Abstract:
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Purpose: Evaluate the effects of signal apodization in the slice-encoding
direction on the reduction of Gibb's ringing in 3D imaging.
Methods and Materials: In Fourier encoded 3D imaging, Gibb's ringing resulting
from discrete sampling occurs not only in the frequency and phase directions
but also in the slice direction. Two specific windows (modified Hanning and
modified Tukey) were analyzed for their effect on measured slice thickness as
well as their apparent effect on ringing both in the original source images and
in orthogonal reformats. The modified Hanning (mHanning) includes an additional
parameter that controls the amount of roll-off. The modified Tukey (mTukey)
window is a piece-wise continuous window in which the central region is flat
and rolls off towards the edge, with the width of the flat region and the
minimum value at the windows ends being controllable. NEMA standard slice
thickness measurements were performed on a Signa Lx 1.5T scanner (GE Medical
Systems, Milwaukee, WI) for a 32 1.5 mm slice thickness volume and compared to
the theoretically calculated full-width half-maximum (FWHM) of the window
frequency response for eleven different mHanning and seven different mTukey
windows. Three new windows (current GE product, one mHanning, and one mTukey)
were tried on an in vivo 3D FIESTA brain and contrast-enhanced carotid
scans for their effects on ringing and apparent increase in slice thickness.
Results: Theoretical and measured slice thickness were in close agreement with
errors ranging from -0.72% to 3.04% and 0.91% to 4.60% for each of the central
two slices, respectively. The measured effective slice thickness was generally
slightly larger than predicted by theory - most likely due to phantom
positioning. As predicted by theory, even when no window is applied the
theoretical FWHM is still 20% larger than the nominal slice thickness. The in
vivo data demonstrated significantly less ringing and increased signal to
noise both in the original source images and in the reformats with the mTukey
window giving the overall best trade-off.
Conclusion: In high contrast applications even with relatively thin slices,
additional apodization in the slice encoding direction is needed to reduce the
inherent ringing. The amount of windowing needed is fairly subjective and also
application dependent. A second finding is that to fairly compare slice
thickness between 2D and 3D images, the inherent differences between the
definitions of slice thickness must first be taken into account. (J.A.P., H.W.,
F.F. are employees of GE Medical Systems.)
Questions about this event email: jason.polzin@med.ge.com
Polzin PhD, J,
Apodization in the Slice Encoding Direction for 3D Imaging. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3102386.html
