Ultrasound Flashcards
Describe process of return of echo signal to a transducer
Piezoelectric effect is responsible for returning echoes - conversion of mechanical energy into electrical energy, which is what we convert into our ultrasound image. PZT (lead zirconium titanate) crystals are surrounded by electrodes + and - on opposing ends, which compress the material - results in current measured by electrodes - current then converted into pixel grey scale value
Reverse piezoelectric effect - Electrical current conversion into mechanical energy - if current run through electrodes- current will force the crystals to change shape - which propagates mechanical energy into tissue. as material expands - regions of compression of sound wave occur. Rarefaction occurs when crystals return to normal shape. The alternating compression and rarefaction result in propagation of sound wave into patient tissue
Explain the physics being time gain compensation (TGC)/ depth gain compensation/ time varied gain/ swept gain
We need to compensate for the attenuation of an incident ultrasound beam as it travels into tissue _ Amplitude is less the further the ultrasound beam travels into tissue despite the reflection/ difference in acoustic impedance throughout tissue being the same
If we don’t compensate, the signal we get back won’t accurately represent the differences in acoustic impedance values, as we have lost intensity just by the ultrasound beam being attenuated
*Attenuation dependent on - distance traveled into tissue ( attenuated more distant it travels, attenuation coefficient (higher frequency more attenuated), tissue itself (scatter and heat loss of and by tissue)
- TGC happens after echo is received - we can increase the gain of the electrical signal dependent on how long it took for the echo to come back. The longer the echo takes to come back, the more we amplify the signal (as depth increases the more we amplify the returning echo increases)
- THC changes if type of tissue imaging changes and if frequency of transducer changes - as these determine how much the incident ultrasound beam is attenuated
- with TGC we are amplifying the electronic signal not the wave
TGC is a way to get equal brightness distribution throughput the depths of our image. Changing gain incorrectly can lead to artifacts.
What is temporal resolution
The ability of a system to display events occurring at different times as separate images (frames per second)
What is temporal resolution reduced by
- Greater number of focal zones
- Having Doppler on
- Deeper object (echo takes longer to reach object and return)
- Large sector width (more space to scan)
What is tissue harmonics
A technique in ultrasound imaging that assists in reducing artifact in image and allowing better identification of body tissues by allowing signals to be sent and received at 2 different frequencies
An electronic filter or pulse inversion technique ensures the fundamental frequency is not returned but instead the harmonic frequencies are used to build the picture.
Eg. A probe will emit 2 MHz but only listen for a 4 MHz frequency response (as body tissue reflects sound at twice the frequency that was initially sent) - results in higher resolution image with less artifact
Pro - useful in obese patients and to improve movement/reverberation artifact
What is compound imaging
Utilizes a phenomenal known as beam steering - where the angle of the ultrasound beam is altered. The beam is transmitted at up to 9 different angles per sweep, in this way the same object is imaged at several different angles. * some beams reach behind the object and return echoes
Vs a transducer listening for a return echo by sending a sound signal directly perpendicular to the probe head
Pro - useful to examine small parts and superficial structures , increase resolution, decrease artifact and increase edge details
Con - takes away useful artifact (acoustic shadow) and reduces frame rate , less effective in very superficial and very deep penetration , slower frame rate the more line of sight , only available in linear and curvilinear transducers , image quality can be better without it
What is Nyquist limit
The Nyquist limit represents the maximum Doppler shift frequency that can be correctly measured without resulting in aliasing in colour or pulsed wave ultrasound.
What is aliasing and how to prevent in Doppler imaging
Incorrect representation of direction and velocity of flow due to blood flow velocity exceeding nyquist limit (sampling frequency must be greater than twice highest frequency of input signal to accurately represent the image - PRF/2)
If velocity greater than limit - Doppler shift exceeds scale - “wrap-around” occurs
Max measurable velocity for pulsed wave Doppler is 1 m/s at 6cm depth. Continuous wave Doppler has no maximum.
What is a sound beam?
A wave that requires a medium to travel from one point to another.
Example: Sound waves in air or water.
How does the velocity of sound depend?
On the nature of the medium in which it travels.
Example: Sound travels faster in solids than in gases.
What happens to the energy of a sound wave as it travels further?
The energy of the wave becomes less.
Example: Sound waves losing intensity as they travel through a medium.
What are the possible outcomes of a generated sound wave?
Absorbed, reflected, or refracted.
Example: Sound waves bouncing off a wall.
How do individual particles move during sound propagation?
Only a few microns back and forth.
Example: Vibrations of air particles during the propagation of sound waves.
How do sound waves propagate in ultrasonography?
By longitudinal motion (compression/expansion), not transverse motion (side-to-side).
Example: Sound waves in medical imaging.
Why do sound waves travel slowly in gases?
Because a particle must move a relatively long distance before affecting a neighboring particle.
Example: Sound waves in air.
Why do sound waves propagate more rapidly in solids and liquids?
Because their molecules are closer, needing to move a short distance to affect a neighbor.
Example: Sound waves in water.
What are specular echoes?
Echoes from large, regularly shaped objects with smooth surfaces.
Example: Reflection from valves.
What are scattered echoes?
Echoes from small, weakly reflective, irregularly shaped objects.
Example: Reflection from blood cells.
What is a transducer?
A device that can convert one form of energy into another.
Example: Conversion of electrical signal into ultrasonic energy.
What is the most important component of an ultrasonic transducer?
A thin piezoelectric crystal element located near the face of the transducer.
Example: Crystal element in ultrasound imaging.
What is the piezoelectric effect?
Materials changing physical dimensions when an electric field is applied.
Example: Piezoelectric crystals in ultrasonography.