Читать книгу Interventional Cardiology - Группа авторов - Страница 205

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.