Biomedical Engineering Reference
In-Depth Information
nonlinear relation between the source pressure and the second harmonic pres-
sure. At the transducer surface, the ultrasound pulse is composed only of the
fundamental frequencies. As soon as it propagates through tissue, however,
energy builds up at the second harmonic frequency. After a few centimeters of
distance, enough energy has been converted from the fundamental frequency
to yield a significant second harmonic frequency energy wave. Because much
of the artifact in an echo image is related to reverberations and scattering
at or near the chest wall, these artifacts contain relatively little harmonic
frequency energy. If imaging is confined to the harmonic range, then most of
the near-field artifacts are eliminated.
2.1.4
The Ultrasonic Characterization of Myocardial Tissue
Abundant indirect information is obtainable by analysis of ultrasonographi-
cally (echocardiographically) assessed abnormal regions of the myocardium;
however, direct, noninvasive identification of the tissue composition of cardiac
structures is still one of the important but unmet goals of cardiac diagnosis.
Ultrasonic cardiac tissue characterization may be defined as the identification
and characterization of abnormalities in the physical or physiological state
of the myocardium based on analyzing interactions between ultrasound and
tissue [1]. The rationale for this field of study is that sucient information
is available in the ultrasound signal passing through or returning from myo-
cardial tissue to identify the tissue as normal or abnormal, and to indicate
the nature of the abnormality. Progress in quantitative myocardial tissue
characterization to date has been possible because of significant advances in
electrical engineering and physics, applied to improving and modifying ultra-
sonic measurement and imaging systems. Quite a few groups have approached
the problem of ultrasonic myocardial characterization with different instru-
mentation techniques and analysis systems, and one important direction of
research has relied on the analysis of unprocessed radio-frequency signals re-
turning from the myocardium. Initially, the degree of ultrasonic attenuation
was quantified in transmission studies, but there are more reflection studies
now in which the extent of ultrasonic backscatter is quantified. This is largely
attributed to the recent advent of a real-time backscatter imaging system.
Biological determinants of tissue acoustic properties have not been fully
understood. As mentioned earlier, when sound waves traverse through a ma-
terial, they are reflected or scattered where local regions of different acoustic
impedance are encountered. Thus, tissue elements responsible for scattering
represent local regions of acoustic impedance mismatch. Early work in this
field identified collagen as a primary determinant of both scattering and at-
tenuation of myocardial tissue [2]. The geometric attributes of myocardial
scatterers have been investigated by a number of groups, who have postu-
lated that myocardial scatterers are comparable in size to cardiac myocytes
[3,4]. Myocardial acoustic properties are also influenced by the orientation
of ventricular muscle fibers. Mottley and Miller have demonstrated that the
