Artifacts - Wrong Location
Wrong Location
This type of artifact is characterized by extra information displayed on the monitor in comparison to the true anatomical correlated. Simply put, there are things on the screen that should not be there.
Four Two Subtypes will be discussed:
1. Side and Grating Lobe Artifacts
2. Range Ambiguity
3. Propagating Speed Error Artifact
4. Mulitpath
Four Two Subtypes will be discussed:
1. Side and Grating Lobe Artifacts
2. Range Ambiguity
3. Propagating Speed Error Artifact
4. Mulitpath
Strong reflectors in the line of sight of these off-axis lobes will be reflected echoes back to transducer and displayed on the monitor. However, the machine will think this reflection is from the Main Beam: falsely displaying the lateral object on the screen. The depth of the artifact is correct, but the lateral position is wrong.
Here, the Heart is in the Main-Axis and displayed on the screen in the appropriate location. However, Smiley, a strong reflector, generates an echo that is detectable by the transducer. The processor, however, is preset to think this signal originated from the Main-Axis, therefore the image is displayed on the monitor though the true anatomical correlate will not be at the located as per displayed.
Attenuate/Mitigate: center the structure of interest at the Focal Zone
Here, the Heart is in the Main-Axis and displayed on the screen in the appropriate location. However, Smiley, a strong reflector, generates an echo that is detectable by the transducer. The processor, however, is preset to think this signal originated from the Main-Axis, therefore the image is displayed on the monitor though the true anatomical correlate will not be at the located as per displayed.
Attenuate/Mitigate: center the structure of interest at the Focal Zone
2. Range Ambiguity:
characterized by the display of the structure of interest in the wrong location. It occurs with high pulse repetition frequency (decreased echo reception phase's duration).
As illustrated by the animation, Echo 1 reflected from the Circle is received simultaneously as Echo 2. Since the processor references echo timing based the current emission/reception cycle (ie. 2nd), Echo 1's distance will be calculated from the beginning of Cycle 2 - not Cycle 1. Hence, the Circle is displayed in at a shallower depth than the anatomical correlate.
Attenuate/Mitigate:by increasing Depth, PRF will decrease. Moving the Focal Zone deep to the structure of interest will, also, attenuate this artifact.
As illustrated by the animation, Echo 1 reflected from the Circle is received simultaneously as Echo 2. Since the processor references echo timing based the current emission/reception cycle (ie. 2nd), Echo 1's distance will be calculated from the beginning of Cycle 2 - not Cycle 1. Hence, the Circle is displayed in at a shallower depth than the anatomical correlate.
Attenuate/Mitigate:by increasing Depth, PRF will decrease. Moving the Focal Zone deep to the structure of interest will, also, attenuate this artifact.
3. Propagating Speed Error Artifact:
objects are displayed either deeper/shallower than their anatomical correlate because different mediums foster different propagation speeds. This violates the assumption that US speed is constant at 1540m/s.
This animation shows the US have to traverse 3 different media to image B. Each medium has it own propagation speed - Fat has slowest, and Muscle has fastest. Consequently, B will be displayed deeper in Fat as it the to-&-fro time of the wave is longer, and vice versa if B was buried in muscle.
Attenuate/Mitigate: View from a different window; Compound Imaging
This animation shows the US have to traverse 3 different media to image B. Each medium has it own propagation speed - Fat has slowest, and Muscle has fastest. Consequently, B will be displayed deeper in Fat as it the to-&-fro time of the wave is longer, and vice versa if B was buried in muscle.
Attenuate/Mitigate: View from a different window; Compound Imaging
4. Multipath Artifacts:
characterizes by the object of interest appearing at an incorrect depth due to oblique reflection of US waves following a longer or shorter path than the incident beam. This violates the assumption US beam travels in a straight line only.
The animation illustrates how a simple trajectory and become complex.
Attenuate/Mitigate: change the angle of insonation
The animation illustrates how a simple trajectory and become complex.
Attenuate/Mitigate: change the angle of insonation
This summarizes artifacts characterized by objects being displayed at incorrect location to the image in comparison to the anatomical correlate and how to attenuate/mitigate them if needed.
Take Home Messages:
- Most of the time, for the purposes of Internal Medicine PoCUS indications, these artifacts will not be troublesome, or be of major clinical significance