Current Status of Intravascular Imaging with IVUS and OCT: The Clinical Implications of Intravascular Imaging

Introduction

As far back as 1991, Davidson and colleagues described the potential use of intravascular ultrasound (IVUS) to examine the coronary vessels and evaluate what was happening during coronary intervention. Using a mechanically rotated 4.8Fr IVUS catheter, in 65 patients, they found that they were better able to identify plaque and the occurrence of dissection than with standard cineangiography. Since that time, interventional cardiologists have used intravascular imaging both as a research and clinical tool to better understand the pathophysiology of coronary disease and the response to coronary intervention / treatments. The technology of IVUS has developed greatly with modern rapid-exchange catheters that are far more trackable. Multiple (64) elements are mounted on the tip of a catheter and the elements are excited one by one or pair-wise, resulting in an electronically rotating beam. The IVUS catheters operate on frequencies between 20MHz (phased array catheters) and 40MHz (mechanical catheters), resulting in a resolution of approximately 80 microns axially and 200-250 microns laterally. Adjunctive use of IVUS has been shown to improve outcomes following stent implantation (Fitzgerald et al., 2000; Oemrawsingh et al., 2003), and may also facilitate the ability to perform optimal percutaneous coronary intervention (PCI) with a reduced volume of contrast.

Optical coherence tomography (OCT) is a catheter-based imaging technique analogous to IVUS but using near infra-red light emission. This generates detailed images which have a much higher resolution (10µm) than those obtained with IVUS. The downside however is a more limited depth of tissue penetration (see Table 1). Until recently, OCT had been viewed predominantly as merely a research tool; however the technology has developed such that real-time images can now be made extremely rapidly without the need for prolonged occlusion of the vessel (OCT cannot image through blood). Using frequency domain OCT catheters with an acquisition speed of 20mm/sec, it is possible to obtain 200 cross-sectional images over a 4-5cm length of vessel in just 3.5 seconds, and the information obtained is proving to have a potentially much broader application in everyday clinical practice. The following chapter aims to compare the modalities of IVUS and OCT, and summarise the potential application of both imaging techniques in specific situations encountered in contemporary clinical practice.

* C7 Dragonfly catheter (St Jude Medical)

** 20MHz Eagle eye from Volcano Corp is compatible with 5Fr (0.056”)