Designing Non-linear Frequency Modulated Signals For Medical Ultrasound Imaging

In this paper a new method for designing non-linear frequency modulated (NLFM) waveforms for ultrasound imaging is proposed. The objective is to control the amplitude spectrum of the designed waveform and still keep a constant transmit amplitude, so that the transmitted energy is maximized. The signal-to-noise-ratio can in this way be optimized.

The waveform design is based on least squares optimization. A desired amplitude spectrum is chosen, hereafter the phase spectrum is chosen, so that the instantaneous frequency takes on the form of a third order polynomial. The finite energy waveform is derived by minimizing the summed squared error between the desired spectrum and the obtained spectrum of the waveform.

Having total control of the waveform spectrum has two advantages: First, it facilitates efficient use of the transducer passband, so that the amount of energy converted to heat in the transducer can be decreased. Secondly, by choosing an appropriate amplitude spectrum, no additional temporal tapering has to be applied to the matched filter to achieve sufficient range sidelobe suppression.

Proper design results in waveforms with a range sidelobe level beyond -80 dB. The design method is tested experimentally using the RASMUS ultrasound system with a 7 MHz linear array transducer. Synthetic transmit aperture ultrasound imaging is applied to acquire data. The proposed design method was compared to a linear FM signal.

Due to more efficient spectral usage, a gain in SNR of 4.3plusmn1.2 dB was measured resulting in an increase of 1 cm in penetration depth. Finally, in-vivo measurements are shown for both methods, where the common carotid artery on a 27 year old healthy male was scanned.