Frequency notched waveforms for medical ultrasound imaging

Modern medical ultrasound scanners allow the user to produce either grey scale images showing the structure of organs inside the patient, or images which are coloured according to the velocity of the blood or tissue. It is often desirable to combine these images to produce a clearer, over-all view of flow in relation to structure. However, the optimum signal requirements for producing high-resolution grey scale images and for low variance velocity estimations are mutually exclusive. Therefore, in order to produce images which combine both modes, the scanner must use a particular pulsing strategy, effectively interleaving the generation of velocity and grey scale data. This results in an overall reduction in the maximum achievable frame rate, which could be undesirable in some circumstances where fast motion must be observed. The work described in this paper outlines a signal coding strategy which uses frequency notched waveforms to allow the two signals to be combined into a single transmission in such a way that they can be effectively separated at the receiver. Results are presented which demonstrate that after the process of combining and separating the two signals, the imaging performance would be reasonably comparable to that of a linear frequency modulated chirp.