Biomedical Engineering Reference
In-Depth Information
The main interest here is in artifacts that give rise to position errors in the
reconstructed data. Because of the way 3D-US data is acquired, a discussion
of image inaccuracies in 3D-US must cover inaccuracies encountered in each
of its constituents, namely: conventional 2D ultrasound scan acquisition,
transducer position measurement, and the 3D acquisition process proper
where 2D slice and position data are acquired dynamically.
5.5.1
Sources of Geometric Inaccuracies and Measures to Prevent
or Minimize Them
5.5.1.1 Conventional 2D Ultrasound Scanning
The 2D ultrasound image acquisition process relies on the following two
assumptions: 1) the ultrasound pulse travels in a straight line and 2) the
speed of sound is constant. Departures from these assumptions will give rise
to refraction and depth errors, both of which may have an impact on regis-
tration. A comprehensive description of 2D ultrasound artifacts is given in
Cosgrove et al.
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5.5.1.1.1 Refraction
If an ultrasound pulse strikes a tissue interface at an oblique angle, the direc-
tion of travel will be changed if the speed of sound is different on either side of
the interface. For most soft tissue interfaces the effect is small. However, if the
propagation path includes fluid (such as in pelvic scans by the transabdominal
route), the effect can be significant. The effect of refraction is to deviate the path
of the ultrasound beam. Since the ultrasound image is built up by assuming
that sound travels in straight lines, regions of tissue which are affected by
refraction will be displayed incorrectly (see Figure 5.6).
It is very difficult to correct for refraction, since it is a fundamental property
of ultrasound imaging. The effect can be reduced by avoiding large angles of
incidence in regions of tissue where refraction is likely to be a problem.
5.5.1.1.2 Depth Errors
Depth is calculated by assuming a fixed speed of sound. Variations in the
speed of sound will give rise to depth errors, which tend to be small for most
soft tissue interfaces. However sound propagates in fat 10 to 15% more slowly
than in most other soft tissues and prosthetic materials show a more marked
deviation. The effect of this artifact can be reduced by avoiding regions in
which it is significant.
5.5.1.1.3 Spatial Resolution
Refraction and depth errors will result in geometric distortion, i.e., image pix-
els will be displayed at incorrect locations. In contrast, variations in spatial res-
olution will change the volume of tissue that corresponds to each image pixel.
