Evaluation of a pulse coding technique for spacial structure characterization

Pulse coding techniques have been used in the past primarily to improve signal to (electronic) noise ratio. However, the flexibility inherent in pulse coding can be exploited to solve several problems in medical imaging and nondestructive testing. We have experimentally examined its potential for spacial structure characterization of a scattering medium on a scale below the resolution of the imaging system. The ability to change the point spread function and the spectral content of the interrogating pulse with frequency modulated (FM) pulse coding has been 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 processing was carried out digitally on a computer. FM pulses with 143 different combinations of center frequency f/sub 0/ and 6 dB bandwidth /spl Delta/f were used. Amplitude signal to noise ratio (SNR/sub A/) was calculated on the envelope detected signal for spacial structure characterization. SNR/sub 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 signature of the underlying semiperiodic scattering structure of the medium.<>