X-ray Detector on CT Sanner Component

The x ray detector is equally important to the performance of a CT Scanner. Like x ray tubes, detector technology has experienced tremendous growth over pas 30 years. The earlier vintage detectors had smaller fields of view mainly to perform head scans. The later vintage detector must have larger fields of view to handle large bodies.

Detector of thirds generation scanner use either high pressure inert gas (usually xenon) or solid state scintillates coupled with photo bodies. The operating principle of xenon detector is illustrated on the picture. The detector is constructed with May thin tungsten plates inside a high pressure xenon chamber. Every other plate is connected to a high voltage dc supply. For example on GD 8800 detector, the plate connected to 500 v. The remaining plates are electricity floating ans are at approximately 0 V. A pair of low –high voltage plates forms a single detector cell. When x ray photon strikes a cell, it causes ionization of xenon gas in a photoelectric interaction. The interaction release energetic photo electrons to ionize more ion gas ions. The ionized gas xenon nuclei are collected by the o V plates and free electrons are collected by positive biased plated (500 V) to produce current signal. The bias voltage across the plates it set high enough such that are ion collected rapidly by plates with little recombination of positive and negative ions in the gas. At the some time, the bias voltage is below the point of the avalanche effect, where a nonlinear magnification of signal occurs. Improperly biased region, the amount of ionization is linearly proportional to the total energy of the absorbed x ray photons.

Solid detectors overcome many of the shortcomings of the xenon detector. The detector is made of small blocks of scintillating materially, such as CdWO4, Gd2O2S or HiLight, coated with reflective material and coupled to set of photodiodes.

The light photon produced by the scintillation process travel in all directions. The scintillatator is coated with highly reflective material to direct the light toward the photodiodes at the bottom of the detector. Because of the reflection and absorption in the scintillator , only small percentage of the light photon reaches the photodiodes to produce electrical signals.