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DK1212_C007 - 7 Ultrasonography Ultrasonography(USG...

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283 7 Ultrasonography Ultrasonography (USG) utilizes ultrasonic waves as the information carrier; these are mechanical longitudinal waves of high inaudible frequencies in the approximate range of 1 to 10 MHz, propagating in tissues. They are emitted artificially by a probe that acts usually as both the emitter and, in time multiplex, the receiver of the ultrasonic energy. The ultrasonic imaging may be based on detecting reflected and scattered waves that are responses to the emitted wave (like in radar or sonar systems); then it is called echo imaging . Alternatively, it is possible to detect the waves penetrating through the imaged object, in which case it is referred to as transmission imaging . The transmission concept, similar in principle to projection tomography, such as computed tomography (CT) or positron emission tomography (PET), enables good specification of the imaged parameter (e.g., ultra- sound attenuation or velocity) extracted from the measured data; also, some kinds of artifacts (nonlinear paths due to refraction, reflection or diffusion, or shadows behind highly attenuating tissues) can be better suppressed this way than in the echo mode. The transmission imaging thus has definite advantages over echo imaging—a better possibility of quantitative imaging and the possibility of applying © 2006 by Taylor & Francis Group, LLC
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284 Jan computational tomographic techniques, at least in principle. However, it is rarely used owing to long imaging times and complicated posi- tioning of probes, with severe practical obstacles in obtaining a reli- able acoustic coupling on opposite surfaces of the object. Echo imaging is practically much simpler to apply, but regard- ing the physics of the measured signals, it is more complicated. Without going into details, let us state that consequently, most standard echo imaging (traditionally called B-scan imaging) is only qualitative , describing only the shape and position, but not any concrete tissue parameters, of the anatomic structures. The infor- mation, so far utilized in commercial systems, is, besides the spatial coordinates, only the intensity of the detected echo. This intensity, however, is dependent on the imaged scene, on the imaging system properties and adjustment, and on the particular circumstances of the measurement in a very complex manner, so that it cannot be considered to describe a particular tissue parameter. A special kind of echo-based imaging describes not primarily the structures, but rather the distribution of blood flow on the image slice or in the imaged volume. On the difference to standard B-scan imaging, the flow imaging depicts the blood flow in vessels (and sometimes also tissue perfusion) quantitatively, yielding a clearly defined quantity per pixel. The value, derived by a special- ized signal analysis, is mostly the local blood velocity (and perhaps also the degree of turbulence) that can be displayed either in a color scale in the echo image or as separate figures. In case of perfusion evaluation based on contrast imaging, the results are
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