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Ultrasound: Sound We Don't Hear

Objectives

  • Flashcard #1
    Term: Frequency
    Definition: The number of cycles per second in a sound wave, measured in Hertz (Hz). Higher frequencies lead to better axial resolution but have less penetration depth due to increased attenuation.

    Flashcard #2
    Term: Ultrasound
    Definition: Sound waves with frequencies higher than 20,000 Hz; widely used in medical imaging for diagnostic purposes beyond human hearing range.

    Flashcard #3
    Term: Harmonic Generation
    Definition: The process by which sound waves, as they propagate through tissue, create harmonics (integer multiples of the fundamental frequency). This occurs because higher pressure portions of the wave travel faster than lower pressure portions, altering the wave shape in a non-linear manner.

    Flashcard #4
    Term: Continuous Ultrasound
    Definition: A mode of ultrasound where sound waves are continuously emitted by the transducer and continuously received. This allows for constant energy transmission and detection but does not allow for depth localization of echoes.

    Flashcard #5
    Term: Pulsed Ultrasound
    Definition: A mode of ultrasound where sound is emitted in discrete, short pulses, followed by a listening period. This allows the transducer to receive returning echoes and provides information about the depth of structures, enabling B-mode imaging and pulsed Doppler capabilities.

    Flashcard #6
    Term: Weakening in Tissue
    Definition: The reduction of ultrasound amplitude and intensity as it propagates through different types of tissue, primarily caused by absorption (conversion of sound energy to heat), reflection, and scattering. This process is commonly known as attenuation.

    Flashcard #7
    Term: Echo Generation
    Definition: The production of returning sound waves that provide information about tissue structure. This occurs when the ultrasound wave encounters an interface between two media with different acoustic impedances, causing a portion of the sound to reflect back to the transducer.

    Flashcard #8
    Term: Sound Wave
    Definition: A mechanical wave (longitudinal wave) that propagates through a medium by causing vibratory motion of its particles, resulting in cyclical variations in acoustic variables such as pressure, density, and particle motion.

    Flashcard #9
    Term: Compression
    Definition: Areas within a sound wave where acoustic variables like pressure and density are at their maximum values, signifying areas where particles are pushed together.

    Flashcard #10
    Term: Rarefaction
    Definition: Areas within a sound wave where acoustic variables like pressure and density are at their minimum values, signifying areas where particles are spread apart.

    Flashcard #11
    Term: Wavelength (λλ)
    Definition: The spatial length of one complete cycle of a sound wave. It is inversely proportional to frequency (λ=c/f)(λ=c/f), where cc is propagation speed and ff is frequency. Shorter wavelengths improve axial resolution.

    Flashcard #12
    Term: Propagation Speed
    Definition: The speed at which a sound wave travels through a medium, determined by the medium's stiffness (or compressibility) and density. For soft tissue, the average propagation speed is approximately 1540m/s1540m/s. This speed is crucial for accurate depth calculation in ultrasound imaging.

    Flashcard #13
    Term: Amplitude
    Definition: The maximum displacement or variation in an acoustic variable (e.g., pressure, density) from its average value. It indicates the strength or intensity of the sound wave and is related to the brightness of the echoes displayed on the image.

    Flashcard #14
    Term: Pulse Repetition Frequency (PRF)
    Definition: The number of ultrasound pulses emitted by the transducer per second. Higher PRF allows for higher frame rates and can mitigate aliasing in Doppler, but it also increases the risk of range ambiguity by reducing the maximum imaging depth.

    Flashcard #15
    Term: Pulse Repetition Period (PRP)
    Definition: The total time from the start of one ultrasound pulse to the start of the next pulse. It is the reciprocal of PRF (PRP=1/PRF)(PRP=1/PRF). A longer PRP allows more time for echoes to return from deeper structures, thus increasing the maximum imaging depth without range ambiguity.

    Flashcard #16
    Term: Pulse Duration (PD)
    Definition: The actual time that a single ultrasound pulse is active or 'on'. It determines the temporal length of the pulse. A shorter pulse duration (e.g., fewer cycles within the pulse) leads to better axial resolution.

    Flashcard #17
    Term: Duty Factor (DF)
    Definition: The fraction or percentage of time that the pulsed ultrasound system is actively transmitting sound. It is calculated as (DF=PD/PRP)(DF=PD/PRP). A lower duty factor is generally desirable for patient safety to minimize thermal effects, especially in diagnostic imaging.

    Flashcard #18
    Term: Spatial Pulse Length (SPL)
    Definition: The physical length of space that one ultrasound pulse occupies from its beginning to its end. It is calculated as (SPL=extnumberofcyclesinpulseimesextwavelength)(SPL=extnumberofcyclesinpulseimesextwavelength). A shorter SPL directly improves axial resolution.

    Flashcard #19
    Term: Bandwidth
    Definition: The range of frequencies contained within a pulse, measured in Hertz. A broader bandwidth typically means a shorter pulse duration, which improves axial resolution, and allows for harmonic imaging.

    Flashcard #20
    Term: Energy
    Definition: The capacity to perform work, measured in Joules. In ultrasound, it refers to the energy contained within the sound wave.

    Flashcard #21
    Term: Work
    Definition: The product of force applied over a distance, resulting in energy transfer. In ultrasound, the sound wave transfers energy to the tissues it propagates through.

    Flashcard #22
    Term: Power
    Definition: The rate at which sound energy is transferred or dissipated per unit of time, measured in Watts. In ultrasound, power directly relates to intensity and is a key factor in potential bioeffects, so it is carefully regulated.

    Flashcard #23
    Term: Intensity
    Definition: The concentration of sound energy, defined as power per unit area (Intensity=Power/Area)(Intensity=Power/Area), measured in W/cm2W/cm2. High intensity can increase the risk of bioeffects, while lower intensity is used for diagnostic purposes, balancing image quality with patient safety.

    Flashcard #24
    Term: Decibels (dB)
    Definition: A logarithmic unit used to express the ratio of two sound intensities or powers (e.g., received intensity versus initial intensity). Decibels describe the relative change (gain or loss) in sound strength, such as attenuation or amplification. For intensity, the formula is dB=10imesextlog10(I2/I1)dB=10imesextlog10(I2/I1); for amplitude, it's dB=20imesextlog10(A2/A1)dB=20imesextlog10(A2/A1).

    Flashcard #25
    Term: Attenuation
    Definition: The reduction in amplitude and intensity of a sound wave as it travels through a medium. It is caused by absorption (conversion to heat), reflection, and scattering. Attenuation increases with higher frequency, longer path length, and specific tissue properties. It is measured in decibels (dBdB).

    Flashcard #26
    Term: Reflection
    Definition: The phenomenon where a portion of the ultrasound wave is redirected back towards the transducer after encountering an interface between two media with different acoustic impedances. This is the primary mechanism for echo generation and image formation. The amount of reflection depends on the impedance mismatch and the angle of incidence.

    Flashcard #27
    Term: Impedance (z)
    Definition: The acoustic resistance of a medium to sound wave propagation, calculated as the product of the medium's density and the propagation speed (z=hoimesc)(z=hoimesc). Differences in impedance at a tissue interface determine the amount of reflection and transmission of sound waves.

    Flashcard #28
    Term: Intensity Reflection Coefficient (IRC)
    Definition: The fraction or percentage of the incident ultrasound intensity that is reflected at a boundary between two media. It is determined by the impedance mismatch at the interface; a larger difference in acoustic impedances (Z1Z1 and Z2Z2) leads to a higher IRC, calculated as IRC=((Z2−Z1)/(Z2+Z1))2IRC=((Z2−Z1)/(Z2+Z1))2.

    Flashcard #29
    Term: Intensity Transmission Coefficient (ITC)
    Definition: The fraction or percentage of the incident ultrasound intensity that continues to propagate into the second medium after encountering a boundary. It is directly related to the Intensity Reflection Coefficient, as ITC=1−IRCITC=1−IRC. A high ITC means most of the sound passes through, while a low ITC means more is reflected.

    Flashcard #30
    Term: Refraction
    Definition: The change in direction (bending) of a sound wave as it passes obliquely from one medium to another with a different propagation speed. Refraction occurs when there is both an oblique incidence angle and different propagation speeds, and it can lead to imaging artifacts (e.g., misregistration of objects, blurry images, or shadows from a structure being imaged at the wrong location).

    Flashcard #31
    Term: Scattering
    Definition: The redirection of a small portion of the sound wave in multiple directions when it encounters small, rough, or irregular interfaces within tissue (e.g., red blood cells, small vessels). Scattering contributes to the texture of the image (speckle) and is a component of attenuation, making structures like the liver appear grainy.

    Flashcard #32
    Term: Contrast Agents
    Definition: Specialized microbubbles (usually gas-filled) injected intravenously to enhance the visualization of blood flow or specific tissues. They increase the echogenicity (ability to reflect sound) of blood, improving signal-to-noise ratio in Doppler imaging and allowing for better visualization of perfusion in organs or detection of liver lesions.

    Flashcard #33
    Term: Range
    Definition: The distance from the transducer to the echo-generating structure. The ultrasound system calculates this distance based on the time it takes for the sound pulse to travel to the structure and return, using the assumed propagation speed (Distance=(SpeedimesTime)/2)(Distance=(SpeedimesTime)/2).

    Flashcard #34
    Term: Harmonics
    Definition: Higher frequency sound waves generated due to nonlinear propagation through tissue; they are integer multiples of the fundamental (transmitted) frequency. Harmonic imaging improves spatial resolution, reduces clutter, and suppresses artifacts because the harmonic signal is stronger and less distorted than the fundamental signal.

    Flashcard #35
    Term: Transducers
    Definition: Devices containing piezoelectric elements that convert electrical energy into ultrasound (transmission) and convert returning ultrasound echoes back into electrical energy (reception). They are the heart of the ultrasound system, determining image quality and functionality through their design and frequency capabilities.

    Flashcard #36
    Term: Piezoelectricity
    Definition: The property of certain crystalline materials (e.g., lead zirconate titanate, PZT) to convert mechanical pressure or deformation into an electrical voltage, and vice versa. This property is fundamental to how ultrasound transducers generate and detect sound waves.

    Flashcard #37
    Term: Capacitive Micromachined Ultrasonic Transducers (CMUTs)
    Definition: A type of ultrasound transducer composed of a large array of miniature, drum-like elements etched onto a silicon wafer. CMUTs offer advantages such as broad bandwidth, improved sensitivity, and ease of integration with other electronics, potentially leading to better image resolution and new applications in miniaturization.

    Flashcard #38
    Term: Damping Material
    Definition: A material (e.g., epoxy resin mixed with tungsten powder) placed on the back of the piezoelectric element within a transducer. Its purpose is to absorb unwanted vibrations and reduce the 'ringing' of the crystal, thereby shortening the pulse duration and spatial pulse length, which directly improves axial resolution.

    Flashcard #39
    Term: Matching Layer
    Definition: One or more layers of material placed on the front face of the piezoelectric element, between the active element and the skin. Its primary function is to reduce the acoustic impedance mismatch between the transducer element and the skin, allowing for more efficient transmission of sound into the patient and reception of echoes.

    Flashcard #40
    Term: Coupling Medium
    Definition: A gel-like substance applied to the patient's skin before scanning. Its crucial role is to eliminate the air layer between the transducer face and the skin, as air causes nearly total reflection of ultrasound. By providing an efficient acoustic pathway, it ensures maximal transmission of sound into the body for imaging.

    Flashcard #41
    Term: Sound Beam
    Definition: The focused region of ultrasound energy emitted from the transducer. Its characteristics, such as width and shape (near zone, focal zone, far zone), are critical for determining lateral resolution and image quality. Narrower beams generally result in better lateral resolution.

    Flashcard #42
    Term: Near Zone (Fresnel Zone)
    Definition: The region of the ultrasound beam closest to the transducer, extending from the transducer face to the focal point. In this zone, the beam diameter gradually narrows. The length of the near zone influences beam focusing and lateral resolution, with longer near zones for higher frequency and larger aperture transducers.

    Flashcard #43
    Term: Focal Length
    Definition: The distance from the transducer's face to the point where the ultrasound beam is narrowest (the focal point). Proper adjustment of focal length is crucial for optimizing lateral resolution at the desired depth of interest within the tissue.

    Flashcard #44
    Term: Axial Resolution
    Definition: The ability of an ultrasound system to distinguish two closely spaced structures that lie parallel to the direction of the ultrasound beam. It is primarily determined by the spatial pulse length (SPL).

    • Degraded by: Long pulse duration, long spatial pulse length, lower frequencies (which result in longer wavelengths).

    • Improved by: Using higher frequency transducers, damping material (backing layer) to shorten pulse duration, and a shorter spatial pulse length.

    Flashcard #45
    Term: Lateral Resolution
    Definition: The ability of an ultrasound system to distinguish two closely spaced structures that lie perpendicular (side-by-side) to the direction of the ultrasound beam. It is primarily determined by the width of the ultrasound beam.

    • Degraded by: Wide beam width, greater depth (beam often widens with depth), lack of proper focusing.

    • Improved by: Focusing (external lenses, internal curvature, electronic focusing), using a high frequency transducer (which naturally produces narrower beams), dynamic aperture, and multiple focal zones (though this may reduce temporal resolution).

    Flashcard #46
    Term: Frame Time
    Definition: The total time required for an ultrasound system to acquire all the scan lines needed to construct a single complete image frame. It is inversely related to the frame rate; longer frame times result in lower frame rates and thus poorer temporal resolution.

    Flashcard #47
    Term: Frame Rate
    Definition: The number of complete image frames that an ultrasound system can display per second. A higher frame rate offers better temporal resolution, allowing for superior visualization of rapidly moving structures (e.g., heart motion). It is limited by imaging depth, line density, and number of focal zones.

    Flashcard #48
    Term: Dynamic Range
    Definition: The ratio of the maximum to minimum signal (or echo amplitude) that an ultrasound system can process and display without distortion. A wide dynamic range allows the display of a greater range of echo strengths, leading to more subtle gray-scale distinctions and a more detailed image, often measured in decibels (dBdB).

    Flashcard #49
    Term: Edge Enhancement
    Definition: A preprocessing technique applied before the image is stored, that sharpens the boundaries or interfaces between different tissues by increasing the contrast at those edges. This can make structures appear more defined but can also amplify noise if overused.

    Flashcard #50
    Term: Pixel Interpolation
    Definition: A post-processing technique used to estimate and fill in the values of missing or unknown pixels in an image, often used to create a smoother appearance, zoom in, or expand the image without introducing jagged edges. It uses the values of surrounding pixels to determine the likely value of the missing pixel.

    Flashcard #51
    Term: Aliasing
    Definition: An artifact encountered in pulsed Doppler imaging where the Doppler shift frequency (velocities) exceeds the Nyquist limit, causing velocities to be incorrectly displayed in the opposite direction or wrapped around. This misrepresentation occurs when the sampling rate (PRF) is too low to accurately capture the true high-frequency Doppler shifts.

    • Caused by: High blood flow velocities, low PRF, high-frequency transducers, deep sample volumes (which necessitate a lower PRF).

    • Improvement: Increasing the PRF (if possible), shifting the baseline, using a lower frequency transducer, increasing the Doppler angle (to reduce the detected shift), or switching to Continuous Wave (CW) Doppler.

    Flashcard #52
    Term: Doppler Angle
    Definition: The angle formed between the direction of the ultrasound sound beam and the direction of blood flow. The Doppler effect is maximized at an angle of 0exto0exto (parallel to flow) and minimized at 90exto90exto (perpendicular to flow). An optimal Doppler angle (typically between 30exto30exto and 60exto60exto) is crucial for accurate velocity measurements, as the cosine of the angle is used in the Doppler equation.

    Flashcard #53
    Term: Color Doppler
    Definition: A pulsed Doppler technique that provides real-time, two-dimensional imaging of blood flow and tissue motion. Blood flow is encoded in color, typically red for flow towards the transducer and blue for flow away, with hues and intensity indicating mean velocity and direction, respectively. It is useful for visualizing flow patterns, vessel patency, and identifying areas of turbulence.

    Flashcard #54
    Term: Spectral Doppler
    Definition: A highly quantitative Doppler technique (including Pulsed Wave and Continuous Wave Doppler) that displays a graph of blood flow velocities over time. The 'spectrum' shows a range of velocities present within the sample volume, with frequency shifts (representing velocity) plotted on the y-axis and time on the x-axis, providing detailed information about flow characteristics like direction, speed, and turbulence.

    Flashcard #55
    Term: Bernoulli Effect
    Definition: A physical principle stating that for a fluid flowing inviscidly, an increase in fluid speed must be accompanied by a decrease in pressure. In medical ultrasound, this applies to stenotic vessels, where blood flow accelerates through the narrowed segment, resulting in a measurable pressure drop across the stenosis. This effect is critical for assessing the severity of vascular narrowing.

    Flashcard #56
    Term: Steady Flow
    Definition: Fluid flow that maintains a constant speed and direction over time. It can be observed in some venous beds, particularly in the periphery, and is characterized by a relatively smooth, unchanging spectral Doppler waveform over the cardiac cycle, showing a narrow range of velocities.

    Flashcard #57
    Term: Pulsatile Flow
    Definition: Fluid flow that changes in speed and often direction during the cardiac cycle, typically observed in arteries and some veins (e.g., hepatic veins). It is characterized by waveforms on spectral Doppler that reflect the rhythmic pumping action of the heart, showing peaks in systole and troughs in diastole.

    Flashcard #58
    Term: Continuity Rule
    Definition: A conservation of mass principle stating that for an incompressible fluid flowing steadily through a vessel, the volumetric flow rate (QQ) must remain constant at any point along the vessel, even if the vessel's diameter changes. This means that if the cross-sectional area decreases, the flow velocity must increase (Q=AreaimesVelocityQ=AreaimesVelocity is constant).

    Flashcard #59
    Term: Nyquist Limit
    Definition: In pulsed Doppler ultrasound, the maximum Doppler frequency shift that can be accurately detected and displayed without aliasing. It is equal to half of the pulse repetition frequency (NyquistLimit=PRF/2)(NyquistLimit=PRF/2). If the detected Doppler shift (which corresponds to blood velocity) exceeds this limit, aliasing will occur.

    Flashcard #60
    Term: Spectral Broadening
    Definition: An increase in the range of Doppler shift frequencies represented on a spectral Doppler display, causing the spectral trace (waveform) to appear 'thickened' or filled-in. It is an indicator of disturbed, turbulent, or non-laminar blood flow, often associated with vessel stenosis or tortuosity, where a wide range of velocities are present within the sample volume.

    Flashcard #61
    Term: Flash Artifact
    Definition: A sudden, transient burst of color (usually mosaic, representing multiple directions and velocities) observed in Color Doppler images, caused by quick, large movements of the transducer or rapid movement of adjacent tissues (e.g., cardiac motion, breathing). It indicates tissue motion rather than true blood flow and can obscure actual flow, making accurate assessment difficult.

    • Caused by: Rapid gross patient or transducer motion.

    • Improvement: Asking the patient to hold breath, using a lower color gain, or adjusting the wall filter to reject low-frequency, high-amplitude signals from tissue motion.

    Flashcard #62
    Term: Coded Excitation
    Definition: A sophisticated transmission technique where a series of complex ultrasound pulses (codes) are transmitted instead of a single short pulse. The echoes received are then decoded, allowing for improved signal-to-noise ratio, deeper penetration, and better axial resolution, without sacrificing temporal resolution, by effectively increasing the amplitude of the signal.

    Flashcard #63
    Term: Spatial Compounding
    Definition: An imaging technique where multiple frames of the same anatomy are acquired from different angles (steered beam directions) and then incoherently averaged together to create a single composite image. This reduces speckle artifact, enhances border definition, and smooths out textures, improving overall image quality, especially in structures with complex geometries.

    Flashcard #64
    Term: Elastography
    Definition: A specialized ultrasound technique that assesses the mechanical properties (stiffness or elasticity) of tissues. It works by measuring the degree of tissue deformation (strain) in response to an applied external or internal force, or by measuring the speed of shear waves through tissue. Stiffer tissues often indicate pathology (e.g., tumors, fibrosis or inflammation).

    Flashcard #65
    Term: Fusion Imaging
    Definition: A technique that combines real-time ultrasound images with previously acquired images from other modalities, such as CT, MRI, or PET, into a single, co-registered display. This allows clinicians to leverage the strengths of each modality (e.g., anatomical detail from MRI with real-time feedback from ultrasound) for improved guidance in procedures or lesion characterization.

    Flashcard #66
    Term: DICOM
    Definition: Digital Imaging and Communications in Medicine (DICOM) is a universally accepted international standard for medical images and related information. It specifies formats for image files, image presentation, and communication protocols, ensuring interoperability between imaging systems, PACS (Picture Archiving and Communication Systems), and other medical devices across different vendors.

    Flashcard #67
    Term: Artifacts
    Definition: Any echo or structure visualized in an ultrasound image that does not correspond to actual anatomical structures, or a misrepresentation of true anatomical information. Artifacts can either obscure real anatomy or create false appearances, potentially leading to misdiagnosis. Understanding their causes is crucial for accurate interpretation.

    • General Improvement: Adjusting transducer position/angle, changing gain/TGC settings, using different imaging presets, or applying advanced imaging techniques like harmonic imaging or spatial compounding.

    Flashcard #68
    Term: Propagation Artifacts
    Definition: A category of artifacts that arise when the ultrasound system's fundamental assumptions about sound propagation (e.g., constant speed of sound of 1540m/s1540m/s, straight line travel, echoes arising from the main beam only) are violated in the patient's body. These include speed error artifact, range ambiguity, and refraction artifact.

    • Caused by: Deviations from the assumed propagation speed, non-linear sound paths, or multiple reflections.

    • Improvement: Often difficult to completely eliminate but can sometimes be minimized by adjusting gain settings, changing the acoustic window, or using specific imaging modes; some are inherent to the physics.

    Flashcard #69
    Term: Reverberation Artifact
    Definition: An artifact caused by multiple, closely spaced reflections occurring between two strong, highly reflective interfaces (e.g., a transducer face and a diaphragm, or air/soft tissue interfaces). These reflections produce a series of equally spaced, diminishing echoes that appear deeper than the actual structure.

    • Caused by: Strong acoustic interfaces, parallel orientation of interfaces relative to the beam.

    • Improvement: Changing the transducer angle, selecting a different acoustic window, adjusting gain settings (reducing gain), or using compound imaging if available.

    Flashcard #70
    Term: Grating Lobes
    Definition: Spurious off-axis beams of sound energy emitted from multi-element array transducers, in addition to the main beam. These weak side lobes can cause echoes from strong reflectors outside the main beam to be registered as if they originated from within the main beam, creating false structures or filling in anechoic areas (like cysts).

    • Caused by: Multi-element transducer arrays (especially linear arrays), the design and spacing of piezoelectric elements.

    • Improvement: Apodization (varying element excitation to reduce side lobe amplitude), tissue harmonics imaging, and using appropriately designed transducer arrays.

    Flashcard #71
    Term: Beam Aberration
    Definition: Distortion or degradation of the ultrasound beam's shape and focus, caused by inhomogeneities in the propagation speed of sound as it travels through different tissues (e.g., fat and muscle). Instead of maintaining a precise focus, the beam becomes smeared, leading to reduced resolution and image quality. This is more pronounced in patients with significant tissue variations.

    • Caused by: Variations in sound speed within the biological medium, leading to phase delays in the returning echoes.

    • Improvement: Adaptive focusing techniques, phase correction algorithms, or techniques like coded excitation and harmonics can help mitigate its effects by correcting for speed variations.

    Flashcard #72
    Term: Dynamic Aperture
    Definition: A technique where the active transducer aperture (the number of piezoelectric elements used to receive echoes) is varied dynamically as echoes return from increasing depths. This optimizes the receive focus at different depths, maintaining a narrower beam and improving lateral resolution throughout the image without mechanically altering the focus.

    Flashcard #73
    Term: Temporal Resolution
    Definition: The ability of an ultrasound system to accurately display rapidly changing anatomical structures or physiological events over time. It is directly related to the frame rate; a higher frame rate means better temporal resolution.

    • Degraded by: Deeper imaging depth (requires longer pulse-listen times), wider scan volume (more scan lines), multiple focal zones (more pulses per scan line), high line density, and the activation of color Doppler.

    • Improved by: Decreasing imaging depth, reducing the number of scan lines (narrower scan width), using single focal zones, reducing line density, and using specific modes that prioritize frame rate, like M-mode.

    Flashcard #74
    Term: Gray-scale Imaging
    Definition: An ultrasound imaging technique that maps different amplitudes of returning echoes to varying shades of gray, from black (no echo, e.g., fluid) to white (strongest echo, e.g., bone). This allows for the visual representation of soft tissue structures and provides detailed anatomical information based on relative tissue echogenicity.

    Flashcard #75
    Term: Young's Modulus
    Definition: A measure of the stiffness or elastic modulus of a material. In elastography, it quantifies a tissue's resistance to elastic deformation, defined as the ratio of stress (force per unit area) to strain (relative deformation). Higher Young's Modulus values indicate stiffer tissues, which can be an indicator of pathology like fibrosis or malignancy.

    Flashcard #76
    Term: Acoustic Radiation Force Impulse (ARFI)
    Definition: A non-invasive elastography technique where a short-duration, high-intensity ultrasound pulse (acoustic radiation force) is used to create a localized push, or 'impulse,' within tissue. The resulting tissue displacement and the speed of the generated shear waves are then measured to quantify tissue stiffness. This technique allows for quantitative elastography without external compression.

Waves

  • Definition: A traveling variation of some quantity (e.g., pressure, density).

  • Compressions: Areas of high pressure and density.

  • Rarefactions: Areas of low pressure and density.

  • Sound Wave: Traveling variation of acoustic variables (pressure, density, particle motion, temperature).

  • Requires a medium to travel.

  • It is a longitudinal mechanical wave.

Descriptive Terms of Continuous Wave (CW)

  • Frequency (ƒ): Number of cycles per second.

  • Cycle: One complete variation.

  • Units: Hertz (Hz), Kilohertz (kHz), Megahertz (MHz).

  • Ranges:

  • Human hearing: 20–20,000 Hz

  • Infrasound: < 20 Hz

  • Ultrasound: > 20,000 Hz

  • Diagnostic Ultrasound: 2-20 MHz

  • Period (T): Time for one cycle.

  • Units: second, microsecond (µs).

  • Wavelength (λ): Length of space one cycle takes up. inversely proportional to frequency so double frequency halves wavelength.

  • Unit: millimeter (mm).

  • Propagation Speed: Speed wave moves through a medium.

  • Units: meter/second; millimeter/microsecond.

  • Soft Tissue Average: 1540 m/s or 1.54 mm/µs.

  • Determinants: Primarily by stiffness (stiffer = higher speed).

  • Amplitude: Maximum variation in an acoustic variable.

  • Units: Dependent on acoustic variable.

Descriptive Terms of Pulse Wave (PW)

  • Pulse Repetition Frequency (PRF): Number of pulses per second. increases acoustic exposure. increasing too much will cause range ambiguity.

  • Unit: kilohertz (kHz).

  • Pulse Repetition Period (PRP): Time from start of one pulse to start of next.

  • Unit: millisecond (ms).

  • Pulse Duration (PD): Time for a pulse to occur.

  • Unit: microsecond (µs).

  • Typical Lengths: Sonographic (2-3 cycles), Doppler (5-30 cycles).

  • Duty Factor (DF): Fraction of time pulsed ultrasound is transmitting sound.

  • Unit: Unitless.

  • Typical Values: Sonography (0.1%–1%), Doppler (0.5%–5%).

  • Spatial Pulse Length (SPL): Length of space a pulse takes up.

  • Unit: millimeter.

  • Bandwidth: Range of frequencies in a pulse. Shorter pulse = broader bandwidth. wider = more range. low q.

  • Fractional Bandwidth: Describes bandwidth size relative to operating frequency.

  • Unit: Unitless.

Energy, Work, and Power

  • Energy: Ability to accomplish work. Unit: Joule (NM). Sound is mechanical energy.

  • Work: Force acting over a displacement.

  • Power: Rate at which energy is transferred. Unit: Watt.

  • Intensity: Rate at which energy passes through a unit area.

  • Units: milliwatts per centimeter squared (mW/cm²) and W/cm².

  • Relationship: Power and intensity are directly proportional to amplitude squared.

Decibels (dB)

  • Purpose: Units for comparison, describe relationship between measured sound levels using logarithms of ratios.

  • Intensity Ratios:

  • 3 dB decrease = 50% reduction in intensity.

  • 6 dB decrease = 75% reduction in intensity.

  • 10 dB decrease = 90% reduction in intensity.

  • Power Ratio Calculation:

  • 1 dB = 1.26

  • 3 dB = 2

  • 10 dB = 10

Attenuation

  • Definition: Weakening of sound as it propagates; reduction in amplitude and intensity.

  • Units: decibels (dB).

  • Compensation: Must be compensated by diagnostic instrument.

  • Components:

  • Absorption: Dominant in soft tissue; sound converted to heat.

  • Reflection: Sound bouncing off a boundary.

  • Scattering: Redirection of sound in multiple directions.

  • Attenuation Coefficient (AC): Attenuation per centimeter.

  • Unit: dB/cm or dB/(cm-MHz).

  • Soft Tissue: 0.5 dB/cm for each MHz.

Reflection

  • Perpendicular Incidence: Incident pulse partially reflected, remainder transmitted.

  • Impedance (z): Property of a medium determining reflection/transmission.

  • Units: rayls.

  • No reflection if impedances are equal.

  • Large difference (e.g., air/tissue) causes nearly total reflection (reason for coupling medium).

  • Intensity Reflection Coefficient (IRC): Fraction of incident intensity reflected.

  • Intensity Transmission Coefficient (ITC): Fraction of incident intensity transmitted.

  • Oblique Incidence: Direction of travel not perpendicular to boundary.

  • Specular Reflector: large, smooth interface. responsible for bright interfaces at boundaries. Incidence angle always equals reflection angle.

  • Refraction: Change in direction of sound when crossing a boundary.

  • Causes lateral position artifact.

  • Requirements: Oblique incidence AND different propagation speeds.

  • Snell's Law: Relates incidence and transmission angles to propagation speeds.

Scattering

  • Definition: Redirection of sound in many directions by rough surfaces or heterogeneous media.

  • Allows imaging of tissue parenchyma and organ boundaries.

  • Strength of Scattered Echoes: Depends on frequency, scatterer density, size, acoustic mismatch.

  • Rayleigh Scattering: Occurs when scatterer size is much smaller than wavelength.

  • Speckle: Form of acoustic noise; grainy appearance from interference patterns.

  • Interference: Constructive (reinforce) or Destructive (cancel).

Contrast Agents

  • Definition: Liquid suspension intravenously injected to increase echogenicity.

  • Composition: Microbubbles of gas stabilized by a shell.

  • Improvements: Left ventricular opacification, lesion detection/characterization, Doppler detection.

Range

  • Definition: Distance from transducer to an echo-generating structure.

  • Requirements for Proper Positioning: Direction from which echo came, distance to reflector.

  • Soft Tissue Constant: 13 µs round trip time for each centimeter of depth.

Harmonics

  • Generation: Higher pressure portions of wave travel faster, changing wave shape and introducing harmonics (even and odd multiples of fundamental frequency).

  • Non-linear Propagation: Energy transferred from fundamental frequency to harmonics.

  • Harmonic Imaging: Transmits fundamental, images using harmonic frequencies (e.g., second harmonic).

  • Advantages: Improved lateral resolution, reduction in grating lobes, reduction in reverberation and clutter artifacts.

  • Disadvantages: Can degrade axial resolution in conventional harmonic imaging due to narrower bandwidths requiring longer pulses.

  • Pulse/Phase Inversion: Uses two fundamental pulses 180 degrees out of phase to cancel fundamental energy, preserving axial resolution. Degrades temporal resolution by a factor of 2.

Transducers: Sending and Receiving

Objectives

  • Describe transducer construction and function of each part.

  • Explain how transducers generate ultrasound pulses and receive echoes.

  • Describe a sound beam and factors affecting it.

  • Discuss beam focusing and automatic scanning.

  • Compare linear, convex, phased, and vector arrays.

  • Define detail resolution and its three aspects.

  • List factors determining detail resolution.

Transducers

  • Definition: Converts one form of energy to another (e.g., electric to ultrasound and vice versa).

Piezoelectricity

  • Principle: Certain materials deform under pressure to produce voltage, and vice versa.

  • Piezoelectric Elements: Also called crystal, active element, or transducer element.

  • Material: Lead Zirconate Titanate (PZT) is common.

  • Function: Thickness changes with applied voltage.

  • Capacitive Micromachined Ultrasonic Transducers (CMUTs):

  • Construction: Miniature elements with two electrically conducting layers.

  • Advantages: Extremely broad bandwidth, improved detail resolution, miniaturization, low impedance.

  • Operating Frequency Determinants:

  • Propagation speed of element material.

  • Thickness of transducer element (thinner = higher frequencies).

  • Plate thickness typically half a wavelength for resonance.

  • Wide-Bandwidth Transducers:

  • Fractional bandwidth of at least 70%.

  • Can operate at multiple frequencies (multi-hertz, dynamic frequency tuning, frequency fusion).

  • Enables harmonic imaging.

Transducer Components

  • Damping (Backing) Material: Attached to rear face. 1-3 cycles.

  • Purpose: Reduces number of cycles in a pulse, broadens bandwidth, reduces pulse duration and SPL (improves axial resolution).

  • Effect: Reduces amplitude, decreases efficiency and sensitivity.

  • Note: CW transducers are not damped.

  • Matching Layer: Material on transducer face. results in inc bw

  • Purpose: Reduces impedance mismatch between element and tissue.

  • Coupling Medium: Gel applied to skin.

  • Purpose: Eliminates air layer, facilitates sound passage.

  • Invasive Transducers: Designed to enter the body.

  • Advantage: Allows higher frequencies and improved resolution.

Sound Beam

  • Definition: Width of a pulse as it travels from the transducer.

  • Characteristics: Width varies with distance; intensity is not uniform.

  • Resolution: Width in scan plane determines lateral resolution; width perpendicular to scan plane determines section thickness artifact.

Near and Far Zones

  • Near Zone (Fresnel Zone/Near Field): Region from transducer to minimum beam width. Beam width decreases.

  • Near Zone Length (NZL): Increases with increasing frequency and element size (aperture).

  • Far Zone (Fraunhofer Zone/Far Field): Region beyond NZL. Beam width increases. Decreases with increased crystal diameter or frequency. more beam divergence in FF results in poor LR.

Mechanical Focusing

  • Purpose: Improves lateral resolution, degrades axial res. in the z-axis (width). Only in the near zone . Decreases beam width in near zone and focal region improves spatial res; widened in far zone.

  • Focal Length: Distance from transducer to center of focal region.

  • Methods: Curved elements, lens, phased arrays.

Resolution

  • Detail Resolution: Ability to resolve physical tissue characteristics in three dimensions. Improves by increasing frequency.

  • Axial Resolution: Minimum reflector separation along sound direction. improves with Increasing frequency, shorter pulse duration, wider bandwidth transducers. constant depth.

  • Lateral Resolution (Angular, Transverse, Azimuthal): Minimum reflector separation perpendicular to beam direction. Narrow Beam width. Best in focal zone. degrades with depth. can be enhanced by multiple transmit foci, dynamic receive, and aperture focusing.

  • Improvement: Focusing (mechanical or electronic). inc f inc nzl reducing bw.

  • Elevational Resolution (Slice Thickness Resolution): Focusing in the section-thickness plane. Usually has the worst resolution.

  • Section Thickness Artifact: Filling in of anechoic structures.

Frame Time and Frame Rate

  • Frame Time: Time to form one image frame.

  • Frame Rate: Number of frames per second (reciprocal of frame time).

  • Temporal Resolution: Ability to resolve events happening in short periods. Higher frame rate = better temporal resolution.

Transducer Evolution

  • Single Disc-shaped Crystal: Doppler, A-mode.

  • Static B-scan: Not real-time; manual transducer sliding.

  • Mechanical Transducers: Simple crystal attached to motor; wobbles for angular sweep.

  • Mechanical Annular Array: Mechanically steered, electronically focused. round transducer, cone shaped, symmetrical to beam axis. its LR = ER.

  • Array Transducers: .

  • 1-D Array: One row of elements; sequencing and/or phasing.

  • 1.5-D Array: 3 or 5 rows of elements; multiple foci in elevational dimension.

  • 2-D Array: Multiple elements in lateral and elevation; steering and focusing in two dimensions.

Arrays: Operation and Types

  • Operation: Sequencing and Phasing.

  • Sequencing: Groups of elements fired in linear sequence.

  • Linear Sequenced (Switched) Array: Straight line of rectangular elements.

  • Convex Sequenced Array: Curved line of elements. scan line density greater in nf.

  • Phasing: Small time delays among excitation pulses to elements. Used for electronic steering and/or focusing.

  • Huygens-Fresnel Principle: Every point on a wavefront is a source of spherical wavelets.

  • Electronic Steering: Steers beam by sending pulses with different phasing.

  • Electronic Focusing: Phased array can focus beam.

  • Multiple Transmit Foci: Achieved by varying curvature. only focus that reduces FR.

  • Variable Aperture: Smaller groups for short focal lengths, larger groups for deeper foci.

  • 1-D Sector Phased Array Transducer: Replaces mechanical sector. mechanically focused, electronically steered. fires all elements for each acoustic scan line using small time delays to steer.

  • Vector Array: Image format similar to curved array but with smaller footprint and flat top.

  • Dynamic Focusing: Continual changing of reception focus for deeper echoes.

  • Dynamic Aperture: Aperture increases during echo reception to maintain constant focal width.

Grating Lobes

  • Cause: Additional weak beams from multi-element array transducers.

  • Effects: Spurious echoes (acoustic noise), widen beam reducing strength, degrade lateral and contrast resolution.

  • Reduction:

  • Apodization: Driving outer elements at lower amplitudes or voltage.

  • Sub-dicing: Cutting each element into small crystals.

Virtual Beam Forming (VBF)

  • Principle 2: Uses weakly focused or non-focused transmit beams and computed reception "beams."

  • Advantages: Images in focus throughout, improved lateral and elevational resolution.

Useful Frequency Range

  • Diagnostic: 2–20 MHz (higher frequencies = increased resolution, decreased depth).

  • Specialized: Up to 50 MHz for ophthalmologic, dermatologic, intravascular imaging.

Instruments: Imaging Anatomy and Principles

Objectives

  • Explain how sonographic instruments work.

  • List primary components and functions.

  • Describe electronic image storage.

  • Compare preprocessing with postprocessing.

  • Compare signal processing and image processing.

  • Explain display mechanisms.

  • List common display modes.

  • Define contrast and temporal resolution and influencing factors.

  • Discuss coded excitation, gain, compensation, detection, compression.

  • Describe elastography vs. gray-scale imaging.

Two Principles of Operation

  • Principle 1: One-to-one correspondence. Physical beam forming directly coupled with displayed scan lines.

  • Limitations: Higher frequency = reduced penetration; multiple transmit focuses = reduced temporal resolution.

  • Principle 2 (Virtual Beam Forming - VBF): Does not rely on one-to-one relationship. Uses weakly focused or nonfocused transmit beams and computed reception "beams."

  • Advantages: Images in focus throughout, improved quality. Improves all imaging characteristics.

Dynamic Range

  • Echo Dynamic Range (after compensation): 50-100 dB.

  • Display Dynamic Range: Up to 30 dB.

  • Human Vision Dynamic Range: Approximately 20 dB.

  • Compression: Process of decreasing differences between smallest and largest echo amplitudes to a usable range. Reduces dynamic range with selective amplification. increase dr by decreasing image contrast.

Principle 1 Instruments

  • Components: Beam Former, Signal Processor, Image Processor, Display.

Beam Former

  • Function: Originates the action.

  • Components:

  • Pulser: Generates voltages driving transducer. Voltage frequency determines ultrasound pulse frequency. Greater voltage amplitude = greater pulse amplitude/intensity. prf.

  • Pulser and Pulse Delays: Carry out sequencing, phase delays (steering, transmit focusing), variations in pulse amplitudes, transmit aperture, transmit apodization.

  • Channels: Independent signal path (transducer element, delay, electronics). More channels = more precise beam control.

  • Transmit/Receive (T/R) Switch: Protects sensitive amplifier components.

  • Amplifiers (Gain): Increase voltage amplitudes. Set subjectively for appropriate brightness.

  • Time Gain Compensation (TGC): Compensates for attenuation. Doesnt affect sound transmission. amplifies echo signals from deeper structures.

  • Analog-to-Digital Converters (ADCs)/Digitizers: Convert analog echo voltages to digital numbers.

  • Nyquist Criterion: Interrogation rate must be twice the highest frequency.

  • Echo Delays and Summer: Accomplish reception steering, reception dynamic focusing, reception apodization, and dynamic aperture.

  • Dynamic Focusing: Continual changing of reception focus for deeper echoes.

  • Dynamic Aperture: Aperture increases during echo reception to maintain constant focal width.

Signal Processor

  • Function: Receives digital signals from beam former, processes them, sends to image processor.

  • Functions:

  • Filtering (Bandpass Filter): Rejects frequencies outside accepted bandwidth. Filters fundamental frequency in harmonic imaging.

  • Detection (Demodulation): Converts echo voltages from radio frequency to video form (retains amplitudes). Includes rectification and smoothing.

  • Compression (Dynamic Range): Reduces dynamic range with selective amplification. Operator adjustable.

Image Processor

  • Function: Converts scan line data into images, processes images before/after storage, converts digital to analog, sends to display.

  • Preprocessing: Image processing done before storing in memory.

  • Edge Enhancement: Sharpens boundaries.

  • Pixel Interpolation: Fills missing pixels.

  • Persistence: Reduces noise, smooths image by frame averaging. Decreases frame rate.

  • Three-dimensional Acquisition: Acquiring 2D scans for 3D volume.

  • Image Memory: Stores image frames.

  • Freeze: Holding and displaying one frame.

  • Cine Loop: Storing last several frames before freezing.

  • Pixels: Image divided into squares.

  • Bit Depth: Number of bits per pixel determines grayscale.

  • Postprocessing: Image processing done after echoes are stored in memory. Operator-controllable.

  • Gray-scale Maps: Assigns display brightness to numbers.

  • B-color: Assigns colors instead of gray shades.

  • Three-dimensional Presentation: Surface renderings, 2D slices, transparent views.

  • Read Zoom: Postprocessing; magnifies existing pixels, makes image pixelated.

  • Write Zoom: Preprocessing; writes smaller anatomic field of view into entire memory, enlarging image without pixelation, improves resolution.

  • Digital-to-Analog Converter (DAC): Converts digital data from image memory to analog voltages for display brightness.

Image Display

  • Presentation Modes: A-mode, B-mode, M-mode.

  • Types: Cathode Ray Tube (CRT), Flat-panel display (LCD, LED).

  • Monitor Frame Rates: Most LCD/LED monitors operate at 60 Hz.

Contrast Resolution

  • Definition: Ability of a gray-scale display to distinguish between echoes of slightly different intensities.

  • Dependence: Number of bits per pixel in image memory.

  • Degradation: Artifacts (grating lobes), noise, coarse speckle.

Temporal Resolution

  • Definition: Ability to distinguish closely spaced events in time and present rapid moving structures correctly.

  • Dependence: Frame rate (higher frame rate = better temporal resolution).

  • Factors: Increasing PRF increases frame rate. Increasing focal zones or imaging depth decreases frame rate.

  • M and A modes: Temporal resolution equals pulse repetition period.

Harmonic Imaging (Axial Resolution Degradation)

  • Cause: To separate fundamental and harmonic bandwidths, a narrower bandwidth of frequencies is transmitted, requiring more cycles, increasing pulse duration and SPL, thus decreasing axial resolution.

Code Excitation

  • Mechanism: Uses series of pulses and gaps (not single pulse) to generate scan line.

  • Results: More sensitive receiving system, separation of harmonic echo bandwidth, increased penetration (improved SNR), speckle reduction (improved contrast resolution), gray-scale imaging of blood flow (B-flow).

Panoramic Imaging

  • Purpose: Expands image beyond transducer's field of view.

  • Processing: Preprocessing.

Spatial Compounding

  • Mechanism: Averaging frames that view anatomy from different angles.

  • Improvements: Image quality, reduction in speckle and clutter, more complete specular surfaces, visualization behind attenuating structures, improved SNR.

  • Temporal Resolution: Degrades temporal resolution.

Elastography

  • Purpose: Presents qualitative/quantitative tissue stiffness information (imaging palpation).

  • Young's Modulus: Ratio of applied stress to resulting strain. Small modulus = soft; large modulus = hard/stiff. Unit: kilopascal (kPa).

  • Stress: Pressure/tension on an object.

  • Strain: Relative change in shape/size due to stress. Unitless.

  • Producing Strain: Manual compression, internal patient motion, Acoustic Radiation Force Impulse (ARFI).

  • Color Coding:

  • Strain (Static) Elastography: Hard tissues (blue), soft tissues (red).

  • Shear Wave (Dynamic) Elastography: Softer tissues (blue), stiffer tissues (red).

Cardiac Strain Imaging

  • Presents information on contraction and relaxation strain/strain rate for myocardium.

Fusion Imaging

  • Combined presentation of sonographic anatomic image with another imaging form.

Output Devices

  • PACS (Picture Archiving and Communications Systems): Electronically communicates images and info to workstations.

  • DICOM (Digital Imaging and Communications in Medicine): Standard protocols for image communication.

Instruments: Imaging Motion and Flow With Principle 1

Objectives

  • List and compare flow types in blood circulation.

  • Explain stenosis effects on flow.

  • Define Doppler effect, Doppler shift, Doppler angle.

  • Explain color encoding of 2D flow.

  • Compare Doppler-shift with Doppler-power displays.

  • Explain flow detection localization with pulsed Doppler.

  • Describe spectral analysis and its application.

Circulatory System

  • Components: Heart, arteries, arterioles, capillaries, venules, veins.

  • Function: Exchange of gases, nutrients, waste products at capillaries.

Fluids

  • Viscosity: Resistance to flow. Units: poise, kg/m-s.

  • Blood Viscosity: Plasma ~50% greater than water. Varies with flow speed and temperature.

Pressure

  • Definition: Force per unit area. Pressure difference divided by distance.

  • Driving Force: Pressure difference required for flow.

Volumetric Flow Rate (Q)

  • Definition: Volume of blood passing a point per unit time. Unit: milliliters (mL/s).

Flow Resistance

  • Poiseuille's Law: Describes flow resistance in a long, straight tube. Arterioles have most resistance.

Types of Flow

  • Laminar Flow: Streamlines are straight and parallel.

  • Plug Flow: Speed constant across tube.

  • Laminar Flow: Speed max at center, min/zero at walls.

  • Parabolic Flow: Average speed = 1/2 max speed.

  • Disturbed Flow: Parallel streamlines altered (e.g., at stenosis or bifurcation).

  • Nonlaminar Flow:

  • Turbulent Flow: Random, chaotic speeds and directions. Occurs with high flow speed or transition from narrow to broad channels.

Steady Flow and Pulsatile Flow

  • Steady Flow: Constant flow (e.g., peripheral venous flow with respiratory variation).

  • Pulsatile Flow: Nonsteady flow with acceleration/deceleration over cardiac cycle (e.g., arterial flow).

  • Windkessel Effect: In aorta, continued forward flow due to aortic valve closure.

  • Flow Reversal: Occurs in distal circulation during diastole.

Continuity Rule

  • Principle: Volumetric flow rate is constant throughout a vessel.

  • Stenosis: Average flow speed increases at stenosis to maintain constant volumetric flow rate. Turbulence can occur distal to it.

Bernoulli Effect

  • Principle: Drop in pressure associated with high flow speed at a stenosis. If flow speed increases, pressure decreases.

  • Mechanism: Pressure/potential energy converted to flow/kinetic energy.

Doppler Effect

  • Definition: Change in frequency caused by motion of sound source, receiver, or reflector.

  • Doppler Shift: Change in frequency caused by motion.

  • Positive if blood moves towards transducer, negative if away.

  • Increases with scatterer speed and source frequency.

  • Proportional to flow speed.

  • Nyquist Limit: Highest frequency (Doppler shift) represented unambiguously. Equal to 1/2 PRF.

  • Aliasing: Undersampling of Doppler shifts in pulsed Doppler. Appearance of Doppler info on wrong side of baseline.

  • Doppler Angle: Angle between sound beam and flow direction.

  • Correction: Flow speed measurements incorporate proper Doppler angle correction.

  • Vascular Doppler: Usually 30°-60°. >60° unacceptable errors; <30° total reflection.

  • Echocardiography: Near zero angles useful.

Doppler Ultrasound

  • Measurement: Sonographic instruments measure Doppler shifts, which are proportional to blood flow speed.

  • Operating Frequencies: Usually lower than 2D scanning.

Doppler Displays

  • Color Doppler: Doppler-shift displays.

  • Power Doppler: Doppler-power displays.

  • Spectral Doppler: Pulsed Wave (PW) Doppler, Continuous Wave (CW) Doppler.

Spectral Doppler

  • Continuous Wave (CW):

  • Detects Doppler shift in overlap region of transmitting and receiving elements.

  • Sample Volume: Large.

  • Advantage: No aliasing.

  • Pulsed Wave (PW):

  • Emits pulses and receives echoes using single element or array.

  • Advantage: Ability to select information from a particular depth (range gating, sample volume).

  • Sample Volume/Range Gate: Selects location for echo reception.

  • Pulse Length: PW pulses (5-30 cycles) are longer than imaging pulses (2-3 cycles) for accurate Doppler shift detection.

  • Phase Quadrature Detector: Determines direction and separates Doppler shift voltages.

  • Fast Fourier Transform (FFT): Mathematical technique to generate Doppler shift spectral displays.

  • Spectral Displays: Vertical axis = Doppler shift frequency; Horizontal axis = time.

  • Spectral Broadening: Vertical thickening of spectral trace, indicates disturbed/turbulent flow.

  • Wall Filter: Rejects low-frequency, high-intensity Doppler shifts (clutter from tissue/vessel wall motion).

  • Duplex: Gray-scale + PW.

  • Triplex: Gray-scale + PW + Color Doppler.

  • Aliasing (PW): Occurs when Nyquist limit is exceeded. Solutions: increase PRF, decrease frequency, decrease depth, shift baseline, use CW.

  • Range Ambiguity (PW): Pulse emitted before all echoes from previous pulse received. Places structures closer than actual.

  • Audio: Doppler shifts in audio range (+/- 10kHz).

Color Doppler

  • Definition: Extension of gray-scale sonography, showing blood flow/tissue motion in color. Real-time.

  • Signal Processor: Uses autocorrelation to rapidly determine mean and variance of Doppler shift.

  • Advantages: Demonstrates blood flow, determines flow direction, demonstrates nonvascular motion.

  • Limitations: Angle dependence, lower frame rates, lack of detailed spectral information.

  • Controls: Gain, Color Window, Steering Angle, Color-map Inversion, Wall Filter, Priority, Baseline Shift, Velocity Range/Scale (PRF), Color Map Selection, Variance, Smoothing (Persistence), Ensemble Lengths (Packet Size).

  • Flow Reversal: Distinguishable from aliasing.

Power Doppler

  • Definition: Color-encodes strength of Doppler shifts. Power determined by concentration of moving scatterers.

  • Advantages: Angle independency, no aliasing, improved sensitivity (deeper penetration, smaller vessels, slower flows).

  • Disadvantages: No directional information, no flow speed/character information, worse temporal resolution, flash artifact.

Color Doppler vs. Spectral Doppler

  • Gating: Color Doppler is scanned, has lateral dimension, series of range gates. Spectral Doppler has a single sample volume.

  • FFT: Not enough data for FFT in Color Doppler.

  • Third Axis: Spectral Doppler (amplitude); Color Doppler (mean velocity estimate).

  • Ultrafast Color Doppler: Plane waves in different angles.

Artifacts: What Can Go Wrong

Objectives

  • List reasons for incorrect presentation in gray-scale images.

  • List reasons for incorrect presentation in Doppler displays.

  • Describe how to recognize specific artifacts.

  • Explain how to deal with artifacts to avoid misdiagnoses.

Artifacts

  • Definition: Any representation in image or spectrum not indicative of "truth."

  • Types: Not real, missing, misplaced, incorrect brightness/shape/size.

  • Usefulness: Some artifacts (enhancement, shadowing, twinkling, ring-down, comet tail) can aid diagnosis.

Assumptions of the Ultrasound System

  • Sound travels in a straight line.

  • Echoes originate only from objects on the beam axis.

  • Amplitude of returning echoes directly related to reflecting/scattering properties.

  • Distance to objects proportional to round-trip travel time.

Propagation Artifacts

Slice Thickness/Partial Volume Artifact

  • Cause: Third dimension, z-axis (beam width perpendicular to scan plane). with linear array transducers is determined by point of mechanical focusing.

  • Resolution: Can be resolved by tissue harmonic imaging.

Speckle

  • Cause: Acoustic noise; grainy appearance from interference pattern of scatterer distribution.

  • Effect: Restricts detection of small changes, obscures fine detail.

Reverberation

  • Cause: Two or more strong reflectors encountered; multiple reflections.

  • Appearance: Equally spaced reflections of diminishing amplitude with increased depth.

  • Types:

  • Comet-tail Artifact: Special form of reverberation; seen with small calcific/crystalline/highly reflective objects (solids). Can have color (twinkle artifact).

  • Ring-down Artifact: Resonance artifact caused by gas bubbles.

  • A-line and B-line: Lung scanning artifacts related to reverberation.

Mirror Image

  • Cause: Duplication of a structure on the opposite side of a strong reflector.

  • Common Locations: Around pleura and diaphragm.

Multi-path Artifact

  • Cause: Transmitted beam reflects off-axis, then off a second reflector back to transducer.

  • Effect: Object appears slightly deeper due to increased path length.

Refraction

  • Cause: Change of direction of sound beam from one medium to the next.

  • Effect: Displaces structures laterally from correct locations. Can image one real structure as two artifactual objects.

  • Requirements: Oblique incidence and different propagation speeds.

Grating Lobes

  • Cause: Additional weaker beams emitted from an array transducer.

  • Effect: Duplicate structures laterally to true ones.

  • Reduction: Apodization, sub-dicing.

Speed Error

  • Cause: Speed of sound in tissue is faster or slower than assumed 1.54 mm/µs.

  • Effect: Slower speeds place echoes deeper; faster speeds place echoes closer.

Range Ambiguity

  • Cause: All echoes not received before next pulse emitted.

  • Effect: Places structures much closer to surface than they should be.

Attenuation Artifacts

Shadowing

  • Cause: Weakening of echoes distal to a strongly attenuating or reflecting structure, or from edges of a refracting structure. From gas. loss of sound energy due to attenuation results in less penetration.

Enhancement

  • Cause: Strengthening of echoes distal to a weakly attenuating structure.

  • Appearance: Increased brightness behind a weakly attenuating structure.

Beam/Phase Aberration

  • Distortion of the beam due to difference in speed of sound in tissue.

Doppler Artifacts

Nyquist Limit

  • Definition: Highest frequency in a sampled signal represented unambiguously. Equal to 1/2 PRF.

Aliasing

  • Cause: Undersampling of Doppler shifts in pulsed Doppler. Occurs when Doppler shift exceeds Nyquist limit.

  • Appearance: Doppler information (spectral or color) on the wrong side of the baseline.

  • Solutions: Increase PRF, decrease operating frequency, decrease imaging depth, shift baseline, use CW Doppler.

Range Ambiguity (Doppler)

  • Cause: Pulse emitted before all echoes from previous pulse received.

  • Effect: Multiple sample volumes appear.

Mirror Image (Doppler)

  • Cause: Duplication of a vessel or Doppler shift on opposite side of a strong reflector.

Flash Artifact

  • Cause: Sudden burst of color Doppler, typically from tissue or transducer motion.

  • Appearance: Extension of color beyond region of blood flow.

  • Power Doppler: More prone due to greater sensitivity.

Color Doppler Dropout

  • Absence of color signal in areas where flow is expected.

Key Formulas and Relationships

1. Wave Characteristics

  • Frequency, Wavelength, and Propagation Speed:

  • Frequency (MHz) = Propagation speed (mm/µs) / Wavelength (mm)

  • Wavelength (mm) = Propagation speed (mm/µs) / Frequency (MHz)

  • Period (T):

  • T (µs) = 1 / Frequency (MHz)

2. Pulsed Wave Parameters

  • Pulse Repetition Period (PRP):

  • PRP (ms) = 1 / PRF (kHz) (PRF = Pulse Repetition Frequency)

  • Pulse Duration (PD):

  • PD (µs) = Period (µs) × Number of cycles in the pulse

  • Duty Factor (DF):

  • Duty factor = Pulse duration (µs) / Pulse repetition period (µs)

  • DF = temporal average intensity / pulse average intensity

  • Spatial Pulse Length (SPL):

  • SPL (mm) = wavelength (mm) × Number of cycles in the pulse

3. Transducer and Beam Properties

  • Fractional Bandwidth:

  • Fractional bandwidth = Bandwidth / Operating frequency

  • Impedance (z):

  • Impedance = Density × Propagation speed (rayls)

4. Imaging and System Performance

  • Amplitude:

  • Amplitude = (max - min) / 2

  • Amplitude = max - mean; mean = (max + min) / 2

  • Intensity:

  • Intensity (mW/cm²) = Power (mW) / Area (cm²)

  • Work:

  • F(n) × D(m) = W(nm) (joule) (Force × Distance = Work)

  • Axial Resolution:

  • Axial resolution (mm) = Spatial pulse length (mm) / 2

  • Frame Time and Frame Rate:

  • Frame time = time per scan line * scan lines per frame

  • time per scan line = 13 microseconds * depth

  • Maximum permissible frame rate (FRm) = 77,000 / [Depth (cm) × Number of lines per frame × Number of foci (if any) × Ensemble length (n)] (where FRm is in Hz)

  • Pulser Imaging Depth Limit:

  • Imaging depth (cm) × Pulse repetition frequency (PRF) (kHz) ≤ 77 (cm/ms)

  • Range Equation (Distance to Reflector):

  • Distance (mm) = ½ [ Propagation speed (mm/µs) × Pulse round trip time (µs)]

  • Contrast Resolution:

  • Contrast resolution (dB per shade) = dynamic range / number of shades

5. Sound Interaction and Attenuation

  • Intensity Reflection Coefficient (IRC):

  • IRC = Reflected intensity (mW/cm²) / Incident intensity (mW/cm²)

  • Intensity Transmission Coefficient (ITC):

  • ITC = 1 − IRC

  • ITC = Transmitted intensity (W/cm²) / Incident intensity (W/cm²)

  • Snell's Law (for Refraction):

  • sin(θ_incident) / sin(θ_transmitted) = speed_medium1 / speed_medium2

6. Flow and Doppler

  • Volumetric Flow Rate (Q):

  • Q (mL/s) = Pressure difference (dyne/cm²) / Resistance to flow (poise)

  • Poiseuille’s Law:

  • Q = (ΔP π r^4) / (8 L η) (where ΔP is pressure difference, r is radius, L is length, η is viscosity)

  • Bernoulli's Equation (Simplified for stenosis):

  • ΔP = 4V^2 (Relates pressure drop to flow velocity)

  • ΔP = 1/2 ρ (V_2^2 - V_1^2) (where ρ is density, V is velocity)

  • Nyquist Limit:

  • Nyquist Limit = PRF / 2

  • Doppler Shift (f_d):

  • Doppler Shift (f_d) = (2 f_0 v * cos(θ)) / c (where f_0 is transmitted frequency, v is scatterer speed, θ is Doppler angle, c is propagation speed)