Principles of Intravascular Ultrasound (IVUS)

• IVUS is an invasive imaging modality that utilizes reflected ultrasound signals to generate high-resolution images of the vessel wall.
• Ultrasound signals are generated by oscillatory movements of a piezoelectric transducer (crystal).
• IVUS utilizes a monorail catheter with an ultrasound transducer at its tip generating two-dimensional cross sectional images of the vessel wall. IVUS images are obtained based on varying acoustic impedance at the interface of different tissue structures.
• There are two major types of IVUS transducers: mechanically rotating single element systems and electronic multi-element array (phased-array) systems.
• Rotational (mechanical) IVUS – the catheter has a single transducer element located at its tip. The element is rotated by a motor drive attached to the proximal end of the catheter. Mechanical transducer catheters require flushing with saline, because even small air bubbles within the protective sheath can affect image quality.
• Solid-state (phased electronic array) – annular array of small stationary piezoelectric transducers, which are sequentially activated.
• Depending on the manufacturer, current IVUS catheters use frequency from 20 to 65 MHz for coronary imaging (Table). With increasing frequency, radial resolution is improved, but tissue penetration is reduced.
• IVUS pullback can be done either manually or using a motorized pullback device which withdraws the catheter at a constant speed between 0.25 and 1 mm/s. Lesion length assessment and volumetric measurements are possible only with motorized transducer pullback.

How to Perform IVUS

• Intracoronary nitroglycerin (100-200 µg) should be given immediately prior IVUS run, and the patient should be adequately anticoagulated
• The preparation of IVUS imaging catheters varies slightly with each manufacturer but mainly consists of flushing the catheter system with saline and connecting the catheter to a console. Follow the manufacturer instructions included in the catheter packaging to minimize the risk of air embolism and to ensure an optimal image quality
• Ideally, imaging should start at least 20 mm distal to lesion and continue across the area of interest to the left main or RCA ostium to include the longest vessel segment possible
• If the catheter does not cross the lesion, balloon pre-dilatation may be used to facilitate image acquisition
• In motorized pullback, a catheter is withdrawn at a constant speed varying between 0.25 and 1 mm/s with the most frequently used speed of 0.5 mm/s. In addition to the settings, ACIST IVUS offers 60 MHz high-speed pullback at 10 mm/s. In manual pullback, the operator performs a slow controlled catheter withdrawal across the lesion and has the ability to remain at a point of interest while acquiring the image sequence

Educational Playlist on Performing IVUS:

https://youtube.com/playlist?list=PLNFad7qfsAGe8CEaI3JvXi09QGViXVk02

IVUS Imaging Systems