This paper describes the design of a transesophageal probe using a matrix (2D array) ultrasonic transducer for 3D echocardiography. To obtain images with sufficient resolution, several thousand elements are required. To reduce the channel count from the transducer to the imaging system, it is necessary to include electronics close to the transducer and use smart signal processing for data reduction. A micro-beamforming method called pre-steering is proposed. All the groups have the same delay configuration, which will simplify the required electronics. For a correct design, delay steps and the maximum delay are of importance. Furthermore, the delay should be programmable for each direction. Simulations are performed to investigate the effect of pre-steering on the received field. The delay and sum operations are realized by an integrated circuit. Simulations show that, for the micro-beamformer of a matrix transducer, the pre-steering approach is an effective method to reduce the complexity of the electronics and the channel count, while maintaining an adequate receive field.
Ultrasound image quality has experienced a significant improvement over the past years with the utilization of harmonic frequencies. This brought the need to understand the physical processes involved in the propagation of finite amplitude sound beams, and the issues for redesigning and optimizing the phased array transducers. New arrays with higher imaging performances are essential for tissue imaging and contrast imaging as well. This study presents measurements and simulations on a 4.6 MHz square transducer. The numerical scheme used solves the KZK equation in the time domain. Comparison of measured and computed data showed good agreement for low and high excitation levels. In a similar way, a numerical simulation was performed on a linear array with 5 elements. The simulation showed that the second harmonic beam is narrower than the fundamental with less energy in the near field. In addition, the grating lobes are significantly lower. Accordingly, selective harmonic imaging shows less near field artifacts and will lower the clutter, resulting in much cleaner images.
When a cluster of randomly distributed blood particles moves across the ultrasound beam, the received echo signals decorrelate as a function of time. This phenomenon may be used to estimate the flow velocity by measuring the decorrelation rate from a sequence of blood scattering signals. A computer model based on the impulse response method was used to study the lateral decorrelation properties of an IVUS transducer from the extreme near-field up to the far-field. An RF decorrelation-based method for measuring local blood velocity and quantifying volume flow from cross-sectional RF IVUS data was developed and tested in vitro with a flow phantom and in vivo in pig experiments. Preliminary results of this study indicate that the proposed decorrelation method is able to extract 2D velocity profiles and volumetric flow.
The ability of transoesophageal echocardiography to visualize the left coronary artery was retrospectively analysed in 60 consecutive patients without clinical evidence of coronary artery disease. The left main coronary artery was visualized in 56 patients, the proximal circumflex in 34 patients and the proximal anterior descending artery in nine patients. Patency of these arteries was established in all these patients. Subsequently, a prospective study was undertaken in 23 patients with angiographically proven left coronary artery disease. Both the left main coronary artery and the circumflex artery were adequately visualized with transoesophageal echocardiography in all 23 patients, whereas the anterior descending artery was identified inthree patients. The extent of stenosis in the left main coronary artery and the circumflex arterywas correctly diagnosed in 18 patients. In five patients the degree of stenosis was overestimated. These findings indicate the potential of transoesophageal echocardiography to detect or exclude stenosis of both the left main coronary artery and circumflex artery.
