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7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results

7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results Magnetic resonance imaging (MRI) plays a key role in the investigation of cerebrovascular diseases. Compared with computed tomography (CT) and digital subtraction angiography (DSA), its advantages in diagnosing cerebrovascular pathology include its superior tissue contrast, its ability to visualize blood vessels without the use of a contrast agent, and its use of magnetic fields and radiofrequency pulses instead of ionizing radiation. In recent years, ultrahigh field MRI at 7 tesla (7 T) has shown promise in the diagnosis of many cerebrovascular diseases. The increased signal-to-noise ratio (SNR; 2.3x and 4.7x increase compared with 3 and 1.5 T, respectively) and contrast-to-noise ratio (CNR) at this higher field strength can be exploited to obtain a higher spatial resolution and higher lesion conspicuousness, enabling assessment of smaller brain structures and lesions. Cerebrovascular diseases can be assessed at different tissue levels; for instance, changes of the arteries feeding the brain can be visualized to determine the cause of ischemic stroke, regional changes in brain perfusion can be mapped to predict outcome after revascularization, and tissue damage, including old and recent ischemic infarcts, can be evaluated as a marker of ischemic burden. For the purpose of this review, we will discriminate 3 levels of assessment of cerebrovascular diseases using MRI: Pipes, Perfusion, and Parenchyma (3 Ps). The term Pipes refers to the brain-feeding arteries from the heart and aortic arch, upwards to the carotid arteries, vertebral arteries, circle of Willis, and smaller intracranial arterial branches. Perfusion is the amount of blood arriving at the brain tissue level, and includes the vascular reserve and perfusion territories. Parenchyma refers to the acute and chronic burden of brain tissue damage, which includes larger infarcts, smaller microinfarcts, and small vessel disease manifestations such as white matter lesions, lacunar infarcts, and microbleeds. In this review, we will describe the key developments in the last decade of 7-T MRI of cerebrovascular diseases, subdivided for these 3 levels of assessment. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Topics in Magnetic Resonance Imaging Wolters Kluwer Health

7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results

7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results


PIPES Extracranial Arteries In the last decade, many developments have taken place in the field of 7-T magnetic resonance imaging (MRI) of the extracranial brain-feeding vasculature, and these can be grossly subdivided into developments in vessel lumen imaging and vessel wall imaging. The Vessel Lumen Imaging of the vessel lumen in cerebrovascular diseases is mainly performed using either phase contrast (PC-)MR angiography (MRA) or time-of-flight (TOF-)MRA. The advantage of ultrahigh field MRI (such as 7 T) in this regard is the longer T 1 relaxation time at these higher field strengths. 1 In TOF-MRA, static spins present in tissues within an excited volume become saturated during repetitive radiofrequency (RF) pulses within their relaxation time T 1 , while flowing spins—like in blood—will only experience a limited number of excitations within this excited volume—being there for only a short period of time—and will therefore result in a high signal. When the T 1 relaxation time becomes longer, the static spins will relax less between RF pulses, thereby achieving faster and better suppression of these tissues and more contrast between flowing blood and the surrounding tissues. 2–8 It might be assumed that this advantage, combined with the increased spatial resolution attainable at 7 T, may result in detailed assessment of the extracranial arteries from the aortic arch upwards to the skull base, including clearer depiction of the arterial lumen and luminal stenosis. However, so far, limited availability of dedicated coils (see next paragraph) for visualization of the neck region currently makes 7 T less useful for “one-stop-shop” imaging of the extracranial arteries. Furthermore, the increased magnetic field inhomogeneities and susceptibility artifacts make the neck region especially challenging for robust imaging of the extracranial arteries at ultrahigh field strength. 9 The Vessel Wall At 1.5 T, extracranial...
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References (141)

Copyright
Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.
Subject
Review Articles
ISSN
0899-3459
eISSN
1536-1004
DOI
10.1097/RMR.0000000000000080
pmid
27049246
Publisher site
See Article on Publisher Site

Abstract

Magnetic resonance imaging (MRI) plays a key role in the investigation of cerebrovascular diseases. Compared with computed tomography (CT) and digital subtraction angiography (DSA), its advantages in diagnosing cerebrovascular pathology include its superior tissue contrast, its ability to visualize blood vessels without the use of a contrast agent, and its use of magnetic fields and radiofrequency pulses instead of ionizing radiation. In recent years, ultrahigh field MRI at 7 tesla (7 T) has shown promise in the diagnosis of many cerebrovascular diseases. The increased signal-to-noise ratio (SNR; 2.3x and 4.7x increase compared with 3 and 1.5 T, respectively) and contrast-to-noise ratio (CNR) at this higher field strength can be exploited to obtain a higher spatial resolution and higher lesion conspicuousness, enabling assessment of smaller brain structures and lesions. Cerebrovascular diseases can be assessed at different tissue levels; for instance, changes of the arteries feeding the brain can be visualized to determine the cause of ischemic stroke, regional changes in brain perfusion can be mapped to predict outcome after revascularization, and tissue damage, including old and recent ischemic infarcts, can be evaluated as a marker of ischemic burden. For the purpose of this review, we will discriminate 3 levels of assessment of cerebrovascular diseases using MRI: Pipes, Perfusion, and Parenchyma (3 Ps). The term Pipes refers to the brain-feeding arteries from the heart and aortic arch, upwards to the carotid arteries, vertebral arteries, circle of Willis, and smaller intracranial arterial branches. Perfusion is the amount of blood arriving at the brain tissue level, and includes the vascular reserve and perfusion territories. Parenchyma refers to the acute and chronic burden of brain tissue damage, which includes larger infarcts, smaller microinfarcts, and small vessel disease manifestations such as white matter lesions, lacunar infarcts, and microbleeds. In this review, we will describe the key developments in the last decade of 7-T MRI of cerebrovascular diseases, subdivided for these 3 levels of assessment.

Journal

Topics in Magnetic Resonance ImagingWolters Kluwer Health

Published: Apr 1, 2016

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