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 'Field Inhomogeneity Artifact' 
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Field Inhomogeneity ArtifactInfoSheet: - Artifacts - 
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Reduction Index, 
etc.MRI Resource Directory:
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Quick Overview
Artifact Information
NAME
Field inhomogeneity
DESCRIPTION
Image distortion signal loss
REASON
HELP
Larger FOV, oversampling
A disturbance of the field homogeneity, because of magnetic material (inside or outside the patient), technical problems or scanning at the edge of the field.
When images were obtained in a progression from the center to the edge of the coil, the homogeneity of the field observed by the imaged volume, changes when the distance from the center of the volume increase. The same problem appears by scanning at a distance from the isocenter in left-right direction or too large field of view.
There are different types of bad image quality, the images are noisy, distorted or the fat suppression doesn't work because of badly set shim currents.
E.g. by using an IR sequence, changes in the T1 recovery rates of the tissues are involved. The inversion time at the center of the imaged volume is appropriate to suppress fat, but at the edge of the coil the same inversion time is sufficient to suppress water. Since the inversion time is not changed, the T1 recovery rates will increase.
mri safety guidance
Image Guidance
Take a smaller imaging volume (and for fat suppression a volume shimming), take care that the imaged region is at the center of the coil and that no magnetic material is inside the imaging volume.
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Further Reading:
  Basics:
MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
MRI Resources 
MRA - MRI Technician and Technologist Career - Calculation - Anatomy - Mass Spectrometry - Image Quality
 
Field of View
 
(FOV) Defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of mm². The FOV is the square image area that contains the object of interest to be measured. The smaller the FOV, the higher the resolution and the smaller the voxel size but the lower the measured signal. Useful for decreasing the scantime is a field of view different in the frequency and phase encoding directions (rectangular field of view - RFOV).
The magnetic field homogeneity decreases as more tissue is imaged (greater FOV). As a result the precessional frequencies change across the imaging volume. That can be a problem for fat suppression imaging. This fat is precessing at the expected frequency only in the center of the imaging volume. E.g. frequency specific fat saturation pulses become less effective when the field of view is increased. It is best to use smaller field of views when applying fat saturation pulses.
mri safety guidance
Image Guidance
Smaller FOV required higher gradient strength and concludes low signal. Therefore you have to find a compromise between these factors. The right choice of the field of view is important for MR image quality. When utilizing small field of views and scanning at a distance from the isocenter (more problems with artifacts) it is obviously important to ensure that the region of interest is within the scanning volume.
A smaller FOV in one direction is available with the function rectangular field of view (RFOV).

See also Field Inhomogeneity Artifact.
 
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Further Reading:
  Basics:
Image Characteristics and Quality
   by www.sprawls.org    
  News & More:
Optimizing Musculoskeletal MR
   by rad.usuhs.mil    
Path Found to a Combined MRI and CT Scanner
Wednesday, 20 March 2013   by spectrum.ieee.org    
MRI Resources 
Examinations - Directories - Equipment - MRA - Societies - Journals
 
Periodically Rotated Overlapping Parallel Lines with Enhanced ReconstructionInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
(PROPELLER) The PROPELLER MRI technique reduces the sensitivity to various sources of image artifacts (e.g., motion artifact, field inhomogeneity artifact, eddy current artifact). PROPELLER can be used with gradient echo and turbo spin echo sequences in a wide range of applications to improve the image quality, for example cardiac MRI, brain MRI, and pediatric examinations.
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Further Reading:
  Basics:
Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction(PROPELLER) MRI; Application to Motion Correction
1999   by cds.ismrm.org    
MR Field Notes
   by www.gehealthcare.com    
Advances in Magnetic Resonance Neuroimaging
Friday, 27 February 2009   by www.ncbi.nlm.nih.gov    
  News & More:
Patient movement during MRI: Additional points to ponder
Tuesday, 5 January 2016   by www.healthimaging.com    
New MR sequence helps radiologists more accurately evaluate abnormalities of the uterus and ovaries
Thursday, 23 April 2009   by www.eurekalert.org    
MRI Resources 
Societies - Health - MR Guided Interventions - Patient Information - Sequences - MRA
 
Gradient EchoForum -
related threads
 
(GE) An echo signal generated from a free induction decay by means of a bipolar switched magnetic gradient. The echo is produced by reversing the direction of a magnetic field gradient or by applying balanced pulses of magnetic field gradient before and after a refocusing RF pulse so as to cancel out the position dependent phase shifts that have accumulated due to the gradient.
In the latter case, the gradient echo is generally adjusted to be coincident with the RF spin echo. When the RF and gradient echoes are not coincident, the time of the gradient echo is denoted echo time (TE) and the difference in time between the echoes is denoted time difference (TD).
Gradient echo does not refocus the effects of main field inhomogeneity and therefore is generally used with a short echo time. Disadvantages of gradient echo imaging are compromised anatomic details and artifacts in regions with varying susceptibility e.g. between the air-containing sinuses and brain and especially between haemorrhages and normal tissue.

See also Susceptibility Artifact.
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• View the DATABASE results for 'Gradient Echo' (121).Open this link in a new window

 
Further Reading:
  Basics:
Mapping of low flip angles in magnetic resonance(.pdf)
Saturday, 1 January 2011   by www.hal.inserm.fr    
A LANTHANIDE LANTHOLOGY(.pdf)
   by www.phy.davidson.edu    
Enhanced Fast GRadient Echo 3-Dimensional (efgre3D) or THRIVE
   by www.mri.tju.edu    
MRI Resources 
Colonography - MR Myelography - Software - Cochlear Implant - Mobile MRI Rental - Fluorescence
 
Spin EchoForum -
related threads
 
(SE) The Reappearance of the MR signal after the FID has apparently died away, as a result of the effective reversal (rephasing) of the dephasing spins by techniques such as specific RF pulse sequences or pairs of field gradient pulses, applied in time shorter than or on the order of T2. Proper selection of the TE time of the pulse sequence can help to control the amount of T1 or T2 contrast present in the image. Pulse sequences of the spin echo type, usually employs a 90° pulse, followed by one or more 180° pulses to eliminate field inhomogeneity and chemical shift effects at the echo. Caused by this 180° refocusing pulse, spin echo or fast spin echo (FSE, TSE) sequences are more robust against e.g. susceptibility artifacts than sequences of the gradient echo type.
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• View the DATABASE results for 'Spin Echo' (96).Open this link in a new window


• View the NEWS results for 'Spin Echo' (1).Open this link in a new window.
 
Further Reading:
  Basics:
Spin echoes, CPMG and T2 relaxation - Introductory NMR & MRI from Magritek
2013   by www.azom.com    
  News & More:
EVALUATION OF HUMAN STROKE BY MR IMAGING
2000
MRI Resources 
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