Basic Physics of Ultrasound

Beth Baughman DuPree M.D. FACS

Medical Director Breast Health Program Holy Redeemer Health System

2011

Financial Disclosures

Faculty/Consultant Ethicon Breast Care Speaker- Myriad Genetics Consultant Precision Therapeutics Faculty- CME at Sea

Breast Ultrasound CertificationStereotactic Biopsy CertificationMastery of Surgery ProgramAPBI Registrywww.breastsurgeons.org

The Changing World of Breast Care

1980 1990 2000 2010

MR Mastectomy BCT DCIS

BCT Lump/ALND XRT DCIS

CHEMO N - SLN BLN

ONCOTYPE

APBI B-39

STEREO MIBB

US MIBB

CONSENSOUS ST

MRI MIBB

OPEN SURGICAL BX

WHOLE BREAST XRT PEM BX

BRCA TESTING

Precision Therapeutics Chemo Fx Assay

Basic Principles

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

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 in the range of 20Hz-20,000Hz

•Sound above the level of human hearing is called ultrasound

Frequency

Diagnostic Ultrasound is measured in mega hertz (MHz)

mega means millions

Imaging transducers are named by their operating frequency

Frequency Range - 2.25 MHz-20 MHz

5 MHz transducer = 5 million cycles/sec.

SoundSource

Incident

Reflected

Transmitted

MEDIUM 1 MEDIUM 2

Acoustic Interface

Reflection

Soft Tissue (1540 m/s) Fat (1459 m/s)

Bone (4080 m/s)Soft Tissue (1540 m/s)

Acoustic interface / Acoustic Mismatch

Getting an Image

The heart of ultrasound is the transducer

Piezo - electric effect

Piezo-Electric Effect

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

Components: Piezoelectric crystal Dampening material Matching layer

covers crystals

Converts electrical energy into sound

7.5 MHz3.5 MHz

Thinner crystal produces smaller sound waves.

The Transducer

Thicker crystal produces bigger sound waves.

The Transducer

The LOWER the frequency the better the penetration

Bigger, Stronger

The HIGHER frequency the less the penetration

Smaller, weaker

3.5 MHz 7.5 MHz

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

Parallel arrangement of the crystals Two-dimensional, rectangular image

Time delay between successive crystal firing can be varied Directing and focusing the beam

B-Mode Ultrasound

Soft Tissue

Soft Tissue

Soft Tissue

Fat

Bone

Cyst

Gray Scale

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.

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.

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

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.

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.

Grayscale Imaging

The more perpendicular the ultrasound beam, the

stronger the returning echo.

Echogenicity

hypoechoic

fat - equivalent

isoechoic

hyperechoic

anechoic

Echogenicity

Anechoic Hypoechoic Isoechoic Hyperechoic

Echogenicity

Anechoic Hypoechoic Isoechoic Hyperechoic

Echogenicity

Anechoic Hypoechoic Isoechoic Hyperechoic

Echogenicity

Anechoic Hypoechoic Isoechoic Hyperechoic

Resolution

Clarity of picture

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.

3.5MHz

7.5 MHz

Axial Resolution

The shorter the pulse, the better

the axial resolution

Increasing the frequency

increases axial resolution

Characteristics of Sound Frequency

Sound

Frequency

Sound

Penetration

Axial

Resolution

High

Low

A transducer with a large surface area will resolve better in the

lateral dimension

Very important for ultrasound guidance with needles/probes

Lateral Resolution

“Fine-tuning” the Image

Gain=Volume

GAIN Control

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

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

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 Depth is patient dependant Depth is transducer

dependant Operator controlled Deep

Increase depth Demonstrate shadowing

Superficial Decrease depth

Image Artifacts

Acoustic Shadowing

Acoustic Enhancement

Used to decide if structures are fluid-filled, solid or a combination.

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

Shadowing Enhancement Reverberation Edge effect

Shadowing

Artifacts and Aberrations

Shadowing Enhancement Reverberation Refraction Edge effect

Posterior enhancementis not proof of a cyst.

Artifacts and Aberrations

Shadowing Enhancement Reverberation Refraction Edge effect

First Reflector

Second Reflector

Reverberation

Artifacts and Aberrations

Shadowing Enhancement Reverberation Refraction Edge effect

IncidentBeam Reflected

Beam

TransmittedBeam

Medium 1Medium 2

Snell’s Law

Artifacts and Aberrations

Shadowing Enhancement Reverberation Refraction Edge effect

S

C

B

R

Edge effect

Summary of Ultrasound Physics

Frequency-”resolution”

Gain-”volume”

Focus-”beam adjustment”

Depth-”field of view”