Page 35 - Feline diagnostic imaging

28 3 Ultrasound: Physical Principles of Ultrasound Imaging
(a) (a)
Period Wavelength
Chart Title

PRESSURE PRESSURE

TIME DISTANCE
Period of a low frequency ultrasound wave. (b)
(b)
Wavelength
Period

PRESSURE PRESSURE

DISTANCE
TIME Period of a high frequency ultrasound wave.
Period of a high frequency ultrasound wave.
Figure 3.2 Wavelength is the length of space one wave cycle
Figure 3.1 Ultrasound is depicted as a sine wave. A sound occupies. This is the distance between two successive peaks or
travels through a medium; the pressure at any point in the two successive valleys in the ultrasound wave. Higher frequency
medium will increase to a maximum value, return to baseline, transducers will have shorter wavelengths. This will result in
decrease to a minimum value and then return to baseline. improved resolution and image detail at the expense of depth
A cycle is one complete positive and negative deflection of this penetration.
wave from baseline. Frequency is how many times these cycles
occur in one second. Higher frequency transducers will have a
shorter spatial pulse length leading to improved resolution.
Table 3.1 Speed of sound, impedance, and density of various
biological tissues
impedance differences occur at interfaces with bone and gas
(Table 3.1) so that the greatest amount of reflection of sound Speed of Density Impedance
occurs at soft tissue–bone and soft tissue–gas interfaces. Tissue sound (m/s) (kg/m ) (Rayls)
3
Since the soft tissue structures within the abdomen have
small differences in acoustic impedance, only a small amount Bone 4080 1912 7.8 × 10 6
of the sound wave is reflected back to the transducer. This is Muscle 1600 1068 1.71 × 10 6
fundamental for our ability to visualize the abdominal con- Kidney 1560 1041 1.63 × 10 6
tents with ultrasound since most of the beam is still available Liver 1555 1061 1.65 × 10 6
for transmission to deeper structures. Soft 1540 1050 1.62 × 10 6
tissues
Water 1480 1000 1.48 × 10 6
3.4 Transducers Fat 1450 924 1.34 × 10 6

Lung 600 300 0.18 × 10 6
Ultrasound transducers convert electric energy into ultra- 6
sound energy and vice versa. Ultrasound transducers oper- Air 330 1.2 0.0004 × 10
ate according to the principle of piezoelectricity [2, 5, 6]. Source: Reference [1].