[Ultrasound Physics] ARTIFACTS

[Contents]

1. Artifacts

2. Resolution Artifacts

3. Propagating Speed/Path Artifacts

4. Attenuation Artifacts

5. Doppler Artifacts

1. Artifacts

1) Any unintended information on an image that does not represent the object

2) Artifacts can be a hindrance or in some cases may be diagnostically helpful

3) Basic Assumptions

- There are several assumptions made by an ultrasound machine

- Artifacts occur when these assumptions are violated

1. Sound travels in a straight line

2. Reflections are produced by structures along the beams main axis

3. Sound travels at exactly 1540 m/s

4. Intensity of a reflection directly corresponds to a reflector’s scattering strength

5. The imaging plane is very thin

6. Sound beams travel directly to a reflector and back to source

2. Resolution Artifacts

1) Axial resolution artifact [★]

- Closely spaced targets of varying distances

 

 

- Axial Resolution

: Ability to differentiate between two objects along the long axis of the ultrasound beam

= SPL/2

- Axial Resolution Artifacts appear when all 3 conditions occur:

1. Two or more reflectors are closer together than SPL/2

2. Only one reflector will appear on the image

3. Reflectors are parallel to the beam axis

- Hindrance

: Produce fewer reflectors on the image

: Actual anatomic data is missing

- Prevention

: by using higher frequency transducers with short distinct pulses

 

2) Lateral Resolution Artifact [★★]

- Measuring lateral width of a target on an ultrasound phantom

 

- Lateral Resolution

: Ability to differentiate between two objects that lie perpendicular to the ultrasound   beam

= beam width

- Lateral Resolution Artifacts appear when all 3 conditions occur:

1. Two or more reflectors are closer together than the width of the beam

2. Only one reflector will appear on the image

3. Reflectors are perpendicular to the beam axis

- Hindrance

: Produce fewer reflectors on the image

: Actual anatomic data is missing

- Prevention

: by using narrower beams

 

3) Elevational Resolution Artifact (= Slice-Thickness A., partial volume artifact) [★★★★★]

-

 

 

 

- Due to beam width perpendicular to the scan plane.

- Affect imaging quality by displaying anatomic structures (reflectors) in the incorrect imaging plane

- These reflections can cause hollow structures to fill in

- Hindrance

: Causes anechoic structures to have low level echoes or false debris

- Example: false appearance of debris in simple cystic structure

- Prevention

1. Turn on Harmonics

: sound beam in this mode is narrower than in regular gray scale mode

2. Disc Shaped Elements

: Provide the thinnest slices and the best elevational resolution.

3. Newer Transducers

- 1.5-D arrays (multirow array transducer) [★★]

· They create thinnest beams with improved slice-thickness

· Have multiple crystals in an up and down direction

→ focus the beam in the thickness plane

· exhibit least amount of volume averaging

 

4) Contrast Resolution Artifact

- Inability of a gray-scale display to distinguish between echoes of slightly different intensities

: How many Shades of gray can be displayed

- Factoring Components

: With a decreasing number of bits per pixel, less shades of gray appear

- Less shades of gray = worse contrast resolution

- Cause: Not enough bits per pixel in image memory

- Poor contrast resolution

: Image appears more black-and-white, with few shades of gray in between

: Showing less detail

- Hindrance                                                                                                                              

: Fewer bits per pixel = fewer shades of gray = degraded contrast resolution

- Prevention: Usage of B Color

 

5) Spatial Resolution Artifacts

- Spatial resolution pertains to the overall detail produced in an image

- Spatial resolution artifacts are created when display monitor fail to produce adequate image detail

- Factoring Components: Spatial resolution artifacts can be created in a number of ways

1. Pixel Density

- Low pixel density (fewer pixels per inch) → larger pixels

: Larger pixels provide blurry and less detailed images

                              - Higher pixel density (more pixels per inch) → smaller pixels

                                             : Smaller pixels provide better detail, better spatial resolution

                              - pixel density cannot be changed

                              - quality may be improved by write magnification (pre-processing technique)

                                             : sonographer chooses a region of interest (ROI) to magnify

                                             : US rescans the image → greater number of pixels, improves SR

2. Line Density

- Low line density creates less detailed images

- controlled by US system, but can be controlled by the operator

- Modern display are equipped with more lines per page providing better SR

3. Number of horizontal lines in a display monitor. 

- Monitors with less horizontal lines degrade spatial resolution

3. Propagating Speed/Path Artifacts

1) Refraction Artifact (= Lateral misregistration) [★★★★]

 

- bending of sound beam due to different media propagation speeds

- ex) while imaging a cyst: shadowing posterior to each lateral border of cyst

- Causes of Refraction

1. Oblique incidence

2. Difference in propagating speed on either side of the boundary

Snell’s law: sinθt/sinθi = Vt/Vi

- angle of sound transmission at an interface between media with different P.S. 

- Propagating Speed (c) through the 2nd medium > 1st medium

               → Transmission angle > Incident angle

- Propagating Speed (c) through the 2nd medium < 1st medium

→ Transmission angle < Incident angle 

 

- Hindrance

: Duplication of a reflector / shows a false structure

: Reflector misplacement

- Prevention

: Change angle, artifact shouldn’t be in same place

 

2) Multipath Artifacts [★]

 

 

 

- Artifact created when the pulse is redirected along different paths before returning to the transducer

- The transmit and returning path are not the same

- Causes: Scattering

: Random redirection of sound in many directions

: Occurs with rough surfaces

: Occurs when tissue interface is small compared to the beam

: Boundary is less or equal to the wavelength of incident beam

: independent of the direction of the incident sound 

- Hindrance

: Subtle, nonspecific changes that cannot be identified on an image

: may cause abnormalities of depth or position of a structure

: result in both axial and lateral displacement of reflector

 

3) Mirror Image Artifact

- a second copy of a true object incorrectly appears on the opposite side of a strong reflector

- Observed in all imaging mode [★]

- Characteristics

: Replica of true reflector

: Artifact will appear deeper than true reflector

: “Mirror” is in straight line between artifact and true reflector

: Artifact and true reflector are equidistant from mirror

- Cause

: sound reflects off of a strong reflector and is redirected towards another structure.

Creating a mirror image of the structure on the opposite side of the strong reflector                                      

- 

 

: Ultrasound will show true object in correct position (arrow showing mass on liver)

: also show a mirror image of the object (hyperechoic duplicate mass below diaphragm in image)

: Artifact will be deeper and behind the strong reflector (curved line-diaphragm)

- same path as true object

: Artifact will be same distance from strong reflector as true object 

- Hindrance

: could mistake mirror image for second object

: unable to correctly view area behind high reflector

- Prevention 

: Change angle of incidence to vary reflectivity of interface

: Adjust focal zone or TGC at level of high reflector causing mirroring to minimize reflectivity

: Scan from multiple windows

: Use spatial compounding

(combining image information from different angles to produce a single image)

 

4) Comet Tail/Ringdown Artifact [★★]

 

 

 

- Due to merging of two closely spaced reverberations

- Similar to a reverberation without the spacing

- Associated with resonance of a gas bubble

- Causes

: Two closely placed strong reflectors parallel to beam axis

: Sound wave bounces between the two reflectors eventually returning to the Transducer

: Most common in mediums with very high propagating speeds

- Solid hyperechoic line directed inferiorly

: Unable to differentiate between individual reverberations

: Appears posterior to actual structure

: Parallel to beams axis

- Hindrance

: False reflectors are displayed

: May obscure visualization of structures posterior to reflectors

- Prevention

: Use an alternative window

- Change beam angle

: Decrease TGC in the near gain

 

5) Reverberation Artifact [★★]

-

 

- Multiple equidistant horizontal bands having decreased brightness with depth

: Only the first two are real

- Resulting in a single structure being displayed repeatedly at greater depths

- Causes

: Two strong reflectors parallel to wave axis

: Sound waves bounce between the two reflectors eventually returning to the Transducer

- Creating a longer go return-time

- Causing incorrect reflector placement on display

- Repeated hyperechoic reflections

: Equal increments of space between artifacts

: “Ladder” or “Venetian blind” appearance

- Hindrance

: False reflectors are displayed

: May obscure visualization of structures posterior to reflectors

- Prevention

: Use an alternative window

- Change beam angle

: Decrease TGC in the near gain.

 

* Water-path scanner

- Advantage of water-path scanner

: near-field reverberations are reduced

- Disadvantage of water-path scanner

               : bubbles in the fluid can inhibit sound transmission into the body

 

6) Propagating Speed Error Artifact/Range Error (= Axial misregistration) [★★]

- Created when sound propagates through medium at a rate other than 1540m/s

- Displays the correct number of reflectors at incorrect depths

: Causing misplaced echoes on image

- Causes

: Error in tissue velocity/velocity calibration of system

: Sound traveling at a speed other than 1540 m/s

- Slower than 1540 m/s (ex: large mass composed primarily of fat)

: Longer go return time than machine expects

: Pulses return slowly

: System places reflections at a greater depth (overestimates distance)

- Faster than 1540 m/s

: Shorter go return time than machine expects

: Pulses return very quickly

: System places reflectors at a shallower depth (underestimates distance)

- Displaces true reflections

- Helpful

: Conveys important information on the image

- Can provide tissue texture

- Hindrance

: Inaccurate placement may appear like pathology

- Need to look at in other views to confirm anatomy

- Prevention

: Currently cannot be prevented

: Use alternate viewing window

- Change beam angle

 

7) Focal Banding/Focal Enhancement Artifact

-

 

- Special form of enhancement

: side to side region of an image appears brighter (hyperechoic) than tissues at other depths

- Occurs in the focal region of the transducer when using multiple focal zones

- Cause

: increased intensity due to multiple foci

- Structures at the focus appear brighter than those at other depths

: An entire horizontal region (band) of tissue appears hyperechoic

- Results from increased intensity at the focus

               : same appearance as incorrect TGC setting

- Hindrance

: The brightening of echoes around the focus (intensity increased by narrowing of the beam)

: Higher intensity causes a hyperechoic horizontal band across the display

→ can be mistaken for a mass

- Prevention

: Decrease the number of foci

: Change the location of multiple foci

4. Attenuation Artifacts

1) Shadowing Artifacts [★]

-

 

- The weakening of echoes distal to a strongly attenuating or reflecting structure [★★]

Or from the edges of a refracting structure

- Causes: result of too much attenuation

: A strongly attenuating or reflecting structure weakens the sound distal to it (attenuation)

→ echoes from the distal region are weak and appear less echogenic (like a shadow)

- Hypoechoic/Anechoic area parallel to sound beam

- Hindrance

: May hide or prevent visualization of a deeper structure

: difficult to obtain information about objects in the far field or within the shadow artifact

: Prevents visualization of true anatomy on the scan, resulting in missed information

- Helpful

: May provide valuable diagnostic information

- helps to characterize tissue

- ex) calcified plaques, stiff breast lesions, and stones

- Prevention

: Image structure in several angles to avoid missing information

: use tissue harmonic imaging (which produces thinner beam reducing slice thickness) [#1]

- Shadowing may not be displayed if beam width is greater than calcification

: due to volume averaging  

: To display acoustic shadow

→ beam width ↓ (frequency ↑ and/or improve focusing)

 

2) Enhancement Artifacts [★★★]

-

- The strengthening of echoes from reflections that lie behind a weakly attenuating structure

- A hyperechoic region that extends beneath structures with abnormally low attenuation

- Opposite of shadowing

- Causes

               : Decreased attenuation through a fluid-filled structure

: Tissues with low attenuation (ex: hematomas and abscesses)

- Helpful

: Provide valuable diagnostic information helping to characterize tissue

- Hinder

: Cause blockage and prevent from seeing something important

- Prevention

: Reduced with spatial compounding  

: Several directional approaches allow the beam to get around the attenuating structure

5. Doppler Artifacts

1)  Aliasing [★★]

- High velocities appear negative

- With PW doppler, high velocity measurements are inaccurate

: if the pulsed doppler sampling rate (PRF) is too low in comparison to messed doppler shift

 

- Appearance of Doppler spectral information on the wrong side of the baseline

- Most common Doppler artifact

: Only occurs with PW

- Very high velocities in one direction are incorrectly displayed as going the opposite direction

- Causes

               : occurs because frequency-shifted signal is sampled

                              (rather than recorded continuously)

               : inadequately sampled shift results in aliasing

: when the Nyquist limit is surpassed

- Nyquist limit (kHz) = PRF/2 [★★]

: sampling frequency needed for detecting the doppler signal unambiguously

: when PRF cannot be increased to a level greater than 2 times doppler frequency

- Nyquist frequency: The doppler frequency at which aliasing occurs

- Peaks will be displayed on the wrong side of the baseline

- Hindrance

: Yields an incorrect direction and value

: Limited ability to correctly measure deep vessels

 - Prevention

: Increase the PRF (and Nyquist frequency), which will lower the baseline

               - can measure high velocities

                              - increased PRF may introduce range ambiguity

: zero baseline shift

: Increase the Doppler angle

: Use a lower operating frequency, which lowers the Doppler shift and shrink the spectrum

: Use a continuous wave device

                              - Aliasing does not occur in CW doppler [★★]

                                             : no limit in maximum velocity

 

2) Ghosting

 

 

- Form of noise

- The presence of false echo signals coming from outside the main beam

- Causes

: Low frequency Doppler shifts created by slowly moving anatomy

: Slow velocity reflectors

: Need to differentiate between anatomy and moving blood cells

- ex: heart muscle, pulsating vessel walls

- Hindrance

: Gives operator false diagnosis or measurement information

: Could interpret as slow-moving blood cells

- Prevention [★★★]

: Use a wall filter to eliminate low frequency Doppler shifts

: Increase PRF – range ambiguity, less sensitivity to slow flow, reduction of color fill

 

* Slow flow [★★]

No color signal detected within portal vein

- color doppler system must be sensitized to detect low frequency shifts

: slow flow produces low frequency shifts

- set PRF lower to increase sensitivity to low frequency shifts

- increasing color doppler transmit frequency

: result in larger frequency shifts from slow flow

                              - improve visibility and sensitivity to slow flow

: decreased penetration to flow in vessels deep within tissues

- Lower the wall filter setting

 

3) Cross talk (spectral mirroring) [★]

-

 

- Identical Doppler spectrum is shown above and below the baseline

- True flow pattern is unidirectional, but flow pattern appears bidirectional on spectrum

               : Spectral Doppler appears on both top and bottom

- Causes

: Equipment malfunction (poorly designed)

: Doppler gain is set too high

: Incident angle is near 90 degrees (between sound beam and flow direction)

- Doppler shifts cannot be obtained perpendicular to the beam

: Operator error

- Hindrance

: Spectral analysis is giving a false information

- Prevention

: Change angle of the transducer

: Apply gel (coupling medium)

: Fix Doppler gain

 

4) Twinkle artifact

 

 

 

- Phenomenon with unclear underlying causes that appears as a rapid alternation of color

- Immediately behind a stationary echogenic object, giving it a false appearance of movement

- Hindrance

: False appearance of movement

- Helpful

: Useful in detection of certain clinical conditions

- Ex: Urolithiasis (twinkle sign → positive reading that a stone is present)

: Can enhance accuracy and sensitivity

- Prevention

: Not having very strong scatters

: Eliminating the phase detection process

: Unfortunately, the operator has no control over these things

 

5) Flash artifact

 

 

 

- high-amplitude, low-frequency shift signal

- reduced by increasing wall filter

               : reduces sensitivity to low frequency shifts ass/w slow flow

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler