Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 13
A B C
FIG. 1.16 Tissue Harmonics. As sound is propagated through tissue, the high-pressure component of the wave travels more rapidly than the
rarefactional component, producing distortion of the wave and generating higher-frequency components (harmonics). (A) The acoustic ield of the
primary frequency is represented in blue. (B) The second harmonic (twice the primary frequency) is represented in red. (C) Using a broad-bandwidth
transducer, the receiver can be tuned to generate an image from the harmonic frequency rather than the primary frequency. As a result, near ield
clutter is reduced since the harmonic only develops at depth in the tissue and the beam proile is improved, leading to better spatial resolution.
archiving and communications system (PACS). Increasingly,
digital storage is being used for archiving of ultrasound images.
SPECIAL IMAGING MODES
Tissue Harmonic Imaging
Variation of the propagation velocity of sound in fat and other
tissues near the transducer results in a phase aberration that
distorts the ultrasound ield, producing noise and clutter in the
ultrasound image. Tissue harmonic imaging provides an approach
for reducing the efects of phase aberrations. 6 Nonlinear propagation
of ultrasound through tissue is associated with the more
rapid propagation of the high-pressure component of the ultrasound
pressure wave than its negative (rarefactional) component.
his results in increasing distortion of the acoustic pulse as it
travels within the tissue and causes the generation of multiples,
or harmonics, of the transmitted frequency (Fig. 1.16).
Tissue harmonic imaging takes advantage of the generation,
at depth, of these harmonics. Because the generation of harmonics
requires interaction of the transmitted ield with the propagating
tissue, harmonic generation is not present near the transducer/
skin interface, and it only becomes important some distance
from the transducer. In most cases the near and far ields of the
image are afected less by harmonics than by intermediate locations.
Using broad-bandwidth transducers and signal iltration
or coded pulses, the harmonic signals relected from tissue
interfaces can be selectively displayed. Because most imaging
artifacts are caused by the interaction of the ultrasound beam
with supericial structures or by aberrations at the edges of the
beam proile, these artifacts are eliminated using harmonic
imaging because the artifact-producing signals do not consist
of suicient energy to generate harmonic frequencies and therefore
are iltered out during image formation. Images generated using
tissue harmonics oten exhibit reduced noise and clutter (Fig.
1.17). Because harmonic beams are narrower than the originally
transmitted beams, spatial resolution is improved and side lobes
are reduced.
Spatial Compounding
An important source of image degradation and loss of contrast
is ultrasound speckle. Speckle results from the constructive and
destructive interaction of the acoustic ields generated by the
scattering of ultrasound from small tissue relectors. his interference
pattern gives ultrasound images their characteristic grainy
appearance (see Fig. 1.6), reducing contrast (Fig. 1.18) and making
the identiication of subtle features more diicult. By summing
images from diferent scanning angles through spatial compounding
(Fig. 1.19), signiicant improvement in the contrast-to-noise
ratio can be achieved (Fig. 1.20). his is because speckle is
random, and the generation of an image by compounding will
reduce speckle noise because only the signal is reinforced. In
addition, spatial compounding may reduce artifacts that result
when an ultrasound beam strikes a specular relector at an angle