Nguyên tắc vật lý của sóng siêu âm Physical Principles of Ultrasound

Ultrasound: Physical Definition• Sound waves greater than 20,000 Hertz or cycles per second Infrasound Ultrasound 20,000 Hz... Ultrasound: Medical Definition• Diagnostic Medical Ultrasou

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

Marlene Kattaron, 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 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 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 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

• 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

• Definition: The distance between consecutive cycles of sound

Transducer frequencyTransducer wavelength

• General abdomen, OB/Gyn

• Vascular, Breast, Gyn

• Breast, Thyroid

• Breast, Thyroid, Superficial veins, Superficial masses

• 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

• 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

• 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 frequencyResolution and image detail

• Higher frequency transducers provide better image resolution

– better gray scale resolution

– improved ability to distinguish fine detail

Frequency and Resolution3.5 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 RangeDecreased 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

– lower frequency transducers for general abdominopelvic uses

2 1

– 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 by

oscillating rotating curved arrays phased arrays

Sector Linear

B-Mode M-Mode

Color Power

Doppler Doppler

Duplex and Triplex Imaging

• Portions of the display

which are not a “true”

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

• Acoustic Shadowing and Acoustic

Enhancement are the two artifacts that

provide the most useful diagnostic

information

• Diminished sound or loss of sound

posterior to a strongly reflecting or strongly attenuating structure

Shadowing

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

structure

• Gain curve expected a certain loss or

attenuation with depth of travel

– Occurs posterior to

• simple cysts or weakly attenuating masses

Enhancement

• Prudent use assures patient safety

• Effects at intensities higher than those used

in diagnostic medical ultrasound include:

cavitation sister 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.”

• 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!