Ultrasound Physics and Instrumentation Artifacts PDF

Ultrasound Physics and Instrumentation MRD535 Artefacts I By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Analyse numerical and visual data related to the physics and inst...

Ultrasound Physics and Instrumentation MRD535 Artefacts I By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Analyse numerical and visual data related to the physics and instrumentation in ultrasonography. (C4) Contents Justify the occurrence of artefacts in a given image in ultrasonography (PLO3, C5). Discuss underlying principles (straight narrow sound beams, simple reflection, constant sound speed). Explain different types of artefacts. Artefacts Artefacts in ultrasound is: an echo, or an reflection that does not correspond to an actual structure; or an echo that is missing; or An echo that is in the wrong location; or An echoes that are displaying the wrong characteristic (size, shape or brightness Artefacts (cont) 7 assumption about sound wave: Sound travel in straight line Sound travels into tissue, encounters a structure and travel back directly to the transducer Sound travel at constant speed in tissue 1540 m/s Echoes arise only from structures within the main ultrasound beam The imaging plane is thin The amount of time it takes an echo to return to the transducer determine the depth of a structure in the body Sound attenuates at an even rate in the tissue. Types of artefacts Ultrasound beam artefacts: Multiple echo artefacts: Reverberation Side lobes Comet tail Grating lobes Ring down Mirror image Beam width artefact Multipath Slice thickness artefact Speckle Velocity error artefacts: Propagation speed artefact Attenuation artefacts: Refraction Shadowing Edge shadowing Enhancement Range ambiguity Attenuation artefacts Attenuation artefacts occurs based on the premise that: Sound attenuates at an even rate in the tissue In reality Sound attenuates unevenly within the tissue (can attenuate differently within each separate structure in the tissue) Types of attenuation artefacts Enhancement Shadowing Enhancement Ultrasound beam encounters a structure (fluid-filled) that attenuates sound to a lesser extent (or not at all) than in the surrounding tissue. The TGC overcompensates through the fluid-filled structure causing deeper tissues to be brighter. Thus, the region behind a liquid-filled structure produces stronger echoes (bright) than are observed from adjacent tissues. Also known as increased through transmission/ posterior acoustic enhancement. The presence of acoustic enhancement aids in the identification of cystic masses Shadowing Refers to the reduction in echo strength distal to a highly attenuating or reflective object. Ultrasound beam encounter a structure that attenuates the sound to a greater extent than in the surrounding tissue. The distal area to these structures is interrogated with a beam of decrease intensity, and thus displayed signals appear to be reduced in brightness compared with the adjacent tissue. Results in a dark shadow posterior to the structure. Shadowing (cont) Shadowing can be useful clinically, often seen with calcification, bone, gas. Clean shadowing the energy is absorbed and is not available for reflections. Partial shadowing Appears as hypoechoic beam and occurs distal to a highly attenuating soft tissue (fat, small stone). Dirty shadowing Distal to a highly reflecting surface (gas). Red arrow = dirty shadowing White arrow = clean shadowing Green arrow = partial shadowing  Enhancement vs shadowing Attenuation is assumed to be uniform, and a baseline TGC is used to amplify echoes from deeper tissues. If a portion of tissue is less attenuating (Blue), the beam distal to this region will be more intense and the tissue deeper will appear bright owing to overcompensation from the TGC. If a portion of tissue is more attenuating or reflecting (Red), the beam distal to this region will be less intense and the tissue deeper will appear dark. Multiple echoes artefacts Multiple echo artifacts occurs based on the premise that: Sounds travels into the tissue, encounters a structure and travels directly back to the transducer. The amount of the time it takes an echo to return to the transducer determines the depth of a structure in the body. In reality: Scattering in the tissue can cause artifactual echoes Strong reflectors can cause sound to bounce between reflectors. Multiple echoes artefacts (cont) Types of multiple echo artefacts: Reverberation Comet tail Ring down Mirror image Multipath Speckle  Reverberation Caused by two highly reflective interfaces in parallel (such as a biopsy needle) Instead of the beam reflecting off a single surface and producing strong echo that returns to the transducer, the sound waves “bounce’ back and forth between the strong reflectors.  Reverberation (cont) The reflected echoes are interpreted as occurring at increasing depths since they take longer to be received by the transducer. Appears as a multiple bright parallel lines. Often caused by reflections between a highly reflective surface in the near field and the transducer itself, and anterior portion of a cyst. Comet tail Comet tail artefact is a subtype of reverberation artefact caused by highly reflective interfaces that are closely spaced that the individual echoes are not discernible. Attenuation of more delayed echoes results in a progressive decreased amplitude and width with increasing depth. Comet tail (cont) Appearing as a tapering echogenic triangle or cone distal to a strongly reflecting structure. Comet-tail artifacts arise from closely spaced interfaces that are not resolvable and decrease in width with depth. Ring down Ring down artifacts is similar to reverberation artifacts, resemble comet tail artifacts but the mechanism is different. Arise from resonant vibrations within trapped air bubbles. As sound wave encounters gas bubbles the sound wave excites the gas bubbles and causing them to vibrate. These resonant vibrations produce a continuous, sound wave transmitted back to the receiver, appearing as a streak or series of parallel bands deep to a focus of gas. Mirror image Mirror image artifacts result from the false assumption that an echo returns to the transducer after a single reflection. This is produced when the beam encounters a target after being reflected off a single strong specular reflector. Mirror image (cont) A portion of the beam is reflected from the target back along its transmitted course, again off the specular reflector and back to the transducer. The second image of the target is therefore generated along that path, deeper than the true location owing to the increased time to echo return An artefactual carbon copy structure to appear above, below or to the side of the strong reflector Mirror image (cont) The resultant image contains two lesions on opposite sides of the diaphragm. May occur adjacent to any strong reflector such as the pleura, bladder, and bowel. Multipath artefact Multipath artifacts result from additional reflections of a portion of the beam on the path to or from a primary reflector. The transmitted beam may encounter a primary reflector, reflect back but off axis, and then reflect off a second adjacent reflector along the path back to the transducer. This results in the object appearing to be slightly deeper than it is because of an increased path length. Speckle Speckle (acoustic interference) refers to the inherent granular appearance within tissues that results from interactions of the beam with small-scale interfaces that are about the size of a wavelength or smaller. Nonspecular reflectors scatter the beam in all directions Speckle Speckle is the grainy (dot-like) echoes on an ultrasound image. It gives tissue it’s textured look. Speckle appears as noise within the tissue, degrading.  Most modern ultrasound has Speckle Reduction Imaging (SRI), a control to increase or decrease the speckle seen on the ultrasound image. SRI making the ultrasound appears smoother on texture. Twinkle artefact Twinkle artefact is classified as color Doppler artefact. It occurs due to alternating colors on Doppler signal posterior to a reflective object (air/ calculus). Irregularities in the reflected ultrasound signal is intertretaed as rapid changes in flow. Thank you