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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5240_Библиотеки_им_академика_М_И_Перельмана.pdf
Figure 4.24: Sidelobe artifacts associated with the fetal skull in a
mid-trimester fetus scanned transabdominally.
Figure 4.25: Sidelobe artifact in a 12-week fetus scanned
transvaginally.
Figure 4.26: A sidelobe artifact associated with a scrotal pearl within
a mild hydrocele.
Figure 4.27: A sidelobe artifact generated by a Foley Catheter is
seen in the mid-sagittal section of the bladder.
Figure 4.28: In the left or right parasagittal section, the sidelobe
artifact caused by the catheter is still visualized (arrow) even though
the causative object is not present in the same plane of section. This
occurs because sidelobes not only exist in the plane of imaging but
also in the elevation plane.
Chapter 5: Refraction
Refraction is the change of beam direction that occurs
when an ultrasound beam encounters a boundary
between two acoustic regions with different
propagation speeds. The angle of incidence must be
non-perpendicular for refraction to occur. Refraction
contributes to several artifacts already discussed in this
book including edge shadowing and anisotropic
shadowing. Unique refraction scenarios occur in the
abdomen due to the symmetrical refracting nature of
the rectus abdominis muscles which behave like a pair
of acoustic lenses. Apparent widening, bending,
duplication or triplication of structures may occur in
the ultrasound image.
5.1 Refraction ghost artifacts
Also known as
Duplication, ghost artifact.
Causative mechanisms
In a transverse view of the abdomen, the rectus abdominis muscles
act as two acoustic lenses and refract the ultrasound beam. Usually,
the refraction effect is towards the midline. Because the machine
assumes a straight beam path, an object which lies in the midline
may be projected laterally which results in widening, duplication or
triplication of this object.
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
Apparent widening, bending, side-by-side duplication or side-by-side
triplication of abdominal or pelvic structures when imaged in the
midline in transverse section. The most classic appearance involves
apparent duplication of the SMA or duplication of a single gestational
sac resulting in the appearance of twin sacs. The artifact disappears
if the operator moves the transducer laterally away from the midline.
Common misconceptions
Students of ultrasound often assume that artifactual duplication of
an object is always caused by the mirror image artifact. Refraction
can cause duplication or triplication of an object. In the case of
refraction, the false objects are always located laterally to the
original object whereas in case of mirror image the false object is
always located behind the acoustic mirror.
Can this artifact be reduced, eliminated or circumvented?
This artifact can be eliminated by sliding the transducer laterally so
that it is not resting over both rectus abdominis muscles.
Is this artifact diagnostically useful?
No. This artifact can lead to confusing anatomical appearances and
is never useful.
Refraction ghost artifact
An optical analogy of the ghost artifact can be demonstrated using
two round glasses and a small object, in this case a tomato.
Depending on the viewing angle (and other physical parameters),
one can demonstrate a range of interesting refraction effects.
Figure 5.1 (a): The objects viewed separately.
Figure 5.1 (b): When the line of sight is directed through one glass
only, only one tomato is seen.
Figure 5.1 (c):If the line of sight is directed through the two closely
adjacent glasses, the following effects can be seen: duplication
(seen here) or triplication (Figure 5.1 (d)).
Figure 5.1 (d): Triplication may occur when the line of sight is
directed through the two closely adjacent glasses.
Figure 5.2: Refraction can also cause distortion and widening of
objects in the image.
Figure 5.3: The superior mesenteric artery (SMA) appears duplicated
in this image. The image is ambiguous. It could represent true
anatomical variants which often occur at this location (such as the
common hepatic artery branching off the SMA instead of the celiac
axis) or the image could represent a refraction artifact.
Figure 5.4: This is the same patient as shown in Figure 5.3. Moving
the transducer laterally has caused the artifact to disappear,
confirming the artifactual nature of the second vessel.