CHAPTER 11 Intravascular and Intracardiac Imaging

Angiography Displaces blood with radiographic contrast media.
Produces images that are silhouettes of the vessel’s internal arterial lumen.
Cannot accurately analyze plaque distribution or morphologic characteristics of a vessel.
Angiography does not always accurately diagnose vascular disease.<br>

Intravascular Ultrasound Uses catheters with an array of transmitters and sensors near their distal tip, facing outwards.
This allows for full 360° visualization of the vessel wall.
Sound waves of frequencies (20–60 MHz) are bounced off the vessel walls, producing echo images from the inside out, not confined to the vessel’s inner lumen.
This provides detailed images of the composition of vessels and lesions.
Can distinguish different plaque morphologies that may not be appreciated with angiography alone.<br>

Angiography vs. IVUS<br>

Mechanical IVUS Systems Have a moveable imaging core within an integrated sheath.
Have short, monorail guide wire lumens.
A single piezoelectric crystal rotates at 1800 rpm.
The distal end of the catheter body allows the imaging core wire to be advanced and retracted 15 cm without manipulating the catheter.
Catheter-handling characteristics remain inferior to solid-state designs. Boston Scientific OptiCross catheter.<br>

Mechanical IVUS System Preparation Catheters are prepped with heparinized saline prior to use.
The telescoping imaging core should be retracted to its most proximal position.
A short extension tube is connected to the proximal hub and a three-way stopcock attached.
A 3-mL and a 10-mL syringe, both filled with heparinized saline, are attached to the stopcock, and the imaging catheter is flushed with the 3-mL syringe.
The 10-mL syringe is used as a fluid reservoir, and the catheter is flushed a second time with the 3-mL syringe to ensure that all air has been expelled.
With poor catheter prepping, the IVUS image intermittently degrades, which may periodically require additional catheter flushing during imaging runs.<br>

Phased-Array IVUS Systems Have electronic elements mounted circumferentially near the distal tip.
Microchips located proximal to the electronic transducer elements control transmission and reception of ultrasound signals.
Imaging elements are 10.5 mm or 2.5 mm proximal to the tip of the catheter.
The catheters have lower profiles, increased flexibility, and superior trackability and steerability than mechanical IVUS imaging catheters. Transducers near the tip of a Philips Eagle Eye Platinum IVUS catheter.<br>

Phased-Array IVUS Systems Preparation The electronic connection for imaging is handed to a nonsterile circulator.
The connector is plugged into the interface module, which connects directly to the IVUS console.
They produce a catheter “ring-down” artifact, which needs to be digitally subtracted from the image.
For optimal imaging, the reference/ring-down should be performed with the IVUS catheter free in the aorta, where tissue signals are not visible.<br>

IVUS Image Interpretation The center of the screen is a black “dead zone” occupied by the IVUS catheter.
Blood appears as gray speckles that swirl or move about.
May demonstrate trilaminar appearance of the intima, media, and adventitia. Normal coronary artery morphology. 1. Lumen border (green). 2. Catheter mask (red).
3. Vessel border (orange).<br>

Lesion Interpretation Lesions that are predominantly composed of lipids produce weakly reflective, dark gray echoes, with a homogenous pattern.
Fibrotic lesions produce highly reflective, brighter, denser gray signals.
Hard, calcified lesions produce very bright white echoes. There is shadowing or echo dropout of the deeper tissue structures beyond. Calcium<br>

Thrombus Visualization Chronic or organized thrombus looks similar to lipid or mildly fibrotic plaques.
Fresh thrombus will appear mobile with an inconsistent form.
Is best seen in a live image, and may appear scintillating, or mobile.<br>

Pullback A three-dimensional image of the section of the vessel can be created by pulling back on the IVUS catheter during image acquisition.
Mechanical pullback units can be used at a constant rate of 0.5–1.0 mm per second.
Imaging systems are equipped with a longitudinal imaging function, which can accurately measure lesion length.<br>

Virtual Histology Imaging Uses spectral analysis of the ultrasound signals to correctly assess plaque components.
The colorized VH IVUS images show four basic plaque types:
Fibrous (green)
Fibro-fatty (green/yellow)
Calcific (white)
Necrotic cores (red) VH IVUS images showing lesion composition.<br>

Near-Infrared Spectroscopy Microscopic mirrors near the end of the catheter deliver infrared light and collect the reflections.
Produces data that can distinguish between collagen and lipid.
Orange to yellow is indicative of lipid in the lesion, making it unstable.
Devices combine IVUS with NIRS to identify vulnerable lesions.<br>

Diagnostic Applications IVUS is an invaluable tool in assessing ambiguous angiographic lesions in symptomatic and in asymptomatic patients.
IVUS assessment frequently reveals significant segments of plaque not visualized with angiography.
Left main trunk disease can be difficult to assess with angiography, but IVUS can provide differentiated, accurate assessment.<br>

IVUS in Interventional Procedures Intimal Flap Plaque morphology can be examined, providing a more “lesion-specific” PCI.
Confirms effective stent placement, with all struts nestled into vessel wall.
Can assess the extent of lesion dissections. Angiographically, the area may appear fuzzy or hazy. With IVUS, a dissection image will clearly show a flap, or dissection arm, extending away from the vessel wall into the lumen.<br>

IVUS and Stenting Incomplete apposition of stent struts to the arterial wall and incomplete stent expansion within the lesion can cause thrombotic vessel occlusion. Stent malapposition (top, white arrows) despite angiography showing a good result (bottom, white arrow). After dilatating with a larger balloon, IVUS reveals complete apposition of the stent to the vessel wall (top), while angiography shows no apparent change in vessel appearance or luminal dimension (bottom).<br>

Optical Coherence Tomography (1 of 2) Uses infrared light to create intravascular tomographic images.
Achieves tissue penetration of 1–3 mm.
Catheter needs a flush system or syringe.
Has 10 times greater image resolution than IVUS
OCT light waves can pass through calcium.<br>

Optical Coherence Tomography (2 of 2) OCT pullback with a Dragonfly Opstar Imaging Catheter.<br>

Intracardiac Echocardiography Uses the same technology as IVUS but used within the cardiac chambers.
Provides superb anatomic detail and aids in diagnosing complex cardiac abnormalities.
Used to guide complex, congenital, and structural heart procedures.<br>

Complications Coronary vasospasm
Patients often receive 100–200 µg of IC nitroglycerin prior to deployment of the imaging catheter to prevent vasospasm.
Coronary dissection
Cardiac perforation leading to tamponade
Ischemia due to the imaging catheter obstructing distal blood flow with severe lesions
Dysrhythmias
Myocardial infarction<br>

Nursing Care Patients undergoing catheter-based imaging procedures have the same needs as those undergoing any other invasive cardiac procedure.
Patients receive 3000–5000 units of IV heparin or other anticoagulant prior to guide wire insertion to decrease the risk of thrombosis.<br>

Summary IVUS, OCT, NIRS, and ICE are technologies that are complementary to angiography.
The technology has experienced tremendous growth.
Cost is the main factor limiting their use.
It has still to be clearly demonstrated that the additional diagnostic information gained by using these technologies improves patient outcomes enough to justify the additional expense.<br>