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ULTRASOUND easy guide

HISTORY OF ULTRASOUND
• 1918 - Sound Navigation and Ranging was used
• Early 1950’s –Water bath immersion technique
• Late 1950’s – First contact compound B-scanner
• 1970’s – Gray scale imaging
• Mid 1970’s – Real time scanning systems
• 1980’s – Doppler technique
SOUND
• A mechanical energy
• Requires a vibrating object to produced
• Cannot travel through a vacuum
ULTRASOUND
• High frequency sound waves
• Above 20,000 cycles per second (20 kHz)
• Inaudible to humans
• Used to scan tissues of the body
• Ultrasound Pulse: 2-10 MHz
• Pulse Duration: 1 microsecond
• Pulse Repetition: 1000 times/second
AUDIBLE SOUND
• 16 Hz to 20,000 Hz
INFRASOUND
• Below 16 Hz
ULTRASOUND GENERATORS
TYPES OF ULTRASOUND WAVES
• Longitudinal/Compression Waves
• Transverse/Shear Waves
• Surface/Rayleigh Waves
ACOUSTIC VARIABLES
• Period (T): the time taken for one complete cycle to occur (s or µs)
• Wavelength (λ): length of space over which one cycle occurs (m or mm)
• Amplitude (Depth): the maximum displacement that occurs in an acoustic variable

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Frequency: cycle per second (Hz)
Velocity: frequency times wavelength

PIEZOELECTRIC CRYSTALS
• Generate ultrasound waves
• Capable
of
changing
(ultrasound) waves

electrical

signals

into

mechanical

PIEZOELECTRIC CRYSTAL AS TRANSMITTER OF SOUND
• Converting electrical energy into mechanical energy (sound)
PIEZOELECTRIC CRYSTAL AS TRANSMITTER OF SOUND
• Converting mechanical energy (sound) into electrical energy
NOTE:
• Small crystal diameter
o Increased beam divergence
• Larger crystal diameter
o Decreased beam divergence
DIFFERENT MODES OF ULTRASOUND
A-MODE
• Echoes are shows as peaks
• Distance between various structures can be measured
• Used to build two-dimensional B-mode image
B-MODE
• Two-dimensional images in which the echo amplitude is depicted as dots of different
brightness
REAL-TIME
• Shows movement as it occurs
M-MODE
• Shows movement as a function of time
• Used in cardiac scanning
DOPPLER ULTRASOUND
• Demonstrates and measures blood flow
DOPPLER EFFECT

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The change in apparent frequency of a wave as a result of relative motion between the
observer and the source
Stationary Reflector: reflected echoes are the same as the transmitted waves
Reflector that Moves Closer: reflected echoes are higher than the transmitted echoes
Reflector that Moves Away: reflected echoes are lower than the transmitted echoes
BASIC TYPES OF DOPPLER ULTRASOUND UNIT

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) Pulsed Wave Doppler Unit
• Ultrasound is transmitted in pulses
• With good depth resolution
• Measures the speed of the blood in a particular vessel
• Cannot measure high blood velocities in deep vessels
• High velocities may be wrongly displayed as low velocities
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) Duplex Doppler System
• Combination of a B-mode and Doppler system
• Allows the Doppler beam to be directed accurately at any particular blood vessels
WAVE PROPAGATION
• The transmission and spread of ultrasound waves to different tissues
• Average Propagation for Soft Tissues: 1540 m/s
• Average Propagation for Soft Tissues: 4620 m/s

WAVELENGTH
• The length of a single cycle of the ultrasound wave
• Inversely proportional to the frequency
• Determines the resolution of the scanner
• Higher the frequency, the shorter the wavelength
FOCUSING
• Adjustment of the ultrasound beam
• To improve resolution
• May be electronic or by a lens attached to the transducer

AMPLIFICATION
• Done by the time-gain-compensation (TGC) amplier
• To compensate for ultrasound attenuation in any part of the body
• To improve the quality of the final image
BOUNDARIES
• The line at the periphery of two tissues which propagate ultrasound differently
• The zone of echoes at the interface

PIEZOELECTRIC EFFECT
• Piezein – “press or pressure”
• Ability of a material to generate an electrical charge un response to applied pressure
PIEZOELECTRIC MATERIALS
• Crystalline materials composed of dipolar molecules
• Quartz – naturally occurring crystals
• Lead zirconate titanate – man made ceramic
• Natural Materials:
o Quartz
o Tourmaline
o Rochelle Salt
• Synthetic Materials:
o Lead zirconate titanate (PZT)
o Barium titanate
o Lead metaniobate
o Ammonium dihydrogen phosphate
o Lithium sulphate
ACOUSTIC IMPEDANCE
• Property of a substance
• Describes how the particles of that substance behave when subjected to pressure wave
• High density substance – high acoustic impedance
• Low density substance – low acoustic impedance
• Formula: Z=pc
o p = density of material (kg/m3)
o c = speed of sound (m/s)
o Z = acoustic impedance (rayls)
ACOUSTIC IMPEDANCE AND REFLECTION
• Substances with same acoustic impedance:
o 100% energy transmission
o No reflection

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Substances with a small difference in acoustic impedance:
o 95% energy transmission
o 5% reflection
Substances with a large difference in acoustic impedance:
o 1% energy transmission
o 99% reflection

TRANSDUCER/PROBE
• A device which converts one form of energy to another
• Converts electrical energy into ultrasound waves and vice versa
• Contains piezoelectric crystals
o Transmit ultrasound beam
o Receive reflected echoes
TRANSDUCERS/SCANNING PROBES
• The most expensive part of any ultrasound unit
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) Sector/Curvilinear Array Transducer
• Provides wide field of view
• Most useful in abdominal and obstetric scanning
• Best suited to image deep lying structures
• 3
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) Convex Transducer
• Wide fan-shaped
• Useful for all parts of the body
• Except for specialized echocardiography
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) PHYSICAL HOUSING
• Contains all individual components

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Provides the necessary structural support
Acts as an electrical and acoustic insulator

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) PIEZOELECTRIC ELEMENTS
• Crystalline minerals that generate voltages when subjected to a mechanical force
• Piezein – “to press or squeeze”
• Piezoelectric Effect – discovered by Jacques and Pierre Curie
• Thinner Piezoelectric Materials
o Higher resonant frequencies
FREQUENCY
• Affects the quality the ultrasound image
• Higher Frequency
o Shorter wavelength
o Better Resolution
o Lower Penetration
o Higher Absorption
• Lower Frequency
o Longer wavelength
o Poor Resolution
o Higher Penetration
o Lower Absorption
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) ACOUSTIC LENS
• Reduce the beam width of the transducer
• Improve image resolution
• Width of the Beam: determines lateral resolution
• Lateral Resolution: the ability to resolve structure across or perpendicular to the beam
axis

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Materials:
o Aluminum
o Perspex
o Polystyrene

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) Obstetric Ultrasound
• Linear or convex transducer
• 3
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0 MHz: best during early pregnancy
• Focused at 7-9 cm

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5 MHz
• Focused at 7-9 cm

3
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0 MHz transducer: for children
• Focused at 5-7 cm
• Sector transducer of 7 MHz:
o Neonatal brain scans
o For adult testis and neck
• Focused at 4-5 cm

ULTRASOUND BEAM
• Area through which the sound energy emitted from the ultrasound transducer
• Three dimensional and symmetrical around its central axis
TWO REGIONS OF ULTRASOUND BEAM
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) Far Field/Fraunhofer zone
• Increasing Frequency
o Longer near field
o Less far field divergence
• Narrow Crystal Diameter
o Narrower near field
o More far field divergence
• Thin Crystal
o Decreased near field
o Increased far field
• Thick Crystal
o Increased near field
o Decreased far field
BEAM INTENSITY
• The power (measured in watts) flowing through a unit area
SIDE LOBES/GRATING LOBES
• Lobes at various angles to the main beam
• Approximately 15% of the energy in the beam
• Cause a degradation of lateral resolution
BEAM WIDTH
• The dimension of the beam in the scan plane
• Affects the spatial resolution
• Narrow Beam Width
o Better spatial resolution
SLICE THICKNESS
• Three dimensional volume displayed as a two dimensional image
RESOLUTION
• The ability of an imaging system to differentiate between structures
• Spatial Resolution: resolution in space
• Contrast Resolution: resolution of gray shades
• Temporal Resolution: resolution in time
SPATIAL RESOLUTION

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Detail Resolution
The ability to display two structures situated close together as separate images
Higher Frequency:
o Better resolution
o Lower penetrability
o Higher absorption
Lower Frequency:
o Poor resolution
o Higher penetrability
o Lower absorption
TWO COMPONENTS OF SPATIAL RESOLUTION

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) LATERAL RESOLUTION
• Azimuthal, Transverse, Angular or Horizontal
• The ability to distinguish two objects perpendicular to the ultrasound beam
• Depends upon the beam diameter
• Smaller Beam Width: better lateral resolution
• Larger Beam Width: poor lateral resolution
CONTRAST RESOLUTION
• The ability of the imaging system to differentiate between body tissue and display them
as different shades of gray
• Optimized by using the correct overall gain
TEMPORAL RESOLUTION
• Frame Rate
• The ability of the imaging system to display events which occurs at different times as
separated images
• Higher Frame Rate: better temporal resolution
ULTRASOUND INTERACTIONS AND ATTENUATIONS
ATTENUATION
• Decrease in the intensity and amplitude of the ultrasound waves as they pass through
tissues

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Unit: decibels per centimeter
FIVE MAIN PROCESSES THAT CAUSE ATTENUATIONS

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) REFLECTION
• Occurs when two large structure of significantly different acoustic impedance form an
interface
• Occurs when a sound wave strikes an object that is larger than the wavelength
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) REFRACTION
• Occurs when the beam encounters an interface between two different tissues at an
oblique angle
• The beam will be deviated as it travels through the tissue
• Occurs due to difference in wave velocity across an interface between two materials
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) REVERBERATION
• Comet tail
• The production of spurious or false echoes due to repeated reflections between two
interfaces with a high acoustic impedance mismatch
• The presence of two or more strong reflecting surfaces
• Often occur at:

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o Skin-transducer interface
o Gas surface and transducer
Prevention/Elimination:
o Increase the amount of gel used
o Used a stand-off gel pad
o Reduce the gain
o Move the position of the transducer

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) ACOUSTIC ENHANCEMENT
• Caused by weakly attenuating structures
•

Often occur at:
o Distal to fluid-filled urinary bladder, gallbladder or cyst
o Fluid-filled mass

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) BEAM WIDTH ARTIFACT
• Variations of all echoes returning to the transducer
• Prevention/Elimination:
o Correct positioning of the focal zone
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) SIDE LOBE ARTIFACT
• Echoes generated by side lobes assumed by the transducer to have arisen form the
central axis of the main lobe
• Appearance can give rise to a false diagnosis
• Inherent characteristic of the transducer

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) DOUBLE IMAGE ARTIFACT
• Caused by refraction of the beam
• Often occur at:
o Rectus abdominis muscle
• Prevention/Elimination:
o Move the transducer slightly to one side to avoid the junction of rectus abdominis
muscle
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