There are two varieties of IVUS catheters: mechanical and phased array (Fig. 31.1)
. Mechanical transducers use electrical current generated through a piezoelectric crystal to produce and receive sound waves. Typically, mechanical transducers have a single transducer element that rotates circumferentially on a drive shaft at approximately 1900 rotations per minute to create a cross-sectional image. Phased array transducers are stationary. They have multiple transducer elements, typically 64 in current catheters, positioned in a radial fashion that fire in circumferential succession. This produces an array of
images, which are processed into a 360-degree cross-sectional image of the vessel lumen and wall.
FIGURE 31.1. Mechanical and phased array transducers. Mechanical transducer images may have artifact related to the wire position. (Courtesy of Boston Scientific Corporation, Natick, MA.)
Both types of transducers have limitations and generate artifacts that the user must be familiar with. Mechanical transducers typically have a wire channel that runs alongside the transducer that generates a wire artifact in the images (Fig. 31.2)
. In addition, mechanical transducers can trap small bubbles that result in air speckling if not flushed properly. Nonuniform rotational distortion (NURD) is a rotational distortion that occurs with mechanical transducers. This is typically seen in tortuous vessels, with kinking of guide catheters, or when hemostatic Tuohy-Borst valves are tightened excessively over the catheter. Excess stress on the IVUS catheter causes bending, and the rotational speed of the transducer varies, causing image acquisition to be distorted. Manufacturers of mechanical transducers have introduced corrective software to prevent this distortion. Phased array transducers can have ring-down artifacts. This represents an area immediately surrounding the catheter obscured by acoustic oscillations within the transducer. Adjustments in the gain of the transducer can be made once the catheter is outside of the sheath or guide catheter and not apposed to the arterial wall to minimize this artifact. Care must be taken to avoid adjusting too much, as true luminal signals can also be suppressed.4
IVUS catheters range in frequency from 8 to 50 MHz. As with all ultrasound probes, higher frequencies provide greater spatial resolution. The trade-off for using higher frequency probes is decreased depth of tissue penetration. Balancing these two factors is imperative when selecting the appropriate catheter for intravascular imaging. Catheters suitable for imaging the aorta and inferior vena cava (IVC) have a frequency range from 9 to 15 MHz. These catheters are typically able to image vessels with a maximum diameter of 5 to 6 cm, depending on the system. Optimal imaging of the iliac and common femoral arteries and veins can be obtained with catheters of 15 to 20 MHz. Smaller vessels, such as the superficial femoral, tibial, and renal arteries, are best imaged with transducers of 20 to 40 MHz (Table 31.1)
. Although vessel size may initially dictate the catheter used, other factors such as delivery system, sheath size, and wire access also factor into the selection of the appropriate catheter for the case at hand.
FIGURE 31.2. Wire artifact associated with mechanical IVUS transducers (red arrow).
TABLE 31.1 CHARACTERISTICS OF CURRENT COMMERCIALLY AVAILABLE PHASED ARRAY AND MECHANICAL IVUS CATHETERS
▪ SMALL VESSEL PHASED ARRAY
▪ MID-VESSEL PHASED ARRAY
▪ LARGE VESSEL PHASED ARRAY
▪ MID-VESSEL MECHANICAL
▪ LARGE VESSEL MECHANICAL
Vessel imaging diameter
Minimum sheath size
Over the wire
Over the wire
IVC, inferior vena cava.
FIGURE 31.3. Virtual histology of peripheral plaque. A, Fibro-fatty plaque with small areas of necrotic core. B, Fibroatheroma with dense areas of calcification.
Interpretation of IVUS imaging can be aided by two specific technologies: virtual histology and ChromaFlo®
. Virtual histology is a computer-generated color map created from the reflected ultrasound signals. This map is displayed on the screen superimposed over the gray-scale image. Each of the four colors used represents a different component of plaque (Fig. 31.3)
. Deep green represents fibrous plaque, lighter green is fibro-fatty plaque, white represents dense calcium, and red is necrotic core. In an attempt to better understand the ability of virtual histology to characterize plaque, the Virtual Histology Intravascular Ultrasound Assessment of Carotid Artery Disease: The Carotid Artery Plaque Virtual Histology Evaluation (CAPITAL) study used virtual histology to assess carotid plaque prior to endarterectomy in 15 patients. Histologic composition of the plaques was compared to the virtual histology generated by IVUS. The authors found the diagnostic accuracy of virtual histology to range from 72.4% for calcified fibroatheroma to 99.4% for thin-cap fibroatheroma. The specificity ranged from 82% for intimal thickening to 100% for thincap fibroatheroma.
FIGURE 31.4. ChromaFlo® imaging showing delineation of an edge dissection after angioplasty. (Images courtesy of Volcano Corporation, San Diego, CA.)
Small and mid-sized vessel phased array catheters are also able to offer color flow (ChromaFlo®
) technology. When the location or patency of the vessel lumen is unclear, color flow imaging can be turned on to display the flow channel of the vessel (Fig. 31.4)
. Applications of this technology include evaluating the luminal irregularity of plaque and stent apposition. This technology has also been mounted on a reentry needle for use in sub-intimal angioplasty.