UT 200 - Artifacts

causes of artifacts

• physics of ultrasound

• operator errors

• improper equipment operation or settings

• violation of imaging assumptions

• equipment malfunction

imaging assumptions

• sound travels in a straight line

• sound travels directly to the reflector and back

• sound travels in soft tissue at exactly 1540 m/s

• reflections arise only from structures positioned in the beam’s main axis

• imaging plane is very thin

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

axial resolution artifact occurs when

long pulse strikes two closely spaced structures, where one is in front of the other, and the two structures combine into one form

for axial resolution artifacts, the images are closer than what size?

½ SPL

different names for axial

longitudinal, axial, range, depth

is axial resolution constant with depth?

yes

how to minimize axial resolution artifacts

transducer design: shorter pulses = higher frequency transducer and increased damping material

which image has a greater axial resolution and why?

the image on the right; it uses a higher frequency probe which results in greater axial resolution

lateral resolution artifacts occur when

a pair of side-by-side reflectors are closer than the width of the sound beam, thus appearing as one

definition of lateral resolution

ability to resolve two structures positioned perpendicular to the sound beam

other names for lateral resolution

longitudinal, azuthimal, transverse, angular

where is lateral resolution artifacts least likely to occur

at the focus of the sound beam where the beam is smallest

the third dimension is dependent on

slice thickness or elevation

reflectors in the 3rd dimension appear as

sludge or debris

slice thickness artifact is due to

transducer design

how to improve slice thickness artifacts

making the imaging planes thinner through focusing with acoustic material (mechanical) or harmonic imaging

what type of artifact?

slice thickness artifact

acoustic speckle looks

bright grainy speckled appearance; like a pseudo tissue texture

acoustic speckle results from

constructive and deconstructive interference of small wavelets

what depths are degraded by acoustic speckle

shallow depths; low contrast structures near the transducer are more difficult to image

what can be used to reduce acoustic speckle

standoff pads

on a machine, what can operators turn on reduce acoustic speckle

speckle reduction

what type of artifact is this and what did we use to fix it on the image to the right

acoustic speckle; turned on speckle reduction on the machine

reverberation is caused by

bouncing of sound waves between two strong reflectors positioned parallel to the ultrasound beam

if there’s reverberation artifact, how many reflections will you see?

multiple

in reverberation artifacts, how are the reflections arranged

in a ladder; each reflection is located at ever-increasing depths

what are the two types of reverberation artifacts?

1. comet tail

2. ring down

what type of artifact is this?

reverberation artifact

what is the difference in reflection location in slice thickness artifacts and reverberation artifacts?

slice thickness artifacts occur at the posteriorly

reverberation artifacts occur anteriorly

what type of artifact is this?

ring-down artifact

where do ring-down artifacts occur?

in air sacs, caused by small air bubbles

what type of artifact is this?

comet tail artifact

what are comet tail artifacts caused by?

mechanical objects within the body

how do we rectify ring-down artifacts?

changing our scanning angle

refraction artifacts occur when

the ultrasound beam changes direction (bends) during transmission after striking a boundary obliquely with 2 different propagation speeds

where is the duplicated structure seen on an image for refraction artifacts?

side by side at the same depth as the real structure (lateral displacement)

how to rectify refraction artifacts?

changing the scanning angle

what type of artifact is this?

refraction artifact

would you be able to distinguish the refraction artifact from the real structure using Doppler?

NO. both structures will fill with color and spectral dopplers signal within both

multipath artifact occurs when

sound pulses glance off a second structure on the way to and from the primary reflector

where is the artifact reflector placed in a multipath artifact?

deeper than its correct position (axial displacement)

how to rectify multipath artifacts?

changing the scanning angle

what type of artifact is this?

multipath artifact

mirror image artifact occurs when

sound reflects off a strong reflector (mirror) and is redirected towards a second structure

what kind of structure does a mirror image artifact create?

a weaker duplicate structure deeper on the opposite side of the reflector

how are mirror artifacts rectified?

changing the scanning angle

what type of artifact is this?

mirror image artifact

what types of transducers create side lobes and grating lobes?

side lobes - single crystal transducer

grating lobes - arrayed (multicrystal) transducers

where are the fake reflections placed in lobe artifacts?

placed at the correct depth but incorrect lateral position

what type of artifact is this?

side lobe artifact

are grating lobes seen much these days? why or why not?

• no, they’re not common

• subdicing (splitting the crystals) and apodization (exciting center elements with higher voltages than outer elements)

range ambiguity occurs when

echoes from one pulse return after another pulse has been transmitted

how do we rectify range ambiguity artifacts?

increasing the depth or lowering PRF

what do range ambiguity artifacts look like?

a grayish haze near the transducer

what kind of artifact is this?

range ambiguity artifact

what happens when sound waves travel through mediums with a propagation speed that ISNT 1540 m/s

systems displays the structures at incorrect depths

what type of artifact appears as “split” or “cut?”

propagation speed error artifacts

propagation speed errors are also known as

range error artifacts

if propagation speed >1540 m/s, echo returns —— and assumes the structure is —— than where it really is

faster; shallower

if propagation speed is <1540 m/s, echo returns —— and assumes the structure is —— than where it really is

later; deeper

what type of artifact is this?

propagation speed error artifact

acoustic shadowing occurs when

the attenuation is higher in the tissue above the shadow than in the surrounding tissue

for acoustic shadowing, the sound beam must hit a —— attenuating/reflecting structure, such as:

• highly

• bone, calcified plaque, metal objects, etc.

T or F: for acoustic shadowing, sound beams are mostly reflected back to the transducer, with little transmission

true

clean shadowing is characterized by:

• posterior to calcification or bone

• high levels of absorption/reflection

• NO transmission

dirty shadowing is characterized by:

• posterior to air-filled structures

• high percentage of reflection

• small percentage of transmission

what type of artifact is this?

clean acoustic shadowing

what types of artifacts can be diagnostic?

• acoustic shadowing

• acoustic enhancement

what type of artifact is this?

clean acoustic shadowing

what type of artifact is this?

clean acoustic shadowing

what type of artifact is this?

dirty acoustic shadowing

acoustic enhancement appears as

a hyperechoic region beneath tissues with low/weakly attenuating structure; abnormally increased brightness posteriorly

opposite of shadowing

enhancement

what type of artifact helps diagnose cystic from solid masses?

acoustic enhancement

what type of artifact is this?

acoustic enhancement

what type of artifact is this?

acoustic enhancement

another name for focal enhancement

focal banding

what does focal enhancement look like?

side to side region of an image that appears brighter than tissues of other depths

how to rectify focal enhacement?

altering the focal zone or increasing the # of focal zones

what type of artifact is this?

focal enhancement/banding

edge shadowing is also known as

refractive edge shadowing

edge shadowing occurs when

ultrasound beam refracts (bends) at the edge of a curved reflector

what does edge shadowing appear as?

hypoechoic region extending down from the edge of a curved reflector

what type of artifact is this?

edge shadowing

what type of artifact is this?

edge shadowing

relationship between coarseness of speckle pattern and transducer frequency

inverse

• coarseness decreases as frequency increases

instrument/electronic noise is caused by

electrical interference from nearby equipment

what do instrument/electronic noise appear as?

arcs of vibrating bands across a monitor

is instrument/electronic noise present when an image is frozen?

NO; it’s only present when image is in motion

what is this type of artifact?

electronic noise

what is this type of artifact?

electronic noise

what happens when there’s inadequate TGC?

horizontal bands of varying brightness on the image

what is this type of artifact?

inadequate TGC artifact

what will we see if there’s dead crystals on the transducer?

vertical dark bands appearing on the image

what type of artifact is this?

dead crystals artifact