Artifacts Artifacts Chapter 20 What are they? Simply put they are an error in imaging These artifacts include reflections that are: not real incorrect shape, size or position incorrect brightness displayed Causes Violations of assumptions Electronic malfunctions or poor design of equipment Operator error The physics involved in ultrasound imaging Common terms used to describe US images Hypoechoic: areas in the image are not as bright as the surrounding structures Hyperechoic: areas in the image are brighter than the surrounding structures. Anechoic: as the prefix implies this is an area that is completely echo free Common terms used to describe US images Isoechoic: sometimes the hardest to distinguish as it has equal brightness to the surrounding structures Homogenous: this describes the area as having an even texture through the image. Heterogeneous: this describes the area as having an uneven texture or echo pattern through the image. Imaging assumptions Most imaging artifacts can be explained by the violation of basic imaging assumptions sound travels in a straight line while traveling in a straight line it also returns in a straight line after it interacts with the reflector all sound travels at 1,540 m/s in soft tissue 1
Imaging assumptions Reflections only come from structure in the center of the imaging beam axis. The imaging plane we use is thin. the characteristics or brightness of the tissue that creates the reflection is related to the strength reflection. Imaging assumptions Artifacts will appear and disappear as we image but actual anatomical structures will not by changing angle, transducer position or patient position the operator can make most artifacts disappear Reverberation Displayed as multiple echo that are equally spaced in depth the result of 2 strong reflectors positioned parallel to the beam this artifact places to many reflection in the image that does not correlate with true anatomical structures. Reverberation what are their characteristics Most always appear in multiples are equally spaced (depth wise) their location is parallel to the axis of the main beam each additional artifact is located deeper the the previous Comet Tail A form of reverberation in which the separations are indistinguishable this creates a hyperechoic line deep to the offending structure some call this ring down occurs when the reflecting structure is located in a medium that has a high propagation speed Can be created by the resonance of small structures like gas bubbles appears as a single hyperechoic echogenic shadow always parallel to the beam main axis Comet Tail 2
Hypoechoic or anechoic region of the image distal to a strong attenuation Gallstones produce this shadow as well as ribs. This is produced when sound is unable to penetrate a structure Shadowing Shadowing characterized by Resulting from too much attenuation hypoechoic or anechoic always located deep to the attenuator precludes visualization of anatomical structures deep to the attenuator A type of artifact that appears at the edge of a curved reflector creating a hypoechoic shadowing deed to the interface prevents the display of true anatomical structures within this shadowing region Edge Shadowing Edge Shadowing How is this caused? The sound beam refracts at intersection of the primary beam and the edge of the curved surface. At the same time it diverges causing a dropout in signal intensity Some call this shadowing by refraction I prefer to think of it a critical angle problem Edge Shadowing characteristics The region is always hypoechoic or anechoic. Results from the bean striking the curved surface a veering off. Extends parallel to the beam and deep to the reflectors edge prevents imaging of true anatomy in this shadow region Appears as a hyperechoic region deep to a low attenuator the reflectors in the region are actually normal they just appear overly bright Enhancement 3
Enhancement characteristics Hyperechoic region deep to a most commonly fluid filled region. Caused by low attenuation in the cyst Always located deep to the low attenuating structure Focal Enhancement This is a form of enhancement that at the same depth in the pie shaped image also termed focal banding can appear as an incorrect TGC setting (one pod too high) Focal Enhancement Prominent in the beams focal zone as the intensity in the focal zone increase so does the intensity of the reflections Projecting a brighter image than the tissues located at different depths characterized by side to side hyperechoic region of the image. As a result of increased intensity in the focal region Created when sound reflects from a strong reflector and the beam is redirected to a different area. This results in a replica of the real image in an incorrect position Mirror Image Mirror Image The erroneous image is located deeper than the real structure. The mirror is always located is a straight line between the transducer and the artifact. This condition occurs with gray scale and color imaging. The real anatomical structure is the shallowest of the two. Mirror Image Characteristics A copy of a true reflector The copy is deeper then the true anatomy The mirror (bright reflector) is located in a straight path between the artifact and the transducer The artifact and real image are equal distant from the mirror 4
It is a mirror artifact from a spectral Doppler display Crosstalk Speed errors There are a correct number of reflectors But they are located at improper depths The system makes an assumption that the speed of the sound is 1.54km/s in soft tissue. When there is a variation in speed portions of the image can be split Speed errors When one medium is faster that soft tissue The sound travels faster than the system assumes The pulses round trip is faster The go-return time is to short The system assumes the reflectors are closer to the transducer than they are Distance is underestimated Structures are placed too shallow in the image Speed errors When the medium is slower than soft tissue The sound travels slower than the system assumes The pulses round trip is slow The go-return time is too long The system assumes the reflectors are further from the transducer than they are Distance is overestimated Structures are placed too deep in the image Speed errors These are propagation speed errors Can also be referred to as Range error artifacts Range ambiguity artifact Lobe artifacts Degrades lateral resolution Occurs when sound energy is transmitted in a directions other than them sound beams primary axis The lobes are weak and seldom create a reflection error that would appear on the image If however a strong reflector is in their path the system will recognize it and assume it arose form a structure in the beams primary path and position it as such 5
Lobe artifacts These lobes are a copy of a true reflector located side by side at the same depth There are 2 types of lobes side lobes and grating lobes Side lobes are created from single PZT transducers such as a mechanical sector probe Lobe artifacts Grating lobes are created by array transducers. Reduction of grating lobe artifacts can be accomplished by subdividing each PZT into smaller elements (subdicing) As you may recall from earlier chapters variations in electronic pulse energies to each PZT can further reduce this problem. A process called apodization Results when the sound beam changes direction during transmission. As the sound strikes a boundary obliquely and the medial on either side has differing propagation speeds the sound beam bends Refraction Refraction Characterized by a second copy of the reflector They are located at the same depth and of course side by side Slice thickness artifacts Traditional systems present anatomy in a two dimensional display. Sonographers erroneously believe that our image is thin when in fact is neither thin nor is it uniformly thick Slice thickness This error relates to the perpendicular dimension of the sound beam. The thickness of this plane is the elevation resolution This appears when the beam dimension is greater than the reflector size This artifact fills in structure that should be cystic Reduced by 1½ dimensional array transducers 6
Lateral resolution Axial resolution Occurs when the beam is wider that the distance between two side by side reflectors. Can result in two smaller reflectors being displayed as one large reflector Least likely to occur at the narrowest focus Created by long pulses striking two closely spaced objects in the beams axis Only one will appear if the they are spaced closer than ½ the spatial pulse length apart Transducers with short pulses, high frequencies and less ringdown minimize this artifact Place photo here Multipath artifacts Created when sound reflects off a second structure on the way to or from the primary reflector This results in an increase go-return time creating nonspecific image changes that are difficult to identify Curved and Oblique reflectors Occurs when the primary beam intersects obliquely with a reflector. This oblique interaction redirects some of the sounds intensity away from the return path. This results in a return echo amplitude that is weaker that is to be expected. Curved and Oblique reflectors This results in absent image quality on the image Appearance of a weak image Images that are not the same as other similar reflecting boundaries Temporal Resolution Artifacts From previous discussion we know temporal resolution is related to frame rate So the best resolution is one with the highest frame rate If the FPS is low there will be less accurate positioning of moving reflectors. 7
Spatial Resolution This is the detail of the overall image It is related to line density or spacing High line densities created great detail Low line density give poorer detail with inferior spatial resolution Analog displays are related to the number of TV scan lines per frame the greater number of line the greater the spatial resolution Digital displays deal with pixel density the higher the pixel densities result in smaller pixels Smaller pixels result in better spatial resolution CRT displays Noise results from small amplitude echo from a myriad of sources Electronic interference (60 cycle interference) Signal processing Spurious reflectors Affects hypoechoic regions rather than hyperechoic region Noise Artifacts Speckle Formed when small amplitude waves interfere with each other Appears as tissue texture close to the transducer that is not related to true anatomical structures Clutter is the noise associated with Doppler imaging Harmonics New technology that reduces noise content in the image Used to improve the signal to noise ratio distinguishing meaningful reflection from unwanted ones 8