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Result : Searchterm 'DWI' found in 1 term [ ] and 40 definitions [ ]
| previous 36 - 40 (of 41) nextResult Pages : [1] [2 3 4 5 6 7 8 9] | ![](gifs/nb.gif) | | ![](gifs/mores_2.gif) | Searchterm 'DWI' was also found in the following services: | | | | |
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Controlling the frequency spectrum ( bandwidth) of a RF pulse (via tailoring) while imposing a magnetic field gradient on spins, such that only a desired region will have an appropriate resonant frequency to be excited.
Originally used to excite all but a desired region; now often used to select only a desired region, such as a plane, for excitation. Used without simultaneous magnetic field gradients, tailored RF pulses can be used to selectively excite a particular spectral line or group of lines. RF and gradient pulse combinations can be designed to select both spatial regions and spectral frequencies. | | ![spacer](gifs/nb.gif) | | | |
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( SNR or S/N) The signal to noise ratio is used in MRI to describe the relative contributions to a detected signal of the true signal and random superimposed signals ('background noise') - a criterion for image quality.
One common method to increase the SNR is to average several measurements of the signal, on the expectation that random contributions will tend to cancel out. The SNR can also be improved by sampling larger volumes (increasing the field of view and slice thickness with a corresponding loss of spatial resolution) or, within limits, by increasing the strength of the magnetic field used. Surface coils can also be used to improve local signal intensity. The SNR will depend, in part, on the electrical properties of the sample or patient being studied.
The SNR increases in proportion to voxel volume (1/resolution), the square root of the number of acquisitions ( NEX), and the square root of the number of scans ( phase encodings). SNR decreases with the field of view squared (FOV2) and wider ban dwidths. See also Signal Intensity and Spin Density.
Measuring SNR:
Record the mean value of a small ROI placed in the most homogeneous area of tissue with high signal intensity (e.g. white matter in thalamus). Calculate the standard deviation for the largest possible ROI placed outside the object in the image background (avoid ghosting/aliasing or eye movement artifact regions).
The SNR is then:
Mean Signal/Standard Deviation of Background Noise | | | | ![spacer](gifs/nb.gif) | | • View the DATABASE results for 'Signal to Noise Ratio' (48).![Open this link in a new window](gifs/nw3_g.gif)
| | | • View the NEWS results for 'Signal to Noise Ratio' (2).![Open this link in a new window. Open this link in a new window.](gifs/nw3_g.gif)
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| | ![spacer](gifs/nb.gif) | | • View the DATABASE results for 'Slice Selection' (8).![Open this link in a new window](gifs/nw3_g.gif)
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RARE Monday, 3 December 2012 by www2.warwick.ac.uk |
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| ![](gifs/nb.gif) | ![](gifs/mores_2.gif) | Searchterm 'DWI' was also found in the following services: | | | | |
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Special imaging primarily means advanced MRI techniques used for qualitative and quantitative measurement of biological metabolism as e.g., spectroscopy, perfusion imaging (PWI, ASL), diffusion weighted imaging ( DWI, DTI, DTT) and brain function ( BOLD, fMRI). This physiological magnetic resonance techniques offer insights into brain structure, function, and metabolism.
Spectroscopy provides functional information related to identification and quantification of e.g. brain metabolites.
MR perfusion imaging has applications in stroke, trauma, and brain neoplasm. MRI provides the high spatial and temporal resolution needed to measure blood flow to the brain. arterial spin labeling techniques utilize the intrinsic protons of blood and brain tissue, labeled by special preparation pulses, rather than exogenous tracers injected into the blood.
MR diffusion tensor imaging characterizes the ability of water to spread across the brain in different directions. Diffusion parallel to nerve fibers has been shown to be greater than diffusion in the perpendicular direction. This provides a tool to study in vivo fiber connectivity in brain MRI.
FMRI allows the detection of a functional activation in the brain because cortical activity is intimately related to local metabolism changes. See also Diffusion Tensor Tractography. | | ![spacer](gifs/nb.gif) | | • View the NEWS results for 'Special Imaging' (14).![Open this link in a new window. Open this link in a new window.](gifs/nw3_g.gif)
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Quick Overview
Materials with magnetic susceptibility cause this artifact. There are in general three kinds of materials with magnetic susceptibility: ferromagnetic materials (iron, nickel etc.) with a strong influence and paramagnetic/diamagnetic (aluminium, platinum etc./gold, water, most organic compounds etc.) materials with a minimal/non influence on magnetic fields. In MRI, susceptibility artifacts are caused for example by medical devices in or near the magnetic field or by implants of the patient. These materials with magnetic susceptibility distort the linear magnetic field gradients, which results in bright areas (misregistered signals) and dark areas (no signal) nearby the magnetic material.
Image Guidance
| | ![spacer](gifs/nb.gif) | | • View the DATABASE results for 'Susceptibility Artifact' (8).![Open this link in a new window](gifs/nw3_g.gif)
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