3D-MRCP for evaluation of intra- and extrahepatic bile ducts: comparison of different acquisition and reconstruction planes

Magnetic resonance cholangiopancreatography (MRCP) is an established technique for the evaluation of intra- and extrahepatic bile ducts in patients with known or suspected hepatobiliary disease [1]. It is considered a reliable non-invasive alternative to diagnostic endoscopic retrograde cholangiopancreatography (ERCP) [2, 3]. Since the first description by Wallner and colleagues in 1991 [4], different acquisition techniques have evolved.

Most current MRCP techniques are based on heavily T2-weighted fast spin echo (FSE) pulse sequences, which yield a luminal image of the bile ducts that is based on the inherent signal of slow-flowing or stationary bile. Both, single shot projections and multislice techniques are available [5], with the latter being distinguished into 2D [6] and 3D techniques [7]. Single shot projections are preferred in individuals who are unable to hold their breath, such as severely sick patients or small children [7]. 3D imaging techniques provide better image quality compared to 2D sequences [1, 8, 9], even though the combination of different MRCP sequences has proven to be valuable in the assessment of bile duct anatomy and pathology [10].

3D FSE sequences are usually acquired with the slab in coronal orientation. Maximum intensity projections (MIP) can then be obtained in any plane [7]. Previous studies have addressed the matter of optimal slice thickness for data acquisition [11] and different techniques regarding respiratory triggering [12]. However, to the best of our knowledge, the ideal acquisition and reconstruction plane, in practical terms meaning best suitable for optimal bile duct visualization with 3D techniques, has not been evaluated. The purpose of this study was to compare different acquisition and reconstruction planes of T2-weighted 3D-MRCP acquisitions for assessment of the intra- and extrahepatic bile ducts.

To the best of our knowledge the ideal acquisition and reconstruction plane for optimal bile duct evaluation with 3D techniques has not yet been evaluated. For single shot FSE techniques it has been suggested that straight coronal and initial left posterior oblique images clearly depict the common hepatic duct and the left hepatic duct, whereas the CBD and right hepatic ducts are seen better on a left posterior images obtained at a steeper angle [15]. Especially in children with segmental liver transplants sagittal oblique planes are preferred due to the more anteroposterior orientation of the neo-porta hepatis [16]. In a first approach towards projection cholangiography by means of MRI in 1991, Wallner and colleagues used a heavily T2-weighted gradient echo sequence for assessment of bile duct dilatation [4]. They concluded that imaging in the coronal plane provided a good view of the biliary system, whereas no additional information was found by imaging in the sagittal plane.

In this study we compared different acquisition and reconstruction planes of T2-weighted 3D-MRCP acquisitions for assessment of the intra- and extrahepatic bile ducts. In contrast to single shot techniques, 3D MRCP has the advantage to facilitate secondary reconstructions. Coronal reconstructions were preferred, regardless of the initial acquisition plane. These findings were supported by good intra- and interobserver agreement. One of the reasons for coronal image preference might be the fact that these images are similar to image impressions of ERCP and conventional cholangiograms.

There are other studies that evaluated secondary reconstruction techniques for MRCP. Schaible and colleagues evaluated selective MIP reconstructions of respiratory-triggered 3D MRCP versus standard MIP reconstructions and single-shot MRCP [17]. Single-shot and standard MIP reconstructions of 3D MRCP were comparable in terms of anatomical bile duct visualization, whereas selective MIP postprocessing proved useful for detection of pathological alterations. In a retrospective study, Morita and colleagues compared volume rendering (VR) and MIP of 3D TSE MRCP sequences to define biliary anatomy mostly in patients without major biliary tract anomaly [18]. Definition of biliary anatomy was found to be more accurate using VR reformation than MIP. However, the assessment of VR images was not the purpose of the present study. One disadvantage of VR reconstructions is that the detection degree of each structure depends on the setting of display parameters, particularly on the lower threshold of the opacity curve. Therefore VR images need to be evaluated interactively [18].

Our study had some limitations. We did not perform quantitative SNR measurements of the MRCP datasets as the focus of our study was to qualitatively evaluate visualization of intra- and extrahepatic bile ducts using multiple readers. This seemed closer to the clinical reality than SNR values that are difficult to measure for small bile ducts. We did not evaluate the added value of acquisition or reconstruction planes. In 1999, Boraschi and colleagues compared axial and coronal 2D FSE sequences with 3D MIP projection images in patients with suspected hepatobiliary disease [19]. A higher global accuracy for axial and coronal FSE T2-weighted sequences was found regarding the diagnosis of the level and probable cause of biliary obstruction in depiction of small intraductal pathology such as calculi or neoplastic lesions.

We have limited our analysis to reconstructed rather thin-slice source images as the purpose of this specific was to directly compare acquisition and reconstruction planes for MIP assessment. A well-known limitation of MIP is that small filling defects may be obscured due to partial volume effects [20]. Further, overestimation of ductal narrowing and pseudostricture may result from the nature of MIP reconstruction [21]. Therefore it is important, that MIP reconstructions should not be appraised separately, but always in combination with the original acquired dataset and in combination with other morphological sequences.

In conclusion, we compared different acquisition and reconstruction planes of T2-weighted 3D-MRCP acquisitions for assessment of the intra- and extrahepatic bile ducts in patients with different hepatobiliary pathologies. The results of our study suggest that for visualization and evaluation of the bile ducts coronal reconstructions are preferred. In this context, the orientation of the primary dataset (coronal or axial) is negligible.