How are X-rays converted into a radiographic image?
Radiographic images are formed when X-ray photons that have passed through the patient interact with an image detector, where their energy is converted into an electrical signal that can be processed and displayed as an image.
Radiographic images are produced when X-ray photons reaching the detector are converted into electrical signals that are processed to form a digital image.
Modern digital radiography systems convert X-ray energy into electronic signals using either indirect conversion detectors, which first convert X-rays to light, or direct conversion detectors, which convert X-rays directly into electrical charge.
Understanding the physics
When an X-ray exposure is made, a beam of photons passes through the patient and is attenuated to different degrees by different tissues. The pattern of transmitted photons reaching the detector contains the information that will form the image.
In digital radiography, the detector converts this pattern of X-ray intensities into an electrical signal that can be processed by a computer. The way this conversion occurs depends on the detector design.
In indirect conversion detectors, the X-ray photons first interact with a scintillator layer, typically made from materials such as cesium iodide (CsI) or gadolinium oxysulfide (Gd₂O₂S). When X-rays interact with the scintillator, their energy is converted into visible light photons.
These light photons then strike an array of photodiodes, which convert the light into electrical charge. The charge is stored and read out by a matrix of thin-film transistors (TFTs), producing a digital signal proportional to the local X-ray intensity.
In direct conversion detectors, the process is slightly different. X-ray photons interact directly with a photoconductor, most commonly amorphous selenium (a-Se). The absorbed X-ray energy generates electron–hole pairs within the photoconductor. An applied electric field causes these charges to move toward electrodes, creating an electrical signal that is collected and read out by the detector electronics.
In both detector types, the resulting electrical signals are digitised and processed to form an image composed of pixels, where the pixel value reflects the number of X-ray photons detected at that location.
Regions where many photons reach the detector appear dark, while areas where fewer photons reach the detector appear bright, producing the contrast seen in the radiographic image.
Where this matters clinically
The efficiency with which a detector converts X-ray photons into a usable signal strongly influences image quality and radiation dose. More efficient detectors can produce high-quality images using fewer photons, which reduces patient exposure.
Modern digital radiography detectors are designed to maximise signal generation while minimising noise, allowing diagnostic images to be produced with relatively low radiation doses.
Understanding how detectors convert X-rays into electrical signals also explains important image quality concepts such as detective quantum efficiency, signal-to-noise ratio, and spatial resolution.