Mechanisms of ultrasound wave propagation Nature of Wave Propagation • The acoustic wave launched by the transducer is propagated in the tissue – as a longitudinal pressure wave that causes local variations of • pressure density and velocity of the medium. Assumptions • The tissue is assumed – to act like an isotropic non absorbing homogeneous fluid • in which shear forces are negligible Wave Equation Final Equation Signal Processing • The interrogating pulse has a – Carrier frequency in the range of 2 to 7 MHz, duration of 2 to 3 cycles, and • With Gaussian envelope • The pulse repetition rate lies • in the range of 1 to 3 KHz • Considering that the velocity of acoustic waves in tissues is approximately that in water (viz. 1540 m/s), the wavelengths lie in the range of 0.20 to 0.75mm. • The attenuation depends on the tissue and frequency; however, as a rule of thumb it can be taken to be 2 db/(depth in cm, the frequency in MHz) • Compensation of attenuation is a prerequisite for a uniformly bright image. This is achieved by Time gain control. (TGC) • Modern ultrasound imaging systems use digital electronics and signal processing. – The sampling rate for the AD converters and delay lines must be chosen at a value at least twice the highest bandwidth of the incoming signals Block Diagram of B Mode Imaging System • The transmitting and receiving piezoelements are activated in groups. • By doing this, the beam is focused region-wise in several regions. • The implementation of this technique of “dynamic focusing” • can be performed elegantly by employing digital signal processing. • The received signals in the individual groups are – digitized by means of the AD converters, • delayed appropriately and finally summed up • The sum is then demodulated using a digital envelope detector, thus – achieving a high SNR even for the low-amplitude signals • The signal is processed to compose the B- mode image, which consists of lines and pixels as in TV frames • Finally, the signal samples are stored in a 2-D array • The gray level of a pixel is coded with 6 to 8 bits
"Ultrasound Wave Propagation"