Diffusion Tensor Imaging
Diffusion tensor imaging (DTI) is an advanced magnetic resonance imaging (MRI) technique that provides detailed information about tissue microstructure such as fiber orientation, axonal density, and degree of myelination. DTI is based on the measurement o
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Diffusion Tensor Imaging Inga Katharina Koerte and Marc Muehlmann
Abbreviations AD ADC ADHD BET CSF DKI DTI DWI FA MD MRI RD ROI TBSS
5.1
Axial diffusivity Apparent diffusion coefficient Attention deficit hyperactivity disorder Brain extraction tool Cerebrospinal fluid Diffusion kurtosis imaging Diffusion tensor imaging Diffusion-weighted imaging Fractional anisotropy Trace, mean diffusivity Magnetic resonance imaging Radial diffusivity Region of interest Tract-based spatial statistics
Introduction
Diffusion tensor imaging (DTI) is an advanced magnetic resonance imaging (MRI) technique that provides detailed information about tissue microstructure such as fiber orientation, axonal density, and degree of myelination. DTI is based on the measurement of the diffusion of water molecules. It was developed in the early 1990s and since then has been applied in a wide
I.K. Koerte, MD (*) • M. Muehlmann, MD Division of Radiology, Ludwig-MaximiliansUniversity, Munich, Germany e-mail: [email protected]; [email protected]
variety of scientific and clinical settings, especially, but not limited to, the investigation of brain pathology in schizophrenia (Shenton et al. 2001; Qiu et al. 2009, 2010), Alzheimer’s disease (Damoiseaux et al. 2009; Mielke et al. 2009; Avants et al. 2010; Gold et al. 2010; Jahng et al. 2011), and autism (Pugliese et al. 2009; Cheng et al. 2010; Fletcher et al. 2010). This chapter describes the basics of the technique, outlines resources needed for acquisition, and focuses on post-processing techniques and statistical analyses with and without a priori hypotheses.
5.2
Basic Principles of Diffusion
5.2.1 Brownian Motion Water molecules are constantly moving due to their thermal energy at temperatures above zero. In 1827, Robert Brown described the phenomena of particles suspended in fluid that move randomly (Brown 1827). Starting from the same position, the movement paths of single water molecules are unpredictable and end up in unforeseeable positions at time x. The process of diffusion depends upon a concentration gradient that was described by Fick’s first law of diffusion. Fick’s second law calculates the distance a particle diffuses in a certain time based on the diffusion coefficient (Fick 1855). Albert Einstein derived Fick’s second law from thermodynamic laws, thereby building toward what has evolved
C. Mulert, M.E. Shenton (eds.), MRI in Psychiatry, DOI 10.1007/978-3-642-54542-9_5, © Springer Berlin Heidelberg 2014
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I.K. Koerte and M. Muehlmann
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λ3
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Fig. 5.1 Isotropic and anisotropic diffusion
to diffusion theory. Further, Einstein was able to mathematically explain Brownian motion by determining the mean squared displacement of a single particle (Einstein 1905).
5.2.2 Isotropic Diffusion During the process of diffusion, the random nature of Brownian motion causes molecules to move passively from a region with high concentration to a region with low concentration. If a drop of ink is dropped in a glass filled with water, the drop seems to s
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