Term
| Define Artifact and give some examples on what is considered artifact. |
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Definition
- Artifact: Any error in imaging
- It includes:
> An image of a structure that isn't supposed to be there
> Missing a structure that is supposed to be there
> Incorrect size and/or shape of a structure
> Improper location of a structure
> Improper brightness of a structure
> Etc. |
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Term
| What are the causes of artifacts? |
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Definition
- Violation of assumption incorporated in the design of the US machine
- Physics of Ultrasound
- Operator Error
- Equipment malfunction or poor design |
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Term
| What are basic assumptions in imaging? |
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Definition
- The speed of sound in tissues is 1540 m/s.
- The image plane is very thin.
- Attenuation of sound is very uniform.
- Sound travels in a straight line.
- Sound travels to the reflector and back.
- All echoes are received by the reflector before the next pulse is sent out. |
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Term
| Give examples of RESOLUTION artifacts |
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Definition
- Axial resolution artifacts
- Lateral resolution artifact
- Speckle
- Slice thickness
- Temporal resolution
- Contrast resoluton |
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Term
| Give examples of ATTENUATION artifacts |
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Definition
- Acoustic shadowing
- Acoustic Enhancement |
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Term
| Give examples of PROPAGATION artifacts |
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Definition
- Reverberation
- "Comet tail"
- Ring down
- Mirror image
- Refraction
- Multipath
- Side Lobe/Gratin Lobe |
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Term
| Give examples of MISCELLANEOUS artifacts |
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Definition
- Range Ambiguity
- Speed error |
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Term
| Explain how artifact is created in an axial resolution artifact |
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Definition
| - When the distance between two objects in the direction of the beam is less than 1/2 the SPL the objects appear as one. |
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Term
| Explain how Lateral resolution artifact is created |
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Definition
- Any two objects spaced LESS than the width of the beam perpendicular to the beam will be shown as one.
> To improve resolution at the DEPTH OF INTEREST
> Dnf = Wcr^2/4wl = Wcr^2 * f / 4v
> (Does NOT improve resolution at the focal point)
> To improve resolution at the FOCAL POINT, decrease the SIZE of the crystal(s)
> Width of the beam at the focal point = 1/2 Width of the crystal |
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Term
| How is a speckle artifact created? |
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Definition
| Created when the reflected waves of reflectors interfere with one another. |
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Term
| What does speckle artifact give us? |
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Definition
| - The internal texture of organs. |
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Term
| How is speckle artifact reduced? |
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Definition
| With the smoothing function. |
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Term
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Definition
- AKA SECTION THICKNESS or PARTIAL VOLUME
- It is assumed that the US imaging plane is ultra thin, but in reality the beam has a small but definite THICKNESS.
- Therefore, if objects lie even just slightly anterior or posterior to the US beam they may be picked up and displayed in the same plane. |
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Term
| Explain Inadequate contrast resolution |
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Definition
- Due to:
- Inadequate computer memory capability (low bits/pixel ratio)
- Inadequate processing of the signal (gain, compression, TGC's, etc) |
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Term
| Explain Inadequate temporal resolution |
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Definition
- If the frame rate is too low, fast moving objects such as heart valve leaflets will appear sluggish and fine details of the motion will be missed.
FR = PRF / l = 1540 m/sec / 2dl |
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Term
| Explain Acoustic Shadowing |
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Definition
| The low signal is caused by the reflection and absorption off a strong reflector (such as bones or calcified plaque). The artifact appears as a shadowing streak underneath the big attenuating object. |
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Term
| Explain Acoustic Enhancement |
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Definition
| - As sound wave travels through weak reflectors (such as a cyst, or urinary bladder) the waves continue on and could eventually hit a strong reflector, and since the strong reflector is hit with very little attenuated strong sound waves it reflects strong waves back to the transducer which makes it's appear very bright on the screen. |
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Term
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Definition
- Ultrasound waves between two closely spaced interfaces are reflected back and forth from one another (ping-pong effect) so the transducer is receiving multiple echoes from the same two interfaces which then causes it to display copies of the original objects at depths below them. These copies are the reverberation artifacts.
- Can be caused by a transducer - interface or between two other interfaces. |
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Term
| Explain Comet Tail Artifact |
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Definition
- A reverberation artifact that occurs due to reverberation WITHIN OR BETWEEN A SMALL VERY REFLECTIVE OBJECT.
- Ex. Would be a cholesterol crystal in the gall bladder. |
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Term
| Explain Ring-Down Artifact |
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Definition
- Reverberation artifact due to OCCURRING IN A GAS OR LIQUID BUBBLE.
- Very low Z
- Similar to Comet tail artifact in appearance |
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Term
| Explain Refraction Artifact |
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Definition
- As the US propagates through tissue it can hit an interface that causes the US to be refracted, the refracted US beam hits a reflector, and the echoes off the reflector will make it back to the transducer.
- Because the machine assumes that sound waves travel in a straight line it displays the object in a straight line, even though the true reflector is off to the side some where. |
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Term
| Explain Refraction shadowing |
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Definition
- Increase or decrease in intensity due to refraction off a curved refracting surface.
- "Catfish whiskers"
- I.E. off a vessel in short axis |
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Term
| Explain Mirror Image Artifact |
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Definition
- Sound is redirected by a strong reflector (such as diaphragm or pleural membrane) to another reflector.
- The 2nd reflector reflects echoes back along the path the sound wave hit it.
- It hits the strong reflector (1st reflector) and goes back to the probe.
- The display shows both the object and the mirror object.
- probe ->1st reflector -> 2nd reflector -> 1st reflector -> probe
- The difference between this and refraction artifact is that the true object is in a different interface with this type of artifact. |
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Term
| Explain Multipath artifact |
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Definition
- Sound is redirected after hitting a strong reflector and then hits a second reflector where the sound then goes straight back to the probe. It doesn't follow the path it came from back to the probe.
- The artifact is then displayed deeper than the depth of the original reflector.
- Only the deep artifact is displayed, not the actual reflector.
- Follows path: probe -> 1st reflector -> 2nd reflector -> probe |
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Term
| Explain Side lobe and Grating lobe's |
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Definition
- Actual shape of the US beam is very complex.
- Although most of the energy in acoustic pulses are sent straight out from the probe, some is sent off from the sides.
- Depending on the type of transducer, energy emitted from the sides of the probe will either be a side lobe or grating lobe. |
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Term
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Definition
- Produced by SINGLE CRYSTAL TRANSDUCER
- Due to the radial vibration of the crystal as opposed to vibration in the thickness direction causing the main beam.
- To reduce side lobe emission a crystal is designed to have a diameter at least 10 times greater than the length of the US wave. |
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Term
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Definition
| - The MULTIPLE SMALL CRYSTAL elements of ELECTRONIC ARRAY TRANSDUCERS may have an effect of a grating lobe. |
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Term
| Explain side lob or grating lobe artifact |
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Definition
- Intensity if the side/grating lobes is low however if there is a highly reflective object on the course of the side/grate lobe sound waves path it can pick up the waves and reflect an echo back to the probe.
- But because the machine assumes sound travels in a straight lines it displays the object (artifact) along the axis of the main beam at the depth of the true reflector (known as "side lobe anatomy". |
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Term
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Definition
- A TECHNIQUE USED TO OVERCOME GRATING LOBES.
- Elements in the transducer are divided into smaller parts, each one being 1/2 the wavelength.
- By each one being 1/2 the wavelength destructive interference then gets ride of the gating lobe. |
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Term
| Explain Range Ambiguity Artifact |
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Definition
- If the PRF is set too high, than the next pulse is sent out before the echoes from the deepest structures have returned.
- The pulse is sent out and then shortly afterwards the echoes from the deepest structures returned so the image is mismapped to a shallow position on the image. |
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Term
| Explain Speed Error artifact |
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Definition
- The assumed speed of sound through tissue is 1540 m/sec. If the actual speed through a tissue is slower than that than it'll take the sound wave longer to get through it and due to RTTT it'll take longer so will be assumed deeper and overestimated in size (such as fat = 1459 m/sec).
- If the actual speed is faster (muscle = 1580 m/sec) than the sound will go quicker and the returning echo will get to the transducer faster and due to RTTT it'll be displayed closer to the transducer and underestimated in size. |
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Term
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Definition
- Occurs due to an insufficient sampling rate and occurs when the doppler shift is greater than half of the pulse repetition frequency (PRF) (Nyquist frequency).
- In color mode it displays as a reversal of color
- In spectral it displays as a wrapping around of the spectral wave, where high velocities are cut off and displayed below the base line. |
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Term
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Definition
- Occurs when echoes are returned from the ambiguous gate (2d,3d,4d, etc.) together with the echoes from the true gate.
- May have different waveform appearances. |
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Term
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Definition
- When the angle of insonation is close to 90 degrees:
> part of the beam that is slightly less than 90 degrees will demonstrate flow towards
> part of the beam that is slightly more than 90 degrees will show flow away
- The result is flow being shown on both sides of the baseline
- A similar artifact can be created when signals are over-gained (electronic noise). |
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Term
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Definition
| - In addition to normal spectral broadening that occurs when a range of velocities are picked up in a vessel, artificial spectral broadening may result due to the SHAPE of the beam. |
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