Image Processing Reference
In-Depth Information
and the Wigner-Vile transform (Claasen and Mecklenbrauker 1980), will be applied in the
following as complementary tools during the combination procedure.
In relation to the other abovementioned parameter defining the combination techniques,
several operators to make the trace combination have been used in previous author's works:
maximum, minimum, mean, median, sum and product. Theoretical and experimental
results obtained by applying these operators indicate that the best performances obtained,
for all the combination methods, were produced when a product operator was employed.
For this reason, we have selected (among all the possible operators) the 2D product between
echo-traces, in order to properly perform the comparison among all the methods considered
in this paper. In the following, the three alternative processing techniques proposed for trace
combination are described, showing their performance in relation to the resultant SNR.
3.1 Time-domain combination technique
This first technique performs the combination using the envelope of the ultrasonic traces.
The first step in this method is the acquisition of the traces from the ultrasonic transducers
involved, which are located over two perpendicular planes in the external part of the
inspected piece. The following step is the matching in time of all the different pairs of traces,
each one with echo-information corresponding to precisely the same volumetric elemental
area, i.e. coming from the two specific transducers which projections define such area. To
reduce problems due to no perfect synchronization of the two matched traces in those pairs,
the signal envelopes are utilized instead of the original signals, because this option is less
sensitive to little time-matching errors. These envelopes are obtained by means of applying
them the Hilbert transform. The final step is the trace combination process, by using the
mentioned 2D product operator.
Briefly, the method can be resumed in four successive steps: first, the collection of the traces
from the different transducers; second, the traces envelope calculation; third, the matching
between the information segments of each perpendicular transducers specifically related to
the same inspection area; and fourth, the combination among all the segment couples. The
corresponding functional scheme is presented in Figure 1 for the particular case of four
Fig. 1. Functional scheme of the time-domain echo-traces combination technique.
