Physics of Ultrasound Part 5-11 Flashcards
What phenomenon occurs when the reflectors dimensions are much smaller than the wavelength of the ultrasound beam?
- When this occurs the sound wave is redirected equally in all directions.
- Clinically this is seen wiht the interaction of RBCs with an ultrasound beam
Raleigh Scattering
What phenomenon occurs when U/S reflections off an irregular surface radiate in more than one direction?
Diffuse Reflection = Backscatter
Which of the following is most responsible for the conversion of U/S energy to heat?
A. Absorption
B. Reflection
C. Refraction
D. Acoustic Vibration
E. Cavitation
Absorption
Which of the following is improved by lowering frequency of an ultrasound beam?
A. Penetration
B. Axial resolution
C. Lateral resolution
D. Temporal resolution
E. Longitudinal resolution
Penetration
Lower frequency ultrasound* has *less attenuation and better penetration
Distractor = Axial resolution improved by higher frequency ultrasound with a higher SPL
Which of the following properties of the media influences acoustic impedence?
A. Velocity
B. Density
C. Elasticity
D. Stiffness
E. All of the above
All of the above
What is the formula for Acoustic Impedence = ?
Acoustic Impedence = Velocity * Density
What is the definition of these respective angles;
Obtuse
Acute
Oblique
Obtuse = 90 - 180
Acute = < 90
Oblique = Not normal (Acute or obtuse, but not 0 or 90)
What does the matching layer do in terms of acoustic impedence?
Matching layer helps to
decrease the change in acoustic impedence from the crystal to the tissue (Helps decrease massive reflections in the probe)
- prevents a large reflection at interface of musosa and TEE probe
- PZT > Matching layer > Gel > Mucosa
What is are the formulas for near field length?
Near field length = (radius)2 / Wavelength
Near field length = (diameter)<span>2</span> / 4* Wavelength
The ability to accurately identify structures that lie close together when one is in front of the other (one deeper than the other) (in tandem) is called:
Longitudinal resolution
Also known as L.A.R.D. (Longitudinal, Axial, Range or Radial, Depth)
What type of resolution is the ability to accurately identify structures that lie close together when one is on top of the other (Along Y axis) (along vertical width of the beam)?
Elevational resolution
What is the relationship between line density and temporal resolution?
Inversely proportional
Increase in temporal resolution = Decrease in line density
What is the definition of line density?
of scan lines / image
What is the relationship between line density, frame rate, and temporal resolution?
Increase line density = Decreased frame rate = Increased temporal resolution
Does decreasing imaging depth affect line density?
No
What is the advantage of a focused ultrasound beam in terms of which resolution and in near/far field?
Improved lateral resolution in the near field
What is the disadvantage of a focused ultrasound beam in terms of which resolution and in near/far field?
Decreased lateral resolution in the far field
(more divergence in the far field)
What is the maximum spatial peak temporal average intensity recommended by the American institute of U/S in medicine to avoid thermal injury for unfocused ultrasound beams is what?
1 Watt / cm2
What is the maximum spatial peak temporal average intensity recommended by the American institute of U/S in medicine to avoid thermal injury for FOCUSED ultrasound beams is what?
Focused = Lower limit because energy isn’t spread out
100 milliwatts (mW)/cm2
= 0.1 W/cm2
What ultrasound modality is an obsolete form of U.S which plots amplitude (Y axis) vs. Depth (X axis)?
A-mode
What does frame rate depend on?
(Name 4 things)
- Line density (# scan lines per image)
- # of foci
- Imaging Depth
- Sector Width
What is the purpose of matching layer in an ultrasound transducer?
Reduces reflection at the tissue transducer interface
(Facilitates improved transmission from probe to the tissue)
What is a piezoelectric crystal made of?
Lead Zirconate Titanate
What is the term for the high temperature at which a piezoelectric crystal loses its function?
Curie Temperature
What is the disadvantage of the backing material?
Decreases sensitivity to reflected echoes
How does the backing material affect axial resolution?
Improves Axial Resolution by decreasing SPL (Spatial pulse length)
How does backing material affect Q factor?
Q Factor = RF / BW
Q factor = (V / 2*Thickness) / BW
Decreases Q factor because it increases bandwidth
How would adjusting PRF affect temporal resolution?
Increase PRF* = *Increase Temporal Resolution
Higher PRF = More images can be formed per second therefore higher fraame rate = # images / second
Increasing frame rate = Increasing temporal resolution
How does pulse duration affect bandwidth?
Shorter pulse = Wider bandwidth
How does damping material affect bandwidth?
Damping material increases bandwidth (BW = range of frequency of the pulses)
What is duty factor?
Unitless number describes amount of time (0% - 100%) U/S machine is producing sound
What is the formula for duty factor?
Pulse Duration* / *Pulse Repetition Period
What is the Q factor?
Unitless number which describes quality of the ultrasound pulse
What is the formula for Q-factor?
Resonant Frequency* / *Bandwidth
Imaging transducers tend to produce short U/S pulses with wide bandwidths, high resonant frequencies and small Q factors
How does the thickness of the piezoelectric crystal increases, how does this affect the frequency and wavelength?
Thicker = Increased frequency* and Smaller *wavelength
How are frequency and attenuation related?
Decreased frequency = Decreased Attenuation
High frequency probes = Can’t penetrate because of attenuation
What is the intensity reflection coefficient?
% of ultrasound intensity that is reflected at the interface of two media
What is the intensity transmission coefficient?
% of ultrasound intensity that is transmitted (Passes forward through the interface of the two media)
What is the formula of the Intensity reflection coefficient?
[Z2 - Z1 / Z2+Z1]2 x 100
Z = Acoustic Impedance
What is the formula for acoustic impedance?
Density of medium * Velocity of U/S in the medium
What is the order of acoustic impedances of materials in the ultrasound?
PZT ~ Backing layer* > *Matching layer* > *Gel* > *Skin
What does the piezeoelectic crystal determine about the wavelength of PWD doppler?
1/2 wavelength = Thickness of crystal
piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength
The frequency of the CWD ultrasound beam is determined by?
The frequency of the PWD ultrasound beam is determined by?
CWD = Electrical frequency of the excitation voltage applied to the crystal
PWD = Thickness of Piezoelectric crystal and the velocity of sound through the crystal
RF = Velocity / (2*Thickness)
What is the formula for near field length?
radius2 / wavelength
In general, do you want a low or a high Q factor?
Low Q factor = Better & Higher quality of pulse
How does the focus change when you have high frequency, short wavelength* transducers vs. *low frequency, long wavelength transducers?
high frequency, short wavelength = Deep focus
low frequency, long wavelength = Shallow focus
What is the frequency of audible sound?
What is the frequency of ultrasound?
Audible sound = 20 Hz - 20,000 Hz
Ultrasound > 20,000 Hz
Selective depth dependent amplificiation is achieved by altering which of the following?
Time Gain Compensation
TGC increases receiver gain with increasing arrival time of echoes to compensate for ultrasound attenuation
- Hence, deeper structures have weaker echos and require more gain for optimal visualization
What is the formula for line density?
of scan lines per image
What is the post image processing method used for CW and PW doppler analysis?
Fast Fourier Transform
How does changing the PRF change the duty factor?
Increase PRF = Increase Duty Factor
(If you increase PRF, you increase the # of pulses per second therefore increase duty factor)
SPL is determined by:
Source?
Medium?
Both?
Both medium and source
What are the components of ultrasound that are determined by sound source only?
“Sound = Seconds”
Period, Frequency, PRF, PRP, Duty Factor, etc
Anything related to time
What are the components of ultrasound that are determined by medium only?
- Velocity
- Acoustic Impedence (density * velocity)
Does a high frequency transducer produce higher quality images because of:
A high or low SPL?
Low SPL
You want this low so that you will improve axial resolution (You want resolution number as small as possible!)
- Paradoxical but true
What is the primary component of attenuation in soft tissue?
Absorption
What ultrasound parameters are determined by sound source & medium?
Parameters with Length (Wavelength), Spatial Pulse Length
In order for refraction to occur, what two componets must be present?
- Oblique Incidence
- Velocity media 1 cannot = Velocity media 2
Rank the following media in order of increasing (Highest velocity last) velocity.
Air
Bone
Fat
Lung
Soft Tissue
(Slowest) Air < Lung < Fat < Soft Tissue < Bone (Fastest)
What is Power doppler tell you?
What is is not tell you?
+/- Presence of a Doppler shift
No speed or direction
What are the synonyms of Power Doppler (Name 2 of them)
- Energy Mode
- Color Angio
What are 3 advantages of Power Doppler?
- Increased sensitivity to low flow
- Unaffected by angle unless 90 degrees
- Unaffected by aliasing since velocity is ignored
What are 3 disadvantages of Power Doppler?
- Lower frame rates than conventional color flow doppler
- No measure of velocity or direction
- Susceptible to motion of the transducer, patient, or soft tissues which may result in a burst of color or flash artifact
What is shown at the green dot in the image?
What does this signify?
L wave
Signifies diastolic dysfunction with impaired relaxation and Elevated LAP
What are the 5 functions of the receiver?
- Amplification
- Compensation
- Compression
- Demodulation
- Rejection
What is the difference between rejection in post image processing vs. Low Pass Wall filters?
Rejection = 2D amplitudes are eliminated
Low Pass Wall filters = Eliminates large doppler shifts (high frequency signals)
Absorption is responsible for what % of attenuation seen in soft tissue?
>80%
What frequencies do most echocardiography machines operate?
2-10 MHz
What intensity is the maximum spatial peak temporal averate intensity (SPTA intensity) recommended for focused ultrasound beams?
100 mW/cm2
What intensity is the maximum spatial peak temporal averate intensity (SPTA intensity) recommended for unfocused ultrasound beams?
1 W / cm2
For every “x” cm will your amplitude be decreased by 1/2?
1 cm
Not sure how he did this calculation
What modality utilizes two or more sample volumes along a scan line which decreases aliasing artifact but introduces range ambiguity?
High pulse repetition frequency PWD
Note: The answer is not CWD
What is the processing method used for modern PWD and CWD analysis?
Fast Fourier Transform
The thickness of tissue required to reduce the intensity by a factor of 0.5 is referred to as the half value thickness layer.
This represents how many decibels (dB) of attenuation?
3 dB
calculate the thickness of tissue required to decrease the power of a 12 MHz TEE by half (half power distance).
Answer = 0.5 cm
What angle of incidence will provide the best 2D gray scale TEE images?
90 degrees
Focused U/S beam improves which type of resolution (Axial, Lateral, Elevational) in which field (Near or Far)?
Improved lateral resolution in the near field
How does line density affect frame rate?
As line density increases, frame rate decreases (Worsens temporal resolution)
Line density = Spacing between sound beams
Line density increases the number of pulses per image
How does the damping material affect the Q factor?
Decreases it
(Increases bandwidth * Decreases Resonant frequency)
What is the formula for attenuation coefficient?
Attenuation coefficient = 1/2 * Frequency
Calculate the attenuation expected to occur at a depth of 7cm when using a 10 MHz TEE probe.
- Calculate attenuation coefficient = 1/2 * (Frequency) = 1/2 * (10Hz) = 5 Hz
- Multiple AC * Path length (7 cm)
3. Answer is 35 dB of attenuation
Selective depth dependent amplification is achieved by altering which of the following?
Time gain compensation* AKA *Depth gain compensation
This increases the receiver gain with increasing arrival of echos to compensate for ultrasound attenuation.
- Deeper structures have weaker echos and require more gain for optimal visualization
- This function compensates for the attenuation that occurs with increasing depth
-
An ultrasound system with a decrease in output by 1/10th the original power can be represented by what decibel reading?
Answer = 10 dB
What determines the frequency of a PWD ultrasound beam?
Thickness of PZT crystal and velocity of sound through crystal
What determines the frequency of a CWD ultrasound beam?
Electrical frequency of the excitation voltatge applied to the PZT crystal
What is the best angle of incidence for 2D TEE images?
What is the best angle of incidence for doppler?
2D images = 90 Degrees
Doppler = 0 degrees where (Cos is 0)
An unfocused CW U/S beam has near zone length of 10cm. At the focus, the beam is 4mm wide. What is the diameter of the PZT crystal in the transducer?
Answer = 8mm
Formula: Unfocused transducer in continuous mode**
Focus diameter (4mm) = [Transducer diameter (In PZT) / 2]
Transducer diameter = 4mm x 2 = 8 mm
What are the formulas for:
Pressure
Amplitude
Power
Intensity
See image
What transducer is capable of creataing multiple focal zones per scan line?
Phased Array Transducer
- Increasing the number of focal zones per scan line will improve lateral resolution
What two words best describe the motion of particles in a medium as sound passes through it?
- Compression
- Rarefaction
What phenomenon is responsible for the doppler determinations of blood flow velocities?
Rayleigh Scattering
What is the relationship of a wavelength and an RBC during Raleigh Scattering?
When Wavelength >> RBC (In terms of length) then Rayleigh scattering occurs
Most TEE Probes are what subtype of array probes?
Linear Phased Array transducers
Newer 3d Probes = Matrix phased aray capable of 3d imaging
