Ultrasound machines assume sound waves travel at a speed of
1540 m/sec
through tissue
1
. In reality, the speed of sound is affected by the density and elasticity of the medium through which it is traveling and these factors are not constant for human tissues.
How is ultrasound velocity calculated?
A clever technique is used to measure the
Doppler shift
in an echo. The frequency of the echoed sound is superimposed on the broadcast frequency, producing beats. The beat frequency is F
B
= |f
1
− f
2
|, and so it is directly proportional to the Doppler shift (f
1
− f
2
) and hence, the reflector’s velocity.
How is ultrasound speed calculated?
The speed at which a sound wave travels through a medium is called the propagation speed or velocity. It is equal to the frequency times the wavelength. In ultrasound it is measured in meters per second (m/s) or
millimeters per microsecond (mm/μ s)
.
Does change in frequency affect ultrasound speed?
The varying speeds of sound modulated at the boundaries of tissues provide the fundamental reason for contrast in ultrasound imaging. Within a given tissue medium,
ultrasound frequency is independent of changes in sound speed
. Hence, the wavelength is determined by frequency and the propagation medium.
How is ultrasound attenuation calculated?
- To calculate attenuation (dB) simply multiply rule of thumb by round trip distance & by frequency.
- 5 MHz sound; 10 cm sound travel.
- 3.5 MHz sound; 4 cm sound travel.
Does ultrasound travel through air?
Sound travels the fastest through bone tissue, and
moves most slowly through air
. The speed at which the sound waves are returned to the transducer, as well as how much of the sound wave returns, is translated by the transducer as different types of tissue.
What type of wave is an ultrasound?
Ultrasound is the name given to
sound waves that have frequencies greater than 20,000Hz (20 kHz)
. This is above the normal hearing range for humans, so we cannot hear ultrasound.
Does ultrasound have any side effects?
Unlike some other scans, such as CT scans, ultrasound scans don’t involve exposure to radiation.
External and internal ultrasound scans don’t have any side effects
and are generally painless, although you may experience some discomfort as the probe is pressed over your skin or inserted into your body.
How do ultrasound waves travel?
The ultrasound machine transmits high-frequency (1 to 5 megahertz)
sound pulses into your body
using a probe. The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).
How fat is the speed of sound?
| Speed of sound (m/s) Average Maximum | Fat (Average Infiltrated) 1440.2 1490.0 | Fat (Not Infiltrated) 1440.2 1490.0 | Gallbladder 1583.6 1583.6 | Heart Lumen 1578.2 1590.0 |
|---|
What absorbs the most ultrasound energy?
Generally, the
tissues with the higher protein content
will absorb ultrasound to a greater extent, thus tissues with high water content and low protein content absorb little of the ultrasound energy (e.g. blood and fat), while those with a lower water content and a higher protein content will absorb ultrasound far more …
Can ultrasound waves be reflected?
Reflections. When ultrasound waves reach a boundary between two substances with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through.
A detector placed near the source of the ultrasound waves is able
to detect the reflected waves.
Is ultrasound high or low frequency?
Ultrasound sound waves have frequencies above those audible to the human ear, that is, greater than approximately
20 MHz
. Ultrasound typically used in clinical settings has frequencies between 2 and 12 MHz.
How far does ultrasound air travel?
That is an enormous distance, so we need to look at the attenuation coefficient instead. This quickly gives us an absolute upper limit of
approximately 200 m
for any ultrasound signal transmission.
What are the causes of attenuation in ultrasound?
Attenuation of ultrasound waves traversing a medium is not only a result of
absorption and scattering within a given tissue
, but also of coherent scattering, including diffraction, refraction, and reflection of the acoustic wave at tissue boundaries.
What affects attenuation in ultrasound?
Absorption
. This is the main factor causing attenuation of the ultrasound beam. The higher the frequency of the sound wave, the greater the amount of absorption that will occur. … The majority of the lost energy will cause a rise in temperature of the tissue through which the sound is traveling.
