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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5240_Библиотеки_им_академика_М_И_Перельмана.pdf
Figure 3.34 (b): A small movement of the transducer into the
intercostal window reveals the echogenic particulate nature of the
fluid collection.
3.8 Edge shadowing
Also known as
Edge effect, refraction shadowing, refraction edge shadowing or
edge defocusing
Causative mechanisms
The cause of edge shadowing is a combination of reflection and
refraction at the margin of a well defined mass. These factors lead
to defocusing of the beam, loss of beam intensity and corresponding
appearance of shadowing beyond the causative margin.
Which system assumptions have been breached?
All of the following assumptions are breached: 1) speed of sound is
constant, 2) sound travels in a straight line and 3) attenuation rate is
constant.
Typical appearance
Shadow originating from the margin of a well defined cystic or solid
structure.
Common misconceptions
Edge shadowing is not only associated with cystic masses but can
also be associated with solid masses or normal organ margins (such
as the margin of the kidney).
Can this artifact be reduced, eliminated or circumvented?
Edge shadowing can be significantly reduced with the use of spatial
compounding. A change in the angle of approach alters the direction
of the shadow which allows investigation of regions previously
affected by shadowing.
Is this artifact diagnostically useful?
No. This artifact is not useful.
Figure 3.35: A cluster of varicose veins demonstrates multiple edge
shadows.
Figure 3.36 (a): Varicose veins scanned without spatial compounding
showing venetian blind shadowing.
Figure 3.36 (b): The same varicose veins scanned with spatial
compounding. Refraction shadowing has been completely eliminated
in the compounded image.
Two examples of edge shadowing associated with solid
masses.
Figure 3.37 (a): Thyroid nodule.
Figure 3.37 (b): Liver metastasis.
Example showing edge shadowing, dirty shadowing and total
shadowing
Figure 3.38: In this image of a breast fibroadenoma, nearly all forms
of shadowing are present. The internal calcification casts a total
shadow (ts), the remainder of the mass causes a dirty shadow (ds)
and edge shadowing is also present especially on the right (es).
3.9 Anisotropic dropout
Also known as
Anisotropy.
Causative mechanisms
Anisotropy is due to variable propagation speed across and along
striated structures such as muscles, tendons and ligaments. When
an anisotropic structure is interrogated from an angle other than
normal (90°) incidence, the incident beam refracts at each interface
within the structure which leads to a rapid defocusing effect
manifesting as loss of echo amplitude, attenuation and dropout. The
problem is well known in musculoskeletal ultrasound.
Which system assumptions have been breached?
All of the following assumptions are breached: 1) speed of sound is
constant, 2) sound travels in a straight line and 3) attenuation rate is
constant.
Typical appearance
Normal striated appearance of the structure of interest (muscle,
tendon, ligament) at normal incidence (90°) but gradual loss of
echogenicity at angles less than 90° with complete drop-out at acute
angles.
Can this artifact be reduced, eliminated or circumvented?
Yes, this artifact can be eliminated if the region of interest can be
scanned at normal incidence by manually angling the transducer,
electronic beam-steering or the use of a favorable acoustic window.
If a 90 approach is not achievable, anisotropy is difficult to eliminate.
Spatial image compounding tends to reduce the effect of anisotropy
because some of the beams will interrogate the region of interest at
a favorable angle.
Is this artifact diagnostically useful?
No. This artifact is a major problem in musculoskeletal imaging. For
example, curved tendons need to be interrogated from a range of
angles because some parts of the tendon will always demonstrate
dropout. Less experienced practitioners can easily confuse
anisotropic shadowing for real pathology such as a tendon tear.
Figure 3.39: Normal supraspinatus tendon demonstrating good
reflectivity and normal fibrillar structure in the region of normal (90°)
incidence. As the tendon curves away from normal (60°, 50°),
visualization of the tendon fibers is progressively lost in an enlarging
zone of shadowing (s).
Figure 3.40: Normal Achilles tendon in longitudinal section
demonstrating varying degrees of anisotropy with dropout at the
tendon insertion.
Chapter 4: Beam Dimension Artifacts
The ultrasound beam is not infinitely thin. Instead it is
a field of energy that has a defined pulse length
(axial), width (beamwidth) and depth (slice thickness).
Reflectors are therefore not represented as discrete
focal points. Instead they can be thought of as bricks
or tiles. They are thin in the axial dimension, but are
variably wide and variably deep.
4.1 Beamwidth artifact
Also known as
Beamwidth.
Causative mechanisms
The ultrasound beam is relatively wide. Lateral resolution is generally
quite poor compared to axial resolution. This problem can easily be
seen in ultrasound images because the image appears to be
composed of fine horizontal lines, not of discrete square reflectors.
Ultrasound reflectors are said to be non-isovolumetric, that is, they
are not cuboidal in their dimensions. Instead, ultrasound reflectors
can be likened to tiles. They are uniformly thin along the beam axis,
variably wide (dependent on scan plane focusing) and variably deep
(dependent on elevation focusing).
Which system assumptions have been breached?
The assumption that beam dimensions are small.
Typical appearance
Wide horizontal dimension of reflectors, lateral smearing, widening
of lateral mass walls and organ boundaries, thickening of vessel
walls when interrogated at angles other than normal incidence (90
degrees). Beamwidth effects are particularly pronounced in the farfield when narrow apertures are used (focus is close to the
transducer).