The international gold standard technique for the detection and evaluation of coronary artery disease is contrast angiography. In recent years a number of limitations of this technique have become apparent; these include the two-dimensional nature of the
The international gold standard technique for the detection and evaluation of coronary artery disease is contrast angiography. In recent years a number of limitations of this technique have become apparent; these include the two-dimensional nature of the images, the absence of information about the blood vessel wall, insensitivity to substantial plaque burden in outwardly remodeled vessels and inability to detect vessel wall disruption during angioplasty. To overcome these limitations, intravascular ultrasound (IVUS) was developed towards the end of the 1980's and has been refined and its clinical importance assessed through the 1990ís 1. Present day catheters are less than 1mm in diameter (having started out at 2.5mm) and operate at 30 to 40 Mhz frequencies and use either phased array or single rotating transducers.
In clinical practice IVUS is most often used as an adjunct to balloon angioplasty to detect dissection, stent underdeployment, stent thrombosis and to predict restenosis risk. It is also used as an accessory to diagnostic angiography to evaluate lesions of uncertain severity (especially in the left main coronary artery) and to detect disease which is not visible on an angiogram (as in the case of transplant coronary artery disease 2). The technique is similar to balloon angioplasty: a 6 to 8 French access sheath and guiding catheter are used through which a conventional angioplasty guidewire is passed to the distal part of the artery under scrutiny. Intravascular ultrasound catheters are available in either over-the-wire or monorail configurations and following administration of intracoronary nitroglycerin (100 to 200 mg) the catheter is advanced distal to the area of interest. The image that appears on the monitor screen is annotated and optimized for gain, zoom and compress settings and attempts made to eliminate any artifact. A slow pullback, either manual or automatic (0.5 to 1 mm/second), with verbal commentary is then performed from the distal vessel back into the guiding catheter. If necessary, the pullback may be repeated several times, or particular areas of interest interrogated in greater detail. At the end of an IVUS procedure, an angiographic run of the coronary artery is recorded to check for disruption due to intravascular instrumentation.
A classic IVUS image consists of three layers around the lumen (in black). The intima is normally a thin layer of endothelial cells supported by smooth muscle cells, fibroblasts and the internal elastic lamina; this layer thickens substantially, and usually eccentrically, in atherosclerosis. Outside the intimal layer a black ring represents the media, mostly composed of vascular smooth muscle cells bounded by the external elastic lamina. Measurements of the vessel area are made by encircling the border of the media with the adventitia. Plaque area is derived by subtracting lumen area from vessel area. Some qualitative information can be gained about plaque composition; in particular calcification is readily visible and may impede accurate measurement of vessel area.
An important aspect of IVUS imaging is in the area of clinical research. The technology is widely accepted as being the new gold standard for follow-up studies of new treatment for coronary artery disease, specifically those designed to prevent restenosis. It is also used for longitudinal studies of other vascular processes, particularly vascular remodeling, progression of transplant coronary artery disease and (to a lesser degree) plaque regression studies.
A number of prospective trials have identified areas where IVUS may have maximum value in the clinical arena. The CLOUT investigators demonstrated that the addition of IVUS to balloon angioplasty permitted safe balloon upsizing in 73% of patients with significant gains in lumen diameter and area 3. One of the implications was that balloon angioplasty without IVUS measurement of vessel area is likely to result in balloon undersizing in most patients. The MUSIC investigators found that IVUS confirmation of optimal stent expansion was associated with low restenosis rates during follow up (less than 10% at 6 months) 4. Finally, the CRUISE study (Can Routine Ultrasound Influence Stent Expansion) 5 showed that IVUS-guided stent deployment resulted in significantly lower target vessel revascularization (TVR) rates at 9 months, compared with angiographic-guided stenting, and that very low TVR rates could be achieved when an in-stent area of more than 9 mm2 was achieved.
Future developments of IVUS-based technology are likely to allow on-line blood-flow measurement, forward views of chronic total occlusions, higher definition discrimination of stable and unstable plaques and the potential for ultrasound-based lysis of culprit plaques in acute myocardial infarction.