Biomedical Engineering Reference
In-Depth Information
FIGURE 5.7
Reconstruction of a circular reflector for a simulated freehand ultrasound scan with increas-
ing latency values. The upper images are a synthesized circular reflector (left) and a simu-
lated freehand scan with zero latency (right). The lower figures show the effect of nonzero
latency with the figure on the right having a larger latency than the left. The black streaks
on the images are gaps in the data that are a consequence of using a simple nearest neighbor
interpolation method. The highest latency value used was chosen in order to illustrate what
could happen in an extreme situation. It is for illustrative purposes only and should not be
regarded as a typical situation.
The effect of the artifact can be minimized by ensuring that the latency is
significantly less than the interslice time interval. In most freehand scanning
the slice acquisition rate will be equal to the scanner frame rate and most
probably 20 to 40 Hz. In this case the interslice time interval will be 25 to 50 ms.
Provided the latency is no more than a few milliseconds, the degree of spatial
distortion is relatively small (a few percent of the interslice time interval). In
principle, an approximate correction could be carried out for the effect of the
latency if the transducer movement can be approximated by a simple func-
tion. However, given that the effect is likely to be relatively small, it is argu-
able whether it is worthwhile.
5.5.1.2.1.2 Uncertainty in Position Sensors
All position sensors will have a
quoted accuracy for both position and orientation measurements. These place
a fundamental limitation on the ultimate spatial resolution of the reconstructed
data and cause nonlinear distortion and blurring. Orientation errors cause an
increase in translation error with depth, since all pixels within a slice are subject
to the same angle error, i.e., depth-dependent blurring
distortion. For a typical
orientation error of 0.5
, the resulting translation error at a depth of 100 mm
will be approximately 1 mm. This will be in addition to the sensor position
error, which is typically 1 mm, so this effect can be significant.
