Phase shift variance imaging for contrast agent detection

A novel ultrasound contrast agent detection method that selectively detects microbubble (MB) destruction is presented. The detection is based on the variance of the phase shift of consecutive echoes. The variance for stationary echoes or motion at constant speed is determined by the noise of the acquisition system and is theoretically predicted. We hypothesize that the phase shift variance of ultrasound echoes increases during MB destruction. It is expected that this PSV is higher than the PSV for stationary echoes or motion thus allowing the detection of the agent. The hypothesis is experimentally verified in a motion phantom. In a second experiment, MB in very low concentrations are imaged to demonstrate the sensitivity of the method. The results show good agreement with theory and exhibit a clear increase in PSV for MB vs. tissue (44.8 dB). Thus, the phase shift variance can be utilized for ultrasound contrast agent detection. Microbubbles in a concentration of 100 bubbles per ml are sufficient to cause detectable signals with a contrast to tissue ratio of up to 34.7 dB. The acquisition is implemented on a free programmable clinical device. No additions or hardware modifications are necessary making the method easily translatable to most clinical systems.