EVALUATION OF A PULSE CODING TECHNIQUE FOR SPATIAL STRUCTURE CHARACTERIZATION

Pulse coding techniques have been used in the past primarily to improv e signal to (electronic) noise ratio. However, the flexibility inheren t in pulse coding can be exploited to solve several problems in medica l imaging and nondestructive testing. We have experimentally examined its potential for spacial structure characterization of a scattering m edium on a scale below the resolution of the imaging system. The abili ty to change the point spread function and the spectral content of the interrogating pulse with frequency modulated (FM) pulse coding has be en utilized. Water filled sponge with pore size much smaller than the resolution cell volume was used as the scattering medium. A nonfocused transducer was driven with FM coded pulses. The pulse compression pro cessing was carried out digitally on a computer. FM pulses with 143 di fferent combinations of center frequency f(0) and 6 Delta B bandwidth nf were used. Amplitude signal to noise ratio (SNR(A)) was calculated on the envelope detected signal for spacial structure characterization . SNR(A) showed significant increase from its Rayleigh limit value of 1.91 at certain specific frequencies. Both simulation and theoretical considerations are used to show that this resonance effect is a signat ure of the underlying semiperiodic scattering structure of the medium.