Physics of Ultrasound 1-4 Flashcards
What is A mode?
Amplitude Mode
What is B-Mode?
Brightness Mode
What is M-Mode?
Motion Mode
(B-Mode vs. Time)
Which ultrasound mode has the best temporal resolution?
M-Mode
What correlates to the strength of the echo on ultrasound?
Brightness of the image
When you look at an M-mode example, what do you see on the:
X-axis?
Y-axis?
X-axis = Time
Y-axis = Depth
What is seen in this M-Mode clip?
Early closure of the aortic leaflets
HOCM
Draw an EKG comparing Systole and Diastole
Draw what you would expect a normal Aortic Valve Appears like in M-mode in AV LAX.
See image
Draw what you would expect a bicuspid Aortic Valve Appears like in M-mode in AV LAX.
See image
What is the period in terms of ultrasounds?
Time it takes to complete a single cycle
What is frequency?
Number of cycles per second
What is the pulse duration?
Time to complete a single pulse
What is the pulse repetition period?
Time is takes to go from pulse to pulse
What is the pulse repetition frequency?
Inverse of Pulse repetition period
OR
Number of pulses per second
What is 1/2 of the Pulse Repetition Frequency?
Nyquist Limit
(Max doppler shift before aliasing occurs)
What is a spatial pulse length?
Length of a single pulse
What is a wavelength
Length (distance of one cycle)
What resolution correlates to spatial pulse length?
Axial Resolution
Axial resolution = 1/2 of the Spatial Pulse Length
What is amplitude?
Difference between the average acoustic variable and the peak acoustic max variable
What is Power?
Amount of Work per unit time
Measured in: Watts or (Joules / second)
Said another way…….
Rate of Energy transfer = Rate at which work is performed
What is intensity?
Power per unit area (Watts/cm2)
What resolution does the pulse repetition frequency determine?
Temporal Resolution
What sound property is important for bioeffects of ultrasound?
Intensity
What is spatial resolution?
Ability to discern the correct space (place) of a structure
Said another way….
Ability to accureately create images of small structures in their correct anatomic position
What are the three types of spatial resolution?
- Axial
- Lateral
- Elevational
Axial resolution is proportional to …?
Pulse Length
Lateral resolution is proportional to …?
Beam Width
Elevational resolution is proportional to …?
Beam Height
What is the equation for axial resolution?
Axial resolution = 1/2 of the Pulse Length
What is the relationship for higher frequency ultrasound for:
Spatial Pulse Length* and *Axial Resolution?
Higher Frequency = Shorter Spatial Pulse Length = Better Axial Resolution
What is Ultrasound in hertz?
>20,KHz
1 KHz = 1 Kilo Hertz = 1000 Hz
What does increasing frequency do to attenuation?
Increased frequency = Increased attenuation
As the ultrasound beam travels through the body it loses energy. The intensity and amplitude of the sound wave decreases, and this process is known as attenuation.
The amount of attenuation that occurs will depend on the type of tissue the sound wave is traveling through. Where the molecules of the tissue are densely packed (such as bone), attenuation will be much greater than in less densely packed tissue (such as fat). Different tissues have different attenuation coefficients depending on the amount of attenuation occurring in the beam of sound.
How does attenuation change with increasing depth
Decreases (See image)
What is temporal resolution?
Abiltiy to accurately determine the position of a structure at a particular instant in time
What is frame rate?
The number of framees/sec = # images /sec
How does the pulse repetition frequency correlate to frame rate?
Proportional (Directly)
What are the 4 acoustic variables of a sound wave?
- Pressure
- Density
- Temperature
- Distance
What are the 3 mechanisms for Bioeffects?
- Thermal Effects
- Cavitation
- Physical Vibration
What is maximal heating related to?
Spatial peak temporal average (SPTA) intensity
What temperature flux do we want to limit the U/S exposure to produce in local tissue temperature?
1 degree Celcius
What is the Maximum Spatial peak temporal average (SPTA) intensity for Unfocused beam?
<1 Watt / cm2
What is the Maximum Spatial peak temporal average (SPTA) intensity for Focused beam?
< 100 milli Watt/cm2 (0.1 Watts)
What is the mechanical index of an ultrasound?
Strength of Ultrasound beam and the ability of the beam to produce cavitation (Bursting of bubbles) of contrast material
What is the formula for mechanical index?
Peak negative pressure / (Frequency) 1/2
What do high mechanical index ultrasound. beams cause?
Microbubbles of contrast materal to be compressed and expand significantly and to even burst (Cavitation)
What does the shrinking and expanding of bubbles create?
New frequencies (Harmonic freqencies) different from the frequency of the original sound beam so the ultrasound transduce will receive echos containing the original frequency and the new harmonic frequency
What two variables does the mechanical index depend on?
- Frequency
- Pressure
How does the mechanical index changes given:
- Changes in frequency?
- Changes in pressure?
MI increases with lower frequency
MI increases with stronger (higher pressure variation)
When are the strongest harmonic frequencies produced under ultrasound?
Mechanical Index (MI) of >1
No harmonics are seen at what Mechanical index value?
MI < 0.1
Linear beahvior of bubble shrinking and expanding and this produces no harmonics but instead produces only backscatter
What are the two types of harmonics?
- Tissue harmonics
- Contrast harmonics
Tissue harmonics only exist at … (what)?
Deeper depths
Center main axis
What is resonance in terms of harmonics?
Uneven shrinking and expanding of bubbles
When you have a MI >1,
what is present?
Strong resonance and Cavitation (Lots of harmonics)
When you have a MI 0.1 0 1
what is present?
Resonance (harmonics generated)
When you have a MI < 0.1,
what is present?
No harmonics
What is different from the Pulse Duation to Pulse Repetition Period?
Pulse Repititon Period = Pulse Duration + Listening time
What is velocity of an ultrasound determined by?
Medium only
(Not the source)
What are main properties of the medium that determine velocity of the medium?
- Stiffness
- Density
How does stiffness of the medium affect velocity?
(Of ultrasound waves)
More stiff = higher velocity
How does density of the medium affect velocity?
(Of ultrasound waves)
Increasing density = Decrease velocity
(
Why do sound waves tend to travel faster in more dense materials?
*Tommy Burch describes this paradox*
Sound travels faster in higher density material because they are more stiff because stiffness differences among materials are usually larger than density differences
What is the relationship between frequency and attenuation?
Frequency increases = Attenuation increases
What is impedance?
Acoustic resistance to sound traveling through a medium
What is the formula for acoustic impedance?
Z = P x V
Z = Acoustic Impedance
P = Density
V = Velocity
What is acoustic impedance dependent on:
Source or Medium?
Medium Only
What does a transducer do in its simplist definition?
Convert one from of energy to another
What is the function of piezoelectric crystals?
Convert voltage to ultrasound
What is the Curie Temperature?
Threshold temperature in which the piezoelectric crystals are dead
What is the backing material on an ultrasound?
Function?
Backing material = Damping material = Deceases “ringing” of crytals
Function = Decreases spatial pulse length and improves axial resolution and decreases sensitivity to reflected echoes
What is Q factor?
Unitless number that represents the ability of the machine to emit a “clean” pulse with a narrow beam length
What is the formula for Q factor?
RF / Bandwidth
Which transducers have a low QF?
Which transducers have a high QF?
Low QF (Imaging transducers) i.e. high bandwidth and short SPL
High QF (Therapeutic transducers)
For continuous wave doppler, what is the frequency determined by?
Electrical frequency of the voltage
For pulse wave doppler, what is the frequency determined by?
U/S System
Resonant Frequency
What is the resonant frequency formula?
RF (Resonant Frequency) = V / 2T
V = Velocity
T = Thickness
What is the bandwidth?
Difference between highest and lowest frequency pulse
What phenomenon will increase your bandwidth?
Damping
How does SPL correlate to bandwidth?
Shorter SPL = Higher BW
How does QF relate to RF and Bandwidth?
QF = RF / Bandwidth
What is the focus of an ultrasound beam?
Most narrow portion of an ultrasound sound beam
said another way
Focus = Location of the Minimum diameter of the beam
What is the near field of a sound beam?
Space between near field and focus of a sound beam
What is the far field of an ultrasound beam?
Space between far field and focus of a sound beam
What is another name for the far field or far zone of the sound beam?
Fraunhofer Zone
What is the formula for the near field length?
Ln = r2/wavelength
Ln = Near Field Length
r = Radius of the crystal
What is another term for near field length?
Focal Length
If you have a higher frequency, how will this affect the focal length?
Longer focal length
(Higher frequency = Lower wavelength)
What are the three types of spatial resoluation?
- LARD - longitudinal, axial, radial, range, and depth resolution
- LATA - Lateral, Angular, Transverse and Azimuthal.
- Elevational
What is the formula for axial resolution?
1/2 Spatial Pulse Length
What is range resolution?
Descibes the ability to discern the specific depth (range ~ location) of a reflector (Describes pulsed U/S)
What is the doppler equation?
Change in Frequency = v * cos(Angle) * 2Ftransmitted / C
Change in Frequency = Doppler shift
v = 1540 m/s
What is fast fourier transform?
Current method of PW and CWD analysis
What are the 5 functions of a receiver?
- Amplification
- Compensation
- Compression
- Demodulation
- Rejection
What does amplification of the receiver do?
Enlargement of the returning signals
AKA
Receiver gain = Overall gain
What does compensation do?
Makes all echos from similar structures appear with similar brightness
AKA
Time Gain Compensation
Depth Gain Compensation
Swept gain compensation
What does compression do?
Reduces the total rnage of signals from smallest to largest
Dynamic range.= Range of signals that can be processed by the ultrasound machine
Compression decreases the dynamic range
What is the function of demodulation?
Changes the shape of the electrical signal to make it recognizable by the image screen
What are the 2 steps of demodulation?
- Rectification (Turn all negative voltages positive)
- Smoothing - smoothes out signals
What is the purpose of rejection?
Low level signals are ignored (Noise)
What are the other two names for rejection?
- Threshold
- Suppression
What is the sound of speed in m/s?
1540 m/s
What is the sound of speed in mm/microsecond?
1.54 mm/microsecond
What are the two types of reverberation?
Ring down artifact
Comet Tail
Image shows Ring down and comet tail artifact
What is seen at the green arrow?
Reverberation artifact
Comet Tail seen here
Why can we not see the septal wall (Green arrows)?
Thick calcified mitral valve causes Acoustic Shadowing
What is the formula for resonant frequency (RF) of a piezoelectric crystal?
RF = Velocity / (2*Thickness)
**Remember, the more thin the crystal, the higher the resonant frequency**
Which of the following is associated with cavitation of microbubbles present in an ultrasound contrast agent?
A. High frequency ultrasound
B. Low pressure ultrasound
C. High mechanical index (MI) ultrasound
D. High velocity ultrasound
E. Low intensity ultrasound
High mechanical index (MI) ultrasound
Mechanical index is an indication of an ultrasound beam’s ability to cause cavitation-related bioeffects, and this is currently thought a reasonable proxy for micromechanical damage. It is “strictly a cavitation index,” but is meant to be interpreted more broadly as tissue mechanical stress/damage.
An ultrasound contrast agent is exposed to an ultrasound beam with a high mechanical index (MI >1). Which is the following is more likely true?
- The sound beam has high frequency
- The sound beam has a low peak negative pressure
- The strength of the sound beam is low
- The contrast agent will experience bubble disruption and production of strong harmonic frequencies
D = The contrast agent will experience bubble disruption and production of strong harmonic frequencies
Which of the following is true with regard to harmonic imaging using contrast agents?
A. Harmonic are only produced when the mechanical index > 0.1
B. The harmonic signals are stronger than tissue harmonics
C. Harmonic signals result from microbubble disruption
D. Harmonic signals result from microbubble resonance
E. All of the above
All of the above
Which of the following is true with regard to tissue harmonic imaging?
A. Tissue harmonics are created during transmission through tissues
B. Tissue harmonics are NOT present as sound leaves the transducer but are only present deeper in the tissue
C. Tissue harmonics are weaker than the harmonics created by contrast agents
E. All of the above
F. None of the above
All of the above
