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The function of the detectors is to transform the transmitted x-rays into electric signal proportionate to the intensity of transmitted x-rays. To accomplish this the detector must satisfy to following conditions :
1. X-ray absorption ability must be efficient
2. The response to the x-ray intensity must have a wide range with good linearity
3. Minimal afterglow and recovery time
4. Good gisgnal reproducibility and stability with long life
5. Small nonsensitive regions between neighboring detectors when multichannel detection is used
6. The signal output has to be sufficient to be converted into digital signals
various types of scintillators, gas ionization detectors, and semiconductor detectors are used for x-ray detection. The scintillators employed in the first second, and fourt-generation ct scanner include NaI(TI), CsI (TI), CaF2(TI), and bismuth germinate oxide (BGO) in combination with photomultipliers or photodiodes. The emitted light spectrum of the scintillator and the response spectrum of the photomultipier or photodiode have to be compatible with each other. The detector’s output signals are, after intergration and A-to-D conversion, put into the computer. These scintillators contain minute amounts of T1 to improve the conversion rate of x-rays into visible light. However, this T1 produces phosphorescence and prolongs the life of the aftrerglow. Since CT sampling time is very short, its values are influenced by the data that proceed it, and accumulated afterglow makes correction rather difficult. Therefore, the life of the afterglow should be as small as possible.
A Xe gas multichannel detector is generally used in third-generation CT scanners. A chamber in which Xe gas of 5-10 atmospheric pressures is filled, and the detector collection field is made by applying DC voltage to the electrodes which are arranged about 1mm or less apart from each other. When x-rays are exposed, the entering photons and Xe gas react with each other and produce pairs of photoelectrons and ions. The electrons produces are collected by the collection electrode, which in turn produces an electric current. Rapid collection of ions and electrons is necessary for efficient CT scanning procedures. The gas pressure is increased and the distance between the electrodes in lengthened, allowing the number of gas molecules per channel to be increased, and raising the sensitivy of the detectors. Undesirable spatial resolution, however, resul when the distance between electrodes is too long, or when recovery time becomes prolonged. An example of a multichannel detector is shown in.
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