Ultrasound Physics and Instrumentation Lecture Notes 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 (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 Ultrasound beam artefact Ultrasound beam artefact occurs based on the premise that: Echoes arise only from structures within the main ultrasound beam The imaging plane is thin (laterally and elevationally) Ultrasound beam artefact (cont) In reality The imaging plane has a third dimension (elevational plane) and the beam has variable width. Echoes can be captured from structure outside the main ultrasound beam (side lobe/ grating lobes). Types of artefacts: Side lobes Grating lobe Beam width artefact Slice thickness artefact  Side lobes/ Grating lobes Occurs when the low energy ultrasound beam is sent out of the transducer at the same time as the main ultrasound beam. Echoes are generated by side lobes just as by the main lobe, but all are assumed to have arisen from the central axis. Results in structures outside the main ultrasound beam being mapped into the main ultrasound beam. Side lobes/ Grating lobes (cont) The machine displays these artifactual echoes as if they had originated within the main ultrasound beam. Causes false echoes to be displayed within the structures on the ultrasound images (echoes within the bladder, gallbladder, cysts, vessels). Echoes generated by these low energy side beam are weak compared to main beam. Multiple falsely displayed echoes within a cystic lesion (blue and white arrow). References: (Cluj-Napoca/RO)  Beam width artefact Related to lateral resolution. Refers to the ability to discriminate 2 closely spaced points at the same depth within the imaging plane. An ultrasound beam begins as the same width as a transducer and gradually narrows in the near filed. The narrowest region is the focal zone with the focus being the smallest width within that region. After the focal zone the ultrasound beam diverges – far filed Beam width artefact (cont) Lateral resolution is limited by the beam width, which varies with depth and is narrowest at the focal zone. Lateral resolution is optimal at the focal zone and deteriorates in the near and far field. Beam width artefact refers to the lateral blurring of a point target that occurs as echoes from the same target are insonated at adjacent beam positions. Beam width artefact (cont) If two objects are separated by a distance less that the beam width, they will appear as one. Beam width decreases as the wavelength decreases; higher frequency has lower transducer has a narrower beam width In the near and far fields, the beam width is wide and will register a target at multiple overlapping beam positions. Thus, two targets appear as one. They may be resolved at focal distance. Slice thickness artefacts Related to elevational resolution (height of the beam). The ultrasound beam is not thin, it has 3 dimensional: Lateral (width) Axial (length Elevational (height) Slice thickness artefacts (cont) The height of the ultrasound beam is determined by the transducer. Structures above or below the beam can mapped into the main ultrasound beam. Results in artifactual echoes within anechoic structures (cyst, bladder, vessels) (a)An object (red bars), at different apparent depths adjacent to the target (ellipses at differing apparent depth) will be superimposed on the target in the near and far fields. (b)Transverse ultrasound image of a normal bladder shows artifactual echoes (arrow) that appear to be within the bladder from an adjacent bowel loops. A side lobe may have also contributed to these echoes. Velocity error artefacts Velocity error artefacts occur based on the premise that sound always: Travels in a straight line Travels at a constant speed in soft tissue (1540m/s) In reality: May travel slower or faster than 1540 m/s depending on the tissue type Does not always travel in a straight line(sound may be refracted or may approach a boundary at an oblique incidence). Types of velocity error artefacts: Propagation speed artefact (speed of sound artefacts) Refraction Edge shadowing Range ambiguity Propagation speed artefact (Speed of sound artefacts) This artefact occurs when the machine assumes that the speed of sound in sift tissue 1540 m/s. When sound travels through a medium with a propagation speed slower than 1540 m/s, it take longer for the echoes to return to the transducer. Echoes placed deeper on the image than their actual; depth. If the speed of sound is greater, the echo will appear closer. Propagation speed artefact (Speed of sound artefacts) (cont) Boundary distortion (speed displacement artifact) occurs when a portion of the beam encounters a region of differential velocity superficial to a smooth interface, giving rise to a distorted appearance of the interface. This is commonly encountered when imaging the liver through a region of focal fat, where the portion of the beam traversing the fat takes longer to return to the transducer. Refraction Ultrasound machine assumes that sound travels in a straight line. A wave changes direction at an interface between mediums having different speeds of sound. If the incident beam is perpendicular to the boundary, no refraction will occur. As the angle of incidence increases and as the difference in the speeds of sound is greater, more refraction will occur. Refraction (cont) Results on echoes being placed to the side of their true location. Refraction needs two things in order to occur: Different propagation speed between two tissue types Oblique incidence Edge shadowing When a sound wave encounters the edge of a curved structure, the wave refracts. This is often observed at the lateral edges of a structure such as a cyst or soft-tissue mass and appears as hypoechoic parallel lines projecting distal to the edges of the structure. Best visualized when enhancement is present As the angle of the boundary increases, the ultrasound beam both reflects and refracts off the surface. The intensity of the beam reaching the tissue immediately distal to the edge of the curve is therefore decreased and appears as a shadow. The hypoechoic bands representing edge shadowing artefact (blue arrow), at the margins of a cystic lesion Reference: (Cluj-Napoca/RO) Range ambiguity Range ambiguity artifact results when the assumption that all returning echoes are generated by the most recently transmitted pulse is violated. Depth is assigned based on the time interval between the transmitted pulse and the received echo. Range ambiguity (cont) If a distant echo of a deep structure from an initial pulse is received after a second pulse is generated, the time delay will be counted from the second pulse emission instead of the first pulse emission. Therefore, the distance will be misregistered as closer to the transducer than it actually is. Thank you