Effect of dynamic receive focusing on Reflex Transmission Imaging (RTI)

RTI aims to display relative attenuation at the focal plane of a single strongly focused transducer by integrating the backscattered power from a reference region, or integration zone, beyond the focus. Ideally RTI should be performed using confocal transmit and receive foci, so that only backscattered echoes returning through the focus are received with maximum gain. Commercial ultrasound imaging systems, however, usually employ dynamic receive focusing. A ray-tracing model of RTI has been developed to compare dynamic receive and confocal focusing systems. When dynamic receive focus adjustment is present, it is predicted that attenuation contrast will be reduced if the integration zone extends beyond a particular depth, as there are echo return paths that do not pass entirely through the attenuating object but for which echoes are received with maximum beamformer gain. Experimental confirmation of this effect and reasonably quantitative agreement with the models predictions were obtained for thin attenuating tiles placed at the depth of the transmit focus (50 mm) of a 3.5 MHz linear array. We have shown that dynamic receive focusing places restrictions on the position of the integration zone for RTI. Good knowledge of beam characteristics and attenuating object size are required to set the RTI system for maximum contrast. In practice however, it may be worth using a large reference region, sacrificing some attenuation contrast in favour of improved contrast to noise ratio and lesion visibility.