Page 19 - Libro vascular I

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PERIPHERAL VASCULAR ULTRASOUND
The path along which the reflected ultrasound travels will also affect the amplitude of the signal detected by the transducer. If the beam is perpen- dicular to the interface, the reflected ultrasound will travel back along the same path to the trans- ducer. If, however, the beam intercepts the inter- face at an angle of less than 90°, then the beam will be reflected along a different path. Figure 2.6 shows that the angle of incidence (i) is the same as the angle of reflection (r) measured from a line perpendicular to the interface. This means that when the beam is at 90° to the interface, all the
reflected ultrasound will travel back towards the transducer, but as the angle of incidence becomes smaller, the beam will be reflected away from the transducer and therefore the transducer will receive less of the reflected ultrasound. The best image of an interface will be obtained when the interface is at right angles to the beam, and like- wise the poorest image will be obtained when the interface is parallel to the beam. Thus, when an artery is imaged in transverse section, the anterior and posterior walls can be seen more clearly than the side, or lateral, walls which are parallel to the beam (Fig. 8.5).
If the ultrasound beam is not perpendicular to the interface and there is a change in the speed of sound in the media on either side of the interface, the path of the beam will be bent. This is known as refraction and is illustrated in Figure 2.7. Refrac- tion causes the beam to change its direction of travel and can lead to artifacts whereby the signal detected by the transducer has originated from a different point in the tissue than that displayed on the image. This is most important where there are large changes in the velocity of sound between media, such as the interface between the uterus and amniotic fluid. It is not usually a major problem
ui ur 0o
Tissue boundary
A
B
ui ur
Transmitted
Lower speed of sound
Higher speed of sound
A: When an ultrasound beam is perpendicular to an interface, the reflected ultrasound will return by the same path. B: If the interface is not perpendicular to the beam then the reflected ultrasound will travel along a different path. The angle of incidence of the beam (i) is equal to the angle of reflection (r).
Figure 2.6
Reflected
Tissue boundary
Tissue boundary
Figure 2.7 Refraction. When a beam is transmitted through an interface between two media in which the sound travels at different speeds and the beam is not perpendicular to the interface, the path of the beam will be bent.