Flat-panel X-ray detectors

A flat-panel X-ray detector converts the X-rays reaching different parts of its surface into a digital image. It contains a scintillator above a two-dimensional array of photodiode pixels. Each pixel produces a signal that represents the radiation received at its position.

From X-rays to a digital image

The scintillator absorbs incident X-ray photons and emits visible light. The photodiodes detect this light and convert it into electrical charge. Where the detector receives more intense radiation, more light is produced and the corresponding pixels generally collect more charge.

Electronics scan the pixel array and read the charge signals. These signals are digitised, converting them into numerical values that form the pixels of the image. Differences in the signals across the array appear as differences in image brightness.

Flat-panel detectors compared with photographic detection

A flat-panel detector produces an image rapidly without the film processing required for photographic detection. The digital image can be stored, transmitted and adjusted electronically.

The detector also has a wide useful exposure range: it can produce usable signals over a relatively wide range of radiation exposures. Together with efficient detection of X-rays, this can reduce the need for repeat exposures.

Photographic detection with an intensifying screen

In photographic X-ray detection, radiation reaching the film produces an exposure pattern. Chemical processing then makes this pattern visible as an image.

An intensifying screen is placed next to the film. It contains a material called a phosphor, which absorbs X-rays and emits visible light. This light also exposes the film, so the screen helps the incident X-rays produce a stronger exposure of the film.

Because the screen makes more effective use of the X-rays, a suitable image can be obtained with a lower X-ray exposure than with film alone. This reduces the patient’s radiation dose.

Fluoroscopic image intensification

An image intensifier converts the X-rays that pass through a patient into a bright visible image. Different amounts of X-rays pass through different parts of the body, creating a pattern that the intensifier turns into an image.

How the image intensifier works

The transmitted X-rays strike an input phosphor, which converts them into visible light.
The light strikes a photocathode, causing it to release electrons in a pattern corresponding to the light image.
An electric field accelerates and focuses the electrons onto a smaller output phosphor.
The electrons strike the output phosphor, producing a brighter visible image that can be displayed.

The electrons gain energy as they accelerate, and focusing them onto the smaller output phosphor concentrates the image. Together, these effects increase the brightness of the output image.

Brightness and real-time viewing

The increased brightness allows a useful image to be obtained at a lower X-ray exposure rate than would otherwise be needed for direct viewing. Fluoroscopy can display a sequence of images in real time, allowing internal movement to be observed.

X-ray contrast enhancement

An X-ray image shows differences in the intensity of X-rays transmitted through the body. If two neighbouring regions transmit different intensities, they can be distinguished on the image. Some soft tissues absorb X-rays by similar amounts, so their boundaries can be difficult to see.

A contrast medium is a substance introduced into a region to make it easier to distinguish from surrounding tissue. An X-ray opaque contrast medium absorbs or attenuates X-rays more strongly than the surrounding tissues. Less X-ray intensity is therefore transmitted through the region containing it, increasing the difference between that region and its surroundings on the image.

X-ray contrast enhancement: barium meal

In a barium meal, the patient swallows a preparation containing barium sulfate. It coats or fills parts of the upper digestive tract and attenuates X-rays more strongly than the surrounding soft tissues. The difference in transmitted intensity makes the outline of those parts visible on the X-ray image.