INTERACTIONS OF ULTRASOUND WITH TISSUE Flashcards
Interactions of Ultrasound
Types of Sound
- _____
- _____
- _____
Types of Incidence
- _____
- _____
Types of Reflection
- _____
- _____
- _____
Types of Interactions
- _____
- _____
- _____
- _____ and _____
- _____
- _____
- _____
Echo Ranging
incident reflected transmitted perpendicular oblique specular diffuse scattering reflection reflectivity refraction divergence diffraction interference attenuation absorption
TYPES OF INCIDENCE – PERPENDICULAR AND OBLIQUE
Incident sound may intersect a boundary either _____ or _____
perpendicularly
obliquely
PERPENDICULAR INCIDENCE
A.K.A.: NORMAL INCIDENCE
When a sound beam (incident sound) intersects a smooth surface (the boundary between two tissues), larger than the width of the beam, at 900 (perpendicular), it may be partially _____ towards the sound source.
If reflection occurs, the angle of reflection equals the angle of _____
reflected
incidence
A sound beam (incident sound) that intersects a smooth surface (the boundary between two tissues), larger than the width of the beam, at LESS THAN 900 (NOT perpendicular) is called:
_____ Incidence
For our purposes always
assume:
there will be \_\_\_\_\_;
also,
the angle of Reflection
equals
the angle of \_\_\_\_\_
oblique
reflection
incidence
To obtain maximum detection of the reflected signal, we must orient the transducer so the generated sound beam will strike an interface _____
perpendicularly
Types of Reflection
- _____
- _____
- _____
specular
diffuse
scattering
SPECULAR REFLECTORS
_____ interfaces, _____ than the beam width
Responsible for major organ outlines as seen by US; _____, _____
Produce _____-intensity, unidirectional reflections
Very _____ dependant
smooth larger diaphragm pericardium high angle
DIFFUSE REFLECTION
Large _____-surface interface deflects beam in multiple directions (as opposed to specular = large smooth-surface)
If interface not _____, sound beams strike interface at various angles of incidence yeilding differing angles of reflection
Some categorize as _____ or nonspecular
_____ the echo returning to the transducer
Mirror/steam; concrete/aggregate
rough
flat
scattering
weakens
SCATTERING
Often referred to as _____ reflection
Occurs because the interfaces are _____ (less than several wavelengths across)
Each interface acts as a separate _____ source and these tiny, punctate interfaces reflect sound in all direction
Responsible for providing the _____ texture of organs in an image
nonspecular
small
sound
internal
SCATTERING
Nonspecular reflectors are very _____ dependent, which makes them useful in characterizing tissue
Responsible for providing the _____ texture of organs in an image
Scattering by small particles in which the linear dimensions are smaller than the wavelength is called _____ scattering, a classic example, RBCs
Some physicists include _____ reflection (due to the roughened surface) in the category of scattering
frequency
internal
rayleigh
diffuse
Types of Interactions
- _____
- _____
- _____
- _____ and _____
- _____
- _____
- _____
reflection reflectivity refraction divergence diffraction interference attenuation absorption
Reflection
Acoustic _____
Impedance _____
_____ Coefficient
_____ Coefficient
_____ Composition
impedance mismatch reflection transmission interface
ACOUSTIC IMPEDANCE
The measure of _____ to sound traveling through a medium
Unit = kg/m2/s or simply, the _____
Impedance = _____ (kg/m3) x _____ _____ (m/s)
Z = pc
Determined by the _____
RESISTANCE Rayl Density Prop Speed MEDIUM
ACOUSTIC IMPEDANCE
Density increase - Impedance _____
Prop. Speed increase - Impedance _____
Impedance is _____ PROPORTIONAL to:
Density and/or Propagation Speed
increase
increase
directly
IMPEDANCE MISMATCH
In Perpendicular Incidence the amount of sound reflected depends on:
Incident _____
AND
Impedance _____ of the 2 media
Does not apply to _____ incidence!!!!!!!!!!
intensity
Mismatch
oblique
PIMP OR PRIMP
PIMP
P = _____ Incidence
IMP = _____
PRIMP
P = _____ Incidence
R = _____
IMP = _____
When an incident beam intersects a media boundary Perpendicularly, Reflection will only occur if there is an acoustic IMPedance _____
Perpendicular Impedance Perpendicular Reflection Impedance mismatch
IMPEDANCE MISMATCH
No impedance difference - No _____
decrease in impedance difference - _____ in reflection
The difference in acoustic impedance causes some portion of the sound to be reflected at an interface, which allows visualization of _____ structures with US.
reflection
decrease
ST
DETERMINING THE AMOUNT OF INCIDENT INTENSITY THAT IS REFLECTED and TRANSMITTED
Expressed BY:
_____ -or- _____
Coefficient
Percentage
DETERMINING THE AMOUNT OF INCIDENT INTENSITY THAT IS REFLECTED and TRANSMITTED
Coefficient
Intensity reflection coefficient IRC
+ Intensity transmission coefficient + ITC
1.0 1.0
Equation
DETERMINING THE AMOUNT OF INCIDENT INTENSITY THAT IS REFLECTED and TRANSMITTED
Percentage
Percent intensity reflected % IR
+ Percent intensity transmitted + % IT
100 % 100%
Equation
INTERFACE COMPOSITION
Doesn’t matter of impedance 1 is greater or less than impedance 2; the same percent of reflection/transmission occurs at the interface
_____ differences in impedences yield large magnitudes of reflection
_____ differences in impedences yield small magnitudes of reflection
The _____ of the medium is not as issue!
large
small
thickness
ECHO-INDUCED SIGNALS
When a device that produces and detects US waves scans a patient, multiple interfaces are encountered
A percent of the beam is _____ and _____ at each interface
A series of echoes is subsequently detected
The relative intensities of the echoes depends on the acoustic impedance mismatch at the interface
- –_____ signals if the acoustic impedance mismatch is small (ST to ST)
- –_____ signals if the acoustic impedance mismatch is large ( air-ST)
- –_____ echoes are produced by diffuse reflection and scattering
reflected transmitted small strong weaker
REFLECTIVITY
The fraction of incident intensity that is reflected at an interface back towards the transducer is influenced by many factors.
Factors include:
- –Acoustic impedance _____
- –_____ of incidence
- –_____ of the structure as compared to the, wavelength
- –_____ of the structure
- –_____ of the surface of the interface
mismatch angle size shape texture
REFLECTIVITY
The combination of these factors is described by the term:
Reflectivity
Differences in reflectivity are partially responsible for patient-to-_____ variations encountered
patient
REFRACTION
When incident sound intersects a media interface of 900, a percent is reflected back through media 1, and a percent is transmitted into media 2 without a change in _____
direction
REFRACTION
When incident sound intersects an interface between two media at an angle _____ (other than 900), the transmitted part
MAY be
_____ or bent
obliquely
refracted
Refraction and Oblique Incidence
If US intersects an interface between two media at an angle other than 900
A portion of the incident sound will be:
_____ away from the boundary
and
_____ through the boundary
Assume it; no ands, ifs, buts or impedance matches!!!
reflected
transmitted
Refraction
The Transmitted:
the angle of transmission MAY _____ the angle of incidence
-or-
the angle of transmission may be _____
(a change in the direction of sound when crossing a boundary)
equal
refracted
SNELL’S LAW
Degree of Refraction is defined by _____ Law
Relates the angle of transmission to the relative PROPAGATION _____ of the 2 media NOT based on acoustic impedance mismatch!!!
snell’s
speeds
SOS OR ROSS
SOS
S = _____
O = _____ Incidence
S = _____ Law
ROSS R = \_\_\_\_\_ O = \_\_\_\_\_ Incidence S = \_\_\_\_\_ S = \_\_\_\_\_ Law
When an incident beam intersects a media boundary at an Refraction will only occur if there is a Oblique angle and a Speed mismatch. The degree of Refraction is defined by _____ Law
speed oblique snell's refraction oblique speed snell's snell's
SNELL’S LAW
sin θi /sin θt = c1 /c2
— OR —
to solve for what we care about
sin θt = (sin θi) c2 /c1
SNELL’S LAW EQUATION
REFRACTION
For refraction to occur there MUST be both:
_____ incidence
AND
propagation _____ differences
between the media
oblique
speed
- If the velocities of 2 media are the same there will be
NO _____!!!
If perpendicular incidence occurs there will be
NO _____!!!
refraction
refraction
The bending occurs because the portion of the wavefront in the 2nd medium travels at a different _____ from the 1st medium
Three scenarios regarding Snell’s Law
velocity
SPEED IN MEDIA 1 IS > (Greater than) MEDIA 2
Then the transmitted (refracted) angle bends _____ the normal
-OR-
If the speed in media 1 is greater than the speed in media 2, then the angle in media 1 is _____ than the angle in media 2
toward
greater
SPEED IN MEDIA 1 IS < (Less than) MEDIA 2
Then the transmitted (refracted) angle bends _____ from the normal
-OR-
If the speed in media 1 is less than the speed in media 2, then the angle in media 1 is _____ than the angle in media 2
away
less
Speed in media 1 is < (Less than) media 2
-AND-
the angle of incidence is _____ than the
CRITICAL ANGLE
The refracted angle will travel along the _____ and no energy enters the second medium
greater
interface
The critical angle is different between any two media and is determined by _____ Law
_____ artifacts / _____ artifacts
snell’s
edge
refraction
DIVERGENCE and DIFFRACTION
_____ causes the US beam to diverge or spread out as it moves further away from the sound source.
Rate of divergence increases:
- As the distance from sound source _____
- As the diameter of sound source _____
- Frequency of the SS
Beam divergence affects the _____ resolution of the beam and the sensitivity of the US system.
_____ occurs after a beam passes through a small aperture on the order of one wavelength. The aperture then acts as a small SS and the beam diverges rapidly
divergence increases decreases lateral diffraction
INTERFERENCE
Phase of a Wave
Locations along a sine (sinusoidal) wave can be expressed in _____.
One complete wavelength is 3600; one half wavelength will be 1800; one quarter wavelength, 900.
If two or more waves with the same frequency have the same starting points, they are said to be “in _____”.
Whether in or out of phase, superposing these waves will result in the _____ summation of the individual waves.
degrees
phase
algebraic
This is known as the
PRINCIPLE OF _____
-or-
_____ PHENOMENA
superposition
interference
CONSTRUCTIVE INTERFERENCE
Results from the superposition of two waves of the same _____, exactly in phase.
The sum of the waves _____ the amplitude of the resultant waveform.
frequency
increases
DESTRUCTIVE INTERFERENCE
Results from the superposition of two waves of the same frequency, that are _____ of phase.
The summation of these waves _____ the amplitude of the resultant waveform.
The superposition of two waves of the same frequency, that are 1800 out of phase results in waveform of _____
out
decreases
zero
The addition of two waves with different frequencies results in complex waveforms that are no longer _____.
Important in the design of an US transducer because it affects the _____ of the beam.
Focusing of the US beam is based on the principle of _____ interference.
sinusiodal
uniformity
wave
SPECKLE
US pulse will simultaneously encounter many scatterers at any point ⇨ generates several echoes at once ⇨ arrive at transducer at same time and may interfere with each other _____ or _____
Results in a displayed “dot” pattern that does not actually represent the scatterers (does not exhibit a one-to-one correspondence to scatters), but an interference pattern called _____.
The speckle pattern is _____ dependant
Speckle is a form of acoustic _____ in US imaging
constructively destructively speckle frequency noise
CONTRAST AGENTS 1.0
Liquid suspensions injected into circulation to _____ echogenicity of vessels and perfused tissue in grey-scale US
increase
CONTRAST AGENTS – 1.0
Small enough to pass through capillaries
Impedance of suspended particles differs from the impedance of the suspending _____
Microbubble especially _____ echoes
Impedance of _____
Bubbles expand and contract producing _____ of the incident sound
medium
strong
gas
harmonics
ATTENUATION
The _____ in the intensity (amplitude) of an US beam as it travels through a medium; the _____ of sound as it propagates
Encompasses _____, _____, and _____
Generation of echoes from reflection and scattering are crucial to US imaging; contribute little to overall _____
The depth of penetration becomes less as frequency is _____, and the ability to observe deep-lying structures is forfeited
Attenuation limits imaging _____ and must be compensated for
reduction weakening absorption scattering reflection attenuation increased depth
ATTENUATION
Absorption
The only process whereby sound energy is dissipated in a medium.
Absorption (conversion of sound to _____) is normally the dominant contribution to _____ (in ST)
Other modes of interactions (_____, _____, scattering, and _____) decrease beam intensity by redirecting its energy
Strongly dependent on _____; rate of absorption is directly related, if frequency _____, absorption _____
heat attenuation reflection refraction scattering divergence frequency doubles doubles
ATTENUATION
Attenuation and Decibels
_____ are good units for comparing relationships between various measured sound levels and the threshold of human hearing
Used for measuring _____, _____ range and _____
Decibels involve _____
Log of a number = number of tens that must be multiplied together to result in that number
decibels output dynamic gain logarithms
ATTENUATION
Attenuation and Decibels
Level (dB) = 10 log10 ( I/I0 )
I = intensity at point of interest I0 = original or reference intensity
_____ dB of attenuation = decrease to ½ the original intensity
_____ dB of attenuation = decrease to ¼ the original intensity
_____ dB of attenuation = decrease to 1/10, or 0.1 the original intensity
_____ dB of attenuation = decrease to 1/100, or 0.01 the original intensity
3
6
10
20
ATTENUATION
Attenuation Units = Decibels = dB
for ST:
½ x frequency(MHz) x pathlength (cm)
-or-
attenuation coefficient x pathlength
As frequency 🡹, attenuation _____
As pathlength 🡹 , attenuation _____
As attenuation coefficient 🡹 , attenuation _____
increases
increases
increases
ATTENUATION
SO WHAT’S AN ATTENUATION COEFFICIENT?
AC ( dB/cm ) = Attenuation (dB)/Separation between two points (cm)
In ST:
AC( dB/cm ) = 0.5 X frequency
The attenuation per unit _____ of sound travel
The attenuation for each _____ of sound travel
Numerical values that express how different materials will attenuate an US beam per _____ length
Different materials have different _____ coefficients
length
cm
path
attenuation
ATTENUATION
In ST: for a 1 MHz transducer, ½ dB of intensity is lost for 1 cm of travel
Half-Value Layer (HVL)
The amount of material required to reduced the intensity by _____ of its original valve
A half-value layer results in a ___-dB reduction in intensity
half
3
ATTENUATION
Attenuation / Intensity Loss
_____ frequency sound waves are attenuated more rapidly than low frequency
Ability to penetrate tissue is reduced at _____ frequencies
Reflectors positioned at increasingly greater depth generate progressively _____ intensity returning echoes
high
higher
lower
ECHO-RANGING PRINCIPLE
In diagnostic US, _____ of the sound beam from interfaces along the US beam path are of primary interest.
US wave ⇒ body ⇒ strikes interface ⇒ reflects/transmit
reflections
A system that can generate an US pulse wave and detect the reflected echo after a measured time permits the distance to the interface to be determined.
This is called a _____ system, and the design is based on the
_____-_____ PRINCIPLE
pulsed
echo - ranging
Echo-Ranging Principle
2 items of info required to properly place echoes on display:
_____ from which the echo came (assumed as a straight line from where the transducer is pointed)
_____ to the reflector where the echo was produced
Distance to the reflector if defined by the
_____ Equation
direction
distance
range
