TABLE OF CONTENTS The tomographic images up to this point have generally been formed by illuminating an object with some form of energy (x-rays, microwaves, or ultrasound) and measuring the energy that passes through the object to the other side. In the case of straight ray propagation, the measurement can be of either the amplitude or the time of arrival of the received signal; an estimate is then formed of a line integral of the object s attenuation coefficient or refractive index. Even when the energy doesn t travel in a straight line it is often possible to use either algebraic techniques or diffraction tomography to form an image.
Transmission tomography is sometimes not possible because of physical constraints. For example, when ultrasound is used for cardiovascular imaging, the transmitted signal is almost immeasurable because of large impedance discontinuities at tissue-bone and air-tissue interfaces and other attenuation losses. For this reason most medical ultrasonic imaging is done using reflected signals. In the most straightforward approach to reflection imaging with ultrasound, the echoes are recorded as in radar; in medical areas this approach goes by the name of B-scan imaging.
The basic aim of reflection tomography is to construct a quantitative cross-sectional image from reflection data. One nice aspect of this form of imaging, especially in comparison with transmission tomography, is that it is not necessary to encircle the object with transmitters and receivers for gathering the projection data; transmission and reception are now done from the same side. The same is of course...
