ULTRASOUND COPY Flashcards
mechanical energy that propagates thru a continuous elastic medium by the compression and rarefaction of particles
Sound
energy propagation occurs as a wavefront in the direction of energy travel
longitudinal wave
distance (mm) bet compressions and rarefactions or bet any 2 points that repeat on the sinusoidal wave of pressure amplitude
wavelength (λ)
of times the wave oscillates thru one cycle each second
frequency (f)
infrasound Hz
<15 Hz
Audible acoustic spectrume Hz
15-20kHz
Ultrasound Hz
> 20 kHz
Medical Ultrasound Hz
2-100 mHz
occurs at tissue boundaries where there is a difference in acoustic impedance of adjacent materials
Reflection
describes the change in direction of the transmitted US energy w/ non perpendicular incidence
Refraction
occurs by reflection or refraction usually by small particles w/in the tissue medium
Scattering
gives rise to the characteristic texture and gray-scale in the image
Scattering
: loss of intensity of beam from absorption and scattering in the medium
Attenuation
process whereby acoustic energy is converted to heat energy –> sound energy is lost and cannot be recovered
Absortpion
Can be likened to the stiffness and flexibility of a compressible medium
Acoustic impedance (z)
Acoustic impedance (z) unit
Rayl
Large difference in acoustic impedance results to
Large Reflection
means for producing an image using pulse echo technique
Acoustic impedance (z)
Type of Transducer
converts electrical energy to mechanical energy by physical deformation of crystal structure
Piezoelectric materials
Type of Transducer
mechanical pressure applied to its surface creates electrical energy
Piezoelectric materials
Type of Transducer
characterized by a well defined molecular arrangement of electrical dipoles
Piezoelectric materials
molecular entities containing (+) and (-) electrical charge w/ overall neutral charge
Electric dipole
Piezoelectric materials are often mad of ?
PZT
Plumbum
Zicornate
Titanate
Type of Transducer
voltage is applied to PZT => PZT initially contract then subsequently vibrates at a natural resonance frequency
Resonance Transducer
higher frequencies are achieved w/ thinner/thicker? elements and lower frequencies w/ thinner/thicker? elements
high frequency - thinner
low frequency - thicker
layered on the back of the PZT and absorbs the backward directed US energy and attenuates stray US signals from the housing
Damping block
creates an US pulse w/ a short spatial pulse length necessary to preserve detail along the beam axis (axial resolution)
Damping block
bandwidth of sound from a transducer
Q factor
Q factor: narrow bandwidth, long SPL
High Q transducer
Q factor: broad bandwidth, short SPL
Low Q transducer
requires a relatively narrow band transducer response to preserve velocity information encoded by changes in the echo frequency relative to the incident frequency
Doppler
Provides the interface bet the raw transducer element and the tissue
Minimizes acoustic impedance difference bet the transducer and the patient
Consists of layers of materials w/ acoustic impedance intermediate to soft tissue and transducer material
Matching layer
is used to eliminate air pockets w/c could attenuate and reflect sound beam
Acoustic coupling gel (acoustic impedance is similar to soft tissue)
Center of frequency can be adjusted in the transmit mode
Piezo element is machined into a large # of small rods and filled w/epoxy resin for smooth surface
Provides greater transmission efficiency w/o multiple matching layers because its acoustic properties are closer to tissue
Non-resonance (broad bandwidth) multifrequency transducer
Sound pulse can be produced at low frequency and echoes received at higher frequency
HARMONIC IMAGING
Greater depth of penetration
Noise and clutter removal
Improved lateral spatial resolution
Native tissue harmonic imaging
Rectangular FOV is produced
Linear arrays
trapezoidal FOV is produced
Curvilinear array
occurs by firing another group of transducer elements
Subsequent A line acquisition
Echoes are detected in the receive mode by acquiring signals from most of the transducer elements
Linear arrays
Simultaneous firing of approx 20 adjacent elements produce the beam adjacent elements produce a synthetic aperture
Linear arrays
256-512 elements – largest
Linear arrays
64-128 elements – smaller
Phased arrays
All elements are activated nearly simultaneously to produce a single beam
Phased arrays
Beam can be steered electronically w/o physically moving the transducer
Phased arrays
All elements detect the returning echoes
Phased arrays
US beams exhibit 2 distinct patterns:
Near field
Far field
Slightly converging beam out to a distance determined by geometry and frequency of the transducer
Near field
Fresnel zone
Near field
Converging
Near field
Occurs due to multiple constructive/destructive interference patterns of US waves from the transducer surface causes the beam profile to be tightly collimated
Near field
Near field length is dependent on
Transducer diameter
Propagation wavelength (thus, frequency)
NFL increases if frequency and diameter are increased
NFL (near field length) increases if __ and __ are increased
frequency and diameter
NEAR FIELD
___ is dependent on the beam diameter and is best at the end of the near field for a single element transducer
Lateral resolution
Ability of the system to resolve objects in a direction perpendicular to the beam direction
Poor in areas close to and far from the transducer surface
Lateral resolution
occurs at the end of the near field
Peak US pressure
Beam is divergent
Far Field
Fraunhofer zone
Far field
US intensity decreases monotonically with distance
Far field
the major factor that limits spatial resolution and visibility of detail is the volume of the acoustic pulse
Spatial resolution
Linear, range, longitudinal or depth resolution
Axial resolution
Ability to discern 2 closely spaced objects in the direction of the beam
Requires that returning echoes be distinct without overlapping
Axial resolution
aka azimuthal resolution
Lateral resolution
Ability to discern as separate 2 closely spaced objects perpendicular to beam direction
Lateral Resolution
Determines lateral resolution
Beam diamter
Depth dependent resolution
Lateral resolution
Far field, beam diverges substantially reduced ____ resolution
Lateral resolution
____ –> improved overall in focus lateral resolution w/ depth –> decrease frame rate
increase # of focal zone
Resolution perpendicular to image plane
Elevational resolution
dependent on transducer element height
Elevational resolution
Display of processed info from the receiver vs time
A mode
Seldom used (ophtha applications for precise distance measurements of the eye)
A mode
Electronic conversion of A mode and A line info into brightness-modulated dots along the A line trajectory
B mode
Brightness of dot is proportional to echo signal amplitude
Used for M mode and 2D gray scale imaging
B mode
Uses B mode info to display echoes from a moving organ
M mode
Provides excellent temporal resolution of motion patterns
M mode
difference between incident and reflected frequency
Doppler shift
Proportional to velocity of the blood cells
Doppler shift
angle bet direction of blood flow and direction of sound
Doppler angle
Without correction, Doppler shift will be less and there will be an underestimate of ___
blood velocity
Doppler Angle: measured Doppler frequency is ½ the actual
> 60 degrees
Doppler angle: measured frequency is 0
At 90 degrees
preferred doppler angle
30-60 degrees
apparent Doppler shift is small
> 60 degrees
Doppler angle
refraction and critical angle interactions can cause problems as can aliasing
< 20 degrees
Provides a 2D visual display of moving blood in the vasculature superimposed upon the conventional gray-scale image
Color flow imaging
determines the processing time necessary to evaluate the color flow data
FOV
Smaller/bigger? FOV delivers a faster frame rate but sacrifices area evaluated
smaller
Error caused by insufficient sampling rate relative to the high frequency Doppler signals generated by fast moving blood
Velocity aliasing
How to eliminate velocity aliasing?
Adjust the velocity scale to a wider range
wraps around to negative amplitude masquerading a reversed flow
Velocity aliasing
Relies on the total strength of the Doppler signal (amplitude)
Ignores directional information
Dependent on amplitude of Doppler regardless of frequency shift
Improves sensitivity to motion at the expense of directional and quantitative information
Not dependent as much to Doppler angle
Aliasing is not a problem
Allows detection of very subtle low blood flow
Slower frame rates
Significant amount of flash artifacts from moving tissue, pt motion or transducer motion
Power doppler
Arise from the incorrect display of anatomy or noise during imaging
Artifacts
Hypointense signal area distal to an object or interface
Shadowing
Caused by objects w/ high attenuation or reflection of the incident beam no return of echoes
Shadowing
Highly attenuating objects (bone/kidney stone) can induce low intensity streaks in the image
Shadowing
Occurs distal to objects having very low US attenuation
Enhancement
Hyperintense signals arise from increased transmission of sound by these structures
Enhancement
Arise from multiple echoes generated bet 2 closely spaced interfaces reflecting US energy back and forth during acquisition of the signal and before the next pulse
Reverberation
Often caused by reflections bet a highly reflective interface and the transducer or bet reflective interface such as metallic objects, calcified tissues or air pockets
Reverberation
Typically manifested as multiple equally spaced boundaries w/ decreasing amplitude along a straight line from the transducer
Reverberation
Arise from resonant vibrations w/in fluid trapped bet a tetrahedron of air bubbles => creates a continuous sound wave that is transmitted back to the transducer => displayed as a series of parallel bands extending posterior to a collection of gas
Ring down artifact
The echoes bounce back and forth between the two
boundaries and produce equally spaced signals of diminishing amplitude in the image
Comet tail artifact
Artifact represented by a rapidly changing mixture of colors
Twinkling
Artifact typically seen distal to a strong reflector (calculus)
Twinkling
Artifact possibly due to echoes from the strong reflector w/ frequency changes due to wide bandwidth of the initial pulse and narrow band ringing caused by the structure
Twinkling
highest instantaneous intensity in the beam
Temporal peak
time averaged intensity over the PRP
Temporal average
highest intensity spatially in the beam
Spatial peak
good indicator of thermal US effects
Spatial peak-temporal average intensity
Indicator of potential mechanical bioeffects and cavitation
Spatial peak-pulse average intensity
the accepted methods of determining power levels
Thermal index (TI) and mechanical index (MI)
