RAB: Ch. 14: Ultrasound Flashcards
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
Wavelength of ultrasound energy
The number of times the wave oscillates through one cycle each second (s)
Frequency (f)
Medical ultrasound uses frequencies in the range of __________ MHz, with specialized ultrasound applications up to 50 MHz
2 to 10 MHz
time duration of one wave cycle and is equal to 1/f, where f is expressed in cycles/s
period
distance traveled by the wave per unit time and is equal to the wavelength divided by the period
Speed of sound
peak maximum or peak minimum value from the average pressure on the medium in the absence of a sound wave
Pressure amplitude (P)
amount of power (energy per unit time) per unit area and is proportional to the square of the pressure amplitude
Intensity
As ultrasound energy propagates through a medium, interactions include _______, ______, _______, and ________.
- Reflection
- Refraction
- Scattering
- Absorption
occurs at tissue boundaries where there is a difference in the acoustic impedance of adjacent materials
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]
describes the change in direction of the transmitted ultrasound energy with nonperpendicular incidence
Refraction
occurs by reflection or refraction, usually by small particles within the tissue medium, causes the beam to diffuse in many directions, and gives rise to the characteristic texture and gray scale in the acoustic image
Scattering
refers to the loss of intensity of the ultrasound beam from absorption and scattering in the medium
Attenuation
process whereby acoustic energy is converted to heat energy, whereby, sound energy is lost and cannot be recovered
Absorption
Major components of ultrasound Transducers
- Piezoelectric material
- Matching layer
- Backing block
- Acoustic absorber
- Insulating cover
- Tuning coil
- Sensor electrodes
- Transducer housing
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
Piezoelectric material (often a crystal or ceramic)
> natural: quartz crystal (ex. Watches)
synthetic
> 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
Damping Block
> (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
Dampening of vibration
Describes the bandwidth of the sound emanating from a transducer
Q factor
Provides the interface between the raw transducer element and the tissue and minimizes the acoustic impedance differences between the transducer and the patient
Matching Layer
> 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
Nonresonance (Broad Bandwidth) “Multifrequency” - Modern transducer design coupled with digital signal processing
Accomplished with a short square wave burst of approximately 150 V with one to three cycles, unlike the voltage spike used for resonance transducers
Excitation of multifrequency transducer
In Transducer Arrays, two modes of activation are used to produce a beam.
- “Linear” (Sequential)
- “Phased” activation/receive modes
> 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
Linear arrays
A rectangular field of view (FOV) is produced with this transducer arrangement
Linear array
A trapezoidal FOV is produced.
Curvilinear array
> 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
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)
- 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
-Also known as the Fraunhofer zone and is where the beam diverges
The Far Field
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
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
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, 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
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
Also known as azimuthal resolution, refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction.
Lateral Resolution
- 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
Weakest measure of resolution for array transducers
Slice thickness
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
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
(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
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
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
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)
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
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
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
The display of the processed information from the receiver versus time (after the receiver processing steps
A-mode (A for amplitude)
- 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)
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)
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
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
A method in which ultrasound information is obtained from several different angles of insonation and combined to produce a single image
Spatial compounding
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