Ultrasound Physics & Instrumentation
4th Edition
Volume II
Companion Presentation
Frank Miele
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Volume II Outline
 Chapter 7: Doppler
 Chapter 8: Artifacts
 Chapter 9: Bioeffects
 Level 2
 Chapter 10: Contrast and Harmonics
 Chapter 11: Quality Assurance
 Chapter 12: Fluid Dynamics
 Chapter 13: Hemodynamics
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Bioeffects
There are two principle mechanisms for bioeffects:
Thermal bioeffects (temperature rise related to):
 temporal average intensity
 duty factor
 scan time
 scanned vs. non-scanned modalities
Mechanical bioeffects (cavitation related to:)
 peak rarefactional pressure
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Intensity
Recall the equation for intensity as derived in Chapter 3:
Power
Intensity 
Area
However, there are many different ways of making an intensity measurement.
For example, the measurement can be made where the beam reaches its peak
in space, its average over space, looking over the shorter duration during which
the transmit is on ( the pulse duration), or during the longer period of time
referred to as the PRP. Since each of these approaches to measuring the
intensity lead to a potentially different indications for the risk of bioeffects, we
will need to look at each approach.
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Common Intensities: Four Concepts
Understanding the common intensities is facilitated by understanding
four basic concepts. These four concepts are detailed in the next 7
slides
Concept 1: A peak is always greater than or equal to an average.
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Concept 1: Peak versus Average
A peak is always greater than or equal to an average.
Therefore an SP is greater than an SA
and
a TP is greater than a TA.
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Concept 2: The First Letter (S)
The first letter (S) refers to how the beam is distributed over physical
space.
Fig. 2: (Pg 628)
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Temporal: Pulsed Wave Timing Diagram
The foundational drawing for PW is helpful in understanding concepts 3
and 4 (the temporal aspects) of bioeffects.
DF
Fig. 1: (Pg 627)
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Concept 3:
Pulse Measurements (PP and PA)
The pulse intensity measurements are made during the shorter period
of time referred to as the pulse duration (PD). The pulse peak is the
highest intensity that occurs during the pulse duration (PD). The pulse
average is the average intensity over the PD.
Fig. 3: (Pg 628)
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Concept 4:
Temporal (PRP) Measurements
The temporal intensity measurements are made during the longer
period of time referred to as the pulse repetition period (PRP). Notice
that the pulse peak and the temporal peak are the same measurement.
Therefore, the pulse peak is not used.
Fig. 4: (Pg 629)
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Combining Temporal Intensity
Measurements
This diagram shows the three different temporal intensity measurements
and the relative amplitudes. The highest intensity is clearly the TP,
followed by the PA, and the TA as the lowest intensity. Note that the ratio
of the TA to the PA is equivalent to the duty factor.
Fig. 5: (Pg 630)
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Review:
Spatial Distribution vs. Time Distribution
Spatial Distribution:
refers to how the beam energy is distributed over physical space in
the body. The beam parameters principally determine the spatial
distribution of the beam.
Temporal Distribution:
refers to how the energy is distributed over time.
For intensity measurements, the first set of two letters refers to the
spatial distribution of energy and the second set of two letters refers
to the spatial distribution of energy:
I ( Spatial
Temporal )
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Spatial Distribution and the BUF
As suggested by the name, the beam uniformity factor is a measure of
beam uniformity. A completely uniform beam would have a BUF equal
to 1. Normally, the BUF is greater than one, since the spatial peak
intensity is usually greater than the average spatial intensity.
SP
B.U .F . 
SA
Fig. 5: (Pg 630)
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Thermal Bioeffects
Non-scanned modalities have a greater risk of thermal bioeffects than
scanned modalities.
Scanned modalities:
 2-D (B-Mode)
 Color Doppler
Non-scanned modalities:
 CW Doppler
 PW Doppler
 M-mode
 A-mode
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Scanned vs. Non-Scanned
Non-scanned modalities transmit repeatedly in the same direction,
allowing for heat to build up, increasing the risk of thermal bioeffects.
PW
B-Mode
Non-scanned
Scanned
For a scanned modality, there is time between transmits being repeated
in the same direction, allowing time for the heat to dissipate.
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Typical Transmit Voltages
Fig. 7 (Pg 635)
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Understanding Intensity Restrictions
Two ways to overflow the bucket:
A High Intensity for a
Short time
A Low Intensity for a
Long Time
Mechanical Bioeffects
Thermal Bioeffects
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Intensity Restrictions (I)
“Highest Transmit Voltage”
B-Mode
“Lowest Duty Factor”
PW
PW
(larger
gate)
“Lowest Transmit Voltage
CW
“Highest Duty Factor”
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Intensity Restrictions (II)
“Greatest Risk of Mechanical” B-Mode
“Lowest Risk of Thermal”
PW
PW
(larger
gate)
“Lowest Risk of Mechanical”
CW
“Highest Risk of Thermal”
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Acoustic Power Measurements
Acoustic Power measurements are made in complex systems as pictured below.
A specialized ultrasound transducer referred to as a hydrophone is placed in the
water tank and aligned to the beam to make the various intensity measurements.
Fig. 8: (Pg 637)
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Membrane Hydrophone
The hydrophone pictured below is called a “membrane” hydrophone.
Fig. 9a: (Pg 638)
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Needle Hydrophones
The four hydrophones pictured below are called “needle” hydrophones.
Fig. 9b: (Pg 638)
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Transducer Flaw and Abnormal Beam
(from Animation CD)
Schlieren Image of a focused beam with a bad element.
(Pg 640 A)
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Schlieren Images: Visualizing Beams
(from Animation CD)
Schlieren Image of a normal and abnormal beam at 3.5 MHz.
(Pg 640 B)
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Schlieren Image of a Pulsed Wave
(from Animation CD)
Schlieren Image of a 5 cycle pulsed wave
(Pg 640 C)
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Pulsed Wave Schlieren Image (Animation)
Notice the beam shape that occurs over time as the 5 cycle pulse travels.
(Pg 640 D)
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Mechanical Index (MI)
With the new output display standard, the best indicator for the risk of
mechanical bioeffects (cavitation) is the mechanical Index (MI).
Peak Rarefactional Pressure
MI 
Operating Frequency
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Thermal Indices (TIS, TIB, TIC)
With the new output display standard, the best indicator for the risk of
thermal bioeffects was replaced with three different thermal indices:
TIS  Thermal Index in Soft Tissue
TIB  Thermal Index in Bone
TIC  Thermal Index in Cranial Bone
The TI values indicate the model predicted worst case temperature
rise (in Celsius) based on the imaging parameters in use (preset,
depth, transmit power, focus, etc.)
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Ultrasound Safety (AIUM Statement)
The official AIUM statement on the safety of diagnostic ultrasound
basically states:
 diagnostic ultrasound has been in use since the 1950’s
 there are clear clinical benefits and recognized efficacy for
medical diagnosis including during pregnancy
 to date there have been no confirmed bioeffects on either the
patient or the instrument operator using currently accepted
power levels and intensities
 although there is a risk that bioeffects will be identified in the
future, current studies indicate that the benefits of prudent
application of diagnostic ultrasound outweigh those risks, if there
are any
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Prudent Use & ALARA Principle
Prudent use implies that steps should be taken to minimize the risk of
bioeffects to patients, including:
 not performing needless scans
 not extending scan times needlessly
 using the minimum amount of power to achieve good clinical
results (ALARA Principle: As Low As Reasonably Achievable)
 if there is a risk of missing clinically relevant information by using
a lower transmit power, increasing the transmit power is
warranted. (Recall that the benefits currently outweigh the risks)
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