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Result : Searchterm 'Bandwidth' found in 1 term [] and 19 definitions []
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Forum  (18)  
 
Selective Excitation
 
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.
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Ultrasound  (9) Open this link in a new window
Signal to Noise Ratio
 
(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 bandwidths. 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
 
Images, Movies, Sliders:
 Brain MRI Images T1  Open this link in a new window
      

 
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• View the DATABASE results for 'Signal to Noise Ratio' (48).Open this link in a new window


• View the NEWS results for 'Signal to Noise Ratio' (2).Open this link in a new window.
 
Further Reading:
  Basics:
Musculoskeletal MRI at 3.0 T: Relaxation Times and Image Contrast
Sunday, 1 August 2004   by www.ajronline.org    
  News & More:
Picture-Perfect Particles Enhance MRI Signal
Friday, 13 April 2012   by cen.acs.org    
Researchers design 'intelligent' metamaterial to make MRIs affordable and accessible
Tuesday, 5 November 2019   by phys.org    
Metamaterials boost sensitivity of MRI machines
Thursday, 14 January 2016   by www.eurekalert.org    
Optimizing Musculoskeletal MR
   by rad.usuhs.mil    
MRI Resources 
Diffusion Weighted Imaging - Safety pool - Jobs - Contrast Enhanced MRI - Nerve Stimulator - Movies
 
Slice SelectionForum -
related threads
 
Exclusive excitation of spins in one slice performed by the coincident combination of a gradient magnetic field and a narrow bandwidth or slice selective RF pulse at a specific Larmor frequency.
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• View the DATABASE results for 'Slice Selection' (8).Open this link in a new window

 
Further Reading:
  Basics:
RARE
Monday, 3 December 2012   by www2.warwick.ac.uk    
  News & More:
Magnetic resonance imaging
   by www.scholarpedia.org    
Searchterm 'Bandwidth' was also found in the following services: 
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Susceptibility ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Artifact Information
DESCRIPTION
Signal dropout, bright spots, spatial distortion
REASON
HELP
Remove the metal, do not take a gradient echo sequence, take a short echo time
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.
mri safety guidance
Image Guidance
Use a spin echo or a fast spin echo sequence, because gradient echo sequences are more sensitve to susceptibility artifacts. A high bandwidth (small water fat shift) and a short echo time help also to reduce this artifact.
In some cases it is even beneficial to use a gradient echo sequence, e.g. a cavernom contains some iron-rich haemosiderin, which also causes a signal void on gradient echo sequences and for this purpose increases the diagnostic image quality.
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• View the DATABASE results for 'Susceptibility Artifact' (8).Open this link in a new window

 
Further Reading:
  Basics:
MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
Susceptibility Artifacts
   by www.mritutor.org    
  News & More:
Metal Artefact Reduction
Thursday, 9 June 2011   by www.revisemri.com    
Ultrashort echo time (UTE) MRI of the spine in thalassaemia
February 2004   by bjr.birjournals.org    
Searchterm 'Bandwidth' was also found in the following service: 
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Ultrasound  (9) Open this link in a new window
Ultrashort Turbo Spin EchoInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.MRI Resource Directory:
 - Sequences -
 
(UTSE) The ultrashort turbo spin echo (TSE / FSE) sequence is a technique with extremely short echo spacing, resulting in shorter scan times. This is an advantage in areas where motion is a problem, for example dynamic or abdominal imaging. The shorter scan time and echo spacing are achieved by using a higher TSE factor and an increased data sampling rate.
Disadvantages are the decrease in SNR (caused through the increase of the bandwidth) and artifacts if minimum echo spacing is used (incomplete dephasing of the 180° pulse FID).

See also Fast Spin Echo.
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Further Reading:
  News & More:
Ultrashort echo time (UTE) MRI of the spine in thalassaemia
February 2004   by bjr.birjournals.org    
MRI Resources 
Quality Advice - Breast Implant - Societies - MR Myelography - Image Quality - Blood Flow Imaging
 
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