Flat panel (FTP) detector including X-ray scintillator, photodiode pixels,…

The idea

A flat panel detector converts an X-ray shadow into a digital image. X-rays passing through the patient strike a scintillator layer, commonly caesium iodide, which absorbs them and emits visible light photons. Beneath it lies a two-dimensional array of photodiode pixels, each a tiny light-sensitive semiconductor element that converts the light into an electrical charge stored locally. The charge at each pixel is therefore proportional to the X-ray intensity reaching that point. Readout electronics scan the array row by row, transferring each pixel's charge to an amplifier and analogue-to-digital converter, producing a numerical value for every pixel. These values form the digital image matrix displayed on a monitor. Scanning is rapid and automatic, so the image appears almost immediately after exposure.

What earns the marks

• The scintillator absorbs incident X-rays and re-emits visible light photons, acting as a converter between X-ray and optical energy.
• The photodiode array is a two-dimensional grid of pixels; each pixel converts received light into a stored electrical charge.
• Charge accumulated at a pixel is proportional to the X-ray intensity incident on that pixel, so spatial variation in X-ray transmission is encoded as charge variation.
• Electronic scanning reads pixels sequentially, typically row by row, using addressing circuitry rather than moving the detector.
• Signals pass through amplifiers and analogue-to-digital conversion so each pixel becomes a numerical grey level in a digital matrix.
• The resulting matrix is processed and displayed, giving an image whose contrast reflects differing X-ray attenuation within the patient.

Common mistakes and how to put them right

• Thinking the scintillator directly produces the electrical signal.

  Put it right: The scintillator emits visible light and the photodiode converts that light into charge.

• Believing the whole panel is exposed and read simultaneously.

  Put it right: Pixels are addressed and read out by electronic scanning, usually row by row.

• Confusing the detector with photographic film by saying X-rays darken the panel.

  Put it right: X-ray intensity is converted to light, then to charge, then to digital numbers.

Exam tips

• Draw a labelled layer diagram: scintillator on top, photodiode pixel array below, addressing and readout electronics at the edge.
• Use the chain X-ray to light to charge to digital number to display, naming the component responsible at each stage.
• Link pixel charge to X-ray attenuation in the patient so the answer explains image formation, not just hardware.