Ch 21- Artifacts

What is an artifact?

an error in imaging

Artifacts include reflections that are described as:

1. ____

2. ____

3. ____

4. ____

5. ____

1. not real

2. not seen on the image

3. incorrect shape or size

4. incorrect position

5. incorrect brightness

What are causes of artifacts?

- violation of assumptions

- equipment malfunction or poor design

- physics of ultrasound

- operator error

What terms are used to describe ultrasound images?

- hyperechoic

- hypoechoic

- anechoic

- isoechoic

- homogenous

- heterogenous

There are __ assumptions incorporated into the design of every ultrasound system. What appears when these assumptions are not true?

6; artifacts

What are the 6 assumptions of imaging systems?

1. sound travels in a straight line

2. sound travels directly to a reflector and back

3. sound travels in soft tissue at exactly 1,540 m/s

4. reflections arise only from structures positioned in the beam's main axis

5. the imaging plane is very thin

6. the strength of a reflection is related to the characteristics of the tissue creating the reflection

Reverberation is caused by ___ bouncing between the ___ and a ___

multiple reflections; transducer; strong reflector

Reverberations appear as ___, ___ spaced echoes

multiple; equally

Reverberation produces ___ echoes of ___ brightness

parallel; decreasing

Which assumption causes reverberation?

sound travels directly to a reflector and back

Comet tail is caused by ___ of ___ or ___ hit by a ___

vibration; small gas bubbles; air; sound beam

When is comet tail artifact created?

when closely spaced reverberations merge

What is comet tail also called?

ring down artifact

Comet tail appears as a ___, ___ line directed ___

solid; hyperechoic; downward

What assumption causes comet tail?

sound travels directly to a reflector and back

Refraction occurs when a sound pulse ___ during ___

changes direction; transmission

Refraction produces a ___ copy of the reflector

side by side

Refraction is ___ with a ___

transmission; bend

What does refraction degrade?

lateral resolution

Which assumption causes refraction?

sound travels in a straight line

Shadowing occurs when sound hits a ___ structure and cannot ___, leaving a ___

highly attenuating; penetrate; shadow

Shadows appear when the attenuation is ___ in the tissue above the shadow than in the surrounding tissue

higher

Shadowing appears as a ___ or ___ region extending ___

hypoechoic; anechoic; downward

When can shadows be useful?

when characterizing tissue (Ex. bone, gallstones)

Which assumption causes shadowing?

the strength of a reflection is related to the characteristics of the tissue creating the reflection

Edge shadow is a ___ artifact caused by sound ___ off of a ___ surface

refraction; refracting; curved

With edge shadow, the sound beam refracts at the edge of a curved reflector

The beam simultaneously ___, resulting in a drop in ___

This decrease causes edge shadowing

diverges; intensity

Edge shadow appears as a ___ region extending along the ___ of a ___ reflector

hypoechoic; edges; curved

Which assumption causes edge shadow?

the strength of a reflection is related to the characteristics of the tissue creating the reflection

Enhancement occurs when sound travels through a ___ structure

weakly attenuating

Enhancement appears as a ___ area beneath ___

hyperechoic; weak reflectors

Weak reflectors are tissues with ___ attenuation

low

Enhancement is a ___ artifact

useful

Which assumption causes enhancement?

the strength of a reflection is related to the characteristics of the tissue creating the reflection

Focal enhancement is a ___ enhancement

side to side / horizontal

What is focal enhancement also called?

focal banding

Focal enhancement is caused by a ___ beam

strongly focused

When a beam is strongly focused, the ___ in the focal zone is increased

intensity

Which assumption causes focal enhancement?

the strength of a reflection is related to the characteristics of the tissue creating the reflection

Mirror image is created when sound reflects off of a ___ and is ___ toward a ___

strong reflector; redirected; second structure

Mirror image artifact is located ___ than the real structure

deeper

What is the mirror image artifact of spectral display?

crosstalk

Which assumption causes mirror image?

1. sound travels in a straight line

6. the strength of a reflection is related to the characteristics of the tissue creating the reflection

Multipath is created when sound pulses ___ off a ___ structure on the way to or from the ___ reflector

bounce; second; primary

Multipath is when the beam makes ___ changes in direction

2 or more

With multipath, the ___ path length differs from the ___ path length

transmit; receive

Multipath often results in ___, ___ changes that can not be identified on the image

subtle; nonspecific

Which assumption causes multipath?

2. sound travels directly to a reflector and back

Speed error is created when a sound wave moves through a medium at a speed other than that of ___ (___)

soft tissue (1.54 km/s or 1540 m/s)

Speed error is created when a sound wave moves through a medium at a speed other than that of soft tissue (1.54 km/s or 1540 m/s).

The correct ___ of reflectors are displayed, but appear at incorrect ___

number; depths

Speed errors appear as a ___, as if structures are ___ or ___

step-off; split; cut

What is another name for speed error?

range error artifact

Which assumption causes speed error?

3. sound travels in soft tissue at exactly 1540 m/s

Lobe artifact appears when sound energy is ___ in a direction other than along the beam's ___

transmitted; main axis

What does lobe artifact degrade?

lateral resolution

Lobes are created when a ___ is in the path of the ___

strong reflector; off-axis lobe

The lobe artifact and the true reflector are located ___

side by side

Which assumption causes lobes?

4. reflections arise only from structures positioned in the beam's main axis

What type of transducer causes side lobes?

mechanical

What type of transducer causes grating lobes?

array

What methods reduce lobes?

subdicing

tissue harmonics

apodization

What is subdicing?

dividing each PZT element into small pieces

What is apodization?

the process of reducing the strength of side and grating lobes- it alters the electrical spike voltages

With slice thickness, reflections from structures ___ or ___ the ___ may appear in the image

above; below; imaging plane

Slice thickness is the result of the beam not being ___

razor thin

With slice thickness, ___ appear in the image as the beam slices through structures ___ to intended reflectors

unintended echoes; adjacent

With slice thickness, cystic structures appear ___

filled in

How is slice thickness artifact reduced?

thinner imaging planes, such as those created by 1 1/2-dimensional array transducers

Which assumption causes slice thickness artifact?

5. the imaging plane is very thin

Lateral resolution artifact occurs when a pair of ___ reflectors are closer than the ___ of the sound beam

side by side; width

Lateral resolution artifact occurs when reflectors are ___ to the beam

perpendicular

With lateral resolution, 2 objects appear as one ___ reflection on the image

horizontal

Axial resolution artifact is created when a long pulse strikes two ___ structures that are ___ of each other or ___ to the beam

closely spaced; in front; parallel

With axial resolution, 2 objects appear as one ___ reflection on the image

vertical

Curved and oblique reflectors are caused by a sound beam striking a ___

curved reflector

With curved and oblique reflectors, a portion of the ___ sound is directed ___ from the transducer

reflected; away

With curved and oblique reflectors, the received sound may be ___ intense than expected

less

What causes temporal resolution artifact?

slow frame rates

Poor temporal resolution results in less accurate ___ of ___

positioning; moving reflectors

What causes spatial resolution artifact?

low line density

Less reflector detail causes ___ spatial resolution

inferior

How do you correct spatial resolution?

use more line density or more pixels

How does noise appear?

as small amplitude echoes

What is noise caused by?

electrical interference

signal processing

spurious reflections

What is speckle caused by?

small amplitude waves interfering with each other

Speckle appears as tissue texture that does not directly correspond to ___

true anatomic scatterers

Speckle causes a ___ appearance of ultrasound images

grainy

What reduces speckle?

harmonic imaging

Clutter is a form of noise in ___

spectral doppler

Ghosting is a form of noise in ___

color doppler

Electronic interference is a ___ on display that appears as ___ moving bands

disturbance; arc-like

What is electronic interference caused by?

ultrasound machine being too close to unshielded electrical equipment