Indirect Conversion Digital Radiography

Indirect Conversion Digital Radiography

A two-step process where a scintillator converts x-ray photons into light, which is then converted into an electrical charge by a photodetector.

Scintillator Layer

A phosphor material, such as gadolinium oxysulphide (Gd2O2S) or cesium iodide (CsI), that rapidly absorbs x-rays and produces light.

Unstructured Scintillator Layer

A type of scintillation layer that produces more scattered light than structured layers, thereby decreasing the efficiency of the detector.

Photodetector

A component, such as a hydrogenated amorphous silicon (a-Si:H) photodiode array, that converts the light from the scintillator into an electric charge.

Step 1 (Indirect Conversion)

X-ray photons are absorbed by a scintillation or phosphor material that converts the incident x-ray photon energy to light.

Step 2 (Indirect Conversion)

A photosensitive array, made of small pixels, converts the light into electrical charges. Each pixel contains a photodiode and a silicon TFT.

2 categories of Indirect

CCD systems and TFT systems

CCD Systems

A system that requires optical coupling to focus the scintillation light from a CsI phosphor onto a small CCD chip. The light is then converted into an electrical signal, resulting in high x-ray capture efficiency and good spatial resolution

TFT (Thin Film Transistor) Systems

A system where a photodiode array below the scintillator converts light photons into an electrical charge. The TFTs are then activated to read the charge from each pixel and send it to the ADC.

TFT Direct Detectors

An alternative form of TFT detectors that convert x-rays directly into an electrical charge without a scintillator.

Photodiode Array

A layer below the scintillator that converts the light photons into an electrical charge.

Detector Element (DEL)

The main component of the Detector Array, arranged in rows and columns in the detector, which is called a matrix.

Fill Factor

The percentage of the pixel face that is sensitive to x-rays.

Fill Factor Percentage

It is approximately 80%, meaning 20% of the x-ray beam does not contribute to the image.

matrix

arranged in rows and columns in the detector

directly proportional

Fill factor, Spatial and Contrast Resolution