2.1 Research objects
Thirty male wistar rats were obtained from Experimental Animal breeding Co., Ltd.(China, Jinan). The rats were 7 weeks old and weighed 170-200g. All rats were acclimatized in a SPF environment for one week before the experiment. At 8, 13, 18, 23, 28, 33 weeks, 5 rats were randomly selected for MRI examination, then killed and acquired pathology.Each rat was given 3 ml of Urethane intraperitoneal injection before MRI to anesthetize.The study was approved by the Institutional Animal Care and Use Committee and was performed in accordance with the National Institutes of Health guidelines for the use of laboratory animals.
2.2 MR Imaging Techniques
All MR imaging was performed using a 3.0-T MR (GE company) and utilizing a single-channel animal coil (GE company). Four standard sequences MR imaging was performed: (A)OAx T2-weighted with fat-saturated[echo time(TE)/repetition time (TR),96.1ms/3000ms; echo train length,16];(B)OCor fs T1 FSE(TE/TR, 13.5ms/500ms; echo train length, 3); (C)DCE-MRI: the fat-saturated contrast enhanced T1-weighted images with LAVA sequence, 1ml gadopentetate dimeglumine contrast agent(BeiLu Pharmaceutical Co., Ltd, Beijing, China) was administered intravenously at a rate of 0.1ml/s, followed by a 2ml saline flush by hand after the acquisition of seven baseline dynamic scans, each 16-slice set was collected at 80 time points for approximately 5 minutes of scanning, and (D)IVIM-DWI: a total of 12 b-values were used:10,20,30,50,80,100,200,300,600,800,1000,1500s/mm2.
2.3 Image Analysis
2.3.1 Analysis of IVIM Parameters
The DWI signal follows the biexponential model to calculate the signal attenuation IVIM, as[10]:
S(b)/S0=(1-f)exp(-bD)+f exp[-b×(D*+D)]
Where Sb is the signal intensity in the pixel with diffusion gradient b, S0 is the signal intensity without diffusion gradient, D is the true diffusion, f is the perfusion fraction related to microcirculation and D* is the pseudo-diffusion coefficient which represented perfusion-related diffusion.
2.3.2 Analysis of DCE-MRI Parameters
For DCE-MRI parameters analysis, all data were quantitatively analyzed using
image processing software, calculated by manually drawing different regions of interests (ROIs) from time-tensity curve (TIC).
Fenh ( %) = ( SImax –SI0) ×100% /SI0;
Senh( %s-1) = ( SImax-SI0) 100 / ( SI0×Tmax);
where SI0 is the baseline SI measured before contrast injection, approaches the maximum signal intensity (SImax) exponentially in the characteristic time. SImax is determined as the maximum SI during the phases and Tmax were defined as the time point of SImax.
2.3.3 Data analysis
The original images were processed using the Advantage Workstation(ADW 4.6 version, GE, US) and post-processed by Functool workstation. Two observers with 15 years and 20 years of experience in MRI were blinded to the information and individually measured the resulting parameter maps. Disagreement of both radiologists was resolved in consensus. All data were measured for 3 times and the average of 3 times is taken to reduce the bias caused by measurement error. The ROI was selected in bilateral sacrum, ilium marrow, synovial membrane of SIJ(the anterior and posterior synovial areas of the bilateral SIJ space) with ranges were 2-4mm2 , total of four ROIs for each layer and then the average values were taken(Figure 1, 2, 3).
2.4 Histologic assessment
After MR examnation, all rats were weighed and each rat was anesthetized with
pentobarbital 100 mg/kg intraperitoneal injection.When heartbeats had not been detected for five minutes, samples of SIJ were cut across the midline and removed, fixed in 10% formalin for one-two day, acid-decalcified with 10% methanoic acid for one week, embedded in paraffin and cutted after dehydration, stained with hematoxylin and eosin. The pathological changes of SIJ were observed under microscope (MODEL BX53F)finally.
2.5 Statistical Analysis
SPSS Statistics version 19.0 was used for statistical analysis. The homoscedasticity of data was tested using the Levene’s tests. All the parameter values were compared by one-way ANOVA. The measured parameters were expressed as means ± standard deviation (SD). P values of less than 0.05 were considered as statistically significant.