Intensities Flashcards by Karen Reitz

If you double the amplitude, what happens to intensity? Amplitude x3 means… ?

It is increased x4 (due to the exponent).

Intensity x9

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What is power?

What are the units for power?

Energy per unit time.

Watts

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What is sound intensity?

What are the units for intensity?

What are the (usual) ultrasound units for intensity?

Power per unit area.

W/m²

mW/cm²

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What is the equation for intensity?

Intensity = power/area,

OR: Intensity = amplitude²/2pV

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True or false:

Amplitude and frequency are dependent on each other.

False, they are independent of each other.

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What are the two basic parameters in measurements of intensity?

Space and Time

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What are the two types of measurements of intensity?

Peak and Average

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Which measurement will be higher?

Peak or Average?

Peak

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What does peak refer to?

The maximum value

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What does average refer to?

The mathematical middle value

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What does temporal refer to?

To all time, Transmit (pulse duration) and Receive. .

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True or false:

A pulsed ultrasound beam does not have the same intensity at different times.

True

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What does pulsed refer to?

Only to the transmit time (pulse duration).

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True or false:

A pulsed ultrasound beam has the same intensity at different times during the pulse.

False: It has does not have the same intensity at different times during the pulse.

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Pulsed is the average intensity for what?

For the pulse duration only. (Ignore listening time).

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If TP = TA, what kind of ultrasound is it?

Continuous wave: always on.

So, peak intensity = average intensity.

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What is I_m?

Another type of intensity measurement.

It’s the average intensity measured at the most intense half-cycle.

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What is pulse average, PA?

And what type of measurement is it?

The average intensity for the duration of the pulse itself.

It is a measurement of time.

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What does spatial refer to?

Location or space. (Distance)

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What is spatial peak intensity, (I_sp)?

The average intensity at the location where it is maximum. (the maximum in space)

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What is spatial average intensity, (I_sa)?

The average intensity across the beams cross sectional area.

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True or false:

Spatial average intensity, or I_sa, is always higher than spatial peak intensity, or I_sp.

False. Spatial peak intensity is always higher.

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What is this all about?

Why do we care?

Sound energy.

We don’t want our sound waves to damage tissue.

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What is beam uniformity coefficient (BUC)?

Another intensity related measurement.

Sometimes called: SP/SA)

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Why is a BUC closer to 1.00 optimal?

It suggests a uniform intensity across the area.

What does a higher peak intensity relative to average intensity mean?

A higher BUC. Less uniformity of intensity.

What is the lowest possible BUC? What are the units for BUC?

1.00 None. BUC is unitless.

What does it mean if the peak intensity isn't very different from the average?

The distribution of intensity is pretty uniform across the area.

What does it mean if there is a significantly higher peak intensity than the average?

The uniformity isn't very good.

What is dosimetry?

The study of effects of exposure of the body to signals used in diagnostic testing, such as ultrasound, X-ray, MRI, etc. (aka: Bioeffects)

What are the two bioeffects of ultrasound?

Heat and Cavitation

With respect to bioeffects, what is heat?

The energy dissipated as ultrasound travels through the tissue.

What is the best index for heat?

SPTA is the best index.

With respect to bioeffects, what is cavitation?

It is the creation of little spaces due to movement of particles.

What is the best index for cavitation?

Temporal peak is the best index.

When is significant heat production good?

Therapeutic ultrasound

True or false: Therapeutic ultrasound uses a higher frequency and lower intensity than diagnostic ultrasound.

False: Therapeutic ultrasound uses a lower frequency and higher intensity.

AIUM found no bad effects with unfocused beams at \_\_\_\_\_\_\_?

100 mW/cm² | (Greater than this is bad)

AIUM found no bad effects with a focused beam at _______ ?

1 W/cm²

What is it we are really doing with intensity conversions?

We are converting space and time units.

How do we convert pulse average to temporal average, PA to TA, using duty factor?

SPPA x DF = SPTA (i.e., PA x proportion of time actually making sound)

What are the units for duty factor, DF?

None. DF is unitless.

How do you solve for DF when you are given SPPA and SPTA?

DF = TA/PA | (Ignore the SP in front)

How do we convert spatial peak to spatial average using BUC?

SA = SP/BUC

How do you solve for BUC when you are given SP and SA?

BUC = SP/SA

When working temporal conversions use _____ .

Duty factor. DF relates peak to average over time.

When working spatial conversions use ______ .

Beam uniformity coefficient. BUC relates peak to average over space.

What is BUC about?

Space

What is DF about?

Time

What is the most relevant intensity with respect to tissue heating?

SPTA

List the intensities from highest to lowest. (There are seven)

SPTP, I_m, SPPA, SPTA, SATP, SAPA, SATA

What does BUC describe? What are its units? What is its value?

The spread of a beam in space. It is unitless. It has a value of 1 or greater.

What does DF describe? What are its units? What is its value?

The relationship of beam intensities with time. It is unitless. It has a value between 0 and 1.

When pulsed and continuous wave sound beams have the same SPTP intensities, which beam has the higher SPTA intensity?

Continuous wave

When pulsed and continuous wave sound beams have the same SATP intensities, which beam has the higher SATA intensity?

Continuous wave

What is temporal peak intensity, or I_tp?

Maximum intensity in time

What is temporal average intensity, or I_max or I_m?

The average intensity during the PRP. | (both the on and off times)

Of all the temporal intensities, which is the highest?

Temporal peak is the highest.

Of all the temporal intensities, which is the lowest?

Temporal average is the lowest.

Why is temporal average the lowest of the temporal intensities?

Because it includes both the on and off time.

What is pulse average intensity, or I_pa?

Taking the average intensity during the pulse duration, the transmit time.

List the temporal intensities from largest to smallest.

I_tp, I_max, I_pa, I_ta

This is an example of what? And what is it measuring?

Temporal peak intensity. TP It is the maximum itensity I_m derived from the most intense half cycle in the pulse.

The shorter line above is an example of what and it measures when? The longer line below is an example of what and it measures when?

Pulse average, PA; Is measured during transmit time only. Temporal average, TA; Is measured during both the transmit and receive times.