the ultrasound image – such as the resolution, depth of penetration, and attenuation of the acoustic – are dependent on the geometric and frequency properties of the transducer. The higher the center frequency, the better the axial resolution, but the lower the depth of penetration. Current IVUS catheters used in the coronary arteries have frequencies ranging 20–60 MHz and 100 to <40 μm axial resolution [6]. High‐definition IVUS with transducer frequency of 60 MHz has also become available, reaching highest axial resolution of 22 microns and lateral resolution from 80–200 to 50–140 μm.
Equipment for IVUS examination
Two different transducer designs are commonly used yielding comparable information: mechanically rotated and electronically activated phased‐array. Mechanical probes use a drive cable to rotate a single‐element transducer at the tip of the catheter at 1800 rpm.
At approximately 1° increments, the transducer sends and receives ultrasound signals providing 256 individual radial scan lines for each image. The mechanical transducer has the advantage of a simple design, greater signal‐to‐noise ratio, and higher temporal and spatial resolution. In electronic systems, multiple tiny transducer elements in an annular array are activated sequentially to generate the cross‐sectional image [4–6].
The IVUS console contains numerous imaging controls such as zoom, gain, TGC (time‐gain‐compensation), gamma curves, compression and reject, and others. With both systems, still frames and video images can be digitally archived on local storage memory or a remote server using DICOM format.
Imaging artifacts
Artifacts often appear in images generated by contemporary IVUS devices and can interfere in imaging interpretation and measurements (Figure 8.1).
Figure 8.1 Four examples of artifacts from Gary Mintz’s Atlas of Intracoronary Ultrasound, CRC Press, 2004. In (a), ring‐down artifacts in an electronic‐array system image, near‐field bright halos (arrows) close to the face of the catheter can obscure the area immediately adjacent to the catheter. In (b), non‐uniform rotation distortion (NURD) occurs only with mechanical systems. Part of the image is expanded causing deformation of the image in its circumferential view—the image appears elliptical (arrows). In (c), reverberations are repetitive echoes of the same structure. This is an example of reverberations from calcium. The arcs of calcium are indicated by the arrows a, and the false structures (reverberations) are indicated by arrows b. (d) Longitudinal image reconstruction (or L‐mode) is shown. There is excessive motion of the transducer (a) relative to the artery, causing zigzag or sawtooth appearance (white arrows). This artifact is more of a problem with the right and circumflex arteries, because of the wide atrioventricular groove movement between systole and diastole.
Ring‐down
Ring‐down artifacts usually appear as a series of parallel bands or halos of variables thickness surrounding the catheter obscuring near field imaging. Phased‐array systems tend to have more ringdown artifacts.
Non‐uniform rotational distortion
Non‐uniform rotational distortion (NURD) arises from frictional forces to the rotating elements in mechanical catheters. NURD creates stretched or compacted portions of the images. Because accurate reconstruction of IVUS two‐dimensional images is dependent on uniform rotation of the catheter, non‐uniform rotation can create errors during IVUS measurements.
Reverberations
Strong spatial tissue heterogeneity creates acoustic noise and pulse reverberations—multiple echoes reaching the transducer before the next pulse transmission to give rise to multiple copies of the anatomy. Reverberation artifacts are more common from strong echoreflectors such as stents, guidewires, guiding catheters, and calcium (especially after rotational atherectomy).
Other artifacts
A few other artifacts can also interfere in IVUS interpretation; side lobes and ghost artifacts also generated from strong echoreflectors such as calcium and stent metal [5]. In longitudinal or L‐mode display, catheter motion artifacts during the pullback results in a “saw tooth” appearance.
Catheter position also has an important role in image quality. Off axis position of the catheter can alter vessel geometry in an elliptical fashion to mislead the operator to overestimate the lumen and vessel area. Axial (antegrade–retrograde) movement of the IVUS probe during the cardiac cycle scrambles consecutive image slices that can have implications for three‐dimensional reconstruction and attempts to assess coronary artery compliance.
Image acquisition and presentation
Two important consensus documents have been published: Clinical use of intracoronary imaging. Part 1: guidance and optimization of coronary interventions. An expert consensus document of the European Association of Percutaneous Cardiovascular Interventions [4] and the Clinical use of intracoronary imaging. Part 2: acute coronary syndromes, ambiguous coronary angiography findings, and guiding interventional decision‐making: an expert consensus document of the European Association of Percutaneous Cardiovascular Interventions [5].
IVUS is displayed as a tomographic cross‐sectional view. A longitudinal view (L‐mode or long‐view) can be also displayed, but this should be done only when using motorized transducer pullback. Longitudinal representation of IVUS images is useful for lengths measurements, for interpolation of shadowed deep arterial structures (i.e. external elastic membrane behind calcium or stent metal). There are advantages and disadvantages to using manual or motorized pullback; however, motorized pullback is usually preferable. Using motorized transducer pullback allows assessment of lesion length, volumetric measurements, consistent and systematic IVUS image acquisition among different operators, and uniform and reproducible image acquisition for multicenter and serial studies.
In standard image acquisition after anticoagulation and intracoronary nitroglycerin administration, the IVUS catheter should be placed distal to the segment of interest (aiming for 20 mm of distal reference), and a continuous pullback to the aorta should be recorded. The preferred pullback speed is 0.5 mm/s but 1 mm/s is often used.
Normal artery morphology
The ultrasound appearance of normal human arteries in vitro and in vivo has been reported [6–8]. In muscular arteries such as the coronary tree there are three layers: intima, media, and adventitia. Normal intima thickness increases with age, from a single endothelial cell at birth to a mean of 60 μm at five years to 220–250 μm at 30–40 years of age [19]. The definition of abnormal intimal thickness by IVUS is still controversial; in general, the threshold of “normal intimal thickness” is <300 μm (0.3 mm). The innermost layer of the intima is relatively echogenic compared with the lumen and media and displayed on the screen as a single bright concentric echo. The lower ultrasound reflectance of the media is due to its homogeneous smooth muscle cells distribution and smaller amounts of collagen, elastic tissue, and proteoglycans. The thickness of media histologically averages 200 μm, but medial thinning occurs in the presence of atherosclerosis [9]. In advanced
