- Artifact
- Any feature that appears in an image that is not present in the original imaged object.
Artifact Cause RF Quadrature Failure of the RF detection circuitry Bo Inhomogeneity Metal object distorting the Bo field Gradient Failure in a magnetic field gradient RF Inhomogeneity Failure of RF coil Motion Movement of the imaged object during the sequence Flow Movement of body fluids during the sequence Chemical Shift Large Bo and chemical shift difference between tissues Partial Volume Large voxel size Wrap Around Improperly chosen field of view
- RF Quadrature
- Bo Inhomogeneity
- All magnetic resonance imaging assumes a homogeneous Bo magnetic field.
Image of water filled straight tubes positioned so as to form a square.
- Gradient
An image where the frequency encoding (left/right encoding) gradient is operating at half of its expected value.
- RF Inhomogeneity
- An RF inhomogeneity problem is a variation in intensity across an image.
Cause: nonuniform B1 field or an nonuniform sensitivity in a receive only coil.
For example, a metal object that prevents the RF field from passing into a tissue will cause a signal void in an image.
RF inhomogeneity artifact in the region of the mouth.
- Motion
- Movement of any portion of the imaged object results in a blurring of that portion of the object across the image.
Example: A single, fixed, signal containing object is imaged.
The central portion of the MX raw data.
FT first in the frequency encoding direction yields a single oscillating peak.
Viewing the data as a function of a phase.
FT in the phase encoding direction.
The same example, except now midway through the acquisition of phase encoding steps the object moves to a new location in the frequency encoding direction.
The central part of the MX raw data.
FT first in the frequency direction gives two oscillating peaks which abruptly stop oscillating.
Viewing the data as a function of a phase.
FT in the phase encoding direction gives several repeating peaks at the two frequencies. This is because the Fourier pair of an abruptly truncated sine wave is a sinc function. The magnitude representation of the data makes all the peaks positive.
Motion artifact from a blood vessel in the posterior side of the head.
Solution: Immobilize the patient or imaged object.Use Cardiac Gating for blood flow motion.
Copyright © 2000 J.P. Hornak.
All Rights Reserved.
All Rights Reserved.