A data
truncation
artifact may occur when the interface between high and low signal intensities is encountered in one imaging plane. The 2D-FT techniques
transform the MR signal to spatial intensity image data with
frequency and
phase information encoding each axis in the plane of the scan. This
artifact is found in both
frequency and
phase axes.
Artifactual ripples adjacent to edges in an image or sharp features in a
spectrum, caused by omission of higher
frequency terms in
Fourier transformation, particularly with the use of
zero filling to replace unsampled higher frequencies.
Complex shapes are specified by series of sine and cosine waves of various frequencies,
phase and
amplitude. Some shapes are more difficult to encode than others. The most difficult shapes to represent with Fourier series of terms are waveforms with instantaneous
transitions, tissue discontinuities or edges. The low-frequency components of the series describe the overall shape of the step function. Higher
frequency components are needed to describe the corners if the step function more accurately.
If not enough samples are taken, these areas cannot be accurately represented.
The
truncation of the infinite data series results in a ringing
artifact because of the inability to accurately approximate this tissue discontinuity with a shorter
truncated data set. Therefore, the ringing that occurs at all tissue boundaries on MR is called
truncation artifact.
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