Imaging Artifact

Artifacts: Any structure in an ultrasound image that does not have a corresponding anatomic tissue structure.
Classification:
1. Missing structures
2. Degraded images
3. Falsely perceived images
4. Structures with misregistered location.
Missing structures:
Reasons:
Reduced resolution – To improve resolution increase the frequency, bring your area of interest into focal zone and decrease the overall gain
Acoustic shadowing – change the window.
Degraded images:
Reverberations – they are secondary reflections that occur along the path of sound pulse. They occur as a result of ultrasound bouncing between the structure and another reflecting surface. The reflecting surface may be the near side of the object, a second object or the transducer itself. The repeated journey produces two or more additional signals which are equally spaced, twice the distance as the original signal. Some times the reverberations are merged together and appear as a single solid line away from the transducer – comet tail/ring down.
Enhancements – reciprocal of image shadowing. If the intervening tissue has low acoustic impedence then the structures beyond appear to be enhanced because the sound signal is minimally attenuated. This can be adjusted by reducing the time gain compensation.
Noise – has many causes – excessive gain, cautery etc.
Falsely perceived objects:
This can be due to refraction or reverberation. Examples include:
Mirror images – Common place of occurrence is descending aorta- called as double barrel aorta. This is believed to be due to aorta-lung interface. Seen at twice the distance from the transducer as the original image.
False dissections
Line artifacts – type of reverberation artifact. Can be identified at twice the distance similar to mirror images. They mimic false intimal flaps. They can be detected as false flaps when they have indistinct borders, do not display rapid oscillatory motion and are located twice the distance from the LA wall. In addition colour Doppler does not show any interruption of flow pattern. Artifacts are more likely if the aortic diameter is >5cm and atrial-aortic ratio ≥0.6.
Reverberation artifacts are also described in the LAA mimicking thrombi. To differentiate -
Thrombus
Artifact
Confined to cavity
Not always
Has an attachment
None
Uniform consistency
Non- uniform
Texture different to LA
Similar
Twice the distance from Coumadin ridge.

Misregistered locations:
Range ambiguity – results in the display of correct structures in the wrong location. It occurs with high PRF(Pulse Repetition Frequency). This results in deeper structures appearing closer. When an unexpected object is seen in the cardiac chamber, it is due to range ambiguity.
Side lobes – are additional multiple beams emerging from the transducer in a diverging manner. Displayed as a curved line and always have a common radius from the transducer. They cross anatomical walls and cavities without regard for natural borders. They disappear with adjustment of depth/angle of the transducer.