Ultrasound physics Flashcards
Rarefaction
Low pressure of a sound wave
opposite of Compression
Period in US
peak to peak (micro second)
Frequency in US
number of events per second (Hz)
Wavelength
peak to peak (distance; mm)
Speed of sound in medium
1540 m/s (1 mile per second)
1MHz frequency = wavelength 1.54 mm
v = frequency x wavelength
Amplitude in US
average to peak value
Strength of wave
Power in US
WATTS
rate at which work is performed
Increase power increases image brightness
(gain)
Intensity in US
Concentration of energy in the cross-section of the sound beam
power / area
1) not uniform
2) temporal variation and spacial variation
Relationship of wavelength, period and frequency
Frequency ~ 1/period or 1/wavelength
Relationship of power, intensity and amplitude
Intensity ~ amplitude^2
Power ~ amplitude ^2
Spatial peak-temporal average (SPTA)
Measure peak spatial and average time intensity
Propagation speed
Distance sound wave travels through medium in 1 second
Speed of sound in body
500-4000 m/s depending on medium
solid > liquid > gas
Soft tissue 1540 m/s
Lung 500 m/s
Bone 3500 m/s
Speed ~ stiffness / density
Attenuation
Weakening of waves as it goes through objects
Higher frequencies weaken more
Longer distances weaken more
Unit = Decibel (relative measure of intensity on a logarithmic scale)
Attenuation coefficient
Intrinsic properties of each tissue with effect on attenuation
In soft tissue this is half transducer frequency (0.5 dB/cm/MHz)
Anisotropy
Directional dependence of the fibers on the attenuation Muscle fibers (perpendicular > parallel) intensity
Types of scatter
Specular reflector - on smooth surface; organized reflection
Diffuse reflectors - rough surface; scattered
Rayleigh scattering - when surface much smaller than wavelength; rock in pond, for RBC’s
Acoustic impedance
Property intrinsic to each tissue that affects the reflected sound
Only factor in perpendicular incidence (difference between impedance of the 2 media)
Refraction define
Reflection with a bend
Absorption
When acoustic energy converted to heat
Increases with higher frequency and scanning depth
Pulse duration
Time from start of purse to end of pulse
Receive time
Time in between pulses
Spatial pulse length
length of each pulse
Pulse repetition period
Start of 1 pulse to the start of the next pulse
Pulse repetition frequency
Number of pulses in 1 second
Function of transducer
Converts electrical energy to acoustic energy and vice versa
Components of an ultrasound
1) Transducer
2) Master synchronizer
3) Pulser
4) Receiver
Receiver functions
1) amplification (brighter)
2) compensation (time gain compensation)
3) compression (reduce signal to 2D image)
4) demodulation (corrects for negative voltages)
5) rejection (eliminate low level noises)
Aperture
When sound beam leaves transucer it is the same diameter (near zone)
it then gets smaller (focal) then widens again (far zone)
Phased-array transducers
Transducers with adjustable focus
Resolution
ability to distinguish two objects that are spacially close to each other
Ways to define resolution
1) distance of two objects
2) spatial (in space: axial/parallel/depth or lateral/perpendicular/side)
3) temporal
4) shades of gray (contrast)
Display modes of US
A-mode (amplitude): used in ophthalmology for precise measurement
B-mode (brightness): varying amplitude converted to dots of varying intensity
M-mode (movement)
Artifacts violated 6 assumptions of an imaging system
1) sound travels straight
2) sounds travels directly to reflector and back
3) sound in soft tissue is exactly 1540 m/s
4) reflections arise only from structures in main axis
5) imaging plane is thin
6) strength of reflection is related to characteristics of the tissue
Reverberation artifact
1) equally spaced echos
2) sound bounce between two strong reflectors
3) first two real, the rest not
Comet tail artifact
Type of reverberation artifact in thoracic ultrasound
Shadowing
Hypoechoic region along edge of curved reflector
Mirror artifact
Reflect off strong reflector then towards second structure
Creates replica of structure that is deeper
Doppler shift equation
Doppler shift = 2vFCostheta / propagation speed
Two most common types of Doppler
1) Continuous wave doppler: good for high velocities
2) pulsed doppler: can identify exact location of a moving RBC
