Evaluation of a pulse coding technique for speckle reduction and 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 two such problems: (i) speckle reduction with multispectral imaging and data averaging; (ii) structural characterization of scattering medium on a scale below the resolution of the imaging system. The ability to change the point spread function and the spectral content with frequency modulated pulse coding has been utilized. Water filled sponge with pore size much smaller than the resolution cell volume was used as a speckle generating medium. A non-focused 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 f₀ and 6 dB bandwidth Δf were used. Normalized decorrelation factors were calculated to evaluate effectiveness for speckle averaging. Significant decorrelation was observed specially at lower Δf, indicating a potential for frequency diversity processing for speckle reduction. Point signal to noise ratio (SNR_A) was also calculated on the envelope detected signal for structural characterization. SNR_A showed significant increase from its high density limit value of 1.91 (fully developed speckle) 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