In 1971, Bom et al1 developed one of the first catheter-based real-time imaging techniques for use in the cardiac system. In placing a set of phased-array ultrasound transducers within the cardiac chambers, Bom and colleagues showed that higher frequencies than those used in transthoracic ultrasound imaging could be used to produce high-resolution images of cardiac structures. By the late 1980s, Yock et al2 had successfully miniaturized a single-transducer system to enable transducer placement within coronary arteries. Since then, intravascular ultrasound (IVUS) has become a pivotal catheter-based imaging technology, having provided practical guidance for percutaneous interventions and scientific insights into vascular biology in clinical settings. Technical developments currently being explored consist of further device improvements, a variety of advanced image analyses, and the extension of this ultrasound-based approach to diverse intravascular imaging techniques with other energy sources. ### Ultrasound-Based Approaches IVUS systems produce tomographic images by performing a series of pulse/echo sequences, or vectors, in which an acoustic pulse is emitted and the subsequent reflections from the tissue are detected. Each vector is acquired by directing the ultrasound beam from the catheter in a slightly different direction from the previous vector by mechanical or electrical means. A gray-scale IVUS image is made with all the vectors (commonly 256 vectors), with each vector acquired at a different angle of rotation. Several clinically relevant properties of the ultrasound image, such as the resolution, depth of penetration and attenuation of the acoustic pulse by tissue, are dependent on the geometric and frequency properties of the transducer. A crystal transducer emits a signal that spans a range of frequencies. The higher the center frequency, the better the radial resolution (Figure 1) but the lower the depth of penetration. Conventional IVUS catheters used in the coronary arteries have center frequencies that range from 20 MHz to 40 …