US systems operate on a basis of certain false assumptions relating to the interaction of the sound beam with soft tissue, including:
a constant speed of sound in the body of 1540m/s
all echoes detected by the transducer originate from the central axis of the beam
the ultrasound beam travels in straight lines
the time taken for an echo from a given interface to return to the transducer is directly related to its distance from the transducer
the rate of attenuation of the beam is constant with depth & throughout the field of view
as a result, artefacts may be produced such as:
echoes in the display which do not appear in the correct position
absence of echoes from parts of the image that would normally generate echoes
structures which are anatomically separate being displayed as joined
brightness of echoes which may not correspond to that expected
single structure being displayed as two separate structures
thin membranes appearing thicker than they really are
Acoustic shadowing:
if the proportion of the beam energy attenuated at a given interface is high, little sound will be transmitted deeper than the interface, thus echoes will not be received from tissue deep to the interface resulting in acoustic shadowing
examples:
soft tissue/gas
shadows from gas are partially filled with reverberations (“dirty”)
soft tissue/bone or calcium
shadows from calcium & bone are usually echo-free (“clean”)
focal zone must be set at the depth of calculus to clearly show a shadow
normal tissue/fibrous tissue (scars, ligts)
Acoustic enhancement:
an attenuation phenomenon resulting in an area of increased brightness relative to echoes from adjacent tissue
this is due to sound passing through a medium of lower sound attenuation than the adjacent tissue resulting in the sound reaching the tissues deep to it having greater amplitude than the sound reaching the adjacent tissues as the same depth
examples:
full bladder - allows better scanning of deeper structures
it is one of the criteria in differentiating b/n a cyst and an echoic tumour (the other criteria are smooth walls & an echo-free interior)
the margins of the area of enhancement are usually sharply defined & may be parallel or divergent depending on the transducer used
Edge shadowing:
sound is attenuated deep to the lateral edges of rounded structures such as aorta
the sound has further to travel within that medium resulting absorption as well as reflection & refraction effects in producing the narrow band of distal edge shadow
Reverberation artefact:
two main types:
multiple linear representations on the display of the same interface equal distances apart as a result of sound echoing backwards & forwards between interfaces with each subsequent echo appearing to be deeper as it has taken longer to get back to the transducer & also less bright as it sequentially loses amplitude
usually occur due to high-level mismatch interfaces when the echo amplitude is high
examples:
deep to subcutaneous fat & muscle layers eg. bladder wall
foreign bodies - the reverberations often merge to form a 'comet tail' artefact
diffuse reverberations deep to a medium that resonates such as gas or to a lesser extent, fat
examples:
deep to bowel gas ⇒ “ring-down” effect
fat ⇒ diffuse reverberations often seen in fundus of GB or superficial regions of anechoic structures such as blood vessels, bladder
can usually reduce this by turning the gain down
Beam width effects:
a group of effects that can cause great difficulty as they display echoes generated from the edge of the beam as though they were generated from the central axis of the beam, resulting in incorrectly placed echoes and “pseudo-pathology”
includes:
beam width
side lobe
grating lobe
slice thickness
examples:
urinary bladder - “false” echoes within the bladder arising from surrounding bowel gas
gall bladder - “pseudo sludge” echoes within the GB lumen arising from adjacent bowel or liver
obstetrics - “false” echoes at the side of the fetal head or femur length which can result in inaccurate measurements
Velocity artefacts:
results from different tissues having different velocities of sound causing incorrect placement of the echoes in the display due to:
inaccurate depth calculation effects
refraction effects
examples:
low velocity lesion in the liver causes displacement of diaphragm deep to it to be deeper than it really is
“lens effect” of well developed parallel paired rectus abdominus muscles results in double virtual images of underlying midline structures such as aorta or gestational sacs which are displayed lon either side of the invisible real image
Mirror image:
structures may be displayed twice - a real image and a virtual image caused by reflecting interface within the field of view which is angled to show the image in the same way as if one looked at an object in the mirror and can see both the object and the virtual image of the object - albeit a different aspect of it.
examples:
diaphragm/lung interface - may show mirroring of liver texture above the diaphragm
bladder/rectum interface when rectum is gas filled - bladder wall is mirrored together with any bladder contents
Operator produced artefacts:
excessive gain:
adds noise to the image
makes like structures appear the same irrespective of depth
poor TGC settings:
may create a band of bright or low level echoes within the field of view
poor skin contact
due to inadequate gel &/or transducer pressure
Problem solving:
turn down the gain
alter angulation of probe and watch effect on the artefact