The present study was approved by our institutional review board and informed consent was waived for every patient.
Patients
We retrospectively reviewed the PACS of 661 consecutive patients with GC who underwent preoperative stomach MRI from January 2017 to November 2020. The present study included patients with pathologically confirmed primary GC who underwent surgery within 1 week following MRI examination and received pathological diagnosis in the end. Patients were excluded if the pathological T stages were T3, T4a or T4b and in case of preoperative therapy, such as chemotherapy, radiotherapy and endoscopic resection. Patients were excluded if they exhibited poor image quality resulting in poor visualization of lesions and in case of lesions that were undetectable in CE-MRI (Fig. 1). Two radiologists (Y.Y. and T.W. both with 10 years of experience in abdominal radiology, respectively) determined the detection efficacy in consensus, according to the criterion of a definite visualization of the GC lesion. Sex, age, CEA level, CA199 level, CA724 level, BMI index of all included patients were recorded.
MRI acquisition
Each patient was fasted for at least 5 h and warm water (500 ml) was administered to dilate the stomach before the MRI examination. No drugs were injected to inhibit gastrointestinal peristalsis. All MRI examinations were performed on one scanner (3.0 T MAGNETOM Skyra, Siemens Healthcare, Germany). 18-channel phased-array body and integrated spine coils were applied to obtain the MRI signal. The sequences were uniformed as a settled image set as follows: axial T2WI, three dimensional T1WI volume interpolated body examination (3D T1WI VIBE) and three phases of CE-T1WI (arterial, venous and delayed phases). The parameters for T2WI were as follows: TR, 4560 ms; TE, 79 ms; FOV, 380 * 380 mm2; matrix, 320 * 320; flip angle 140°; slice thickness, 6 mm; gap, 1.2 mm; fat saturation, yes; acquisition time 3–4 min. The following parameters were used for CE-T1WI: TR, 3.46 ms; TE, 1.32 ms; FOV, 308 * 380 mm2; matrix, 195 * 320; flip angle 12°; slice thickness, 3 mm; gap, 0 mm; fat saturation, yes; acquisition time, 14 s. The CE-T1W images were obtained at 30, 60, 90 s following contrast injection, which consisted of 0.2 ml/kg body weight Gd-DTPA (Beilu, Beijing, China) delivered using an automatic power injector (Medrad Spectris Solaris EP MR Injector System, PA, USA) at 2 ml/s followed by a 20 ml saline flush at the same rate.
Image analysis
All images were transferred to GE PACS RA1000 for further analysis. Two abdominal radiologists (Y.Y. and T.W. both with 10 years of experience in abdominal radiology, respectively) who were aware of the existence but not aware of any other clinical information of all GC lesions were selected for independent evaluation of the location (three categories of fundus, body and antrum), the MRI T stage (subjective evaluation of ≤ T1 or ≥ T2 with combination of T2W and CE-T1WI). The researchers reached a consensus when encountering inconsistent diagnosis. The MRI T stage was determined according to the AJCC 8th edition GC staging. The MRI T stage criteria were as follows: enhanced tissues not exceeding the submucosal layer were termed T1a–T1b stage, whereas enhanced tissues exceeding the submucosal layer were termed T2 stage [10, 20].
The parameters thickness, maximum area and volume were evaluated based on CE-T1WI (venous phase). All the lesions were moved to the center of the screen and zoomed in two times, so that two radiologists could independently measure the lesions in order to achieve more precise measurements within one week. Thickness was measured at the thickest part of the maximum area which presented as abnormal high signal intensity perpendicular to the gastric wall direction. The regions of interest (ROIs) were drawn along the edge of the lesion according to abnormal signal intensity compared with the adjacent normal gastric wall on each slice within one week. The maximum area was selected from the ROIs (Fig. 2). The volume was estimated by the following formula: volume = sum of area of ROIs × slice thickness. The average value of thickness, maximum area and volume was provided by two radiologists for final analysis.
Pathological evaluation
All specimens were pathologically confirmed as GC. The location and T stage were assessed. The T stage was determined according to the American Joint Committee on Cancer (AJCC, 8th edition). The tumor invasion of the lamina propria or muscularis mucosae was considered T1a, whereas the tumor invasion of the submucosa was considered T1b and the tumor invasion of the muscularis propria was considered T2 [20]. T1a and T1b were grouped together, whereas T2 was an independent group.
Statistical analysis
The Kolmogorov–Smirnov statistical test was performed to assess the normality for all continuous variables. The sample t test or Mann–Whitney U test were performed to compare continuous variables. The Chi-square test was performed to compare qualitative data. The parameters AUC, optimal cutoff (if required), sensitivity, specificity, accuracy, positive likelihood ratio (PLR), negative likelihood ratio (NLR), positive predictive value (PPV), negative predicative value (NPV) of MRI T stage, thickness, maximum area and volume for differentiating T1a–T1b from T2 lesions were calculated. The Delong test was performed to compare the ROC curves. Kappa or weighted Kappa coefficient was estimated to assess the interobserver agreement in the qualitative data. A kappa value > 0.8 indicated excellent agreement, whereas those from 0.6 to 0.8, 0.4 to 0.6, 0.2 to 0.4 and 0.0 to 0.2 indicated substantial, moderate, fair and slight agreement, respectively. A kappa value < 0.0 indicated poor agreement [21]. The intraclass correlation coefficient (ICC) was performed for continuous variables (ICC = 0 to 0.49, poor agreement; ICC = 0.50 to 0.75, moderate agreement; ICC = 0.76 to 0.90, good agreement; ICC = 0.91 to 1.00, excellent agreement) [22]. The Bland–Altman analysis was performed for continuous variables to evaluate the interobserver agreements of two radiologists. P < 0.05 was considered to indicate significant differences. The MedCalc software (version 19.6.1) and the SPSS software (version 20.0) was used to calculate the weighted Kappa coefficient, perform ROCs and other analysis. The decision curve analysis (DCA) was performed with R (R version 3.3.3).