AIM: Ch 14: Ultrasound Flashcards
Term that describes sound waves of frequencies exceeding the range of human hearing and their propagation in a medium
Ultrasound
Modality that uses ultrasound energy and the acoustic properties of the body to produce an image from stationary and moving tissues
Medical diagnostic ultrasound
It generates the sound pulses and detects returning echoes to be converted to 2D tomographic image using ultrasound acquisition system
Transducer
Medical uses of ultrasound came about shortly after the close of which era?
World War II
Wave front by which energy propagation in ultrasound occurs in the direction of energy travel
Longitudinal wave
It refers to the distance between compressions or rarefactions, or between any two points that repeat on the sinusoidal wave of pressure amplitude
Wavelength
Unit of wavelength
mm or um
It refers to the number of times the wave oscillates through one cycle each second
Frequency
Unit of frequency
1/s or Hertz (Hz)
Identify the frequency range of the following:
1. Infrasound
2. Audible sound
3. Ultrasound
4. Medical ultrasound
- Infrasound: <15 Hz
- Audible sound: 15-20 kHz
- Ultrasound: >20 kHz
- Medical ultrasound: 2-10 MHz up to 50 MHz
It is the distance traveled by the wave per unit time and is equal to the wavelength divided by the period.
Speed of sound
Unit of speed of sound
m/s
Relationship of period and frequency
Inversely related
Two factors that affect wave speed
- Bulk modulus
- Density of the medium
A measure of the stiffness of a medium and its resistance to being compressed
Bulk modulus
SI units for B and density of the medium
kg/(m-s2), kg/m3
Average speed of sound for the following:
1. Soft tissue
2. Fatty tissue
3. Air
- 1540 m/s
- 1450 m/s
- 330 m/s
It is a fundamental property that generates echoes and (contrast) in an ultrasound image
Difference in the speed of sound at tissue boundaries
Which speed of sound is being used by medical ultrasound machines when determining localization of reflectors and creating the acoustic image
Speed of sound of soft tissues: 1540 m/s
The spatial resolution of the ultrasound image depends on what factor?
Wavelength
Ultrasound wavelength affects the spatial resolution achievable along the direction of the beam
The attenuation of the ultrasound beam energy depends on what factor?
Frequency
A high-frequency ultrasound beam (small wavelength) provides better resolution and image detail than a low-frequency beam; however, the depth of beam penetration is significantly reduced at higher frequency
Most important factors that determine the amount of constructive or destructive interference of the interacting beams are:
Phase: position of the periodic wave with respect to a reference point
Amplitude of the interacting beams
It is defined as the peak maximum or peak minimum value from the average pressure on the medium in the absence of a sound wave
Pressure amplitude
T/F: In most diagnostic ultrasound applications, the compressional amplitude significantly exceeds the rarefactional amplitude
True
SI unit of pressure amplitude
Pascal (Pa) = N/m2
It is defined as amount of power per unit area
Intensity, I
T/F: Intensity is inversely proportional to pressure amplitude
False
It is proportional to the SQUARE of the pressure amplitude
SI unit of medical diagnostic ultrasound intensity
milliwatts/cm2
It describes the logarithmic ratio of relative intensity and pressure levels
Decibel (dB)
T/F: The absolute intensity level depends upon the method of ultrasound production
True
This refers to the tissue thickness that reduces the ultrasound intensity by 3 dB
“Half-value” thickness (HVT)
A loss of 3 dB (-3 dB) represents how much loss of signal intensity
50%
It occurs at tissue boundaries where there is a difference in the acoustic impedance of adjacent materials.
Reflection
T/F: For non-normal incidence, the incident angle is that made relative to normal incidence; the reflected angle is equal to the incident angle
True
It describes the change in direction of the transmitted ultrasound energy with nonperpendicular incidence
Refraction
It 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
It refers to the loss of intensity of the ultrasound beam from absorption and scattering in the medium.
Attenuation
It refers to the process whereby acoustic energy is converted to heat energy, whereby, sound energy is lost and cannot be recovered.
Absorption
What does this equation represent:
Acoustic impedance (Z)
delta = density, c = speed
What is the SI unit of acoustic impedance (Z)?
Rayl = 1 kg/(m2 * s)
T/F: Acoustic impedance gives rise to differences in transmission and refraction of ultrasound energy, which is the means for producing an image using pulse-echo techniques.
False
Acoustic impedance gives rise to differences in transmission and reflection of ultrasound energy, which is the means for producing an image using pulse-echo techniques.
What is the acoustic impedance of air?
0.0004 x 10^6 Rayl
What is the acoustic impedance of water?
1.48 x 10^6 Rayl
What is the acoustic impedance of bone?
7.8 x 10^6 Rayl
T/F: The reflection of ultrasound energy at a boundary between two tissues occurs because of the differences in the acoustic impedances of the two tissues.
True
It describes the fraction of sound intensity incident on an interface that is reflected.
Incident reflection coefficient (Ri)
Defined as fraction of the incident intensity that is transmitted across an interface.
Intensity transmission coefficient (Ti)
T/F: RI = 1 − TI
False
A conduit of tissue that allows ultrasound transmission through structures such as the lung is known as what?
Acoustic window
When the beam is perpendicular to the tissue boundary, the sound is returned back to the transducer as what?
Echo
As sound travels from a medium of higher acoustic impedance into a medium of lower acoustic impedance, the reflected wave experiences a 180-degree phase shift in pressure amplitude, as shown by the negative sign on the pressure amplitude values
False
However, if you use the formula, the statement in in Q card is actually true because in that case Z2 < Z1, which means their difference is a negative number. Nevertheless, the above discussion assumes a “smooth” interface between tissues, where the wavelength of the ultrasound beam is much greater than the structural variations of the boundary. With higher frequency ultrasound beams, the wavelength becomes smaller, and the boundary no longer appears smooth. In this case, returning echoes are diffusely scattered throughout the medium, and only a small fraction of the incident intensity returns to the ultrasound transducer.
T/F: For nonperpendicular incidence at an angle i, the ultrasound energy is reflected at an angle r equal to the incident angle, i = r Echoes are directed away from the source of ultrasound and are thus undetected.
True
It describes the change in direction of the transmitted ultrasound energy at a tissue boundary when the beam is not perpendicular to the boundary.
Refraction
What law describes the relationship of the angle of refraction relative to the change of speed that occurs at the boundary?
Snell’s Law
What are the 2 circumstances where no refraction occurs
- c1=c2, or
- Perpendicular incidence
This situation occurs when c2 > c1 and the angle of incidence of the sound beam with the boundary between two media exceeds an angle called the critical angle
Total reflection
In this case, the sound beam does not penetrate the second medium at all but travels along the boundary.
How do you compute the critical angle?
See below
It 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.
Acoustic scattering
It is 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
In general, the echo signal amplitude from the insonated tissues depends on 4 factors. Name them.
- Number of scatterers per unit volume
- Acoustic impedance differences at the scatterer interfaces
- Sizes of the scatterers
- Ultrasonic frequency
Hyperechoic (higher scatter amplitude) and hypoechoic (lower scatter amplitude). Hyperechoic areas usually have greater numbers of scatterers, larger acoustic impedance differences, and larger scatterers.
These terms are used for describing the scatter characteristics relative to the average background signal:
Hyperechoic
Hypoechoic
T/F: Acoustic scattering from specular reflectors increases with frequency, while nonspecular reflection is relatively independent of frequency.
False
Acoustic scattering from nonspecular reflectors increases with frequency, while specular reflection is relatively independent of frequency.
It is the loss of acoustic energy with distance traveled, and is caused chiefly by scattering and tissue absorption of the incident beam.
Ultrasound attenuation
T/F: Absorbed acoustic energy is converted to heat in the tissue.
True
Unit of attenuation coefficient, u
dB/cm
Ultrasound attenuation expressed in dB is approximately proportional to what factor?
Frequency
T/F: Since the dB scale progresses logarithmically, the beam intensity is exponentially attenuated with distance
True
What is the relationship of HVT and frequency?
Inversely proportional
As the frequency increases, the HVT decreases, as demonstrated by the examples above.
What device produces and detects ultrasound, and is comprised of one or more ceramic elements with electromechanical properties and peripheral components?
Transducer
It converts electrical energy into mechanical energy to produce ultrasound and mechanical energy into electrical energy for ultrasound detection.
Ceramic elements
It is the functional component of the transducer.
Piezoelectric material
Often a crystal or ceramic, it converts electrical energy into mechanical (sound) energy by physical deformation of the crystal structure. Conversely, mechanical pressure applied to its surface creates electrical energy.
These are characterized by a well-defined molecular arrangement of electrical dipoles
Piezoelectric materials
What comprises a single-element ultrasound transducer assembly?
See below
This is an example of a natural piezoelectric material commonly used in watches and other timepieces to provide a mechanical vibration source at 32.768 kHz for interval timing.
Quartz crystal
Discuss the mechanism by which a potential difference (voltage) is created across the piezoelectric element with one surface maintaining a net positive charge and one surface a net negative charge:
a. Under the influence of mechanical pressure from an adjacent medium (e.g., an ultrasound echo)
b. When an external voltage source applied
See below
Piezoelectric materials are characterized by a well-defined molecular arrangement of electrical dipoles, as shown in Figure 14-9. Electrical dipoles are molecular entities containing positive and negative electric charges that have an overall neutral charge. 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. A potential difference (voltage) is created across the element with one surface maintaining a net positive charge and one surface a net negative charge. Surface electrodes measure the magnitude of voltage, which is proportional to the incident mechanical pressure amplitude. Conversely, application of an external voltage through conductors attached to the surface electrodes induces the mechanical expansion and contraction of the transducer element.
Ultrasound transducers for medical imaging applications employ this synthetic piezoelectric ceramic, which is a compound with the structure of molecular dipoles.
Lead-zirconate-titanate (PZT)
This process permanently maintain the dipole orientation
Cooling
For PZT, these processes cause the dipoles to align in the ceramic
Heating the material past its “Curie temperature” (e.g., 328°C to 365°C) and applying an external voltage
Curie temperature
328°C to 365°C
T/F: For PZT in its natural state, piezoelectric properties are exhibited.
False
For PZT in its natural state, NO piezoelectric properties are exhibited
What mode do resonance transducers for pulse-echo ultrasound imaging operate, whereby a voltage of very short duration is applied, causing the piezoelectric material to initially contract and then subsequently vibrate at a natural resonance frequency?
Resonance mode
What is the usual voltage (and its duration) used for resonance mode?
Voltage: 150 V
Duration: 1 us
The natural resonance frequency is selected by the (a) of the PZT, due to the preferential emission of (b)-wavelength ultrasound waves in the piezoelectric material
a. Thickness cut
b. 1/2-wavelength
Usually, you have a given x-MHz transducer with speed of sound of PZT material ~4,000 m/s. From the formular of speed, c = wavelenght x frequency, you’ll be able to compute wavelength. For half of the computed wavelength will be the required transducer element thickness.
What factors determine the operating frequency for resonance transducers?
Speed of sound of, and the thickness of the piezoelectric material
Resonance transducers transmit and receive preferentially at a (blank)
Single center frequency
It is layered on the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing.
Damping block or backing block
The damping block is the component that also dampens the transducer vibration to create an ultrasound pulse with a SHORT (a), which is necessary to preserve detail along the (b)
a. Short pulse length (SPL)
b. Beam axis (axial resolution)
This process lessens the purity of the resonance frequency and introduces a broadband frequency spectrum.
Dampening of the vibration (aka “ring-down”)
It describes the bandwidth of the sound emanating from a transducer
Q factor
A “high Q” transducer has a narrow bandwidth (i.e., very little damping) and a corresponding long SPL. A “low Q” transducer has a wide bandwidth and short SPL.
Describe the required bandwidth transducer for the following and the reason why so:
1. Medical imaging
2. Blood velocity measurements by Doppler instrumentation
3. Continuous-wave ultrasound transducers
- Imaging applications require a broad bandwidth transducer in order to achieve high spatial resolution along the direction of beam travel.
- Blood velocity measurements by Doppler instrumentation require a relatively narrow-band transducer response in order to preserve velocity information encoded by changes in the echo frequency relative to the incident frequency.
- Continuous-wave ultrasound transducers have a very high Q characteristic.
T/F: Since the Q factor is derived from the term “quality factor,” it means that a transducer with a low Q does imply poor quality in the signal.
False
While the Q factor is derived from the term “quality factor,” a transducer with a low Q does not imply poor quality in the signal.
It provides the interface between the raw transducer element and the tissue and minimizes the acoustic impedance differences between the transducer and the patient.
Describe the relationship of Q factor and SPL
See below
What is the acoustic impedance of the acoustic coupling gel?
Acoustic impedance similar to soft tissues
What is the purpose of the acoustic coupling gel?
It used between the transducer and the skin of the patient to eliminate air pockets that could attenuate and reflect the ultrasound beam.
The thickness of each layer of the matching layer is equal to (a) wavelength, determined from the (b) of the transducer and (c) characteristics of the matching layer.
a. 1/4
b. Center operating frequency
c. Speed
Broadband multifrequency transducers have bandwidths that exceed (blank) of the center frequency
80%
Describe nonresonance (broad bandwidth) “Multifrequency” transducers
- Center frequency can be adjusted in the transmit mode
- 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 a greater transmission efficiency of the ultrasound beam without resorting to multiple matching layers.
How is excitation of the multifrequency transducer accomplished?
Short square wave burst of approximately 150 V with one to three cycles
This allows the center frequency to be selected within the limits of the transducer bandwidth. Likewise, the broad bandwidth response permits the reception of echoes within a wide range of frequencies. For instance, ultrasound pulses can be produced at a low frequency, and the echoes received at higher frequency.
Recent application of the concept of excitation of multifrequency transducer where lower frequency ultrasound is transmitted into the patient, and the higher frequency harmonics (e.g., two times the transmitted center frequency), created from the interaction with contrast agents and tissues, are received as echoes.
Harmonics imaging
Name 3 advantages of native harmonic imaging
- Greater depth of penetration
- Noise and clutter removal, and
- Improved lateral spatial resolution
Typical number of individual rectangular elements that comprise the transducer assembly.
128 to 512
Each rectangular element in a transducer assembly has a width typically less than (a) wavelength and a length of several millimeters
a. 1/2
Two modes of activation are used to produce a beam:
a. Linear (sequential)
b. “Phased” activation/received modes
Physically these are the the largest transducer assemblies, and contain how many elements?
Linear array transducers with 256 to 512 elements
In operation, the simultaneous firing of a small group of approximately (a) adjacent elements produces the ultrasound beam.
a. 20
This is produced by simultaneous activation, and defined by the number of active elements.
Synthetic aperture (effective transducer width)
This mode is when echoes are detected by acquiring signals from most of the transducer elements.
Receive mode
It occurs by firing another group of transducer elements displaced by one or two elements.
“A line” acquisition
Describe the field of view (FOV) produced for the following arrays:
1. Linear array
2. Curvilinear array
- Rectangular FOV
- Trapezodal FOV
Give the numer of individual elements in the following transducers:
1. Linear
2. Phased
- 256 to 512
- 64 to 128
Describe by which ultrasound beam is produced in the following transducers:
1. Linear
2. Phased
- Simultaneous firing of a small group of approximately 20 adjacent elements
- All transducer elements are activated nearly simultaneously
For a given transducer element, which of the following is on the order of 1/2 of the wavelength:
a. height
b. width
c. thickness
b. width
For a given transducer element, which of the following depends on the transducer design and slice-thickness requirements:
a. height
b. width
c. thickness
a. height
These high-frequency ultrasound devices, first investigated in the early 1990s, are silicon-based electrostatic transducers, recently shown to be competitive with the lead-zirconate-titanate for producing and receiving ultrasonic data for patient imaging.
Capacitive micromachined ultrasound transducers (CMUT)
Basic elements of CMUT
- Capacitor cell with a fixed electrode (backplate)
- Free electrode (membrane)
Principle of operation of CMUT, whereby 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
In terms of depth of penetration, which technology is better, piezoelectric or CMUT?
Piezoelectric array
Lateral dimensions of the membranes of CMUT (a) microns and a thickness of about (b)
a. 10 microns
b. 1 to 2 um
Two distinct beam patterns:
- A slightly converging beam (near field)
- A slightly diverging beam (far field)
What factors determine the converging beam?
- Geometry
- Frequency of the transducer
What factors determine the distance of the near field in an unfocused, single-element transducer?
- Transducer diameter
- Frequency of the transmitted sound
T/F: For multiple transducer element arrays, an “effective” transducer diameter is determined by the excitation of a group of transducer elements.
True
This zone is adjacent to the transducer face and has a converging beam profile
Near field or Fresnel zone
It (a) describes a large transducer surface as an infinite number of point sources of sound energy where each point is characterized as a (b).
a. Huygens’ Principle
b. Radial emitter
The ultrasound beam path is thus largely confined to the dimensions of the active portion of the transducer surface, with the beam diameter converging to approximately (a) the transducer diameter at the end of the near field.
a. half
Factors which the near field length is dependent on?
- Transducer diameter
- Propagation wavelength
What is the formula for near field length?
See below
If you increase the diameter by two times, the NFL will increase by how much?
4x
If you increase the radius by two times, the NFL will increase by how much?
4x
This describes the ability of the system to resolve objects in a direction perpendicular to the beam direction
Lateral resolution
Lateral resolution is dependent on what factor?
Beam diameter
For a single-element transducer, lateral resolution is:
a. best at what point/s?
b. poor at what point/s?
a. At the end of the near field
b. Areas close to and far from the transducer surface
Peak (a) occurs at the end of the near field, corresponding to the minimum (b) for a single-element transducer
a. ultrasound pressure
b. beam diameter
T/F: Only when the near field is reached do the ultrasound pressure variations decrease continuously
False
Only when the FAR field is reached do the ultrasound pressure variations decrease continuously
It is the field of ultrasound beam propagation where it diverges
Far field or Fraunhofer zone
This formula describes the relationship of the angle of ultrasound beam divergence in relation to the frequency and beam diameter for a large-area single-element transducer.
See below
For a large-area single-element transducer, what factors will result to less beam divergence?
- High frequency
- Large-diameter transducers
where c (constant) = wavelength (constant) x frequency
In the (a) field, the beam intensity varies from maximum to minimum to maximum in a converging beam, while ultrasound intensity in the (b) field decreases monotonically with distance.
a. Near
b. Far
Typical wavelength of a narrow piezoelectric element width
1/2 to 1 wavelength
In a transducer array, the narrow piezoelectric element width produces a diverging beam at what distance?
Very close to the transducer face
Transducer elements in a ____ array that are fired simultaneously produce an effective transducer width ____ to the sum of the widths of the individual elements.
Linear, equal
With a ____-array transducer, the beam is formed by interaction of the individual wave fronts from each transducer, each with a slight difference in excitation time. Minor phase differences of adjacent beams form constructive and destructive wave summations that “steer” or “focus” the beam profile.
Phased
Which transducer has an unchangeable focal depth?
Single transducer or group of simultaneously fired transducers in a linear array
It is a function of the transducer diameter, the center operating frequency, and the presence of any acoustic lenses attached to the element surface.
Focal distance
This process allows selectable focal distance in phased array transducers and many linear array transducers, and case the beam to converge at a specified distance.
Specific timing delays between transducer elements
A shallow focal zone (close to the transducer surface) is produced by firing ____ transducers in the array before the ____ transducers in a ____ pattern
outer, inner, symmetrical
Greater focal distances are achieved by ____ the delay time differences amongst the transducer elements, resulting in more distal beam convergence.
Reducing
It is a method to rephase the signals by dynamically introducing electronic delays as function of depth (time).
Dynamic receive focusing
At ____ depths, rephasing delays between adjacent transducer elements are greatest.
Shallow
With greater depth, there is less difference, so the phase delay circuitry for the receiver varies as a function of ____
Echo listening time
It is the process that 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
A lateral spatial resolution of the linear array beam varies with ____, dependent on the ____
a. depth
b. linear dimension of the transducer width (aperture)
These 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
Side lobes
T/F: In the receive mode of transducer operation, echoes generated from the side lobes are unavoidably remapped along the main beam, which can introduce artifacts in the image.
True
Given the mode operation, describe frequency bandwidth, Q, and quantify side lobe energy:
1. Continuous mode operation
2. Pulsed mode operation
- Continuous mode operation, narrow bandwidth, high Q, high side lobe energy
- Pulsed mode operation, high bandwidth, low Q, reduced lobe energy
In continuous mode operation, the narrow frequency bandwidth of the transducer (high Q) causes the side lobe energy to be a significant fraction of the total beam.
In pulsed mode operation, the low Q, broadband ultrasound beam produces a spectrum of acoustic wavelengths that reduce the emission of side lobe energy.
Give 2 methods to reduce side lobe emissions
- Keep individual transducer element widths small (less than 1/2 wavelength)
- Reduce the amplitude of the peripheral transducer element excitations relative to the central element excitations
These 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
In ultrasound, what major factor limits the spatial resolution and visibility of detail?
Volume of the acoustic pulse
These dimensions determine the minimal volume element
Axial, lateral and elevational (slice-thickness) dimensions
It 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)
How to achieve good axial resolution?
Returning echoes be distinct without overlap
What is the minimal required separation distance between two reflectors to avoid the overlap of returning echoes?
1/2 of the SPL
T/F: The distance traveled between two reflectors is twice the separation distance.
True
How many cycles are typically there for every ultrasound pulse?
3
How to achieve shorter pulses?
- Greater damping of the transducer element (to reduce the pulse duration and number of cycles)
- Higher frequency (to reduce wavelength)
Number of cycles emitted per pulse by the transducer multiplied by the wavelength
SPL
T/F: At a given frequency, shorter pulse lengths require heavy damping and low Q, broad bandwidth operation.
True
T/F: For a constant damping factor, higher frequencies (shorter wavelengths) give better axial resolution, but the imaging depth is increased.
False
For a constant damping factor, higher frequencies (shorter wavelengths) give better axial resolution, but the imaging depth is reduced.
T/F: Axial resolution remains constant with depth.
True
It refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction.
Lateral resolution, also known as azimuthal resolution
For both single-element transducers and multielement array transducers, what determines the lateral resolution?
Beam diameter
T/F: Since the beam diameter varies with distance from the transducer in the near and far field, the lateral resolution is depth independent.
False
Since the beam diameter varies with distance from the transducer in the near and far field, the lateral resolution is depth DEPENDENT.
The best lateral resolution occurs at what point?
near field-far field interface
At the point where the best lateral resolution occurs, what is the estimated effective beam diameter?
Approximately 1/2 the transducer diameter
T/F: In the near field, the beam diverges and substantially reduces the lateral resolution.
False
In the far field, the beam diverges and substantially reduces the lateral resolution.
NOTE: Pls revisit p525-526
Last 2 paragraphs
T/F: Increasing the number of focal zones improves overall in-focus lateral resolution with depth, but the amount of time required to produce an image increases, with a consequent reduction in frame rate and/or number of scan lines per image (see section on image quality).
True
This dimension of the ultrasound beam is perpendicular to the image plane.
Elevational or slice-thickness dimension
What factors are the following dependent on:
1. Elevational dimension
2. Lateral resolution
- Transducer element height
- Transducer element width
This is typically the weakest measure of resolution for array transducers.
Slice thickness
These are multiple linear array transducers with five to seven rows, and have the ability to steer and focus the beam in the elevational dimension.
5D transducer arrays
Image formation using the pulse-echo approach requires a number of hardware components (5):
- Beam former
- Pulser
- Receiver
- Amplifier
- Scan converter/image memory, and
- Display system
Components of the ultrasound imager. This schematic depicts the design of a digital acquisition/digital beam former system, where each of the transducer elements in the array has a pulser, transmit-receive switch, preamplifier, and ADC (e.g., for a 128-element phased array, there are 128 components as shaded boxes). Swept gain reduces the dynamic range of the signals prior to digitization. The beam former provides focusing, steering, and summation of the beam; the receiver processes the data for optimal display, and the scan converter produces the output image rendered on the monitor. Thick lines indicate the path of ultrasound data through the system.
It 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.
Beam former
It controls application-specific integrated circuits that provide transmit/receive switches, digital-to-analog and analog-to-digital converters (ADCs), and preamplification and time gain compensation (TGC) circuitry for each of the transducer elements in the array.
Digital beam former
It provides the electrical voltage for exciting the piezoelectric transducer elements and controls the output transmit power by adjustment of the applied voltage.
Pulser
Another term for pulser
Transmitter
In digital former beam, this determines the amplitude of the voltage
Digital-to-analog converter (DAC)
Determine the effect to the following factors if there is an increase in the transmit amplitude:
1. Intensity sound
2. Echo detection from weaker reflectors
3. Signal-to-noise ratio
4. Power deposition to the patient
- Higher
- Improved
- Higher
- Higher
It is synchronized with the pulser, and 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
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 the returning echoes, over a period up to about ____
1,000 μs (1 ms)
In the pulse-echo mode of transducer operation, 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. This event is sometimes known as the ____
Main bang
T/F: One pulse-echo sequence produces one amplitude modulated (A-line) of image data.
True
The number of times the transducer is pulsed per second is known as the ____
PRF (Pulse repetition frequency)
For imaging, the PRF typically ranges from ____
2,000 to 4,000 pulses per second (2 to 4 kHz).
What is pulse repetition period?
The time between pulses, and is equal to the inverse of the PRF
T/F: An increase in PRF results in an increase in echo listening time.
False
An increase in PRF results in a DECREASE in echo listening time.
T/F: The maximum PRF is determined by the time required for echoes from the most distant structures to reach the transducer.
True
It is determined from the product of the speed of sound and the PRP divided by 2 (the factor of 2 accounts for round-trip distance):
Maximal range
T/F: Higher ultrasound frequency operation has limited penetration depth, allowing high PRFs.
True
Conversely, lower frequency operation requires lower PRFs because echoes can return from greater depths.
It 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
Also known as the fraction of “on” time, and is equal to the pulse duration divided by the PRP.
Duty cycle
For realtime imaging applications, the duty cycle is typically ____
0.2% to 0.4%
This indicates that greater than 99.5% of the scan time is spent “listening” to echoes as opposed to producing acoustic energy.
T/F: In state-of-the-art ultrasound units, each piezoelectric element has its own preamplifier and ADC. A typical sampling rate of 40 to 60 MHz with 16 to 32 bits of precision is used
False
In state-of-the-art ultrasound units, each piezoelectric element has its own preamplifier and ADC. A typical sampling rate of 20 to 40 MHz with 8 to 12 bits of precision is used
Typical PRF, PRP and Duty Cycle for:
M-mode
500, 2000, 0.05
Typical PRF, PRP and Duty Cycle for:
Real-time
2000 – 4000, 500 – 250, 0.2 – 0.4
Which operation mode has the highest PRP?
M-mode
Which operation mode has the lowest PRP?
Pulsed doppler
T/F: ADCs with larger bit depths and sampling rates are necessary for systems that digitize the signals directly from the preamplification stage. In systems where digitization of the signal occurs after analog beam formation and summing, a single ADC with less demanding requirements is typically employed.
True
It is a user-adjustable amplification of the returning echo signals as a function of time, to further compensate for beam attenuation.
TGC (also known as time varied gain, depth gain compensation, and swept gain)
This accepts data from the beam former during the PRP, which represents echo information as a function of time (depth).
Receiver
T/F: The ideal TGC curve makes all equally reflective boundaries equal in signal amplitude, regardless of the depth of the boundary.
True
User adjustment of TGC is typically achieved by: (2)
- Multiple slider potentiometers: where each slider represents a given depth in the image, or
- 3-knob TGC control: which controls the initial gain, slope, and far gain of the echo signals
T/F: The TGC amplification effectively reduces the maximum to minimum range of the echo voltages as a function of time to approximately by how much?
50 dB (300:1)
It is 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.
Dynamic frequency tuning
What is beam softening?
Increase in the attenuation of higher frequencies in a broad bandwidth pulse occurs as a function of depth
What is the purpose of dynamic frequency tuning?
To accommodate beam softening
It defines the effective operational range of an electronic device from the threshold signal level to the saturation level
Dynamic range
Which parts of signal processing of data receved beam former during PRP are user adjustable?
TGC, noisre rejection level
Key components in the ultrasound detection and display that are most affected by a wide dynamic range include the ____ and ____
ADC, display
Thus, after TGC, the signals must be reduced to ____ to ____ dB, which is accomplished by compression using ____ to increase the smallest echo amplitudes and to decrease the largest amplitudes.
20 to 30 db, logarithmic amplification
T/F: Logarithmic amplification produces an output signal proportional to the logarithm of the input signal.
True
It inverts the negative amplitude signals of the echo to positive values.
Rectification
These processes convert the rectified amplitudes of the echo into a smoothed, single pulse.
Demodulation and envelope detection
This process sets the threshold of signal amplitudes allowed to pass to the digitization and display subsystems. This removes a significant amount of undesirable low-level noise and clutter generated from scattered sound or by the electronics.
Rejection level adjustment
These are optimized for gray-scale range and viewing on the limited dynamic range monitors, so that subsequent adjustments to the images are unnecessary.
Processed images
T/F: Inappropriate adjustment of TGC can lead to artifactual enhancement of tissue boundaries and tissue texture as well as uniform response versus depth.
False
Inappropriate adjustment of TGC can lead to artifactual enhancement of tissue boundaries and tissue texture as well as NONuniform response versus depth.
The earliest uses of ultrasound in medicine used A-mode information
To determine the midline position of the brain for revealing possible mass effect of brain tumors
Current use of -mode and A-line information
Ophthalmology applications for precise distance measurements of the eye
This mode is the display of the processed information from the receiver versus time
A-mode
In A-mode, as echoes return from tissue boundaries and scatterers (a function of the acoustic impedance differences in the tissues), a digital signal proportional to ____ is produced as a function of ____
echo amplitude, time
Result of A-mode
One “A-line” of data per PRP
Mode where electronic conversion of the A-mode and A-line information into brightness-modulated dots along the A-line trajectory occurs.
B-mode
In B-mode, In general, the ____ of the dot is proportional to the echo signal ____ (depending upon signal processing parameters).
brightness, amplitude
B-mode is used for 2 purposes
- M-mode
- 2D gray-scale imaging
It is 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
In M-mode, the echo data from a single ultrasound beam passing through moving anatomy are acquired and displayed as a function of time, represented by reflector ____ on the vertical axis (beam path direction) and ____ on the horizontal axis
depth, time
T/F: Only one anatomical dimension is represented by the M-mode technique
True
This creates 2D images from echo information from distinct beam directions and to perform scan conversion to enable image data to be viewed on video display monitors.
Scan converter
These are matrix of small picture elements that represent a rectangular coordinate display.
Pixels
These factors determine the correct pixel addresses (matrix coordinates) in which to deposit the digital information.
Transducer beam, orientation, and echo delay times
The final image is most often recorded with ____, representing about ____ Mbytes of data. For color display, the bit depth is often as much as ____ bits (1 byte per primary color).
512 x 512 x 8 bits per pixel, ¼, 24
2D ultrasound image is acquired by sweeping a pulsed ultrasound beam over the volume of interest and displaying echo signals using ____-mode conversion of the ____-mode signals.
B, A
This scanning produces an acoustic tomographic slice of the body.
Articulating arm B-mode scanning
This was achieved by implementing periodic mechanical motion of the transducer.
Dynamic scanning with “real-time” display
They 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.
Linear and curvilinear array
These 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
It is a method in which ultrasound information is obtained from several different angles of insonation and combined to produce a single image.
Spatial compounding
In linear array transducer systems, ____ allows the insonation of tissues from multiple angles, and by averaging the data, the resultant compound image improves image quality in a variety of applications including breast imaging, thyroid, atherosclerotic plaque, and musculoskeletal ultrasound imaging.
Electronic beam steering
T/F: One downside to spatial compounding is the persistence effect of frame averaging, the loss of temporal resolution, and the increase in spatial blurring of moving objects, so it is not particularly useful in situations with voluntary and involuntary patient motion.
True
This is a random source of image variation, and is reduced by the averaging process of forming the compound image, with a corresponding increase in signal-to-noise ratio.
Speckle noise
Higher frame rates can be achieved by ____ the imaging depth, number of lines, or FOV
Reducing
The spatial sampling (LD) of the ultrasound beam ____ with depth for sector and trapezoidal scan formats and remains ____ with depth for the rectangular format (linear array).
decreases, constant
Another factor that affects frame rate is ____, whereby the ultrasound beam (each A-line) is focused at multiple depths for improved lateral resolution
Transmit focusing
T/F: The frame rate will be decreased by a factor approximately equal to the number of transmit focal zones placed on the image, since the beam former electronics must transmit an independent set of pulses for each focal zone.
True
This feature can enhance the image information to improve and delineate details within the image that are otherwise blurred
“Zoom” feature
It enlarges a user-defined region of the stored image and expands the information over a larger number of pixels in the displayed image.
“Read” zoom
This requires the operator to rescan the area of the patient that corresponds to the userselectable area.
“Write” zoom
T/F: In “read” zoom, even though the displayed region becomes larger, the resolution of the image itself does not change.
True
T/F: In a “write” zoom resampling of the image content, the latter allows a greater LD, and higher sampling across the FOV provides improved resolution and image quality.
True
Ultrasound images are typically comprised of ____ or ____
640 x 480 or 512 x 512 pixels
Each pixel has a depth of ____ bits (1 byte) of digital data, providing up to ____ levels of gray scale. Image storage (without compression) is approximately ____ MB per image.
8, 256, ¼
How many frames per second is required for real time imaging
10 to 30 frames per second
It is based on the shift of frequency in an ultrasound wave caused by a moving reflector, such as blood cells in the vasculature
Doppler ultrasound
It is the difference between the incident frequency and reflected frequency.
Doppler shift
The angle between the direction of blood flow and the direction of the sound is called
Doppler angle
T/F: The component of the velocity vector directed toward the transducer is less than the velocity vector along the vessel axis by the cosine of the angle, cos theta. Without correction for this discrepancy, the Doppler shift will be less and an underestimate of the actual blood velocity will occur.
True
The preferred Doppler angle ranges from ____ degrees.
30 to 60
It is the simplest and least expensive device for measuring blood velocity.
Continuous wave Doppler system
In continuous wave Doppler system, this compares the returning frequency to the incident frequency
Demodulator
It measures the magnitude of the Doppler shift but does not reveal the direction of the Doppler shift, that is, whether the flow is toward or away from the transducers.
Demodulation technique
It is phase sensitive and can indicate the direction of flow either toward or away from the transducers.
Quadrature detection
It combines the velocity determination of continuous wave Doppler systems and the range discrimination of pulse-echo imaging.
Pulsed Doppler ultrasound
According to sampling theory, a signal can be reconstructed unambiguously as long as the true frequency (e.g., the Doppler shift) is less than ____ the sampling rate. Thus, the PRF must be at least ____ the maximal Doppler frequency shift encountered in the measurement.
half, twice
For Doppler shift frequencies exceeding one-half the PRF, ____ will occur, causing a potentially significant error in the velocity estimation of the blood
Aliasing
It refers to the combination of 2D B-mode imaging and pulsed Doppler data acquisition.
Duplex scanning
This represents the variation of flow velocity within the vessel lumen.
Velocity profile
It provides a 2D visual display of moving blood in the vasculature, superimposed upon the conventional gray-scale image
Color flow imaging
It is a technique to measure the similarity of one scan line measurement to another when the maximum correlation (overlap) occurs.
Phase-shift autocorrelation
It is an alternate method for color flow imaging. It is based upon the measurement that a reflector has moved over a time (delta)t between consecutive pulse-echo acquisitions
Time-domain correlation
Which correlation technique has a better axial resolution and less prone to aliasing effects?
Time-domain correlation
T/F: Fast, laminar flow exists in the center of large, smooth wall vessels, while slower blood flow occurs near the vessel walls, due to frictional forces.
True
It is an error caused by an insufficient sampling rate (PRF) relative to the high-frequency Doppler signals generated by fast-moving blood.
Aliasing
This represents 0 velocity or 0 Doppler shift
Spectral baseline
It is a signal processing method that relies on the total strength of the Doppler signal (amplitude) and ignores directional (phase) information.
Power Doppler
It is dependent on the amplitude of all Doppler signals, regardless of the frequency shift
Power (aka energy) mode
Is Aliasing a problem in Power Doppler?
No as only the strength of the frequency shifted signals are analyzed, and not the phase
The compressibility produces shifts in the returning frequency of the echoes, called ____
Frequency harmonics
These are integral multiples of the frequencies contained in an ultrasound pulse.
Harmonic frequencies
For harmonic imaging, ____ pulse lengths are often used to achieve a higher transducer Q factor.
Longer
This allows an easier separation of the frequency harmonics from the fundamental frequency. Although the longer SPL degrades axial resolution, the benefits of harmonic imaging overcome the slight degradation in axial resolution.
Imaging higher frequency harmonics produced by tissues when using lower frequency incident sound waves
“Native tissue” harmonic imaging
Means to do volume sampling for 3D Imaging
(1) linear translation, (2) freeform motion with external localizers to a reference position, (3) rocking motion, and (4) rotation of the scan
Measures of ultrasound image quality include (4)
Spatial resolution
Contrast resolution
Image uniformity, and
Noise characteristics.
T/F: Image artifacts are common phenomena that can enhance or degrade the diagnostic value of the ultrasound image.
True
It is determined by the frequency of the ultrasound and the damping factor of the transducer, which together determine the spatial pulse length.
Axial resolution
These are 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
Increasing the depth of lateral resolution focus involves a trade-off of ____ resolution for realtime imaging.
Temporal
Which of the following resolutions are independent of depth?
a. Axial
b. Lateral
c. Elevational
a. Axial
Lateral and elevational resolutions are strongly dependent on depth.
T/F: The minimum resolution in the lateral/elevational directions is typically three to five times greater than axial resolution.
True
LE > A
It 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.
Elevational resolution
T/F: Contrast resolution also depends upon spatial resolution.
True
Detection of subtle anatomy in the patient is dependent on the ____.
Contrast-to-noise ratio
Exponential attenuation of the ultrasound beam, which reduces contrast and increases noise with depth, requires ____ to improve depth uniformity.
TGC
Image processing that specifically reduces noise, such as temporal or spatial averaging, can ____ the contrast-to-noise ratio; however, trade-offs include ____ frame rates and/or poorer spatial resolution.
increase, lower
In low-power operation (e.g. OB setting), increasing the ____ and/or the ____ 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.
transmit power, PRF
These arise from the incorrect display of anatomy or noise during imaging.
Artifacts
This artifact refers to objects appearing and disappearing with slight differences in orientation of the beam. This occurs as a result of change in the transmitted ultrasound pulse direction at a boundary with nonperpendicular incidence, when the two tissues support a different speed of sound
Refraction artifacts resulting to misplaced anatomy
It refers to 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
It 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
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.
Reverberation artifacts
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.
Reverberation echoes are typically manifested as multiple, equally spaced boundaries with decreasing amplitude along a straight line from the transducer
Give two forms of reverberation artifacts
Comet tail artifacts
Ring-down artifacts
Name the artifact
Refraction artifact
Name the artifact
Acoustic shadowing
Name the artifact
Acoustic enhancement
Name the artifact
Comet tail artifact
Name the artifact
Ring-down artifact
This is caused by the variability of speed of sound in different tissues.
Speed displacement artifact
Name the artifact
Speed displacement artifact
Speed of sound variation in the tissues can cause a mismapping of anatomy. In the case of fatty tissues, the slower speed of sound in fat (1,450 m/s) results in a displacement of the returning echoes from distal anatomy by about 6% of the distance traveled through the mass.
“Pseudo-sludge” is a results of this phenomenon
Side lobes
These are emissions of the ultrasound energy that occur in a direction slightly offaxis from the main beam and arise from the expansion of the piezoelectric elements orthogonal to the main beam.
Side lobes
These occur with multielement array transducers and result from the division of a smooth transducer surface into a large number of small elements. The misdirected energy can create ghost images of off-axis high-contrast objects.
Grating lobe artifacts
Name the artifact
Mirror image artifact
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).
Picture: A mirror image artifact arises from multiple beam reflections between a mass and a strong reflector, such as the diaphragm. Multiple echoes result in the creation of a mirror image of the mass beyond the diaphragm. Clinical images show mirror artifacts for color Doppler (top) and B-mode scan (bottom), where the lower arrow points to the artifact mirrored by the diaphragm.
Mismapping of very deep echoes to shallow positions can occur as a result of a high PRF limiting the amount of time spent listening for echoes during the PRP.
Ambiguity artifacts
It 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
Name the artifact
Twinkling artifact
It 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. Consequences include 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.
Slice thickness artifacts
The most frequently reported source of performance instability of an ultrasound system is related to ____
Display on maladjusted video monitors.
It is the rate of energy production, absorption, or flow.
Power
It is 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
It is the 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
It 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
It is the value that estimates the likelihood of cavitation by the ultrasound beam.
Mechanical index, MI
T/F: No bioeffects have been shown below ISPTA of 100 mW/cm2
True
It 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).
Stable cavitation
T/F: Chiefly related to the peak rarefactional pressure, the MI is an estimate for producing cavitation.
True
