Ultrasound Flashcards
Mechanical energy that propagates thru a continuous, elastic medium by the compression and rarefaction of particles that comprise it
Sound
Generated by a mechanical displacement in compressible medium, which is modeled as an elastic spring
Ultrasound energy
Shown as a function of time, resulting in areas of compression and rarefaction with corresponding variations in positive and negative pressure amplitude
Energy propagation
Distance between compressions or rarefactions or between any two points that repeat on the sinusoidal wave of pressure amplitude
Wavelength
Number of times the wave oscillates thru one cycle each second
Frequency
Sound waves with frequencies less than 15 cycles per second are called
Infrasound
Frequency range of 15 and 20 kHz comprises the
Audible acoustic spectrum
Frequency range above 20 kHz are
Ultrasound
Medical ultrasound is at what frequency range
2-10 MHz
Specialized medical ultrasound applications are up to what frequency range
50 MHz
Time duration of one wave cycle an is equal to 1/f
Period
Distance travelled by wave per unit time and is equal to the wavelength divided by the period
Speed of sound
A highly compressible medium such as air, has a high or low speed of sound?
Low speed of sound
A less compressible medium such as bone, has a higher or lower speed of sound?
Higher speed of sound
True or false: ultrasound frequency is unaffected by changes in sound speed as the acoustic beam propagates thru different media
True
True or false: ultrasound wavelength affects the spatial resolution achievable along the direction of the beam
True
Provides better resolution and image detail than a low frequency beam
High frequency ultrasound beam
Interaction of 2 or more separate ultrasound beams in a medium can result in
Constructive and/or destructive wave interference
Position of the periodic wave with respect to a reference point
Phase
Amount of constructive or destructive interference depends on several factors, but the most are the
Phase and amplitude of interacting beams
Defined as the peak maximum or peak minimum value from the average pressure on the medium in the absence of soundwave
Pressure amplitude
In diagnostic ultrasound applications, the compressional amplitude significantly _______ the rarefactional amplitude
Exceeds
SI unit for pressure is the
Pascal (Pa) defined as 1 newton per square meter
Relative intensity and pressure levels are described as a logarithmic ratio, the
Decibel
Occurs at tissue boundaries where there is a difference in the acoustic impedance of adjacent materials
Reflection
Describes the direction of the transmitted ultrasound energy with nonperpendicular incidence
Reflection
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
Refers to loss of intensity of the ultrasound beam from absorption and scattering in the medium
Attenuation
Process whereby acoustic energy is converted to heat energy, whereby sound energy is lost and cannot be recovered
Absorption
SI unit for acoustic impedance is
Kg/m2s
Gives rise to differences in transmission and reflection of ultrasound energy, which is the means for producing an image using pulse echo techniques
Acoustic impedance
Describes the fractiom of sound intensity incident on an interface that is reflected
Reflection coefficient
Change in the direction of transmitted ultrasound energy at a tissue boundary when the beam is not perpendicular to the boundary
Refraction
Arises from objects and interfaces within a tissue that are about the size of the wavelength or smaller, and represent a rough or nonspecular reflector surface
Scattering
A smooth boundary between two media, where the dimension of the boundary between two media, where the dimensions of the boundary are much larger than the wavelength of the incident ultrasound energy
Specular reflector
These areas usually have a greater number of scatterers, larger acoustic impedance differences and larger scatterers
Hyperechoic areas
Specular reflection is dependent or independent? of frequency
Independent
Attenuation coefficient is expressed in units of
dB/cm
Ultrasound is produced and detected with a ______, comprised of one or more ceramic elements with electromechanical properties and peripheral components
Transducer
Part of the transducer that converts electrical energy into mechanical energy to produce ultrasound and mechanical energy into electrical energy for ultrasound detection
Ceramic element
Functional component of the transducer, which is often a crystal or ceramic
Piezoelectric material
Measures the magnitude of voltage, which is proportional to the incident mechanical pressure amplitude
Surface electrodes
Common piezoelectric ceramic in medical imaging
Lead-zirconate-titanium
Higher frequencies are achieved with thinner/thicker elements
Thinner elements
Thicker elements produce what type of frequency
Lower frequencies
Layered at the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing
Damping block
Creates short spatial pulse length, which is necessary to preserve detail along the beam axis (axis resolution)
Damping block
High Q transducer has a _____bandwidth and corresponding ______ spatial pulse length
Narrow bandwidth
Long SPL
A low Q Transducer has a _____ bandwidth and ______ spatial pulse length
Wide bandwidth and short spatial pulse length
Continuous-wave ultrasound transducers have a very ______ q characteristics
High
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
Broadband multi frequency transducers have bandwidth that exceeds ____% of the center frequency
80%
Located adjacent to the backside of the transducer and limits the vibration of the element to a small number of cycles
Damping block
Described as percentage of the center frequency
Bandwidth
Recently introduced technique that uses this ability— lower frequency ultrasound is transmitted into the patient, and the higher frequency harmonics, created from the interaction with contrast agents and tissues, are received as echoes
Harmonic imaging
Typically, how many individual rectangular elements comprise the transducer assembly
128 to 512
Linear array transducers typically contain how many elements
256 to 512
Largest transducer assemblies
Linear arrays
Usually comprised of 64-128 individual elements in a smaller package than a linear array transducer
Phased arrays
Its basic element is a capacitor cell, with a fixed electrode (backplate) and a free electrode (membrane). The principle of operatjon is electrostatic transduction
Capacitive micromachined ultrasonic transducers
An alternating voltage is applied between the membrane and the backplate, and the modulation of the electrostatic force results in membrane vibration with the generation of ultrasound
Electrostatic transduction
Its main advantage is better acoustic matching with propagation medium, which allows wider bandwidth capabilities, improved resolution
Capacitative micromachined ultrasonic transducers
Slightly converging beam out to a distance determined by geometry and frequency of the transducer
Near field
Diverging beam beyond the point
Far field
Also known as the fresnel zone, is adjacent to the transducer face and has a converging beam profile
Near field
Describes a large transducer surface as an infinite number of point sources of sound energy where each point is characterised as a radial emitter
Huygen’s principle
Near field distance is increased, as the physical diameter and operation frequency of the transducer are increased or decreased?
Increased
Peak ultrasound pressure happens in the
At the end of the near field
Also known as the fraunhofer zone, and is where the beam diverges
Far field
Less beam divergence occurs with ____ frequency, ____ diameter transducers
High frequency
Large diameter
Method to rephase the signals by dynamically introducing electronic delays as function of depth (time)
Dynamic receive focusing
At shallow/deep depths, rephasing delays between adjacent transducer elements are greatest
Shallow
Process that increases the number of active receiving elements in the array with reflector depth, so that the lateral resolution does nor degrade with depth of propagation
Dynamic aperture
Unwanted emissions of ultrasound energy directed away from the main pulse, caused by the radial expansion and contraction of the transducer element during thickness contraction and expansion
Side lobes
How to reduce side lobes
Keep the individual transducer element widths small (less than 1/2 wavelength), reduce amplitude if peripheral transducer element excitations relative to the central element excitations
Results when ultrasound energy us emitted far off-axis by multi-element arrays and are a consequence of the noncontinuous transducer surface of the discrete elements
Grating lobes
Direction of side lobes
Forward directed
Characteristics of grating lobes
Emitted from the array surface at very large angles
Major factor that limits the spatial resolution and visibility of detail is the
Volume of acoustic pulse
Dimensions that determine the minimal volume element
Axial, lateral, elevational (slice-thickness)
Refers to ability to discern two closely spaced objects in the direction of the beam
Axial
Also known as linear, range, longitudinal or depth resolution
Axial resolution
The minimal required separation distance between two reflectors is
1/2 of SPL
Number of cycles emitted per pulse by the transducer multiplied by the wavelength
SPL
Also known as azimuthal resolution, refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction
Lateral resolution
Best lateral resolution occurs at the
Near field-far field interface
The elevational or slice-thickness dimension of ultrasound beam is ______ to the image plane
Perpendicular
Weakest measure of resolution of array transducers
Slice thickness
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 applied voltage
Pulser
It is synchronized with the pulser, isolates the high voltage associated with pulsing (~150 V) from the sensitive amplification stages during receive mode, with induced voltages ranging from approximately 1 to 2 uV from returning echoes
Transmit/receive switch
In this transducer operation, the ultrasound beam is intermittently transmitted, with a majority of time occupied by listening for echoes
Pulse-echo mode
Number of times the transducer is pulsed per second is known as the
pulse repetition frequency (PRF)
For imaging, PRF range from
2000 to 4000 pulses per second (2-4 kHz)
Time between pulse is the
Pulse repetition period
Ratio of number of cycles in the pulse to the transducer frequency and is equal to the instantaneous “on” time
Pulse duration
The fraction of “on” time, is equal to the pulse duration divided by the PRP
duty cycle
User adjustable amplification of the returning echo signals as a function of time, to further compensate for beam attenuation
Time varied gain, depth gain compensation and swept gain (TGC)
Effectively reduces the maximum to minimum range of echo voltages as a function of time to approximately 50dB
TGC amplification
Feature of some broadband receivers that changes the sensitivity of the tuner bandwidth with time, so that echoes from shallow depths are tuned to a higher frequency range, while echoes from deeper structures are tuned to lower frequencies
Dynamic frequency tuning
Defines the effective operational range of an electronic device from the threshold signal level to the saturation level
Dynamic range
Inverts the negative amplitude signals of the echo to positive values
Rectification
Convert the rectified amplitudes of echo into smoothed, single pulse
Demodulation and envelope detection
Display of the processed information from the receiver versus time. This is currently used in ophthalmology applications for precise distance measurements of the eye
A mode (amplitude)
Electronic conversion of the A-mode and A-line information into brightness-modulated dots along the A-line trajectory
B-mode (brightness)
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 (motion)
Method in which ultrasound information is obtained from several different angles of insonation and combined to produce a single image
Spatial compounding
Based on shift of frequency in an ultrasound wave caused by a moving reflector, such as blood cells in the vasculature
Doppler ultrasound
Difference between the incident frequency and reflected frequency
Doppler shift
Refers to vector quantity describing both the distance travelled per unit time (speed) and the direction of movement such as blood flow
Velocity
Preferred Doppler angle ranges from
30- 60 degrees
In this angle, doppler shift is small, and minor errors in angle accuracy can result in large errors in velocity
More than 60 degrees
At this doppler angle, refraction and critical angle interactions can cause problems, as can aliasing of the signal in pulsed Doppler studies
Less than 20 degrees
Simplest and least expensive device for measuring blood velocity
Continuous wave Doppler system
A method of signal processing called _______ is phase sensitive and can indicate the direction of flow either toward or away from the transducers
Quadrature technique
Combines the velocity determination of continuous wave Doppler systems and the range discrimination of pulse echo imaging
Pulse doppler ultrasound
A signal can be reconstructed unambiguously as long as the true frequency is _______
Less than half the sampling rate
Combination of 2D B-mode imaging and pulsed Doppler acquisition
Duplex scanning
2D visual display of moving blood in the vasculature, superimposed upon the conventional gray-scale image
Color flow imaging
Technique to measure the similarity of one scan line measurement to another when the maximum correlation (overlap) occurs
Phase-shift autocorrelation
Alternate method for color flow imaging. It is based upon the measurement that a reflector has moved over time, between executive pulse echo acquisitions
Time domain correlation
Direction of flow is best determined with a
Small Doppler angle (about 30 degrees)
Error caused by an insufficient sampling rate (PRF) relative to the high frequency Doppler signals generated by fast-moving blood.
Aliasing
Signal processing method that relies on the total strength of the Doppler signal (amplitude) and ignores directional (phase) information
Power doppler
Artifacts seen un power doppler imaging which are related to color signals arising from moving tissues, patient motion or transducer motion
Flash artifacts
Most common ultrasound contrast agents
Encapsulated microbubbles of 3-5 um diameter containing air, nitrogen or insoluble gaseous compounds such as perfluorocarbons, encapsulation materials such as human albumin
Hypointense signal area distal to an object or interface and is caused by objects with high attenuation or reflection of the incident beam without the return of echoes
Shadowing
Arise from multiple echoes generated between two closely spaced interfaces reflecting ultrasound energy back and forth during the acquisition of the signal and before the next pulse
Reverberation artifact
These artifacts are often caused by reflections between a highly reflective interface and the transducer or between reflective interfaces such as metallic objects, calcified tissues or air pocket/partial liquid areas of anatomy
Reverberation
Comet tail artifact is a form of
Reverberation artifact
Arise from resonant vibrations within fluid trapped between tetrahedron of air bubbles, which creates a continuous sound wave that is transmitted back to the transducer and displayed as 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
Created when a high PRF limits the amount of time spent listening for echoes during the PRP
Ambiguity
Represented as a rapidly changing mixture of colors, typically seen distal to a strong reflector such as a calculus, and is often mistaken for an aneurysm when evaluating vessels
Twinkling artifact
Determined with the first high-contrast target (positioned at several depths from 0- ~1cm) visible in the image
Dead zone depth
Rate at which sound energy flows thru a unit area and is usually expressed in units of watts per square centimeter or milliwatts per square centimeter
Acoustic intensity
Accepted method of determining power levels for real time instruments that provide the operator with quantitative estimates of power deposition in the patient
Thermal and mechanical indices of ultrasound operation
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
Consequence of negative pressures (rarefaction of the mechanical wave) that induce bubble formation from the extraction of dissolved gas in the medium
Cavitation
Value that estimates the likelihood of cavitation by ultrasound beam
Mechanical index
It is directly proportional to the peak rarefaction (negative) pressure and inversely proportional to the square root of the ultrasound frequency
Mechanical index
Generally refers to the pulsation (expansion and contraction) of persistent bubbles in the tissue that occur at low and intermediate ultrasound intensities
Stable cavitation
At higher ultrasound intensity levels, ________ can occur, whereby the bubbles respond nonlinearly to the driving force, causing a collapse approaching the speed of sound
Transient cavitation
