Physical Principles of Ultrasound

©2000 UIC All Rights Reserved.

Grateful appreciation to Richard A. Lopchinsky, MD, FACS and Nancy H. Van Name, RDMS, RTR, and MarleneKattaron, RDMS

Course Objectives

• Identify history & define ultrasound

• Define piezoelectric effect

• Define frequency & wavelength; identify their relationship

• Define bandwidth

• Define attenuation; identify relationship to frequency

• Define resolution & its components; identify relationship to frequency

• Identify basic transducer types

• Define electronic array

• Differentiate between sector & linear array

• Identify types of image display

• Identify artifacts useful to diagnosis

• Discuss safety of medical ultrasound

History of Ultrasound

• Piezoelectricity discovered by the Curies in

1880 using natural quartz

• SONAR was first used in 1940’s war-time

• Diagnostic Medical applications in use

since late 1950’s

Ultrasound: Physical Definition

• Sound waves greater than 20,000 Hertz or cycles per second

Infrasound Ultrasound

<20 Hz >20,000 Hz

Ultrasound: Medical Definition

• Diagnostic Medical Ultrasound is the use of high frequency sound to aid in the diagnosis and treatment of patients.

• Frequency ranges used in medical ultrasound imaging are 2 - 15 MHz

Piezoelectric Effect

• Definition: The principle of converting energy by applying pressure to a crystal.

• The reverse of the piezoelectric effect converts the energy back to its original form.

Piezoelectric Effect and Ultrasound Transducers

• A transducer converts one type of energy into another.

• Based upon the pulse-echo principle occurring with ultrasound piezoelectric crystals, ultrasound transducers convert:– Electricity into sound = pulse

– Sound into electricity = echo

Pulse

• Pulse of sound is sent to soft tissues

• Sound interaction with soft tissue = bioeffects

• Pulsing is determined by the transducer or probe crystal(s) and is not operator controlled

Echo

• Echo produced by soft tissues

• Tissue interaction with sound = acoustic propagation properties

• Echoes are received by the transducer crystals

• Echoes are interpreted and processed by the ultrasound machine

Frequency

• Number of complete cycles per unit of time

• Man-made transducer frequency is predetermined by design

• Ultrasound transducers are referred to by the operating, resonant or main frequency

Frequency Units

• One cycle per second = one Hertz (Hz)

• One thousand Hertz = One kilohertz (KHz)

• One million Hertz = One megahertz (MHz)

Example: a 7.5 MHz transducer operates at 7,500,000 cycles per second

Wavelength

• Definition: The distance between consecutive cycles of sound.

Transducer frequency

Transducer wavelength

Transducer Frequencies

• 2.5 MHz

• 3.5MHz

• 5.0 MHz

• 7.5 MHz

• 10.0 MHz

• Deep abdomen, OB/Gyn

• General abdomen, OB/Gyn

• Vascular, Breast, Gyn

• Breast, Thyroid

• Breast, Thyroid, Superficial veins, Superficial masses

Bandwidth

• All ultrasound transducers contain a range of frequencies, termed bandwidth

• Broad bandwidth technology produces medical transducers that contain more than one operating frequency, for example:– 2.5 - 3.5 MHz for general abdominal imaging

– 5.0 - 7.5 MHz for superficial imaging

Attenuation

• Definition: The reduction in power and intensity as sound travels through a medium.

Transducer frequency

Depth of penetration

• Higher frequencies attenuate, or are absorbed, faster than lower frequencies

Attenuation

Time Gain Compensation

• Operator controlled adjustment to compensate for the attenuation of sound as it travels into the tissue

• Must be adjusted manually for each tissue type examined and may be manipulated throughout an exam to optimize the image

RESOLUTION

• The ability to differentiate between structures that are closely related, both in terms of space and echo amplitude

• Wavelength (frequency) dependent

– Gray Scale Resolution

– Axial Resolution

– Lateral Resolution

Frequency vs. Resolution

Transducer frequency

Resolution and image detail

• Higher frequency transducers provide better image resolution – better gray scale resolution

– improved ability to distinguish fine detail

Frequency and Resolution

3.5 MHz 7.5 MHz

Gray Scale Resolution

• Adequate gray scale resolution allows for the differentiation of subtle changes in the tissues

• Dynamic Range determines how many shades of gray are demonstrated on an image

Dynamic Range

Decreased DR Increased DR

Axial & Lateral Resolution

• Spatial Resolution describes how physically close two objects can be and displayed separately.

– Axial: along the beam path

– Lateral: perpendicular to beam path

• All current equipment has an overall spatial resolution of 1.0 mm or less.

Frequency Summary

• High frequency– improved

resolution

– depth of penetration loss

– higher frequency transducers for superficial uses

• Low frequency– poorer resolution

– full depth of penetration

– lower frequency transducers for general abdominopelvic uses

Machine Components

Transducer

Beam Former

Receiver

Memory

Display

Transducer Types

• Mechanical– Oscillating

– Rotating

• Electronic– Linear Arrays

– Curved Arrays

– Phased Arrays

Electronic Arrays

• Groups of piezoelectric material working singly or in groups

Transducer

126873 4 5 621

Electronic Transducers

• Sector Array– crystals are placed

parallel or in concentric rings

– transducer face is curved

– produces sector or pie-shaped image

• Linear Array– crystals are placed

parallel

– transducer face is flat

– produces rectangular image

Display Field of View

• Field Of View -- the display of the echo amplitudes

• shape dependent on transducer type and function

Field of View Shapes

• SECTOR FOV

• produced byoscillatingrotatingcurved arraysphased arrays

• typically used in abdominal application

• LINEAR FOV

• produced bylinear arrays

• typically used in superficial application

Sector Linear

Display Modes

• B Mode

• B Color

• M Mode

• D Mode or Doppler– spectral

– audio

– color

• Color/Doppler/PowerAngio -- slow flow

B-Mode M-Mode

Color PowerDoppler Doppler

Duplex and Triplex Imaging

Artifacts

• Portions of the display which are not a “true” representation of the tissue imaged

• Medical Diagnostic Ultrasound imaging utilizes certain artifacts to characterize tissue

Artifacts

• The ability to differentiate solid vs. cystic tissue is the hallmark of ultrasound imaging

• Acoustic Shadowingand Acoustic Enhancementare the two artifacts that provide the most useful diagnostic information

Shadowing

• Diminished sound or loss of sound posterior to a strongly reflecting or strongly attenuating structure– Strong reflectors

• large calcifications, bone

– Strong attenuators • solid tissue, significantly dense or malignant masses

Shadowing

Enhancement

• Increased through transmission of the sound wave posterior to a weakly attenuating structure

• Gain curve expecteda certain loss or attenuation with depth of travel – Occurs posterior to

• simple cysts or weakly attenuating masses

Enhancement

Bioeffects

• Prudent use assures patient safety

• Effects at intensities higher than those used in diagnostic medical ultrasound include:

cavitationsister chromatid exchange

AIUM Statement

• “No confirmed biological effects on patients or operators caused by exposure at intensities typical of diagnostic ultrasound…

• ...current data indicate that the benefits… outweigh the risks.”

Summary

• Ultrasound > 20,000 Hz

• Piezoelectric Effect = pulse-echo principle

• Frequency & wavelength are inversely proportional

• Broad bandwidth enables multihertz probes

• Attenuation & frequency are inversely related

• Resolution determines image clarity

• Electronic Arrays may be sector or linear

• Display mode chosen determines how image is registered

• Shadowing & Enhancement are the artifacts most used in ultrasound diagnosis

• Diagnostic Medical Ultrasound is safe!