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Result : Searchterm 'Magnetic Field' found in 5 terms [] and 219 definitions []
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Searchterm 'Magnetic Field' was also found in the following services: 
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News  (76)  Resources  (16)  Forum  (18)  
 
Chemical Shift ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Please note that there are different common names for this artifact.
Artifact Information
NAME
Chemical shift, black boundary, spatial misregistration, relief
DESCRIPTION
Black or bright band
During frequency encoding, fat protons precess slower than water protons in the same slice because of their magnetic shielding. Through the difference in resonance frequency between water and fat, protons at the same location are misregistrated (dislocated) by the Fourier transformation, when converting MRI signals from frequency to spatial domain. This chemical shift misregistration cause accentuation of any fat-water interfaces along the frequency axis and may be mistaken for pathology. Where fat and water are in the same location, this artifact can be seen as a bright or dark band at the edge of the anatomy.
Protons in fat and water molecules are separated by a chemical shift of about 3.5 ppm. The actual shift in Hertz (Hz) depends on the magnetic field strength of the magnet being used. Higher field strength increases the misregistration, while in contrast a higher gradient strength has a positive effect. For a 0.3 T system operating at 12.8 MHz the shift will be 44.8 Hz compared with a 223.6 Hz shift for a 1.5 T system operating at 63.9 MHz.
mri safety guidance
Image Guidance
For artifact reduction helps a smaller water fat shift (higher bandwidth), a higher matrix, an in phase TE or a spin echo technique. Since the misregistration offset is present in the read out axis the patient may be rescanned with this axis parallel to the fat-water interface. Steeper gradient may be employed to reduce the chemical shift offset in mm. Another strategy is to employ specialized pulse sequences such as fat saturation or inversion recovery imaging. Fat suppression techniques eliminate chemical shift artifacts caused by the lack of fat signal.

See also Black Boundary Artifact and Magnetic Resonance Spectroscopy.
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• Related Searches:
    • Bandwidth
    • Dual Echo Fast Gradient Echo
    • Matrix Size
    • Chemical Shift
    • Frequency Encoding
 
Further Reading:
  Basics:
MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
  News & More:
What is chemical shift artefact? Why does it occur? How many Hz at 1.5 T?
   by www.revisemri.com    
Abdominal MRI at 3.0 T: The Basics Revisited
Wednesday, 20 July 2005   by www.ajronline.org    
Searchterm 'Magnetic Field' was also found in the following services: 
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Radiology  (9) Open this link in a new windowUltrasound  (2) Open this link in a new window
Coherent Gradient EchoInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.MRI Resource Directory:
 - Sequences -
 
Coherent gradient echo sequences can measure the free induction decay (FID), generated just after each excitation pulse or the echo formed prior to the next pulse. Coherent gradient echo sequences are very sensitive to magnetic field inhomogeneity. An alternative to spoiling is to incorporate residual transverse magnetization directly into the longitudinal steady state. These GRE sequences use a refocusing gradient in the phase encoding direction during the end module to maximize remaining transverse (xy) magnetization at the time when the next excitation is due, while the other two gradients are, in any case, balanced.
When the next excitation pulse is sent into the system with an opposed phase, it tilts the magnetization in the -a direction. As a result the z-magnetization is again partly tilted into the xy-plane, while the remaining xy-magnetization is tilted partly into the z-direction.
A fully refocused sequence with a properly selected and uniform f would yield higher signal, especially for tissues with long T2 relaxation times (high water content) so it is used in angiographic, myelographic or arthrographic examinations and is used for T2* weighting. The repetition time for this sequence has to be short. With short TR, coherent GE is also useable for breath hold and 3D technique. If the repetition time is about 200 msec there's no difference between spoiled or unspoiled GE. T1 weighting is better with spoiled techniques.
The common types include GRASS, FISP, FAST, and FFE.
The T2* component decreases with long TR and short TE. The T1 time is controlled by flip angle. The common TR is less than 50 ms and the common TE less than 15 ms
Other types have stronger T2 dependence but lower SNR. They include SSFP, CE-FAST, PSIF, and CE-FFE-T2.
Examples of fully refocused FID sequences are TrueFISP, bFFE and bTFE.
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• View the DATABASE results for 'Coherent Gradient Echo' (6).Open this link in a new window

MRI Resources 
Nerve Stimulator - Mobile MRI Rental - Anatomy - Hospitals - Service and Support - Artifacts
 
Continuous Wave NMR
 
(CW) A technique for studying NMR by continuously applying RF radiation to the sample and slowly sweeping either the RF frequency or the magnetic field through the resonance values; now largely superceded by pulse MR technique.
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• View the DATABASE results for 'Continuous Wave NMR' (2).Open this link in a new window

Searchterm 'Magnetic Field' was also found in the following services: 
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News  (76)  Resources  (16)  Forum  (18)  
 
ContrastForum -
related threads
 
Contrast is the relative difference of signal intensities in two adjacent regions of an image.
Due to the T1 and T2 relaxation properties in magnetic resonance imaging, differentiation between various tissues in the body is possible. Tissue contrast is affected by not only the T1 and T2 values of specific tissues, but also the differences in the magnetic field strength, temperature changes, and many other factors. Good tissue contrast relies on optimal selection of appropriate pulse sequences (spin echo, inversion recovery, gradient echo, turbo sequences and slice profile).
Important pulse sequence parameters are TR (repetition time), TE (time to echo or echo time), TI (time for inversion or inversion time) and flip angle. They are associated with such parameters as proton density and T1 or T2 relaxation times. The values of these parameters are influenced differently by different tissues and by healthy and diseased sections of the same tissue.
For the T1 weighting it is important to select a correct TR or TI. T2 weighted images depend on a correct choice of the TE. Tissues vary in their T1 and T2 times, which are manipulated in MRI by selection of TR, TI, and TE, respectively. Flip angles mainly affect the strength of the signal measured, but also affect the TR/TI/TE parameters.
Conditions necessary to produce different weighted images:
T1 Weighted Image: TR value equal or less than the tissue specific T1 time - TE value less than the tissue specific T2 time.
T2 Weighted Image: TR value much greater than the tissue specific T1 time - TE value greater or equal than the tissue specific T2 time.
Proton Density Weighted Image: TR value much greater than the tissue specific T1 time - TE value less than the tissue specific T2 time.

See also Image Contrast Characteristics, Contrast Reversal, Contrast Resolution, and Contrast to Noise Ratio.
 
Images, Movies, Sliders:
 Fetus (Brain) and Dermoid in Mother  Open this link in a new window
      

Courtesy of  Robert R. Edelman

 Circle of Willis, Time of Flight, MIP  Open this link in a new window
    
SlidersSliders Overview

 Anatomic MRI of the Knee 1  Open this link in a new window
    
SlidersSliders Overview

 Anatomic Imaging of the Liver  Open this link in a new window
      

 Brain MRI Inversion Recovery  Open this link in a new window
    
 
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• View the DATABASE results for 'Contrast' (373).Open this link in a new window


• View the NEWS results for 'Contrast' (77).Open this link in a new window.
 
Further Reading:
  Basics:
Magnetic resonance imaging
   by www.scholarpedia.org    
MRI's inside story
Thursday, 4 December 2003   by www.economist.com    
Image Characteristics and Quality
   by www.sprawls.org    
  News & More:
A natural boost for MRI scans
Monday, 21 October 2013   by www.eurekalert.org    
A groundbreaking new graphene-based MRI contrast agent
Friday, 8 June 2012   by www.nanowerk.com    
New MRI Chemical Offers Amazing Contrast
Friday, 22 January 2010   by news.softpedia.com    
Searchterm 'Magnetic Field' was also found in the following services: 
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Radiology  (9) Open this link in a new windowUltrasound  (2) Open this link in a new window
Coupling Constant
 
Spectral lines are split by spin spin coupling into multiplets whose frequencies are separated by an amount depending on the coupling constant, J. The magnitude of J is independent of the strength of the applied magnetic field and is given in units of frequency, Hz.
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• View the DATABASE results for 'Coupling Constant' (2).Open this link in a new window

 
Further Reading:
  News & More:
Spin Spin Coupling:The splitting of NMR Signals
   by iis.dmhcsm.edu.hk    
MRI Resources 
Online Books - Patient Information - MR Myelography - Research Labs - Intraoperative MRI - RIS
 
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