This document discusses the physical principles of ultrasound used in medical imaging. It defines key terms like frequency, wavelength, attenuation and resolution. It describes how piezoelectric transducers convert electrical pulses to ultrasound pulses and echoes. It explains how sector and linear array transducers work and the different display modes. It also discusses artifacts and the safety of diagnostic medical ultrasound.
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
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
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
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
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
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
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
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
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
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!
