unit 11 Flashcards
What are Artifacts?
Answer: Artifacts are errors in imaging which are not actual structures displayed in an image. The ultrasound systems are designed based on certain assumptions. Artifacts are caused by violations of these assumptions. The ultrasound artifacts include:
- images that have reflectors of improper shape
- images of reflectors in an inappropriate location
- images of reflectors of incorrect brightness
- images that do not correspond to anatomical structures
The artifacts can be caused by equipment malfunction, operator error, or patient motion.
What is an axial resolution artifact?
Answer: Axial Resolution Artifact is the failure to resolve two separate reflectors located parallel to the main axis of the ultrasound beam. The axial resolution artifact appears when spatial pulse length is longer than the distance between two reflectors that are located in front of each other.
The two separate reflectors appear as one reflection.
Two ultrasound transducers are used to perform an exam.
Transducer A has frequency of 5 MHz and 2 cycles per pulse.
Transducer B has a frequency of 2.5 MHz and 4 cycles
Answer: The transducer A will have better axial resolution.
The axial resolution artifact appears when spatial pulse length is longer than the distance between two reflectors that are located in front of each other. The two separate reflectors appear as one reflection. The transducers with fewer cycles in an ultrasound pulse have better axial resolution.
How can the axial resolution artifact be reduced?
Answer: The axial resolution artifact appears when a long ultrasound pulse strikes two closely spaced structures.
The axial resolution artifact can be reduced by the use of high frequency transducers. The high frequency transducers produce shorter pulses.
Shorter pulses improve axial resolution and create higher quality images.
What is a lateral resolution artifact?
Answer: Lateral Resolution Artifact is the failure to resolve two separate reflectors located perpendicular to the main axis of the ultrasound beam.
The lateral resolution artifact appears when ultrasound beam is wider than the distance between two reflectors that are located side by side.
The two separate reflectors appear as one reflection.
The lateral resolution is related to the diameter or width of the ultrasound beam.
Two ultrasound transducers are used to perform an exam.
Transducer A has frequency of 7.5 MHz ultrasound beam diameter of 3 mm. Transducer B has a frequency of 5 MHz and ultrasound beam diameter of 8 mm. Which transducer will have better lateral resolution?
Answer: The transducer A will have better lateral resolution.
The lateral resolution is related to the diameter or width of the ultrasound beam. The lateral resolution artifact can be reduced by the use of ultrasound transducers which produce smaller diameter or narrow ultrasound beam.
The ultrasound beam with smaller diameter will have better lateral resolution.
How can the lateral resolution artifact be reduced?
Answer: The lateral resolution is related to the width of the ultrasound beam.
The factors that affect ultrasound beam width are:
- diameter of the piezoelectric element
- frequency of the ultrasound beam
- focusing of the ultrasound beam
Small diameter PZT crystals produce narrower ultrasound beams compared to large diameter PZT crystals which produce wider ultrasound beam. The ultrasound beams created by a small diameter crystal will have better lateral resolution and will produce better quality images.
High frequency ultrasound beams are narrower than low frequency ultrasound beams and have better lateral resolution.Focusing of ultrasound beam narrows its diameter within the focal zone area.
Narrower ultrasound beams have better lateral resolution and produce high quality images.
What are reverberations?
Answer: Reverberations are the multiple echoes appearing on the display as a result of ultrasound pulse bouncing between two reflectors.
Reverberations are produced by a pair of strong reflectors that lie along the main axis of an ultrasound beam.
The reverberations are seen on the image as equally spaced bands of diminishing amplitude.
What is a comet tail artifact?
Answer: The comet tail artifact is composed of thin lines of closely spaced echoes.
It appears as a single solid hyperechoic line directed downward.
The reflectors are located parallel to the main axis of the ultrasound beam. Comet tail artifact is produced by a strong reflector and is similar in appearance to reverberation.
The comet tail artifact can be seen posterior to an object that has either a much lower or much higher acoustic impedance than its surroundings.
The reflections of a comet tail artifact are closely spaced because of strong reflectors with high propagation speed and lying close to each other.
What is the fundamental mechanism of comet tail artifact formation?
Answer: The fundamental mechanism in the formation of comet tail artifact is reflection. The ultrasound beam bounces between two closely spaced strong reflectors.
What are the characteristics of the medium that produces comet tail artifact?
Answer: The characteristics of the medium that produces comet tail artifact are high propagation speed and closely spaced strong reflectors.
What is a Shadowing Artifact?
Answer: Shadowing artifact occurs when ultrasound beam is unable to pass through a structure because the structure has a higher than usual attenuation.
Shadowing artifact appears as a hypoechoic or anechoic region extending downward from a very strong attenuating medium. The structures that lie deeper are not imaged or displayed. Structures such as gallstones, renal calculi, and bone produce shadowing artifact.
How is shadowing artifact expressed?
Answer: The shadowing artifact is expressed as reflectors being absent on the image.
Shadowing artifact occurs when ultrasound beam is unable to pass through a structure because the structure has a higher than usual attenuation. The structures that lie deeper than the initial structure are not imaged or displayed.
How is the shadowing artifact produced?
Answer: Shadowing artifact occurs due to attenuation of ultrasound wave.
When ultrasound waves strike a highly attenuating structure such as gall stone, the strength of the reflections from the structures located behind the attenuating structure decreases. Therefore, the structures located behind the attenuating structure cannot be displayed on the image. For example shadowing caused by a prosthetic valve can obscure the visualization of structures behind the prosthetic valve.
The weakening of ultrasound wave can occur with refraction, reflection, and absorption.
What is an enhancement artifact?
Answer: Enhancement artifact appears as a hyperechoic region beneath tissues with low attenuation. Enhancement artifact is an increase in amplitude of echoes located behind a weak attenuating structure. Stronger reflections may be seen behind a fluid filled structure such as urine filled bladder.
. Acoustic focusing of an ultrasound beam may create which artifact?
Answer: Acoustic focusing of an ultrasound beam may create focal enhancement or banding artifact.
Focal enhancement or banding is a form of acoustic enhancement within the focal zone. This results from the increase in beam intensity within the focal zone of a sound beam.
What is vertical misregistration artifact?
Answer: Vertical misregistration occurs when ultrasound pulses travel at the speed less than 1540 m/sec in the body. The reflections on the image are placed deeper than the actual position of the reflector in the body.
Which artifact is created by sound moving through a medium with a propagation speed less than soft tissue?
Answer: Vertical misregistration artifact is created by sound travelling through a medium with a propagation speed less than soft tissue.
If the sound beam passes from a medium of higher speed into a medium of a lesser speed, then the distance is calculated more than the actual distance and the reflectors are placed too deep which is called vertical misregistration.
What is vertical misregistration artifact?
Answer: Vertical misregistration occurs when ultrasound pulses travel at the speed less than 1540 m/sec in the body. The reflections on the image are placed deeper than the actual position of the reflector in the body.
An ultrasound wave travels through a mass at a speed of 1000 m/sec. What will happen to the position of echoes on the image produced from reflectors distal to the mass?
Answer: The reflected echoes on the image will appear farther than the actual position of the reflector.
The propagation speed of mass is less than the propagation speed of soft tissue (1540 m/sec). Therefore, the go return time from the structures located behind the mass will be longer. The ultrasound machine assumes that a long go return time for ultrasound pulse means that it came from a reflector that is located deep in the body. All structures that lie distal to the mass will be placed too deep on the image. The reflected echoes on the image will appear farther than the actual position of the reflector.
An ultrasound wave travels through a large cyst at a speed of 1700 m/sec. What will happen to the position of echoes on the image produced from reflectors distal to the cyst?
Answer: The reflected echoes on the image will appear closer than the actual position of the reflector.
**The propagation speed of sound in the cyst is higher than the propagation speed of soft tissue (1540 m/sec). **Therefore, the go return time from the structures located behind the cyst will be shorter. The ultrasound machine assumes that a short go return time for ultrasound pulse means that it came from a reflector that is located shallower in the body. All structures that lie distal to the cyst will be placed shallower on the image. The reflected echoes on the image will appear closer than the actual position of the reflector.
During an exam, the distance measured between two reflectors on an image is 120 mm. The actual distance between the two reflectors is 100 mm. If the actual distance between two reflectors is 50 mm, what will be the measured distance on the image by the same ultrasound system?
Answer: The measured distance will be 60 mm.
In this example, the sound is travelling in a medium which has propagation speed 20% less than the soft tissue and the system is over reporting distance by 20%.
Therefore, if two structures are 50 mm apart, the system will overestimate the distance by 20% and will be 60 mm when measured on the image.
During an exam, the distance measured between two reflectors on an image is 40 mm. The actual distance between the two reflectors is 50 mm. If the actual distance between two reflectors is 100 mm, what will be the measured distance on the image by the same ultrasound system?
Answer: The measured distance will be 80 mm.
In this example, the sound is travelling in a medium which has propagation speed 20% faster than the soft tissue and the system is under reporting distance by 20%.
Therefore, if two structures are 100 mm apart, the system will underestimate the distance by 20% and will be 80 mm when measured on the image.
Which artifacts are affected by the shape or dimensions of an ultrasound pulse?
Answer: Lateral resolution artifact, longitudinal resolution artifact and slice thickness artifact are affected by the shape or dimensions of an ultrasound pulse.
