[Quantitative recording of renal function with magnetic resonance tomography]

Functional renal imaging: nonvascular renal disease

Functional renal imaging-a fast-growing field of MR-imaging-applies different sequence types to gather information about the kidneys other than morphology and angiography. This update article presents the current status of different functional imaging approaches and presents current and potential clinical applications. Apart from conventional in-phase and opposed-phase imaging, which already yields information about the tissue composition, BOLD (blood-oxygenation level dependent) sequences, DWI (diffusion-weighted imaging) sequences, perfusion measurements, and dedicated contrast agents are used.

Dynamic MRI contrast enhancement of renal cortex: a functional assessment of renovascular disease in patients with renal artery stenosis

PURPOSE

To evaluate differences in the magnitude and time course of renal cortical contrast uptake in patients with minimal, moderate, and severe renal artery stenosis (RAS) using contrast-enhanced magnetic resonance renography (CE-MRR).

MATERIALS AND METHODS

CE-MRR was performed on 56 patients with renovascular disease using a three-dimensional volume interpolated breath-hold examination (VIBE) perfusion sequence. After administration of 2 mL of contrast, nine sequential axial VIBE datasets were acquired: at baseline, 7, 14, 21, 45, 60, 120, 180, and 240 seconds. Aortic peak signal enhancement and cortical peak signal enhancement through the mid portion of each kidney was recorded, along with the time delay between each peak. Each renal artery was subsequently examined using three-dimensional contrast-enhanced MR angiography, and graded as being minimally (0%-30%), moderately (31%-70%), or severely (71%-100%) stenotic.

RESULTS

When the data were subdivided by RAS category, the cortical to aortic peak enhancement ratio (CAPR) reduced with increasing RAS. Further, the cortical to aortic time delay (CATD) increased with increasing RAS. These measurements were statistically significant between patients with minimal and moderate RAS compared to severe RAS CONCLUSION: CE-MRR can assist in the differentiation of patients with minimal or moderate RAS from those with severe RAS.

Current issues in the diagnosis and management of patients with renal artery stenosis: a cardiologic perspective

Renal artery stenosis most often is caused by atherosclerosis. Although patients with renal artery stenosis can be managed conservatively, renal revascularization may be indicated, particularly in patients with refractory hypertension on a multidrug regimen and patients with declining renal function. Duplex ultrasonography of the renal arteries and magnetic resonance angiography are currently the most efficient noninvasive methods for the evaluation of renal artery stenosis. Selective digital subtraction renal arteriography remains the gold standard for the definitive diagnosis. In selected patients undergoing coronary studies and angiography immediately after the coronary procedure can be efficient. Atherosclerotic renal artery lesions, which commonly affect the renal artery ostium, can be treated safely and effectively with balloon-expandable stents. Successful angioplasty commonly results in improved control of hypertension, but an overall benefit on renal function and/or patient survival has not been shown. Generally the risk/benefit ratio of renal artery stenting seems favorable, but further randomized studies are needed for evidence-based decision making. All patients with atherosclerotic renal artery stenosis should receive rigorous secondary prevention measures including platelet inhibitors, statins, and beta-blockers.

Noninvasive measurement of absolute renal perfusion by contrast medium-enhanced magnetic resonance imaging

OBJECTIVE

The aim of this study was to validate the quantification of absolute renal perfusion (RP) determined by dynamic magnetic resonance imaging (MRI) and contrast media using an experimental model in the rabbit and a transit-timed ultrasound flow probe around the left renal artery as comparison.

MATERIAL AND METHODS

An MR-compatible ultrasonic time-of-flight flow-probe was placed around the left renal artery in 9 New Zealand white rabbits. Absolute RP in basal state, after mechanical renal artery stenosis, intravenous dopamine, angiotensin II, or colloid infusion was measured using dynamic MRI and intravenous injection of gadoteridol. The results were correlated to the renal artery flow measured inside the magnet with the transit-timed flow-probe. For the signal intensity concentration conversion, we applied different calibrations according to various velocities measured in the aorta by a phase contrast sequence to correct for inflow effect. MRI-derived RP (in mL/min) was calculated by the maximum upslope method, where RP/volume was defined as the ratio of the cortex contrast enhancement slope over the maximum of the arterial input function determined in the aorta.

RESULTS

Reproducible arterial and renal transit curve with excellent contrast to noise ratio were obtained. The MRI derived perfusion was systematically underestimated by comparison to the ultrasonic transit-timed flow-probe but was linearly correlated with these measures (r = 0.80, P < 0.001).

CONCLUSIONS

Using a flow-sensitive calibration, an accurate arterial input function can be measured from the blood MR signal and used in a realistic model to assess the RP. There was a good correlation between the MR-derived RP and the renal artery blood flow measured by the flow-meter. This experimental study validates absolute RP quantification by MRI and contrast media injection and justifies further clinical studies.

MR techniques for renal imaging

This article describes the principles, attributes, and pitfalls of the many MR imaging approaches available for assessment of renal-related disorders. Tables 1 and 2 summarize the specific approach and rationale.