1.
Physical Principles of Ultrasound
Grateful appreciation to Richard A. Lopchinsky, MD,
FACS and Nancy H. Van Name, RDMS, RTR, and
Marlene Kattaron, RDMS
©2000 UIC All Rights Reserved.

2.
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

3.
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

4.
Ultrasound: Physical Definition
• Sound waves greater than 20,000 Hertz or
cycles per second
Infrasound Ultrasound
<20 Hz >20,000 Hz

5.
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

6.
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.

7.
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

8.
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

9.
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

10.
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

11.
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

12.
Wavelength
• Definition: The distance between
consecutive cycles of sound.
Transducer frequency
Transducer wavelength

13.
Transducer Frequencies
• 2.5 MHz • Deep abdomen,
OB/Gyn
• 3.5MHz • General abdomen,
OB/Gyn
• 5.0 MHz • Vascular, Breast, Gyn
• 7.5 MHz • Breast, Thyroid
• 10.0 MHz • Breast, Thyroid,
Superficial veins,
Superficial masses

14.
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

15.
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

16.
Attenuation

17.
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

18.
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

19.
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

20.
Frequency and Resolution
3.5 MHz 7.5 MHz

21.
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

22.
Dynamic Range
Decreased DR Increased DR

23.
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.

24.
Frequency Summary
• High frequency • Low frequency
– improved – poorer resolution
resolution
– depth of penetration – full depth of
loss penetration
– higher frequency – lower frequency
transducers for transducers for general
superficial uses abdominopelvic uses

25.
Machine Components
Transducer
Beam Former
Receiver
Memory
Display

26.
Transducer Types
• Mechanical • Electronic
– Oscillating – Linear Arrays
– Rotating – Curved Arrays
– Phased Arrays

27.
Electronic Arrays
• Groups of piezoelectric material working
singly or in groups
Transducer
1 2 3 4 5 6 7 8 126

28.
Electronic Transducers
• Sector Array • Linear Array
– crystals are placed – crystals are placed
parallel or in parallel
concentric rings
– transducer face is – transducer face is
curved flat
– produces sector or – produces
pie-shaped image rectangular image

29.
Display Field of View
• Field Of View -- the display of the echo
amplitudes
• shape dependent on transducer type and
function

30.
Field of View Shapes
• SECTOR FOV • LINEAR FOV
• produced by • produced by
oscillating linear arrays
rotating
curved arrays
phased arrays
• typically used in • typically used in
abdominal application superficial application

31.
Sector Linear

32.
Display Modes
• B Mode
• B Color
• M Mode
• D Mode or Doppler
– spectral
– audio
– color
• Color/Doppler/PowerAngio -- slow flow

33.
B-Mode M-Mode

34.
Color Power
Doppler Doppler

35.
Duplex and Triplex Imaging

36.
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

37.
Artifacts
• 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

38.
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

39.
Shadowing

40.
Enhancement
• 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

41.
Enhancement

42.
Bioeffects
• Prudent use assures patient safety
• Effects at intensities higher than those used
in diagnostic medical ultrasound include:
cavitation
sister chromatid exchange

43.
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.”

44.
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!