Generating Ultrasound Images Flashcards
What are the steps of generating an ultrasound image?
- A signal is generated by electrical impulses and converted to ultrasound waves in the transducer.
- The ultrasound signal is transmitted through the tissues.
- The signal is reflected back to the transducer.
- The transducer converts the ultrasound waves back to electrical impulses which are interpreted as an image.
How do piezoelectric crystals work?
Piezoelectric crystals (artifical ceramic) are assembled in the transducer head and individually connected to electric wires. Electricity deforms the crystal, causing it to produce a pressure wave.
The crystals also act as receivers, converting the returning wave into electrical signals.
What is ultrasound wave frequency and what is their unit of measurement?
The number of wave cycles generated. This is measured as cycles per second, and the unit of measurement is Hertz (1Hz = 1sec). Ultrasound produces millions of cycles/sec, or megahertz (MHz).
What is the ultrasound wave period?
The length of time for one wave cycle to occur, from beginning to end of a wave.
For ultrasound this is less than 1 millisecond.
What is ultrasound wavelength and how does this relate to frequency?
The distance from beginning to end of a wave. Wavelength is inversely proportional to frequency.
Ultrasound wavelength is less than 1mm.
What is ultrasound amplitude, what does it represent and how is it expressed?
The height of the ultrasound wave. This represents the magnitude of compression (energy), and is expressed in decibels.
How does an ultrasound wave move through the body?
The wave moves energy, not physical objects. The energy hits a particle which moves and transfers the energy to the next particle and then moves back to it’s original position. This process continues forward transferring the energy along.
What is ultrasound rarefaction and compression?
Compression occures when the particles transferring soundwave energy have been pushed close together. The temporary gap left is rarefaction.
How does transducer in pulsed mode operate?
Pulses of sound, usually 2-3 cycles long (doppler is 5-20), are sent out. The transducer waits for returning echos in between pulses before sending the next pulse.
What is Pulse Repetition Frequency (PRF) and what is it’s unit of measurement?
PRF describes the number of pulses occuring in 1 second. This is expressed in kHz i.e. thousands of pulses per second.
What is Pulse Repetition Period (PRP)?
PRP is the time from the beginning of one pulse to the beginning of the next and includes the listening time. It is usually expressed in microseconds (µs).
A typical pulse duration may be 0.5-3 µs with a total PRP of 100µs, meaning 99µs are spent listening.
What is Duty Factor?
The fraction of time (expressed as a percentage) spent sending sound verses the total time including receiving.
As the PRF increases, the PRP gets shorter and the Duty Factor increases.
I.e. for pulse duration of 1 microseconds with total PRP of 100 microseconds the Duty Factor would be 1/100 or 1%.
What is Spatial Pulse Length (SPL)?
The length or distance of each pulse of a few cycles of sound.
Typical values are 0.1 to 1mm. Shorter pulses (higher frequency) create higher resolution images.
Explain Continuous Wave Doppler
Two transducer elements adjacent to each other (within the one transducer) are used: one to transmit ultrasound continuously and the second to receive continuously.
It is used to measure high velocity blood flow such as aortic flow.
What is the relationship between image resolution and wavelength?
Resolution is proportional to wavelength (and hence frequency).
The smallest object or distance discernible is proportional to 1-2 wavelengths. Higher frequency (shorter wavelength and SPL) ultrasound has greater image resolution than lower frequency ultrasound.
What is propagation speed and what causes it to vary?
Propagation speed is the speed at which a wave moves through a medium.
It varies with the medium, depending on the density and stiffness or elasticity of the medium.
What is the average propagation speed of ultrasound through soft tissue, and how much does this vary by?
1540m/s or 1.54mm/microsecond. This only varies by about 10% between different soft tissues.
What is attenuation?
Attenuation is the decrease in amplitude (energy) of ultrasound waves as a result of interaction with tissue and tissue boundaries.
Low frequencies travel further before attenuation, giving them greater (better) penetration.
What is the relationship between frequency, resolution and penetration?
The higher the frequency, the better the resolution but also the lower the penetration.
The lower the frequency, the better the penetration.
What is acoustic impedance?
The level of resistance encountered by an ultrasound wave when it passes through a tissue.
It is related to the density of the tissue and the speed of sound through that tissue.
When ultrasound waves encounter a large change in impedance (i.e. muscle to bone or air) they are reflected.
What is shadowing artefact?
Areas where ultrasound is not penetrating, and is commonly because it has been reflected away from an object.
There is usually shadow behind bony structures or anything with a high calcium content, such as gallstones in the gallbladder.
Shadowing due to renal calculi
What is reverberation artifact?
When an ultrasound beam reflects back and forth between two strong parallel reflective structures, it is interpreted as a deeper structure due to some of the signal having a longer return time that is a multiple of the correct time.
What is edge artifact?
Edge artifact occurs due to refraction of the ultrasound beam along the edge of a curved structure that results in a decrease in intensity of the ultrasound beam posterior to the curved edge.
What is acoustic enhancement?
Increased echogenicity (whiteness) posterior to a fluid filled area. This is because fluid attenuates sound significantly less than tissue, causing TGC to overcompensate through a fluid-filled structure causing the deeper tissues to be brighter.
This usually occurs due to structures such as the urinary bladder, gallbladder or a cyst.
Explain Comet-tail artifact.
Comet-tail artefact is a specific type of imaging artifact that arises due to acoustic reverberation. It presents as an inverted triangular acoustic enhancement posterior to the hyperechoic focus, which exhibits a gradual decrease in strength and thickness
It is primarily generated at interfaces where there is a significant difference in acoustic impedance, such as between air and fluid or soft tissue. This difference causes the ultrasound waves to reflect back and forth, creating the appearance of multiple echoes. The reverberation focus is a tiny area of interstitial fibrous tissue or fluid with non parallel surfaces.
- pulmonary oedema (shows increased extravascular lung water)
- Small renal/uteteric calculi
- Small bile duct stones
- Gallbladder adenomyomatosis
- thyroid colloid nodules
- Pancreatic calcifications
- Foreign bodies such as surgical clips, glass/metal
Thyroid colloid cyst
Consider possilbe ring down artifact.
Explain ring down artifact.
It appears similar to comet-tail artifact however is due to a completely different mechanism.
The artifact is only associated with gas bubbles, when a pulse hits “bugle” shaped fluid collection that is trapped between an inverted tetrahedron of 4 bubbles (3 on top and 1 nestled deep to them). The trapped fluid resonates, emitting a continuous signal back to the transducer. The returning signal consists of one or more discrete (resonant) frequencies. “Beats” between these frequencies produce the variable appearance of the ring down. There is no “reverberation” ( i.e. multiple reflectances).
What causes mirror image artifact?
This occurs when the ultrasound beam hits a highly reflective surface (e.g. diaphragm), reflects off it and changes direction. It then reflects off another structure, hits the first reflective surface again, and then returns to the transducer.
This is commonly seen around the heart or diaphragm.
A mirror image of the liver above the diaphragm while performing abdominal ultrasound is normal and caused by the signal reflecting from the interface between diaphragm and air filled lung. If there is fluid/lung consolidation above the diaphragm the reflective interface is lost and consolidation is now seen.
