatial resolution of DWI due to intestinal motion artifacts at the 2nd part of the duodenum [8].
Our results were lower than that of Cetiner et al. [12] who reported that all the malignant cases were diagnosed by DWI, all their malignant lesion were correctly diagnosed by DWI with sensitivity and specificity of 100%. This may be due to the larger size of the malignant tumors as their study did not include infiltrative cholangiocarcinoma [12].
In our study ADC values were measured in the malignant strictures except for two cases because the very small size of the lesions precluded accurate measurement of the ADC values, the mean ADC value was 0.87 × 10−3 (range 0.68–1.27 × 10−3).
In agreement with our results, Sim et al. investigated the diagnostic efficacy of the DWI in differentiating benign and malignant lesions and reported ADC values of 0.928x 10−3 (0.841–0.976x10−3) among the malignant strictures [15].
Cetiner et al. measured the ADC values in fourteen patients with malignant lesions; among them the mean ADC value was 1.306±0.30×10−3 mm2/s (range 0.9–1.9×10−3 mm2/s), explaining higher values ADC values seen in low-grade adenocarcinoma and three of their malignant tumors had a cystic component [12].
By using Roc curve analysis the ADC cutoff value to differentiate between the benign and malignant lesions which has the highest sensitivity (100%) and specificity (100%) was ≤ 1.27 × 10−3.
The previous study by Zhong et al. comparing the accuracy of the different biliary imaging modalities in diagnosing the cause of the biliary obstruction in 82 patients, the accuracy of them was as follows: US (57.3%), CT (73.7%), ERCP (85.0%), and MRCP (87.8%) [16].
Matching with their results in the current study, the MRCP showed 88.3% accuracy in the diagnosis of the biliary obstruction, with 88.4% sensitivity and 88.2% specificity.
Another earlier prospective study by Rosch et al. [17] comparing the diagnostic accuracy of four imaging methods (i.e., ERCP, EUS, MRCP, and CT) reported no statistically significant difference between the sensitively of the MRCP and ERCP for diagnosing malignant strictures, the sensitivity and specificity for the diagnosis of malignancy in the 50 patients were as follows: ERCP showed 85% sensitivity and 75% specificity, the MRCP showed 85% sensitivity and 71% specificity, the CT showed 77% sensitivity and 63% specificity, and finally, the EUS showed 79% sensitivity and 62% specificity.
A few previous studies demonstrated that; the specificity of MRCP for characterizing malignant biliary strictures was improved by combining MRCP with either EUS or contrast-enhanced MRI. Specifically, the combination of MRCP and EUS helped to demonstrate the presence of a visible mass, whereas that of MRCP and contrast-enhanced MRI allowed better depiction of the bile duct walls and extension of malignant lesions [14].
Although combining MRCP and contrast-enhanced MRI or EUS may facilitate the differential diagnosis of biliary strictures, the benefit of using contrast may not outweigh its risk for the development of nephrogenic systemic fibrosis in patients with poor renal function, and the routine use of EUS may also increase the medical costs and complexity of the diagnostic workup [18].
Our results showed that the addition of DWI significantly improved the overall diagnostic accuracy of the conventional MRCP from 88.33 to 98.3% (P< 0.001). The addition of DWI led to correct changes of the final diagnosis of 8.3% of the patients (5/60).
The sensitivity and specificity of diagnosis benign and malignant strictures were significantly higher with combined T2WI, MRCP, and DWI than with conventional T2WI and MRCP, with values increasing from 88.4% and 88.2% to 96% and 100%, respectively.
Our results were comparable to the results of a study by Tsai et al. [8] which reported an improvement of the diagnostic accuracy of MRCP by adding the DWI from 93 to 98%, also the diagnostic sensitivity increased from 79 to 98%.
Our study had some limitations. First, in this study, we did not include a dynamic MRI study as a routine part of the examination; it was only done in two of the malignant cases as requested by the referring physician, and their results confirmed our diagnosis revealing enhancing lesions; however, it did not change our final diagnosis.
Therefore, the diagnostic value of a combined MRCP and dynamic study with or without DWI was not investigated; however, in an earlier study by Yoo et al., their results revealed that both the sensitivity and specificity of combined MRCP and DWI did not significantly differ from that of MRCP, DWI, and contrast-enhanced T1WI combined [18].
The second limitation, not all the malignant lesions were confirmed by pathology or ERCP. Four out of our malignant cases had advanced malignancy showing hepatic deposits and non-regional nodal metastasis, and they underwent palliative biliary drainage and died before confirmation; however, they were considered malignant depending on the complementary clinical and laboratory data.
Also, most of our cases with benign strictures were not pathologically confirmed. However, we can confidently assume these cases to be benign because they were stable or regressed on a follow-up study, and some patients had symptoms resolution after conservative medical therapy.
Third, our DWIs were obtained using respiratory-triggered methods with b values of 0, 400, and 800 s/mm2. Some authors have demonstrated a preference for respiratory-triggered DWI over breath-hold DWI for focal hepatic lesion detection. However, the relevance of this application is not completely clear with regard to the biliary system and has some disadvantages include poor signal-to-noise ratio (SNR), artifacts due to pulsatility and susceptibility, and only a limited number of b values can be obtained per breath-hold [9].
A study by Lee et al. [19] applied free-breathing EPI DWIs; as non-breath hold DWI techniques provide improved SNR and contrast-to-noise ratios, allowing for higher spatial resolution, thin slice partitions (4–5 mm) are possible so the possibility of multiplanar reformatting, multiple b values can also be easily obtained, but this technique is prone to partial volume averaging artifact and can be time-consuming.
Finally, retrospective studies might exhibit a certain degree of selection bias or recall bias, despite implementing thorough inclusion and exclusion criteria and research methods.