Technical Aspects of the IVUS

The images of ultrasound [7] are based on the reception and transmission of high-frequency sound waves. The transmitted wave propagates through the material until it strikes the reflecting object. The reflected wave returns and is received by a transducer. The time interval between the transmission and the reception of the wave is directly related to the distance between the source and the reflector. The advantage of ultrasound is that it can travel through water smooth tissue. Additionally, ultrasound is not harmful at the levels of energy used for the generation of the image. A standard configuration of IVUS acquisition images consists of three components. Figure 1.3 shows a scheme of a catheter with a miniaturized piezoelectric transducer, the pull-back unit, and the console for reconstructing the images. IVUS catheter has a rank of measures that oscillates between 2.9 and 3.5 F (0.96-1.17 mm) in diameter. The quality of the image depends on the operation frequency, which is of the order of 20-50 MHz; the lateral resolution is approximately of the order of 113 ¡xm and the axial resolution is of the order of 80 ¡xm [8]. The IVUS images acquisition process is initiated when the catheter is manually (guided by the angiography) inserted within the artery (Fig. 1.3(a)). The catheter pull-back is made at linear constant velocity (usually 0.5 mm/sec)

Figure 1.3: The IVUS catheter is manually positioned within the artery (a) and extracted by a pull-back unit at a constant linear velocity and rotated at a constant angular velocity. The information is transformed by the IVUS console as a unique cross-section artery gray-levels image (d) or a longitudinal image sequence (e).

Figure 1.3: The IVUS catheter is manually positioned within the artery (a) and extracted by a pull-back unit at a constant linear velocity and rotated at a constant angular velocity. The information is transformed by the IVUS console as a unique cross-section artery gray-levels image (d) or a longitudinal image sequence (e).

and constant angular velocity of 1800 rev/min. The pivoting transducer sends a radially focused beam of ultrasound and receives its corresponding echoes. The radial lines obtained for different transducer angular positions are adequately processed, giving, as a result, a 2D cross-section artery image (Fig. 1.3(d)). The sequence can be shown as a longitudinal sequence, which gives a longitudinal artery cut (Fig. 1.3(e)). The resolution of an ultrasound image is directly related to the ultrasound signal frequency: high frequencies allow one to obtain better resolution. Nevertheless, when the frequency is increased, the attenuation of the waves of ultrasound increases while penetrating the biological tissue. The typical frequencies in the IVUS technique are in the rank of 20-50 MHz, with inferior resolutions of 50 ¡m.

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