Real-time 3-D echocardiography:challenges of parallel transmission and acquisition

Abstract

Ultrasound imaging is a well-established, noninvasive and relatively low cost method for assessing the cardiac function. By providing dense image volumes in real-time, three-dimensional echocardiography could improve and automatize the diagnostic by professionals. But the limited speed of sound still prevents the acquisition of large fields of view at frame rates sufficient to assess the complex dynamics of a beating heart. To increase the frame rate, parallelizations of both the transmission and reception processes is being proposed. In this thesis, the challenges of Multi-Line Transmission (MLT) and two Synthetic Transmit Aperture (STA) techniques for imaging the heart in real-time 3-D when an optimal image quality is required are addressed across four scientific publications.

When transmitting multiple focused ultrasound pulses in parallel, interactions between the simultaneous beams are prone to generate artifacts, aka. MLT crosstalks. Cross-talks appear as additional clutter in the image, notably at every location where the edge waves of one beam overlap with the main-lobe of another beam, both on transmission and reception. To isolate the parallel beams, we propose to combine MLT with the low transmit side-lobe levels of second harmonic imaging. 10 to 15 dB reductions of transmit cross-talks are observed both in vitro and in vivo. Beam isolation may also be achieved using 3-D dispositions. When aligning the beams along the transducer transverse diagonal, reductions of up to 30 dB of both transmit and receive cross-talks are observed in simulations backed-up by water tank measurements. This allow a potential increase of the frame rate by a factor five without visible image degradation.

Using parallel receive beams with STA focusing, the coherent combination of data acquired over successive transmit events allow to recover the spatial resolution and contrast while maintaining higher frame rates. However, the success of this combination is no longer guaranteed in presence of rapid displacements such as observed in the myocardium or the blood. The influence of motion is studied for STA applications combining two or many transmit events: Synthetic Transmit Beamformation, both in 2-D and 3-D, and Coherent Plane Wave Compounding. In presence of significant axial motion, lateral image shifts and deteriorations of the focusing capability are observed, resulting in losses of contrast and SNR. Motion compensation algorithms based on cross-correlation are introduced to correct for the axial motion component. Such algorithms are robust, computationally inexpensive and their necessity is demonstrated through both simulations and in vivo experiments.