Ultrasound 1 Flashcards
Sound is a longitudinal wave which requires:
- Needs a medium to travel
- Particles in the medium move
to carry the sound
Compression is when
regions in the air where the air particles are compressed together
Rarefaction is when
regions where the air particles are spread apart
Humans can detect frequencies ranging from
20-20,000 Hz or 20 Hz to 20kHz
Infrasound is
f < 20 Hz, below the threshold of human hearing, i.e., earthquakes.
Audible sound is
f = 20 Hz-20 kHz, range of frequencies audible to a healthy ear, we lose this range over time.
Ultrasound is
f > 20 kHz, above the threshold of human hearing.
Wavelength is the
distance between crest to crest or trough to trough
Amplitude is the
Magnitude of the disturbance travelling through the medium
The amplitude of a sound wave is proportional to the change in pressure
Frequency is
the number of complete wave cycles that pass a fixed-point in 1 s
Destructive interference is when
Waves that are out of phase can “cancel out” each other
Constructive interference is when
Waves that are in phase can add together
How does density and temperature affect the speed of sound?
Density – higher the density the higher the speed of sound. Less distance between molecules.
Temperature – higher temperature results in higher wave speed. Molecules can vibrate faster.
Longitudinal waves, such as sound is when particles oscillate…
in the direction parallel to wave propagation
Transverse waves, such as light, is when particles oscillate…
in the direction perpendicular to the wave propagation
Why are ultrasounds used?
Ultrasound is used in medicine as a fast, cheap & non-invasive diagnostic imaging technique.
How do we know which frequency to choose in regards to ultrasound?
In general, the shorter the wavelength, the greater the image resolution. This means a high frequency is desirable, remember 𝑐=𝑓𝜆
The problem with this is that higher frequencies are attenuated (absorbed) quicker.
This means that a compromise must be made, generally frequencies of 1-10 MHz are used. Below are rough guides:
7.5-10 MHz used for superficial structures (tendons, ligaments)
5 MHz for vascular imaging
3.5-5 MHz for deeper structures, bladder, pelvis
Lateral resolution indicates the ability to distinguish to points ___________ to the path of the ultrasound beam.
perpendicular
Axial resolution is the ability to distinguish between two points _________ to the path of the ultrasound beam.
parallel
True or false: The narrower the beam, the higher resolution (in general).
True
True or false: A shorter wavelength means longer pulses and worse axial resolution.
False, shorter pulses and better axial
Axial resolution =
1/2 pulse length
Elevation Resolution is the
The ability to distinguish between two points perpendicular to the scan plan. Depends on depth.
Pulse length =
Pulse velocity x pulse duration
The key component is the piezo element, which consists of
a piezoelectric crystal that can both produce ultrasound and generate a signal from the ultrasound via the piezoelectric effect.
How does the piezoelectric effect work?
A piezoelectric crystal changes dimension if a voltage is applied along one direction (can make it vibrate)
Alternatively, if you apply pressure along one direction of a Piezo-electric crystal it generates a voltage (vibrations hitting the crystal will induce a voltage which is interpreted as a signal)
What three things can happen to an ultrasound pulse that meets a boundary between two media?
Transmission – wave passed through
Reflection – waves bounces off
Absorption – wave is absorbed
What is acoustic impedance (Z) and what depends on it?
The amount of reflection, and transmission that takes place, depends on the Acoustic Impedance (Z) of the two media.
Acoustic Impedance is a measure of how hard it is for the ultrasound wave to travel through the medium.
Why is ultrasound gel used?
If the impedance is matched correctly, most of the ultrasound will travel through the material and won’t be reflected.
Ultrasound gel is used to “couple” the probe and the skin to improve transmission.
Briefly explain the A-mode scan. Make sure to highlight the key factors which differ it from other modes of ultrasound:
Largely outdated but the method of scanning is useful to study.
Velocity = distance/time
distance = (velocity)(time)
Speed of ultrasound in the medium is known so the distance from the probe is easily determined from the echo time.
Were used to look at the eye before optical coherence tomography (OCT) became more popular.
1D form of ultrasound imaging
Discuss the B-mode scan:
Mode that is most usually associated with ultrasound images.
A 2D image is generated of the slice of the patient immediately below the probe head.
