Optimization of contrast-to-tissue ratio by adaptation of transmitted ternary signal in ultrasound pulse inversion imaging

Abstract

Ultrasound contrast imaging has provided more accurate medical diagnoses thanks to the development of innovating modalities like the pulse inversion imaging. However, this latter modality that improves the contrast-to-tissue ratio (CTR) is not optimal, since the frequency is manually chosen jointly with the probe. However, an optimal choice of this command is possible, but it requires precise information about the transducer and the medium which can be experimentally difficult to obtain, even inaccessible. It turns out that the optimization can become more complex by taking into account the kind of generators, since the generators of electrical signals in a conventional ultrasound scanner can be unipolar, bipolar, or tripolar. Our aim was to seek the ternary command which maximized the CTR. By combining a genetic algorithm and a closed loop, the system automatically proposed the optimal ternary command. In simulation, the gain compared with the usual ternary signal could reach about 3.9 dB. Another interesting finding was that, in contrast to what is generally accepted, the optimal command was not a fixed-frequency signal but had harmonic components.

Figure 1

Block diagram of CTR optimization in pulse inversion imaging.

Figure 2

Simulation of automatic optimization of the contrast-to-tissue ratio (CTR) by a transmitted ternary signal. The optimization was compared to two ternary signals, where the transmit frequency was at the optimal frequency and at two-thirds of the central frequency of the transducer.

Figure 3

(a) Optimal transmitted ternary signal x ₁(n) obtained by genetic algorithm, (b) Signal p(n) at the transducer output (Figure 1), when w(n) was the optimal ternary signal, (c) the radiofrequency line, and (d) their spectra.