Ultrasound Flashcards
describe a sound wave
Sound is a longitudinal wave that requires a medium for propagation.
This wave consists of a series of compressions and rarefactions.
What happens to medium particles with the soundwaves?
The particles in the medium simply oscillate back and forth as the sound wave propagates through the tissue
The range of ultrasound frequnecies
2 - 10 MHz
What does the higher the frequency mean?
the more superficial the wavelength travels into soft tissue
What is acoustic impedance?
It is a measure of how “easy” it is for sound to pass through a medium
Why is acoustic impedance important?
as differences in acoustic impedance between tissues cause sound wave reflection, which allows the transducer to detect echoes and form images.
What effect will a big difference in acoustic impedance e.g., tissue and air?
It will result in strong reflection of the soundwave
Ultrasound is a wave motion. True or False?
True
How are US images constructed?
By computing the time it takes for the beam to travel and reflect back to the transducer.
This provides depth data.
The magnitude of the echo modulates the brightness of the display
How does refraction of ultrasound obey similar laws to light?
Sound changes
velocity in going from one medium to another (wavelength changes).
What does refraction result in?
The beam bends as it passes from one medium to another
Beam axis no longer lies beneath transducer axis. Usually does not cause significant artefact
What is diffraction?
the bending of the ultrasound beam into the shadow of a
strong absorber
It occurs at the absorber edge e.g. gall stone
Can cause minor artefact
What is the basis of ultrasound
Images are constructed by computing the time it takes for the beam to
travel from the transducer and return from a reflecting surface.
This gives us our depth data (based on the velocity of sound in tissue)
The magnitude of the echo modulates the brightness of the display
What is a transducer?
A device which converts one form of energy to another form
What is used for the ultrasound transducer?
Piezoelectric crystal
How does this crystal work for the transducer?
By applying a voltage across the crystal the molecular dipoles
change orientation, changing the shape of the crystal.
The reciprocal effect also happens. By mechanically altering the
shape of the crystal a small voltage is generated across the crystal.
Thus the piezoelectric crystal can be used to generate and detect an ultrasound signal
What two modes can the transducer have?
- Transmit more
- Receive mode
What occurs in transmit mode?
a high pulsed voltage is applied to the transducer
What occurs in receive mode?
the small voltage generated by deformation of the crystal
is detected and amplified
What mode is the transducer in 99.9% of the time?
Receive mode
What is the function of backing block of of the transducer?
dampens reverberations in the crystal and absorbs backward transmission of the ultrasound pulse
Why is there a plastic matching layer positioned between the crystal and the skin in the transducer?
As there is a huge difference in acoustic impedance between the transducer crystal and the tissues, it improves sound transmission
The crystal thickness is ____ the wavelength of the US beam
Half
What is the use of the acoustic insulator in the transducer?
An acoustic insulator such as rubberized cork separates the crystal and backing block from the casing. This prevents artefacts due to casing vibrations
Why is ultrasound gel used?
to make good contact between the probe and the skin by eliminating the air layer. If we did not use ultrasound gel one would get almost 100% reflection
What kind of wave is used in ultrasound in modern days?
Pulse wave
How many wavelengths should an ultrasound pulse have?
Ideally, 2-3 wavelengths
“The deeper the area of interest in ultrasound, the ____ the spatial resolution is”
Worse
What is pulse repetition frequency?
This is the number of pulses per second.
It is set by a master clock in the ultrasound unit.
What drives the transducer?
an electrical pulse generator
By increasing the PRF we increase ____ but shortens ____
Image information rate (Echoes received), shortens the receive time
What determines spatial resolution in ultrasound?
Spatial Pulse Length (SPL)
What is SPL?
the lenght of the pulse (mm)
“an increase in SPL results in a _____ of spatial resolution”
decrease
What determines the maximum depth which can be imaged?
PRF
The range of frequencies within a pulse is called…
a Bandwidth
By increasing the SPL we _______ the bandwith
Decrease
What is specular reflection?
occurs when the boundary between two structures is smooth. It is responsible for generating the echoes that define organ boundaries
Nonspecular reflection
occurs when the surface irregularities are similar in size
to the beam’s wavelength. echoes are must smaller than in specular reflection
How does scatter occur in ultrasound?
When sound waves encounter structures that are much smaller than their wavelength they are scattered more or less equally in all directions.
The scattered echoes interfere and produce a” Speckle” appearance which is characteristic of the tissue structure, could be mistook for pathology
What is an important artefact in ultrasound?
Mirror artefact
What is mirror artefact?
When the reflecting surface meets the beam at a large angle of incidence (e.g. diaphragm)
Part of the beam is not reflected directly back to the transducer. This can result in a mirror image of a structure
Fresnel zone
Near field
Frauenhoffer zone
Far field
Which is more superior, Axial or Lateral reolution?
Axial
Axial resolution is…
The ability to distinguish two structures parallel to the US beam. It depends on the SPL (ie., shorter pulses improve resolution)
Lateral resolution is…
the ability to distinguish 2 structure perpendicular to the US beam. It is determined by the beam width (ie., narrower beam improves resolution)
What is Time Gain Control?
it amplifies the signal proportional to the time delay between transmission and detection of the echo
It allows us to see all signals at once, rather than just looking at the signal most recently received.
What is the Doppler effect?
a change in the perceived frequency of sound emitted by a moving source (eg blood flow)
