Interventional Cardiology. Группа авторов

      CHAPTER 8

      Intravascular Ultrasound: Principles, Image Interpretation, and Clinical Applications

       Adriano Caixeta, Akiko Maehara, and Gary S. Mintz

      Medical uses of ultrasound came shortly after the end of World War II. However, real‐time ultrasound imaging originated in the late 1960s and early 1970s when Bom et al. [1] pioneered the development of linear array transducers for use in the cardiovascular system. The first two‐dimensional catheter imaging system was designed in 1972 using a solid‐state transducer array of 32 elements arranged radially at the tip of a 9 Fr catheter [2]. By the late 1980s, Yock et al. [3] had successfully miniaturized a single‐transducer system that could be placed within coronary arteries. Ever since, intravascular ultrasound (IVUS) has become an increasingly important catheter‐based imaging technology providing both practical guidance for percutaneous coronary interventions (PCI) as well as many different clinical and research insights [4,5]. IVUS directly images the atheroma within the vessel wall, allowing reproducible measurement of plaque size, distribution, and to some extent its composition.

Principles of IVUS imaging

      Ultrasound is acoustic energy with a frequency above human hearing. The highest frequency that the human ear can detect is approximately 20 thousand cycles per second (20 000 Hz). This is where the sonic range ends and where the ultrasonic range begins. In medical imaging, high‐frequency acoustic energy is the range of millions of cycles per second (megahertz; MHz).

      IVUS supplements angiography by providing a tomographic perspective of lumen geometry and vessel wall structure. The equipment required to perform intracoronary ultrasound consists of a catheter incorporating a miniaturized transducer and a console to reconstruct the images. The IVUS transducer converts electrical energy into acoustical energy through a piezo‐electric (pressure‐electric) crystalline material that expands and contracts to produce sound waves when electrically excited (i.e. a series of pulse/echo sequences or vectors). After reflection from tissue, part of the ultrasound energy returns to the transducer; the transducer then generates an electrical impulse that is converted into moving pictures [6]. All materials in the body reflect sound waves. Sound waves bounce back at various intervals depending on the type of material and the distance from the transducer. It is the variation in reflective sound waves that creates the ultrasound image on the console.

      The intensity of reflected (or backscattered) ultrasound depends on a number of variables including the intensity of the transmitted signal, the attenuation of the signal by the tissue, the distance from the transducer to the target, the angle of the signal relative to the target, and the density of the tissue. Several clinically relevant properties of 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,