Direct vs Indirect Digital Radiography (DR) will describe the physic behind digital radiography system . Digital detectors are also used for fluoroscopy, interventional and mobile x-ray systems in place of image intensifier systems.
Digital Radiography (DR) detectors are classified as indirect flat panels or direct flat panels based on the underlying physics for each type of panel. Indirect flat panels convert x-rays to light photons that are then measured by a CCD/CMOS or Si photodiode and TFT array. Direct flat panels convert the x-rays directly to electrons that are measured by a TFT. Direct flat panels achieve higher spatial resolution by skipping the visible light step that can blur the images as the light spreads.
The first figure demonstrates the different categories of flat panel detectors where there are two categories indirect and direct. The indirect flat panels are further differentiated into the TFT type and the CCD/CMOS type.
Indirect flat panels measure the x-ray signal by converting from x-rays to: visible light, to charge (i.e. electrons) and then to the digital signal. Direct flat panels skip the visible light step and the x-rays are converted directly to electrons, and then the charge is digitized
Indirect Flat Panels Scintillation Layer (GOS vs CsI):
There are some types of crystals which absorb x-rays and produce visible light. These are called scintillators and they are the first step in indirect panels. The two most frequently used materials for the scintillation layer are Gd2O2S (Gadolinium OxiSulfate, also know as: GadOx or GOS) and Cesium Iodine.
See howradiologyworks.com for more rad physics fun.
CsI on the other hand has less light spread since the crystal has a columnar structure so the light will follow the columns of the CsI as it passes through the crystal. This reduced blurring leads to higher spatial resolution for CsI based systems compared with GOS systems.
Indirect TFT Flat Panels:
The most common type of Digital Radiography system is the Indirect flat panel that is read out with an array of Thin Film Transistor (TFTs). TFT flat panels use the same basic technology that are used to control LCDs (Liquid Crystal Displays) in many common flat panel monitors and TVs. The TFT array enables each pixel in an LCD to be turned on and off independently and likewise when it is used in an x-ray detector it allows each detector element (DEL) to be read out independently.
The visible light photons are then measured on a photodiode that converts from the light signal to an electrical signal. The photodiodes are typically made of amorphous Si. When the light photons hit the photodiode they produce electron and hole pairs.
The material for the photodiode (Si) is chosen because the light photons coming out of the scintillator are best matched to generate a photocurrent, due the the material properties (i.e. the bandgap).
Finally, after the photodiode converts the light to an electrical signal a TFT (thin film transistor) array is used to readout the measured charge. The TFT array is based on a glass substrate and silicon is deposited as a conductor and silicon dioxide as an insulator to build up the circuit so that all of the electronic circuits are flat and built-in to the TFT.
Each detector element in the TFT stores the charge in a capacitor and uses a TFT switch or gate to control when the charge is read out.
Within each detector element (DEL) there is an active area shown in blue in the figure. This is the region where change can actively be deposited. There is also the need for some other electronics such as a capacitor (to store the charge) and a TFT switch (to open up when it is time to readout the value from that DEL).
Since the signal is read out from the active area a higher fill fraction is desired and leads to a more efficient detector as more of the x-rays that pass through the patient will contribute to measurements on the detector. This efficiency contributes directly to the dose efficiency of the system. This factor limits how small each individual DEL can be as we also want high dose efficiency.
Indirect CCD/CMOS Flat Panels
The other alternative for an indirect flat panel detector is to make use of the technology that is used in digital cameras, namely CCDs (Charge Coupled Devices) or CMOS (Complementary Metal-Oxide-Semiconductor). CCDs are well designed for the purpose of measuring visible light as they are used as the sensor on many digital cameras. CCDs also have the advantage that they can be read out quickly. However, unfortunately the size of the CCDs does not match the size of the flat panel detector.
Direct Flat panel
A voltage is applied across the detector such that the electrons will flow to one side and the holes will flow more slowly to the other side. The electrons which are pulled to the positive side can then be measured. The flow of electrons is also referred to as an electric current.