Renal relevant radiology: renal functional magnetic resonance imaging

Abstract

Because of its noninvasive nature and provision of quantitative measures of a wide variety of physiologic parameters, functional magnetic resonance imaging (MRI) shows great potential for research and clinical applications. Over the past decade, application of functional MRI extended beyond detection of cerebral activity, and techniques for abdominal functional MRI evolved. Assessment of renal perfusion, glomerular filtration, interstitial diffusion, and parenchymal oxygenation turned this modality into an essential research and potentially diagnostic tool. Variations in many renal physiologic markers can be detected using functional MRI before morphologic changes become evident in anatomic magnetic resonance images. Moreover, the framework of functional MRI opened a window of opportunity to develop novel pathophysiologic markers. This article reviews applications of some well validated functional MRI techniques, including perfusion, diffusion-weighted imaging, and blood oxygen level-dependent MRI, as well as some emerging new techniques such as magnetic resonance elastography, which might evolve into clinically useful tools.

Figure 1.

Dynamic contrast-enhanced magnetic resonance imaging–derived images in a swine unilateral renal artery stenosis. (A) The stenotic (right) and the contralateral (left) kidneys at baseline (a) and at the vascular (b) and tubular (c) phases. High cortical blood flow results in high contrast between the cortex and medulla during the vascular phase. (a and b) The white arrow shows the arrival of the contrast bolus in the aorta (a) and in the cortex (b) (during the contrast agent transition), whereas the red arrow shows the medulla. (c) The physiologically hypoperfused medulla shows visible enhancement mainly during the tubular phase. (d) Finally, as contrast media wash out from the tissue, their appearance in the stenotic kidney calyx is delayed compared with the contralateral kidney. (B) Typical magnetic resonance concentration-time curves, with vascular, proximal tubular, Loop of Henle, and distal tubular transitions. D, distal tubular; L, Loop of Henle; P, proximal tubular; V, vascular.

Figure 2.

Diffusion-weighted magnetic resonance imaging, with different maps that reflect the diverse parameters that can be derived. (A) Anatomic MR image. (B) Biexponential signal decay consistent with pseudodiffusivity (fast decaying perfusion and tubular flow dependence) and diffusivity (slowly decaying pure tissue diffusion dependence) components. (C) Apparent diffusion constant map from monoexponential decay model. (D) Pure tissue diffusivity map. (E) Pseudodiffusivity map representing tubular fluid and microvascular blood velocity. (F) Perfusion fraction (fluid fraction) map calculated using a intravoxel incoherent motion biexponential decay model. DWI, diffusion-weighted imaging.

Figure 3.

Renal BOLD MRI and physiological restricting factors. (A) Representative anatomic reference (a) and BOLD maps before (b) and after (c) administration of furosemide. Furosemide reduces oxygen-dependent tubular transport in the medulla and improves medullary oxygenation (drops on the scale toward green-blue shades). The response to furosemide is measurable by comparing the areas of the hypoxic regions and/or the change in average R₂* magnitude. (B) Parameters that may affect BOLD magnetic resonance images. Fibrotic tissues can restrict oxygen exchange between the microvasculature and tissue, so that due to low oxygen diffusivity the vascular oxygenation (sampled by BOLD) remains high despite tissue hypoxia. The three bottom squares represent relative contrast in a T₂*-weighted image, which can be affected by density of capillaries or hemoglobin (e.g., hematocrit). Higher concentration of hemoglobin results in faster decay of the magnetic resonance signal and gives rise to dark regions interpreted as hypoxic tissue, without necessarily representing tissue oxygenation. BOLD, blood oxygen level–dependent.

Figure 4.

Magnetic resonance elastography in swine unilateral renal artery stenosis. Lower stiffness (less red color) in the cortex of the stenotic kidney (right) compared with the contralateral kidney (left), despite greater fibrosis, is a consequence of lower perfusion pressure (and turgor) distal to the stenosis. CLK, contralateral kidney; STK, stenotic kidney.

Figure 5.

Fat quantification by magnetic resonance imaging. Water-only (A) and fat-only (B) images and the fat-ratio map (C) calculated from in-phase and out-of-phase images (not shown).