Cadmium telluride (CdTe) is a stable crystalline compound formed from cadmium and tellurium and can be used as a compound semiconductor. CdTe is used in the next generation X-ray detectors. It absorbs radiation and efficiently converts it to an electronic signal.
CdTe is a direct conversion detector and as such does not need to turn the X-ray photon into a light photon before recording it. This means the position of the conversion can be more accurately recorded and the signal is directly related to the energy of the converted photon. Other radiation detectors use scintillators to convert X-rays to light, this process causes significant blur and loss of efficiency.
CdTe detectors have very good spatial resolution and are very efficient which allows them to be used in very low dose situations. The crystal is radiation hard and robust against environmental effects.
A scintillator is a substance such as Gadolinium Oxysulfide or Cesium Iodide that converts an X-ray photon into a light photon. The light photons can then be recorded by a normal CMOS or CCD camera. Light photons travel in all directions causing a loss of accuracy in spatial resolution. Typically scintillation technologies are not as efficient as CdTe at stopping X-rays.
CdTe is a compound semiconductor made from, Cadmium and Tellurium. Since the atomic number of both materials is high (Cd:48, Te:52) and CdTe has high density (5.85g/cm3), it can absorb radiation more effectively.
X-ray detectors can be divided into two classes. They either use a direct technique (direct-conversion detectors) or an indirect technique (indirect-conversion detectors) for converting X-rays into an electric charge. Direct-conversion detectors have an X-ray photoconductor, such as CdTe, that directly converts X-ray photons into an electric charge. Indirect-conversion detectors, on the other hand, have a scintillator that first converts X-rays into visible light. This light is then converted into an electric charge by means of photodetectors such as amorphous silicon photodiode arrays or CCDs. Thin-film transistor (TFT) arrays may be used in both direct- and indirect-conversion detectors.
In both direct- and indirect-conversion detectors, the electric charge pattern that remains after the X-ray exposure is sensed by an electronic readout mechanism, and analog-to-digital conversion is performed to produce the digital image. Some direct conversion detectors can count photons rather than integrate the charge as an indirect-conversion detector must.
Photon counting is where a direct conversion detector counts each electronic pulse caused by the conversion of the converted X-ray photon. This means there is no analogue to digital conversion and the output of the detector is direct digital and free from electronic read-out noise.
Photon counting has a higher dynamic range and the read out is normally noise free, so summing images can create a more and more dynamic range. Some photon counting devices can also compare the size of the electronic pulse and equate it with the energy of the converted X-ray photon. This is because the electron shower created by the converted X-ray photon is directly proportional to the energy of the converted X-ray photon. The shower is made unidirectional by a bias voyage across the crystal meaning all of the electrons are recorded on the same electrode which acts as the pixel.
Time-delay Summation is when the X-ray detector is imaging a moving object and sampling at a rate equal to one row per resolution step. The detector is read out at each step and the images summed with a one line offset, thereby summing the signal for each line as it passes the same spot on the object of interest.
Dual energy imaging records all the X-ray conversions and at the same time all the conversions above a threshold which equates to a specific energy. The two images are recorded and read out simultaneously. The images can be used to calculate the low energy element by subtraction of the high energy image from the total energy image. Image processing of these images allows for material separation such as removing bones from a chest X-ray
Because some X-ray photons will land between electrodes and may travel across two electrodes spreading the signal out, the detector needs a way to prevent this effect from being recorded as two events. Anti-coincidence technology combines events happening simultaneously and assigns the event to the electrode with the highest signal at that event time.
Anti-coincidence works by associating events happening at the same time with a single electrode (pixel). This is done in real time in the hardware which allows the detector to be more accurate for energy specifications and increases the spatial resolution of the resulting image.
Acquisition time is the time the sensor spends counting before reading out. The lower limit is 10us but is truly limited by the X-ray flux. The upper limit is 4s but the real limit is the 12 bit counter. Summing individual acquisitions is a noise free operation and so can extend the overall image acquisition time.
The dominant factor is the X-ray flux. The sensor has a 12bit counter and the user should try to keep each exposure within, on average, 50% of this range adjusting the acquisition time to achieve this. Summing frames is noise free so splitting an acquisition into several shorter acquisitions is a good way to deal with high flux.
This is the amount of single acquisitions the sensors should take per second and is generally expressed in Hz.