MRI - DTI - CBIC, Amy Chan:
Brief description of DTI (diffusion tensor imaging) basic principals and applications (such as the mapping of neural tracts). CBIC Web Notes provided by Amy Chan, BS Copyright 2006.
fMRI (Functional Magnetic Resonance Imaging) and its relation to DTI
fMRI is a non-invasive in vivo method for characterizing neuronal function. MRI signal is sensitive to changes in blood oxygenation. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. The magnetic state of hemoglobin (Hb) depends upon its oxygenation, so that changes in oxygen saturation of Hb produce a small change in the local MR signal, the blood oxygenation level-dependent (BOLD) effect. Specifically, deoxygenated Hb is paramagnetic and tends to reduce the local MR signal by creating microscopic field gradients within and around the blood vessels. With fMRI, we will be able to see areas of the brain that work together during a particular perception, cognition, memory, or action. We can link these activated areas of cortex by mapping cortical interconnections with tract tracing techniques, such as diffusion tensor maps.
DWI/ DTI (Diffusion Weighted Imaging/Diffusion Tensor Imaging) Diffusion is a random molecular motion, also known as Brownian motion. Biological systems depend on diffusion for the normal functioning. An example is the transport of metabolites into cells if facilitated by diffusion. The study of diffusion will provide insights into both cell physiology and cell structure. Unlike other MR parameters, such as the longitudinal and transverse relaxation time constants (T1 and T2) that are affected by experimental MR parameters, diffusion is an intrinsic property that is independent of the MR procedure employed to measure it.
Diffusion Weighted Imaging (DWI) makes use of the Brownian motion of water molecules in biological tissue. In biological tissues, diffusion is not truly random because tissue has structure. Cell membraes, vascular structures and axon cylinders limit or restrict the amount of diffusion. Therefore, in the brain, water diffusion is referred to as “apparent diffusion” coefficient (ADC). To obtain diffusion-weighted images, a pair of strong gradient pulses are added to the pulse sequence. The first pulse dephases the spins, and the second pulse rephases the spins if no net movement occurs. If net movement of spins occurs between the gradient pulses, signal attenuation occurs. The degree of attenuation depends on the magnitude of molecular translation and diffusion weighting. The amount of diffusion weighting is determined by the strength of the diffusion gradients, the duration of the gradients, and the time between the gradient pulses. The more attenuated the image is at a give position, the more diffusion there is locally. However, this image intensity varies when the spatial direction or the diffusion gradient is changed. As a result, diffusion tensor (see below) models are used to account for the changes.
Image courtesy of J.M. Tszyska
Diffusion imaging is currently the most sensitive way to image an acute infarct which has a DECREASED apparent diffusion coefficient. On a diffusion weighted image, an acute infarct is hyperintense. Cerebral Spinal Fluid (CSF), which has a higher diffusion coefficient, is hypointense. DTI is useful to diagnose vascular strokes in the brain, study diseases of the white matter and to see connectivity of the brain.
Diffusion Tensor Imaging (DTI) is the measure of tensor directly from diffusion-weighted data. A tensor is used to describe diffusion in anisotropic systems. DTI allows us to visualize the location, orientation and anisotropy of the brain’s white matter tracts. White matter diffusion property preferentially orients in one direction called anisotropic diffusion. Applying diffusion gradients in diffusion MRI, in at least 6 directions, it is possible to calculate a tensor (i.e. a 3 x 3 matrix) that describes the 3-dimensional shape of diffusion. The fiber direction will be indicated by the tensor’s main eigenvector. DTI is useful in studying tractography within white matter.
Reference: Jones, D. (2005). Fundamentals of diffusion MR Imaging. Clinical MR Neuroimaging: Diffusion, Perfusion and Spectroscopy. Gillard J, Waldman A and Barker P. Cambridge, Cambridge University Press: 54-85.
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