Magnetic Resonance Angiography. Techniques, Indications and Practical Applications

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3 Magnetic Resonance Angiography Techniques, Indications and Practical Applications

4 G.Schneider M.R.Prince J.F.M.Meaney V.B.Ho (Eds) Magnetic Resonance Angiography Techniques, Indications and Practical Applications Foreword by E. J. Potchen

5 Günther Schneider Department of Diagnostic and Interventional Radiology Saarland University Hospital Homburg/Saar, Germany Martin R. Prince Departments of Radiology Weill Medical College of Cornell University and Columbia College of Physicians and Surgeons New York, USA James F. M. Meaney MRI Department St. James s Hospital Dublin, Ireland Vincent B. Ho and Radiological Sciences Uniformed Services University of the Health Sciences Bethesda, USA Anatomical drawings by Nadia Simeoni (Turin, Italy) ISBN Springer Milan Berlin Heidelberg New York Library of Congress Control Number: This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the Italian Copyright Law in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the Italian Copyright Law. Springer is a part of Springer Science+Business Media springeronline.com Springer-Verlag Italia 2005 Printed in Italy The use of general descriptive names, registered names, trademarks,etc. in this publication does not imply, even in the absence of a speci.c statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Product liability: The publishers cannot guarantee the accuracy of any information about dosage and application contained in this book. In every individual case the user must check such information by consulting the relevant literature. Cover design: Simona Colombo, Milan Typesetting: Compostudio, Cernusco s/n (Milan) Printing and binding: Arti Grafiche Nidasio, Assago (Milan)

6 Foreword Those of us involved in the development of magnetic resonance angiography (MRA) in the late 1980 s could hardly envision the routine application of MRA in every MR facility everyday. In those years there was spectacular development of many new MR clinical applications. Many pioneering researchers investigated various strategies exploiting the effects of blood flow on the MR signal to optimize clinical MRA. Remarkable successes were demonstrated in rapid succession. It is very alluring to attempt to catalogue the significant contributions in the founding of clinical MRA here, but that comprehensive effort is best relegated to the careful authors of history chapters in MRA books. The following are a few milestones from the early years of MRA development. The first research meeting devoted to Magnetic Resonance Angiography was hosted by Roberto Passariello in L Aquila, Italy in This meeting gave rise to formation of the MR Angio Club, which then held its first meeting at Michigan State University in Those were the days when three-dimensional phase contrast MRA would take some 19 hours from the time the patient entered the magnet until images could be seen: one hour to acquire the image and 18 hours of overnight image post processing. Seeing vasculature for the first time in 3D display is when we all realized the future potential clinical utility of MRA. Computational capabilities of modern equipment have reduced the delay to a few seconds. Post processing now has taken a more central role in the communication of enormous amounts of data with less cumbersome two- or three-dimensional projections. Many variations on the MRA theme have been presented over the ensuing 15 years. For example, Dennis Parker developed the 3D multi-slab time-of-flight MRA technique which remains in routine clinical use to this day. Pulse sequence design plays a major role in the continuing advancements in the field, most notably as a consequence of more sophisticated and novel k-space filling strategies. The work of Kent Yucel and Martin Prince at the Massachusetts General Hospital in 1992 brought gadolinium-enhanced MR angiography to clinical utility. The first-pass dynamic contrast-enhanced MRA method provides robust and reproducible imaging results that have propelled the adoption of MRA into wider clinical use. This advance reliably produced images of sufficient quality to replace invasive catheter-based x-ray contrast angiography for most diagnostic purposes. Now it is possible to acquire a high quality MR angiography study in seconds. The advent of very high field clinical scanners operating at 3.0 Tesla is now reinvigorating earlier non-contrast methods. 3.0 T MRA benefits from two key phenomena: (1) the signal to noise of 3.0 T is twice that of the 1.5 T, offering the opportunity to either increase the spatial resolution or to shorten scan times by up to a factor of four, and, (2) the longer T1 s of tissues at 3.0 T, ~20-40% higher than 1.5T, provides better background suppression, additional inflow enhancement, and improved contrast-to-noise. Magnetization transfer would normally be considered SAR prohibitive at 3.0 T, but novel pulse sequence design has overcome this challenge. The appropriate choice of imaging parameters can minimize artifacts and exploit T1 prolongation at higher fields for better quality MRA. The availability of scanners capable of parallel imaging along with growing availability of multi-channel coils is coincident with the arrival of these very high field scanners. The future potential is bright. Early results using these combined advancements for intracranial MRA yield spatial resolution exceeding invasive DSA and provide breathtaking visualization of small arteries such as the lenticulostriate vasculature. The efficiency of the parallel imaging approach will also compliment the quality of time resolved MR techniques.

7 VI Magnetic Resonance Angiography Where are we going from here? Highest on the cardiovascular unresolved diagnostic problem list is the localization and assessment of unstable plaques. Specifically designed contrast agent(s) targeted to a characteristic within the unstable plaque will comprise a Molecular MRA procedure. Clearly, the domain of MRA is embracing this pursuit. It is remarkable that, after 15 years, we are still searching for the MRA technique to completely assess atherosclerotic disease in the carotid artery. Insights into bifurcation disease drive the quest for ever higher spatial resolution and SNR to assess plaque structure and stability. In this regard, carotid MRA will require integrating the newly available technologies to achieve the necessary spatial resolution. Fusion MRA can refer to integrated multidimensional presentations of the MRA anatomy merged with other anatomical and functional modalities. We are now beginning to see presentations of MR and CT coronary angiography fused with PET myocardial perfusion images, or short-axis MR cardiac function images, or MR perfusion reserve images. Fusing MRA images to MRI, MR CSI, PET, nuclear medicine, and CT will be a direction that this field will take. How do we optimize the present value of this potential? The persistent need for comprehensive outcome studies for MRA endures.a persistent challenge, however, is that by the time these studies are completed, the best methods may well have changed. We, who are students of changing technologies and best practices, need to further develop methodologies to measure the merits of alternative diagnostic procedures. MRA has the probability of becoming the standard for future non-invasive technologies. This book presents an up-to-date treatise, a much needed presentation of the current practice of clinical MRA fully exploiting the benefits of dynamic contrast-enhanced MRA. I compliment Drs. Schneider, Prince, Meaney, and Ho on producing a definitive work on a rapidly moving target. This book provides the benchmark against which future MRA developments will be measured. E. James Potchen, M.D. Michigan State University, USA

8 Contents SECTION I Technical Background I.1. Unenhanced MR Angiography M. Backens, B. Schmitz I.2. Contrast-Enhanced MR Angiography: Theory and Technical Optimization V.B.Ho,W.R.Corse,J.H.Maki I.3. Time-resolved MR Angiography F. S. Pereles,V. B. Ho I.4. Image Processing in Contrast-Enhanced MR Angiography P. C. Douek, M. Hernández-Hoyos, M. Orkisz SECTION II Contrast Agents II. Contrast Agents for MR Angiography: Current Status and Future Perspectives M. V. Knopp, M. A. Kirchin SECTION III Head and Neck Vessels III.1. MR Angiography of Extracranial Carotid and Vertebral Arteries K. R. Maravilla, B. Chu III.2. Intracranial MR Angiography N. Anzalone, A. Tartaro SECTION IV Thorax IV.1. MR Angiography of the Thoracic Aorta G. Schneider IV.2. MR Angiography of the Pulmonary Vasculature J. P. Goldman SECTION V Coronary Arteries V. MR Angiography of the Coronary Arteries M.Y.Desai,M.Stuber

9 VIII Contents SECTION VI Abdomen VI.1. Contrast-Enhanced MR Angiography of the Abdominal Aorta P. C. Douek VI.2. MR Angiography of the Renal Arteries H. Zhang, S. Schoenberg, M. R. Prince VI.3. MR Angiography of the Mesenteric Arteries M. Goyen VI.4. MR Angiography of the Portal Venous System M. Goyen SECTION VII Peripheral Arteries VII.1. MR Angiography of Peripheral Arteries: Upper Extremities M. N. Wasser VII.2. MR Angiography of Peripheral Arteries: Lower Extremities J. F. M. Meaney VII.3. Pedal MR Angiography J. H. Maki VII.4. Whole Body 3D MR Angiography M. Goyen SECTION VIII MR Angiography in Pediatrics VIII. MR Angiography in Pediatric Patients P. Fries, R. Seidel, G. Schneider SECTION IX MR Venography IX. MR Venography S. G. Ruehm SECTION X Clinical Implications X. Impact of MR Angiography on Endovascular Therapy R. A. Lookstein SUBJECT INDEX

10 Contributors Nicoletta Anzalone Department of Neuroradiology Scientific Institute and University H. S. Raffaele Milan, Italy Martin Backens Centre for Radiology Work Group Magnetic Resonance Imaging Saarland University Hospital Homburg/Saar, Germany Baocheng Chu University of Washington Seattle, USA William R. Corse Doylestown Hospital Doylestown, USA Jeffrey P. Goldman Mount Sinai School of Medicine Dept. of Radiology New York, USA Mathias Goyen University Medical Center Hamburg-Eppendorf Hamburg, Germany Marcela Hernández-Hoyos CREATIS, CNRS Research Unit (UMR 5515) affiliated to INSERM Lyon, France Vincent B. Ho and Radiological Sciences Uniformed Services University of the Health Sciences Bethesda, USA Milind Y. Desai Departments of Medicine and Radiology Johns Hopkins University Baltimore, USA Philippe C. Douek CREATIS, CNRS Research Unit (UMR 5515) affiliated to INSERM Lyon, France Peter Fries Department of Diagnostic and Interventional Radiology Saarland University Hospital Homburg/Saar, Germany Robert A. Lookstein Division of Interventional Radiology Mount Sinai Medical Center New York, USA Miles A. Kirchin Worldwide Medical Affairs Bracco Imaging SpA Milan, Italy Michael V. Knopp Ohio State University Hospital Columbus, USA

11 X Contributors Jeffrey H. Maki University of Washington Seattle, USA Kenneth R. Maravilla University of Washington Seattle, USA James F. M. Meaney MRI Department St. James s Hospital Dublin, Ireland Maciej Orkisz CREATIS, CNRS Research Unit (UMR 5515) affiliated to INSERM Lyon, France F. Scott Pereles Feinberg School of Medicine Northwestern University Chicago, USA Martin R. Prince Departments of Radiology Weill Medical College of Cornell University and Columbia College of Physicians and Surgeons New York, USA Stefan G. Ruehm David Geffen School of Medicine at UCLA Los Angeles, USA Bernd Schmitz University Hospitals Ulm Ulm, Germany Günther Schneider Department of Diagnostic and Interventional Radiology Saarland University Hospital Homburg/Saar, Germany Stefan Schoenberg Institute of Clinical Radiology University Hospitals Grosshadern Munich, Germany Roland Seidel Department of Diagnostic and Interventional Radiology Saarland University Hospital Homburg/Saar, Germany Matthias Stuber Departments of Medicine, Radiology and Electrical Engineering Johns Hopkins University Baltimore, USA Armando Tartaro Department of Clinical Sciences and Bioimaging Section of Radiological Sciences G. D Annunzio Foundation University of Chieti (Pescara), Italy Martin N. Wasser Leiden University Medical Center Leiden, The Netherlands Honglei Zhang Weill Medical College of Cornell University New York, USA

12 SECTION I Technical Background

13 I.1 Unenhanced MR Angiography Martin Backens and Bernd Schmitz Introduction In conventional x-ray digital subtraction angiography (DSA), administration of contrast agent is necessary in order to depict blood vessels. After arterial catheterization and injection of an iodinated contrast agent, two-dimensional projection images of the lumen of the vessel are acquired from chosen angles. For every new projection this procedure has to be repeated. The availability of 3D rotational angiography and CT angiography may help to overcome this problem but at the expense of high radiation doses. Unenhanced MR angiography (MRA) differs from DSA and other angiographic techniques in that blood vessels are depicted non-invasively in the absence of contrast agent injection. Unenhanced MR techniques allow the acquisition of 3D datasets or stacks of 2D images that contain all vessels in the volume of interest. The acquired images included in the 3D data set are called source images. Projectional angiographic displays of the vessel are subsequently reconstructed from the data using the maximum intensity projection (MIP) postprocessing algorithm, which generates angiogram-like images from the entire dataset or a subset from any desired viewing angle without the need for further measurement. Another advantage of MRA versus x-ray angiography derives from the fact that extravascular tissue is depicted together with the vessels, thereby permitting the correlation of blood flow abnormalities with associated soft tissue pathologies (Table 1). Contrast in MR images depends principally on static tissue parameters: longitudinal relaxation time T1, transverse relaxation time T2, and proton density. In addition, the MR signal is sensitive to flow and movement which frequently leads to artifacts in MR imaging. MR angiographic sequences, however, use flow-induced signal variations to depict blood vessels or even to obtain quantitative information about blood flow in terms of velocity and direction. Unenhanced MRA comprises those MR techniques that rely solely on flow effects. Unlike contrast-enhanced MRA (CE MRA) and x-ray angiography, which depict the vessel lumen filled with contrast agent, it is just the movement of blood that is seen in the unenhanced MR-angiogram. Flow effects can be grouped into two fundamentally different categories: Amplitude effects (time-of-flight) Blood flowing into or out of a chosen slice has a different longitudinal magnetization compared to stationary spins, depending on the duration of stay (time-of-flight) in the slice. Phase effects Blood flowing along the direction of a magnetic field gradient is subject to changes of its transverse magnetization compared to stationary spins. In principle, both flow phenomena are effective simultaneously leading either to a decrease or an increase of the MR signal depending on the type of sequence used. Appropriate sequence techniques have been developed which emphasize one of the flow effects and suppress the other. Typically, MR angiography techniques are designed in such a way that flowing blood produces hyperintense signal while the background signal from stationary tissue remains largely suppressed ( bright-blood angiography). Alternatively, flowing spins can be Table 1. Advantages of MR angiography Advantages of MR angiography No ionizing radiation Non-invasive Any projection can be reconstructed from 3D datasets Depiction of extravascular tissue Flow quantification in terms of velocity and direction

14 4 Magnetic Resonance Angiography Table 2. Techniques of unenhanced MR angiography Time-of-Flight (TOF) angiography: Flow changes longitudinal magnetization Phase-contrast (PC) angiography: Flow changes transverse magnetization made to appear hypointense compared to the stationary background ( black-blood angiography). In the clinical setting bright-blood MRA is the more widely accepted of the two techniques. There are two approaches to performing bright-blood MRA: time-of-flight (TOF) and phase-contrast (PC) angiography (Table 2). Although intravenous contrast administration is not required in TOF and PC MRA, it can be applied in certain situations to improve vessel contrast. Since unenhanced MRA is based on complex flow phenomena, physiological conditions of flow in the vascular territory of interest are of major importance for the applicability of the method. Advantageous conditions are found especially in the vessels of the brain because of the nearly laminar flow in this territory. Moreover, flow is largely constant during the heart cycle (Fig. 1), making ECG triggering unnecessary for imaging of brain vessels. The high velocity of arterial flow ( cm/sec) provides good vessel-background contrast and moderate acquisition times. In clinical routine, unenhanced MRA has proven to be a robust and versatile method for non-invasively imaging of brain vessels (circle of wilis, sagittal sinus). In addition, this technique is also suitable for depicting extracranial carotid arteries and short segments of peripheral vessels (e.g. lower leg). A major limitation of unenhanced MRA, however, is a susceptibility to signal loss in areas of turbulent or very slow flow. In severe cases, this may lead to a misdiagnosis of the pathologic condition (stenosis, aneurysm). Additionally, TOF and PC angiography are highly sensitive to motion artifacts. Although motion artifacts often arise due to patient movement because of the need for relatively long acquisition times, they may also occur in areas of very pulsatile flow such as that occurring in the carotids, aorta, and especially, in the peripheral arteries, and in the thoracic and abdominal regions due to breathing and heart actions. Whereas ECG triggering may reduce or eliminate those artifacts associated with pulsatile flow, the availability of contrast-enhanced MR angiography (CE MRA) has largely made unenhanced MRA redundant for most vascular territories outside of the brain. However if unenhanced MRA techniques are performed, a thorough understanding of the underlying physical and technical mechanisms is prerequisite to performing imaging and to correctly interpreting the acquired angiograms. Understanding Flow Effects Outflow-related Signal Loss (washout effect, T2 flow void) When images are obtained with a spin-echo (SE) pulse sequence, blood flowing at a high velocity perpendicular to the imaging plane produces a weaker signal than the surrounding stationary tissue. This phenomenon is caused by the washout of flowing spins from the slice during the imaging process. Spin-echo techniques are characterized by a sequence of slice-selective 90 and 180 radio frequency (RF) pulses. Only those tissue components that are affected by both pulses can provide an MR signal. Moving material, such as blood in the vessels, flowing through the excited slice at a suffi- Fig. 1. Comparison of flow profiles in the external carotid artery (ECA), the internal carotid artery (ICA) and the middle cerebral artery (MCA). There is very pulsatile flow in the ECA. However, in the intracranial vessels, the variation of flow during heart cycle is much less pronounced [Courtesy of Steffi Behnke, MD, Dept. of Neurology, Saarland University Hospitals]

15 I.1 Unenhanced MRA 5 Fig. 2. In spin-echo sequences, blood flowing out of the measured slice in the time between 90 - and 180 -radiofrequency pulses leads to signal loss ciently high velocity, is affected by only one of these pulses, and therefore does not contribute to the MR signal. This is the so-called flow void (Fig. 2). The intensity of the vascular signal declines with decreasing slice thickness, s, increasing echo time, TE, increasing flow velocity, v. If the blood flow velocity is so high that all spins leave the slice between the 90 - and 180 pulses (v s/(te/2)), there will be no signal at all and the vessel will appear dark. Spins flowing within the imaging plane are not affected by this phenomenon. The washout effect is observed only for SE sequences and is most pronounced on T2- weighted imaging because of the long echo times used. With gradient-echo (GRE) techniques, the echo is refocused without a 180 pulse simply by reversing the imaging gradients. Since only one RF pulse is needed to form an echo, the washout effect does not occur. With standard SE sequences the washout effect provides valuable and reliable information about blood flow. The absence of a flow void on T2- weighted imaging should be considered as indicative of very slow flow or even occlusion of the vessel (Fig. 3). On the other hand, occlusion of the vessel can be excluded if a flow void is present. Fig. 3. Axial T2-weighted spin-echo image of the pons region. Left: Missing flow void in the basilar artery indicates thrombosis of the vessel. Right: After thrombolysis, the vessel (arrow) appears dark due to restored flow