Module title = Tutorial: Ultrasound Physics without Physics

Lesson title = Artifacts

Artifacts occur when assumptions about physics are not true.
 
One assumption is that sound travels in a straight line. We can see this example at play if you put a spoon into a glass of water. The handle of the spoon underwater will appear to not line up with the portion that is above water. Light travels in a straight line too, but water and glass can refract and curve the light beam so that not all the light beams travel in a perfectly straight line.
 
The same principle is true for sound. A single sound beam sent from one crystal should generate an echo that returns to that same crystal. If the echo does not travel in a straight line, but rather it “bends”, then the echo will land on a different crystal. This will create an artifact.


Refraction
Some surfaces that are smooth and round shaped will deflect the sound beams. The crystals that sent out those beams will get no echo coming back to them. Remember that echo sound waves make a white signal on the screen, so the absence of echo sound waves will make a black signal. Refraction causes black areas on the screen.
 
Refraction is an important and common artifact and could be mistaken for fluid (which also appears black). Refraction occurs when the ultrasound waves are deflected from their original path by passing close to a large, curved, smooth-walled structure. The result is a shadow-like image that seems to project from the edges of the curved structure, also called edge artifact.

Here is our sequence of images to help you understand why refraction occurs. Think of the ultrasound beam as being made of many skinny ultrasound lines. This image shows one skinny ultasound "line" generated by one crystal:


Putting many skinny lines together produces a sheet or plane of ultrasound. Each red beam that shoots out of the transducer is expected to return to the same crystal that generated it.


If a curved smooth surface deflects some of these skinny lines toward other crystals, then the crystals that sent those sounds will not hear them come back and without an echo, a black image is produced on the screen. This is how refraction artifact or edge artifact is created. The blue wavy lines in the image below represent returning echos that are deflected to other crystals.


Here is an example of refraction artifact. Dotted yellow lines indicate the black artifact:
 


Shadowing

Some structures, like bone, do not allow ultrasound to pass through them. Therefore, structures behind them (further from the probe) do not receive any ultrasound beams and therefore appear black. Shadows cast "black bars" behind the structures. Note that both refraction and shadowing artifacts cause blackout, but for different reasons.

Ribs are a good example to show shadowing (yellow dotted lines):
 


Enhancement
Enhancement, or “through transmission”, is the opposite of shadowing. When ultrasound waves go through an area of low resistance, namely fluid, the tissues on the far side glow more brightly than the tissues beside them. The bladder, the gall bladder and any fluid-filled cystic or vascular structure can do this. The waves go through the fluid without any difficulty and therefore retain nearly all of their energy. Upon entering denser tissue on the far side, this excess energy makes the far wall of the fluid-filled structure light up more brightly than adjacent tissues. 
 


Reverberation
Let’s play ping pong!  A sound signal hits a structure (maybe even a small air bubble) that is very close to the transducer. The sound beam then bounces off the structure and hits the transducer. But wait! It’s not finished. It bounces off the transducer now and back to the structure and off the structure and returns to the transducer a second time. Every time it returns to the transducer, the crystal can hear the echo and generates a signal on the monitor. Repeat this process many times, and you have just created many reverberations. The distance from the transducer and structure is very very short. Therefore, these events occur very fast and result in artifacts that rain down in tiny but symmetrical waves. They are ALWAYS at the top of the screen because the structures that create reverberation are always near the transducer. Can you see the reverberation in this image?
 


Mirroring
Mirroring will create 2 images of one structure. One image is the real image. The other image is created by reflection of ultrasound beams so that crystals that should NOT be "seeing" echos of the structure end up seeing the structure. How does this work? The ultrasound beams must travel sideways at an angle so that their echo hits a different part of the transducer, therefore vibrating a different set of crystals.

The video below shows two copies of the the IVC within the liver:

How does this happen? There is a structure in the body that acts like a reflector or a mirror. The straight edge of the liver, for example, can act like a mirror. If this mirror is at just the right angle, it will reflect ultraound beams sideways from the transducer to the structure and back again.

Therefore, ultrasound beams that travel in a straight line will hit the structure and a second set of ultrasound beams that are deflected sideways will also hit the structure. These 2 sets of ultraound beams will return to 2 different locations on the transducer and create 2 separate images. In this image, the blue arrow points to the liver edge which is the reflector. The blue dot represents the real IVC. The yellow dotted line represents the mirror image. The schematic of the ultrasound probe shows the path of ultrasound beam takes that creates the MIRRORED image.

 

• the image that is further away (lower) from the probe is ALWAYS the mirror image