医学影像学英文课件：Basics of Ultrasound

Physical characteristics of ultrasound
6.Attenuation:
As sound passes through tissue, it loses energy, and the pressure waves decrease in amplitude as they travel farther from their source
Physical characteristics of ultrasound
Figure 1 Sound waves
1.Sound waves: Mechanical
oscillations that are transmitted by particles
2. Wavelength (λ) , Wave period (T) and frequency (f)
Physical characteristics of ultrasound
7. Doppler effect
When ultrasound is transmitted towards a stationary reflector, the reflected waves will be of the same frequency as those originally transmitted
Basics of Ultrasound and Echocardiography
Part I Fundamental physical principles
of diagnostic ultrasound
DeHale Waihona Puke Baiduinition of ultrasound
Ultrasound refers to high-frequency sound waves, over 20000 cycles per second (20 000Hz or 20kHz)
Scatter occurs when the dimension of reflectors is much smaller than the wavelenth of sound wave θ1 =70°
θ2 =86° Figure 3 Reflection/Refraction/Scatter
Most diagnostic applications employ frequencies of 1 to 13 MHz
The unit of frequency is hertz (Hz); kilohertz (kHz; 1 kHz =1000 Hz) ; megahertz (MHz; 1 MHz = 1,000,000 Hz)
4. Acoustic Impedance
Resistance offered by tissue to movement of particles caused by ultrasound waves
Acoustic impedance (Z) is determined by product of the density (ρ) of the medium propagating the sound and the propagation velocity (c) of sound in that medium
Z = ρc
Physical characteristics of ultrasound
5. Reflection/Refraction/Scatter
Direction of sound wave will change when it strikes a boundary of two different mediums with different acoustic impedance
Attenuation is the result of the
combined effects of absorption, scattering and reflection
Figure 4 Tissue attenuation causes gradual loss of display of deeper tissues
Physical characteristics of ultrasound
3. Velocity of sound (C)
Propagation velocity of sound is determined by the physical properties of tissue. In the body, propagation velocity of sound is assumed to be 1540 meters per second (m/sec).
λ: The distance occupied by each cycle
T : The time which is required to produce each cycle
f : The number of cycles completed per second. The frequency of the wave is 1/T
The propagation velocity of sound (c) is related to frequency and wavelength by the following simple equation:
c = fλ
Figure 2 Propagation velocity
Physical characteristics of ultrasound
When sound passes from a tissue with one acoustic propagation velocity to a tissue with a higher or lower sound velocity, there is a change in the direction of sound wave