Physics Of Ultrasound Flashcards
Imaging modes
- A-mode — amplitude mode
- B-mode — brightness mode
- brightness of image = strength of echo
- depth = position
• M-mode — motion mode
- B-mode vs time
- x-axis —> time ; y-axis —> depth
- brightness of reflector = strength of echo
- best temporal resolution
• 2D grayscale
- multiple M-mode scan line
- multiple pulses from multiple crystals
- real time 2D images on screen
- 2D phased array
•3D imaging
- cone / pyramid of M-mode scan lines
Basic U/S Physics
- sound = mechanical longitudinal wave
- medium vibrates as sound travels L—>R
- sound waves = compressions and rarefactions in the molecules of the medium
Time (seconds) —> sound source
•Period — time to complete a single cycle
- period = 1 / frequency
- similar to wavelength but a measure of time
•Frequency — number of cycles per second
- frequency = 1 / period
- u/s > 20 KHz
- increased frequency —> better axial resolution but more attenuation
•Pulse duration — time to complete a single pulse
- similar to SPL but a measure of time
- # of cycles x period
•Pulse repetition period — time from beginning of one pulse to beginning of the next
- PRP = pulse duration + listening time
- deeper the structure —> longer the listening time —> longer the PRP
•Pulse repetition frequency — number of pulses per second
- PRF = 1 / PRP
- analogous to frequency
- 1/2 PRF = Nyquist limit [max doppler shift before aliasing]
- important for temporal resolution (higher PRF —> higher frame rate —> better resolution)
- depth dependent since it includes listening time
Length (distance) —> sound source & medium
- Wavelength — length of a single cycle
- Spatial pulse length — length of a single pulse
- important for axial resolution
- 1/2 SPL = axial resolution
Strength —> sound source
•Amplitude — difference between average acoustic and max acoustic variable
-acoustic variables: pressure, density, temperature, distance
•Power — amount of work done per unit time
- proportional to amplitude^2
•Intensity — power per unit area
- proportional to amplitude^2
- most important measure for bio effects (thermal, cavitation)
Velocity —> medium only
•stiffness —> proportional to velocity
•density —> inversely proportional to velocity
- paradoxically sound usually travels faster in higher density material because they tend to be stiffer and stiffness differences are usually larger than density differences
Attenuation
- As u/s travels through tissue the strength / amplitude decreases
- higher frequency attenuated more than lower frequency
Impedance
- acoustic resistance to sound traveling through a medium
- Z = P x V [p = density , v = velocity]
- determined by the medium only
- important for reflection — with perpendicular incidence, reflection only occurs when there is different impedance
Transducers
•converts one form of energy to another
•piezoelectric crystals convert voltage to u/s and back to voltage
- voltage applied to crystals —> vibrations —> sound pulses —> sends out —> bounce & reflect back —> vibrations —> voltage difference —> image —> displayed on screen
•distance related to time
•brightness related to strength
Backing Material
•damping material
•decreases ringing —> short pulses —> decreases SPL —> improved axial resolution
•increases band width
•decreases Q factor (QF = RF / BW)
- unitless # that represents ability of machine to emit a ‘clean’ pulse w/ narrow BW
- imaging transducers have a low QF (high BW , short SPL)
- therapeutic transducers having a high QF
- short pulses have a broad range of frequency = high BW , low QF
•decreases sensitivity to reflected echoes
Matching Layer
- larger the difference in Z between two media the larger the reflection at the interface of those media
- has Z between mucosa and PZT
- prevents a large reflection at interface of mucosa and probe
- Z : PZT > matching layer > gel > mucosa
- Z = velocity x density [V determined by stiffness & density]
Transducer Frequency
•different for CW and PW
•CW —> frequency = electrical frequency of the voltage
•PW —> determined by the u/s system
- crystal properties determine the resonant frequency (thickness & velocity)
- RF = V / 2T
Band Width
- difference between highest and lowest frequency pulse
- damping —> increase band width
- shorter the SPL the higher the BW
- QF = RF / BW …
- RF = V / 2T …
- QF = (V / 2T) / BW
Near Field (Fresnel Zone) Far Field (Fraunhofer Zone) Focal Zone (near the focus)
•Ln = r^2/lambda
Ln —> length ,
r —> crystal radius
Lambda —> wavelength
- larger crystal radius —> longer focal length
- higher frequency / lower wavelength —> longer focal length
•crystal diameter and divergence
- crystal diameter decreases —> divergence increases
- smaller d —> shorter focal length
- smaller d —> more divergence
•crystal frequency and divergence
- higher frequency / lower wavelength —> deeper focal length
- higher frequency —> less divergence
Focusing
•Methods:
- lens
- curved PZT crystal
- focusing mirror
- electronic (used in TEE)
•Effects:
- improved lateral resolution
- shorter focal zone
- shallower focal depth = near zone
