RAB: Ch. 14: Ultrasound Flashcards
1
Q
The distance (usually expressed in units of mm or um) between compressions or rarefactions, or between any two points that repeat on the sinusoidal wave of pressure amplitude
A
Wavelength of ultrasound energy
2
Q
The number of times the wave oscillates through one cycle each second (s)
A
Frequency (f)
3
Q
Medical ultrasound uses frequencies in the range of __________ MHz, with specialized ultrasound applications up to 50 MHz
A
2 to 10 MHz
4
Q
time duration of one wave cycle and is equal to 1/f, where f is expressed in cycles/s
A
period
5
Q
distance traveled by the wave per unit time and is equal to the wavelength divided by the period
A
Speed of sound
6
Q
peak maximum or peak minimum value from the average pressure on the medium in the absence of a sound wave
A
Pressure amplitude (P)
7
Q
amount of power (energy per unit time) per unit area and is proportional to the square of the pressure amplitude
A
Intensity
8
Q
As ultrasound energy propagates through a medium, interactions include _______, ______, _______, and ________.
A
- Reflection
- Refraction
- Scattering
- Absorption
9
Q
occurs at tissue boundaries where there is a difference in the acoustic impedance of adjacent materials
A
Reflection
[When the incident beam is perpendicular to the boundary, a fraction of the beam (an echo) returns directly back to the source; the transmitted fraction of the beam continues in the initial direction]
10
Q
describes the change in direction of the transmitted ultrasound energy with nonperpendicular incidence
A
Refraction
11
Q
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
A
Scattering
12
Q
refers to the loss of intensity of the ultrasound beam from absorption and scattering in the medium
A
Attenuation
13
Q
process whereby acoustic energy is converted to heat energy, whereby, sound energy is lost and cannot be recovered
A
Absorption
14
Q
Major components of ultrasound Transducers
A
- Piezoelectric material
- Matching layer
- Backing block
- Acoustic absorber
- Insulating cover
- Tuning coil
- Sensor electrodes
- Transducer housing
15
Q
Is the functional component of the transducer.
It converts electrical energy into mechanical (sound) energy by physical deformation of the crystal structure.
Mechanical pressure applied to its surface creates electrical energy.
Characterized by a well-defined molecular arrangement of electrical dipoles
A
Piezoelectric material (often a crystal or ceramic)
> natural: quartz crystal (ex. Watches)
synthetic
16
Q
> layered on the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing.
This component also dampens the transducer vibration to create an ultrasound pulse with a short spatial pulse length (SPL), which is necessary to preserve detail along the beam axis (axial resolution
A
Damping Block
17
Q
> (also known as “ring-down”) lessens the purity of the resonance frequency and introduces a broadband frequency spectrum.
> With ring-down, an increase in the bandwidth (range of frequencies) of the ultrasound pulse occurs by introducing higher and lower frequencies above and below the center (resonance) frequency
A
Dampening of vibration
18
Q
Describes the bandwidth of the sound emanating from a transducer
A
Q factor
19
Q
Provides the interface between the raw transducer element and the tissue and minimizes the acoustic impedance differences between the transducer and the patient
A
Matching Layer
20
Q
> Unlike the resonance transducer design, the piezoelectric element is intricately machined into a large number of small “rods” and then filled with an epoxy resin to create a smooth surface.
The acoustic properties are closer to tissue than a pure PZT material and thus provide greater transmission efficiency of utz beam.
these have transducers have bandwidths that exceed 80% of the center frequency
A
Nonresonance (Broad Bandwidth) “Multifrequency” - Modern transducer design coupled with digital signal processing
21
Q
Accomplished with a short square wave burst of approximately 150 V with one to three cycles, unlike the voltage spike used for resonance transducers
A
Excitation of multifrequency transducer
22
Q
In Transducer Arrays, two modes of activation are used to produce a beam.
A
- “Linear” (Sequential)
- “Phased” activation/receive modes
23
Q
> these transducers typically contain 256 to 512 elements; physically these are the largest transducer assemblies
the simultaneous firing of a small group of approximately 20 adjacent elements produces the ultrasound beam
The simultaneous activation produces a synthetic aperture (effective transducer width) defined by the number of active elements
Echoes are detected in the receive mode by acquiring signals from most of the transducer elements. Subsequent “A-line” (see Section 14.5) acquisition occurs by firing another group of transducer elements displaced by one or two elements
A
Linear arrays
24
Q
A rectangular field of view (FOV) is produced with this transducer arrangement
A
Linear array
25
A trapezoidal FOV is produced.
Curvilinear array
26
>comprised of 64 to 128 individual elements in a smaller package than a linear array transducer
> All transducer elements are activated nearly simultaneously to produce a single ultrasound beam
>uses time delays which allows it to be steered and focused electronically without moving transducer on patient
Phased-array transducer
27
Compared to PZT, are better acoustic matching with the propagation medium, which allows wider bandwidth capabilities, improved resolution, potentially lower costs with easier fabrication, and the ability to have integrated circuits on the same “wafer
CMUT (capacitive micromachined ultrasonic transducers)
28
- also known as the Fresnel zone, is adjacent to the transducer face and has a converging beam profile
- Beam convergence occurs here because of multiple constructive and destructive interference patterns of the ultrasound waves from the transducer surface
The Near Field
29
-Also known as the Fraunhofer zone and is where the beam diverges
The Far Field
30
A process termed __________ 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
31
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
32
Result when ultrasound energy is emitted far off-axis by multielement arrays and are a consequence of the noncontinuous transducer surface of the discrete elements
Grating lobes
33
In ultrasound, the major factor that limits the spatial resolution and visibility of detail is the _______ of the acoustic pulse. The axial, lateral, and elevational (slicethickness) dimensions determine the minimal volume element
Volume
34
Also known as linear, range, longitudinal, or depth resolution), refers to the ability to DISCERN two closely spaced objects in the DIRECTION OF THE beam
Axial Resolution
35
Also known as azimuthal resolution, refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction.
Lateral Resolution
36
- The slice-thickness dimension of the ultrasound beam is perpendicular to the image plane. Slice thickness plays a significant part in image resolution, particularly with respect to volume averaging of acoustic details in the regions close to the transducer and in the far field beyond the focal zone.
- This resolution is dependent on the transducer element height in much the same way that the lateral resolution is dependent on the transducer element width
Elevational Resolution
37
Weakest measure of resolution for array transducers
Slice thickness
38
Ultrasound Images are created using a _____ mode format of ultrasound production and detection
-Image formation using this approach requires a number of hardware components: the beam former, pulser, receiver, amplifier, scan converter/image memory and display system
Pulse-echo mode
39
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
40
(Also known as the transmitter) PROVIDES ELECTRICAL VOLTAGE for exciting the piezoelectric transducer elements and controls the output transmit power by adjustment of the applied voltage
Pulser
41
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 V to 2 µV from the returning echoes
Transmit/Receive Switch
42
The ultrasound beam is intermittently transmitted, with a majority of the time occupied by listening for echoes. The ultrasound pulse is created with a short voltage waveform provided by the pulser of the ultrasound system.
Event is sometimes known a s the main bang
Pulse-echo mode of transducer operation
43
the ultrasound beam is intermittently transmitted, with a majority of the time occupied by listening for echoes. The ultrasound pulse is created with a short voltage waveform provided by the pulser of the ultrasound system
pulse repetition frequency (PRF)
44
is the ratio of the number of cycles in the pulse to the transducer frequency and is equal to the instantaneous “on” time
Pulse duration
45
A user-adjustable amplification of the returning echo signals as a function of time, to further compensate for beam attenuation
Its ideal curve makes all equally reflective boundaries equal in signal amplitude, regardless of depth of the boundary.
TGC (also known as time varied gain, depth gain compensation, and swept gain
46
A 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
47
The display of the processed information from the receiver versus time (after the receiver processing steps
A-mode (A for amplitude)
48
- The display of the processed information from the receiver versus time (after the receiver processing steps
- the brightness of the dot is proportional to the echo signal amplitude (depending upon signal processing parameters)
-its display is used for M-mode and 2D gray-scale imaging
B-mode (B for brightness)
49
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)
50
Are typically composed of 256 to 512 discrete transducer elements of ½ to 1 wavelength width each in an enclosure from about 6 to 8 cm wide. A small group of adjacent elements (~15 to 20) is simultaneously activated to create an active transducer area defined by the width (sum of the individual element widths in the group) and the height of the elements
Linear and curvilinear array
51
Typically comprised of a tightly grouped array of 64, 128, or 256 transducer elements in a 3- to 5-cm-wide enclosure
Phased-array transducers
52
A method in which ultrasound information is obtained from several different angles of insonation and combined to produce a single image
Spatial compounding
53
Image Display
For digital flat-panel displays, digital information from the scan converter can be directly converted into a viewable image.
For analog monitor displays, the digital scan converter memory requires a ___ and electronics to produce a compatible video signal
DAC
54
based on the shift of frequency in an ultrasound wave caused by a moving reflector, such as blood cells in the vasculature
Doppler ultrasound
55
It is the difference between the incident frequency and reflected frequency.
When the reflector is moving directly away from or toward the source of sound, the Doppler frequency shift (fd) is calculated as
Doppler Shift
56
Blood moving towards transducer produces ____ frequency echoes
Blood moving away from transducer produces _____ frequency echoes
Sound waves reflected from a moving object are compressed (higher frequency) when moving toward the transducer and expanded (lower frequency) when moving away from the transducer compared to the incident sound wave frequency
Higher
Lower
57
When the sound waves and blood cells are not moving in parallel directions, the equation must be modified to account for less Doppler shift
True
58
The angle between the direction of blood flow and the direction of the sound is called
Doppler angle
59
The preferred Doppler angle ranges from ___ to ___ degrees. At too large an angle (greater than 60 degrees), the apparent Doppler shift is small, and minor errors in angle accuracy can result in large errors in velocity (Table 14-7). At too small an angle (e.g., less than 20 degrees), refraction and critical angle interactions can cause problems, as can aliasing of the signal in pulsed Doppler studies
30 to 60 degrees
60
This combines the velocity determination of continuous wave Doppler systems and the range discrimination of pulse-echo imaging
Pulsed doppler operation
61
A 180-degree phase shift in the Doppler frequency represents blood that is moving ___ from the transducer
Away from the transducer
62
This refers to the combination of 2D B-mode imaging and pulsed Doppler data acquisition.
It allows estimation of the blood velocity directly from the Doppler shift frequency, since the velocity of sound and the transducer frequency are known, while the Doppler angle can be estimated from the B-mode image and input into the scanner computer for calculation
Duplex scanning
63
Provides a 2D visual display of moving blood in the vasculature, superimposed upon the conventional gray-scale image as shown
Color Flow Imaging
64
In color flow imaging, Velocities and directions are determined for multiple positions within a subarea of the image and then color encoded (e.g., shades of red for blood moving ______ the transducer, and shades of blue for blood moving _____ from the transducer).
BART
Blue
Away from the transducer
Red
Towards the transducer
65
In duplex scanning, this is positioned over the vessel of interest with size (length and width).
The ______ (sample area) could be mispositioned or of inappropriate size, such that the velocities are an overestimate (gate area too small) or underestimate (gate area too large) of the average velocity
Doppler GATE
66
Not from BUSHBERG:
What is the ideal DOPPLER GATE (SAMPLE SIZE) or GATE SIZE?
2/3 the diameter of the vessel lumen and placed witha Doppler insonation angle of fewer than 60 degrees.
Failure to do so may increase the velocities calculated.
67
With the pulsatile nature of blood flow, the spectral characteristics vary with time. Interpretation of the frequency shifts and direction of blood flow is accomplished with the fast Fourier transform, which mathematically analyzes the detected signals and generates amplitude versus frequency distribution profile known as the _________.
Doppler Spectrum
68
The ______ is typically represented by a spectrum of frequencies resulting from a range of velocities contained within the sampling gate at a specific point in time
Doppler Signal
69
______, laminar flow exists in the center of large, smooth wall vessels, while ______ blood flow occurs near the vessel walls, due to frictional forces
FAST
SLOW
70
______ flow occurs at disruptions in the vessel wall caused by plaque buildup and stenosis
Turbulent
71
T or F
A large Doppler gate that is positioned to encompass the entire lumen of the vessel will contain a large range of blood velocities, while a smaller gate positioned in the center of the vessel will have a smaller, faster range of velocities. A Doppler gate positioned near a stenosis in the turbulent flow pattern will measure the largest range of velocities
True
72
Interpretation of the spectral display provides the ability to determine the ___ of flow, the ___ of flow, and ___ of the flow
presence, direction and characteristic
73
T or F
It is more difficult to determine a lack of flow, since it is also necessary to ensure that the lack of signal is not due to other acoustical or electrical system parameters or problems
True
74
A broad spectrum (bandwidth) represents ____ flow, while a narrow spectrum represents _____ flow within the Doppler gate
Turbulent
Laminar
75
The direction of flow (positive or negative Doppler shift) is best determined with a small Doppler angle (about ___degrees). Normal flow is typically characterized by a specific spectral Doppler display waveform, which is a consequence of the hemodynamic features of particular vessels. Disturbed and turbulent flow produce Doppler spectra that are correlated with disease processes
30 degrees
76
Pertinent quantitative measures, such as pulsatility index and resistive index are dependent on the characteristics of the Doppler spectral display
Formula of Pulsality Index (PI)?
(Max-min)/
Average
77
Pertinent quantitative measures, such as pulsatility index and resistive index are dependent on the characteristics of the Doppler spectral display
Formula of Resistive Index (RI)?
(Max-min)/
Max
78
This is an error caused by an insufficient sampling rate (PRF) relative to the high-frequency Doppler signals generated by fast-moving blood
Velocity ALIASING
79
A minimum of ___ samples per cycle of Doppler shift frequency is required to unambiguously determine the corresponding velocity
Two
80
The most straightforward method to reduce or eliminate the aliasing error is for the user to adjust the velocity _____ to a wider range, as most instruments have the PRF of the Doppler unit linked to the scale setting (a wide range delivers a high PRF)
Velocity Scale
81
It is a signal processing method that relies on the total strength of the Doppler signal (amplitude) and ignores directional (phase) information. The ____ (also known as energy) mode of signal acquisition is dependent on the amplitude of all Doppler signals, regardless of the frequency shift. This dramatically improves the sensitivity to motion (e.g., slow blood flow) at the expense of directional and quantitative flow information
Power Doppler;
Power
82
Compared to conventional color flow imaging, power Doppler produces images that have more sensitivity to _____ and are not affected as much by the Doppler angle (largely nondirectional). Aliasing is not a problem as only the strength of the frequency shifted signals are analyzed, and not the phase
Motion
83
On the other hand, frame rates tend to be slower for the power Doppler imaging mode, and a significant amount of “___ artifacts” occur, which are related to color signals arising from moving tissues, patient motion, or transducer motion
Flash artifacts
84
The name “power Doppler” is sometimes mistakenly understood as implying the use of increased transmit power to the patient, but, in fact, the power levels are typically the same as in a standard color flow procedure. The difference is in the processing of the returning signals, where sensitivity is achieved at the expense of direction and quantitation
True; the difference is in the processing of the returning signals
85
This enhances contrast agent imaging by using a lowfrequency incident pulse and tuning the receiver (using a multifrequency transducer) to higher frequency harmonics. This approach allows removal of “echo clutter” from fundamental frequency reflections in the near field to improve the sensitivity to the ultrasound contrast agent
Harmonic Imaging
86
This type of probe provide an inside out acoustic mapping of soft tissue anatomy.
Intracavitary probe
87
linear array abdominal transducer of relatively ____ frequency = good penetration
small-parts phased-array transducer of ____ frequency = high spatial resolution
Low frequency = good penetration
High frequency = high spatial resolution
88
Three-dimensional image acquisition as a function of time (___D) allows visualization of motion during the scan and rendering of the 3D data
4D = 3D image as a function of time
89
acquires 2D tomographic image data in a series of individual B-scans of a volume of tissue
3D ultrasound imaging
90
Ultrasound Spatial Resolution components in 3 directions
1. Axial
2. Lateral
3. Elevational
91
is determined by the frequency of the ultrasound and the damping factor of the transducer, which together determine the spatial pulse length
Axial resolution
92
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 resolution
93
Axial and lateral resolutions are in the plane of the image and plainly discernable, while elevational (slice-thickness) resolution is perpendicular to the plane of the image and not as easy to understand or to interpret
Axial and Lateral resolution = In the plane of the image
Elevational (slice-thickness) resolution = Perpendicular to the plane of the image
94
Resolution in the axial direction (along the beam) is equal to ½ SPL and ______ of depth.
Lateral and elevational resolutions are strongly ____ on depth. The minimum resolution in the lateral/elevational directions is typically three to five times greater than axial resolution
Axial = independent to depth
Lateral and Elevational = dependent to depth
95
This resolution is a function of the height of the transducer array and is depth dependent as dictated by the near field/far field beam characteristics of the fixed transducer height.
Poor _____ resolution occurs adjacent to the transducer array and beyond the near/far field interface
Elevational Resolution
96
Detection of subtle anatomy in the patient is dependent on the ___-to-___ ratio.
Low-power operation (e.g., an obstetrics power setting with low transmit gain) requires higher electronic signal amplification to increase the weak echo amplitudes to useful levels and results in a higher noise level and lower contrast-to-noise ratio. Increasing the transmit power and/or the PRF can improve contrast resolution, but there is a limit with respect to transducer capabilities, and, furthermore, the intensity must be restricted to levels unlikely to cause biological damage
Contrast-to-Noise Ratio
Low power operation = requires higher electronic signal amplification = higher noise, lower contrast
High power = improve contrast resolution
97
provides multiple beam angles to better depict tissue boundaries, as well as provide averaging to reduce stochastic speckle and electronic noise
Spatial Compounding
98
In contrast resolution, this give rise to reflections that delineate tissue boundaries and internal architectures
Acoustic impedance differences
99
In contrast resolution, this produce a specific “texture” (or lack of texture) that provides recognition and detection capabilities over the FOV
Density and size of scatters within tissues or organs
100
In contrast resolution, this introduces additive signals from areas other than from echoes generated by specular reflectors and degrades image contrast
Ultrasound Scattering
101
These arise from the incorrect display of anatomy or noise during imaging. The causes are machine and operator related, as well as intrinsic to the characteristics and interaction of ultrasound with the tissues
Artifacts
102
Artifact caused by a change in the transmitted ultrasound pulse direction at a boundary with nonperpendicular incidence, when the two tissues support a different speed of sound.
Misplaced anatomy often occurs in the image during the scan.
Anatomical displacement due to these artifacts will change with the position of the transducer and angle of incidence with the tissue boundaries
Refraction
103
This artifact is a 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
104
This artifact occurs distal to objects having very low ultrasound attenuation, such as fluid-filled cavities (e.g., a filled bladder or cysts). Hyperintense signals (“through transmission”) arise from increased transmission of sound by these structures
Enhancement
105
These artifacts 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. These artifacts are often caused by reflections between a highly reflective interface and the transducer or between reflective interfaces such as metallic objects (e.g., bullet fragments), calcified tissues, or air pocket/ partial liquid areas of the anatomy.
These echoes are typically manifested as multiple, equally spaced boundaries with decreasing amplitude along a straight line from the transducer
Reverberation
106
Comet Tail artifact is a form of ____ artifact.
Reverberation
107
Also a form of reverberation or resonance artifacts; these artifacts arise from resonant vibrations within fluid trapped 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
108
echogenic intramural foci from which emanate V-shaped comet tail reverberation artifacts are highly specific for _____________, representing the unique acoustic signature of cholesterol crystals within the lumina of Rokitansky-Aschoff sinuses
-radiopedia
Adenomyomatosis
109
a special type of resonance artifact. Its appearance is similar to the ladder-like reverberation of comet-tail artifact, but it is produced by a completely different mechanism.
The artifact is only associated with gas bubbles, and occurs when an ultrasound pulse encounters a "horn" or "bugle" shaped fluid collection that is trapped between an inverted tetrahedron of 4 bubbles (3 on top and 1 nestled deep to them). The trapped fluid resonates, emitting a continuous signal back to the transducer. Whereas the transducer pulse is broad spectrum, the returning signal consists of one or more discrete (resonant) frequencies. "Beats" between these frequencies produce the variable appearance of the ring down. There is no "reverberation" ( i.e. multiple reflectances).
This artifact can be eliminated by angling the ultrasound probe.
Ring down artifact
110
An artifact that is caused by the variability of speed of sound in different tissues. In particular, the lower speed of sound in fat (1,450 m/s) causes edge discontinuities of organ borders distal to fatty tissue.
Range and distance uncertainty result from this artifact, and this also reduces the accuracy of spatial measurements made with ultrasound
Speed displacement artifact
111
These are emissions of the ultrasound 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. [still under artifacts]
One type of this artifact occurs near a highly reflective surface just out of the main beam
This occurs, for instance, in imaging of the gall bladder, where _____ produce artifactual “pseudosludge” in an otherwise echo-free organ
Side lobe artifact
112
These artifacts occur with multielement array transducers and result from the division of a smooth transducer surface into a large number of small elements.
Ultrasound energy is produced at a large angle relative to the main beam. This misdirected energy can create ghost images of off-axis high-contrast objects. _______ artifacts are reduced by using very closely spaced elements in the array (less than one-half wavelength apart). Linear array transducers are more prone to _____ artifacts than phased-array transducers, chiefly due to the larger width and spacing of the individual elements
Grating lobe artifacts
113
Near highly reflective surfaces, multiple beam reflections and refractions can find their way back to the transducer. The anatomy involved in these reflections is misplaced on the beam axis more distal to the actual position caused by delays of the echoes returning from the reflector(s)
The back and forth travel distance of the second echo set from the mass produces an artifact in the image that resembles a mirror image of the mass, placed beyond the diaphragm
Multipath Reflection and mirror images
114
These artifacts are created when a high PRF limits the amount of time spent listening for echoes during the PRP. As the PRF increases, the PRP decreases, with returning echoes still arriving from a greater depth after the next pulse is initiated. Mismapping of very deep echoes to shallow positions can occur in the image
Ambiguity artifacts
115
This artifactual appearance is possibly due to echoes from the strong reflector with frequency changes due to the wide bandwidth of the initial pulse and the narrow band “ringing” caused by the structure.
This artifact is 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
116
Artifact that is 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.
The thickness of the slice profile varies with depth, being broad close to the transducer, narrowing at the elevational focal zone, and widening with distance beyond the focal zone.
Consequences of this slice-thickness shape are loss of signal from objects that are much smaller than the volume element due to partial volume averaging and inclusion of signals from highly reflective objects that are not in the imaging plane. These artifacts are most significant at distances close and far from the transducer
Slice thickness;
Consequences are loss of signal from objects that are muchsmaller than volume elemtn due to PARTIAL VOLUME AVERAGING
117
The most frequently reported source of performance instability of an ultrasound system
Display or maladjusted video monitors
118
In UTZ quality control:
This is evaluated with “gray-scale” objects of lower and higher attenuation than the tissue-mimicking gel
Contrast resolution
119
In UTZ quality control:
THis is evaluated by measuring the lateral spread of the high-contrast targets as a function of depth and transmit focus
Lateral resolution
120
In UTZ quality control:
These are evaluated with the “sphere” module (Fig. 14-53B). The ultrasound image of the spherical targets illustrates the effects of slice-thickness variation with depth and the dependence of resolvability on object size
Elevational resolution and partial volume effects
121
_____ is the rate of energy production, absorption, or flow. The SI unit of ____ is the watt (W), defined as one joule of energy per second
Power
122
the rate at which sound energy flows through 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
123
the highest instantaneous intensity in the beam
Temporal peak
124
the time-averaged intensity over the PRP
Temporal average
125
Average intensity of the pulse
Pulse average
126
is the highest intensity spatially in the beam
spatial peak
127
Average intesity over the beam area, taken to be the area of the transducer
spatial average
128
acoustic power contained in the ultrasound beam (watts), averaged over at least one PRP and divided by the beam area (usually the area of the transducer face)
spatial average–temporal average intensity ISATA
129
ratio of the acoustical power produced by the transducer to the power required to raise tissue in the beam area by 1°C
Thermal index
130
In mechanical index, this is a consequence of the negative pressures (rarefaction of the mechanical wave) that induce bubble formation from the extraction of dissolved gases in the medium
Cavitation
131
A value that estimates the likelihood of cavitation by the ultrasound beam
It is directly proportional to the peak rarefactional (negative) pressure and inversely proportional to the square root of the ultrasound frequency (in MHz)
As the ultrasound output power (transmit pulse amplitude) is increased, the MI increases linearly, while an increase in the transducer frequency (say from 2 to 8 MHz) decreases the MI by the square root of 4 or by a factor of two
Mechanical index (MI)
132
With higher energy deposition over a short period, ______ can occur, broadly defined as sonically generated activity of highly compressible bodies composed of gas and/or vapor
Cavitation
133
______ cavitation generally refers to the pulsation (expansion and contraction) of persistent bubbles in the tissue that occur at low and intermediate ultrasound intensities (as used clinically). Chiefly related to the peak rarefactional pressure, the MI is an estimate for producing cavitation.
At higher ultrasound intensity levels, _____ cavitation can occur, whereby the bubbles respond nonlinearly to the driving force, causing a collapse approaching the speed of sound
Stable cavitation;
Transient cavitation
