Physics of Ultrasound learing notes - Impedance and Artefacts

Physics of Ultrasound - Artefacts - BATS (an acronym for "Better Anaesthesia Through Sonography")

http://www.bats.ac.nz/resources/physics.php

The ultrasound machine makes various assumptions in generating an image. These include -

The ultrasound beam only travels in a straight line with a constant rate of attenuation

The speed of sound in all body tissues is 1540m/s

The ultrasound beam is infinitely thin with all echoes originating from its central axis

The depth of a reflector is accurately determined by the time taken for sound to travel from the transducer to the reflector and return

Reverberation Artefact

This occurs when ultrasound is repeatedly reflected between two highly reflective surfaces. If for example, the transducer acts as another reflective surface to a returning echo, this echo will then be re-reflected and retrace itself resulting in an artefactual image identical, but at twice the distance from the transducer, as the real image. Due to the process of attenuation, each subsequent echo is weaker than the first.

A is the true anatomical structure strongly reflective of ultrasound

A1 is the artefact generated by the returning ultrasound beam being re-reflected from the transducer and again reflected from the true anatomical structure at A.
As this doubly reflected beam has travelled twice as far it is seen at twice the distance from the transducer. It is weaker but otherwise identical to the real structure at A

Mirror Image Artefact

This is a type of reverberation artefact occurring at highly reflective air / fluid interfaces such as the lung and descending aorta. The first image is displayed in the correct position while a false image is produced on the other side of the reflector due to its mirror like effect.

Side Lobe Artefacts

Side lobe beams are generated from the edges of the transducer element and project in a different direction from the main beam. These echoes are much weaker than those of the main beam but if a very strong reflector is encountered they may be strong enough on returning to the transducer to be displayed prominently on the image. Because any returning echoes are always assumed by the machine to have been generated from the main beam, their position on the display is incorrect (although at the right depth as time taken to and from transducer is the same. Multiple side lobe echoes as from a rapidly oscillating beam, are displayed as a curved line equidistant from the transducer along its length.

The two most important features that distinguish a side lobe artefact from an anatomical structure are that it is equidistant to the transducer along its length and it passes through anatomical structures.

Multipath Artefact

This arises when the path of the ultrasound beam to and from a reflector is different. Part of the original echo returns to the transducer while part is reflected off a second interface before returning. As the second echo takes longer to return it is displayed deeper, but along the same line as the first.

Beamwidth Artefacts

We assume the ultrasound beam is razor thin but in fact it has a measurable thickness. Any echoes generated from the beam's edge will be displayed as though they have arisen from its centre. This is most apparent when most of the beam travels through fluid but part of the beam interacts with adjacent soft tissue. Echoes from this interface will be displayed as having arisen from inside the chamber. The intensity of the artefact decreases with distance.

Propagation Speed Errors

Occur when the media through which the ultrasound beam passes does not propagate at 1540 m/s resulting in these echoes appearing at incorrect depths on the display. These speed errors can make the image appear "split" or "cut."

Acoustic Shadowing

A shadow results when an ultrasound beam is unable to pass through an area deep to a strongly reflecting or attenuating structure. It occurs deep to a region of high acoustic impedance mismatch such as soft tissue / gas or soft tissue / calcification interfaces. It is seen for example, deep to areas of calcification and prosthetic material. Tissue dropout occurs due to poor beam penetration and may result in misdiagnoses of defects such as septal defect.

Incorrect Gain

Excess gain can create or obscure information and reduce lateral resolution. This is of great importance when attempting to planimeter calcified valves as increased gain will artificially reduce the valve area. Incorrect gain or focal zone setting may give an erroneous appearance of spontaneous echo contrast (SEC). True SEC can be distinguished by a "swirling" pattern as distinct from the grainy appearance of high gain settings.

Near Field Clutter

This is due to acoustic noise near the transducer, resulting from high amplitude oscillations of the piezoelectric elements, and can make distinguishing structures within 1cm of the transducer very difficult.

In general, artefacts change their appearance and appear or disappear depending on the view. Real structures will remain constant and can be seen in multiple views. By using appropriate settings, understanding the imaging principles and knowing echocardiographic anatomy, it should be possible to readily distinguish artefacts and pitfalls from real structures.

Reference

This article is based on "Ultrasound Physics," a chapter in the Westmead TOE Manual by Dr Lenore George, Consultant Anaesthetist, Westmead Hospital, Sydney.





Ultrasound Impedance - NDT

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/reflectiontransmission.htm



INTERACTION of ULTRASOUND WITH MATTER

http://www.isradiology.org/isr/docs_books/basic/Chapter3.pdf




Handheld impedance-controlled ultrasound probe

http://web.mit.edu/matthewg/www/projects/PROTOTYPE4.htm

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