Diffusion-weighted imaging: basic concepts and application in cerebral stroke and head trauma

Diffusion-weighted imaging (DWI) of the brain represents a new imaging technique that extends imaging from depiction of neuroanatomy to the level of function and physiology. DWI measures a fundamentally different physiological parameter compared with conventional MRI. Image contrast is related to differences in the diffusion rate of water molecules rather than to changes in total tissue water. DWI can reveal pathology in cases where conventional MRI remains unremarkable. DWI has proven to be highly sensitive in the early detection of acute cerebral ischemia and seems promising in the evaluation of traumatic brain injury. DWI can differentiate between lesions with decreased and increased diffusion. In addition, full-tensor DWI can evaluate the microscopic architecture of the brain, in particular white matter tracts, by measuring the degree and spatial distribution of anisotropic diffusion within the brain. This article reviews the basic concepts of DWI and its application in cerebral ischemia and traumatic brain injur

Cerebral Infarct/Intracranial Cerebrovascular Disease

Imaging goals for intracranial cerebral vascular disease are (1) to assess the degree of parenchymal injury and identify intraparenchymal hemorrhage; (2) to determine if there are areas of altered perfusion that may be at risk for future injury; and (3) to assess the intracranial arteries (patency as well as direction of flow). This unit describes a that can be used to evaluate stable patients with acute, subacute, or chronic cerebrovascular symptoms. An is also given for cases of hyperacute strokes or cerebrovascular symptoms in an unstable patient.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145276/1/cpmia0101.pd

Cerebral Infarct/Intracranial Cerebrovascular Disease

Imaging goals for intracranial cerebral vascular disease are (1) to assess the degree of parenchymal injury and identify intraparenchymal hemorrhage; (2) to determine if there are areas of altered perfusion that may be at risk for future injury; and (3) to assess the intracranial arteries (patency as well as direction of flow). This unit describes a Basic Protocol that can be used to evaluate stable patients with acute, subacute, or chronic cerebrovascular symptoms. An Alternate Protocol is also given for cases of hyperacute strokes or cerebrovascular symptoms in an unstable patient.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145411/1/cpmia0101.pd

Arterial spin labelling magnetic resonance imaging of the brain: techniques and development

This thesis centres on the development of arterial spin labelling (ASL) MRI, a non-invasive technique to image cerebral perfusion. In the first chapter I explain the principles of cerebral blood flow (CBF) quantification using ASL beginning with the original implementation through to the most recent advances. I proceed to describe the established theory behind the key additional MRI contrast mechanisms and techniques that underpin the novel experiments described in this thesis (T2 and T1 relaxation, diffusion imaging and half-Fourier acquisition and reconstruction). In Chapter 2 I describe work undertaken to sample the transverse relaxation of the ASL perfusion-weighted and control images acquired with and without vascular crusher gradients at a range of post-labelling delay times and tagging durations, to estimate the intra-vascular, intra-cellular and extra-cellular distribution of labelled water in the rat cortex. The results provide evidence for rapid exchange of labelled water into the intra-cellular space relative to the transit-time through the vascular bed, and provide a more solid foundation for CBF quantification using ASL techniques. In Chapter 3 the performance of image de-noising techniques for reducing errors in ASL CBF and arterial transit time estimates is investigated. I show that noise reduction methods can suppress random and systematic errors, improving both the precision and accuracy of CBF measurements and the precision of transit time maps. In Chapter 4 I present the first in-vivo demonstration of Hadamard-encoded continuous ASL (H-CASL); an efficient method of imaging small volumes of labelled blood water in the brain at multiple post labelling delay times. I present evidence that H-CASL is viable for in-vivo application and can improve the precision of δa estimation in 2/3 of the imaging time required for standard multi post labelling delay continuous ASL

Microstructural imaging of the human brain with a 'super-scanner': 10 key advantages of ultra-strong gradients for diffusion MRI

The key component of a microstructural diffusion MRI 'super-scanner' is a dedicated high-strength gradient system that enables stronger diffusion weightings per unit time compared to conventional gradient designs. This can, in turn, drastically shorten the time needed for diffusion encoding, increase the signal-to-noise ratio, and facilitate measurements at shorter diffusion times. This review, written from the perspective of the UK National Facility for In Vivo MR Imaging of Human Tissue Microstructure, an initiative to establish a shared 300 mT/m-gradient facility amongst the microstructural imaging community, describes ten advantages of ultra-strong gradients for microstructural imaging. Specifically, we will discuss how the increase of the accessible measurement space compared to a lower-gradient systems (in terms of Δ, b-value, and TE) can accelerate developments in the areas of 1) axon diameter distribution mapping; 2) microstructural parameter estimation; 3) mapping micro-vs macroscopic anisotropy features with gradient waveforms beyond a single pair of pulsed-gradients; 4) multi-contrast experiments, e.g. diffusion-relaxometry; 5) tractography and high-resolution imaging in vivo and 6) post mortem; 7) diffusion-weighted spectroscopy of metabolites other than water; 8) tumour characterisation; 9) functional diffusion MRI; and 10) quality enhancement of images acquired on lower-gradient systems. We finally discuss practical barriers in the use of ultra-strong gradients, and provide an outlook on the next generation of 'super-scanners'

Finsler Active Contours

©2008 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or distribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder.DOI: 10.1109/TPAMI.2007.70713In this paper, we propose an image segmentation technique based on augmenting the conformal (or geodesic) active contour framework with directional information. In the isotropic case, the euclidean metric is locally multiplied by a scalar conformal factor based on image information such that the weighted length of curves lying on points of interest (typically edges) is small. The conformal factor that is chosen depends only upon position and is in this sense isotropic. Although directional information has been studied previously for other segmentation frameworks, here, we show that if one desires to add directionality in the conformal active contour framework, then one gets a well-defined minimization problem in the case that the factor defines a Finsler metric. Optimal curves may be obtained using the calculus of variations or dynamic programming-based schemes. Finally, we demonstrate the technique by extracting roads from aerial imagery, blood vessels from medical angiograms, and neural tracts from diffusion-weighted magnetic resonance imagery

MRI of cerebral micro-vascular flow patterns: A multi-direction diffusion-weighted ASL approach.

The study and clinical assessment of brain disease is currently hindered by a lack of non-invasive methods for the detailed and accurate evaluation of cerebral vascular pathology. Angiography can detect aberrant flow in larger feeding arteries/arterioles but cannot resolve the micro-vascular network. Small vessels are a key site of vascular pathology that can lead to haemorrhage and infarction, which may in turn trigger or exacerbate neurodegenerative processes. In this study, we describe a method to investigate microvascular flow anisotropy using a hybrid arterial spin labelling and multi-direction diffusion-weighted MRI sequence. We present evidence that the technique is sensitive to the mean/predominant direction of microvascular flow in localised regions of the rat cortex. The data provide proof of principle for a novel and non-invasive imaging tool to investigate cerebral micro-vascular flow patterns in healthy and disease states

Quantification of venous blood signal contribution to BOLD functional activation in the auditory cortex at 3 T

Most modern techniques for functional magnetic resonance imaging (fMRI) rely on blood-oxygen-level-dependent (BOLD) contrast as the basic principle for detecting neuronal activation. However, the measured BOLD effect depends on a transfer function related to neurophysiological changes accompanying electrical neural activation. The spatial accuracy and extension of the region of interest are determined by vascular effect, which introduces incertitude on real neuronal activation maps. Our efforts have been directed towards the development of a new methodology that is capable of combining morphological, vascular and functional information; obtaining new insight regarding foci of activation; and distinguishing the nature of activation on a pixel-by-pixel basis. Six healthy volunteers were studied in a parametric auditory functional experiment at 3 T; activation maps were overlaid on a high-resolution brain venography obtained through a novel technique. The BOLD signal intensities of vascular and nonvascular activated voxels were analyzed and compared: it was shown that nonvascular active voxels have lower values for signal peak (Pb10−7) and area (Pb10−8) with respect to vascular voxels. The analysis showed how venous blood influenced the measured BOLD signals, supplying a technique to filter possible venous artifacts that potentially can lead to misinterpretation of fMRI results. This methodology, although validated in the auditory cortex activation, maintains a general applicability to any cortical fMRI study, as the basic concepts on which it relies on are not limited to this cortical region. The results obtained in this study can represent the basis for new methodologies and tools that are capable of adding further characterization to the BOLD signal properties