Chapter 3 Ultrasound Physics Flashcards
What is rarefaction in ultrasound imaging?
Low pressure
Rarefaction refers to the phase of a wave where the pressure is lower than the ambient pressure.
What is compression in ultrasound imaging?
High pressure
Compression is the phase of a wave where the pressure is higher than the ambient pressure.
Define frequency in the context of ultrasound.
Number of times a wave is repeated per second.
What is wavelength in ultrasound?
Distance travelled by a sound wave in one cycle.
What determines the velocity of sound in an acoustic medium?
Physical density and stiffness.
What is the average velocity of sound within soft tissue?
1.54 mm/us.
Define acoustic impedance.
The product of the tissue’s physical density and sound velocity within the tissue.
What does amplitude represent in ultrasound imaging?
Proportional to the difference in acoustic impedance in two adjacent tissues.
What is the angle of incidence?
The angle at which a sound wave encounters a medium.
What happens if the angle of incidence is not perpendicular?
The angle of reflectance equals the angle of incidence.
Fill in the blank: The amount of attenuation is determined by the _______ and the frequency of the sound wave.
Distance travelled
List the three components that determine attenuation.
- Absorption
- Reflection
- Scattering
What do transducers contain to convert electric current into sound waves?
Piezoelectric crystals.
What is pulse repetition frequency (PRF)?
The number of times sending and receiving is repeated within 1 second.
Define spatial pulse length (SPL).
The length of space in one pulse of ultrasound.
What is axial resolution?
The ability to distinguish two separate reflectors along the direction of the sound wave.
How is axial resolution related to spatial pulse length?
It is equal to half of the SPL.
Define lateral resolution.
The ability to distinguish two separate reflectors perpendicular to the direction of the sound wave.
List the types of array transducers.
- Linear
- Convex
- Annular
What is broad bandwidth technology in ultrasound?
Allows transducers to emit a wide range of frequencies.
What is harmonic imaging?
Integer multiple of the frequency of the transmitted ultrasound pulse.
What does spatial compounding do?
Generates a composite image by combining multiple frames from ultrasound beams at different angles.
Name the two modes of echo display.
- Brightness mode (B-mode)
- Motion mode (M-mode)
List the basic scanner controls in ultrasound.
- General gain
- Time gain compensation (TGC)
- Frequency
- Focal point or region
- Depth of field
What is doppler shift?
The frequency difference between the incident sound waves and the reflected sound waves.
What is the condition for measuring doppler shift accurately?
The doppler angle should be less than 60 degrees.
What is continuous-wave doppler?
Uses two separate crystals in one transducer, one emitting sound continuously and one receiving echoes continuously.
What is pulsed-wave doppler?
Uses the same crystal for both sending and receiving sounds.
What does colour doppler indicate?
Red indicates net blood flow toward the transducer; blue indicates net blood flow away.
What is power doppler?
Analyzes the total strength of the doppler signal determined by the concentration of moving red blood cells.
What does microvascular imaging help to show?
Low-velocity and small-diameter blood vessel flow.
What are microbubbles used for in contrast-enhanced ultrasound?
Enhance the ultrasound signal.
What is ultrasound elastography used for?
Evaluates tissue elasticity/stiffness.
Define acoustic shadowing.
Regions of decreased echogenicity distal to structures of high reflectivity.
What is acoustic enhancement?
Regions of increased echogenicity distal to structures of low attenuation.
What is the comet tail artifact?
Reverberation caused by two closely spaced, discrete highly reflective surfaces.
Define mirror-image artifact.
Duplication of normal structure on the opposite side of a strong reflector.
What are side lobes and grating lobes?
Secondary sound beams that result in an error in the positioning of the returning echo.
What is range ambiguity?
Occurs when PRF is high enough that not all returning echoes are received before the next pulse.
What happens during aliasing in pulsed wave doppler?
A portion of the tracing wraps around on the spectral tracing to the opposite direction.
