Diffusion-weighted magnetic resonance imaging to assess diffuse renal pathology: a systematic review and statement paper

Abstract

Diffusion-weighted magnetic resonance imaging (DWI) is a non-invasive method sensitive to local water motion in the tissue. As a tool to probe the microstructure, including the presence and potentially the degree of renal fibrosis, DWI has the potential to become an effective imaging biomarker. The aim of this review is to discuss the current status of renal DWI in diffuse renal diseases. DWI biomarkers can be classified in the following three main categories: (i) the apparent diffusion coefficient-an overall measure of water diffusion and microcirculation in the tissue; (ii) true diffusion, pseudodiffusion and flowing fraction-providing separate information on diffusion and perfusion or tubular flow; and (iii) fractional anisotropy-measuring the microstructural orientation. An overview of human studies applying renal DWI in diffuse pathologies is given, demonstrating not only the feasibility and intra-study reproducibility of DWI but also highlighting the need for standardization of methods, additional validation and qualification. The current and future role of renal DWI in clinical practice is reviewed, emphasizing its potential as a surrogate and monitoring biomarker for interstitial fibrosis in chronic kidney disease, as well as a surrogate biomarker for the inflammation in acute kidney diseases that may impact patient selection for renal biopsy in acute graft rejection. As part of the international COST (European Cooperation in Science and Technology) action PARENCHIMA (Magnetic Resonance Imaging Biomarkers for Chronic Kidney Disease), aimed at eliminating the barriers to the clinical use of functional renal magnetic resonance imaging, this article provides practical recommendations for future design of clinical studies and the use of renal DWI in clinical practice.

FIGURE 1:

Illustration of the principles of renal DWI in a normal volunteer and a CKD patient with 60% of renal fibrosis. First, a series of renal MRI with varying degrees of diffusion weighting are acquired as shown in the first row. Then, the ADC, an estimation of the water molecule motion in the tissue, is extracted from these images by curve fitting. An example of the ADC fit is presented in the graphs in the middle row. Note the difference in the cortical and medulla fits between the normal volunteer without fibrosis and the CKD patient. The medulla curve in the CKD patient has a stronger curvature reflecting an increased ADC. Finally, the result of fit for each pixel is displayed in a new image called an ADC map (bottom row). In the normal volunteer (on the left), the ADC is higher in the cortex (blue arrow) than in the medulla (red arrow). The opposite is found in the CKD patient (on the right), where the ADC is higher in the medulla than in the cortex in agreement with the curve fits. This inversion of cortico-medullary ADC difference is correlated with the amount of fibrosis present in the CKD patient.