Renal artery stenosis: grading of hemodynamic changes with cine phase-contrast MR blood flow measurements

Abdominal Magnetic Resonance Angiography

MR imaging hardware and software improvements have led to new applications for contrast-enhanced and noncontrast-enhanced magnetic resonance angiography in the abdomen and pelvis. Higher magnetic field strength MR imaging scanners have greater signal-to-noise ratio and contrast-to-noise ratio, which is used to improve spatial resolution or temporal resolution for these techniques. New noncontrast-enhanced sequences offer high-resolution magnetic resonance angiography without contrast and provide additional hemodynamic information. Magnetic resonance angiography is particularly well suited to imaging patients with chronic mesenteric ischemia, renal vascular disease, pelvic congestion syndrome, and vascular malformations.

4D flow imaging with UNFOLD in a reduced FOV

PURPOSE

Two-dimensional selective excitation (2DRF) allows shortening 4D flow scan times by reducing the FOV, but the longer 2DRF pulse duration decreases the temporal resolution, yielding underestimated peak flow values. Multiple k-space lines per cardiac phase, n_l ≥ 2, are commonly applied in 4D flow MRI to shorten the inherent long scan times. We demonstrate that 2DRF 4D flow with n_l ≥ 2 can be easily combined with UNFOLD (UNaliasing by Fourier-encoding the Overlaps using the temporaL Dimension), a technique that allows regaining nominally the temporal resolution of the respective acquisition with n_l = 1, to assure peak flow quantification.

METHODS

Two different 2DRF pulses with spiral k-space trajectories were designed and integrated into a 4D flow sequence. Flow phantom experiments and 7 healthy control 4D flow in vivo measurements, with and without UNFOLD reconstructions, were compared with conventional reconstruction and 1D slab-selective excitation (1DRF) by evaluating time-resolved flow curves, peak flow, peak velocity, blood flow volume per cardiac cycle, and spatial aliasing.

RESULTS

Applying UNFOLD to 4D flow imaging with 2DRF and reduced FOV increased the quantified in vivo peak flow values significantly by 3.7% ± 2.3% to 5.2% ± 2.4% (P < .05). Accordingly, the peak flow underestimation of 2DRF scans compared with conventional 1DRF scans decreased with UNFOLD. Finally, 2DRF combined with UNFOLD accelerated the 4D flow acquisition 3.5 ± 1.4 fold by reducing the FOV and increasing the effective temporal resolution by 6.7% compared with conventional 1D selective excitation, with 2 k-space lines per cardiac phase.

CONCLUSION

Two-dimensional selective excitation combined with UNFOLD allows limiting the FOV to shorten 4D flow scan times and compensates for the loss in temporal resolution with 2DRF (Δt = 64.8 ms) compared with 1DRF (Δt = 43.2 ms), yielding an effective resolution of Δt_eff = 40.5 ms to enhance peak flow quantification.

Magnetization Transfer Imaging Is Unaffected by Decreases in Renal Perfusion in Swine

OBJECTIVES

Multiparametric renal magnetic resonance imaging (MRI), including diffusion-weighted imaging, magnetic resonance elastography, and magnetization transfer imaging (MTI), is valuable in the noninvasive assessment of renal fibrosis. However, hemodynamic changes in diseased kidneys may impede their ability to measure renal fibrosis. Because MTI assesses directly tissue content of macromolecules, we test the hypothesis that MTI would be insensitive to renal hemodynamic changes in swine kidneys with acute graded ischemia.

MATERIALS AND METHODS

Seven domestic pigs underwent placement of an inflatable silicone cuff around the right renal artery to induce graded renal ischemia. Multiparametric MRI was performed at baseline, 50%, 75%, and 100% renal artery stenosis as well as reperfusion. Measurements included regional perfusion, R2*, apparent diffusion coefficient (ADC), stiffness, and magnetization transfer ratio (MTR) using arterial spin-labeled MRI, blood oxygenation-dependent MRI, diffusion-weighted imaging, magnetic resonance elastography, and MTI, respectively. Histology was performed to rule out renal fibrosis.

RESULTS

During graded ischemia, decreases in renal perfusion were accompanied with elevated R2*, decreased ADC, and stiffness, whereas no statistically significant changes were observed in the MTR. No fibrosis was detected by histology. After release of the obstruction, renal perfusion showed only partial recovery, associated with return of kidney R2*, ADC, and stiffness to baseline levels, whereas cortical MTR decreased slightly.

CONCLUSIONS

Renal MTI is insensitive to decreases in renal perfusion and may offer reliable assessment of renal structural changes.

Technical recommendations for clinical translation of renal MRI: a consensus project of the Cooperation in Science and Technology Action PARENCHIMA

Purpose

The potential of renal MRI biomarkers has been increasingly recognised, but clinical translation requires more standardisation. The PARENCHIMA consensus project aims to develop and apply a process for generating technical recommendations on renal MRI.

Methods

A task force was formed in July 2018 focused on five methods. A draft process for attaining consensus was distributed publicly for consultation and finalised at an open meeting (Prague, October 2018). Four expert panels completed surveys between October 2018 and March 2019, discussed results and refined the surveys at a face-to-face meeting (Aarhus, March 2019) and completed a second round (May 2019).

Results

A seven-stage process was defined: (1) formation of expert panels; (2) definition of the context of use; (3) literature review; (4) collection and comparison of MRI protocols; (5) consensus generation by an approximate Delphi method; (6) reporting of results in vendor-neutral and vendor-specific terms; (7) ongoing review and updating. Application of the process resulted in 166 consensus statements.

Conclusion

The process generated meaningful technical recommendations across very different MRI methods, while allowing for improvement and refinement as open issues are resolved. The results are likely to be widely supported by the renal MRI community and thereby promote more harmonisation.

Phase-contrast magnetic resonance imaging to assess renal perfusion: a systematic review and statement paper

Objective

Phase-contrast magnetic resonance imaging (PC-MRI) is a non-invasive method used to compute blood flow velocity and volume. This systematic review aims to discuss the current status of renal PC-MRI and provide practical recommendations which could inform future clinical studies and its adoption in clinical practice.

Methodology

A comprehensive search of all the PC-MRI studies in human healthy subjects or patients related to the kidneys was performed.

Results

A total of 39 studies were included in which PC-MRI was used to measure renal blood flow (RBF) alongside other derivative hemodynamic parameters. PC-MRI generally showed good correlation with gold standard methods of RBF measurement, both in vitro and in vivo, and good reproducibility. Despite PC-MRI not being routinely used in clinical practice, there are several clinical studies showing its potential to support diagnosis and monitoring of renal diseases, in particular renovascular disease, chronic kidney disease and autosomal dominant polycystic kidney disease.

Discussion

Renal PC-MRI shows promise as a non-invasive technique to reliably measure RBF, both in healthy volunteers and in patients with renal disease. Future multicentric studies are needed to provide definitive normative ranges and to demonstrate the clinical potential of PC-MRI, likely as part of a multi-parametric renal MRI protocol.

Electronic supplementary material

The online version of this article (10.1007/s10334-019-00772-0) contains supplementary material, which is available to authorized users.

Magnetization Transfer Magnetic Resonance Imaging Noninvasively Detects Renal Fibrosis in Swine Atherosclerotic Renal Artery Stenosis at 3.0 T

OBJECTIVES

Renal fibrosis is a useful biomarker for diagnosis and evaluation of therapeutic interventions of renal diseases but often requires invasive testing. Magnetization transfer magnetic resonance imaging (MT-MRI), which evaluates the presence of macromolecules, offers a noninvasive tool to probe renal fibrosis in murine renal artery stenosis (RAS) at 16.4 T. In this study, we aimed to identify appropriate imaging parameters for collagen detection at 3.0 T MRI and to test the utility of MT-MRI in measuring renal fibrosis in a swine model of atherosclerotic RAS (ARAS).

MATERIALS AND METHODS

To select the appropriate offset frequency, an MT-MRI study was performed on a phantom containing 0% to 40% collagen I and III with offset frequencies from -1600 to +1600 Hz and other MT parameters empirically set as pulse width at 16 milliseconds and flip angle at 800 degrees. Then selected MT parameters were used in vivo on pigs 12 weeks after sham (n = 8) or RAS (n = 10) surgeries. The ARAS pigs were fed with high-cholesterol diet to induce atherosclerosis. The MT ratio (MTR) was compared with ex vivo renal fibrosis measured using Sirius-red staining.

RESULTS

Offset frequencies at 600 and 1000 Hz were selected for collagen detection without direct saturation of free water signal, and subsequently applied in vivo. The ARAS kidneys showed mild cortical and medullary fibrosis by Sirius-red staining. The cortical and medullary MTRs at 600 and 1000 Hz were both increased. Renal fibrosis measured ex vivo showed good linear correlations with MTR at 600 (cortex: Pearson correlation coefficient r = 0.87, P < 0.001; medulla: r = 0.70, P = 0.001) and 1000 Hz (cortex: r = 0.75, P < 0.001; medulla: r = 0.83, P < 0.001).

CONCLUSIONS

Magnetization transfer magnetic resonance imaging can noninvasively detect renal fibrosis in the stenotic swine kidney at 3.0 T. Therefore, MT-MRI may potentially be clinically applicable and useful for detection and monitoring of renal pathology in subjects with RAS.

Simulation of phase contrast angiography for renal arterial models

BACKGROUND

With the development of versatile magnetic resonance acquisition techniques there arises a need for more advanced imaging simulation tools to enable adequate image appearance prediction, measurement sequence design and testing thereof. Recently, there is a growing interest in phase contrast angiography (PCA) sequence due to the capabilities of blood flow quantification that it offers. Moreover, as it is a non-contrast enhanced protocol, it has become an attractive option in areas, where usage of invasive contrast agents is not indifferent for the imaged tissue. Monitoring of the kidney function is an example of such an application.

RESULTS

We present a computer framework for simulation of the PCA protocol, both conventional and accelerated with echo-planar imaging (EPI) readout, and its application to the numerical models of kidney vasculatures. Eight patient-specific renal arterial trees were reconstructed following vessel segmentation in real computed tomography angiograms. In addition, a synthetic model was designed using a vascular tree growth simulation algorithm. The results embrace a series of synthetic PCA images of the renal arterial trees giving insight into the image formation and quantification of kidney hemodynamics.

CONCLUSIONS

The designed simulation framework enables quantification of the PCA measurement error in relation to ground-truth flow velocity data. The mean velocity measurement error for the reconstructed renal arterial trees range from 1.5 to 12.8% of the aliasing velocity value, depending on image resolution and flip angle. No statistically significant difference was observed between measurements obtained using EPI with a number of echos (NETL) = 4 and conventional PCA. In case of higher NETL factors peak velocity values can be underestimated up to 34%.

Unenhanced renal magnetic resonance angiography in patients with chronic kidney disease & suspected renovascular hypertension: Can it affect patient management?

Background & objectives:

Renal artery stenosis (RAS) is an important cause of severe hypertension in patients with chronic kidney disease (CKD). It is important to detect RAS early as it can reverse hypertension and stop rapid deterioration of renal function. The potential drawbacks of various imaging modalities used to detect RAS including contrast-related adverse effects associated with diagnostic angiography have led to increasing interest in unenhanced magnetic resonance (MR) renal angiography. The aim of this study was to detect and grade RAS in patients with CKD and suspected renovascular hypertension using unenhanced MR angiography (UMRA) and to identify patients with significant RAS (>70%) who would subsequently require further investigation and revascularization.

Methods:

Thirty five CKD patients with suspected RAS were subjected to UMRA using non-contrast MR angiography of ArTery and VEins 3D True fast imaging with steady state precession technique over a three year period. Patients with RAS >70 per cent on UMRA were subjected to digital subtraction angiography (DSA) with intervention if indicated.

Results:

In all, 76 renal arteries were evaluated using UMRA in 35 patients, of which 18 arteries showed stenosis and 11 were haemodynamically significant (eight patients). Seven patients (10 renal arteries) underwent DSA.

Interpretation & conclusions:

An association between UMRA and DSA findings was obtained in six patients (nine renal arteries), and these patients were stented. Post-procedure follow up showed good improvement in blood pressure and renal function. UMRA was found to be a useful non-invasive imaging modality to detect RAS in CKD patients. It can identify patients who require further invasive angiography and revascularization.

Phase-contrast MRI evaluation of haemodynamic changes induces by a coeliac axis stenosis in the gastroduodenal artery

OBJECTIVE

To evaluate the correlation between the gastroduodenal artery (GDA) haemodynamic changes and the degree of coeliac axis (CA) stenosis using phase-contrast MRI.

METHODS

The study was institutional review board approved, and written informed consent was obtained from patients included prospectively. A two-dimensional phase-contrast MRI was performed in 23 patients scheduled for a potential complex supramesocolic surgery, in a plane perpendicular to the GDA, during inspiration and expiration. The peak and mean velocities (V_p and V_m), mean flow rate (Q_m) and flow direction at inspiration and at expiration have been correlated with the degree of CA stenosis evaluated by CT.

RESULTS

13 of 23 patients presented CA stenosis due to the median arcuate ligament (34-80% of stenosis), 4 of them had associated atheromatous calcifications. V_p, V_m and Q_m of GDA presented a significant and linear relationship with the degree of CA stenosis, at inspiration as well as at expiration (r > 0.74, p < 0.001). Velocities and flow rates presented a different pattern depending on the degree of CA stenosis: <34% (n = 10), flow direction remained physiological with low velocities and flow rates; >60% of stenosis (n = 5), a reverse flow direction with increased velocities and flow rates were observed; variable patterns between 34% and 60%.

CONCLUSION

Phase-contrast MRI permits the evaluation of haemodynamic changes in GDA induced by CA stenosis, including median arcuate ligament compression, and could be of great interest in therapeutic decision making in supramesocolic surgery, such as liver transplantation or duodenopancreatectomy, by detecting haemodynamically significant stenoses. Advances in knowledge: Physiological phase-contrast MRI detects haemodynamically significant stenoses of the CA by evaluating haemodynamic parameters in the GDA. Physiological phase-contrast MRI demonstrates that flow in the GDA can be a direct marker of the real haemodynamic impact of a CA stenosis on the hepatic vascularization and could thus participate in the pre-surgical work-up of duodenopancreatectomy or hepatic graft, whereas existing pre-operative imaging are only morphological.

Assessment of renal artery stenosis using intravoxel incoherent motion diffusion-weighted magnetic resonance imaging analysis

OBJECTIVES

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).

MATERIALS AND METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

4D flow MRI

Traditionally, magnetic resonance imaging (MRI) of flow using phase contrast (PC) methods is accomplished using methods that resolve single-directional flow in two spatial dimensions (2D) of an individual slice. More recently, three-dimensional (3D) spatial encoding combined with three-directional velocity-encoded phase contrast MRI (here termed 4D flow MRI) has drawn increased attention. 4D flow MRI offers the ability to measure and to visualize the temporal evolution of complex blood flow patterns within an acquired 3D volume. Various methodological improvements permit the acquisition of 4D flow MRI data encompassing individual vascular structures and entire vascular territories such as the heart, the adjacent aorta, the carotid arteries, abdominal, or peripheral vessels within reasonable scan times. To subsequently analyze the flow data by quantitative means and visualization of complex, three-directional blood flow patterns, various tools have been proposed. This review intends to introduce currently used 4D flow MRI methods, including Cartesian and radial data acquisition, approaches for accelerated data acquisition, cardiac gating, and respiration control. Based on these developments, an overview is provided over the potential this new imaging technique has in different parts of the body from the head to the peripheral arteries.

Combined renal MRA and perfusion with a single dose of contrast

Both anatomical and functional scans are often performed when diagnosing renovascular diseases, which in many cases require two separate contrast injections. With nephrogenic systemic fibrosis being associated with gadolinium, minimizing contrast injection dosage is desirable. In this study, a technique which performs time-resolved renal magnetic resonance angiography (MRA) and perfusion with a single scan and single dose of contrast has been evaluated in six healthy volunteers. A previously developed three-dimensional MRA technique called Contrast-enhanced Angiography with Multi-Echo and Radial k-space (CAMERA) has been used to acquire images, and perfusion analysis was performed using deconvolution methods. Time-resolved MRA, as well as renal blood flow, renal volume of distribution and mean transit time maps, were acquired.

Renal arteries: isotropic, high-spatial-resolution, unenhanced MR angiography with three-dimensional radial phase contrast

PURPOSE

To prospectively compare a new three-dimensional (3D) radial phase-contrast magnetic resonance (MR) angiographic method with contrast material-enhanced MR angiography for anatomic assessment of the renal arteries.

MATERIALS AND METHODS

An institutional review board approved this prospective HIPAA-compliant study. Informed consent was obtained. Twenty-seven subjects (mean age, 52.6 years ± 20.5 [standard deviation]) were imaged with respiratory-gated phase-contrast vastly undersampled isotropic projection reconstruction (VIPR) prior to contrast-enhanced MR angiographic acquisition with a 3.0-T clinical system. The imaging duration for phase-contrast VIPR was 10 minutes and provided magnitude and complex difference ("angiographic") images with 3D volumetric (320 mm) coverage and isotropic high spatial resolution (1.25 mm(3)). Quantitative analysis consisted of comparing vessel diameters between the two techniques. Qualitative assessment included evaluation of the phase-contrast VIPR and contrast-enhanced MR angiographic techniques for artifacts, noise, and image quality. Bland-Altman analysis was used for comparison of quantitative measurements, and the Wilcoxon signed rank test was used for comparison of qualitative scores.

RESULTS

Phase-contrast VIPR images were successfully acquired in all subjects. The vessel diameters measured with phase-contrast VIPR were slightly greater than those measured with contrast-enhanced MR angiography (mean bias = 0.09 mm). Differences in mean artifact, quality scores for the proximal renal arteries, and overall image quality scores between phase-contrast VIPR and contrast-enhanced MR angiographic techniques were not statistically significant (P = .31 and .29, .27 and .39, and .43 and .69 for readers 1 and 2, respectively). The quality scores for the segmental renal arteries were higher for phase-contrast VIPR than for contrast-enhanced MR angiography (P < .05). Although the noise scores were higher with phase-contrast VIPR than with contrast-enhanced MR angiography and were statistically significant (P < .05), the presence of noise did not interfere with the ability to interpret the images.

CONCLUSION

Isotropic, high-spatial-resolution, unenhanced MR angiography of the renal arteries is feasible with 3D radial undersampling.

Renal arterial blood flow measurement by breath-held MRI: Accuracy in phantom scans and reproducibility in healthy subjects

This study evaluates reliability of current technology for measurement of renal arterial blood flow by breath-held velocity-encoded MRI. Overall accuracy was determined by comparing MRI measurements with known flow in controlled-flow-loop phantom studies. Measurements using prospective and retrospective gating methods were compared in phantom studies with pulsatile flow, not revealing significant differences. Phantom study results showed good accuracy, with deviations from true flow consistently below 13% for vessel diameters 3mm and above. Reproducibility in human subjects was evaluated by repeated studies in six healthy control subjects, comparing immediate repetition of the scan, repetition of the scan plane scouting, and week-to-week variation in repeated studies. The standard deviation in the 4-week protocol of repeated in vivo measurements of single-kidney renal flow in normal subjects was 59.7 mL/min, corresponding with an average coefficient of variation of 10.55%. Comparison of renal arterial blood flow reproducibility with and without gadolinium contrast showed no significant differences in mean or standard deviation. A breakdown among error components showed corresponding marginal standard deviations (coefficients of variation) 23.8 mL/min (4.21%) for immediate repetition of the breath-held flow scan, 39.13 mL/min (6.90%) for repeated plane scouting, and 40.76 mL/min (7.20%) for weekly fluctuations in renal blood flow.

Renovascular imaging in the NSF Era

The detection of the association between nephrogenic systemic fibrosis (NSF), a rare but potentially life-threatening disease only encountered in patients with severely impaired renal function, and the previous administration of some Gd-chelates has cast a shadow on the administration of Gd-chelates in patients with chronic renal failure. So far, contrast-enhanced MR-angiography (MRA) was considered the best diagnostic modality in patients with suspected renal disease. This review explores the most appropriate use of renal MRA with a focus on newly developed nonenhanced MRA techniques. Nonenhanced MRA techniques mainly based on SSFP with ECG-gating allow for acceptable spatial resolution to visualize at least the proximal parts of the renal arteries. In addition functional renal imaging techniques and their current clinical role are critically appreciated. J. Magn. Reson. Imaging 2009;30:1323-1334. (c) 2009 Wiley-Liss, Inc.

[Analysis of flow in artificial stenosis models of mid-sized arteries using 3D PC-MRI]

Phase contrast MRI allows access to tri-directional encoded velocity information and therefore, measurement of flow in the human hemodynamic system. The aim of this work was to investigate whether this technology could be applied to support the grading of stenosis in mid-size arteries. Using a specially constructed flow phantom and a stenosis model with tube diameter of 5mm and 8mm and a stenosis of 50%, experiments at different flow rates (180-640 ml/min), slice thickness (1-4 mm), field strength (1.5 and 3.0 T), and multi-slice as well as 3D volume acquisition were performed. The observations were assessed visually and evaluated by signal-to-noise (SNR) ratios in regions before and after the stenosis. The obtained results show that examinations should be performed at high field (3.0 T) and at flow rates up to 500 ml/min without hampering the measurements by areas of signal loss. In comparison, no detectable differences in the flow patterns of the two acquisition schemes could be observed. However, the SNR was higher using the 3D volume acquisition and thick slices. In summary, 3D PC-MRI of mid-size vessels with stenosis is feasible for certain flow rates. The presented results could be seen as guidance for in vivo situations to assess if an examination of a patient is reasonable in terms of outcome.

Non-contrast-enhanced MR imaging of renal artery stenosis at 1.5 tesla

Balanced steady-state free precession (Bal-SSFP) techniques produce excellent anatomic images of renal arteries without the use of contrast agents and are relatively flow-insensitive. Electrocardiography (ECG)-triggered and non-ECG-triggered sequences have been shown to be quite sensitive for detection of regional arterial stenosis (RAS), and the already high specificity is likely to increase with further refinement of the techniques. Bal-SSFP sequences can be used as a screening tool or as an alternative to contrast-enhanced (CE) magnetic resonance angiography (MRA) when contrast agents are contraindicated. In addition to morphologic imaging of RAS, non-CE techniques can be used in functional assessment of hemodynamic significance. The complimentary tools can be used alone or in combination with CE-MRA for MR imaging of renal vascular hypertension.

Advances in contrast-enhanced MR angiography of the renal arteries

Renal artery stenosis (RAS) is a potentially curable cause of renovascular hypertension (RVH) and is caused by either atherosclerosis or fibromuscular dysplasia in the vast majority of patients. Although intra-arterial digital subtraction angiography is still considered the standard of reference test for the anatomic diagnosis of RAS, MR angiography and functional renal MR imaging are promising alternatives that also allow for functional characterization of RAS. This article provides an overview of these techniques and discusses their relative merits and shortcomings. Because missing RVH may have serious consequences the most important requirement for an alternative test is that it has high sensitivity. An unresolved issue is the prediction of functional recovery after therapy.

Determining exercise-induced blood flow reserve in lower extremities using phase contrast MRI

PURPOSE

To study the changes in limb blood flow during lower extremity exercise using phase contrast (PC) MRI in normal volunteers.

MATERIALS AND METHODS

Healthy volunteers performed plantar flexion exercise (<1 W) for four minutes. Flow velocity was measured using cardiac-gated, cine PC-MRI sequences (fast gradient recalled echo [GRE]; multishot echo planar imaging [EPI]) on a 3T scanner at the level of the superficial femoral artery (SFA): 1) preexercise; 2) immediately postexercise; 3) during three minutes recovery; and 4) postrecovery.

RESULTS

At rest there was a triphasic flow waveform in the SFA. During exercise it changed to a monophasic pattern with an increase in total flow; there were variable changes in vessel size and flow velocity. The waveform regained the triphasic pattern during recovery. The exercise-induced flow reserve (FR) was 167 +/- 90%.

CONCLUSION

PC-MRI demonstrates that the resting triphasic flow waveform transforms into a monophasic pattern with submaximal exercise and returns to baseline with recovery. This increase in the regional blood flow allows for measurement of exercise-induced FR in the SFA.

Nonenhanced MR angiography

While nonenhanced magnetic resonance (MR) angiographic methods have been available since the earliest days of MR imaging, prolonged acquisition times and image artifacts have generally limited their use in favor of gadolinium-enhanced MR angiographic techniques. However, the combination of recent technical advances and new concerns about the safety of gadolinium-based contrast agents has spurred a resurgence of interest in methods that do not require exogenous contrast material. After a review of basic considerations in vascular imaging, the established methods for nonenhanced MR angiographic techniques, such as time of flight and phase contrast, are considered and their advantages and disadvantages are discussed. This article then focuses on new techniques that are becoming commercially available, such as electrocardiographically gated partial-Fourier fast spin-echo methods and balanced steady-state free precession imaging both with and without arterial spin labeling. Challenges facing these methods and possible solutions are considered. Since different imaging techniques rely on different mechanisms of image contrast, recommendations are offered for which strategies may work best for specific angiographic applications. Developments on the horizon include techniques that provide time-resolved imaging for assessment of flow dynamics by using nonenhanced approaches.

Renal Artery Stenosis in Swine: Feasibility of MR Assessment of Renal Function during Percutaneous Transluminal Angioplasty

PURPOSE

To prospectively test--in a swine model of renal artery stenosis (RAS)--the hypothesis that magnetic resonance (MR) imaging can reveal changes in renal function at the time of percutaneous transluminal angioplasty (PTA).

MATERIALS AND METHODS

In this animal care and use committee-approved study, high-grade unilateral RAS was surgically induced in six pigs. MR imaging at 3.0 T was used for intraprocedural assessment of the anatomic and physiologic changes induced by x-ray-guided PTA. With use of MR imaging, changes in single-kidney glomerular filtration rate, extraction fraction, and renal blood flow were assessed during PTA. The arterial diameter of stenosis before and after PTA was assessed by using conventional digital subtraction angiography. Mean changes in functional and anatomic parameters were compared by using the Wilcoxon signed rank test (alpha = .05).

RESULTS

At digital subtraction angiography, the mean percentage of stenosis was 69% +/- 10 (standard deviation) before PTA and 26% +/- 10 after PTA (P<.03). Mean pre- and post-PTA extraction fraction values were 0.11 +/- 0.03 and 0.19 +/- 0.06, respectively (P<.03). The mean single-kidney glomerular filtration rate before PTA, 19 mL/min +/- 13, increased to 41 mL/min +/- 33 after PTA (P<.03). There was no significant change in mean renal blood flow after PTA (P=.44).

CONCLUSION

In swine, MR imaging can reveal changes in renal function after x-ray-guided PTA for unilateral RAS.

Visualization of iliac and proximal femoral artery hemodynamics using time-resolved 3D phase contrast MRI at 3T

PURPOSE

To demonstrate the feasibility of time-resolved 3D MR velocity mapping at 3 Tesla for the visualization of vascular hemodynamics in normal iliac and femoral arteries.

MATERIALS AND METHODS

Electrocardiographically (ECG) synchronized three-dimensional (3D) CINE phase-contrast MRI with three-directional flow encoding was adapted to analyze flow in peripheral arteries at 3T. Visualization of peripheral arterial hemodynamics within the acquired data volume included 3D streamlines and time-resolved 3D particle traces within the major vessels and localized analysis of flow profiles using 2D-vector graphs. Data was visually compared to results from color-coded duplex ultrasound (US).

RESULTS

Global and detailed local blood flow characteristics were successfully analyzed in all subjects. In agreement with US findings, normal laminar flow patterns without flow acceleration or disturbances were visualized in all healthy individuals. In an exemplary patient measurement multiple segmental flow accelerations could be demonstrated. MRI additionally revealed complex helical flow alterations distal to a moderate stenosis.

CONCLUSION

Due to the full spatial and temporal coverage of the arteries of interest, 3D CINE phase contrast MRI at 3T is a promising tool for the evaluation of vascular hemodynamics in peripheral arteries. Future methodological improvements will be directed to improve spatial and temporal resolution as well as quantitative data analysis. Moreover, the technique will have to be evaluated in patients in comparison to standard diagnostic tools.

Renal magnetic resonance angiography at 3.0 T: technical feasibility and clinical perspectives

The increased signal-to-noise ratio at 3.0 T holds promise for high-spatial resolution renal magnetic resonance angiography (MRA). Today, state-of-the-art renal MRA is feasible with submillimeter isotropic spatial resolution in less than 20 seconds acquisition time with sufficient signal-to-noise ratio. This article explains the fundamentals of 3.0-T imaging that are essential for renal MRA, with a focus on the clinical implications. Protocol and imaging recommendations are given based on the physical principles of 3.0-T imaging and underlined by current clinical cases. Apart from pure morphological imaging, the value of functional renal imaging such as renal flow measurements and renal perfusion measurements for a comprehensive 3.0-T renal MRA protocol is discussed.

Cine flow measurements using phase contrast with undersampled projections: in vitro validation and preliminary results in vivo

PURPOSE

To assess the accuracy of flow measurements in vitro and in vivo during scan times shorter than a breath-hold using a 2D cine phase contrast (PC) undersampled radial acquisition method, which may be useful for measuring flow, especially in vessels subject to motion during respiration.

MATERIALS AND METHODS

For in vitro assessment, a flow phantom was imaged at various flow rates and undersampling factors. For in vivo assessment, five normal subjects were imaged and the flow rate in the aorta was compared with the sum of the flow rates in the iliac arteries.

RESULTS

For results in vitro, the accuracy of flow measurements was maintained with scan times as low as 13-17 seconds. For results in vivo, scans acquired in less than 25 seconds provided flow measurements in the aorta that corresponded well to the sum of flow measurements in the iliac arteries.

CONCLUSION

The undersampled radial acquisition cine PC technique provided accurate flow measurements in a flow phantom and in healthy human volunteers in scan times shorter than a typical breath-hold.

Functional magnetic resonance imaging in renal artery stenosis

Renal artery stenosis (RAS) is the leading cause of secondary hypertension. Magnetic resonance (MR) imaging and in particular MR angiography have evolved into important diagnostic tools for the detection and grading of RAS due to the lack of ionizing radiation and nephrotoxic contrast agent. This review describes state-of-the-art MR angiographic techniques and introduces the reader to current concepts of RAS grading with MR angiography. We compare MR angiography with conventional angiography and intravascular ultrasound as a standard of reference. The technical basis of functional imaging techniques such as arterial spin labeling perfusion measurements, contrast-enhanced perfusion measurements, and MR flow measurements are explained. Their value for the grading and detection of RAS and for the differentiation of renovascular from renal parenchymal disease is discussed. An overview about imaging during and after interventional therapy of RAS and an introduction to the current understanding of prediction of successful interventional therapy finishes this review.

Comparison of intraarterial MR angiography at 3.0 T with X-ray digital subtraction angiography for detection of renal artery stenosis in swine

PURPOSE

To compare the accuracy of catheter-directed intraarterial (IA) magnetic resonance (MR) angiography at 3.0 T with that of x-ray digital subtraction angiography (DSA) for the measurement of renal artery stenosis (RAS) in swine.

MATERIALS AND METHODS

Unilateral hemodynamically significant RAS (>50%) was induced surgically in six pigs with use of reverse cable ties. One to two weeks after surgery, each pig underwent x-ray DSA and MR angiography before and after percutaneous transluminal balloon angioplasty (PTA). X-ray DSA was performed before and after PTA of RAS by injection of iodinated contrast agent through a 5-F multiple-side hole angiographic catheter placed in the abdominal aorta under fluoroscopic guidance. MR angiography of RAS was performed before and after PTA of RAS on a 3.0-T clinical MR imager with use of gadolinium-based contrast agent. MR angiography and DSA images were analyzed with the full width at half maximum method. Percent stenosis measurements between x-ray DSA and MR angiography were compared with a paired t test and were correlated with linear regression and Bland Altman analysis (alpha = 0.05).

RESULTS

Six cases of RAS were induced and imaged successfully with DSA and MR angiography techniques before and after PTA. On x-ray DSA, median stenoses was 64% (95% CI 57%-80%) before PTA and 20% (95% CI 5%-32%) after PTA. Corresponding MR angiography median stenosis measurement was 69% (95% CI 58%-80%) before PTA and 26% (95% CI 16%-36%) after PTA. A paired t test comparison did not show a difference between DSA and MR angiography (P = .16). RAS measurements on MR angiography correlated closely (P < .01) with DSA measurements (r(2) = 0.92).

CONCLUSION

In swine, the accuracy of catheter-directed IA MR angiography with use of a clinical 3.0-T MR imaging unit for the measurement of RAS was similar to that of conventional x-ray DSA.

Technology insight: Clinical role of magnetic resonance angiography in the diagnosis and management of renal artery stenosis

Noninvasive modalities, including duplex ultrasonography, renal scintigraphy, CT angiography and magnetic resonance angiography (MRA), may usefully contribute to diagnosis and treatment planning in patients with suspected renal artery stenosis. Important technical developments have increased the accuracy and feasibility of MRA for the detection of renal artery stenosis. A number of different MRA techniques can be applied to the study of renal arteries, but contrast-enhanced MRA represents the most valuable approach; several studies corroborate the high diagnostic accuracy of this technique, especially for the detection of atherosclerotic renal artery stenosis. A combined MRA protocol, which might include angiographic information provided by contrast-enhanced technique in addition to renal flow information derived from phase-contrast imaging, could help in classifying patients appropriately. Limitations of renal MRA include low accuracy in the evaluation of renal fibromuscular dysplasia and in the assessment of patients who undergo stenting of the renal arteries. This review describes the MRA techniques applied to the study of renal artery stenosis, including the technical features of current approaches and forthcoming developments. An overview of the clinical role of MRA, in conjunction with the other diagnostic modalities, in the identification and management of patients with renal artery stenosis, is also presented.

Angiografia por ressonância magnética na avaliação das artérias renais: achados de imagem

OBJETIVO: Descrever as indicações, os principais diagnósticos e os achados de imagem nas angiografias por ressonância magnética das artérias renais. MATERIAIS E MÉTODOS: Estudo retrospectivo, no período de 6/12/2001 a 11/3/2004, num total de 56 exames, totalizando 111 artérias renais estudadas. Os exames foram realizados em um equipamento de 1,5 tesla, segundo o protocolo do Serviço. RESULTADOS: Foi demonstrado que 55,4% (n = 31) pacientes eram masculinos e 44,6% (n = 25), femininos. O paciente mais novo tinha 12 anos e o mais velho, 88 anos. De um total de 25 diferentes indicações, a hipertensão arterial sistêmica com 26,7% (n = 15) foi a principal, seguida de dor abdominal e/ou lombar com 12,5% (n = 7), aneurisma da aorta abdominal com 10,7% (n = 6), estenose da artéria renal com 8,9% (n = 5), e outros. No que se refere às imagens, 43 (76,7%) exames tiveram algum tipo de alteração e 13 (23,2%) foram normais. Dentre os que tinham alterações, a maioria se deu no calibre, e dentre elas, as irregularidades parietais, aneurismas e estenoses foram as mais comuns. Na artéria renal direita as alterações mais comuns foram as irregularidades parietais com 17,87% (n = 10), e na artéria renal esquerda foram as estenoses com 25,45% (n = 14). CONCLUSÃO: A angio-RM mostrou-se excelente método no estudo das artérias renais, devido à sua sensibilidade e capacidade multiplanar para avaliar as estruturas vasculares.

MR Imaging– versus Conventional X-ray Fluoroscopy–guided Renal Angioplasty in Swine: Prospective Randomized Comparison

PURPOSE

To test the hypothesis that the technical success rates, complication rates, and procedural times for magnetic resonance (MR) imaging-guided percutaneous transluminal angioplasty (PTA) and conventional (x-ray) fluoroscopy-guided PTA for treatment of renal artery stenosis are similar.

MATERIALS AND METHODS

The study was animal care and use committee approved. After surgically inducing bilateral renal artery stenosis in 11 swine, the authors performed baseline digital subtraction angiography. They transferred each animal to a 1.5-T MR imaging unit and randomly decided which artery would be treated with MR-guided PTA. With MR imaging guidance, angioplastic devices were tracked by using active and passive techniques. Vascular depiction was achieved by using catheter-directed MR angiography. Stenotic vessels were dilated by using 5-6-mm-diameter balloon catheters. PTA was then performed in the contralateral artery by using conventional fluoroscopy-guided techniques. With the intention to treat, the authors compared the technical success (residual stenosis < 50%) rates, complication rates, and procedural times for each guidance method. They compared technical successes and complications by using the McNemar test and procedural times by using a paired t test, with P < .05 indicating a significant difference.

RESULTS

The authors successfully dilated nine (82%) of 11 renal arteries with MR guidance and all 11 arteries (100%) with conventional fluoroscopic guidance. The difference was not significant (P = .5). Complications occurred in three (27%) arteries with MR guidance and in one (9%) artery with fluoroscopic guidance, with no significant differences (P = .5). The mean MR-guided PTA procedural time was 46 minutes longer than the fluoroscopy-guided PTA procedural time; this difference was significant (P = .01).

CONCLUSION

In a small cohort of swine, the authors did not observe a significant difference between MR imaging- and conventional fluoroscopy-guided renal artery PTA in terms of success and complication rates. However, no evidence of similarity between the techniques should be assumed. Procedural times differed significantly.

Superficial femoral artery occlusive disease severity correlates with MR cine phase-contrast flow measurements

PURPOSE

To evaluate how cine phase-contrast (PC) flow data correlate with the severity of peripheral vascular disease (PVD).

MATERIALS AND METHODS

Flow waveforms were obtained in 48 patients proximal and distal to superficial femoral artery (SFA) disease using the 2D cine PC technique with velocity encoding (venc) = 100 cm/second. Flow data were correlated with SFA disease severity and compared with data from nine healthy volunteers.

RESULTS

Of 96 arterial segments in 48 patients, 26 were patent or only mildly stenotic, 35 had moderate-to-severe stenosis, and 35 were occluded. The flow patterns tended to become low-resistant below severe stenoses or occlusion. The mean peak flow velocity above/below SFA lesions was significantly higher in patients with severe disease (1.9 +/- 1.0, P = 0.01) or occlusion (2.0 +/- 1.0, P = 0.003) compared to normal volunteers (1.4 +/- 0.6). The delay in peak velocity below the lesions showed a significant positive correlation with lesion severity (r = 0.65, P < 0.001). The mean flow volume ratio above/below SFA lesions was greater in patients with occluded vessels compared to normal volunteers (3.9 and 2.3 respectively; P = 0.04).

CONCLUSION

Cine PC flow waveform changes across atherosclerotic lesions correlate with disease severity. This may help determine which lesions are hemodynamically significant.

Renal blood flow measurements with use of phase-contrast magnetic resonance imaging: normal values and reproducibility

PURPOSE

To assess the validity and the direct, short-term, and long-term reproducibility of renal blood flow (RBF) measurements with phase-contrast (PC) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

In 20 healthy volunteers, RBF measurements were repeated with and without repositioning. Internal validity was assessed by comparing the total RBF with the difference in aortic flow above and below the renal arteries. In 19 healthy volunteers, RBF measurements were performed at two different occasions. In 40 healthy volunteers, RBF measurements were performed to assess normal values as a function of age. Analyses were performed according to Bland and Altman.

RESULTS

The technical success rate ranged from 78% to 85%. Total RBF and the difference in aortic flow rates showed good agreement (Pearson correlation coefficient, 0.72; P = .002). Directly repeated measurements had a mean difference of 54 mL/min in total RBF with a coefficient of variation (CV) of 17%. For repeated measurements with repositioning, the mean difference in total RBF was 74 mL/min (CV, 23%). Repeated measurements on different occasions showed a CV of 20%. The mean total RBF of the 40 healthy volunteers was 838 mL/min +/- 244 (SD).

CONCLUSIONS

RBF measurement with PC MR has a success rate greater than 75%. The demonstrated internal reliability of this method and fair reproducibility of the flow parameters is crucial for further studies of the renal artery with MR imaging.

Applications of phase-contrast flow and velocity imaging in cardiovascular MRI

A review of cardiovascular clinical and research applications of MRI phase-contrast velocity imaging, also known as velocity mapping or flow imaging. Phase-contrast basic principles, advantages, limitations, common pitfalls and artefacts are described. It can measure many different aspects of the complicated blood flow in the heart and vessels: volume flow (cardiac output, shunt, valve regurgitation), peak blood velocity (for stenosis), patterns and timings of velocity waveforms and flow distributions within heart chambers (abnormal ventricular function) and vessels (pulse-wave velocity, vessel wall disease). The review includes phase-contrast applications in cardiac function, heart valves, congenital heart diseases, major blood vessels, coronary arteries and myocardial wall velocity.

Contemporary imaging techniques for the diagnosis of renal artery stenosis

Renal artery stenosis (RAS) is a potentially curable cause of renovascular hypertension (RVH) and is caused by either atherosclerosis or fibromuscular dysplasia (FMD) in the vast majority of patients. Although intra-arterial digital subtraction angiography (IA-DSA) is still considered the standard of reference test for the anatomical diagnosis of RAS, noninvasive techniques such as MR angiography, CT angiography, and color-aided duplex ultrasonography are promising alternatives that also allow functional characterization of RAS. We provide an overview of these techniques and discuss their relative merits and shortcomings. Analysis of high-quality studies shows that both MR and CT angiography are significantly more accurate for the diagnosis of at least 50% atherosclerotic RAS than ultrasonographic techniques. The primary strength of ultrasonography at present is its suggested ability to predict functional recovery based on preinterventional resistance index measurements. A still unresolved issue is the detection of FMD. Because missing RVH may have serious consequences the most important requirement for a screening test is that it has high sensitivity.

The limitations of magnetic resonance angiography in the diagnosis of renal artery stenosis: Comparative analysis with conventional arteriography

PURPOSE

Gadolinium-enhanced magnetic resonance angiography (MRA) is commonly used as a screening modality for the detection of renal artery stenosis. However, evidence supporting its utility in clinical practice is lacking; few rigorous studies have compared MRA with contrast arteriography (CA). After making anecdotal clinical observations that MRA sometimes overestimated the degree of renal artery stenosis, we decided to determine the interobserver variability, sensitivity, specificity, and diagnostic accuracy of MRA compared with CA.

METHODS

From September 1999 to April 2003, we evaluated 68 renal arteries in 34 patients with clinically suspected renal artery stenosis using both MRA and CA. All studies were independently reviewed by four blinded observers. Renal arteries were categorized by MRA as normal, <50%, and >50% stenosis/occlusion. The sensitivity, specificity, and accuracy of MRA detection of renal artery stenosis were compared to CA as the gold standard. Interobserver variability (kappa) was also calculated.

RESULTS

MRA demonstrated 87% sensitivity, 69% specificity, 85% accuracy, 95% negative predictive value, and 51% positive predictive value for the diagnosis of renal artery stenosis. Interobserver agreement was moderate for MRA (kappa = 0.53) and good for CA (kappa = 0.76). In 21 arteries (31%), MRA was falsely positive.

CONCLUSIONS

In patients with a high clinical suspicion of renal artery stenosis, MRA is 87% sensitive in the detection of >50% stenosis. However, MRA is relatively nonspecific compared with CA and results in significant overestimation of renal artery stenosis in nearly one third of patients. To reduce unnecessary CA, clinicians should consider supplemental studies.

Catheter-directed MR Angiography and Cross-sectional Imaging for the Assessment of Renal Artery Stenosis

PURPOSE

Catheter-directed intraarterial (IA) gadolinium (Gd)-enhanced gradient-echo (GRE) imaging has been used in the setting of magnetic resonance (MR) imaging-guided endovascular procedures for two-dimensional (2D) or three-dimensional (3D) depiction of blood vessels. In a swine model, the hypothesis was tested that the combination of 2D IA GRE and 2D cross-sectional steady-state free precession (SSFP) imaging improves assessment of renal artery stenosis (RAS) compared with 3D IA GRE imaging alone.

MATERIALS AND METHODS

Bilateral RAS was surgically induced in seven pigs. Detection of stenoses was then compared between the combination of 2D projection IA GRE and cross-sectional 2D SSFP imaging without contrast agent and 3D IA GRE alone. Radiographic digital subtraction angiography (DSA) was employed as the reference standard. Linear regression was used to compare stenosis measurements, with an alpha of 0.05.

RESULTS

Radiographic DSA and MR imaging were successful in the seven animals (14 stenoses). With use of linear regression analysis, the combination of 2D IA GRE and 2D SSFP imaging had a higher r(2) (0.87 vs 0.72) and a slope closer to unity (1.02 vs 0.77) compared with 3D IA GRE imaging alone. When comparing intercepts, the regression line for SSFP significantly differed from that of 3D IA GRE imaging (P < .05).

CONCLUSION

The combination of 2D IA GRE and cross-sectional 2D SSFP imaging improves the accuracy of RAS detection compared with IA 3D IA-GRE alone.

Magnetic resonance angiography in children: technique, indications, and imaging findings

Non-invasive imaging of cardiovascular disease in pediatric subjects has been an elusive and much anticipated development for many years. Magnetic resonance angiography (MRA) has now been established in many institutions as an important diagnostic method for evaluating vascular disease in adults. However, MRA techniques have disseminated more slowly in children owing to significant technical challenges in the pediatric population, including motion, low signal-to-noise ratio, and suboptimal temporal or spatial resolution. Recent technical developments in MRA have addressed many of these issues, and the MRA acquisition methods are far more robust than previously used techniques. The objective of this manuscript is to discuss the indications for MR imaging, the techniques employed, and the imaging findings expected on MRA of children with vascular disease.

Renal Disease: Value of Functional Magnetic Resonance Imaging With Flow and Perfusion Measurements

PURPOSE

To differentiate healthy kidneys from diseased kidneys, we propose a combined magnetic resonance (MR) examination that includes measurements of renal arterial blood flow and parenchymal perfusion.

MATERIALS AND METHODS

A total of 130 kidneys (patients/healthy volunteers: 83/47) were examined using renal artery MR flow measurements and renal parenchymal perfusion measurements, as well as contrast-enhanced MR angiography. Cine phase-contrast-flow measurements were performed using an ECG-gated fast low angle shot pulse sequence; perfusion was measured with an arterial spin labeling flow-sensitive alternating inversion recovery technique. Contrast-enhanced MR angiography was performed with a fast 3D gradient echo sequence in a single breath hold. For evaluation, kidneys were divided into groups based on nephrologic diagnosis of the patient. Recursive partitioning and Wilcoxon rank-sum tests were used to separate the different groups.

RESULTS

Significant differences in mean renal artery flow and parenchymal perfusion were found in kidneys with renal artery stenosis as well as parenchymal disease as compared with healthy kidneys. Using a classification tree derived from the recursive partitioning, a specificity of 99% and sensitivity of 69% with a positive/negative predictive value of 97%/84% was achieved for the separation of healthy kidneys from kidneys with vascular, parenchymal or combined disease. The overall accuracy was 88%.

CONCLUSION

The combination of cine PC flow measurements and MR perfusion measurements offers a comprehensive assessment of both renovascular and renoparenchymal disease and provide a noninvasive approach to differentiate between these kidneys and normal kidneys.

Renal MR angiography

The high accuracy of renal MR angiography makes it well suited for diagnosing renal vascular disease. A comprehensive examination includes three-dimensional gadolinium MR angiography to assess lumenal anatomy and functional techniques to assess the hemodynamic significance of any stenosis identified. Postprocessing is critical to provide reformations, maximum intensity projections, and optional volume-rendered images to display arteries in an angiographic format for optimal demonstration of any vascular lesions. It is important to review source images to avoid missing pathologic findings. As MR imaging continues to develop, the renal MR angiography examination will likely expand to include extensive functional information about creatinine clearance, flow, and response to pharmacologic agents as well as spectroscopy, diffusion, perfusion, phase contrast, and other techniques.

Quand, comment et pourquoi réaliser une exploration des artères rénales ?

The objectives of this course are both: to describe acquisition, injection and reconstruction parameters of volumic images for renal arteries examination and specific signs; to discuss the role of the different images in the diagnosis and in the therapeutic management. Ultrasound is one of the best imaging for the analysis of renal arteries in the detection of stenosis even if the sensitivity is less (around 85%)compared to CT Angiography (95%) and MR Angiography (90%). Because of this advantage and of 3D evaluation, CTA and MRA are sometimes in the first line for renal artery evaluation and can assess morphology before angioplasty. Renal scintigraphy with Captopril test and renin dosage are only used for small kidney evaluation. Arteriogram is systematically followed by angioplasty if possible. With the new endovascular materials, complications decrease (less than 5% with a major reduction in cholesterol emboli) and indications of endoprosthesis increase (71% of stenting with half of it in direct stenting technique). This course will give practical tools for imaging acquisition, specifically 3D imaging, for indications and management of lesion in accordance to symptoms and morphology.