**3.2. Line by line CST**

4 Will-be-set-by-IN-TECH

plane and the detector is connected to a multichannel analyzer. When the axis of a collimated point-like detector is made to intersect the incident pencil beam, the intersection site is actually a scattering site of the Compton effect. So by measuring the scattered photon flux density at a given energy, by estimating the strength of the attenuation factors, and by evaluating the beam spreading factors one can obtain *n*(**M**). The process is then repeated for all sites in a trans-axial slice. It is interesting to note that equation 3 has been recast in the framework of an inverse problem by E. M. A. Hussein et al.[33], who have set up a discretization scheme to

At the turn of the twentieth century, the idea of using scattered gamma rays for investigating hidden structures in tissues seems to have been proposed for the first time by F. W. Spiers [52]. In 1959, P. G. Lale [40], realizing that radiographic images of organs fail to reveal their inner structure, has suggested a technique using Compton scattered radiation from a thin pencil of X-rays. Using equation 1, it is possible to move a collimated detector in space to measure the scattered radiation flux density and deduce the electron density at a precise point on the incoming pencil of X-rays. When this measurement can be performed in a planar slice of an object, it said that an image of the slice is obtained by Compton scatter tomography (CST). Of

An analogous problem arises in nuclear industry. There heat transfer research in circulating two-phase fluids requires knowledge and measurement of their densities without disturbing or arresting their flow. It was realized that penetrating gamma-radiation would be the most suitable for this purpose. As matter density is responsible for traversing radiation attenuation (since most of the attenuation occurs as a result of Compton scattering) measurements of radiation attenuation along linear paths crossing the section of a pipe can be easily done. D. Kershaw [36] has shown how the matter density distribution can be reconstructed from these measurements. In fact this problem is practically identical to the medical computed tomography which was developed simultaneously at that time and Kershaw has actually performed the mathematical inversion of the classical Radon transform, but most probably without being aware of the seminal works of J. Radon [49] and A. M. Cormack [15]. But it was N. N. Kondic [37], who has first suggested the use of scattered radiation for obtaining the

In 1973, R. L. Clarke et al. [13] used both transmitted and scattered gamma-rays to measure bone mineral content. In [14], they have introduced the term gamma-tomography for their apparatus which is made of a fixed collimated pencil source and four focussing collimated detectors positioned so as the scattering angle is about 450. The investigated object is placed on

course this is clearly a point by point procedure which is quite time consuming.

solve it.

**Figure 2.** CST point by point scanning

two-phase fluid density in a pipe.

In 1971, F. T. Farmer & M. P. Collins [19] advocated the use of a *wide angle* collimated detector limited by two plates parallel to the incident radiation pencil. Fig. 3 shows how schematically how this procedure works. The detector is coupled to a multichannel analyzer. This design allows to obtain rapidly the electron density profile along the probing incident radiation pencil. This is the reason why it is called line scanning technique. A global image of an object slice is realized when parallel lines are put together. This technique has been refined in [20].

**Figure 3.** CST with line by line scanning

Variants of this design have been developed by the NDE community to combat the low efficiency of the single voxel technique. In [25], G. Harding described a Compton scatter imaging system for NDE with horizontal scanning and incident pencil beam perpendicular to the scanning direction. Scatter detectors are disposed on both sides of the incident beam. Fig. 4 shows a sketch of the COMSCAN (Compton Scatter Scanner), developed by the Philips Research Laboratories in Hamburg, Germany which was quite successful (see [26]).
