Direct digital radiography (DR)

Direct digital radiography (DR) systems use flat-panel detectors that convert X-rays into digital signals immediately, without cassette handling or scanning.

These detectors measure the intensity of X-rays transmitted through the patient at each location across the field of view, and convert that information into electronic signals representing the local exposure level. The resulting electrical data are digitised and displayed almost instantaneously as an image.

DR represents the current standard in diagnostic imaging. It provides rapid image acquisition, high dose efficiency, and consistent image quality across examinations.

There are two main types of DR detectors, defined by how X-rays are converted into electrical signals:

1. Indirect conversion detectors:
   • X-rays → visible light (scintillator) → electrical signal.
   • Used in most general radiography and fluoroscopy systems.
2. Direct conversion detectors:
   • X-rays → electrical charge directly (no light stage).
   • Used mainly in mammography for high-resolution imaging.

Structure of a Flat-Panel Detector

All DR detectors share a common layered design:

1. X-ray conversion layer (indirect scintillator or direct semiconductor)
2. Photodiode or charge collection layer
3. Thin-Film Transistor (TFT) array. Switches that control readout from each pixel.
4. Readout electronics and analogue-to-digital converter (ADC).
5. Protective and support layers for mechanical stability.

Let’s start by looking at indirect detectors.

Indirect Conversion Detectors

Mechanism

1. X-ray absorption
   • The incoming X-rays are absorbed by a scintillator layer such as cesium iodide (CsI:Ti) or gadolinium oxysulfide (Gd₂O₂S:Tb).
   • The scintillator converts X-ray energy into visible light photons.
2. Light-to-charge conversion
   • Beneath the scintillator is an amorphous silicon (a-Si) photodiode layer.
   • The emitted light photons generate electron–hole pairs within the photodiode.
3. Charge storage and readout
   • The resulting electrical charge is collected and stored temporarily in a capacitor associated with each pixel.
   • The TFT array acts as an electronic “gate” to release the charge sequentially for readout and digitisation.

Scintillator materials

Material	Structure	Light spread	Applications
CsI:Ti	Columnar (needle-like crystals)	Minimal → high resolution	Common in DR and fluoroscopy
Gd₂O₂S:Tb (Gadox)	Granular	More diffuse → slightly lower resolution	More robust, cheaper

Columnar CsI acts like optical fibre bundles, channeling light toward the photodiodes and limiting lateral spread, thus preserving sharpness.

How do direct detectors differ? Essentially they DO NOT have a scintillation layer.

Direct Conversion Detectors

Mechanism

1. X-ray absorption
   • The detector uses a semiconductor layer of amorphous selenium (a-Se) as the photoconductor. Note this is different from the amorphous silicon (a-Si) photodiode layer in indirect detectors.
   • X-ray photons ionise selenium atoms, creating electron–hole pairs directly within the material.
2. Charge collection
   • A high electric field is applied across the a-Se layer.
   • Electrons and holes drift in opposite directions and are collected at electrodes, generating a charge pattern proportional to the local X-ray intensity.
3. Readout
   • Each pixel’s charge is stored on a capacitor connected to the TFT array and read out sequentially for digitisation. Same as in indirect detectors.

A few notes about direct detectors:

• No light emission or spread → excellent spatial resolution.
• Thickness of a-Se layer limits absorption efficiency (good for low-energy photons but less efficient for higher energies).
• Used mainly in mammography and high-detail applications.

Readout via Thin-Film Transistor (TFT) Array

This is a common pathway for both indirect and direct detectors.

• Each pixel is connected to a TFT switch and a storage capacitor.
• When the readout sequence begins, a row of TFTs is activated simultaneously, discharging stored charge into column readout amplifiers.
• The analogue voltage signals are converted to digital numbers by an analogue-to-digital converter (ADC).
• The resulting digital matrix forms the radiographic image.

Pixel pitch (size) typically ranges from 100–200 μm for general radiography and 50–100 μm for mammography. Smaller pixels → higher spatial resolution but more electronic noise.

Comparison: Direct vs Indirect Detectors

Feature	Indirect (CsI / Gadox)	Direct (a-Se)
Conversion	X-rays → Light → Charge	X-rays → Charge
Intermediate step	Scintillation (light emission)	None
Light spread	Some (less with CsI)	None
Spatial resolution	Slightly lower	Higher
Absorption efficiency	Higher (thicker scintillator)	Lower (thin layer)
Noise performance (DQE)	Very good	Excellent (low noise)
Best suited for	General radiography, fluoroscopy	Mammography, fine-detail imaging

Key Takeaways and Exam Tips:

• DR uses flat-panel detectors with TFT arrays for instant digital readout.
• Two main types: indirect (scintillator-based) and direct (a-Se).
• Indirect detectors (CsI/Gd₂O₂S) are used in general imaging; direct detectors (a-Se) in mammography.
• Indirect = X-rays → Light → Charge
• Direct = X-rays → Charge.
• Common exam question: “Compare direct and indirect digital detectors in terms of design, image quality, and clinical applications.”

Up Next

Next, we’ll move on to Image Formation and Display, where we’ll explore how the electronic signal from the detector is digitised, processed, and displayed. We’ll cover sampling, bit depth, matrix size, and windowing in digital imaging.