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The Essential Physics of Medical Imaging > Ultrasound > Flashcards

Ultrasound Flashcards

1
Q

It is the term that describes sound waves of frequencies exceeding the range of human hearing and their propagation in a medium.

A

Ultrasound

Medical diagnostic ultrasound is a modality that uses ultrasound energy and the acoustic properties of the body to produce an image from stationary and moving tissues.

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2
Q

This is a mechanic energy that propagates through a continuous, elastic medium by the compression and rarefaction of “particles” that comprise it.

A

Sound

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3
Q

This is caused by a mechanical deformation induced by an external force, with resultant increase in the pressure of the medium.

A

Compression

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4
Q

This occurs following compression event -as the backward motion of the “piston” reverses the force, the compressed particles transfer their energy to adjacent particles with a subsequent reduction in the local pressure amplitude.

A

Rarefaction

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5
Q

Energy propagation occurs as a wave front in the direction of energy travel, known as a what?.

A

Longitudinal wave

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6
Q

The is the distance between compressions or rarefactions, or between any two points that repeat on the sinusoid always wave of pressure amplitude.

A

Wavelength

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7
Q

This is the number of times the wave oscillates through one cycle each second.

A

Frequency

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8
Q

Sound waves with frequencies less than 15 cycles per second (Hz) are called what?.

A

Infrasound

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9
Q

The range between 15 Hz and 20 kHz comprises what?.

A

Audible acoustic spectrum

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10
Q

Ultrasound represents the frequency range above how many kHz?

A

20 kHz

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11
Q

Medical ultrasound uses frequencies in the range of how many MHz?

A

2 to 10 MHz

With speciliazed ultrasound applications up to 50 MHz.

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12
Q

This is the time duration of one wave cycle.

A

Period

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13
Q

This is the distance traveled by the wave per unit time and is equal to the wavelength divided by the period.

A

Speed of sound

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14
Q

This is determined by the ration of the bulk modulus (a measure of the stiffness of a medium and its resistance to being compressed) and the medium.

A

Wave speed

A highly compressible medium, such as air, has a low speed of sound, while a less compressible medium, such as bone, has a higher speed of sound.

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15
Q

What are the average speed for “soft tissue”, fatty tissue, and air?

A

Soft tissue = 1,540 m/s

Fatty tissue = 1,450 m/s

Air = 330 m/s

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16
Q

Higher frequency has _______ wavelength.

Shorter or longer?

A

Shorter

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17
Q

For body parts requiring greater travel distance of sound waves (e.g. abdominal imaging), lower frequency ultrasound is used to image significant depths.

Approximately how many MHz?

A

3.5 to 5 MHz

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18
Q

For small body parts or organs close to the skin surface (e.g., thyroid, breast), higher frequency ultrasound is selected.

Approximately how many MHz

A

7.5 to 10 MHz

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19
Q

Most medical imaging applications use ultrasound frequencies in what range?

A

2 to 10 MHz

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20
Q

Interaction of two or more separate ultrasound beams in a medium can result in what?.

A

Constructive and/or destructive wave interferences

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21
Q

The amount f constructive or destructive wave interference depends on several factors, but most important are the _________ and ________ of the interacting beams.

A

Phase (position of the periodic wave with respect to a reference point) and amplitude

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22
Q

The spatial resolution of the ultrasound image and the attenuation of the ultrasound beam energy depend on the ________ and _______, respectively.

A

Wavelength and frequency

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23
Q

This interference occurs with two ultrasound waves of the same frequency and phase, resulting in a higher amplitude output wave.

A

Constructive interference

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24
Q

This interference occurs with the waves 180 degrees out-of-phase, resulting in lower amplitude output wave

A

Destructive interference

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25
Q

This interference occurs when waves of slightly different frequency interact, resulting in an output waveform of higher and lower amplitude.

A

Complex interference

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26
Q

The SI unit of pressure is the what?

It is defined as one Newton per square meter.

A

Pascal (Pa)

The average atmospheric pressure on earth at sea level is approximately equal to 100,000 Pa. Diagnostic ultrasound beams typically deliver peak pressure levels that exceed ten times the earth’s atmosphere pressure, or about one MPa (mega Pascal).

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27
Q

This is the amount of power (energy per unit time) per unit area and is proportional to the square of the pressure amplitude.

A

Intensity

Medical diagnostic ultrasound intensity levels are described in units of milliwatts/cm2 -the amount of energy per unit time per unit area.

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28
Q

As ultrasound energy propagates through a medium, interactions includes:

A

Reflection, refraction, scattering, and absorption

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29
Q

This occurs at tissue boundaries where there is a difference in the acoustic impedance of adjacent materials.

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.

A

Reflection

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30
Q

This describes the change in direction of the transmitted ultrasound energy with nonperpendicular incidence.

A

Refraction

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31
Q

This occurs by reflection or refraction, the usually by small particles within the tissue medium, causes the beam to diffuse in may directions, and gives rise to the characteristic texture and gray scale in the acoustic image.

A

Scattering

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32
Q

This refers to the loss of intensity of the ultrasound beam from absorption and scattering in the medium.

A

Attenuation

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33
Q

This is the process whereby acoustic energy is converted to heat energy, whereby, sound energy is lost and cannot be recovered.

A

Absorption

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34
Q

This occurs because of the differences in acoustic impedances of the two tissues.

A

Reflection - of ultrasound energy between two tissues

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35
Q

This describes the fraction of sound intensity incident on an interface that is reflected.

A

Reflection coefficient

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36
Q

This is defined as the fraction of the incident intensity that is transmitted across an interface.

A

Intensity transmission coefficient

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37
Q

A conduit of tissue that allows ultrasound transmission through structures such as the lung is known as what?

A

Acoustic window

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38
Q

This describes the change in direction of the transmitted ultrasound energy at a tissue boundary when the beam is not perpendicular to the boundary.

A

Refraction

Straight-line propagation is assumed in ultrasound signal processing, and when refraction does occur, misplacement of anatomy in the image can result.

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39
Q

This arises from objects and interfaces within a tissue that are about the size of the wavelength or smaller and represents a rough or nonspecular reflector surface.

A

Acoustic scattering

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40
Q

These are terms used for describing the scatter characteristics relative to the average background signal.

A

Hyperechoic (higher scatter amplitude) and hypoechoic (lower scatter amplitude)

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41
Q

It is caused chiefly by scattering and tissue absorption of the incident beam.

A

Ultrasound attenuation

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42
Q

Absorbed acoustic energy is converted to ________ in the tissue.

A

Heat

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43
Q

This is the relative intensity loss per centimeter of travel for a given medium.

A

Attenuation coefficient

Expressed in units of dB/cm.

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44
Q

Ultrasound is produced and detected with what?

It is comprised of one or more ceramic elements with electromechanical properties and peripheral components.

A

Transducer

The ceramic element converts electrical energy into mechanical energy to produce ultrasound and mechanical energy into electric energy for ultrasound detection.

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45
Q

What are the major components of a transducer?

A 
Piezoelectric material
Matching layer 
Blacking block 
Acoustic absorber 
Insulating cover
Tuning coil
Sensor electrodes
Transducer housing
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46
Q

This is the functional component of the transducer.

It converts electrical energy into mechanical (sound) energy by physical deformations the crystal structure.

A

Piezoelectric material (often a crystal or ceramic)

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47
Q

These are molecular entities containing positive and negative electric charges that have an overall neutral charge.

A

Electrical dipoles

When mechanically compressed by an externally applied pressure, the alignment of the dipoles is disturbed from the equilibrium position to cause an imbalance of the charge distribution.

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48
Q

This is an example of a natural piezoelectric material.

Commonly used in watches and other timepieces to provide mechanical vibration source at 32.768 kHz.

A

Quartz crystal

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49
Q

Ultrasound transducers for medical imaging applications employ a synthetic piezoelectric ceramic.

What is the most often material used?

A

Lead-zirconate-titanate (PZT)

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50
Q

For Lead-zirconate-titanate (PZT) in its natural state, no piezoelectric properties are exhibited; however, heating the material past its __________ and applying an external voltage causes the dipoles to align in the ceramic.

A

“Curie temperature” (e.g., 328 to 365 C)

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51
Q

This transducer is for pulse-echo ultrasound imaging which operate in a “resonance” mode, whereby a voltage (usually 150 V) of very short duration (a voltage spike of 1 us) is applied, causing piezoelectric material to initially contract and the subsequently vibrate at a natural resonance frequency

A

Resonance transducer

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52
Q

The is layered on the back of the piezoelectric element, and absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing.

A

Damping block

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)

53
Q

Dampening of the vibration lessens the purity of the resonance frequency and introduces a broadband frequency spectrum.

This is also known as what?

A

“Ring-down”

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.

54
Q

This describes the bandwidth of the sound emanating from a transducer

A

“Q factor”

A “high Q” transducer has a narrow bandwidth (i.e., very little damping) and a corresponding long SPL.

55
Q

This provides the interface between the raw transducer element and the tissue and minimize the acoustic impedance differences between the transducer and the patient.

It consist of layers of materials with acoustic impedances that are intermediate to soft tissue and the transducer material.

A

Matching layer

The thickness of each layer is equal 1/4 wavelength, determined from the center operating frequency of the transducer and speed characteristic of the matching layer.

56
Q

Modern transducer design coupled with digital signal processing enables ________ transducer operation, whereby the center frequency can be adjusted in the transmit mode.

A

“Mutifrequency” or “multihertz” transducer

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.

57
Q

These transducers contain 256 to 512 elements; physically these are the largest transducer assemblies.

A

Linear array

In operation, the simultaneous firing of a small group of approximately 20 adjacent elements produces the ultrasound beam.

58
Q

This transducer is usually 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.

A

Phased-array

By using time delays in the electrical activation of the discrete elements across the face of the transducer, the ultrasound can be stored and focused electronically without physically moving the transducer on the patient.

59
Q

This is also known as the Fresnel zone.

It is adjacent to the transducer face and has a converging beam profile.

A

Near field

Beam convergence in the near field occurs because of multiple constructive and destructive interference patterns of the ultrasound waves from the transducer surface.

60
Q

This principle describes a large transducer surface as an infinite number of point source of sound energy where each point is characterized as a radial emitter.

A

“Huygens’ principle”

61
Q

This is also known as the Fraunhofer zone and is where the beam diverges.

A

Far field

Less beam divergence occurs with high-frequency, large-diameter transducers.

62
Q

This is a method to rephase he signals by dynamically introducing electronic delays as function of depth (time).

A

Dynamic receive focusing

At shallow depths, rephasing delays between adjacent transducer elements are greatest.

63
Q

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 propagation.

A

Dynamic aperture

64
Q

This are 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.

A

Side lobes

In the receive mode transducer operation, echoes generated from the side lobes are unavoidable remapped along the main beam, which can introduce artifacts in the images.

65
Q

This result when ultrasound energy is emitted far off-axis by multi-element arrays and are a consequence of the non-continuous transducer surface of the discrete elements.

A

Grating lobes

The grating lobe effect is equivalent to placing a grafting in front of a continuous transducer element, producing coherent waves directed at a large angle away from the main beam.

66
Q

In ultrasound, the major factor that limits the spatial resolution and visibility of detail is what?

A

Volume of the acoustic pulse

67
Q

This refers to the ability discern two closely spaced objects in the direction of the beam.

A

Axial resolution

68
Q

This is also known as linear, range, longitudinal, or depth resolution.

A

Axial resolution

69
Q

This resolution, along the direction of the beam, is independent of depth.

A

Axial resolution

This remain constant with depth

70
Q

This refers to the ability to discern as separate two closely spaced objects perpendicular to the beam.

A

Lateral resolution

For both single-element transducers and multielement array transducers, the beam diameter determines the lateral resolution.

71
Q

This is also known as azimuthal resolution.

A

Lateral resolution

72
Q

The best lateral resolution occurs at where?

A

Near field-far field interface

73
Q

This dimension of the ultrasound beam is perpendicular to the image plane.

This 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.

A

Elevational or slide thickness

Elevational resolution is dependent on the transducer element height in much the same way that the lateral resolution is dependent on the transducer element width.

74
Q

This is typically the weakest measure of resolution for array transducers.

A

Slice thickness

75
Q

Multiple linear array transducers with five to seven rows, known as what?

They have the ability to steer and focus the beam in the elevational dimension.

A

1.5D transducer arrays

76
Q

Image formation using the pulse-echo approach requires a number of hardware components. What are these components?

A 
Beam former
Pulse
Receiver
Amplifier
Scan converter/image memory
Display system
77
Q

This is 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.

A

Beam former

Most modern, high-end ultrasound equipment incorporates a digital beam former.

78
Q

This provides the electrical voltage for exciting the piezoelectric transducer elements and controls the output transmit power by adjustment of applied voltage.

This is also known as the transmitter.

A

Pulser

79
Q

This is synchronized with the pulser, and isolates the high voltage associated with pulsing (150 V) from the sensitive amplification stages during received mode, with induced voltages ranging from approximately 1 V to 2 uV from returning echoes.

A

Transmit/receive switch

After the ring-down time, when vibration of the piezoelectric material has stopped , the transducer electronics are switched to sensing surface charge variations of mechanical deflection caused by returning echoes, over a period up to about 1,000us (1 ms)

80
Q

Mode of transducer operation, where the ultrasound is intermittently transmitted, with majority of the time occupied by listening for echoes?

A

Pulse-echo

81
Q

The ultrasound pulse is created with short voltage waveform provided by the pulser of the ultrasound system.

This event is sometimes known as the __________.

A

Main bang

82
Q

The number of times the transducer is pulsed per second is known as what?

A

Pulse repetition frequency (PRF)

For imaging, the PRF typically ranges from 2,000 to 4,000 pulses per second (2 to 4 kHz).

83
Q

The time between pulses is called what?

It is equal to the inverse of pulse repetition frequency (PRF).

A

Pulse repetition period (PRP)

The maximum PRF is determined by the time required for echoes from the most distant structures to the transducer.

84
Q

This is determined from the product of the speed of sound and the PRP divides by 2 (the factor of 2 accounts for round-trip distance).

A

Maximal range

85
Q

The ultrasound frequency is calibrated in MHz, whereas PRF is in kHz, and the ultrasound period is measured in _________ compared to milliseconds for the PRP.

A

Microsecond

86
Q

This is the ratio of the number of cycles in the pulse to the transducer frequency and is equal to the instantaneous “on” time.

A

Pulse duration

Pulse consisting of two cycles with a center frequency of 2 MHz has a duration of 1 us.

87
Q

This is equal to the pulse duration divided by the PRP.

This is the fraction of “on” time.

A

Duty cycle

For realtime imaging applications, the duty cycle is typically 0.2 to 0.4%, indicating that greater than 99.5% of the scan time is spent “listening” to echoes as opposed to producing acoustic energy.

88
Q

This accepts data from the beam former during PRP, which represents echo information as a function of time (depth).

A

Receiver

89
Q

This is a user-adjustable amplification of the returning echo signals as a function of time, to further compensate for beam attenuation

A

Time gain compression (TGC)

Also known as time varied gain, depth gain compensation, and swept gain.

The ideal TGC curve makes all equally reflective boundaries equal in signal amplitude,regardless of the depth of the boundary.

90
Q

This is 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.

A

Dynamic frequency tuning

The purpose of this is to accommodate for beam softening, where increased attenuation of higher frequencies in a broad bandwidth pulse occurs as a function of depth.

Dynamic frequency tuning allows the receiver to make the most efficient use of the ultrasound frequencies incident on the transducer.

91
Q

This defines the effective operational range of an electronic range of an electronic device from the threshold signal level to the saturation level.

A

Dynamic range

Dynamic range (logarithmic) compression

92
Q

This inverts the negative amplitude signals of the echo to positive values.

A

Rectification

93
Q

These convert the rectified amplitudes of the echo into smoothed, single pulse.

A

Demodulation and envelope detection

94
Q

This sets the threshold of signal amplitudes allowed to pass to the digitization and display subsystems.

A

Rejection - level adjusements

This removes a significant amount of undesirable low-level noise and clutter generated from scattered sound or by the electronics.

95
Q

This are optimized for gray-scale range and viewing on the limited dynamic range monitors, so that subsequent adjustments to the images are unnecessary.

A

Processed images

96
Q

The receiver processes the data streaming from the beam former .

What are the steps include?

A
  1. Time gain compensation
  2. Logarithmic compression
  3. Demodulation and “envelope” detection
  4. Noise reduction
  5. Processed signal
97
Q

This is the display of the processed information from the receiver versus time.

A

A mode (A for amplitude)

98
Q

A-mode and A-line information is currently used in what applications?

A

Ophthalmology, for precise distance measurement of the eye.

Otherwise, A-mode display by itself is seldom used.

99
Q

This is the electronic conversion of the A-mode and A-line information into-brightness modulated dots along the A-line trajectory.

A

B-mode (B for brightness)

In general, the brightness of the dot is proportional to the echo signal amplitude (depending upon signal processing parameters.

The B-mode display is used for M-mode and 2-D gray-scale imaging.

100
Q

This is a technique that uses B-mode information to display the echoes from a moving organ, such as the myocardium and the valve leaflets, from a fixed transducer position and beam direction on the patient.

A

M-mode (M for motion)

101
Q

This is to create 2D images from the echo information from distinct beam directions.

A

Scan converter

To perform scan conversion to enable image data to be viewed on video on video display motions.

Scan conversion is necessary because the image acquisition and display occur in different formats.

102
Q

This is a method in which ultrasound information is obtained from several different angles of intonation and combined to produce a single image.

A

Spatial compounding

103
Q

This is based on the shift of frequency in an ultrasound wave caused by a moving reflector, such as blood cells in the vasculature.

A

Doppler ultrasound

104
Q

This is the difference incident frequency and the reflected frequency.

A

Doppler shift

105
Q

This is the simplest and least expensive device measuring blood velocity.

Two transducers are required, with one transmitting the incident ultrasound and the other detecting the resultant continuous echoes.

A

Continuous wave Doppler system

106
Q

This combines the velocity determination of continuous wave Doppler systems and the range of discrimination of pulse-echo imaging.

A

Pulsed Doppler ultrasound

107
Q

This refers to the combination of 2D B-mode imaging and pulses Doppler data acquisition.

A

Duplex scanning

108
Q

This provides a 2D visual display of moving blood in the vasculature, superimposed upon the conventions gray-scale image.

A

Color flow imaging

109
Q

Interpretation of the frequency shifts and direction of blood flow is accomplished with the fast Fourier transforms, which mathematically analyses the detected signals and generates amplitude versus frequency distribution profile known as what?

A

Doppler spectrum.

In clinical instrument, the Doppler spectrum is continuously updated in a real-time spectral Doppler display.

110
Q

This is an error caused by an insufficient sampling rate (PRF) relative to the high-frequency Doppler signals generated by fast-moving blood.

A

Aliasing

A minimum of two samples per cycle of Doppler shift frequency is required to unambiguously determine the corresponding velocity.

111
Q

This is a signal processing method that relies on the total strength of the Doppler signal (amplitude) and ignores directional (phase) information.

A

Power Doppler

112
Q

This is a change in the transmitted ultrasound pulse direction at a boundary with nonperpendicular incidence, when two tissues support a different speed of sound.

Misplaced anatomy often occurs in the image.

A

Refraction

113
Q

This 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.

A

Shadowing

114
Q

This occurs distal to objects having very low ultrasound attenuation, such as fluid-filled cavities.

A

Enhancement

115
Q

This arise from multiple echoes generated between two closely spaced interfaces reflecting ultrasound energy back and fourth during the acquisition of the signal and before the next pulse.

A

Reverberation artifacts

116
Q

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.

A

Reverberation artifacts

117
Q

Comet tail artifact is a form of ________.

A

Reverberation

118
Q

This arise from resonant vibrations within fluid trapped between w tetrahedron 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.

A

Ring-down artifacts

119
Q

This caused by the variability of speed of sound in different tissues.

A

Speed displacement artifact

120
Q

This occur, for instance, in imaging of the gallbladder, where the _______ produce artifactual “pseudosludge”in an otherwise echo-free organ.

A

Side lobes

This are emissions of the ultrasound energy that occur in a direction slightly off-axis from the main beam and arise for the expansion of the piezoelectric elements orthogonal to the main beam.

121
Q

This occurs with multielement array transducers and result from the division of a smooth transducer surface into a large number of small elements.

A

Grating lobes

122
Q

These artifacts are created when a high PRF limits the amount of time spent listening for echoes during the PRP.

A

Ambiguity artifacts

123
Q

This 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.

A

Twinkling artifact

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.

124
Q

This is determined by the beam width of the transducer array perpendicular to the image plane and is greater than the beam wider in the image plane.

A

Slice thickness

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.

125
Q

This is the ratio of the acoustical power produced by the transducer to the power required to raise tissue in the beam area by 1C.

A

Thermal index

126
Q

This is a consequence of the negative pressure (rarefaction of the mechanical wave) that induce bubble formation from the extraction of dissolved gases in he medium.

A

Cavitation

127
Q

This is a value that estimates the likelihood of cavitation by the ultrasound beam.

A

Mechanical index

128
Q

This generally refers to the pulsation (expansion and contraction) of persistent bubbles in the tissue that occur at low and intermediate ultrasound intensities.

A

Stable cavitation

129
Q

At higher ultrasound intensity levels, this can occur, whereby the bubbles respond nonlinearly to the driving force, causing collapse approaching the speed of sound.

A

Transient cavitation

The Essential Physics of Medical Imaging (11 decks)

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