Biomedical Engineering Reference
In-Depth Information
interslice distance with depth, especially if the acquisition involves “rocking”
the transducer. One possible solution is to employ a technique called “spatial
compounding,” where tissue volume is repeatedly scanned from different
directions. This reduces coherent noise (speckle) and increases the pixel den-
sity, thus reducing the interslice distance. Spatial compounding has been
described in Rohling et al.
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5.5.1.3.2 Tissue Motion
Ultrasound is a contact scanning method and therefore a small amount of tis-
sue compression will always occur, causing a greater degree of spatial distor-
tion for more superficial scanning. Clearly the degree of compression can be
minimized by using as little pressure as possible.
5.5.1.3.3 Nonuniform Sampling
In general, 3D ultrasound images which have been obtained by moving the
ultrasound beam through a tissue volume will have nonuniform sampling.
For images produced by moving the transducer under computer control (i.e.,
fixed geometry), the slices will be parallel and therefore the interslice sam-
pling interval will be fixed. However different sampling intervals in the three
directions are likely. For images that have been produced by a freehand
sweep, the slices will not, in general, be parallel and the interslice sampling
interval will not be fixed. Each of these types of nonuniform sampling will be
considered in turn.
5.5.1.3.3.1 Fixed Geometry
Here the sampling interval is constant, but has
a different value in each of the three directions. This occurs frequently, since
the interslice separation is often much larger than either of the in-plane
sampling intervals. Provided the interslice separation is small enough to
sample the tissue volume adequately, there are no special problems with
this type of data. Precisely what constitutes “adequate sampling” will
depend on the minimum size of the structure which needs to be resolved.
For data display, it may be necessary to interpolate the data to give a cubic
voxel.
5.5.1.3.3.2 Freehand Geometry
There are two effects to consider here: the aver-
age interslice separation will vary and adjacent slices will not be parallel. The
combination of these two effects produces a 3D dataset where the separation
between data points varies in all three directions. Since the effect of a small
change in transducer orientation increases with depth, large separations
between data points will occur at large depths. Some degree of oversampling
will therefore be necessary at smaller depths to compensate for this effect, in
which case the acquisition method known as spatial compounding
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can be used.
It is generally necessary to interpolate the data to a regular voxel array so
that it can be displayed and
or compared with other imaging modalities; a
number of different algorithms have been proposed.
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