ABSTRACT
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., ultrasound 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
computational tomographic techniques, at least in principle. However, it is rarely used owing to long imaging times and complicated positioning of probes, with severe practical obstacles in obtaining a reliable acoustic coupling on opposite surfaces of the object.
