Artifacts are ultrasound imagesÂ on the screen that do not correspond exactly what is in the body. Artifacts can be useful in determining true anatomy:
1. The presence of some artifacts can help us to identify anatomy:Â e.g. “an aorta” isÂ “the aorta”Â because it’s resting on the spine, which is “the spine” because it casts a shadow (what if the spine does not cast a shadow….?)
2. The absence of artifacts can also reveal pathology:Â e.g. inÂ Â FAST with right hemothorax, loss of the mirror image of the liver above the diaphragm not only reveals the blood and superior aspect of the diaphragm, it also allowsÂ Â the vertebral column (above the diaphragm) to show up! The spine above the diaphragm is never seen because the normal aerated lung scatters all of the ultrasound energy above the diaphragm.
3. Both the real image and artifact arise because of certainÂ assumptions that that ultrasound machine makes. When they are all met, you get a real image; whenÂ any assumptionÂ is not, well, you get an artifact. And thankfully, there are only four such assumptions. Here’s a quick review of themÂ as we begin this series of what’sÂ real and what’s not.
A pulse of ultrasound beam emitted by the transducer travels in a straight line, is reflected atÂ an interface, and travels back to the transducer (exactly along the path it was emitted, only in the reverse direction)
All the returning echoes of the beam are presumed to have arisen only from the center (i.e. axis) of the beam and hence are displayed as such (i.e. along a vertical line on the screen that represents the axis)
The speed of ultrasound beam (emitted and/or reflected) is always and exactly 1540m/s
The intensity of the displayed echo is dependent on the acoustic properties and size of the interface where it is being reflected
And with that, we’ll make good use of what’s notÂ really there to find out what’s really going on.