Brief Summary
Alright, so this video is basically a crash course on intravascular ultrasound (IVUS) imaging. The doc breaks down everything from the basic principles of how IVUS works to interpreting the images and spotting common artifacts. Key takeaways include understanding how ultrasound interacts with different tissues, recognizing normal coronary anatomy versus diseased states on IVUS, and avoiding misinterpretations due to artifacts.
- IVUS provides a detailed view of the vessel wall, unlike angiography which only shows the lumen.
- Image interpretation relies on echogenicity, shadowing, attenuation, reverberation, and intimal thickness.
- Recognizing artifacts is crucial to avoid misdiagnoses.
Intro to IVUS Imaging
The video will cover basic aspects of IVUS imaging, including the imaging system, image production, interpretation, and potential artifacts. The session will not cover IVUS guidance for interventions, but that might be a topic for a future webinar. Coronary angiography is useful for visualizing coronary anatomy and making decisions about treatment, but IVUS is needed for more detailed information about the vessel wall.
Why IVUS is Important
Coronary angiography, while useful, only provides a 2D image of the lumen and doesn't give information about the vessel wall, plaque distribution, or vessel remodeling. It's also limited by foreshortening and angle dependency, making it difficult to accurately quantify stenosis or stent expansion. IVUS is needed to better understand what's happening inside the vessel wall and around stents.
Understanding the IVUS System
The IVUS system has three main parts: the console (provides power, display, and storage), the motor drive unit (rotates the catheter and pulls it back at a constant speed), and the IVUS catheter (contains a transducer for image production). The system works by sending electrical energy to piezoelectric crystals in the transducer, which then emit ultrasound waves. These waves reflect off tissue interfaces and are converted back into electrical signals to create cross-sectional images. There are two types of IVUS systems: mechanical (single transducer element) and phased array (64 elements).
How Ultrasound Waves Interact with Tissues
Understanding how ultrasound waves interact with tissues requires knowing about tissue interfaces (borders between media with different acoustic impedance) and acoustic impedance (resistance to sound passage). The interaction depends on the angle of incidence, surface smoothness, and the difference in acoustic impedance between the media. Perpendicular incidence on a smooth surface results in most waves reflecting back to the transducer. A large impedance difference causes more reflection at the first interface. Irregular surfaces cause scattering.
Shadowing, Attenuation and Image Formation
Shadowing occurs when a large acoustic impedance difference causes most ultrasound waves to reflect off the surface, obscuring deeper tissues. Attenuation is the decrease in ultrasound wave strength as it travels through tissue due to absorption, scattering, and reflection. Speckling is caused by scattering from structures smaller than the wavelength, like RBCs. Image formation requires information on the position of structures and their reflectivity. Depth is determined by the time it takes for the ultrasound signal to return, and brightness depends on the strength of the reflection.
IVUS Image Orientation and Normal Coronary Anatomy
IVUS images are like histological sections, oriented as if you're viewing from inside the sinus of Valsalva into the coronary artery. There's no absolute anterior/posterior orientation; abnormalities are described using clock positions. Normal coronary arteries have a lumen and a three-layered vessel wall. The lumen appears echo-lucent with blood speckles. The three layers are: intima (bright), media (dark), and adventitia (bright).
Atherosclerosis and Plaque Morphology
Atherosclerosis involves plaque accumulation in the intima, increasing its thickness. Plaque echogenicity depends on its composition (lipid, fibrous tissue, or calcium). Plaque can accumulate concentrically or eccentrically. Blood speckles are finely textured echoes in a swirling pattern, appearing darker with faster blood flow and brighter with stagnant blood.
Key Anatomical Landmarks on IVUS
The pericardium appears as a stripe of tissue that moves back and forth with systole, showing spoke-like reverberations. The pericardial space is an echo-free space around the coronary artery. Veins are oval structures that exhibit solid compression, running parallel or perpendicular to the artery without joining it, and have brighter blood speckles. Side branches, unlike veins, join the main vessel.
IVUS Anatomy of Different Coronary Arteries
The left main coronary artery originates from the aortic sinus and runs through the transverse sinus behind the pulmonary artery. The osteum has the oblique aorta in its view, the middle part travels through the transverse sinus, and the distal part bifurcates into the LAD and LCX. The LAD runs in the anterior interventricular groove, related anteriorly to the pericardium and posteriorly to the myocardium. The LCX has the great cardiac vein lying on its atrial side. RCA venous structures move perpendicular to the artery, creating a horseshoe shape.
Differentiating LAD Branches and IVUS Grayscale Characteristics
Diagonal branches run along with the LAD for a distance and are often seen in the far field, located about 90 degrees from the pericardium. Septal branches originate and go perpendicularly into the myocardium. Grayscale IVUS characterization uses five parameters: echogenicity (compared to adventitia), shadowing, attenuation, reverberation, and intimal thickness.
Interpreting IVUS Images: Plaque Types
Isoechoic plaque is fibrous. Hyperechoic plaque with shadowing and reverberation is fibrocalcific. Hypoechoic plaque is soft. Isoechoic plaque with deep signal attenuation is attenuated. Plaque with interrupt plaque equilucency is a collusion plot. Mixed plaque has multiple characteristics. Plaque rupture is an intra-plaque cavity communicating with the lumen. Thrombus is a freely mobile intra-luminal mass with clear separation from the intimal layer.
More on Attenuated Plaque and Classifying Fibrocalcific Plaque
Deeper attenuated plaque usually indicates lipid or early necrosis, while superficial attenuation suggests advanced necrotic block or thin cap fibrotheroma. Fibrocalcific plaque is classified as superficial (closer to the lumen) or deeper (closer to the adventitia). Calcium is quantified by measuring the degrees it encompasses along the circumference. Spotty calcium (less than 90 degrees) and protruding calcification are associated with plaque instability.
Specific Morphologies in CTO and Dissections
Recanalized thrombus shows multiple channels, called lotus root or Swiss cheese appearance. In chronic total occlusion (CTO), the lumen is occluded with intimal plaque. Subintimal space is the medial space that expands easily with wire movement, often showing a hematoma and collapsed intimal plaque on IVUS. Dissection is a breach in the lumen continuity, classified by axial, circumferential, and longitudinal extent.
Intramural Hematoma, Malapposition, and Tissue Prolapse
Intramural hematoma is blood accumulation in the medial space, often from balloon dilatation or stenting. Malapposition is the lack of contact between the stent and vessel wall, with RBC movement in between. Tissue prolapse is the relapse of acrylothrombotic material into the stent. Muscle bridge is when the muscle layer goes around the artery, causing systolic compression.
Aneurysms, Remodeling, and Radiofrequency IVUS
Coronary aneurysm is a part more than 1.5 times the diameter of the reference segment. Remodeling is the change in vessel size at the lesion site compared to reference segments (positive or negative). Radiofrequency IVUS uses radiofrequency signals for tissue characterization, with algorithms like virtual histology (Philips), iMap (Boston Scientific), and integrated backscatter (cerumo IVUS).
Image Artifacts: Non-Uniform Rotation and Tru-and-Fro
Non-uniform rotational distortion (mechanical systems) results from mechanical binding, causing circumferential distortion. Tru-and-fro artifacts (AV groove arteries) occur when the artery moves during cardiac cycle before a full circle image is formed. Kite catheter artifact is when the guide catheter is deeply encased in the artery, which should not be mistaken for a calcified block.
Ringdown, Blood Speckle, and Shadowing Artifacts
Ringdown artifact is bright halos surrounding the catheter, creating uncertainty near the vessel wall. Blood speckle artifact impairs identifying the lumen-intima interface; inject contrast or saline to remove it. Shadowing occurs when ultrasound waves reflect from the surface, obscuring deeper structures (caused by calcium, air bubbles, or guide wires).
Air Bubbles, Reverberation, and Sidelobe Artifacts
Air bubbles are a big enemy of IVUS, causing diffuse brightness and reverberation artifacts; flush the catheter to remove them. Reverberation is repeated reflections between the catheter and reflective surfaces (stent, calcium), creating ghost images. Sidelobe artifacts occur when sidelobes encounter strong reflecting surfaces, causing false appearances of stent malapposition or dissection.
Stent Ghost Artifact and Q&A
Stent ghost artifacts are reflections from stent metals on the opposite side, which can be mistaken for malapposition. The Q&A covers topics like managing tissue prolapse after DES, using IVUS in carotid arteries, avoiding ringdown artifacts, measuring vessel size, identifying the lumen-vessel wall interface, differentiating shadow and sidelobe, choosing between OCT and IVUS, and selecting stent size.
