***Chapter 14 - Ultrasound Flashcards
Transmitter and receiver of ultrasound pulses
Transducer array
Mechanical energy that propagates through a continuous, elastic medium by the compression and rarefaction of “particles” that comprise it
Sound
Energy propagation occurs as a wave front in the direction of energy travel
Longitudinal wave
Expressed in mm or um; distance between compressions or rarefactions, or between any two points that repeat on the sinusoidal wave of pressure amplitude
Wavelength
The number of times the waves oscillates through one cycle each second
Frequency
Sound waves with frequencies less than 15 cycles per second (Hz)
Infrasound
Comprises the range between 15 Hz and 20kHz
Audible acoustic spectrum
Represents the frequency range above 20kHz
Ultrasound
Uses frequencies in the range of 2 to 10 MHz,with specialized ultrasound applications up to 50 MHz
Medical ultrasound
Time duration of one wave cycle
Period
Distance traveled by the wave per unit time and is equal to the wavelength divided by the period
Speed of sound
Determined by the ratio of the bulk modulus and the density of the medium
Wave speed
A highly compressible medium such as AIR, has a ____speed of sound
Low
*while a less compressible medium(e.g.bone),has a higher speed of sound
A less dense medium (e.g.dry air) has a ______ speed of sound than a denser medium (e.g.humid air)
Higher
Average speed of sound for:
A. Soft tissue
B. Fatty tissue
C. Air
A. 1,540 m/s
B. 1,450 m/s
C. 330 m/s
Fundamental property that generates echoes in an ultrasound image
Difference on the speed of sound at tissue boundaries
Higher frequency sound has _____ wavelength
SHORTER
Spatial resolution of the ultrasound image depend on the ____
Wavelength
Attenuation of the ultrasound sound beam energy depend on the _______
Frequency
High frequency utz beam (small wavelength) provides _____ resolution and image detail than a lower frequency beam
Better
*the depth of the beam penetration is significantly reduced at higher frequency
Position of the periodic wave with respect to a reference point
Phase
Defined as the peak maximum or peak minimum value from the average pressure on the medium in the absence of sound wave
Pressure amplitude
SI unit of pressure; defined as one Newton per square meter (N/m2)
Pascal (Pa)
Occur with two utz waves of the same frequency and phase, resulting in a higher amplitude output wave
Constructive interference
Occurs with the waves 180 degrees out-of-phase, resulting in a lower amplitude output wave
Destructive interference
Occurs when waves of slightly different frequency interact, resulting in an output waveform of higher and lower amplitude
Complex interference
Amount of power (energy per unit time) per unit area
Intensity,I
*described in milliwatts/cm2
Relative intensity and pressure levels described as a logarithmic ratio
Decibel (dB)
When the intensity ratio is >1 the dB values are _____;
When the intensity ratio is <1 the dB values are ____
Positive; negative
The tissue thickness that reduces the ultrasound intensity by 3 dB is considered the _____
Half-value thickness (HVT)
*a loss of 3 dB (-3 dB) represents a 50% loss of signal intensity
Occurs at tissue boundaries where there is a difference in the acoustic impedance of adjacent materials
Reflection
Describes the change in direction of the transmitted ultrasound energy with nonperpendicular incidence
Refraction
Occurs by reflection or refraction,usually by small particles within the tissue medium, causes the beam to diffuse in many directions, and gives rise to the characteristic texture and gray scale in the acoustic image
Scattering
Loss of intensity of the utz beam from absorption and scattering in the medium;
Loss of acoustic energy with distance traveled
Attenuation
Process whereby acoustic energy is converted to heat energy, whereby the sound energy is lost and cannot be recovered
Absorption
Likened to the stiffness and flexibility of a compressible medium
Density X speed of sound; SI Unit is kg/m2s,
rayl(special name).
*one rayl is equal to 1 kg/(m2s)
Acoustic impedance
*gives rise to difference is transmission and reflection of ultrasound energy
Describes the fraction of sound intensity incident on an interface that is reflected
Reflection coefficient
Defined as the fraction of the incident intensity that is transmitted across an interface
Intensity transmission coefficient
A conduit of tissue that allows ultrasound transmission through structures such as the lung is known as an _____
Acoustic window
A smooth boundary between two media, where the dimensions of the boundary are much larger than the wavelength of the incident ultrasound energy
Specular reflector
Terms used for describing the scatter characteristics relative to the average background signal
Hyperechoic (higher scatter amplitude);
Hypoechoic (lower scatter amplitude)
2 chief causes of attenuation
- Scattering
2. Tissue absorption of the incident beam
The relative intensity loss per centimeter of travel for a given medium
Attenuation coefficient
An approx. rule of thumb for soft tissues…
- 5 dB per cm per MHz OR
0. 5 (dB/cm)/MHz
Utz attenuation occur ____ with penetration depth and
____ with increased frequency
Exponentially; increases
Functional component of the transducer
Piezoelectric material (often a crystal or ceramic)
Molecular entities containing positive and negative electrical charges that have an overall neutral charge
Electrical dipoles
Natural piezoelectric material
Quartz crystal
Synthetic piezoelectric ceramic
Lead-zirconate-titanate(PZT)
Low frequency oscillation is produced with a _____ piezoelectric element
Thicker
Layered on the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing; also dampens the transducer vibration
Damping block
Lessens the purity of the resonance frequency and introduces a broadband frequency spectrum
Dampening of the vibration (also known as “ring-down”)
Describes the bandwidth of the sound emanating from the transducer
Q factor
- high Q transducer-narrow bandwidth,long SPL
- caused by light damping
- low Q transducer-wide bandwidth,short SPL
- caused by heavy damping
Provides the interface between the raw transducer element and the tissue and minimizes the acoustic impedance differences between the transducer and the patient
Matching layer
Transducers which contain 256 to 512 elements; physically these are the largest transducer assemblies
-produces a beam by firing a subset of the total number of transducer elements as a group
Linear arrays
- produces a rectangular field of view(FOV)
- for a curvilinear array, a trapezoidal FOV is produced
Produces a beam from ALL of the transducer elements fired with fractional time delays in order to steer and focus the beam
Phased array transducer
CMUT
- principle of operation: electrostatic transduction
- basic element of a CMUT: a capacitor cell with a fixed electrode (backplate) and a free electrode (membrane)
Capacitive Micromachined Ultrasonic transducers
UTZ beam pattern that is adjacent to the transducer face and has a converging beam profile
Near field (Fresnel Zone)
Describes a large transducer surface as an infinite number of point sources of sound energy where each point is characterized as a radial emitter
Huygen principle
Near field length is dependent on (2):
Transducer diameter and propagation wavelength
The ability of the system to resolve objects in a direction perpendicular to the beam direction
Lateral resolution
Where the ultrasound beam diverges…
Far field (Fraunhofer zone)
A method to rephase the signals by dynamically introducing electronic delays as function of depth (time)
Dynamic receive focusing
A process which increases the number of active receiving elements in the array with reflector depth, so that the lateral resolution does not degrade with depth of propagation
Dynamic aperture
Unwanted emissions of ultrasound energy directed away from the main pulse, caused by radial expansion and contraction of the transducer element during thickness contraction and expansion.
Side lobes
Result when ultrasound energy is emitted far off-axis by multi element arrays and are a consequence of the noncontinuous transducer surface of the discrete elements
Grating lobes
Off-axis energy emissions produced by linear and phased array transducers (2)
Side lobes (are forward directed)
Grating lobes (emitted from the array surface at very large angles)
(In UTZ), the major factor that limits spatial resolution and visibility of detail…
Volume of the acoustic pulse
Determines the minimal volume element in UTZ (3)
Axial, lateral and elevational(slice thickness) dimensions
Refers to the ability to discern two closely spaced objects in the direction of the beam
Axial resolution (aka linear, range, longitudinal or depth resolution)
The number of cycles emitted per pulse by the transducer multiplied by the wavelength
Spatial pulse length (SPL)
Refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction
Lateral resolution (aka Azimuthal resolution)
The best lateral resolution occurs at the ______
Near field-far field interface
Dimension of the ultrasound beam that is perpendicular to the image plane
Elevational or slice-thickness dimension
Typically the weakest measure of resolution for array transducers
Slice thickness
Multiple linear array transducers with five to seven rows; have the ability to steer and focus the beam in the elevational dimension
1.5 D transducer arrays
Responsible for generating the electronic delays for individual transducer elements in an array to achieve transmit and receive focusing and , in phased arrays , beam steering.
Beam former
Provides the electrical voltage for exciting the piezoelectric transducer elements and controls the output transmit power by adjustment of the applied voltage.
Pulser (aka the transmitter)
(In digital beam former systems),a ______ determines the amplitude of the voltage
Digital-to-analog converter (DAC)
Isolates the high voltage associated with pulsing (150 V) from the sensitive amplification stages during receive mode
Transmit/receive switch
The ultrasound beam is intermittently transmitted, with a majority of the time occupied by listening for echoes
Pulse-echo mode of transducer operation
Event where the ultrasound pulse is created with a short voltage waveform provided by the pulser of the ultrasound system
Main bang
Number of times the transducer is pulsed per second
Pulse repetition frequency (PRF)
The time between pulses; equal to the inverse of the PRF (pulse repetition frequency)
Pulse repetition period (PRP)
Determined from the product of the speed of sound and the PRP divided by 2
Maximal range
A. UTZ frequency is calibrated in ____
B. UTZ period is measured in _____
A. MHz
B. microseconds
A. PULSE REPETITION FREQUENCY (PRF) is calibrated in ___
B. PULSE REPETITION PERIOD (PRP) is measured in ____
A. kHz
B. milliseconds
Ratio of the number of cycles in the pulse to the transducer frequency; equal to the instantaneous “on” time
Pulse Duration
The fraction of “on” time; equal to the pulse duration divided by the PRP
Duty cycle
A feature of some broadband receivers that changes the sensitivity of the tuner bandwidth with time; echoes from the shallow depths are tuned to a higher frequency range,while echoes from deeper structures are tuned to lower frequencies
Dynamic frequency tuning
A user adjustable amplification of the returning echo signals as a function of time, to further compensate for beam attenuation
TGC (aka time varied gain, depth gain compensation, swept gain)
Defines the effective operational range of an electronic device from the threshold signal level to the saturation level
Dynamic range
Increases the smallest echo amplitudes and decreases the largest amplitudes
Logarithmic amplification
Inverts the negative amplitude signals of the echo to positive values
Rectification
Convert the rectified amplitudes of the echo into a smoothed single pulse
Demodulation and enveloped detection
______ level adjustment sets the threshold of signal amplitudes allowed to pass to the digitization and display subsystems
Rejection
2 RECEIVER subsequent signal processing steps that the operator can control …
- TGC
2. Noise/clutter rejection
Display of the processed information from the receiver versus time (after the receiver processing steps)
A-mode (A for amplitude)
*A-mode and A-line info is currently used in OPHTHA applications for precise distance measurements of the eye
Electronic conversion of the A-mode and A-line information into brightness modulated dots along the A-line trajectory
B-Mode (B for Brightness)
- used for M-mode and 2D gray-scale imaging
A technique that uses B-Mode information to display the echoes from a moving organ, such as the myocardium and valve leaflets, from a fixed transducer position and beam direction on the patient
M-Mode (M for motion)
Used to create 2D images from echo info from distinct beam directions and to perform scan conversion to enable image data to be viewed on video display monitors
Scan converter
UTZ spatial resolution has components in three directions, namely…
Axial, lateral and elevational
UTZ resolution type determined by the frequency of the UTZ and the damping factor of the transducer
Axial resolution
UTZ resolution types determined by the dimensions (width and height,respectively) of the transducer aperture, the depth of the object, and mechanical and electronic focusing
Lateral and elevational resolutions
A change in the transmitted UTZ pulse directional boundary with nonperpendicular distance;
Misplaced anatomy often occurs in the image during the scan
Refraction
A hypointense signal area distal to an object or interface and is caused by objects with high attenuation or reflection of the incident beam w/o the return of echoes
Shadowing
Occurs distal to objects having very low UTZ attenuation
Enhancement
Arise from multiple echoes generated between two closely spaced interfaces reflecting UTZ energy back and forth during the acquisition of the signal and before the next pulse
Reverberation
*typically manifested as multiple,equally spaced boundaries with decreasing amplitude along a straight line from the transducer
A form of reverberation
Comet tail artifact
Arise from resonant vibrations within fluid trapped between between a tetrahedron of air bubbles, which creates a continuous sound wave that is transmitted back to the transducer and displayed as a series of parallel bands extending posterior to a collection of gas
Ring-down artifacts
Caused by the variability of speed of sound in different tissues
Speed displacement
Emissions of the UTZ energy that occur in a direction slightly off-axis from the main beam and arise from the expansion of the piezoelectric elements orthogonal to the main beam
Side lobes
Occur with multielement array transducers and result from the division of a smooth transducer surface into a large number of small elements
Grating lobes
***page 567
Multipath reflection and mirror image
Created when a high PRF limits the amount of time spent listening for echoes during the PRP
Ambiguity artifacts
Represented as a rapidly changing mixture of colors, is typically seen distal to a strong reflector such as a calculus, and is often mistaken for an aneurysm when evaluating vessels
Twinkling artifact
*possibly due to echoes from a strong reflector with frequency changes due to wide bandwidth of the initial pulse and the narrow band “ringing” caused by the structure
Determined by the beam width of the transducer array perpendicular to the image plane and is greater than the beam width in the image plane
Slice thickness
Rate of energy production, absorption or flow
Power
*SI unit: watt(W) - one joule of energy per second
Rate at which sound energy flows thru a unit area and is usually expressed in units of watts per square centimeter (W/cm2) or milliwatts per square centimeter (mW/cm2)
Acoustic intensity
A device containing a small piezoelectric element coupled to external conductors and mounted in a protective housing; measures UTZ pressure amplitude within a beam
Hydrophone
Highest instantaneous intensity in the beam
Temporal peak
Time-averaged intensity over the PRP
Temporal average
Average intensity of the pulse
Pulse average
Highest intensity spatially in the beam
Spatial peak
Average intensity over the beam area, usually taken to be the area of the transducer
Spatial average
The acoustic power contained in the UTZ beam, averaged over at least one PRP and divided by the beam area
Spatial average-temporal average intensity(I sata)
Ratio of the acoustical power produced by the transducer to the power required to raise tissue in the beam area by 1 degree Celsius
Thermal Index (TI)
A consequence of the negative pressures that induce bubble formation from the extraction of dissolved gases in the medium
Cavitation
A value that estimates the likelihood of cavitation by ultrasound beam
Mechanical index (MI)
Best indicator of heat deposition (2)
- I spta
2. Calculated TI value
Generally refers to the pulsation (expansion and contraction) of persistent bubbles in the tissue that occur at low and intermediate UTZ intensities
Stable cavitation
Occurs at higher UTZ intensity levels, where the bubble respond linearly to the driving force, causing a collapse approaching the speed of sound. At this point, the bubbles might dissolve, disintegrate or rebound
Transient cavitation
