Artifacts

Chapter 9 Artifacts

LEARNING OBJECTIVES

1. List the basic assumptions made by ultrasound equipment which gives rise to artifacts.

2. Describe the appearances of the more common artifacts encountered during ultrasound scanning.

3. Explain the causes of the commonly arising artifacts.

An ultrasound artifact is a structure in an image which does not directly correlate with actual tissue being scanned. Artifacts assume different forms, including:

• structures in the image that are not actually present

• objects that should be represented but are missing from the image

• structures which are misregistered on the image.

Where an operator is not aware of the presence of artifacts, it is possible that pathology will not be demonstrated and will therefore not be diagnosed (false negative result), or that pathology will be diagnosed where it does not exist (false positive result).

Understanding the basic mechanisms behind ultrasound artifacts, recognizing situations in which they are likely to arise, and becoming familiar with their appearance all help to eliminate misdiagnoses that may otherwise occur.

ASSUMPTIONS MADE BY ULTRASOUND EQUIPMENT

Ultrasound machines have to make certain assumptions in order to operate. These are:

• The speed of sound propagation is the same in all tissue types (usually equipment assumes this to be equal to 1540 m/s)

• The ultrasound beam travels in straight lines

• The time taken after emission of a pulse for an echo to return to the transducer from an interface is directly related to the distance of the interface from the transducer

• Attenuation of sound in tissue is uniform

• All echoes detected by the transducer have arisen from the central axis of the beam

• An echo generated at an interface returns directly in a straight line to the transducer without generating any secondary echoes

• The intensity or strength of the returning echo is proportional to the density of the reflector generating the echo.

These assumptions are often incorrect and will give rise to appearances which do not correspond to actual anatomy or tissue in the scan plane. It is these appearances that we term artifacts.

Several possible artifactual appearances may occur:

• There may be areas incorrectly showing absence of echoes

• Echoes may appear in areas which do not represent actual interfaces

• Separate structures may appear joined

• A single structure may appear as separate structures

• The brightness or textural appearance of echoes may not correlate with actual tissue structure

• Thin structures may appear thicker than reality or vice versa.

There are a multitude of different artifacts which may occur, but the more commonly occurring artifacts which will be discussed in this chapter are:

• reverberation

• acoustic shadowing

• acoustic enhancement

• edge shadowing

• beam width artifact

• slice thickness artifact

• side lobe artifacts

• mirror image

• double image

• equipment-generated artifacts.

REVERBERATION

This is the production of spurious (false) echoes due to repeated reflections between two interfaces with a high acoustic impedance mismatch. The echo from the interface is received by the transducer and displayed on the image. Some of the energy in the returned echo is reflected at the transducer face, and returns to the reflecting interface as if it was a weak transmitted pulse, returning as a second echo (reverberation). As the time taken for the second echo to arrive is twice that taken by the first echo, the image will display it at twice the depth. This sequence of reflection and transmission can occur many times, with the third echo taking three times as long to return to the transducer and being displayed at three times the depth, and so on. The reverberation echoes will be equally spaced because the time for each additional echo is a multiple of the time of return of the first echo. These reverberation echoes will be strong because of the high acoustic mismatch (see Fig. 9.1).

Fig. 9.1 Repeated reflections between two interfaces cause reverberation of the displayed echoes

This artifact will often be seen:

• at the skin–transducer interface when the sound reverberates between the subcutaneous fat/muscle layer and the transducer (see Figs 9.2 and 9.3)

• behind bowel gas when the sound reverberates between the gas surface and the transducer (also known as ringdown) (see Fig. 9.4).

Fig. 9.2 Reverberation occurring at the skin–transducer interface

Fig. 9.3 Reverberation arising at the skin–transducer interface and appearing in the bladder

Fig. 9.4 Reverberation occurring between bowel gas and transducer, appearing in the image distal to the bowel gas, and known as ringdown

The artifact can be differentiated from real echoes due to the lack of breathing movement occurring.

To help to eliminate this artifact it may be useful to:

• increase the amount of gel used

• use a stand-off pad (such as a commercially produced gel pad or a bag of saline)

• reduce the gain

• move the position of the transducer.

ACOUSTIC SHADOWING

This appears as an area of low amplitude echoes (hypoechoic or anechoic) behind an area of strongly attenuating tissue. It is caused by severe attenuation of the beam at an interface, resulting in very little sound being transmitted beyond (see Fig. 9.5). The attenuation can be due to either absorption or reflection of the sound waves, or a combination of the two.

Fig. 9.5 Acoustic shadowing posterior to a strongly attenuating gallstone

Acoustic shadowing will occur at interfaces with a large acoustic mismatch such as:

• soft tissue and gas (where the degree of attenuation will be due to reflection of 99.9% of the beam)

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Tags: Ultrasound Physics and Technology How, Why and When

Mar 10, 2016 | Posted by admin in ULTRASONOGRAPHY | Comments Off

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