Pulse-echo instrumentation
anything not properly indicative of anatomy or motion imaged.
binary digit; smallest amount of computer memory.
group of eight bits of computer memory.
imaging display where the strength of the electron beam determines the brightness.
an independent signal path consisting of a transducer element, delay, and other electronic components.
storage of the last several real-time frames.
a series of pulses and gaps allowing multiple focal zones and harmonic frequencies.
a series of closely spaced reverberation echoes behind a strong reflector.
the ratio of the largest to the smallest amplitude that the ultrasound system can handle.
loss in intensity from bending of the sound beam at a curved surface.
the increase in reflection amplitude from structures that lie behind a weakly attenuating structure.
displayed image of the returning echoes.
a complete scan of the ultrasound beam; individual image composed of multiple scan lines.
the number of complete scans (images) displayed per second.
holding and displaying one frame of the real-time sequence.
ratio of amplifier output to input of electric power.
secondary sound beams produced by a multielement transducer.
number of scan lines per frame; scan-line density.
denotes the rows and columns of pixels in a digital image.
an artifactual gray-scale, color-flow, or Doppler signal appearing on the opposite side of a strong reflector.
the path toward and away from a reflector are different.
disturbance that reduces the clarity of the signal.
the minimum number of samples required to avoid aliasing; Doppler shift frequency above which aliasing occurs.
an expanded image display beyond the normal limits of the transducer.
picture element; smallest portion of a digital image.
number of picture elements per inch.
assigning a brightness value to a missing pixel.
a harmonic imaging technique using two pulses per scan line where the second pulse is an inverse of the first pulse.
the number of voltage pulses sent to the transducer each second.
time from the beginning of one voltage pulse to the start of the next voltage pulse.
allows access of stored data in an unsystematic order.
produced when echoes are placed too superficially because a second pulse was emitted before all reflections have returned from the first pulse.
stored data cannot be modified.
two-dimensional imaging of the motion of moving structures.
portion of the sound reflected from the boundary of a medium.
change of sound direction on passing from one medium to another.
multiple reflections between a structure and the transducer or within a structure.
redirection of sound in several directions on encountering a rough surface.
reduction of reflective amplitude from reflectors that lie behind a strongly reflecting or attenuating structure.
comparison of meaningful information in an image (signal) to the amount of signal disturbance (noise).
averaging of frames that view anatomy from different angles.
the smallest distinguishable part of a three-dimensional image
Display modes
A-mode
• One-dimensional (1-D) quantitative image using a single sound beam.
• Displays vertically the amplitude of the returning echo (y-axis), and distance is along the horizontal axis (x-axis).
B-mode
• Creates a 2-D qualitative, cross-sectional image using multiple sound beams.
• Displays the strength of the returning echoes as pixels in various shades of gray.
• The vertical or the y-axis represents increasing depth and the horizontal or the x-axis represents the side-to-side or superior-to-inferior aspects of the body.
Real-time imaging
• Multiple frames per second make up multiple scan lines per frame.
• Imaging depth determines when the next pulse is transmitted.
• Echo brightness increases with echo amplitude.
• Echo position is determined by the round-trip time of the reflector.
Limitations
• Penetration depth is limited by the propagation speed of the medium.
• Exact imaging plane cannot be systematically reproduced.
• Measurement of structures larger than the field of view is estimated.
| PARAMETER | DESCRIPTION | UNITS | RELATIONSHIP |
| Field of view | Size of the displayed image | N/A | Directly related to the pulse repetition frequency (PRF)Inversely related to frame rate and temporal resolutionOperator-adjustable using depth and region-of-interest settings |
| Frame rate | Number of images per sTypically 30-60 frames/s are used in real-time imagingHuman eye detects fewer than 15-20 frames/s | HzFrames/s | Determines temporal resolutionDetermined by the propagation speed of the medium and imaging depthProportional to the PRFInversely proportional to the number of focal zones used, imaging depth, and lines per frame (beam width)Operator adjustable using depth and PRF settings |
| Line density | Concentration of scan lines within the field of view | Lines/cmLines/degrees | Directly related to PRF and spatial resolutionInversely related to the frame rate and temporal resolution |
| Maximum imaging depth | Maximum penetration depth for the overall parameters used | cm | Dependent on the frame rate, number of lines per frame, and the number of focal zones usedInversely related to the PRF |
| Pulse repetition frequency | Determines the number of scan lines per frameEqual to the voltage PRFTypically 2.0-15.0 kHz is used in real-time imaging | HzkHz | Inversely related to the operating frequency and imaging depthIndirectly adjusted by the operator using imaging depth setting |
| TYPE | DESCRIPTION |
| Coded excitation | Uses a series of pulses and gaps rather than a single driving pulseEnsembles of pulses drive the transducer to generate a scan lineImproves contrast, spatial, and axial resolutionOccurs in the pulser |
| Extended field of view (panoramic) | Expansion of the image display beyond the normal limits of the transducer diameterRetains previous echo information while adding new echo information parallel to the scanning plane |
| Four-dimensional imaging | Real-time presentation of a three-dimensional imageFourth dimension of time is combined with rapidly acquired volumetric data. |
| Harmonic frequencies (MHz) | Even and odd multiples of the fundamental frequencyGenerated at a deeper imaging depth reducing reverberation artifactGenerated in the highest intensity and narrowest portion of the beamReturning harmonic signals are processed separate from the operating signalsImproves lateral resolutionDecreases contrast resolutionReduces grating lobes |
| Multifocal imaging | Ability to use multiple focal zones during real-time imagingDirectly related to lateral resolution and pulse repetition frequencyInversely related to the frame rate and temporal resolution |
| Pixel interpolation | Assigns a brightness value to missed pixelsBased on the average brightness of adjacent pixelsCommonly used in sector scanning |
| Presets | Setup of grayscale, depth, and Doppler imaging controls to exam to be performed |
| Pulse inversion | A technique in harmonic imaging using two pulses per scan, where the second pulse is the inversion of the first pulseAllows for a broader bandwidth and shorter pulsesImproves axial resolutionReduces temporal resolution |
| Spatial compounding | Scan lines are directed in multiple directionsImproves visualization of structures beneath a highly attenuating structureSmoothes specular surfacesReduces speckle and noiseUses phasing to interrogate the structures more than once |
| Three-dimensional imaging |
Power
Transducer
• Produces ultrasound pulses for each electrical pulse applied.
• Receives returning echo reflections, producing an electrical voltage.
• Delivers electrical voltages to the memory.
• Generates a small voltage signal (radio frequency) proportional to the amplitude of the returning echo.
• Radio frequency signals are processed by the system.
Channels
• Individual signal paths for transmission and reception of the sound beam.
• Number of channels equals the number of transducer elements.
• In ultrasound, typically 64, 128, or 196 channels are used.
• Controlling the characteristics of the sound beam is directly related to the number of channels employed.
• Independent pulse delay and element combination constitutes a transmission channel.
• Each independent element, amplifier, analog-to-digital converter, and delay path constitutes a reception channel.
Pulser (transmitter)
• Generates the electric pulses to the crystal producing pulsed ultrasound waves.
• Determines the pulse repetition frequency, pulse repetition period, and pulse amplitude.
• Drives the transducer through the pulse delays with one voltage pulse per scan line.
• Adjusts the PRF appropriately for imaging depth.
• Communicates with the receiver the moment the crystal is excited to help determine the distance to the reflector.
Transmit and receiver switch (T/R switch)
Receiver
• Receives, amplifies, and modifies echo information returning from the transducer.
• Five functions of the receiver:
Time gain compensation (fig. 4-1)
• Mechanism that compensates for the loss of echo strength caused by the depth of the reflector.
• Operator adjustable using time-gain compensation or depth-gain compensation centimeter division slide controls (adjusts variable depths of the image).
• Provides equal amplitude for all similar structures regardless of depth.
• Compensates for attenuation by boosting amplitudes of deep reflections and suppressing superficial reflections.
• Near field—area of minimum amplification.
• Delay—depth at which variable compensation begins.
• Slope—available region for depth compensation.
• Knee—deepest region attenuation compensation can occur.
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