Artifacts and Pitfalls

Chapter 18


Artifacts and Pitfalls



Christopher J. Gallagher and Gian Paparcuri


When you are in the OR, you are forever asking yourself, “Is it real, or is it Memorex?”. We get seduced by the great images, and you have to shake yourself and say, “This is not a REAL LIVE picture, this is an ULTRASOUND CREATION OF A PICTURE, and ultrasound can fool you”.


Let’s grind through these monsters. Pay close, close, close attention to the aortic dissection artifacts; that is what will scare the hell out of you in the middle of the night.


Useful for any and all artifacts is the mantra, “Change the viewing angle.”. That may allow you to see around a calcified or prosthetic valve. Plus, if you see the same thing from a bunch of different angles, guess what? It’s really there! (Maybe. We live in an uncertain world.)


Artifacts change their appearance and appear or disappear depending on the view.


Real structures will remain constant and can be seen in multiple views.



(See also CFD artifacts, pitfalls, and masses.)


Acoustic imaging assumptions (to assign the location and intensity of received echoes):



image Reflections arise only from structures along the beam’s main axis. Echoes detected originated from within the main ultrasound beam. Problem: side lobes, grating lobes.


image Sound travels directly to a reflector and back (straight line). Problem: refraction.


image Echo returns to transducer after a single reflection. Problem: mirror images, reverberations.


image Sound (sound beam and its echo) travels in a straight line.


image Sound travels exactly at 1540 m/sec (constant, average speed → time is distance). Problem: when the propagation speed errors occurs, reflectors are placed in improper positions or at incorrect depth (time is not equal to distance).


image The depth of an object is directly related to the amount of time for an ultrasound pulse to return to the transducer as an echo (round trip). The length of time for a single round trip of an echo is related only to the distance traveled by the echo.


image Reflection’s intensity (strength) is related to tissue characteristics. Problem: attenuation.


image Imaging plane is extremely thin. Problem: ultrasound beam has some thickness in the perpendicular beam plane, reflections from structures above or below a target reflector may be placed with the target reflector on the display screen.


image Acoustic energy in an ultrasound field is uniformly attenuated.



Artifacts





Artifacts Associated with US Beam Characteristics


US beam exits transducer as a complex 3-D bowtie shape (below figure) with additional off-axis low-energy beams (side lobes and grating lobes). Strong reflector (highly reflective object) located outside the main US beam (peripheral field) may generate detectable echoes that will be displayed as having originated from within the main beam. Misplaced echoes overlapping anechoic structure. Adjust focal zone to the level of interest and place transducer at the center of the object of interest.



image


Beam width: distal beam (widened beam) widens beyond the actual transducer width (below figures).



image



image


Side lobe: multiple beams of low-amplitude ultrasound energy projecting radially (off axis) from the main beam axis (from the edges of the transducer elements).


Multiple beams encountering same object duplicate the structure (false targets), generating artifactual echoes of the true reflector, at its correct depth but laterally from the true anatomy (the system assumes they originated from the main beam).


Weaker than the main beam, less acoustic energy, weaker returning echoes, many are not actually seen (overshadowed by true echoes). Evident when they do not conflict with real echoes or originate from strong reflectors.


If the beam is oscillating rapidly, the multiple artifactual echos produced by the side lobes are displayed as a curved line at the same level as the true object.


Options: reduce power (below figures).



image



image


Edge shadow: shadowing as a result of refraction at the edge of a circular structure (dropout phenomena). US beam intercepting tangentially (poorly imaged around 10 and 2 o’clock).


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Jun 4, 2016 | Posted by in CARDIOLOGY | Comments Off on Artifacts and Pitfalls

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