Ebrahimi Behzad, Rihal Naveen, Woollard John R, Krier James D, Eirin Alfonso, Lerman Lilach O
From the Division of Nephrology and Hypertension, Mayo Clinic, Rochester, MN.
Invest Radiol. 2014 Oct;49(10):640-6. doi: 10.1097/RLI.0000000000000066.
Diffusion-weighted magnetic resonance imaging is a powerful tool to assess renal morphology. However, its quantitative index, apparent diffusion coefficient (ADC), derived from a conventional monoexponential model can vary with both functional and structural alterations as well as the choice of b values. In contrast, the intravoxel incoherent motion (IVIM) biexponential model provides independent parameters that may represent broader aspects of renal pathophysiology. We hypothesized that IVIM analysis is capable of detecting early morphological and functional changes in the swine kidney distal to renal artery stenosis (RAS).
Domestic pigs divided into 3 groups (n = 6-7 each) were studied for 16 weeks. Unilateral RAS was induced in 2 groups, of which 1 group was fed with a high-cholesterol diet to induce early atherosclerosis (ARAS), whereas the other (RAS) consumed regular diet. The third group included healthy pigs that served as control sham. Renal function, hemodynamics, tubular function, and morphology were assessed using multidetector computed tomography and histology. Diffusion-weighted magnetic resonance images were acquired at 3T and analyzed using monoexponential and biexponential models. Parameters of ADC and IVIM (diffusivity [D(t)], flow-dependent pseudodiffusivity [D(p)], and fluid fraction [f(p)]) were calculated in the cortex and medulla of the stenotic (STK) and contralateral kidneys (CLKs). Results were analyzed using analysis of variance, Student t test, and regression analysis.
In both RAS and ARAS, the STK shrank and the CLK underwent hypertrophy. Glomerular filtration rate and renal blood flow declined in STKs, and CLKs manifested hyperfiltration. In addition, ARAS kidneys showed reduced mean transit time in distal tubular segments. Apparent diffusion coefficient and diffusivity both decreased in STK of RAS and ARAS. D(p) and f(p) were elevated in both the STK and CLK of RAS and more prominently in ARAS. The STK cortical ADC and D(t) correlated inversely with the degree of fibrosis and directly with glomerular filtration rate. Furthermore, D(p) correlated with tubular injury score in all kidneys.
Apparent diffusion constant and D(t) both correlated with cortical and medullary fibrosis; however, IVIM-derived parameters can detect subtle functional and structural changes in the post-STK and may also serve as markers for tubular injury.
扩散加权磁共振成像（DWI）是评估肾脏形态的有力工具。然而，其基于传统单指数模型得出的定量指标——表观扩散系数（ADC），会因功能和结构改变以及b值的选择而有所不同。相比之下，体素内不相干运动（IVIM）双指数模型提供的独立参数可能代表肾脏病理生理学的更广泛方面。我们假设IVIM分析能够检测猪肾动脉狭窄（RAS）远端肾脏的早期形态和功能变化。
将家猪分为3组（每组n = 6 - 7头），研究16周。2组诱导单侧RAS，其中1组喂食高胆固醇饮食以诱导早期动脉粥样硬化（ARAS），另一组（RAS）喂食常规饮食。第三组包括健康猪作为对照假手术组。使用多排螺旋计算机断层扫描和组织学评估肾功能、血流动力学、肾小管功能和形态。在3T条件下采集扩散加权磁共振图像，并使用单指数和双指数模型进行分析。计算狭窄侧肾脏（STK）和对侧肾脏（CLK）皮质和髓质的ADC和IVIM参数（扩散系数[D(t)]、血流依赖的伪扩散系数[D(p)]和流体分数[f(p)]）。使用方差分析、学生t检验和回归分析对结果进行分析。
在RAS组和ARAS组中，STK均萎缩，CLK肥大。STK的肾小球滤过率和肾血流量下降，CLK表现为超滤过。此外，ARAS组肾脏远端肾小管段的平均通过时间缩短。RAS组和ARAS组STK的ADC和D(t)均降低。RAS组的STK和CLK中D(p)和f(p)均升高，在ARAS组中更为明显。STK皮质ADC和D(t)与纤维化程度呈负相关，与肾小球滤过率呈正相关。此外，所有肾脏中D(p)与肾小管损伤评分相关。
表观扩散常数和D(t)均与皮质和髓质纤维化相关；然而，IVIM衍生参数可检测STK后的细微功能和结构变化，也可作为肾小管损伤的标志物。
