Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 15
A
B
FIG. 1.20 Spatial Compounding. (A) Conventional image and (B) compound image of the thyroid. Note the reduced speckle as well as
better deinition of regions (arrows) such as supericial tissue as well as small cysts and calciications.
greater or less than 90 degrees. In conventional real-time imaging,
each scan line used to generate the image strikes the target at a
constant, ixed angle. As a result, strong relectors that are not
perpendicular to the ultrasound beam scatter sound in directions
that prevent their clear detection and display. his in turn results
in poor margin deinition and less distinct boundaries for cysts
and other masses. Compounding has been found to reduce these
artifacts. Limitations of compounding are diminished visibility
of shadowing and enhancement; however, these are ofset by the
ability to evaluate lesions, both with and without compounding,
preserving shadowing and enhancement when these features
are important to diagnosis. 7
Three-Dimensional Ultrasound
Dedicated 3-D scanners used for fetal (Fig. 1.21), gynecologic,
and cardiac scanning may employ hardware-based image registration,
high-density 2-D arrays, or sotware registration of scan
planes as a tissue volume is acquired. 3-D imaging permits volume
data to be viewed in multiple imaging planes and allows accurate
measurement of lesion volume.
Ultrasound Elastography
Palpation is an efective method for detection of tissue abnormality
based on detection of changes in tissue stifness or elasticity
and may provide the earliest indication of disease, even when
conventional imaging studies are normal. Ultrasound elastography
provides a noninvasive method for evaluation of tissue stifness. 8
Tissue contrast in conventional ultrasound imaging is based
on the bulk modulus determined by the molecular composition
of tissue, whereas elastography relects shear properties that are
determined by a higher level of tissue organization, the strain
modulus. his higher level of tissue organization is most likely to
be altered by disease. he dynamic range of the strain modulus
is several orders of magnitude greater than the bulk modulus,
permitting contrast resolution far exceeding conventional ultrasound
imaging. 9 Elastography therefore ofers the potential for a