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Diagnostic ultrasound ( PDFDrive )

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APPENDIX

Ultrasound Artifacts: A Virtual Chapter

or comet-tail occur. And if the direction of the beam or its echo

is altered, refraction or anisotropy artifact may be produced.

Beam Proile

An assumption is made that the sound beam generated by the

transducer is a narrow line. When the beam is not suiciency

narrow in the imaging plane or in the elevation plane (i.e., along

the short axis of the transducer), side lobe or grating lobe or

partial volume averaging artifacts may occur.

PROPAGATION VELOCITY ARTIFACT

large structure composed of tissue that propagates speed at a

diferent velocity is encountered, then propagation velocity artifact

can occur. 1 In the case of a fatty lesion, the slower speed of

propagation (approximately 1450 m/sec) means that the echo

will take longer to return to the transducer; thus the lesion is

displayed deeper in the image than its true location. Some newer

ultrasound machines with multibeam (spatial compounding)

features and improved signal processing can minimize this

artifact.

Click here to see an explanatory video of propagation velocity

artifact (Video A.1).

Ultrasound image processing assumes that the speed of sound

in tissue is a constant 1540 m/sec. However, when a suiciently

Air

Fat

Water

Soft tissue

(average)

Liver

Kidney

Blood

Muscle

Bone

330

1450

1480

1540

1550

1560

1570

1580

4080

1400 1500 1600 1700 1800

Propagation velocity (meters/second)

FIG. A.1 Propagation Velocity. Propagation velocity of different

body tissues. 2 (See Chapter 1, Fig. 1.2.)

FIG. A.2 Propagation Velocity. When sound passes through a fatty mass, in this case a myelolipoma, its speed slows down to 1450 m/sec.

Because the ultrasound scanner assumes that sound is being propagated at the average velocity of 1540 m/sec, the delay in echo return is

interpreted as indicating a deeper target. Therefore the inal image shows a misregistration artifact in which the diaphragm (arrow) is shown in

a deeper position than expected in this patient with a right adrenal myelolipoma. The computed tomography image depicts the myelolipoma, conirming

its fatty nature.

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