Physics of Ultrasound Part 5-11 Flashcards by Tim Montrief

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

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What phenomenon occurs when U/S reflections off an irregular surface radiate in more than one direction?

Diffuse Reflection = Backscatter

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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

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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

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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

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What is the formula for Acoustic Impedence = ?

Acoustic Impedence = Velocity * Density

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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)

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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

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What is are the formulas for near field length?

Near field length = (radius)² / Wavelength

Near field length = (diameter)^{<span>2</span>}/ 4* Wavelength

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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)

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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

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What is the relationship between line density and temporal resolution?

Inversely proportional

Increase in temporal resolution = Decrease in line density

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What is the definition of line density?

of scan lines / image

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What is the relationship between line density, frame rate, and temporal resolution?

Increase line density = Decreased frame rate = Increased temporal resolution

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Does decreasing imaging depth affect line density?

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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

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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)

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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

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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

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What ultrasound modality is an obsolete form of U.S which plots amplitude (Y axis) vs. Depth (X axis)?

A-mode

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What does frame rate depend on?

(Name 4 things)

Line density (# scan lines per image)
# of foci
Imaging Depth
Sector Width

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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)

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What is a piezoelectric crystal made of?

Lead Zirconate Titanate

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What is the term for the high temperature at which a piezoelectric crystal loses its function?

Curie Temperature

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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?**

"**S**ound = **S**econds" **Period, Frequency, PRF, PRP, Duty Factor, etc** Anything related to time

What are the components of ultrasound that are determined by **medium only?**

1. **Velocity** 2. **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?

1. Oblique Incidence 2. 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)**

1. Energy Mode 2. Color Angio

What are 3 **advantages** of Power Doppler?

1. Increased sensitivity to low flow 2. Unaffected by angle unless 90 degrees 3. Unaffected by aliasing since velocity is ignored

What are 3 **disadvantages** of Power Doppler?

1. Lower frame rates than conventional color flow doppler 2. No measure of velocity or direction 3. 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?

1. Amplification 2. Compensation 3. Compression 4. Demodulation 5. 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.

1. Calculate attenuation coefficient = 1/2 \* (Frequency) = 1/2 \* (10Hz) = 5 Hz 2. 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 1/10^th 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?

1. Compression 2. 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