Magnetic Resonance Imaging

Joseph P. Hornak, Ph.D.

Image Artifacts - II

Flow

The timing diagram and side view of the imaged slice.

Flow during the slice selective 90^o and the 180^o pulses.

Example

In a multislice sequence, vessels in some slices might look bright.

Chemical Shift

A chemical shift artifact is caused by the difference in chemical shift (Larmor frequency) of fat and water. The artifact manifests itself as a misregistration between the fat and water pixels in an image.
The difference in chemical shift is approximately 3.5 ppm which at 1.5 Tesla corresponds to a frequency difference between that of fat and water is approximately 220 Hz.
The slice selection process when two chemical shift components present.

Assume 9 voxels with water. The center one has some fat.

In a uniform magnetic field the vectors precess at their own Larmor frequency.

Application of the phase encoding gradient.

The phase encoding gradient is turned off.

Application of the frequency encoding gradient.

The resultant image places the fat in the upper right voxel rather than in the center.

The magnitude of the effect is proportional to B_o and inversely proportional to the sampling rate. At a 16 kHz and 256 pixels, the effect is 3.5 pixels.
In this axial slice image through the legs there is a chemical shift artifact between the fat and muscle in the legs.

Partial Volume

A partial volume artifact is any artifact which is caused by the size of the image voxel.

3 mm 10 mm

Wrap Around

The occurrence of a part of the imaged anatomy, which is located outside of the field of view, inside of the field of view.

Digital Filtering
The application of oversampling, digital filtering, and decimation to reduce the wrap around problem.
Oversampling creates a larger FOV, but generates too much data to be conveniently stored.
Digital filtering eliminates the high frequency components from the data.
Decimation reduces the size of the data set.

Explanation using the convolution theorem.

Decimation is the elimination of data points from a data set. A decimation ratio of 4/5 means that 4 out of every 5 data points are deleted, or every fifth data point is saved.
High speed digitizers, capable of digitizing at 2 MHz, and dedicated high speed integrated circuits, capable of performing the convolution on the time domain data as it is being recorded, are used to realize this procedure.

Copyright © 2000 J.P. Hornak.
All Rights Reserved.