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 Certification Stereotactic Biopsy Certification Mastery of Surgery Program APBI Registry

The Changing World of Breast Care 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 XRTPEM 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 MHz-20 MHz 5 MHz transducer = 5 million cycles/sec.

Sound Source Incident Reflected Transmitted MEDIUM 1MEDIUM 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 MHz 3.5 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 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 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 gainToo 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 enhancement is 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 Incident Beam Reflected Beam Transmitted Beam Medium 1 Medium 2 Snell’s Law

Artifacts and Aberrations Shadowing Enhancement Reverberation Refraction Edge effect

S C B R

Summary of Ultrasound Physics Frequency-”resolution” Gain-”volume” Focus-”beam adjustment” Depth-”field of view”