Image Quality And Artifacts In Ultrasound

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SAHANA KAYASTHA

MSC MIT 1^ST YR

ROLL NO: 26

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Introduction

• Ultrasonography in the last few decades has undergone massive transformation and today occupies a crucial role in practice of most of the domains of medicine.
• Based on the detection and display of acoustic energy reflected from interfaces within the body.
• Provides the information needed to generate high-resolution, gray-scale images of the body, as well as display information related to blood flow.
• Unique imaging attributes have made ultrasound an important and versatile medical imaging tool.

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Ultrasound Transducer

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Ultrasound Beam Characteristics

• Exhibits two distinct wave patterns:
• Slightly converging beam (near field)
• Diverging beam beyond a point (far field).

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Ultrasonic Image Display

• A-mode
• B-mode
• M-mode
• Over the years, imaging has evolved from simple A-mode and bi-stable display to high-resolution, real-time, grayscale imaging.
• Modern equipment uses a pulse-echo approach with a brightness-mode (B-mode) display.

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A- Mode

• “Amplitude mode” Echoes returning from the body are displayed as signals on an oscilloscope.
• Echoes displayed as spikes from a baseline.
• Baseline identifies the central axis of the beam.
• Spike height proportional to echo intensity
• Represents the echo amplitude only along the single line of sight of the mid beam of the transducer at any point of time.

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A- Mode

• Contains information about the depth of structure and the amplitude of the returning echoes.
• Only the position and strength of a reflecting structure are recorded.
• Displays depth on the horizontal axis and echo intensity (pulse amplitude) on the vertical axis.
• Used in Opthalmology

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B-Mode

• Also K/a Brightness mode
• Variations in display intensity or brightness are used to indicate reflected signals of differing amplitude.
• A series of dots appears along the line of sight.
• Brightness of each dot proportional to amplitude of returning echoes.

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B-Mode

• The distance of any particular dot from the left side of the display=distance from the transducer face.
• When an ultrasound image is displayed on a black background, signals of greatest intensity appear as white; absence of signal is shown as black; and signals of intermediate intensity appear as shades of gray.
• The mainstay for real-time, 2-D gray-scale imaging, which the echo was generated.

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M-Mode

• Time motion(TM mode)
• Displays echo amplitude and shows the position of moving reflectors.
• Uses the brightness of the display to indicate the intensity of the reflected signal.
• The time base of the display can be adjusted to allow for varying degrees of temporal resolution, as dictated by clinical application.

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M-Mode

• Interpreted by assessing motion patterns of specific reflectors and determining anatomic relationships from characteristic patterns of motion.
• Currently, used in evaluation of the rapid motion of cardiac valves and of cardiac chamber and vessel walls.
• May play a future role in measurement of subtle changes in vessel wall elasticity accompanying atherogenesis.

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M-Mode

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US Image Characteristics

• Key determinants of the quality of an ultrasound image:
• Spatial Resolution
• Contrast Resolution
• Temporal resolution
• Noise
• Freedom from certain artifacts

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Spatial Resolution

• Ability of an imaging system to display two adjacent objects as discrete entities.
• Considered in 3 different planes with determinants of resolution for each planes.
• Consists of
• Axial Resolution
• Lateral Resolution
• Elevational Resolution
• The axial, lateral, and elevational resolution dimensions determine the minimal volume element

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Axial Resolution

• 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.
• Achieving good axial resolution requires that the returning echoes be distinct without overlap.
• Most important factor affecting axial resolution: Pulse length
• Short pulses, achieved by damping of the transducer, are essential for good axial resolution performance.
• Ultrasound frequency and wavelength are inversely related, the pulse length decreases as the imaging frequency increases, finally increasing resolution.

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Axial Resolution

• Transducer frequency is the determinant of axial resolution.
• High-frequency transducers must be used for good axial resolution.
• Ultrasound imaging has a fundamental trade-off between spatial resolution and maximum imaging depth.
• High axial resolution requires minimizing the distance between the transducer and objects of interest.
• High-resolution imaging is achieved in breast ultrasound using 8-or 10-MHz transducers.

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Axial Resolution

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Lateral Resolution

• Aka azimuthal resolution.Ability to resolve two adjacent objects perpendicular to the beam direction.
• Limiting aspect of spatial resolution.
• Factors affecting lateral resolution include:
• Beam width
• Frame rate
• Performance is determined by the ultrasound beam width.
• Improved by using focused transducers.
• Best lateral resolution performance is obtained within the focal zone.
• Focal zone is the area where the beam is 3 to 4 wavelengths wide.

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Lateral Resolution

• Lateral resolution can be controlled by adjusting the focal position.
• Multiple focal lengths(array transducers)may be used to improve lateral resolution.
• Use of multiple focal lengths is generally at the expense of a reduced frame rate.
• The number of scan lines contributing to the image affects lateral resolution.
• Sampling the tissues at closer intervals improves resolution.
• The higher the lines per frame, the better the lateral resolution.
• Measured using phantoms and worse than axial resolution.

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Lateral Resolution

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Lateral resolution usually becomes worse at larger distances from the transducer.

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Elevational Resolution

• Resolution in the plane perpendicular to the image plane.
• Slice thickness is another term for elevational resolution.
• Transducer height is directly related to elevational resolution.
• Elevational focusing can be achieved using an acoustic lens.
• Elevational resolution is generally image depth dependent.
• Slice thickness can be improved by the use of 1.5D arrays.
• A disadvantage of elevational focusing is a frame rate reduction penalty required for multiple excitations to build one image.
• The increased width of the transducer array also limits positioning flexibility.

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Elevational Resolution

• Extension to future full 2D transducer arrays with enhancements in computational power will allow 3D imaging with uniform resolution throughout the image volume.
• Lateral resolution and elevational resolution are generally comparable.

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Resolution in 3-D Space

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Contrast Resolution

• The ability to distinguish between signal sizes.
• In the ultrasound image, this translates to differentiating between the intensities of the dots representing echoes of different size at the display.
• The ability to detect small changes in the characteristic echo pattern of an organ may well depend on the level of contrast resolution.
• The electronics in the imaging system, and the inherent contrast properties of the display and recording devices, affect the contrast resolution to a large extent.
• The use of digital image processing techniques is contributing to improved contrast resolution of the ultrasound image.

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Contrast Resolution

• Introduction of microbubble contrast agents improves the visualization of the vasculature and tissue perfusion.
• Spatial compounding provides multiple beam angles to better depict tissue boundaries, as well as providing averaging to reduce speckle and noise.
• Harmonic imaging improves image contrast by eliminating unimportant or degrading signals from lower frequency echoes.
• Doppler imaging techniques uses moving anatomy and sophisticated processing techniques to generate contrast.

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Noise

• All imaging systems are prone to two types of noise random and structured.
• Result from random signals produced in the electronic preamplifier of the transducer,
• Seen as fluctuating moving grey spots on the image producing a snow storm appearance.
• Averaging successive images reduces noise.

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Speckle

• Various other sources of‘noise’ within ultrasound images, Perhaps the most important is speckle.
• Constructive and destructive interference of these acoustic fields result in speckle.
• Textured appearance that results from small closely spaced structures.
• Structures producing speckle are too small to be normally resolved.
• Source of image degradation and loss of contrast in ultrasound image.
• Reduced using techniques that reduce noise (i.e, higher-frequency transducer, real-time compounding, adaptive post-processing, and harmonic imaging

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Temporal Resolution

• The ability to separate events in time.
• Important in real-time imaging.
• The rate at which image frames are generated and viewed affects the visualization of moving structures.
• The limit of temporal resolution for the human eye is about 40 ms.
• Events occurring within a time interval of less than 40 ms are viewed as taking place "simultaneously".

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Temporal Resolution

• This level of temporal resolution dictates that the framing rate required for real-time imaging to observe moving structures should be 25 frames per second (f.p.s) or more (if one image frame is generated every 40 ms, then in a total time of 1 sec, or 1000 ms, 25 image frames will be generated).
• Framing rates below about 20 fps are associated with a phenomenon called image flicker, which arises from the ability of the eye to distinguish the resulting image frames as being separate in time

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Optimization of US Image

• Image formation in ultrasound depends on the physical properties of ultrasound pulse formation, transmission and interaction with tissues.
• Diagnostic applications of ultrasound are based on detection and display of acoustic energy reflected from interfaces within the body
• New technical developments and remarkable improvement in image quality have marked several new applications for diagnostic ultrasound.
• Nonetheless, it does not by itself guarantee, high quality ultrasound images.

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Optimization of US image

• Optimization of image is an essential prerequisite for diagnostic ultrasound imaging.
• Image optimization includes:
• Factory Presets
• Transducer Selection
• Overall Gain
• Time Gain Compensation
• Depth Setting
• Focal Zone Setting
• Zooming
• Dynamic Range

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Optimization of US image

• Factory Presets:
• The more complicated machine settings are saved as presets.
• Provide a useful starting point, settings which are optimized for patients of average body habitus
• Further optimization of the image by manual adjustments is invariably required to increase diagnostic confidence and avoid artifacts.
• A single touch image optimization is also available on some systems however this only resets the parameters to the chosen preset.

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Optimization of US image

Transducer Selection

• Transducer characteristics, such as frequency and shape, determine ultrasound image quality.
• Choosing the appropriate transducer both in terms of frequency as well as shape(footprint) is extremely important for a good examination.
• Higher frequencies: better resolution,attenuated more (Att:f⁴)
• For superficial scans (Thyroid, musculoskeletal system or testicular examination); linear transducer with a high frequency (7-10 mHz or even 15-17 MHz).
• For deeper structures: Infra-clavicular and popliteal regions; lower frequency as 2.5 to 3 MHz is used

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Optimization of Image Characteristics

• Spatial resolution versus tissue depth
• Conflict of interest between tissue depth and spatial resolution which necessitates a compromise.
• Use of multi-frequency transducers
• Broad band transducers
• Translates into a wide range of frequencies in the beam, thus combining the advantages of high spatial resolution from the higher frequency components with those of deeper penetration from the lower frequency components.

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Trade-Offs

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Frequency Vs Resolution Vs Penetration

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Optimization of US Image

Overall Gain:

• Amplifies all the returning echoes/signals uniformly.
• An excessively high gain can result in a washed out image and can obscure many details.

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Optimization of US Image

Time Gain Compensation

• Also referred to as distance gain compensation (DGC) or spatial time compensation (STC).
• Signals that arrive later, i.e. from greater depths are amplified more than earlier signals to compensate for the attenuating effect of tissues.
• TGC controls permit the user to selectively amplify the signals from deeper structures or suppress signals from superficial tissues so that a smooth gray scale picture can be obtained.
• Although most newer machines provide for some automatic TGC, manual adjustment by the user is one of the most important factors that may have a profound effect on image quality.

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Optimization of US Image

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Fig: Improper TGC setting resulting in a dark band over the deep aspect of the liver (A).

With the correct TGC setting the whole liver shows a uniform echopattern (B)

Optimization of US Image

Depth Setting:

• Selects the depth of the imaging field
• Selection of too much depth: smaller image and the area of interest difficult to visualize, measurements may also be inaccurate.
• Too shallow depth setting: non visulization of the deeper structures, pathology at a deeper levels may be completely missed.
• Practically, adjusted by starting at a higher depth, subsequently depths should be decreased so that the area of interest in at about three- fourths the depth of the screen.

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Effect of Depth

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Fig: Improper depth setting in (A) non-visualization of posterior most portion of liver with consequently missed metastatic focus, which is easily appreciated (arrow) with proper depth setting(B)

Optimization of US image

Focal Zone Setting:

• To improve resolution diagnostic transducers are focused electronically.
• Best resolution of ultrasound in the focal zone.
• The focal zone should thus be set over the area of interest.
• A significant difference in image resolution depending on the focal zone setting is the hallmark of a well- focused beam.
• Use of multiple focal zones in modern scanners results in decreased frame rate and hence decreased temporal resolution.

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Effect of Focal Zone

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1. Fig: blurred appearance of the posteriorly placed hepatic cyst and diaphragm.
2. Structures sharply defined with proper focal zone positioning over the area of interest

Optimization of US image

Zooming:

• used to magnify the area of interest.
• useful for accurate measurement of small structures.
• Two types of zoom available.

Read zoom:

• used to enlarge a frozen image
• Gray level values are read only from a small part of the computer memory
• Pixels gets enlarged while displaying(magnifying glass).
• Magnification factor (too large): Image matrix becomes obvious and renders the image to grainy.

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Optimization of US image

Write Zoom:

• Used to enlarge the display magnification, when scanning is taking place.
• Use is made of the fact to receive echo samples much more faster for several samples to occur in each pixel.
• Each pixel, whose size remains unchanged, displays less tissue(magnification without loss of definition)
• If magnification scale is increased, Requirement of further interpolation and smoothening to maintain an acceptable image.

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• More real scan lines can be introduced using sophisticated signal processing so that definition is improved.
• One such option is referred to as high density (HD) zoom.
• Increases the line density by redistributing and reformatting all scan lines for the defined region of interest, finally magnifies the region of interest
• Compared to Read zoom, Write zoom is preferred.

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Optimization of US image

Dynamic Range:

• Ratio of the highest to the lowest amplitudes that can be displayed (db)decibels.
• Dynamic range adjustment changes the contrast.
• Widest dynamic range permitting best differentiation of subtle differences in echo intensity is preferred for the most applications.
• The narrower ranges increase the conspicuity of larger echo differences

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Optimization of US image

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Fig; Images of the liver displayed at dynamic ranges of 70 and 36 dB resp.

Sensitivity of the Imaging system

• Represents the capability of the imaging system to detect very small signal sizes in relation to the energy expended in the effort to generate the signals.
• Determines the ability of the imaging system to detect small echoes.
• Affects the amount of energy absorbed within the tissues of the subject during an examination.

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Sensitivity of the Imaging system

• Since a large proportion of diagnostic information is carried by weak echoes, system sensitivity is a critical aspect of ultrasonic imaging.
• Factors influencing the sensitivity of the imaging system includes transducer factors, beam characteristics, signal processing, and image display and recording.

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Artifacts

• Term used to describe any unwanted information generated in the process of image formation.
• In US, used to describe any part of an image that does not accurately represent the anatomic structures present within the subject being evaluated.
• May cause structures to appear in an image that are not present anatomically or a structure that is present anatomically may be missing from the image.
• US is prone to numerous imaging artifacts, occuring commonly encountered in our daily clinical practice.
• Have the greatest potential to interfere with image interpretation.

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Assumptions

• Ultrasound display equipment relies on physical assumptions to assign the location and intensity of each received echo.
• Assumptions:
• Echoes detected, originated from within the main ultrasound beam.
• An echo returns to the transducer after a single reflection,
• The depth of an object is directly related to the amount of time for an ultrasound pulse to return to the transducer as an echo,
• The speed of sound in human tissue is constant, the sound beam and its echo travel in a straight path
• The acoustic energy in an ultrasound field is uniformly attenuated.

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Contd..

• In clinical sonography, these assumptions are often not maintained; when this occurs, echoes may be displayed erroneously and perceived as artifact.
• Artifacts thus arise secondary to errors.
• inherent to the ultrasound beam characteristics,
• the presence of multiple echo paths
• velocity errors
• attenuation errors

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Classification

Classification based on different factors:

• Artifacts Associated with Ultrasound Beam Characteristics
• Artifacts Associated with Multiple Echoes
• Artifacts Associated with Velocity Errors
• Artifacts Associated with Attenuation Errors
• Equipment generated Artifacts
• Miscellaneous Artifacts

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Classification of US Artifacts

Artifacts Associated with Ultrasound Beam Characteristics:

• Beam artifacts :
• Why do they occur?
• Based on the assumption that : echoes arise only from structure within main US beam
• Imaging plane is thin(laterally and elevationally)
• How they occur?
• In reality; 1) the imaging plane has a third dimension (elevational plane) and the beam has variable width

2)Echoes can be captured outside the main US beam(side/ grating lobes)

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Beam Artifact

• Types of US beam artifacts:
• Side lobe
• Grating lobe
• Beam width artifact
• Slice thickness artifact

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Side Lobe and Grating Lobes

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• The main ultrasound beam narrows as it approaches the focal zone and then diverges and appears as a complex three-dimensional bow-tie shape with additional off-axis low energy beams, which are referred to as side lobes and grating lobes.

Side/Grating Lobe Artifacts

• Multiple beams of low-amplitude ultrasound energy that project radially from the main beam axis .
• Side lobe energy is generated from the radial expansion of piezoelectric crystals orthogonal to the main beam
• Results in structures outside the main US beam being mapped into the main US beam
• and is seen primarily in linear-array transducers .

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Side Lobe and Grating Lobes

• Strong reflectors present in the path of these low-energy, off-axis beams may create echoes detectable by the transducer.
• These echoes will be displayed as having originated from within the main beam in the side lobe artifact .
• As with beam width artifact, this phenomenon is most likely to be recognized as extraneous echoes present within an expected anechoic structure such as the bladder.
• Often, referred to as Chinese Hat Artifacts (convex shaped).

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Side Lobe and Grating Lobes

• Causes false (artifactual) echoes to be displayed within structures on th US image(echoes within the bladder, gallbladder, cysts, vessels, etc)
• Eg: GB imaging,where side lobes produce artifactual pseudosludge in an otherwise echo- free region.
• Side lobes are present with all types of single and multielement transducer assemblies and are more forward directed.

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Side/Grating Lobe Artifacts

• Grating lobe occurs with multielement array transducer mainly linear than phased- array due to larger width and spacing of individual elements.
• Can create ghost images of off-axis high-contrast objects.
• Can be reduced by using closely spaced elements in array(less than 1/2ƛ apart).

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Beam Width Artifact

• Related to lateral resolution
• A highly reflective object located within the widened beam beyond the margin of the transducer may generate detectable echoes.
• The ultrasound display assumes that these echoes originated from within the narrow imaging plane and displays them as echoes.

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Beam Width Artifact

• Clinically, beam width artifact may be recognized when a structure that should be anechoic such as the bladder contains peripheral echoes.
• Image quality may be improved by adjusting the focal zone to the level of interest and by placing the transducer at the center of the object of interest

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Beam Width Artifact

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Fig; 1) US image of a partially filled bladder shows echoes (arrow) in the expected anechoic urine. 2)US image obtained after adjustment of the focal zone and optimal placement of the transducer shows resolution

Slice thickness artifact

• Related to elevational resolution(height of beam)
• The US beam is not thin(flat like a sheet of paper, it is 3 dimensional)
• 3 dimensions:- lateral(width)

Axial(length)

Elevational(height)

• The height of the US beam is determined by the transducer
• Structures above and below the beam can be mapped into the main US beam
• Results in artifactual echoes within anechoic structures(cysts, vessels, bladder, ect)

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Classification of Artifacts

Artifacts Associated with Multiple Echoes:

• These occurs based on the premise that :
• Sound travels into the tissue, encounters a structure and travels directly back to the transducer.
• The amount of time it takes an echo to return to the transducer determines the depth of a structure in the body.
• But in reality; 1) scattering in the tissue(strong reflectors) can cause artifactual echoes

2) Strong reflectors can cause sound to bounce between the reflectors

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• Types of multiple echo artifacts:
• Reverberation artifact
• Comet Tail artifact
• Ring down artifact
• Mirror image artifact

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Reverberation Artifact�

• Caused by one or more strong reflectors(such as a biopsy needle)
• The sound waves bounce back and forth between the strong reflector and the transducer or between two strong reflectors.
• Produces multiple lines of artifact inferiorly to the strong reflectors
• Appearance- Multiple equidistantly spaced linear reflections , ladder shaped, echogenic horizontal lines
• Located posterior to the strong reflectors
• Often visualize in the anterior portion of cysts.
• Prevention – Decrease TGC near in the near gain , Change beam angle /alternative window

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Reverberation Artifact

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Reverberation Artifact�

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Comet Tail Artifact�

• Caused by a strong reflector ( calcifications, surgical clips, IUCD and in adenomyosis of gallbladder)
• Appearance - Series of multiple, closely spaced small bands of echoes .
• Form of reverberation , two reflective interfaces and thus sequential echoes are closely spaced that individual signals are not perceivable in the image .
• Prevention – decrease TGC near in the near gain .
• Change beam angle / alternative window.

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Comet Tail Artifact�

Fig: The artifact is diagnostic of adenomyosis

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Ring Down Artifact

• Thought to be a variant of comet tail artifact.
• Based on the assumption of often similar appearance of the two artifacts.
• US energy causes resonant vibrations of the air bubbles create a continuous sound wave that is transmitted back to the transducer .
• Appearance - A line or series of parallel bands extending posterior to a gas collection.
• Occurs - Posterior to collections of gas (eg, pneumobilia, portal venous gas, gas in abscesses

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Ring Down Artifact

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Ring Down Artifact

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Mirror Image Artifacts

• Multipath artifact
• Generated by the false assumption that an echo returns to the transducer after a single reflection.
• The return of sound beams is delayed, and therefore the structures from which these delayed beams are reflected are displayed at a greater depth than their true anatomic depth
• Appearance - Duplicated structure equidistant from a strongly reflective interface
• Occurs - Diaphragm with liver lesions with the air-pleural interface acting as a strong reflector or the liver itself being duplicated.
• Prevention – Scan from different angle , adjust focal zone or TGC at the level of the diaphragm , scan from multiple windows

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Mirror Image Artifacts

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Mirror Image Artifacts

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Fig: US image obtained at the level of the right hepatic lobe shows an echogenic lesion in the right hepatic lobe (cursors) and a duplicated echogenic lesion (arrow) equidistant from the diaphragm overlying the expected location of lung parenchyma.

Air artifact�

• On US, air (in the stomach, bowel or introduced via a procedure) appears as white hazy areas.
• There can be shadowing, reverberation, comet tail or ring down artifacts posterior to the air.
• Air can be a helpful artifact( leaves behind a white “track” which shows where a biopsy needle has been
• Air can also be a hindrance (by obscuring anatomyor pathology)

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Speckle

• Speckle is the grainy( dot- like) echoes on an US image
• Speckle echoes do not correspond to actual tissue or structures
• Gives tissue it’s textured look
• Speckle is a type of noise artifact(extraneous echoes)
• Caused by echoes( sound waves thata re returning to the transducer) interacting with areas of scattering in the tissue(from other US pulses of sound)resultimg in false echoes being sent back to the transducer.

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Classification of Artifacts

Artifacts Associated with Velocity Errors:

Why do they occur:

Based on the premise that sound always:

Travels in a straight line

Travels at aconstant speed in soft tissue (1540m/s)

How they occur:

In reality, sound 1) may travel slower or faster than 1540m/s depending on the tissue type.

2) Doesn’t always travel in a straight line( the sound may be refracted, or may approach a boundary at an oblique incidence)

Types of velocity error artifacts:

• Speed displacement artifact
• Refraction Artifact

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Speed Displacement Artifact�

• Propagation velocity artifact.
• Caused by the variability of the speed of sound in different tissues.
• The speed of sound within a material is dependent on its density and elastic properties.
• US image processing assumes a constant speed of sound in human tissue of 1540 m/sec.
• In clinical sonography, the ultrasound beam may encounter a variety of materials such as air, fluid, fat, soft tissue, and bone.
• Then sound travels through material with a velocity significantly slower than the assumed 1540 m/sec, the returning echo will take longer to return to the transducer.

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Speed Displacement Artifact�

• The image processor assumes that the length of time for a single round trip of an echo is related only to the distance traveled by the echo.
• The echoes are thus displayed deeper on the image than they really are, referred to as the speed displacement artifact in clinical imaging,
• Often occurs when the ultrasound beam encounters an area of focal fat

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Speed Displacement Artifact�

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Refraction Artifacts

• Refraction: Change of the ultrasound sound direction on passing from one medium to another.
• US beam travelling through two adjacent tissues with different density and elastic properties, changes its velocity, may produce a refraction artifact.
• When a sound wave approaches a boundary at an oblique angle, and when the propagation speeds of the two tissue is different ,the sound wave will change direction.
• Degree of this change in direction is dependent on both the angle of the incident ultrasound beam and the difference in velocity between the two media
• Snells Law: Sin r/Sin i = c₂/c₁

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Refraction Artifacts

• Occurs in pelvic structures deep to the junction of the rectus muscles and midline fat.
• Cause structures to appear wider than they actually are or may cause an apparent duplication of structures.
• Cause spatial distortion and loss of resolution in the image.
• Repositioning the transducer eliminates this artifact.

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Classification of Artifacts

Artifacts Associated with Attenuation Errors:

• Why they occur?
• Based on the premise that:
• Sound attenuates at an even rates in the tissue.
• How they ocuur?
• In reality: sound attenuates unevenly within the tissue( can attenuate differently within each separate structure in the tissue)
• Types of attenuation errors:
• Shadowing (Acoustic and Edge)
• Increased through transmission (Enhancement)

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Acoustic Shadowing

• Aka decreased through transmission.
• Appears as Dark or hypoechoic band deep to a highly attenuating structure , clean shadowing
• US beam encounters a tissue that attenuates the sound to a greater or lesser extent than in the surrounding tissue.
• Strength of the beam distal to this structure will be either weaker or stronger than in the surrounding field.
• Occurs - Calcified lesions, dense tumors
• Are of diagnostic significance and are useful for diagnosis of calcification or calculi
• Prevention - Image structure in different angles

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Acoustic Shadowing

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Acoustic shadows: Seen as dark bands posterior to multiple gallbladder calculi

Edge Shadowing

Dirty Shadowing

• Refraction of the ultrasound beam as it strikes a curved surface and increased attenuation at the edges
• Sometime seen as fine dark lines extending distal to the edges of strongly curved surfaces, such as cyst walls, fetal skull and vessels.
• Fine dark line even more striking in the case of a cyst due to the increased through transmission behind the cyst itself.

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• Increased attenuation at the edges because the ultrasound beam passing through the edge travels a larger distance through the cyst wall than that passing through the diameter of the cyst.
• Commonly occur in situations where shadowing erroneously suggests calcification, e.g. vessel walls in renal sinus or malignancy, Coopers ligaments in the breast.
• It is important to recognize them as artifactual and dismiss them.

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Edge Shadowing

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Refractive and attenuation model to explain mechanism of edge shadowing (A).

Subtle edge shadowing seen along the edges of fibroadenoma breast (B).

Increased Through Transmission or Distal Bright Up

• Acoustic Enhancement, attenuation errors occur if a tissue attenuates less than its surrounding tissues (e.g. a full bladder or a simple cyst).
• Overcorrection of the echoes of the fluid containing structures and distal to it , appearing as a bright band
• Often termed ‘distal enhancement’ but in view of the enhancing effect of ultrasound contrast agents it is better termed increased through transmission or distal bright up
• Appears as Hyperechoic area behind Fluid-containing structures attenuate the sound much less than solid structures.
• Prevention – Reduced with spatial compounding different direction.

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Increased Through Transmission or Distal Bright Up

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Anisotropy

• Transducer’s angle of incidence is not perpendicular to the structure.
• Appearance - Hypoechoic area in a structure
• Occurs -Tendons, and to a lesser extent muscles, ligaments, and nerves

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Summary

• Ultrasound imaging with its inherent flexibility, low cost and real time physiologic measurement capability with no known bioeffects, at intensities used in medical imaging, plays a vital role in diagnostics.
• Sonography examination relies on the skill, knowledge, and accuracy of the sonographer who must pay attention to the texture, outline, size, and shape of both normal and abnormal structures.
• The quality of an ultrasound image depends on its spatial resolution, temporal resolution, contrast resolution and freedom from certain artifacts or errors.

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• Often, these errors in image display are unavoidable and occur secondary to intrinsic physical properties of the ultrasound beam and its echo and to limitations of the display equipment.
• Recognition of these unavoidable artifacts is important because they may be clues to tissue composition and aid in diagnosis.
• Ability to recognize and remedy potentially correctable artifacts is important for image quality improvement and optimal patient care.

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References

• Diagnostic Ultrasound; 4^th Edition; Carol M. Rumack, MD, FACR
• Diagnostic Radiology: Recent Advances and Applied Physics in Imaging; 2^nd Edition Arun Kumar Gupta, Niranjan Khandelwal
• US Artifacts; Myra K. Feldman, MD , Sanjeev Katyal,MD Margaret S. Blackwood, MS
• The Essential Physics of Medical Imaging ,JERROLD T. BUSHBERG, PhD.
• Various Websites

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Thank you

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