Image Artifacts Flashcards
Describe the mirror image artifact
A form of reverberation. A strongly reflective structure returns echoes back to the transducer. Due to scattering some of these echoes return to the strongly reflective structure which once again reflects this energy back to the source. The returning echoes will be displayed further away than the actual distance to the target
Describe the image artifact called refraction
The change in the direction of sound as it crosses a boundary. May cause a reflector to be displaced laterally.
May cause shadowing at the edges of a structure which are large and curved. This is referred to as edge shadowing
Describe the image artifact called slice (section) thickness artifact
The ultrasound beam transmit has a certain width (beam diameter) and a certain elevational (slice, section) thickness. The thickness of the beam causes a core sample to be taken and that core sample is placed on the display screen, which is a flat surface. This may lead to an inappropriate display of structures.
Ex: A fluid filled cyst may appear filled in because of the surrounding soft tissue
Explain the difference between side lobe image artifact and grating lobe image artifact
Transducers produce a primary beam with some off axis beams as well. The side beams are called side lobes. Side lobes produce secondary images on both sides of the primary image. Any type of transducer may produce side lobes.
Grating lobes are similar to side lobes in that off axis beams are produced from the edges of elements in a multielement transducer. Grating lobes can produce images on both sides of the image just like the side lobes. Only electronic transducers produce grating lobes. Apodization is a manufacturing process which reduces grating lobes
Explain the speed of sound artifact
The ultrasound machine is calibrated to assume that the propagation speed in soft tissue is 1540 m/s. If the medium in which the sound beam is traveling is slower than 1540 m/s, the returning echo will take longer to return and therefore be displayed farther away from the transducer than it actually is. If the propagation speed is greater than 1540 m/s, the returning echo will return sooner to the transducer and thus be placed too close to the transducer
Describe the image artifact shadowing
Shadowing is the loss of echo amplitude due to an intervening highly attenuating structure. There will be an artifactual loss of echo strength due to shadowing
Describe the image artifact called enhancement
The increase in echo amplitude due to an intervening low attenuating structure
Describe the image artifact called reverberation
The display of multiple echoes, equally spaced. May be seen near the top of the image display as ultrasound energy is trapped between the transducer and surface of the body
What is the comet tail image artifact?
A type of reverberation where a highly reflective structure (e.g. bullet) results in a series of multiple echoes, equally spaced
What is the ring down image artifact?
Similar to comet tail except it originates from a gas bubble in a fluid medium
Explain the image artifact of axial resolution
Occurs when two reflectors that lie parallel to the sound beam are displayed as one reflector
Explain the image artifact of lateral resolution
Occurs when two reflectors which are perpendicular to sound beam travel are displayed as one reflector
