US physics Flashcards
How does US works and can create images
- Sound waves are to the body → reflected from soft tissue structures
o Many continuous and rapid waves = generates many 2D images/min
Moving image (real-time B-mode)
o One continuous wave = generates image only with structures associated with that line
M-mode
Define cycle
- Molecules on a line of sound wave: compressed and refracted = 1 cycle
Define wavelength
Distance travelled by sound wave during a cycle = wavelength
Define frequency
- # cycles/sec → 1 cycle/s = 1 Hertzo Transducer frequency = MHz (1 000 000 cycles/s)
o Doppler frequency = KHz (1 000 cycles/s) - ↑ frequency = ↓ wavelength
Frequency affects
image resolution
tissue penetration
Doppler signal
Velocity of US in tissue
- Speed that US travels the tissue
o Depends on stiffness/density of medium
↑ density = travels faster (bone = 3000m/s)
↓ density = travel slower (air = 700m/s)
o Velocity is constant in a homogenous substance - Average velocity in soft tissue = 1540m/s (myocardium, blood, liver, fat…)
o Qualibrated to US machine → calculation of distance to cardiac structures
o D = V x (T/2)
A-mode
Amplitude mode
* Amplitude of reflected US vs depth
M-mode
Motion mode
* Single line of sight
o PRF limited to the time to travel to maximal depth and back
o High resolution of structures possible
* Guided by 2D imaging to ensure appropriate angle btwn M line and cardiac structures
B-mode
2D echo
- For each scan line: short pulses/burst of US emitted at a fixed pulse rate frequency (PRF)
o PRF determined by time to travel to max image depth: ↓ at greatest image depth
o Signal received by piezoelectric crystals → generate images
Image formation = depend on time delay btw US transmission and return signal
Tissue harmonic imaging
o Based on harmonic frequencies generated by US waves propagation
Non linear effects of US propagation
Reduce near field and side lobe artifacts + improves endocardial definition
o Key properties
Strength ↑ with depth propagation
Maximal at typical cardiac imaging depth
Stronger fundamental frequencies = stronger harmonics
Focusing of probe
o Unfocused: width equal to transducer and diverge as travel to tissues
Distance from transducer → divergence = near field
* Near field length: α to beam diameter, iα to wavelength
* ↓ transducer diameter = ↓ divergence in far field
Area beyond = far field
o Focused = gated acquisition
Artifacts
- Patient movement/breathing artifacts
- Side lobe artifact (beam width)
- Reverberation/mirror image artifact/range ambiguity
- Acoustic shadowing
- Refraction
Side lobe artifact (beam width)
o Central + peripheral beam
Peripheral beam directed laterally → can reflect sound waves back
Transducer cannot differentiate central vs peripheral reflected waves
o Superimposition of lateral structures in central beam = side lobe artifact
o Most commonly in dilated chambers: empty space allows weaker side lobes to be displayed
Reverberation/mirror image artifact/range ambiguity
o Strong reflectors encountered in thorax
Send strong echoes back = received + reflected from transducer
Transducer receive same sound wave twice → perceived as taken twice the time cause travelled the heart again
o Mirror image below the first one
Can also happen with Doppler: if both side of baseline
* Created by high gain settings
o Minimize in adjusting depth settings
Refraction
beam is deviated → reflected image assumed to come from original path = double image
Transducer physics
- Contain piezoelectric crystals:
o Deformed by electrical voltage → generate sound
o Receive sound → convert to electrical E - Thickness of crystals: basic operating frequency of transducer
o Wavelength = ½ of crystal thickness
o ↓ thickness = ↓ wavelength = ↑ frequency
Pulse Repetition Frequency (PRF)
- Pulsed US: transmit sound waves in short bursts → receive sounds
o # pulses/sec = pulse repetition frequency (Hz)
Determine max velocity w/o ambiguity
o Usually 2-3cycles/pulse → controlled by damping material in transducer - Pulse length: ↓ if ↑f (shorter wavelengths)
- Should ↓ for deeper structures
Instrument settings
Gain
Power output
Depth
Dynamic range
Gain
Adjust displayed amplitude of received signals
o Time gain compensation (TGC): differential adjustment of gain along length of US beam
Compensate the effects of attenuation
Near field gain can be lower, gradually increase gain as deepens
Power output
US energy delivered by transducer → ↑ amplitude of reflected signals
Depth
affects PRF and frame rate
Dynamic range
of grey level in the image
o Contrast btwn light and dark areas
Acoustic impedance
- Opposition/resistance to sound propagation
o Depend on density/stiffness of medium
↑ density = ↑ impedance (and ↑ velocity)
Bone have higher impedance because inability to refract/compress easily
o Independent of frequency - High acoustic impedance → high sound reflection at bony/air interface
o Shadow on US image = lack of further sound transmission
Impedance equation
Density x speed
Reflection
- Return of US signal from smooth tissue boundary
o Interface with different acoustic impedance → portion of the sound reflected back to transducer
o Greatest when beam perpendicular to tissue: 2D, M-mode images
Reflection depend on
o Angle of incidence: angle at which the US strikes the surface = determines angle of reflection
o Acoustic impedance difference btw tissues (↑ = ↑ reflection angle)
o Size of structures: must be ¼ the size of wavelength to occur
Higher f transducer = higher resolution images (can reflect smaller strctures)
* ↑ frequency = ↓ wavelength = ↑ resolution
Refraction
- Change of direction when change of medium
o Acoustic impedance mismatch
o Can result in artefacts: image deviated (deepest = ↑ deviation)
Attenuation
- Sound travelling weakened by reflection, refraction, scattering, absorption of heat in tissues
o Loss of E = attenuation - ↑ frequency = ↑ attenuation = ↓ depth
o Wavelengths interact w more structures - ½ power distance of a tissue = distance travelled before ½ of sound E is attenuated
Scattering
- Radiation of US in multiple directions from a small structure
o When radius of target < wavelength
o Basis of Doppler US: change in signal from moving target (RBCs)
Extent of scattering depends on
Size of target
o # of particles
o US transducer frequency = #1 determinant
o Compressibility of RBCs and plasma
Types of resolution
Axial
Lateral
Elevational
Temporal
Axial resolution
- Ability to identify 2 structures as different along length of US beam
o = depth/longitudinal resolution - Smaller axial resolution = ↑ detail image
o ↑frequency = ↑ resolution - Axial resultion = ½ pulse length (2 structures must be > ½ pulse length ditance)
o ↑ frequency = ↑ resolution - Most precise: measures should be done on structures perpendicular to beam ideally
Lateral resolution
- Ability to identify 2 structures as different perpendicularly to US beam
- Beam width: affected by
o Focusing sound waves generated by transducer
o Transducer diameter + frequency - ↓ beam width = ↑ resolution
o Better in near field where beam width is narrowest (approaches axial resolution) - ↑ frequency = ↑ resolution
o Longer near field
Elevational resolution
- Refers to the thickness of the image slice
o Cardiac US usually have a thickness of 3-10mm
o Strong reflectors adjacent to image plane can appear to be in the image
Temporal resolution
- Frame rate = # of real time images/min
o Depend on PRF: ↑ frame rate = ↑ PRF
o ↑ frame rate = ↑ temporal resolution - Rapidly moving structures = need fast frame rate
o ↓ sector width + image depth = ↑ PRF (less time required to generate next frame)
Information provided by Doppler analysis
- Detection/analysis of moving RBCs/myocardium
o Direction, velocity, character, timing of blood/muscle motion
Doppler shift
change of frequency btw sound transmitted and received
o ↑ frequency when moving toward sound source
RBCs moving toward probe = reflect higher number of sound waves
Received frequency > transmitted frequency = positive frequency shift
o ↓ frequency when moving away from sound source
RBCs moving away from probe = reflect lower number of sound waves
Received frequency < transmitted frequency = negative frequency shift
Doppler tracing
o Positive frequency shift = produce waves up from baseline
o Negative frequency shift = produce waves down from baseline
Doppler equation
determine RBCs velocity
o V = speed of sound in tissues
o Fd = frequency shift, Fo = transmitted frequency by transducer
o Cos = intercept angle
Intercept angle: closer to // the transmitted wave is to blood flow = ↑ accuracy of velocity measure
Interrogation angles > 15-20 = result in significant errors
V = ((C x fd))/(2 (fo)) x cos
Nyquist limit
maximum Doppler shift = ½ PRF
o Limit which produces aliasing = signal ambiguity
Nyquist limit is influenced by
transducer frequency
o Maximum velocity recorded at given depth is iα to frequency
o Higher velocity jets at any depth = better recorded with lower frequency transducer
o Exceeded sooner at deeper gates for any given frequency
reduce aliasing
o Move baseline up/down
o Find imaging plane with less depth for structure of interest
o ↓ transducer frequency
o Switch to CW Dopper
o ↑ PRF for depth
4 types of Doppler
PW
CW
Color flow
TDI
PW doppler
o Site specific: receive signal for time interval of specific sample depth
o No sound wave transmission until received echoes from previous burst
o Limited capacity to detect higher frequency shifts
Characteristic of PW Doppler envelope
o Spectral broadening on PW signal
Blood flow: usually laminar
Slower velocities at periphery vs center of flow stream
* Improper gain settings
* Large intercept angle
* Turbulent flow: many velocities, flow direction
High PRF Doppler
use of range ambiguity to ↑ max velocity measured
Signals twice as far will reach transducer during the next cycle = can be analyzed
Range ambiguity
- Echo signal from earlier pulse cycle reach the transducer the next cycle
- Deep structures appear closer to the transducer
- Double image on vertical axis
CW doppler signal
o Use 2 separate crystals: 1 transmit, 1 receive
o Continuously send and receive sound
Detect frequency shifts along beam with no range of resolution
o Can detect higher frequency shifts
o Not possible to selectively interrogate at specific depths
Characteristic of CW Doppler envelope
o Full envelops from multiple velocities = spectral broadening
Color flow Doppler
o Form of pulsed wave Doppler: hundreds of interrogation lines/gates
o Perceived as moving towards or away from transducer (negative or positive frequency shift)
Toward = red → from deep red for slow to yellow for fast
Away = blue → from deep blue for slow to white for fast
Color flow Doppler: quality
depends on PRF + frequency
Intercept angle: no color if perpendicular flow
Color flow Doppler: aliasing
mosaic/mixing of colors
Can occur in normal flow with high frequency transducers
Turbulent flow = green
Color flow Doppler: frame rate
can be improved with reducing wedge size
Optimize Color flow Doppler image
↓ frequency
↓ color sector width
Eliminate real time image
↑ packet size (will ↓ frame rate)
↓ packet size: ↓ sampling time, good for high HR
TDi
o Analyzes myocardial velocities
Myocardial motion: ↑ amplitude signals at low velocities (opposite for RBCs)
Bypass low velocity filter
o Narrow sector of color: PW gate placed over this sector
Spectral trace of myocardial motion in real time
o Highest temporal and velocity range resolution
