Study population
A case control study was conducted prospectively on two groups of patients; the first one included 25 patients known to have different portal vein pathological conditions whereas the second group included age-matched 25 patients with no previous history of portal vein disease to act as a control group. Both groups were examined first by color duplex ultrasonography to document portal vein status followed by non-contrast MR portography using time-spatial labeling inversion pulse technique. The diseased group underwent dynamic contrast-enhanced MRI of the liver as a part of their routine imaging work-up. The study duration was 1 year starting from January 2018 to December 2018.
Informed consent was obtained from individuals participating in this study after explanation of the benefits and possible risks of the procedure. The participants were screened carefully to exclude those with contraindication to MRI examination. The study was approved by our local institutional ethical committee.
Color duplex ultrasonography
All participants in this study underwent color duplex ultrasonography of the portal vein before non-contrast MR portography exam, using ultrasound machine Aplio 500 (Canon Medical Systems) equipped with 3.5 MHz convex array transducer. Portal vein size, patency, peak systolic velocity, and flow direction were documented. Only patients with satisfactory visualization of the portal vein and its pathology were included in the study.
Non-contrast MR portography technique
All participants lied supine on MRI table with arms raised above head and their feet entering the magnet bore first. Respiratory bellows was placed around the upper abdomen to acquire respiratory-triggered sequences with meticulous care to ensure it yields a good respiratory monitoring trace on scanner screen. A phased array surface body coil was placed on the lower chest and upper abdomen. Reassurance and brief explanation of the procedure were done to alleviate anxiety and to ensure participant’s cooperation. IV cannula was placed for those who underwent contrast-enhanced dynamic MRI study.
Scanning protocol
All studies were performed using 1.5 T MRI scanner (Vantage Titan, Canon Medical Systems). After 3-plane localizers, axial and coronal half-Fourier T2 images with breath hold were obtained to delineate the liver and portal vein. Parameters are as follows: axial images (TR = 14,700 ms, TE = 120 ms, matrix = 192 × 256, FOV = 38 × 42 cm, acceleration factor = 2) and coronal images (TR = 9500 ms, TE = 120 ms, matrix = 224 × 256, FOV = 35 × 40 cm, acceleration factor = 2). Non-contrast MR portography sequence was acquired in coronal plane using respiratory-triggered 3D balanced steady-state free precession (b-SSFP) sequence combined with time-spatial labeling inversion pulse with flow-out technique. The obtained axial and coronal T2 images were used to plan the non-contrast MR portography sequence; axial images were used to ensure adequate number of slices to cover portal vein and its intrahepatic branches while coronal images were used for placement of the tagging inversion pulse which was placed obliquely parallel to the inferior liver edge. Additional saturation pulse was applied superiorly above the liver to block the inflow effect of aorta, and another saturation pulse was placed inferiorly in oblique orientation to block the inflow effect of inferior vena cava without blocking the inflow effect of superior mesenteric vein which contributes to portal vein flow (Fig. 1). Acquisition parameters were as follows: TR = 5.2, TE = 2.6 ms, FOV = 40 × 37 cm, matrix size = 256 × 256, and slice thickness = 3 mm, and black blood time interval (BBTI) was set at 1200 ms. Scan time was variable according to the number of slices acquired and respiratory rate of the participant.
Image post-processing
The obtained coronal source images were processed on MRI scanner console to generate 3D maximum intensity projection images (MIP) and colored 3D volume-rendered images in different planes after image clipping and manual removal of undesirable structures.
Dynamic contrast-enhanced MRI protocol
All 25 participants with known portal vein pathology underwent contrast-enhanced dynamic MRI of the liver, and 3D gradient-echo T1-weighted fat-suppressed axial images (TR = 4.8 ms, TE = 1.9 ms, flip angle = 15°, matrix = 192 × 256, FOV = 40 × 35, slice thickness = 7 mm, and scan time = 13–16 s) were acquired with breath holding after manual IV injection of gadolinium-based contrast agent, gadopentetate dimeglumine (Magnevist, Bayer Healthcare), at a dose of 0.1 mmol/kg followed by 10 cc saline flushing. The arrival of contrast material at abdominal aorta was detected by real-time bolus tracking built-in software (Visual prep). Actual MR acquisition started 8 s after contrast detection at abdominal aorta to acquire late arterial phase. Portal-venous phase was acquired 10 s after late arterial one, and delayed phase acquired 2 min thereafter.
Image analysis of non-contrast MR portography
Assessment of image quality
Non-contrast MR portography studies were assessed in conjoint sessions by three radiologist with total experience of 15 years in interpreting abdominal MRI studies; they were aware of duplex ultrasound results before interpreting images. Images were assessed first for image quality by scrolling through source images to detect respiratory and voluntary motion artifacts, to assess the degree of portal circulation flow-related enhancement, and to rule out the presence of any intra-abdominal fluid material like ascites, fluid-filled stomach, distended GB, or biliary radicals that may interfere with adequate visualization of portal vein at MIP reconstructions. Accordingly, each study was assigned a quality score as follows: (1) good quality scan—absent respiratory and voluntary motion artifacts, adequate flow-related enhancement at portal vein or its tributaries, and absent fluid-filled material obscuring portal vein at MIP images; (2) moderate quality scan—presence of mild to moderate respiratory/voluntary motion artifacts, adequate flow-related enhancement at portal vein or its tributaries, and/or presence of fluid material obscuring portal vein at MIP images; and (3) poor quality non-diagnostic scan—presence of severe respiratory/voluntary motion artifacts and/or inadequate flow-related enhancement at portal vein or its tributaries. Good and moderate quality scans were considered interpretable and were grouped together when performing statistical analysis. The non-interpretable scans were discarded.
Assessment of portal vein
Good and moderate quality scans were selected for portal vein assessment by analysis of reconstructed MIP and volume-rendered images, and scrolling through coronal source images. The presence of intraluminal filling defect, eccentric or concentric narrowing, luminal dilatation, and cavernous transformation were reported. Any associated dilatation of splenic vein and coronary vein or the presence of varices was reported as well. All findings of non-contrast MR portography were compared to findings of color duplex ultrasonography as a reference. Only patients with good delineation of the portal vein and/or its pathology on color Doppler ultrasound were included in the study. In addition, the findings of diseased portal vein at non-contrast MR portography images were also compared to portal-phase dynamic contrast-enhanced MRI images.
Statistical analysis
Two sample t test for equal means was used for comparison between the groups, and chi-square test was used for testing relationships between categorical variables. Calculation of sensitivity, specificity, and accuracy of T-SLIP MR portography was also done. All tests were performed on Minitab 17 statistical software package (Minitab Inc.).