Ultrasonography Flashcards
What is an artifact?
An artifact is a structural defect that appears in an image due to the methods and technology used to create the image.
Artifacts may show something that is not actually present in the object being imaged, or misrepresent the object.
What are some common artifacts encountered in ultrasound?
(1) Reverberation
(2) Acoustic Shadowing
(3) Acoustic Enhancement
(4) Edge Shadowing
(5) Beam Width artifact
(6) Slice Thickness artifact
(7) Side lobe artifact
(8) Mirror image
(9) Double image
(10) Equipment-generated artifact
(11) Refraction artifact
Explain reverberation.
Reverberation is the production of false echoes, due to repeated reflections between two interfaces with high acoustic impedance mismatch.
An echo will then take longer to return to the transducer and be detected.
The first echo, a normal ultrasound echo will return at the expected time for a given depth of the interface. The second echo between the interfaces would take twice the time to return to the transducer and be detected and so, would be shown at twice the depth on the image.
A similar explanation follows for the third , fourth and rest of the reverberation echoes between the same interfaces.
Reverberation occurs typically at the skin-transducer interface and behind bowel gas as a ring-down artifact.
Rectification:
(1) Use more gel
(2) Reduce gain
(3) Move transducer
(4) Use a standoff pad
Explain acoustic shadowing.
Acoustic shadowing is the appearance of an area of low amplitude echoes behind a region of highly attenuating tissue.
It arises due to absorption and/or reflection of the sound waves by the tissue giving rise to severe attenuation of the beam behind it.
It occurs at the soft tissue-gas interface and soft tissue-bone/calculus interface. There is high acoustic impedance mismatch at these interfaces.
Acoustic shadowing appears as a dark area behind the object.
If it also contains ring-down artifacts and the bowel region is being imaged then it may be bowel gas.
It could be used to identify gallstones, kidney stones etc. and so can aid in diagnosis.
Explain acoustic enhancement.
Acoustic enhancement is the appearance of an area of increased echo amplitude behind a low-attenuating tissue. This will show as increased brightness behind or around a dark area in the image. It arises due to time-gain compensation (tgc) of low-attenuating tissues. Such tissues may be fluid-filled for example, the urinary bladder, the gallbladder and cysts.
It can useful in diagnosis.
Explain edge shadowing.
Edge shadowing appears a region of darkness behind a curved object. It arises due to reflection and refraction of a beam at a curved surface and difference in speed of sound in the tissue. This refraction may cause the tissue behind the point of refraction to receive little sound waves and so little signal from this region.
This artifact may occur, for example, behind a cyst.
How are sound waves classified by their frequency?
Normal sound is that which can be detected by the human ear: 20 Hz- 20 kHz
Infrasound (subsonic frequency): <20 Hz
Ultrasound (ultrasonic frequency): >20 kHz
Explain beam width artifact.
Echoes are seen in a typically echo-free area.
Caused by widening of the beam after the focal spot.
Rectification:
Correct positioning of the focal spot.
What range of frequencies are typically used in diagnostic ultrasound?
2-20 MHz
What are the main components of an ultrasound system?
(1) Transducer containing piezo-electric crystal
(2) CPU or computer system
(3) Monitor or Image display
List components of a transducer.
(1) Transducer housing
(2) Protective front layer
(3) Matching layer
(4) Acoustic lens
(5) Crystal
(6) Backing material
List components of a transducer.
(1) Transducer housing
(2) Protective front layer
(3) Matching layer
(4) Acoustic lens
(5) Crystal
(6) Backing material (tungsten powder and plastic or epoxy resin)
What is ‘pulse length’ and ‘pulse duration’ ?
A pulse is typically 2-3 wavelengths.
What is ‘pulse length’, ‘pulse duration’ and ‘pulse repetition frequency’ (PRF) ?
A pulse is typically 2-3 wavelengths.
Explain slice thickness artifact.
Similar to beam width artifact.
Typically observed in transverse views of bladder.
Echoes are seen as if they are occurring within bladder when they are not.
Occurs when slice thickness is wider than the scanned structure.
aka Partial volume averaging artifact.
Cannot be rectified apart from changing the transducer itself.
Explain side lobe artifact.
“Side lobe artifacts occur where side lobes reflect sound from strong reflector that is outside of the central beam, and where the echoes are displayed as if they originated from within the central beam.”
“Ultrasound transducer crystals expand and contract to produce primary ultrasound beams in the direction of the expansion and contraction. Secondary beams also occur because the crystals also expand and contract radially. These radial beams are called side lobe beams. Side lobe beams are low-intensity beams that surround the central beam.
Side lobe artifacts are echogenic, linear or curvilinear artifacts. Strong reflectors include bowel gas adjacent to the gallbladder or urinary bladder.”
Explain mirror image artifact.
A mirror image of a structure occurs in an ultrasound image. Due to specular reflection of the beam at a large smooth surface. Occurs where there is a large acoustic impedance mismatch.
Eg. bladder, liver etc.
Explain double image artifact.
Two refracted beams send back echoes of the same structure.
Eg. aorta
Explain equipment-generated artifact.
Arise due to misuse of controls like gain or tgc.
Areas are recorded as too light or too dark.
Explain refraction artifact.
Refraction artifacts causes spatial distortion and loss of resolution.
arises due to change in the velocity of the sound wave in crossing the interface of two tissues at an an oblique angle.
Formula for near zone length
NZL= D^2/(4*lambda)
= D^2f/(4c)
