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