Ultrasound - artefacts and settings Flashcards
Ultrasound frequency used in medicine
2-15 MHz
wavelength in tissue is short
Sound, in general, moves 1540 m/sec.
First U/S machine made by who in what year
1957 by Ian Donald
First color doppler in 1980.
Define Wavelength(λ):
length of space over which one cycle occurs ( mm)
Define Frequency ( f):
how many cycles in a second. US > 20 kHz
Define Period ( T):
time it takes for one cycle to occur (microsec)
As sound waves propagate through a medium, different interactions occur; such as (4)
reflection,
refraction,
diffraction,
attenuation
The product of the tissue`s density and the sound velocity within the tissue is known as the
tissue’s acoustic impedence.
Acoustic impendence refers to
the reflection or transmission characteristics of a tissue.
Picture quality is affected by (3)
the higher the frequency the better the resolution
the higher the frequency the shorter the wave length which in turn means the penetration depth of the ultrasound is LOWER
the shorter the wave length, the smaller the penetration
the higher the frequency the better the
resolution
the higher the frequency the shorter the wave length which in turn means the penetration depth is…
LOWER
the shorter the wave length, the smaller the
penetration
Types of display modes used in U/S (4)
A-mode (amplitude modulation) (used in ophthamology)
B-mode (brightness modulation) (everyday mode)
M-mode (motion modulation)
Doppler
The A-mode is
the oldest ultrasound technique and was invented in 1930.
The transducer sends a single pulse of ultrasound into the medium. Consequently, a one-dimensional simplest ultrasound image is created on which a series of vertical peaks is generated after ultrasound beams encounter the boundary of the different tissue.
The distance between the echoed spikes can be calculated by dividing the speed of ultrasound in the tissue (1540 m/s) by half the elapsed time, but it provides little information on the spatial relationship of imaged structures.
This one is used in ophthamology.
Describe B mode.
In B-mode (brightness mode) ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen.
More commonly known as 2D mode now.
This one is our (everyday mode).
Describe M mode.
In M-mode (motion mode) ultrasound, pulses are emitted in quick succession – each time, either an A-mode or B-mode image is taken.
As the organ boundaries that produce reflections move relative to the probe, this can be used to determine the velocity of specific organ structures.
Describe doppler.
Phenomenon by which the frequency of a wave received after reflection by a moving target is shifted from that of the source.
Occurs when the distance between the observer (transducer) and the source (blood cells) is changing with time.
Frequency is decreased (negative frequency shift) when blood moves away from transducer (blue = away).
Frequency is increased (positive frequency shift) when blood moves toward the transducer (red = towards). The greater the velocity, the greater the frequency shift.
The color depends on the direction the fluid or blood is moving (not on whether its an artery or vein).
Types of U/S transducer. (4)
microcurve array
curved array
linear array
phased array
linear array transducer / probe
curvilinear array sector transducer / probe
phased-array sector transducer / probe
Name and describe.
Curved array probe:
Large investigative sensor area
Low frequency - see deeper structures
Abdominal ultrasound
Name and describe.
Convex probe:
Smaller sensor investigative area (fits between ribs)
Low frequency
Echo or abdominal ultrasound
Name and describe.
Linear array probe:
Flat sensor area
High frequency
Maximum depth 10-13 cm
Musculoskeletal or vessel ultrasound
U/S scanning levels: (3)
sagittal and parasagittal
transverse
+ other oblique levels
Sagittal and parasagittal views are
parallel with the body
The transverse scanning level is
“crosswise” with the body (across)
Define Echogenicity
the ability to bounce an echo
Echostructure =
“grain-icity”
Anechogenic =
tissues producing no echoes, appearing black on the image (usually fluid).
Hypoechogenic =
tissues producing few echoes, appearing grey on the image (liver, renal medulla, intestine, muscle).
Hyperechogenic =
tissues producing strong echoes, appearing bright on the image (bone, air, stones).
Homogenic =
Isoechogenic =
Mixed echogenic =
Homogenic = same structure across the image
Isoechogenic = similar structure or density e.g. kidney cortex is isoechongenic with the liver parenchyma
Mixed echogenic = self-explanatory
Describe Noise artefacts.
Appear as small amplitude echoes
Results from:
- Electrical interference
- Signal processing
- Spurious reflections
- More likely to affect low-level hypoechoic regions rather than bright echogenic areas.
Describe acoustic shadowing artefact.
Hypoechoic or anechoic region extending downward from highly attenuating structure.
Same color as image background.
Too much attenuation occurs, deep reflecting surfaces don’t appear on image.
Prevents display of true anatomic structures.
May provide valuable diagnostic information that helps to characterize tissue.
Describe Acoustic enhancement artefact.
Hyperechoic region beneath tissues with abnormally low attenuation.
Hyperechoic regions are brighter and same color as foreground (echoes) of image.
Number of reflectors on image is correct, some are overly bright.
Assumption that intensity of reflection is related to the tissue creating reflection is violated.
Opposite of shadowing
Describe Reverberation artefact.
Multiple, equally spaced echoes caused by bouncing of sound wave between two strong reflectors positioned parallel to ultrasound beam.
Assumption that sound travels directly to reflector and back is violated.
Appears:
- In multiples
- Equally spaced
- Located parallel to sound beams’ main axis
- Located at ever-increasing depths
Describe Ring-down artifact
Is a part of previous artefact.
Is due to metal for example biopsy needle.
Describe Comet-tail artefact.
Solid hyperechoic line directed downward (reverberation with spaces squeezed out).
Created when closely spaced reverberations merge.
Arise from resonance, or vibration, of small structures such as gas bubbles after they have been bombarded by sound pulse.
Assumption that sound travels directly to reflector and back is violated.
Characteristics:
Appears as single long hyperechoic echo
Located parallel to sound beam’s main axis
Describe Mirror-image artefact.
Sound reflects off a strong reflector (mirror) and is redirected toward a second structure.
Causes replica of structure to incorrectly appear on image. Located deeper than real structure.
Always located along a straight line between tissue and artifact.
Characteristics:
- Second copy of true reflector
- Artifact appears deeper than true reflector
- Bright reflector (mirror) lies on a straight line between artifact and (T).
- True reflector and artifact are equal distances from mirror.
Describe edge-shadowing artefact.
Special form of shadowing that appears as a hypoechoic region extending down from edge of a curved reflector.
Prevents display of true anatomic structure that are positioned within extended hypoechoic region.
Decrease in intensity causes edge shadowing.
Assumption that intensity of reflection is related to the tissue creating reflection is violated.
Characteristics:
- Hypo- or anechoic
- Result when beam spreads after striking a curved reflector.
- Extends downward from curved reflector’s edge, parallel to beam.
- Prevents visualization of true anatomy on scan.
Describe Slice-thickness/beam-width artefact.
Related to the dimension of beam that is perpendicular to imaging plane.
Elevational resolution determined by thickness of imaging plane.
True reflector lies either above or below assumed imaging plan.
Displayed within image
Fills in hollow structures such as cysts
Reduced with thinner imaging planes
(will make it look like there’s sediment so jiggle the bladder to see if its true)
What artefact do you see?
Reverberation artefact
What artefact do you see?
Edge-shadowing artefact
What artefact do you see?
Acoustic enhancement artefact
What do you see (not an artefact)?
Multifocal hypoechogenic lesions (not artefacts)
What do you see (not an artefact)?
Hyperechogenic area within splenic capsule
What do you see (not an artefact)?
Isoechogenic, kidney and liver parenchyma
What artefact do you see?
Acoustic shadowing artefact due to urolith
Preparing the patient for U/S exam. (2)
Should be good contact between probe and skin, without air: should be clipped.
Ultrasound gel or chlorhexidine/alcohol should be used.
Abdominal study in what position?
Dorsal recumbency unless
Laterally if gas is disturbing
Cardiac in lat
Review common acoustic windows for major structures and regions.
The further away a reflective interface is from the transducer, the… (finish the sentence).
the weaker the returning echo will be.
The U/S scanner’s controls are designed either to increase the intensity of sound transmitted into tissues or to electronically amplify returning echoes.
The prime objective of U/S is to produce a uniform image brightness throughout the near and far field.
Describe focus on U/S machines.
Focusing narrows the U/S beam width to improve lateral resolution and sensitivity.
Describe gain on U/S machines.
GAIN affects the amplification of returning echoes.
May be changed looking at different structures.
If too high; signal is saturated, loss of contrast. (left image)
If too low; image too dark, loss of contrast. (right image)
Describe Power in U/S
POWER control modifies the voltage applied to pulse the piezolectric crystal.
The power should be set as low as possible to obtain the best resolution and prevent artefacts. This is done by choosing the appropriate transducer frequency
Describe the TIME-GAIN-COMPENSATION feature.
Echoes returning from deeper structures are weaker than those arising from superficial structures because of increased sound attenuation.
The TGC function is to adjust gain applied at various depths.
(this image an example of what not to do)
