Ultrasound Artifacts

ULTRASOUND ARTIFACT

ULTRASOUND PHYSICS LECTURE 4

ARTIFACTS

DEFINITION

In ultrasound imaging, an artifact is an appearance on the image which does not faithfully represent the structures present in the subject being scanned.

Therefore the echoes are a false or misleading image which need to be properly understood and interpreted.

Artifacts may appear as structures seen in the image which do not exist in the subject, or structures which should be seen in the image but are not. Structures may also be represented in the wrong location in the image ( mis -registration ).

Artifacts may be caused by improper function or use of equipment, or they may simply be a characteristic of the physical properties of ultrasound waves and their interaction with tissues.

COMMON ARTIFACTS

• Reverberation
• Acoustic shadowing
• Acoustic enhancement
• Edge shadowing
• Beam width artifact
• Slice thickness artifact
• Side lobe artifacts
• Mirror image artifacts
• Double image

Equipment generated artifacts

REVERBERATION

Reverberation is the production of false echoes due to repeated reflections back and forth between the transducer face and a strong reflector in the subject.

The first returning echo is displayed correctly, but some of the returning sound bounces off the face of the transducer back into the subject like a weak transmitted pulse. This returning pulse meets the strong reflector again causing another returning echo from the same interface.

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 results in a series of equally spaced false echoes to be displayed on the ultrasound image.

REVERBERRATION

REVERBERATION

The bouncing of the sound back and forth (reverberation) often occurs between the transducer face and the subcutaneous fat / muscle layer.

It also commonly occurs when the sound waves reverberate between two anatomic reflecting surfaces in the subject, such as between the near and far surface of an area of gas. In this case the echoes are placed much closer together in the image, and the appearance is termed comet tail.

Small gas bubbles create an almost continuous band of reverberation called Ring down

FORMATION OF COMET TAIL ARTIFACT

COMET TAIL ARTIFACT

RING DOWN ARTIFACT

ACOUSTIC SHADOWING

Acoustic shadowing appears as an area of low level echoes or no echoes at all distal to an interface with a large acoustic mismatch. It is caused by severe attenuation of the beam at the interface.

This will occur where the beam passes from soft tissue to gas (99.9 % of the beam is reflected) It also occurs where the beam passes from soft tissue to bone (40% reflection and 60% absorption by the bone)

ACOUSTIC SHADOWING

GAS OR STONES?

ACOUSTIC ENHANCEMENT

This artifact appears as an area of increased echo brightness distal to an area of low attenuation.

Enhancement is normally seen distal to structures containing fluid such as the urinary bladder, gall bladder, or cyst.

Time –gain compensation is applied equally to areas of the subject with minimal attenuation such as a cyst, and to adjacent areas of tissue.

Therefore echoes from within the cyst are increased by the TGC unnecessarily resulting in a distal brightness.

ACOUSTIC ENHANCEMENT

EDGE SHADOWING

When the sound beam meets a curved interface both refraction and reflection occur which results in an edge shadowing artifact.

As the ultrasound beam meets the curved surface, some is reflected away from its true course and some continues into the second medium, but is refracted or bent from its original course This results in an acoustic shadow distal to the curved edges of the mass.

EDGE SHADOWING

BEAM WIDTH ARTIFACT

The beam width can vary depending upon focusing, divergence etc.

Since the width of the beam is not always as small as the objects being imaged, echoes arising from the edge of the beam will be displayed as having arisen from the beam centre.

A small reflector will generate an echo for as long as it remains in the beam and so will be represented as a line in the display rather than a dot.

Some echoes from outside the assumed plane will be displayed as if from within the plane.

We may for example, see some echoes arising from outside the bladder but displayed within the bladder

BEAM WIDTH ARTIFACT

SLICE THICKNESS ARTIFACT

This is similar because it is related to the size of the ultrasound bream, and creates false echoes in areas that should be echo free.

The ultrasound machine assumes the echoes received by the transducer are from a very thin slice of tissue. However, an image is actually composed from several slices, and therefore echoes from slices either side of the intended slice may be displayed in the image.

This is often seen in transverse views of the urinary bladder. Structures adjacent to the slice through the bladder will be included within the bladder.

SLICE THICKNESS ARTIFACT

SIDE LOBE ARTIFACT

As well as the main beam used to form the ultrasound image, the transducer also produces many smaller and weaker side lobes. These are not normally used to form the primary image, but they can cause artifacts These also cause echoes to be displayed in areas which should be echo free. This is more apparent when there are very strong reflectors , such as gas, adjacent to an echo free area. This can give a false impression of a septation within the urinary bladder or gall bladder.

SIDE LOBE ARTIFACT

MIRROR IMAGE ARTIFACT

Some large smooth reflectors, such as the diaphragm, return such a strong echo to the transducer that they are read by the machine as part of the normal returning echoes.

As these sound waves are attenuated for a second time on their return through the body, they create a mirror image.

These echoes are displayed as if coming from beyond the strong reflecting surface

MIRROR IMAGE ARTIFACT

AIR IN THE RECTUM CREATING A MIRROR IMAGE OF THE URINARY BLADDER

MIRROR IMAGE ARTIFACT OF LIVER

DIAPHRAGM CAUSING MIRROR IMAGE ARTIFACT OF LIVER

DOUBLE IMAGE ARTIFACT

This artifact occurs when refraction of the ultrasound beam is caused in areas such as the rectus abdominis muscle of the anterior abdominal wall.

The muscles act as a lens, dividing the beam into two and creating a double image of a single object.

DOUBLE IMAGE ARTIFACT

DOUBLE IMAGE ARTIFACT: of aorta and a gestation sac

EQUIPMENT GENERATED ARTIFACTS

Incorrect use of the controls of the ultrasound machine can lead to the formation of various artifacts in the images.

In particular, incorrect use of the gain and TGC controls causes areas of incorrect brightness or darkness in the image.

Too much gain or TGC may fill in cystic or fluid filled areas by causing echoes to appear where they should not be.

Too little gain or TGC may under fill some areas and give a false impression of a cystic or anechoic area.

INCORRECT USE OF GAIN CONTROL

INCORRECT USE OF TGC