Image Detection and Enhancement

Producing an X-ray beam is only half the story. Once that beam has passed through a patient, something has to catch it, turn it into a usable picture, and — wherever possible — do that while keeping the radiation dose as low as it can be. This lesson covers the detectors, screens and contrast media that make that possible.

Flat panel detectors versus film

The modern standard for capturing an X-ray image is the flat panel detector, a large, thin electronic array built from two functional layers. A scintillator layer sits on top, and its job is to absorb the incoming X-ray photons that have passed through the patient and convert their energy into a burst of visible light. Beneath the scintillator sits an array of photodiode pixels: each individual photodiode sits directly under a small patch of scintillator, and converts the local light produced immediately above it into an electrical signal. Because the array is subdivided into a fine grid of individual pixels, the pattern of electrical signals across the whole array captures the spatial pattern of the X-ray beam that reached the detector. Electronic scanning circuitry then reads out each pixel's signal in turn, very rapidly, assembling all of these individual readings into one complete digital image.

This is a fundamentally different process from older photographic film, where X-ray exposure directly and chemically alters the film's emulsion, darkening it in proportion to the X-ray dose it received. Film has to be physically removed from its cassette and chemically processed (developed) before the image can even be seen, and the result is a fixed physical object rather than digital data.

Flat panel detectors offer several practical advantages over film as a result: the image is available immediately, in digital form, without any chemical processing step; it can be viewed, enhanced, stored and transmitted electronically; and flat panel systems are generally more dose-efficient than plain film, meaning less radiation is needed to form a usable image of a given quality.

Intensifying screens

Before flat panel detectors became standard, a related idea was already in wide use to solve a closely related problem: photographic film on its own is fairly inefficient at directly absorbing X-ray photons, so producing a well-exposed film image directly from the X-ray beam alone would require a relatively high dose. An intensifying screen is a fluorescent screen placed in close, direct contact with the film, on one or both sides. X-ray photons that would otherwise pass through or barely interact with the film are absorbed by the screen instead, and the screen converts each absorbed X-ray photon's energy into a burst of many visible light photons. Because film is far more efficient at responding to visible light than to X-rays directly, this light exposes the film far more effectively than the original X-ray beam would have on its own — substantially reducing the X-ray dose needed to produce a usable image.

Fluoroscopic intensification

Fluoroscopy provides continuous, real-time X-ray imaging, rather than producing a single still image the way a conventional radiograph does. This is essential for applications such as watching a swallowed contrast medium move through the digestive tract in real time, or guiding a live medical procedure as it happens. The challenge with continuous imaging is that it would, in principle, require a continuous X-ray beam running for the whole procedure — and doing this at a high enough intensity to produce a usable image directly would deliver an impractically large radiation dose over the course of a procedure lasting minutes.

An image intensifier solves this by amplifying the very faint light output produced by the fluorescent detection stage, so that a sufficiently bright, real-time image can be viewed or recorded on a screen using a much lower continuous X-ray intensity than would otherwise be needed. This amplification step is what makes continuous fluoroscopic imaging practical without an unacceptably high dose.

Barium and iodine contrast media

Some internal structures don't naturally attenuate X-rays very differently from the tissue surrounding them. Soft tissue structures, blood vessels and the digestive tract, for example, often have attenuation coefficients close enough to their surroundings that they produce poor natural contrast on a plain X-ray image — the structure of interest simply doesn't stand out clearly from what's around it.

A contrast medium is introduced specifically to fix this. Contrast media contain a high-atomic-number element, which strongly increases X-ray absorption wherever the medium is present, so that the structure filled with contrast medium shows up with much greater contrast against the surrounding tissue.

• Barium compounds (such as barium sulfate) are typically swallowed as a "barium meal" or introduced as an enema, coating and filling the digestive tract so that its shape, lining and any abnormalities show up clearly against the surrounding soft tissue.
• Iodine-based contrast media are typically injected into the bloodstream, outlining blood vessels (useful for detecting blockages or abnormalities) or concentrating in particular organs or tumours, highlighting them against surrounding tissue.

In both cases, the physical principle is the same one covered in the next lesson on attenuation: introducing a strongly absorbing medium locally increases the attenuation coefficient of that region, so the beam emerging from it carries a much clearer, higher-contrast signature of exactly where the medium has reached.

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