Coded pulse excitation for ultrasonic strain imaging

Decorrelation noise in strain images can be significantly reduced using coded pulse excitation. Linear, frequency-modulated chirp pulses were applied to image tissue mimicking phantoms with 2.5-mm-diameter inclusions that mimick the elastic properdes of breast lesions. We observed a 5-10 dB reduction in echo SNR (eSNR) that led to a doubling of the depth of focus for strain images without a significant loss of axial resolution. Coded excitation allows use of higher transmission frequencies and shorter correlation windows that improve spatial resolution. Strain imaging performance is quantified using SNR _strain, modulation transfer function (MTF) and contrast to noise ratio (CNR _strain). The performance of chirp and short pulses were compared using simulations and phantom measurements. The challenge is to design robust pulse sequences that can be decoded despite distortions from tissue deformation.