10 Ultrasound Flashcards
Sound waves
Pressure disturbance that propagates through a material
The amplitude of a wave
The size of pressure difference from the equilibrium value.
Wavelength (λ)
The distance between successive wave crests.
Frequency (f)
- The number of oscillations in each second.
* The number of wavelengths that pass a given point each second.
Longitudinal waves in ultrasound
Have vibrations along their travel direction.
Transverse waves in ultrasound
Have vibrations perpendicular to the travel direction.
The period
The time between successive oscillations.
Diagnostic ultrasound uses transducers with frequencies ranging
1 to 20 MHz.
velocity (v).
v = f ×λ (m/s).
The ultrasound intensity
Measuring the energy flowing through a given cross-sectional area each second.
Relative sound intensity
Measured on a logarithmic scale and may be expressed in decibel (dB).
Negative decibel values
Correspond to signal attenuation.
Positive decibel values
Correspond to signal amplification.
The acoustic impedance
- Describes how much resistance an ultrasound beam encounters as it passes through a tissue.
- The acoustic impedance (Z) of a material is the product of the density (ρ) and the sound velocity (v) in the material.
- Z = ρ × v
- The acoustic impedance unit is called the Rayl.
Reflections in ultrasound
A portion of the ultrasound beam is reflected at tissue interfaces.
[(Z2 − Z1)/(Z2 + Z1)]^2.
Nonspecular reflections in ultrasound
Diffuse scatter from rough surfaces where the irregular contours are bigger than the ultrasound wavelength.
Specular reflections in ultrasound
Occur from large smooth surfaces. Specular reflection intensity is independent of frequency.
echo
The sound reflected back toward the transducer
Gel is applied between the transducer and skin
To displace the air and minimize large reflections that would interfere with ultrasound transmission into the patient.
shadowing.
Lack of transmissions beyond these interfaces results in areas void of echoes
Scattering in ultrasound
Occurs when ultrasound encounters objects that are smaller than the ultrasound wavelength.
Hyperechoic
A higher scatter amplitude relative to the background signal.
Hypoechoic
There is a lower acoustic scatter intensity relative to the average background signal.
Organs showing black in ultrasound
Organs that contain fluids, such as the bladder, and cysts have no internal structure and almost no echoes
Refraction in ultrasound
The change in direction of an ultrasound beam when passing from one tissue to another having a different speed of sound.
Refraction is described by
Snell’s law: sinθi/sinθt = v1/v2, where θi is the angle of incidence, θt is the transmitted angle, v1 is the velocity in tissue 1, and v2 is the velocity in tissue 2.
Attenuation in ultrasound
The composite effect of loss by scatter and absorption.
Attenuation in ultrasound is normally expressed in terms of
dB and depends on the distance the ultrasound beam has traveled in tissue.
A transducer
Device that can convert one form of energy into another.
The Doppler effect
Refers to changes in frequency resulting from a moving sound source.
–Objects moving toward the detector reflect sound that has a higher frequency.
–The increase in frequency is associated with a reduction in wavelength.
–Objects moving away from the detector reflect sound that has a lower frequency.
–The reduction in frequency is associated with an increase in wavelength.
Doppler ultrasound
Used to identify and evaluate blood flow in vessels based on the backscatter of blood cells.
Attenuation of
−10 dB
−20 dB
−30 dB
- 10%.
- 1%.
- 0.1%.
Depth gain compensation accounts for tissue attenuation
By increasing echo amplification for later echoes.
The average velocity of sound in soft tissue
1,540 m/s.
Velocity is inversely proportional to the square root of
The material compressibility.
Intensity transmitted in ultrasound
(4Z1× Z2)/(Z1 + Z2)^2.
Attenuation of ultrasound in soft tissue at 1 MHz is most likely
0.5 dB/cm.
Increasing the transducer thickness is most likely to increase the sound
Wavelength, since the crystal thickness is one half of the ultrasound wavelength.
The damping material behind the crystal transducer reduces the
The pulse length.
The pulse repetition period in ultrasound (listening time) is given by
1/PRF (i.e., 1/4,000seconds).
Ultrasound signals are converted to a video monitor display using:
Scan converts.
It takes 13 μs to get an echo from an interface from a depth of
1 cm.
The thermal index (TI) value indicates the possible increase in tissue
Temperature.
