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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5240_Библиотеки_им_академика_М_И_Перельмана.pdf
Figure 13.11: The initial transabdominal scan revealed an unusual
intracranial appearance.
Figure 13.12: The sonographer then switched to transvaginal
imaging, which was unhelpful.
Figure 13.13: Concerned that these appearances may represent true
intracranial abnormality, the sonographer organized a postnatal
ultrasound. This revealed a perfectly normal brain in this healthy
newborn. The cause of the unusual antenatal appearance almost
certainly stems from unfavorable fetal lie and the inability of the
sonographer to achieve a good angle of approach transabdominally
or transvaginally.
Chapter 14: System Artifacts and Malfunctions
The digital nature of modern ultrasound machines
makes these systems highly reliable. If a major
component of the system malfunctions, the effect is
usually a failure of the system to operate altogether.
Minor system malfunctions and errors in image
processing can produce a wide range of image defects.
Electrical interference
Ultrasound transducers and electronic circuit boards contain highly
sensitive components. Strong electrical interference from other
nearby electrical devices can, on rare occasion, produce visible
interference patterns, as shown below.
Faulty elements
Faulty transducer elements, poor electrical connections at the
transducer port or faulty electronics affecting the performance of
individual channels can produce a range of image defects. In the
past, missing vertical scan lines were the most evident defects. With
the advent of spatial image compounding as a default scanning
mode, however, missing scan lines are often masked by active scan
lines from adjacent apertures and are much less obvious.
Figure 14.1: A simple uniformity test that best reveals the presence
of faulty elements involves applying a small amount of acoustic gel
to the surface of the transducer and then running a finger back and
forth along the scan head. Faulty elements leave a visible gap.
Figure 14.2 (a): A uniformity test performed on this transducer
revealed a faulty element.
Figure 14.2 (b): The defect is not noticeable at all in the clinical 2D
image. This is because a single element makes only a modest
contribution to beam formation.
Figure 14.2 (c): The defect is not noticeable in the clinical color
Doppler image either, for the same reason.
Stitching artifact
Figure 14.3 (a): Images obtained in multifocal imaging mode are
horizontally stitched from echo information belonging to different
beams focused at different levels in the image. When automatic gain
compensation algorithms fail to accurately amplify echoes, there
may be an obvious horizontal break in the image, known as a
stitching artifact (see arrow in Figure 14.3 (b)).
Figure 14.3 (b)
Panoramic imaging spatial correlation algorithm problems
The spatial correlation algorithm that tracks image movement in
panoramic imaging may fail to correctly construct an image,
resulting in discontinuity, steps, empty wedges and other unsightly
defects. This can happen for a number of reasons:
1. The sonographer did not move the transducer smoothly and
consistently.
2. There was a loss of contact between the transducer and the
patient (for example due to a lack of gel or a gas bubble in gel).