Basic Physics of
Ultrasound
Beth Baughman DuPree M.D. FACS
Medical Director Breast Health Program Holy
Redeemer Health System
2011
Financial Disclosures
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Faculty/Consultant Ethicon Breast Care
Speaker- Myriad Genetics
Consultant Precision Therapeutics
Faculty- CME at Sea
Breast Ultrasound Certification
 Stereotactic Biopsy Certification
 Mastery of Surgery Program
 APBI Registry
 www.breastsurgeons.org

The Changing World of Breast
Care
Precision Therapeutics Chemo Fx
Assay
BCT DCIS
MR Mastectomy
BCT Lump/ALND
XRT DCIS
CHEMO N -
SLN
WHOLE BREAST XRT
OPEN SURGICAL BX
BRCA TESTING
BLN
APBI B-39 PEM BX
STEREO MIBB
US MIBB
1980
1990
ONCOTYPE
2000
CONSENSOUS ST
MRI MIBB
2010
Basic Principles
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Sound waves are mechanical waves that require a
medium through which to propagate
Sound cannot travel through a vacuum
Different materials have different acoustic properties
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Varies the ability to transmit sound waves
Varies the ability to reflect sound at interfaces
Frequency
The number of cycles completed per second.
1 cycle per second is called Hertz (Hz)
•Humans hear frequencies •Sound above the level of
in the range of 20Hzhuman hearing is called
20,000Hz
ultrasound
Frequency
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Diagnostic Ultrasound is measured in mega
hertz (MHz)
mega means millions
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Imaging transducers are named by their
operating frequency
Frequency Range - 2.25 MHz-20 MHz
5 MHz transducer = 5 million cycles/sec.
Sound
Source
Incident
Transmitted
Reflected
MEDIUM 1
MEDIUM 2
Acoustic Interface
Reflection
Soft Tissue (1540 m/s)
Fat (1459 m/s)
Acoustic interface / Acoustic Mismatch
Soft Tissue (1540 m/s)
Bone (4080 m/s)
Getting an Image
The heart of ultrasound is the transducer
Piezo - electric effect
Piezo-Electric Effect
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The crystal is mounted on a rotational axis
It is driven by an electric motor
A sound pulse is transmitted and received
Results in a specific focal zone
Some transducers contain several crystals
Hence 8-14mHz probes have several crystals
The Transducer
Converts electrical energy into sound
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Components:
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Piezoelectric crystal
Dampening material
Matching layer
covers crystals
The Transducer
3.5 MHz
Thicker crystal
produces bigger
sound waves.
7.5 MHz
Thinner crystal produces
smaller sound waves.
The Transducer
3.5 MHz
7.5 MHz
The LOWER the
frequency the better
the penetration
The HIGHER frequency
the less the penetration
Bigger, Stronger
Smaller, weaker
The Transducer
Short pulses of sound are sent (transmits) into
the body and then the transducer listens for the
returning signals (receives).
The ultrasound system processes the returning
signals into images that are displayed on the
ultrasound monitor
Transmits
Waits
Receives
Linear Array Transducer
Electronic Linear-Array
Transducer
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Parallel arrangement of the crystals
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Two-dimensional, rectangular image
Time delay between successive crystal firing
can be varied
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Directing and focusing the beam
Gray Scale
B-Mode Ultrasound
Soft Tissue
Fat
Soft Tissue
Bone
Soft Tissue
Cyst
Grayscale Imaging
Propagation speed is how fast the sound
travels through a medium.
The system keeps track of when the
pulse is sent and when the echo
returns and places the pixel at a
depth represented by the time
difference.
Grayscale Imaging
The strength of the returning echoes also
depends on the differences in the acoustic
impedance between various structures.
Acoustic impedance relates to tissue density.
The greater the difference in density
between two structures, the stronger the
returning echo
Examples:
different:
aorta and liver
same:
kidney and liver
Grayscale Imaging
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Attenuation:
A decrease in the strength of the sound wave as it passes
through tissue and further into the body.
Acoustic
Impedance:
The resistance of the sound wave traveling through
tissue
Each tissue has its own acoustic impedance due to the density of
the tissue.
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Through Transmission
There is no attenuation of the sound wave traveling
through the tissue.
Grayscale Imaging
WHITE DOTS = STRONG = e.g., bone
BLACK DOTS = NO reflections = e.g., fluid
GRAY (different shades) = WEAKER
reflections
Grayscale Imaging
The strength of the returning echo is
directly related to the angle at which the
ultrasound beam strikes an interface.
The more perpendicular the
ultrasound beam, the stronger
the returning echo.
Echogenicity
fat - equivalent
hypoechoic
isoechoic
hyperechoic
anechoic
Echogenicity
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Anechoic
Hypoechoic
Isoechoic
Hyperechoic
Echogenicity
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Anechoic
Hypoechoic
Isoechoic
Hyperechoic
Echogenicity
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Anechoic
Hypoechoic
Isoechoic
Hyperechoic
Echogenicity
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Anechoic
Hypoechoic
Isoechoic
Hyperechoic
Resolution
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Clarity of picture
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Ability of equipment to detect
2 separate reflectors in tissue and to display
them as 2 separate reflectors on the monitor
without merging them.
Image Resolution
Types of Resolution
The ability to identify structures very
close together:
Axial
Ability to identify structures that are one in
front of the other
Lateral
Ability to identify structures that are side by
side
Temporal
Ability to accurately locate a moving structure
Spatial
Ability to display very small structures in their
correct anatomic location.
Axial Resolution
3.5MHz
7.5 MHz
The shorter the
pulse, the better
the axial resolution
Increasing the
frequency
increases axial
resolution
Characteristics of Sound
Frequency
Sound
Sound
Axial
Frequency
Penetration
Resolution
High
Low
Lateral Resolution
Very important for ultrasound
guidance with needles/probes
A transducer
with a large
surface area
will resolve
better in the
lateral
dimension
“Fine-tuning” the Image
Gain=Volume
GAIN Control
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Controls the
brightness of the
whole image
Not enough gain
Too much gain
Time Gain Compensation (TGC)
Depth Gain Compensation
• Compensates for tissue
attenuation
• Controls the brightness
in portions of the
image
• Distributed over depth
Poor TGC adjustment
Good TGC adjustment
Focus
Focal Zones
• Decreases the beam diameter
• Adjustable by operator.
• Place in area of interest
Focus the Image
Focal Zones
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Image of a solid
mass with the focal
zone placed
incorrectly
The focal zone depicted
by the caret is at the
bottom of the image.
Focal Zones
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Image of the same solid
mass with the focal
zone placed correctly
The focal zone depicted by
the caret is at the top of the
image near the lesion.
Depth
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Depth is patient dependant
Depth is transducer
dependant
Operator controlled
Deep
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Increase depth
Demonstrate shadowing
Superficial
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Decrease depth
Image Artifacts
Used to decide if structures are fluid-filled,
solid or a combination.
Acoustic Shadowing
Acoustic Enhancement
Acoustic Shadow = decrease in the intensity of the
echoes behind the attenuating structure
Acoustic Enhancement = increase in the intensity
of the echoes behind the structure
Artifacts and Aberrations
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Shadowing
Enhancement
Reverberation
Edge effect
Shadowing
Artifacts and Aberrations
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Shadowing
Enhancement
Reverberation
Refraction
Edge effect
Posterior enhancement
is not proof of a cyst.
Artifacts and Aberrations
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Shadowing
Enhancement
Reverberation
Refraction
Edge effect
Reverberation
First Reflector
Second Reflector
Artifacts and Aberrations
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Shadowing
Enhancement
Reverberation
Refraction
Edge effect
Incident
Beam
Reflected
Beam
Medium 1
Medium 2
Transmitted
Beam
Snell’s Law
Artifacts and Aberrations
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Shadowing
Enhancement
Reverberation
Refraction
Edge effect
Edge effect
B
C
R
S
Summary of Ultrasound Physics
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Frequency-”resolution”
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Gain-”volume”
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Focus-”beam adjustment”
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Depth-”field of view”