In tomography the 3D distribution of some physical quantity in the object is determined.
[3] Tomography involves taking measurements around the periphery of an object (e.g. process vessel or patient) to determine what is going on inside.
The best known technique is CT scanning in medicine; however, process tomography instrumentation needs to be cheaper, faster and more robust.
In all cases external sensors are used to detect signals from boundary of the object, and the three-dimensional material distribution or the velocity field is computed using the measured data.
Process tomography is an area of rapid growth both in terms of research and applications.