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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5240_Библиотеки_им_академика_М_И_Перельмана.pdf
Figure 5.5: Another nice example of apparent duplication of the SMA
(left frame).
Figure 5.6: In this image the refraction artifact gives the impression
of a twin pregnancy by duplicating a single gestational sac.
Figure 5.7: The same patient as shown in the previous figure. The
probe has been moved and the apparent duplication has
disappeared.
Figure 5.8: The refraction artifact causes duplication of the yolk sac
(left image) giving the impression of a monochorionic twin
pregnancy. Repositioning of the transducer yields an improved view
showing a solitary yolk sac.
Figure 5.9: Partial ghosting of a Foley catheter with 3 copies of the
left lateral catheter wall.
Figure 5.10 (a): Duplication of the left hepatic vein branches is seen
in this image.
Figure 5.10 (b): Repositioning of the transducer laterally eliminates
the artifact.
Not a refraction ghost
Figure 5.11: Refraction ghosting usually only occurs in the abdomen
and is due to the double lens effect of the two rectus abdominis
muscles. There is no other anatomical region prone to this type of
refraction. This image shows true anatomical duplication of the small
saphenous vein.
5.2 Refraction: distortion and defocusing
Causative mechanisms
The bending of an ultrasound beam due to refraction can lead to
distortion and defocusing of parts of the image. For instance, if a
large anatomical structure spans parts of the image that are not
affected by refraction as well as parts of the image that are affected
by refraction, the structure may appear distorted.
Which system assumptions have been breached?
The assumption that the speed of sound is constant and the
assumption that the beam and echoes travel along a straight path.
Typical appearance
The appearance is variable, but in general the image appears
distorted, blurred or defocused.
Can this artifact be reduced, eliminated or circumvented?
Refraction artifacts can be successfully eliminated by using different
acoustic window. Often a very slight change in transducer position
and angulation can eliminate the artifact.
Is this artifact diagnostically useful?
No. This artifact is never useful.
Refraction - distortion and defocusing
Figure 5.12: Dual image showing the same 12 week fetal head.
Refraction defocusing is apparent in the left image with partial
refraction ghosting of the anterior skull edge. A slight lateral change
in transducer position resulted in the improved diagnostic image on
the right.
Refraction - distortion
Figure 5.13 (a): This image of a fetal femur shows a bowed femoral
contour suggestive of a bone fracture such as can occur with
osteogenesis imperfecta. The appearance is artifactual and is due to
refraction.
Figure 5.13 (b): Repositioning of the transducer reveals a normal
straight femur in this normal healthy mid-trimester fetus.
Figure 5.14 (a): This image of a normal gallbladder is affected by
refraction.
Figure 5.14 (b): The effect disappears after slight change of
transducer position where a more uniform acoustic window exists.