This lesson was derived

from pages 22 through 24

in the textbook:

Lesson 05:

Intensity Measurements

and Bioeffects

This lesson contains 18 slides

plus 6 multiple-choice

questions.

Intensity

Measurements

and Bioeffects

SATA

SPTA

SATP (SAPA)

SPTP (SPPA)

How are intensity measurements made?

Many different methods may be used to measure the strength of a sound

beam. Differences in the results of these measurements are as varied as the

number of different measuring methods. Before intensity measurements are

performed, two decisions must be made.

The first decision is: Where in the beam’s cross

section will the measurement be made?

The Spatial Peak (SP) method of measuring the intensity in an ultrasound

beam provides a result that is greater than that obtained when using the Spatial

Average (SA) method. ISP > ISA

The Beam Uniformity Ratio (B.U.R.) is the ratio of Spatial Peak to Spatial

Average.

The second decision is: When (during the pulse -

receive interval) will the measurement be made?

The Temporal Peak (TP), which is more appropriately designated as Pulse

Average (PA), is the method of measuring the intensity of the beam that

provides a result that is greater than that obtained when using the Temporal

Average (TA) method. ITP > IPA > ITA

For pulse-echo measurements, the result obtained with the TA method is

affected by the duty factor. For CW measurements, the TP, PA and TA methods

provide identical results.

TA intensity can be measured with a force balance. TP (PA) intensity and

beam profile can be measured with a hydrophone that measures acoustic

pressure variations.

SA TA

SP TP(PA)

SP or SA may be used with either TP (PA) or TA.

WHERE WHEN

SATA

SPTA

SATP(PA)

SPTP(PA)

SP or SA may be used with either TP (PA) or TA.

WHERE - WHEN

ISATA ISPTA ISATP (ISAPA) ISPTP (ISPPA)

Lowest Highest

Which method produces the lowest result?

Which method produces the highest result?

For the four possible combinations, the order of intensity measurement results

(from lowest to highest) is:

SATA, SPTA, SATP (SAPA), and SPTP (SPPA).

HEAT (Thermal)

CAVITATION (Mechanical)

Bioeffects are the biological effects of ultrasound on tissue. Basic bioeffects of

ultrasound energy are heat (thermal) and cavitation (mechanical).

BIOEFFECTS

HEAT (Thermal)

CAVITATION (Mechanical)

Heat is the effect that is caused by friction produced as a result of vibration in

tissue. Heating is negligible when using pulsed ultrasound for medical

diagnostic purposes, but is significant when ultrasound is used for therapeutic

purposes.

BIOEFFECTS

HEAT (Thermal)

CAVITATION (Mechanical)

Stable

Transient

Cavitation is the result of the production of gas bubbles in the tissue. It is

possible when higher than normal ultrasound intensities are used, particularly

due to an increase in peak pressure that accompanies rarefaction.

BIOEFFECTS

HEAT (Thermal)

CAVITATION (Mechanical)

Stable

Transient

Stable cavitation, which could be produced when intensity levels are low may

result in some stress to cells, but is relatively minor.

BIOEFFECTS

HEAT (Thermal)

CAVITATION (Mechanical)

Stable

Transient

Transient cavitation, which is common when very high intensities are used,

such as with a lithotripter, may cause the bubbles to collapse or burst, thus

providing a potential for significant cellular damage.

BIOEFFECTS

MECHANICAL INDEX

THERMAL INDEX

SAFETY INDICES

Safety indices have been developed to predict the potential for adverse

bioeffects.

MI

MECHANICAL INDEX

The mechanical index (MI) value, which is related to the maximum value of

negative peak pressure is used as a guide in predicting mechanical bioeffects

(mainly cavitation). For a given intensity, lower frequencies produce higher MI

values.

TI

THERMAL INDEX

The thermal index (TI) value, which is related to absorption is an estimate of

the rise in tissue temperature in °C. Three different thermal indices are used

for different combinations of soft tissue and bone in the area to be examined.

TIS is the thermal index in soft tissue. TIB is the thermal index in bone near the

beam’s focal zone. TIC is the thermal index in bone near the surface. There

have been no adverse biological effects observed due to temperature

increases less than or equal to 2 degrees above normal.

Nonfocused: SPTA < 100 mW / cm2 (0.1 Watt per square centimeter)

Focused: SPTA < 1 W / cm2

Bioeffects have been determined experimentally, through animal and invitro

studies, but have not been confirmed for nonfocused SPTA intensities below

100mW/cm2 (0.1 watt per square centimeter) or focused SPTA intensities below

1W/cm2 . Although many transducer measurements are performed using

temporal average methods, temporal peak values are important when

assessing cavitation. Assuming that there could be a minimal risk of bioeffects

when using diagnostic ultrasound, the best course of action is to use ultrasound

when the expected benefit outweighs the potential risk. Additionally, a

reduction in the time of an examination will reduce the chances of potential

bioeffects.

BIOEFFECTS

OUTPUT POWER AT MAXIMUM OUTPUT POWER AT ONE - HALF

Depending on the manufacturer, the ultrasound system’s display may indicate

the relative output power or intensity as a percentage of the system’s

maximum, as a decibel (dB) difference, the estimated SPTA intensity, or as

mechanical index and thermal index values. Any displayed values should be

interpreted as relative information to help achieve the ALARA principle.

TYPICAL METHODS FOR DISPLAY OF RELATIVE INTENSITIES

ALARA: the guiding

principle regarding

output intensity,

defined as “as low

as reasonably

achievable”

decibel (dB): a

parameter used to

compare the

relative powers,

intensities, or

amplitudes of two

ultrasound energy

levels

Sound energy is typically measured in watts, which is a parameter of power.

Although the unit for power is the watt, the milliwatt (mW) is normally used.

Intensity also represents the amount of sound energy, but the cross-sectional

area of the beam is specified.

INTENSITY

power: the rate of doing

work

INTENSITY = POWER DIVIDED BY AREA

POWER BEAM DIMENSIONS AREA INTENSITY

100 mw 1 cm x 1 cm 1 cm2 100 mW / cm2

100 mw 1.414 cm X 1.414 cm 2 cm2 50 mW / cm2

100 mw 2 cm x 2 cm 4 cm2 25 mW / cm2

50 mw 1.414 cm x 1.414 cm 2 cm2 25 mW / cm2

Answers to the following

SIX practice questions

were derived from

material in the textbook:

Question 1

Which intensity measuring method provides the highest result?

SPTA

SPTP

SATA

SAPA

SATP

Page 22

Question 1

Which intensity measuring method provides the highest result?

SPTA

SPTP

SATA

SAPA

SATP

Page 22

Question 2

A hydrophone probe can be used to measure the

ultrasound intensity

beamformer voltage

bandwidth

ultrasound frequency

axial resolution

Page 22

Question 2

A hydrophone probe can be used to measure the

ultrasound intensity

beamformer voltage

bandwidth

ultrasound frequency

axial resolution

Page 22

Question 3

Thermal bioeffects of ultrasound energy have been

determined experimentally by using

epidemiological and cavitation studies

cavitation detectors

animal and in vitro studies

hydrophones

statistical predictions

Page 23

Question 3

Thermal bioeffects of ultrasound energy have been

determined experimentally by using

epidemiological and cavitation studies

cavitation detectors

animal and in vitro studies

hydrophones

statistical predictions

Page 23

Question 4

Potential bioeffects of ultrasound can be minimized by

using high pulse repetition frequencies

increasing the mechanical index

increasing the thermal index

using highly focused transducer

reducing the time of the examination

Page 23

Question 4

Potential bioeffects of ultrasound can be minimized by

using high pulse repetition frequencies

increasing the mechanical index

increasing the thermal index

using highly focused transducer

reducing the time of the examination

Page 23

Question 5

Since very little is known about bioeffects and assuming that

there could be a minimal risk of bioeffects when using

diagnostic ultrasound, what would be the best course of

action?

Perform fewer ultrasound examinations but increase

the time of each examination

Perform ultrasound examinations only when a crash

cart is available.

Use pulsed Doppler only for obstetrical studies.

Use ultrasound when the expected benefit outweighs

the potential risk

Page 23

Question 5

Since very little is known about bioeffects and assuming that

there could be a minimal risk of bioeffects when using

diagnostic ultrasound, what would be the best course of

action?

Perform fewer ultrasound examinations but increase

the time of each examination

Perform ultrasound examinations only when a crash

cart is available.

Use pulsed Doppler only for obstetrical studies.

Use ultrasound when the expected benefit outweighs

the potential risk

Page 23

Question 6

Sound power is measured in units of

dB/cm

mW/cm

watts/m3

watts

dB

Page 24

Question 6

Sound power is measured in units of

dB/cm

mW/cm

watts/m3

watts

dB

Page 24

END OF LESSON 05