Data Acquisition for Computed Tomography (CT)

Tomography originates from Greek, with the meaning 'recording slices'. The process has been developed in answer to the emerging interest in learning the inner structure of objects and beings. Simple X-ray radiography only delivers integrated information on layers of materials along a line, but the change of absorption in two or three dimensions can not be determined. The mathematical foundations of CT has been laid by Johann Radon in 1917, though at that time technical advance has not reached levels sufficient for constructing such a device. One reason to that was the lack of information technology needed for data acquisition and computing, the other the underdevelopment of the X-ray devices of the age. The inner structure of an object can be investigated by applying common radiography, but instead of a single direction projections should be taken from multiple angles. Every measurement results in a different image depending on the position of the point of view. The essence of CT is that from the results of projections taken at different angles, the image of the inner structure of an object can be (re-)constructed, namely the distribution of the absorption coefficient 8. The more detailed image we would like to obtain the more angles we need to take. For the image reconstruction problem A. M. Cormack published a possible solution in 1963 and the practical realization has been provided by G. N. Hounsfield and for these results the medical Nobel-prize has been awarded in 1972.
A common X-ray system can also be used for imaging layers envisaged by the physicist Gusztáv Grossmann. The idea is that during the imaging both the source and the film cassette are moved in the opposite direction simultaneously and the image will only be sharp in a single layer. With the parameters of moving the X-ray tube and the film the position of the layer can be selected.

Fig. 23.The first CT device of Hounsfield

 
In the first generation of CT equipment the X-ray tube and the detector moved simultaneously along opposite parallel lines, while the X-ray beam was transmitted through the object in-between. The signal strength of the detector opposite the source depends on the absorption along a line. When the whole object volume is projected the sample has to be turned along one of the axis. With this new orientation setup (in relation to the detector and the source) the object again has to be projected with the linearly moving detector-tube couple, and this has to be repeated until the whole 360 degrees angular domain is covered.

Fig. 24 Working principle of the first generation CT.

 
The finer the resolution of the linear projection and the rotation is, the sharper the obtained image of the inside of an object becomes. The bounding factor on one hand is the dose limit on the other the acquisition time. The first CT device measured in 160 linear position and at 180 angles, while modern-day CTs apply 750 projections at each angle. As translational movements are harder to realize in the same precision as in the rotational, the first CTs were extremely sensitive to mechanical instabilities and vibrations resulting in image quality degradation.
The second generation CT scanners did not eliminate translational movements but the beam was formed such that it was not parallel anymore but formed a fan-shape with angle of 5-10 degrees. This technical solution allowed for applying multiple detectors. With this technological advance the acquisition time substantially decreased, as the measurement time at different angles could be lowered.
The third generation scanners are applied since 1977, where the beam from is kept fan-shaped and the X-ray source and the detector are located in the opposite sides and the whole system rotates around the object and can transmit the beam from every direction 9. The opening angle of the fan now can reach 30-50 degrees and the number of detectors about 1000. This setup let the acquisition time decrease to a couple of seconds. The fourth generation is constituted by a moving source and by increasing the number of detectors to 1000-5000 thus they can be fixed in position.

Fig. 24. Schematics of a second and third generation scanners.