Types of Flat Panel Detector and How to Choose Between Them

Layers of a digital flat panel detector

Most panels share a similar structure: a conversion layer that reacts to X-rays, an active matrix of thin-film transistors (a TFT flat panel detector) that stores and reads out the charge pixel by pixel, readout electronics, and a housing with a protective cover. A carbon-fibre front plate keeps absorption of the beam low.

Image size, pixel pitch and bit depth are set by the matrix and electronics. The conversion layer is where the main technology split occurs.

Flat panel detector direct vs indirect conversion

In direct conversion, a photoconductor such as amorphous selenium turns X-ray photons straight into electric charge, which the TFT matrix collects. There is little light spread, so resolution can be very high, and this design is common in mammography. In indirect conversion a scintillator first turns X-rays into visible light, which a photodiode array and TFT matrix then convert to charge.

Indirect panels based on amorphous silicon dominate general radiography and fluoroscopy, because they offer a good balance of efficiency, ruggedness and price. Direct panels are more sensitive to temperature and handling.

CsI vs GOS scintillators

The two scintillator materials used in indirect panels are caesium iodide (CsI) and gadolinium oxysulfide (GOS). CsI is grown in needle-like columns that guide light toward the photodiodes, which limits spread and generally gives better sharpness and dose efficiency at the same thickness. GOS is a powder layer that is cheaper to manufacture and robust, but the scattered light lowers resolution somewhat.

For high-volume general radiography, a CsI panel is usually preferred when dose efficiency matters, while GOS panels can serve cost-sensitive rooms. Always compare measured detective quantum efficiency and modulation transfer function rather than the scintillator name alone.

• CsI: needle structure, higher sharpness and dose efficiency
• GOS: powder phosphor, lower cost, slightly lower resolution
• Amorphous selenium: direct conversion, specialised applications

Flat panel detector vs CCD and CMOS

CCD-based systems image a scintillator through a lens or fibre-optic reducer onto a small sensor, which loses light and limits field size. They are found in some dental and small-field products, while large-area general radiography almost always uses TFT flat panels. CMOS sensors offer fast readout and are used in some compact and high-frame-rate panels.

For fluoroscopy, you need a dynamic panel that can read out many frames a second with low electronic noise. The best-known example in our range is the 43 × 43 cm dynamic detector of Apollo DRF, which supports continuous and pulsed fluoroscopy as well as radiography. More on this is in flat panel detectors for fluoroscopy.

Fixed or wireless, and what to ask before buying

Radiography rooms often combine fixed panels in the table and stand with a wireless one. Villa's Moviplan iC DR is configured with fixed and wireless flat-panel detectors in the table bucky and chest stand. If mobility matters, read our guide to the wireless flat panel detector.

Also assess resistance to shocks and liquids, the housing protection class, warm-up time, the calibration procedure and how the supplier provides a loan detector during repair. These points rarely appear in a brochure, but they decide how smoothly a panel works in a busy room.

Ask for size, pixel pitch, scintillator, DQE and MTF data, frame rate if used in fluoroscopy, weight, the warranty and the cost of replacement. Check also the current UAE requirements for medical device registration with the relevant authorities. For a configuration proposal, contact Beckfinity.