Pseudo-inverse filtering, Range resolution, Low center frequency pulse, Directivity
In the ultrasonic diagnosis, a higher center frequency pulse has usually better range resolution
than a low frequency pulse. However, it is difficult to obtain the reflected echo signal of far target
because the attenuation of the media may be proportional to the frequency. In the other hand, a
ringing less pulse has the good range resolution like a high frequency pulse, in spite of a low center
frequency. The pseudo-inverse filtering method in the DFT domain is good for transmitting and/or
receiving a pulse without ringing. In degassed water or in caster oil, we could actually project
ringing less pulses of 0.5 μs whose center frequencies are 2 to 8 MHz. The objective pulse of 2 MHz
had a wavelet and 8 MHz pulse had 4 wavelets, respectivly, because its duration time should be 0.5 μs.
The experimental pulse duration time or the range resolution was measured with and without filtering
technique. This technique much improved the range resolution at on-axis. The off-axis resolution
was also measured as the function of an angle from the center axis. It was shown that the range
resolution with filtering technique changed more smoothly than without it. The directivity of a 5 mm
disk transmitter was measured using pulse with and without filtering, in order to compare with the
continuous sinusoidal wave. Three kinds of directivities had almost the same beam widths. Each
result coincided with the calculated value, respectively.