"Intravascular Ultrasound (IVUS) – A View from Inside"

William Kongto Hau, PhD

Abstract

Intravascular ultrasound (IVUS) is now established as an important adjunct technique to the coronary angiography for the diagnosis of arterial diseases. IVUS uses high frequency ultrasound to image not only the coronary lumen, but also the structure of the vessel wall together with the atherosclerotic plaque burden within the vessel, enabling the interventionist to strategize therapy according to lesion- specific criteria.

The IVUS catheter is approximately 1 mm in diameter, incorporating a miniaturized ultrasound transducer that operates at frequencies in the range of 10 – 45Mhz, and generates cross-sectional images at 30 frames per second. Currently, there are two configurations of IVUS catheter: mechanical and solid-state. Mechanical catheter design traditionally uses a higher frequency ultrasound transducer (up to 45Mhz), and is commonly known as the high-frequency catheter; while for solid-state catheter design, which uses a lower frequency (20Mhz) ultrasound transducer, is commonly known as the low-frequency catheter. Due to technological differences, there are advantages, disadvantages and qualitative differences in image presentation between the two catheter designs. However, in terms of image quality and measurement accuracy, both catheter configurations are able to produce an image quality and measurement accuracy sufficient for everyday clinical practise.

Recently, the introduction of a new ultrasound tissue characterization technique termed Virtual Histology (VH-IVUS), which uses spectral analysis of the radiofrequency ultrasound signals to reconstruct tissue maps, aims to provide a detailed classification of atherosclerotic plaque composition. Both in vitro and in vivo studies have shown that VH-IVUS is an effective method for characterizing coronary atherosclerotic plaques, with an overall predictive accuracy of 90% to identify the four possible basic tissue types: fibrotic, fibro-fatty, dense calcium, and necrotic core. VH-IVUS is only commercially available with the solid-state catheter design, Each of the four tissue types mentioned above are assigned a colour code and presented on the VH-IVUS interface screen; fibrotic, fibro-fatty, calcium and necrotic tissue are labeled green, light green, white and red, respectively. Since accurate assessment of atherosclerotic plaque composition was previously impossible with grayscale IVUS images, the technique of VH-IVUS has the potential to revolutionize the way interventional cardiology is practised.