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Machine Imperfection 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 Machine imperfection, data error
DESCRIPTION Striped ghosts with a shift of half the field of view
REASON Non-uniform sampling, phase differences
HELP Data correction

Machine imperfection-based artifacts manifest themselves due to the fact that the odd k-space lines are acquired in a different direction than the even k-space lines. Slight differences in timing result in shifts of the echo in the acquisition window. By the shift theorem, such shifts in the time domain data then produce linear phase differences in the frequency domain data.
Without correction, such phase differences in every second line produce striped ghosts with a shift of half the field of view, so-called Nyquist ghosts. Shifts in the applied magnetic field can also produce similar (but constant in amplitude) ghosts.
This artifact is commonly seen in an EPI image and can arise from both, hardware and sample imperfections.
A further source of machine-based artifact arises from the need to acquire the signal as quickly as possible. For this reason the EPI signal is often acquired during times when the gradients are being switched. Such sampling effectively means that the k-space sampling is not uniform, resulting in ringing artifacts in the image.

Image Guidance
Such artifacts can be minimized by careful setup of the spectrometer and/or correction of the data. For this reasons reference data are often collected, either as a separate scan or embedded in the imaging data. The non-uniform sampling can be removed by knowing the form of the gradient switching. It is possible to regrid the data onto a uniform k-space grid.

• View the DATABASE results for 'Machine Imperfection Artifact' (2).Open this link in a new window

Further Reading:
MRI Artifact Gallery
Macroscopic Magnetic Moment
See Macroscopic Magnetization Vector.
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Macroscopic Magnetization Vector
Net magnetic moment per unit volume (a vector quantity) of a sample in a given region, considered as the integrated effect of all the individual microscopic nuclear magnetic moments.

• View the DATABASE results for 'Macroscopic Magnetization Vector' (9).Open this link in a new window

Magic Angle Effect (Artifact)InfoSheet: - Artifacts -
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
Quick Overview

Artifact Information
NAME Magic angle
DESCRIPTION Increase of the T2 time, bright signal in tendons
REASON Angle about 55° to the main magnetic field
HELP Angle not about 55°

The magic angle is a precisely defined angle, the value is approximately 54.7°. Hence, two nuclei with a dipolar coupling vector at an angle of approximately 54.7° to a strong external magnetic field have zero dipolar coupling.
Magic angle spinning is a technique in solid-state NMR spectroscopy, which employs this principle to remove or reduce dipolar couplings, thereby increasing spectral resolution. In MRI, the magic angle effect visualizes as bright spots through an increased T2 time on short echo time (TE) images, for e.g. collagen fibers of tendons and ligaments, which are oriented at the magic angle of approximately 54.7° to the magnetic field.

Image Guidance
Take care that tendons and ligaments are not oriented at about a 54.7° angle to the main magnetic field.
Further Reading:
Magic angle
Magic Angle Effects
Magic Pixie Dust
Your MRI system is often sensitive. It needs a lot of service to keep it in a good mood. Magic Pixie Dust makes it smile. It is pure magic.
Further Reading:
  News & More:
A powder to enhance NMR signals
Thursday, 12 December 2013   by    
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