Study population

Between January 1, 2015 and December 31, 2018, we identified 165 kidney transplant patients (195 examinations, 158 men and 37 women, range 19–72 years old), who were suspected of having TRAS. The main reasons for patients for IFIR examination are as follows: (1) routine follow-up after kidney transplantation; (2) decreased renal function; (3) hypertension. All arteries were evaluated using IFIR as part of the routine clinical protocol. In the study, twenty-seven arteries with TRAS were evaluated with DSA. Among them, balloon dilatation was performed on nine patients, and balloon dilatation coupled with stent implantation was performed on the remaining 18 patients. The remaining patients did not undergo DSA for the following reasons: (1) no clinical symptoms or signs; (2) normal renal function/artery; (3) after clinical conservative treatment, satisfactory results were achieved. The study was approved by the ethics committees of our hospital (Table 1).

Table 1 Demographic and summaries of study population (n = 195)

MRI examination

All IFIR measurements were performed using a 3.0 T whole-body scanner (Discovery 750, General Electrics (GE), Milwaukee, WI, USA). The standard MRI protocol for kidney imaging started with an axial respirator-triggered single-shot fast spin-echo T2-weighted sequence and an axial breath-hold fast-spoiled gradient T1-weighted sequence. The imaging sequence was performed with breath-hold. The scan parameters were applied as follows: TE 1.5 ms, TR 3.10 ms, flip angle 60°, TI = 1300 ms, receiver bandwidth 125 Hz/pixel; field of view 340 × 255 mm for covering both kidneys, slice thickness 2.4 mm, locations per slab 32, frequency matrix 224, phase matrix 160, number of excitation 1, phase FOV 0.75, and an acquisition time of 10–12 s [14].

DSA examination

DSA was performed with a GE DSA system (Innova; GE Healthcare, Waukesha, WI). Angiography was performed by experienced interventional radiologists through the femoral arterial route using a 5-F pigtail catheter to inject 30 ml of iodinated contrast medium (Visipaque 320; Amersham Health, Princeton, NJ) at a flow rate of 15 ml/s (Mark V; Medrad, Indianola, PA). The optimal DSA images were obtained by C arm X-ray roentgenscope in multiple directions and angles (frontal, left, and right oblique views). During the period between IFIR and DSA, the renal function of each patient was monitored and strictly controlled [16].

Image analysis

We used the scale measurement function/3D reconstruction/multiplanar reformation of the post-processing software (Discovery 750, General Electrics (GE), Milwaukee, WI, USA) to measure the diameter of arteries. The IFIR and DSA data were measured three times by two experienced radiologists separately. Mean value ± standard deviation (SD) was taken of the IFIR and DSA data. Each radiologist completed the measurement independently. They were blinded to each other and they were blinded to relevant diagnosis before measurement.

The quality of IFIR images was assessed at four levels: excellent, good, moderate, and non-diagnostic (Fig. 1). The following measurements were made for each artery: The percentage of stenosis = (1 − (S/R)) × 100, where S is the minimum diameter of the lesion and R is the diameter of the reference site; the latter was defined as the normal-looking portion of the stenotic vessel distal or proximal to the lesion.

Fig. 1
figure1

Shows representative IFIR images of different quality: a non diagnostic, b moderate, c good, d excellent. IFIR: Inflow inversion recovery

The extents of the TRAS were then graded as follows: grade 1 (< 25%); grade 2 (25–49%); grade 3 (50–74%); grade 4 (75–99%); grade 5 (100%) [14, 17].

Statistical analysis

The data were analyzed in SPSS 17.0 statistical software and GraphPad Prism. The nonparametric Wilcoxon signed-rank test was used to compare IFIR to DSA of TRAS. The agreement between IFIR and DSA was assessed by Bland–Altman plots. The significance level was set to 0.05.

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