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1. The study was approved by the Institutional Review Board and the Ethics Committee of the Affiliated Hospital of North Sichuan Medical College. It is difficult to obtain informed consent from all of the patients involved in the study because of the retrospective nature of this study and the health care conditions in Nanchong. The Ethics Committee considered that the experimental design and the program of the study would not cause harm or risk to the subjects. The data collected were analyzed anonymously. The Ethics Committee of our Hospital waived the need for written informed consent from the participants. The study complies with the ethical principles of the Helsinki Declaration of 1964, as revised by the World Medical Organization in Edinburgh in 2000.
2. The patients with AP admitted to our institution between April 2010 and March 2012 were considered for inclusion in this study. The diagnosis of acute pancreatitis was based on the presence of typical abdominal pain combined with three-fold elevated amylase or lipase. All of the readers were blinded to the clinical presentation, blood work and outcomes of the patients. The inclusion criteria for the patients in this study were as follows: (a) in-patient; (b) acute onset of abdominal pain; (c) pancreatitis at first onset; (d) three-fold elevated amylase or lipase, excluding other causes of elevated enzymes; and (d) abdominal MRI examination. The exclusion criteria in this study were as follows: (a) resistance to MR imaging (including contraindications to MRI, claustrophobia, inability to control breathing); (b) a history of chronic pancreatitis; (c) AP resulting from pancreatic carcinoma; and (d) the presence of other diseases with mesenteric involvement (including hypoproteinemi, chronic liver disease, peritoneal dialysis). A total of 210 patients with AP were enrolled in this study. All of the patients had a clinical assessment and laboratory workup on admission. In the same period, 40 continuous patients with no mesentery disorders visible on MRI between September 2011 and March 2012 served as the control group. The subjects had no abdominal symptoms and signs, and on MRI, we found nothing that could cause mesentery involvement. An upper-abdominal examination (plain-scan and dynamic-enhanced) was performed in our hospital as part of a physical examination or to test for other diseases, and the results included 19 patients with no abnormal findings in the abdomen, eight patients with hepatic cysts, five patients with hepatic hemangioma and eight patients with renal cysts visualized on MRI.
3. All of the MR examinations were performed during suspended respiration with a 1.5-T system and a phased-array coil (Signa, GE Medical Systems, Milwaukee, WI, USA). The sequences included two-dimensional coronal and axial single-shot fast spin-echo (SSFSE) T2-weighted, axial fast-recovery fast-spin-echo (FRFSE) T2-weighted with fat suppressed, axial spoiled dual gradient-echo (GRE T1-weighted in- and out-of-phase, axial-slab three-dimensional (3D) spoiled gradient-echo (SPGR) dynamic contrast-enhanced with fat-suppressed and SSFSE radial series slab MRCP. The coronal SSFSE T2-weighted MR images were obtained in two or more breath-holds. The following parameters were used: repetition time [TR] ms/echo time [TE] ms, 2500∼3500/80–100; section thickness, 7 mm; intersection gap, 0; matrix, 256×192; signal acquired, one half; and field of view (FOV), 32×32 cm. The axial SSFSE T2-weighted images were obtained in one or two breath-holds. The following parameters were used: TR ms/TE ms, 2500∼3500/80–100; section thickness, 5 mm; intersection gap, 0; matrix, 256×160; signal acquired, one half; and FOV, 32×24 cm. The axial two-dimensional multi-section SPGR T1-weighted images were obtained during breath holding. The following parameters were used: TR ms/TE ms, 120/4.2 (in-phase) and 2.1 (out-of-phase); flip angle, 90°; section thickness, 8 mm; intersection gap, 0; matrix, 256×192; signal acquired, 1; and field of view, 32×24 cm. SSFSE radial oblique slabs were obtained for MR cholangiopancreatography (MRCP) with the following parameters: TR ms/TE ms 6000/700, fat saturation, 40-mm section thickness, 256×192 matrix, half signal acquired, and 24×24 cm FOV. The axial three-dimensional SPGR dynamic MR images were obtained with TR ms/TE ms, 6.1/2.1; flip angle, 15–20°; matrix, 256 ×128; signal acquired, 1; section thickness, 5 mm; overlap, 2.5 mm; and FOV, 32×24 cm. Three-dimensional SPGR was obtained at 2.5 mm increments with zero-fill interpolation for dynamic enhancement. Twenty milliliters of gadolinium (Magnevist; Schering Guangzhou Co., China) was administered intravenously with a pressure injector (Spectris MR Injection System, Medrad, Inc., USA) at 2–3 ml/s and followed by a 20-ml saline solution flush. The first-pass arterial enhancement was optimized with a timing bolus sequence (axial FMPSPGR). The dynamic imaging was performed during breath holding before the injection (unenhanced), immediately after the injection (hepatic arterial phase), 30 s after the injection (early venous phase) and 1 min after the injection (late venous phase). The delayed phases (90 s after the injection) were acquired with axial fast spoiled gradient echo (FSPGR) and 3D SPGR T1-weighted sequences.
4. The original MRI data were loaded to a workstation (GE, AW 4.4) for observation and measurement. The MR images were reviewed by two observers (with more than three years of experience in abdominal MR imaging), who were blinded to the laboratory data and clinical outcomes. 4.1. Normal transverse mesocolon on MRI: The features of the normal transverse mesocolon observed on MRI included the transverse mesocolon adipose tissue signal (uniformity, consistency with the subcutaneous fat signal on the T2-weighted image in the axial and coronal views); the shape and direction of the mesenteric vessels, with a focus on the middle colic artery, middle colic vein and gastrocolic trunk in the axial and coronal views; the diameter of the above three vessels on dynamic enhanced MRI; the lymph nodes attached to the transverse mesocolon; and the size of the root of the transverse mesocolon (if larger than 5 mm in the short diameter). 4.2. MRI findings in acute pancreatitis on MRI: AP was defined as edematous and necrotic pancreatitis on the MR images [17], [18], [19]. The severity of AP on MRI was graded according to the MR-severity index (MRSI), which was derived from the CT-severity index [20]. AP was graded as mild (0–3 points), moderate (4–6 points) or severe (7–10 points), according to the MRSI [20], [21], [22].
4.3. The findings of transverse-mesocolon involvement observed on MRI included transverse-mesocolon edema, thickening, effusion and transverse-mesocolon vessel involvement. We measured the diameter of the middle colic artery, middle colic vein and gastrocolic trunk in the patients with AP who had received dynamic enhanced MR imaging. The severity of TMI was graded as Grade I (included no abnormalities and transverse-mesocolon vessels involvement, zero points) Grade II (linear and patchy signal in the transverse mesocolon, one point); or Grade III (transverse-mesocolon effusion, two points). The transverse mesocolon is divided into right and left by the centerline of the body.
5. The APACHE II is widely used to evaluate the severity of acute pancreatitis in clinical practice [23]. An APACHE-II score of eight is a cut-off point to differentiate between mild AP (zero to seven points) and severe AP (≥eight points) [4]. Two physicians with five years of experience in treating digestive diseases calculated the scores according to the laboratory data and clinical outcomes.
6. The MRSI and TMI scores on the MRI were averaged between the two observers. Any discrepancies between the readers were settled by consensus. Kappa statistics were used to assess the inter-rater reliability. The continuous variables were expressed as the mean ± SD and the range, and the T-test was used to analyze the difference between the AP group and the control group in the blood vessels of the transverse mesocolon and patient age. The matched four-fold table chi-square test was used to analyze the difference in the sex ratio between the AP group and the control group. Chi-squared tests were used to analyze the differences in the transverse-mesocolon involvement between mild, moderate and severe AP according to the MRSI and between mild AP and severe AP according to the APACHE-II scoring system. The Spearman rank-correlation coefficient was calculated to test the correlation of the transverse-mesocolon involvement score with the MRSI and the APACHE-II score. The statistical tests were performed using the Statistical Package for Social Sciences (SPSS) for Windows (Version 13.0, Chicago, IL, USA). P-values <0.05 were considered significant.
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