Noninvasive comprehensive characterization of renal artery stenosis by combination of STAR angiography and EPISTAR perfusion imaging

Thrombosis Models: An Overview of Common In Vivo and In Vitro Models of Thrombosis

Occlusions in the blood vessels caused by blood clots, referred to as thrombosis, and the subsequent outcomes are leading causes of morbidity and mortality worldwide. In vitro and in vivo models of thrombosis have advanced our understanding of the complex pathways involved in its development and allowed the evaluation of different therapeutic approaches for its management. This review summarizes different commonly used approaches to induce thrombosis in vivo and in vitro, without detailing the protocols for each technique or the mechanism of thrombus development. For ease of flow, a schematic illustration of the models mentioned in the review is shown below. Considering the number of available approaches, we emphasize the importance of standardizing thrombosis models in research per study aim and application, as different pathophysiological mechanisms are involved in each model, and they exert varying responses to the same carried tests. For the time being, the selection of the appropriate model depends on several factors, including the available settings and research facilities, the aim of the research and its application, and the researchers' experience and ability to perform surgical interventions if needed.

Evaluation of Renal Blood Flow in Chronic Kidney Disease Using Arterial Spin Labeling Perfusion Magnetic Resonance Imaging

Introduction

Chronic kidney disease (CKD) is known to be associated with reduced renal blood flow. However, data in humans are limited to date.

Methods

In this study, noninvasive arterial spin labeling magnetic resonance imaging data were acquired in 33 patients with diabetes and stage 3 CKD as well as in 30 healthy controls.

Results

A significantly lower renal blood flow in both the cortex (108.4 ± 36.4 vs. 207.3 ± 41.8; P < 0.001, d = 2.52) and medulla (23.2 ± 8.9 vs. 42.6 ± 15.8; P < 0.001, d = 1.5) was observed. Both cortical (ρ = 0.67, P < 0.001) and medullary (ρ = 0.62, P < 0.001) blood flow were correlated with estimated glomerular filtration rate, and cortical blood flow was found to be confounded by age and body mass index. However, in a subset of subjects who were matched for age and body mass index (n = 6), the differences between CKD patients and control subjects remained significant in both the cortex (107.4 ± 42.8 vs. 187.51 ± 20.44; P = 0.002) and medulla (15.43 ± 8.43 vs. 39.18 ± 11.13; P = 0.002). A threshold value to separate healthy controls and CKD patients was estimated to be a cortical blood flow of 142.9 and a medullary blood flow of 24.1.

Discussion

These results support the use of arterial spin labeling in the evaluation of renal blood flow in patients with a moderate level of CKD. Whether these measurements can identify patients at risk for progressive CKD requires further longitudinal follow-up.

Flow measurement in MRI using arterial spin labeling with cumulative readout pulses--theory and validation

PURPOSE

This article systematically examines arterial spin labeling (ASL) as a flow quantification technique through theoretical simulation, in vitro, and in vivo experiment. The authors present a novel imaging pulse sequence design consisting of a single ASL magnetization preparation followed by Look-Locker-like image readouts. Bloch-equation-based modeling has been developed and validated using a hemodialyzer as a tissue-mimicking flow phantom.

METHODS

After the single in-plane slice-selective double inversion magnetization preparation, multiple TFL readouts are acquired with linear k-space ordering, causing a signal variation that depends on through-slice flow velocity. Computer simulations were performed to assess the behavior of the flow-dependent ASL signal as a function of varying imaging parameters. The signal was optimized by choosing imaging parameters that maximize the simulated flow-sensitive signal. Furthermore, a hemodialyzer which mimics blood flow in human tissues was tested with a wide range of flow rates. An exponential curve fitting of the flow-sensitive dynamics to the model derived from Bloch equations provides a method to estimate through-slice velocity for varying flow rates on the hemodialyzer and in vivo human brain.

RESULTS

The flow dependency of the ASL signal and the sensitivity of the ASL signal to imaging parameters were demonstrated. Experimental results from a hemodialyzer when fitted with a Bloch-equation-based model provide flow measurements that are consistent with ground truth velocities. Human brain velocity mapping was obtained as well.

CONCLUSIONS

The results provide evidence that the proposed pulse sequence design is an effective technique to measure total fluid flow through image voxels. The unique combination of the two main features, multiple-image readout after a single ASL preparation and linear acquisition ordering in the phase encoding direction in TFL imaging, make this technique an appealing flow imaging method to quantify through-plane flow in a time-efficient manner.

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.

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.

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.

Non-invasive measurement of perfusion: a critical review of arterial spin labelling techniques

The non-invasive nature of arterial spin labelling (ASL) has opened a unique window into human brain function and perfusion physiology. High spatial and temporal resolution makes the technique very appealing not only for the diagnosis of vascular diseases, but also in basic neuroscience where the aim is to develop a more comprehensive picture of the physiological events accompanying neuronal activation. However, low signal-to-noise ratio and the complexity of flow quantification make ASL one of the more demanding disciplines within MRI. In this review, the theoretical background and main implementations of ASL are revisited. In particular, the perfusion quantification methods, including the problems and pitfalls involved, are thoroughly discussed in this article. Finally, a brief summary of applications is provided.

Functional MRI of the kidney: tools for translational studies of pathophysiology of renal disease

Magnetic resonance imaging (MRI) provides exquisite anatomic detail of various organs and is capable of providing additional functional information. This combination allows for comprehensive diagnostic evaluation of pathologies such as ischemic renal disease. Noninvasive MRI techniques could facilitate translation of many studies performed in controlled animal models using technologies that are invasive to humans. Such a translation is being recognized as essential because many proposed interventions and drugs that prove efficacious in animal models fail to do so in humans. In this article, we review the state-of-the-art functional MRI technique as applied to the kidneys.

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.

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.

Comparison of X-ray fluoroscopy and interventional magnetic resonance imaging for the assessment of coronary artery stenoses in swine

The accuracy of a two-step interventional MRI protocol to quantify coronary artery disease was compared to the clinical gold standard, X-ray angiography. Studies were conducted in nine swine with a surgically induced stenosis in the proximal left circumflex coronary artery. The two-step protocol consisted of catheter-directed magnetic resonance angiography (MRA), which was first used to localize the stenosis, followed by MRI cross-sectional images to quantify the degree of stenosis without the use of contrast agent. Line signal intensity profiles were drawn across the vessel diameter at the stenosis site and proximal to the stenosis for each data set to measure percentage stenosis for each animal. Catheter-directed MRA successfully detected eight of nine stenoses. Cross-sectional MRI accurately quantified each stenosis, with strong agreement to the measurements made using X-ray fluoroscopy (intraclass correlation coefficient = 0.955; P < 0.05). This study demonstrates that in the future interventional MRI may be an alternative to X-ray angiography for the detection and quantification of coronary artery disease.

Free-breathing renal magnetic resonance angiography with steady-state free-precession and slab-selective spin inversion combined with radialk-space sampling and water-selective excitation

The impact of radial k-space sampling and water-selective excitation on a novel navigator-gated cardiac-triggered slab-selective inversion prepared 3D steady-state free-precession (SSFP) renal MR angiography (MRA) sequence was investigated. Renal MRA was performed on a 1.5-T MR system using three inversion prepared SSFP approaches: Cartesian (TR/TE: 5.7/2.8 ms, FA: 85 degrees), radial (TR/TE: 5.5/2.7 ms, FA: 85 degrees) SSFP, and radial SSFP combined with water-selective excitation (TR/TE: 9.9/4.9 ms, FA: 85 degrees). Radial data acquisition lead to significantly reduced motion artifacts (P < 0.05). SNR and CNR were best using Cartesian SSFP (P < 0.05). Vessel sharpness and vessel length were comparable in all sequences. The addition of a water-selective excitation could not improve image quality. In conclusion, radial k-space sampling reduces motion artifacts significantly in slab-selective inversion prepared renal MRA, while SNR and CNR are decreased. The addition of water-selective excitation could not improve the lower CNR in radial scanning.

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.

Free-breathing renal MR angiography with steady-state free-precession (SSFP) and slab-selective spin inversion: Initial results

BACKGROUND

The aim of our study was the investigation of a novel navigator-gated three-dimensional (3D) steady-state free-precession (SSFP) sequence for free-breathing renal magnetic resonance angiography (MRA) without contrast medium, and to examine the advantage of an additional inversion prepulse for improved contrast.

METHODS

Eight healthy volunteers (mean age 29 years) and eight patients (mean age 53 years) were investigated on a 1.5 Tesla MR system (ACS-NT, Philips, Best, The Netherlands). Renal MRA was performed using three navigator-gated free-breathing cardiac-triggered 3D SSFP sequences [repetition time (TR) = 4.4 ms, echo time (TE) = 2.2 ms, flip angle 85 degrees, spatial resolution 1.25 x 1.25 x 4.0 mm(3), scanning time approximately 1 minute 30 seconds]. The same sequence was performed without magnetization preparation, with a non-slab selective and a slab-selective inversion prepulse. Signal-to-noise ratio (SNR), contrast-to-noise (CNR) vessel length, and subjective image quality were compared.

RESULTS

Three-dimensional SSFP imaging combined with a slab-selective inversion prepulse enabled selective and high contrast visualization of the renal arteries, including the more distal branches. Standard SSFP imaging without magnetization preparation demonstrated overlay by veins and renal parenchyma. A non-slab-selective prepulse abolished vessel visualization. CNR in SSFP with slab-selective inversion was 43.6 versus 10.6 (SSFP without magnetization preparation) and 0.4 (SSFP with non-slab-selective inversion), P < 0.008.

CONCLUSION

Navigator-gated free-breathing cardiac-triggered 3D SSFP imaging combined with a slab-selective inversion prepulse is a novel, fast renal MRA technique without the need for contrast media.

Functional renal MR imaging

MR imaging is the only noninvasive test that may provide a complete picture of renal status with minimal risk to the patient, simultaneously improving diagnosis and lowering costs. This article reviews several MR renography techniques, including approaches for quantifying renal perfusion and glomerular filtration rate. Also discussed are clinical applications for the diagnosis and follow-up of renovascular disease, hydronephrosis,and renal transplant dysfunction. The article concludes with an overview of technical problems and challenges facing MR renography.

Perfusion imaging using arterial spin labeling

Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

MR imaging of renal function

MR imaging is the only single noninvasive test that can potentially provide a complete picture of renal status with minimal risk to the patient, simultaneously improving diagnosis while lowering medical costs by virtue of its being a single test. The strengths of MR imaging lie in its high spatial and temporal resolution and its lack of exposure to ionizing radiation and nephrotoxic contrast agents. This article reviews the use of MR imaging for quantification of renal functional parameters and its application to clinical problems, such as RVD, hydronephrosis, and renal transplantation. Although advances in both the technical and clinical aspects of functional renal MR imaging have been made, much remains to be done. The preliminary results reported in the many studies reviewed are exciting, but these techniques need to be validated against accepted standards where such standards exist. In addition, and perhaps more important, the effects of these new diagnostic methods on patient outcomes must be studied. Finally, further progress in image processing and analysis must be made to make functional renal MR imaging truly practical. With these advances, one can expect functional renal MR imaging to play an ever-expanding and influential role in the care and management of the patient with renal disease.

Quantification of renal perfusion using an intravascular contrast agent (part 1): results in a canine model

In this work absolute values of regional renal blood volume (rRBV) and flow (rRBF) are assessed by means of contrast-enhanced (CE) MRI using an intravascular superparamagnetic contrast agent. In an animal study, eight foxhounds underwent dynamic susceptibility-weighted MRI upon injection of contrast agent. Using principles of indicator dilution theory and deconvolution analysis, parametric images of rRBV, rRBF, and mean transit time (MTT) were computed. For comparison, whole-organ blood flow was determined invasively by means of an implanted flow probe, and the weight of the kidneys was evaluated postmortem. A mean rBV value of 28 ml/100 g was found in the renal cortex, with a corresponding mean rBF value of 524 ml/100 g/min and an average MTT of about 3.4 s. Although there was a systematic difference between the absolute blood flow values determined by MRI and the ultrasonic probe, a significant correlation (r(s) = 0.72, P < 0.05) was established. The influence of the arterial input function (AIF), T(1) relaxation effects, and repeated measurements on the precision of the perfusion quantitation is discussed.

Quantification of renal perfusion abnormalities using an intravascular contrast agent (part 2): results in animals and humans with renal artery stenosis

The interrelation between the morphologic degree of renal artery stenosis and changes in parenchymal perfusion is assessed using an intravascular contrast agent. In seven adult foxhounds, different degrees of renal artery stenosis were created with an inflatable clamp implanted around the renal artery. Dynamic susceptibility-weighted gradient-echo imaging was used to measure signal-time curves in the renal artery and the renal parenchyma during administration of 1.5 mg/kg BW of an intravascular ultrasmall particle iron oxide (USPIO) contrast agent. From the dynamic series, regional renal blood volume (rRBV), regional renal blood flow (rRBF), and mean transit time (MTT) were calculated. The morphologic degree of stenosis was measured in the steady state using a high-resolution 3D contrast-enhanced (CE) MR angiography (MRA) sequence (voxel size = 0.7 x 0.7 x 1 mm(3)). Five patients with renoparenchymal damage due to long-standing renal artery stenosis were evaluated. In the animal stenosis model, cortical perfusion remained unchanged for degrees of renal artery stenosis up to 80%. With degrees of stenoses > 80%, cortical perfusion dropped to 151 +/- 54 ml/100 g of tissue per minute as compared to a baseline of 513 +/- 76 ml/100 g/min. In the patients, a substantial difference in the cortical perfusion of more than 200 +/- 40 ml/100 g/min between the normal and the ischemic kidneys was found. The results show that quantitative renal perfusion measurements in combination with 3D-CE-MRA allow the functional significance of a renal artery stenosis to be determined in a single MR exam. Differentiation between renovascular and renoparenchymal disease thus becomes feasible.

Renal arteries: navigator-gated balanced fast field-echo projection MR angiography with aortic spin labeling: initial experience

A cardiac-triggered free-breathing three-dimensional balanced fast field-echo projection magnetic resonance (MR) angiographic sequence with a two-dimensional pencil-beam aortic labeling pulse was developed for the renal arteries. For data acquisition during free breathing in eight healthy adults and seven consecutive patients with renal artery disease, real-time navigator technology was implemented. This technique allows high-spatial-resolution and high-contrast renal MR angiography and visualization of renal artery stenosis without exogenous contrast agent or breath hold. Initial promising results warrant larger clinical studies.

MR angiography of the renal arteries

During, the past decade. MRA has evolved from an cxperimental technique into the modality of choice for the noninvasive evaluation of renovascular disease. The recent widespread application of MRA for these indications has been driven primarily by the advent of 3D contrast-enhanced MRA. which provides a fast, reliable technique for imaging large vascular territories and generates images, after postprocessing, similar in appearance to digital subtraction angiography. The cross-sectional volumetric nature of contrast-enhanced MRA affords some advantages over conventional catheter angiography. Although 3D contrast-enhanced MRA forms the backbone of vascular MR studies, several adjunctive sequences are employed to maximize the diagnostic yield of the examination. For example. flow-dependant imaging is used to complement the morphologic images of contrast-enhanced MRA by providing hemodynamic information. As such, MRA is unique among noninvasive imaging modalities in that it offers a comprehensive evaluation of anatomy and function. The availability and reliability of MRA extend renal artery screening to a wider spectrum of patients. Current applications of renal MRA range from detection of renal artery stenosis to evaluation for renal transplant donors.

Morphologic and functional magnetic resonance imaging of renal artery stenosis: a multireader tricenter study

The effect of combined morphologic and functional magnetic resonance (MR) imaging on the interobserver and intermodality variability for the grading of renal artery stenosis is assessed. In a randomized, blinded tricenter analysis, seven readers evaluated 43 renal arteries on x-ray digital subtraction angiography (DSA), 3D-Gadolinium MR angiography (3D-Gd-MRA), cine phase-contrast flow measurement (PC-flow), and a combined analysis of the last two. Interobserver variability was assessed for the grading of renal artery stenosis as well as regional vessel visibility. Intermodality variability for stenosis grading was analyzed in cases in which the readers agreed on the degree of stenosis in DSA. DSA had a substantial interobserver variability for the grading of stenosis (mean kappa kappa 0.64). 3D-Gd-MRA revealed a slightly improved interobserver variability but incorrectly graded 6 of 34 stenoses on a two-point scale (<50%, > or =50%). The combined approach of 3D-Gd-MRA and PC-flow revealed the best (P = 0.0003) interobserver variability (median kappa = 0.75) and almost perfect intermodality agreement with DSA (97% of cases). These findings were confirmed in a prospective analysis of 97 renal arteries. The vessel visibility of the renal artery ostium was significantly better in 3D-Gd-MRA than in DSA, whereas the visibility of the hilar and intrarenal vessels was significantly worse (P = 0.0001). A combined morphologic and functional MR examination significantly reduces interobserver variability and offers reliable and reproducible grading of renal artery stenosis based on stenosis morphology and hemodynamic changes. It can be considered a safe and noninvasive alternative for diagnostic DSA in cases that do not require assessment of intrarenal vessels.

Captopril MR renography in a swine model: toward a comprehensive evaluation of renal arterial stenosis

PURPOSE

To test the feasibility of captopril magnetic resonance (MR) renography and to validate the technique in an animal model of renal arterial stenosis.

MATERIALS AND METHODS

Seven pigs with induced renal arterial stenosis were studied. MR renography was performed with a T1-weighted approach by using three-dimensional fast imaging with steady-state precession, or FISP, sequences after administration of a bolus of 0.1 mmol of gadopentetate dimeglumine per kilogram of body weight. Captopril was administered to improve the specificity.

RESULTS

The results demonstrate that differences in renographic curves and indices are observed only if an anatomically substantial stenosis, typically a diameter reduction of more than 70%, is present and captopril is administered.

CONCLUSION

In this preliminary experience in an animal model, captopril MR renography provided data consistent with expectations based on conventional renographic results.

MR-guided angioplasty of renal artery stenosis in a pig model: a feasibility study

PURPOSE

To test the hypothesis that magnetic resonance (MR) imaging can guide the percutaneous treatment of renal artery stenosis in a pig model.

MATERIALS AND METHODS

Ameroid constrictors were surgically placed around six renal arteries in four pigs. After 30-36 days, all stenoses were documented by conventional x-ray aortograms. MR-guided renal angioplasty was attempted for three stenoses. For these pigs, MR angiography was performed with use of contrast-enhanced three-dimensional (3D) techniques. The authors visualized catheters by filling them with dilute 4% gadolinium and imaging with two-dimensional (2D) and 3D MR fast spoiled gradient recalled echo techniques. Under MR guidance, the authors advanced a selective catheter into the affected renal artery and crossed the stenosis with a nitinol guide wire. Angioplasty was performed with a balloon catheter filled with dilute gadolinium. Stenosis and luminal diameter measurements were compared before and after angioplasty.

RESULTS

After ameroid constrictor placement, four significant stenoses, one mild stenosis, and one occlusion developed. Under MR guidance, the authors achieved technical success in performing three of three (100%) attempted dilations. After MR-guided angioplasty, the mean reduction in stenosis was 35% and the mean increase in luminal diameter was 1.6 mm.

CONCLUSION

Use of MR guidance for the angioplasty of renal artery stenosis in pigs is feasible.

Dynamic three-dimensional magnetic resonance abdominal angiography and perfusion: implementation and preliminary experience

Implementation of and preliminary experience with an ultra-fast partial-Fourier radiofrequency (RF) spoiled gradient-echo sequence for gadolinium-enhanced imaging are presented. Three-dimensional angiograms can be acquired in less than 6 seconds. Repetition of the acquisition allows the three-dimensional visualization of several distinct vascular phases. Feasibility is demonstrated in three healthy volunteers. The trade-offs among spatial resolution, temporal resolution, and spatial coverage as well as the technical aspects of gadolinium-enhanced pulse sequences are discussed.

Perfusion of the kidney using extraslice spin tagging (EST) magnetic resonance imaging

This study was undertaken to determine whether extraslice spin tagging (EST) perfusion-weighted magnetic resonance imaging is suitable for screening persons for renal perfusion deficits. Six normal and seven patient volunteers with suspected decreased renal perfusion due to renal vascular disease were imaged. X-ray angiograms were also obtained on all patients. The normalized EST signal intensity showed a linear correlation with respect to the percent stenosis measured from the X-ray angiograms. This demonstrates the potential utility of using EST for the evaluation of kidney perfusion, which was done without the need for exogenous MR contrast agents. EST is fast and less expensive than contrast-based methods. These features make EST a candidate for routine screening of patients for renal vascular disease and for the assessment of angiographically equivocal renal artery stenoses. J. Magn. Reson. Imaging 1999;10:886-891.

First-pass renal perfusion imaging using MS-325, an albumin-targeted MRI contrast agent

RATIONALE AND OBJECTIVES

MR angiography is proving to be a useful clinical study for the diagnosis of vascular disorders of renal arteries. However, its utility in terms of stenosis characterization is still limited. Renal perfusion could provide supplemental information that could allow for a comprehensive evaluation of renal artery stenosis by MR imaging.

METHODS

MS-325 is a small-molecule blood pool agent that reversibly binds with serum albumin and hence leads to higher relaxivity and longer residence times in the blood. In this study, the authors evaluated the use of MS-325 to perform first-pass perfusion imaging and contrast-enhanced MR angiography in the characterization of renal artery stenosis in an animal model.

RESULTS

Quantitative perfusion estimates were obtained in the renal cortex (258 +/- 19.8 mL/min/100 g) and are comparable to microsphere measurements (198 +/- 12.2 mL/min/100 g), given the practical constraints. Based on these measurements, perfusion showed minimal changes even when the diameter reductions reached 75%.

CONCLUSIONS

MS-325 could provide quantitative perfusion estimates that when combined with MR angiography may lead to comprehensive evaluation of renal artery stenosis.

Functional imaging of the kidneys with fast MRI techniques

Availability of faster and stronger gradient systems have given rise to a multitude of fast MRI data acquisition strategies which have tremendously increased the scope of MRI applications. These have led to the realization of long desired comprehensive approaches to evaluate anatomy and function using a single modality. In this work, we describe some of our own experiences with functional evaluation of the kidneys using MRI. Examples that suggest the feasibility of comprehensive approaches for evaluation of renal disease are also provided. We also introduce BOLD renal MRI, a method that may allow basic understanding of human renal physiology and pathophysiology in a way that has not been previously possible.