indirect detection DR system

The Indirct Detection Flat Pannel System consists of a scintillation phosphor, amorphous silicon photo diode (a-Si) and flat TFT arrays. CsI: Tl or Gd2O2S: Tb ​​scintillation crystals are used in this. This scintillation crystal incident converts x-ray photon into light photon. Its function is similar to an intensifying screen in a cassette.

Phosphor Material

Generally, two types of phosphor material are used in it.

Terbium-dopped Gadolium Oxisulfide (Gd2O2S: Tb)

Thalium-dopped Cesium iodode (CsI: Tl)

Gd2O2S: Tb ​​produces unstructured crystal. It is derived from phosphor screen-film technology. In this system there is loss of light through scatter which is by lateral spread of light photon. Due to which the light photon reacts with the nearby pixel. Hence the spatial resolution of the image is reduced.

CsI: Tl image is derived by intensifier technology. It has different monoclinic needles 5–10 µm wide and 600 µm long. These phosphor crystals are hygroscopic and degrade quickly. So they are completely sealed. Due to these crystal needles, light goes in the forward direction (light direction), which reduces the lateral spread of light photons. In this, the thickness of phosphor can be taken more, which leads to greater interaction of x-ray photon with phosphor and more quantum effeciency.

The base glass of the detector is made of substrate, on which light sensitive a-Si, capacitor and TFT are in the form of pixel. The topmost part of it is scintillation phosphor.

Flat panel array

The a-Si is in a liquid state that is spread over the glass substrate in the form of paint. This entire assembley is kept inside a protective cover outside which the cable connection sockets are also attached.

Indirect conversion

TFT has three connections.

Gate

Source

Drain

The source is a capacotoe. The Drain is connected to the readout line (Vertical line) and the gate is connected to the horizontal (rows). A TFT is usually an electronic switch that can be turned ON or OFF. When negative voltage is applied at the ends of the gate, the TFT switch is in the OFF state. And if positive voltage is applied then it is in ON state.

readout array

First the capacitor of each detector element which stores the charge is earthed, so that the residual charge goes into the ground. Now it is exposed to X-rays then the scintillation emits crystal visible light. This visible light exposes photo-diode (a-Si). Through which electrons come out of the photo diode. Which creates a charge in each detector element. This charge is stored in the capacitor. The charge is then readout into each detector element.

Flat pannel system

During the X-ray exposure, all transistors are placed in the OFF position by appplying negative voltage at the gate. The charge generated in each detector element is stored in the capacitor. During the readout process, the positive voltage at the gate is charged in such a way that one row gate is in ON condition at a time. This switch is connected to the digitizer via vertical wire c1, c2 via S1, S2. Multiplexer selects one column at a time as a sequence. The received charge is amplified and transferred to the digitizer.

analog to digital converter

So in each detector element, gate selects the row and selects the multiplexer column. Thus the charge in each detector element is readout and the signal is digitized and stored for image analysis.