Download
1 / 96
990 likes | 1.35k Vues
A r t i f a c t s. Artifacts. - are echoes that appear on the image but do not have a true correspondence to an anatomical structure. It is important to recognize them so that they may be ignored, eliminated or made useful. Artifacts. Not real Missing information Improperly represented
E N D
Artifacts - are echoes that appear on the image but do not have a true correspondence to an anatomical structure. It is important to recognize them so that they may be ignored, eliminated or made useful.
Artifacts • Not real • Missing information • Improperly represented • Location • Size • Brightness • Shape
Causes of Artifacts: • Ultrasound equipment assumptions • Equipment malfunction or design • Operator assumptions and/or errors
Ultrasound Equipment Assumptions • Sound travels in a straight line • Reflections are produced only by structures along the main axis of the sound beam • Intensity of an echo corresponds to a reflector’s scattering strength • Sound travels directly to & from a reflector • Imaging plane is thin • Sound travels at exactly 1540 m/sec
Effects of Artifacts • Measurement errors can occur because the equipment is calibrated to 1540 m/s (range ambiguity) • Interpretation errors can occur due to artifacts (localizing, cyst vs. solid, turbulence) • Aids in determining cyst vs. solid, (enhancement & shadowing)
Artifacts Associated with Resolution • Axial resolution • Lateral resolution • Acoustic speckle • Section thickness
Axial Resolution - the ability to distinguish 2 structures that are in close proximity to each other from anterior to posterior • Multiple structures along the main axis of the beam appear only as 1 reflector on the image • Contributes to incorrect representation of size & shape of interfaces & to missing interfaces
Lateral Resolution • resolution is the minimum distance of 2 side-by-side structures that can be separated & still produce 2 distinct echoes • can contribute to incorrect representation of size & shape of interfaces & to missing interfaces
Acoustic Speckle - is produced by wavelet interference resulting in image degradation • Appears as grainy images & spectral displays • Dominates near the face of the transducer • Interferes with the ability of the system to detect low-contrast objects
Section Thickness • AKA slice thickness, out-of-plane focusing, elevational resolution, or width focusing • is the thickness of the scanned tissue volume perpendicular to the scan plane • Extra echoes appear in the image making a structure more echogenic than it really is
Artifacts Associated with Propagation • Reverberation • Mirror image artifact • Multipath artifact • Refraction artifacts • Side lobes • Range ambiguity • Propagation Speed Error Artifacts
Reverberation Artifacts • appear as ‘echoes’ of echoes • A portion of the sound beam reverberates between 2 highly reflected surfaces sending back multiple echoes from the same 2 interfaces • Appear as multiple, equally-spaced reflections on the image, decreasing in intensity; however somewhat compensated by the gain control • Only the first two reflections closest to the transducer are real
Comet-tail - is a form of reverberation artifact caused by 2 closely spaced strong reflectors in a soft tissue medium with a high propagation speed (surgical clips) & appear as multiple small bands
Ring-down Artifact - is another type of reverberation that is produced by small gas bubbles, such as air & appear as a single, long, strong echo behind the reflector
Mirror Image Artifact - occur when a structure is located in front of a highly reflective surface (i.e.. pleura, diaphragm,& bowel) causing the anatomy to be reproduced or duplicated on the other side of the interface • Reflector & object (true & false image) are equidistant from strong reflector • Mirror image duplicate always appears deeper than the true anatomic structure
Multipath Artifact - occurs when the beam strikes an interface at an angle & is reflected from a 2nd (or 3rd) interface before being reflected to the transducer resulting in incorrect axial location of an interface due to time in reaching the receiver
Refraction Artifacts - are caused be refracted reflections to appear in improper locations • AKA - ghost image artifact • Appears as a replication of the anatomical region to be placed side-by side to the real anatomy
Edge Refraction - produces shadowing at the edges of structures that are large compared to the width of the ultrasound beam because the refracted beam diverged to much & do not appear as echoes on the image from the area they would have demonstrated reflections from. Region beyond the refraction appear anechoic
Side Lobes • off the main axis of single crystal transducers • can introduce positioning artifacts of highly reflecting structures appearing as a duplicate of the true reflector lateral to & at the same depth as the anatomy • Array transducers produce off-axis grating lobes (these beams are stronger than side lobes) that can also cause reflectors to be displayed in improper locations. • Sub-dicing has for most part eliminated this artifact
Range Ambiguity Artifact - is the misplacement of an interface when the assumption “each echo is derived from the most recent pulse” is violated The structure is placed closer to the surface than it should be
Propagation Speed Error Artifacts • If the speed of sound > 1540 m/s, reflector is placed too close to the transducer • If the speed of sound < 1540 m/s, reflector is placed too far from the transducer. Example: lesion in the posterior liver may cause the diaphragm posterior to the lesion to be displaced too far from the transducer
Artifacts Associated with Attenuation • Acoustic shadowing • Enhancement
Acoustic Shadowing - the absence or reduction of echo intensity distal to a reflector • caused by the sound beam passing through a highly attenuating structure (i.e. calcium, calcified plaque) • useful in diagnosing, e.g., cholelithiasis • harmful by not demonstrating disease due to lack of echo reflection
Enhancement - when the sound beam passing through an area of very low attenuation (i.e. fluid) causes an increase in echo brightness distal to the structure, especially when compared to the same type of tissue on either side True tissue enhancement is useful in distinguishing cystic from solid masses
Horizontal Enhancement • horizontal banding produced from focusing • due to the increased intensity of the beam in the focal zone Adjust the TGC to correct this artifact
Banding Artifact - can be produced by improper TGC settings Adjust the TGC to correct this artifact
Artifacts Associated with Doppler & Color Flow Instrumentation • Aliasing • Flash artifact • Color bleed • Color noise
Aliasing - occurs when the Doppler shift frequency exceeds ½ PRF Correct by the PRF or the Doppler shift frequency
Flash Artifact - the sudden burst of color that encompasses the frame caused by anatomical motion changing the interface position Can be suppressed by the color filter, the persistence & reducing the width of the color field of view
Color Bleed - the extension of color beyond the region of flow to the adjacent tissue Eliminate by the transmit power& color gain
Color Noise - is random variation in signal detection causing areas with no flow to be color encoded Eliminate by the transmit power& color gain
Equipment Artifacts Electronic noise (from the equipment itself or other electrical equipment) may cause low-level echoes to appear as vibrating vertical bands on the image filling in cystic structures and affect contrast resolution.
