Diffusion Magnetic Resonance Imaging for Cancer Treatment Response Assessment
Martin G. Pomper, Juri G. Gelovani, Benjamin Tsui, Kathleen Gabrielson, Richard Wahl, S. Sam Gambhir, Jeff Bulte, Raymond Gibson, William C. Eckelman in Molecular Imaging in Oncology, 2008
Magnetic resonance imaging (MRI) can be used to obtain information related to biophysical, physiological, metabolical, anatomical, or functional properties of tissues. This chapter will highlight the application of diffusion MRI as a molecular imaging approach for oncological imaging. Applications of diffusion MRI range from assessment of cellular status (1), cellular density (2), diagnostic screening (3), and microstructural organization (4,5), all of which are used in clinical and research studies. Specific attention will be given to the application of this imaging approach for assessment of cancer treatment response. The overall concept for this application in cancer imaging is that diffusion MR can be used to quantify the mobility or diffusion of water molecules within tissue. Because the diffusion of individual water molecules relies on interactions which occur at the cellular level, subtle alterations in, for example, cellular density results in a corresponding change in the diffusion rate of water molecules which makes diffusion MRI a sensitive imaging biomarker (6,7). The goal of this chapter will be to provide the reader with an up to date summary of the basic methods and applications of applying diffusion MRI in oncological research and clinical practice.
Mapping the Injured Brain
Yu Chen, Babak Kateb in Neurophotonics and Brain Mapping, 2017
Diffusion MRI has long been used as an important tool in studying neurological disorders because it provides in vivo measurements of tissue microstructure changes that are unable to be detected using conventional CT or MR imaging. Diffusion tensor imaging (DTI) entails measuring water diffusion in at least six directions to obtain an appropriate representation of a diffusion tensor, describing the preferential diffusion direction and an ellipsoidal diffusivity profile. Briefly speaking, common measurements in DTI include mean diffusivity (MD) and apparent diffusion coefficient, which both measure the average magnitude of diffusivity, and fractional anisotropy (FA), which measures the disproportion of diffusion along the three principal axes of the diffusion ellipsoid. The diffusion properties of different brain tissues (GM, WM, CSF) exhibit unique structural properties. Diffusion in CSF is similar to free diffusion in water, so MD is extremely high (~3 × 10−3 mm2/s) and FA is almost 0. Diffusion in GM is generally nondirectional or isotropic because it is mostly composed of neurons and glial cells (FA < 0.2). On the other hand, in WM, diffusion is highly anisotropic due to the myelinated axons, which restrict water diffusion in the direction perpendicular to the axon. Therefore, axial diffusivity (AD, diffusion measured along the axon) can be as much as seven times the radial diffusivity (RD, diffusion measured perpendicular to the axon averaged across two axes) (FA ~ 0.45–0.8) (Song et al., 2003). Figure 14.1 shows an example of water diffusion in WM axons and the effect of axon membrane injury to water diffusion, which leads to increased RD and reduced FA.
Diffusion Imaging in Brain Tumors and Treatment Response
Andrei I. Holodny in Functional Neuroimaging, 2019
The key to understanding diffusion MRI is realizing that it is fundamentally different from routine MR sequences such as T1 and T2 and, in fact, conceptually easier to understand. T1- and T2-weighted images are generated from the time it takes for molecules to return to their original resting state after undergoing a series of excitations. Diffusion MRI is based on visualizing the relative speeds at which water molecules diffuse through tissue.
Cellular and extracellular white matter alterations indicate conversion to psychosis among individuals at clinical high-risk for psychosis
Published in The World Journal of Biological Psychiatry, 2021
Felix L. Nägele, Ofer Pasternak, Lisa V. Bitzan, Marius Mußmann, Jonas Rauh, Marek Kubicki, Gregor Leicht, Martha E. Shenton, Amanda E. Lyall, Christoph Mulert
The diffusion MRI-based free-water imaging technique addresses these limitations by decomposing the diffusion signal into two compartments (Pasternak et al. 2009). The first compartment models the fractional volume of isotropic, unrestricted diffusion in the extracellular space (free-water, FW) which can be responsive to pathologies such as edoema and atrophy (Pasternak et al. 2012; Lyall et al. 2018). The second compartment models hindered/restricted diffusion in close proximity to cellular membranes, from which FA of the tissue (FAT) is derived, reflecting more closely changes in myelination and axonal membrane health than the conventional DTI metric FA (Pasternak et al. 2009). By differentiating these compartments this model has proven to be successful in providing new insights to potential biological mechanisms underlying the observed FA reductions in schizophrenia (Pasternak et al. 2012, 2015; Lyall et al. 2018).
Impact of Amblyopia on the Central Nervous System
Published in Journal of Binocular Vision and Ocular Motility, 2020
Nathaniel P. Miller, Breanna Aldred, Melanie A. Schmitt, Bas Rokers
However, in recent decades, advances in magnetic resonance imaging (MRI) methods have enabled more precise in vivo assessment of the structure and function of the brain beyond the retina. Functional integrity can be assessed using functional MRI (fMRI) in which neural activity is estimated based on metabolic demand via blood oxygen level-dependent (BOLD) signals.35 Structural integrity can be assessed with voxel-based morphometry (VBM) or diffusion MRI (dMRI). VBM is based on MRI intensity differences produced by different kinds of brain tissue and can assess local gray and white-matter volume. Diffusion MRI measures the diffusion of water molecules in tissue, enabling the identification of white-matter pathways in the brain and the estimation of their structural integrity.
Neurosurgical applications of tractography in the UK
Published in British Journal of Neurosurgery, 2021
Sebastian M. Toescu, Patrick W. Hales, Martin M. Tisdall, Kristian Aquilina, Christopher A. Clark
Tractography derived from diffusion MRI is a useful tool in the arsenal of the modern neurosurgeon. In this UK-based survey of practising neurosurgeons, we show that predominantly DTI-based reconstructions are used, that tumour resection remains the most frequent use of the technique, and that large tracts such as the corticospinal tract are most frequently identified. The results point out a number of limitations with the technique, many of which are inherent, such as inaccuracy in representing underlying anatomy, and intra-operative brain shift. The advent of iMRI and rapid-acquisition high angular resolution imaging may mitigate some of the perceived limitations of tractography described in this report. We urge units using tractography to adopt standardised procedures for tract reconstruction, and hope that broader collaboration in the field can lead to the development of ‘best practice’ in this area.
Related Knowledge Centers
- Contrast
- Diffusion
- In Vivo
- Macromolecule
- Tissue
- Tractography
- White Matter
- Mri Sequence
- Biological Membrane
- Spin–Lattice Relaxation