Frontiers in Intravascular Imaging Technologies

Y. Honda, P. J. Fitzgerald
2008 Circulation  
I n 1971, Bom et al 1 developed one of the first catheterbased 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 al 2 had successfully miniaturized a single-transducer system to enable transducer
more » ... 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. Principles and Device Developments 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 MHz, thus providing theoretical lower limits of resolution (calculated as half the wavelength) of 39 and 19 m, respectively. In practice, the radial resolution is at least 2 to 5 times poorer, as determined by factors such as the length of the emitted pulse and the position of the imaged structures relative to the transducer. For peripheral and intracardiac echocardiographic (ICE) applications, larger imaging catheters with lower center frequencies (8 to 20 MHz) are produced in both mechanical and electrical configurations. In addition, a phased-array intracardiac echocardiography catheter is available that provides a sector image with color/spectral Doppler and multiple frequency imaging (5.0 to 10 MHz) capabilities. Optical Approaches Angioscopy Intracoronary angioscopy is an endoscopic technology that allows direct visualization of the surface color and superficial morphology of atherosclerotic plaque, thrombus, neointima, or stent struts. The light source emits a high-intensity white light to illuminate the target object through the fiber optic catheter. The imaging catheter contains a flexible fiber optic bundle of several thousand pixels; the latest-generation catheter, which incorporates 6000 fibers, is 0.75 mm in outer diameter with a microlens that provides a 70°field of view and a focused depth that ranges from 1 to 5 mm. Although conventional delivery systems were equipped with a distal balloon to create a blood-free field for optical imaging, an alternative system uses a smaller catheter to continuously flush an optically clear liquid in front of the tip of the angioscope for transient blood displacement. To circumvent the subjectivity of color interpretation, quantitative colorimetric methods have been proposed. One research group is also developing a side-view imaging catheter to overcome several inherent limitations of the conventional forward-view catheter configuration (Figure 2 ). Optical Coherence Tomography Optical coherence tomography (OCT) generates real-time tomographic images from backscattered reflections of infrared light. The greatest advantage of this optical technology is its resolution that is significantly higher (10 to 15-m axial resolution and 20 to 25-m lateral and out-of-plane resolution) than ultrasound-based approaches. This improvement comes at the expense of poorer penetration through blood and tissue (1 to 3 mm). The imaging catheter includes a fiber optic core with a microlens and a prism at the distal tip to generate a focused scanning beam perpendicular to the catheter axis.
doi:10.1161/circulationaha.105.551804 pmid:18413510 fatcat:whslay2bbvfgvjmtiprvq5niuu