Achieving high frame rates in intravascular ultrasound imaging
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Intravascular ultrasound (IVUS) is a catheter-based imaging modality that allows not only visualization of coronary arteries but also characterization of blood vessel walls. IVUS imaging -- an acoustics-based visualization technique, has many advantages that have fueled a wealth of research in recent years: relative low cost, high speed, safety, and portability. Furthermore, IVUS imaging is a real-time imaging technique -- current clinical IVUS scanners provide images at video rate (30 frames per second). However, ultrasound research facilities need reliable, high frame rate IVUS systems to image live arteries of laboratory animals with fast heart rates in excess of 200 beats per minute. In such systems, the acquisition rate must reach up to 100 frames per second. Using IVUS imaging catheters with a mechanically rotated, single element ultrasound transducer, several challenges arise at this frame rate: (a) The catheter needs to deliver this fast rotational motion to the transducer. (b) The pulser/receiver (PR) needs to support a high pulse repetition frequency. (c) The analog-to-digital converter (ADC) needs to have a very fast memory transfer protocol to store the samples at high speeds. (d) The motor system needs to rotate at speeds up to 6000 rpm and also provide fast and accurate triggers to the ADC and the PR with no significant interference to the signal. At this speed, many components of the motor system can be a source of electromagnetic interference (EMI). This interference propagates in air and couples to the transducer wires, decreasing SNR, degrading image quality and introducing artifacts. This thesis work includes engineering design, integration, and testing of a high frame rate, low noise, mechanically-rotated intravascular ultrasound imaging system. The system is capable of imaging at 100 frames per second with approximately 25 dB SNR nominal. Hardware description, resulted signals and images, and practical design considerations are presented and discussed.