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.
What are the imaging modes in ultrasonography and explain them
A-mode - amplitude
One beam of ultrasound is passed through the material and the returning echoes are recorded giving a 1D representation of the structures the beam passes through. A-mode was the first use of medical ultrasound and was used to show midline shift in the brain. An ultrasound beam was passed through the skull and the bones and falx would return the echoes showing their position. Now, it is mainly used in ophthalmolo- gy to investigate retinal detachment etc.
B-mode - brightness
This is now the main mode of ultrasound used. The echoes returned are shown on screen in a grey-scale corresponding to their intensity. The structures are shown as a 2D image on screen
, and
M-mode - motion
Ultrasound waves are released in quick succession in A or B-mode and recorded. This creates an image analogous to a video recording.
As organ boundaries reflecting the sound waves move, the velocity can be calculated eg. Heart valves
Example: Different display modes in ultrasound imaging.
Compare the different transducers
Linear transducers produce a rectangular field of view with uniform beam density throughout. They are useful for imaging shallow structures and small parts. produce an ultrasound beam that is emitted at 90 degrees to the transducer head. contain 256-512 elements (crystals), making them the largest assembly. Frequency range 4-12 MHz
The sensitivity of the image reduces at extremes of steering and lateral resolution is best in the centre of the field of view due to a larger effective aperture.
The benefits of a phased array include; a small faced transducer allowing for imaging in small spaces and being able to change the focus of the ultrasound beam.
A phased array ultrasound transducer is typically 2-3 cm long, consisting of 64-128 elements with its surface . It is a smaller assembly than a sequential array and can be either linear or curvilinear. Small surface area but large field of view. Frequency range 2Mhz – 7.5Mhz
Convex (sequential) arrays, also known as curvilinear or curved linear arrays, are similar to linear arrays but with piezoelectric elements arranged along with a curved transducer head. Ultrasound beams are emitted at 90 degrees to the transducer head. This arrangement results in a trapezoidal field of view due to the divergence of the ultrasound beam with increasing depth. This allows for a wider field of view but with decreased line density at depth and reduced lateral resolution. Frequency range 2.5-7.5 MHz
- As a general rule, if the shape at the top of the images matches the shape at the bottom of the image it is a sequential array. If the shapes are different (e.g. rectangular at the top and curved at the bottom) it is a phased array.
- endocavitaty probe - very large FOV for small transducer surface - transvaginal, transrectal
Endovascular - within vessels, hollow visci
- types - single (1PZT crystal that makes ultrasound wave- old ) element vs array
Describe the anatomy and properties of an ultrasound wave
Particles closer together - compression
Far apart - rarefaction
Frequency - 2-18 MHz (1 Hz = 1 wavelength per second)
Velocity - dependent on, and constant for, the material through which the wave is passing.
c = √ (ƙ / ρ) Where: c = speed
ƙ = rigidity ρ = density
Wavelength - distance between points of peak compression
Intensity - watts per metre2
Describe basic physical principles underlying use of Doppler effect
Doppler effect refers to a shift in frequency perceived by an object in motion relative to the source of a wave.
When applying this to ultrasonography, the transducer is a stationary source of ultrasound waves of a known frequency (depending on the mode) and velocity (1540 meters/second) which reflect off tissues, and specifically for doppler ultrasound, red blood cells (RBCs).
Many of these sound waves are scattered in all directions, some are absorbed by tissues, and some of them are even dissipated as heat. The waves that are reflected back to the ultrasound transducer are used to construct the image.
Explain the artifact aliasing
The Nyquist limit states that the sampling fre- quency must be greater than twice the high- est frequency of the input signal in order to be able to accurately represent the image.
Nyquist limit = PRF / 2
If the velocity of the flow is greater than the Nyquist limit, the Doppler shift exceeds the scale and “wrap-around” occurs.
Describe spectral broadening artifact
Blood flowing closer the inside of the vessel wall is slower than flow in the middle of the vessel. This large range of frequencies in a particular moment in time produces a widen- ing of the spectral graph and different colours in colour Doppler. This also occurs with turbulent flow (e.g. stenotic vessels) as the turbu- lence creates flow of different velocities and directions.
Describe reflection/ mirror image artifact
Sound bounces off a strongly reflecting object which acts as a mirror and reflects the pulse to another tissue interface.
The inter- pretation of the image is that the second in- terface is beyond the first surface, much like the reverberation artifact
This most often happens at the diaphragm wherein the liver is seen in the chest cavity due to sound waves being reflected off the diaphragm.
Describe reverberation artifact
Multiple reflections to and fro between the transducer face and a relatively strongly re- flecting interface near the surface produces a series of delayed echoes. These look like stripes within a fluid filled structure.
Two types of reverberation artefact exist:
- Comet tail: from metal or calcified objects
- Ring down: from a collection of gas bubbles
Ring-down” is an ultrasound artifact that appears as a solid streak or a series of parallel bands radiating away from abdominal gas collections.
What is resonance in ultrasound?
Resonance is the condition that arises when, at a specific frequency (or frequencies), the reflection of sound waves within the resonating item provide constructive interference or re-inforcement of vibrations within the item.
If 2 sound waves of the same wavelength cross in the same phase, they combine and are reinforced (constructive interference). If, how- ever, they are in different phases they cancel each other out (destructive interference).
What is diffraction
Diffraction is a phenomenon observed by all waves (electromagnetic, light, sound, vibration, gravity, etc.). Diffraction is what happens as a wave propagates. Beams spread, waves scatter around obstacles, can deflect light, change its frequency and modulate its phase and amplitude.
What is side lobe artifact
Side lobe artifacts occur where side lobes reflect sound from a strong reflector that is outside of the central beam, and where the echoes are displayed as if they originated from within the central beam.
Ultrasound transducer crystals expand and contract to produce primary ultrasound beams in the direction of expansion and contraction. Secondary beams occur because the crystals also expand and contract radially. These radial beams are called side lobe beams. Side lobe beams are low-intensity beams that surround the central beam.
Side lobe artifacts are echogenic, linear or curvilinear artifacts. Strong reflectors include bowel gas adjacent to the gallbladder or urinary bladder.
What is speed artifact
Refraction - sound speed error
