Types of Digital Radiography Systems

There are two main categories:

System	Acronym	Detection Process	Readout
Computed Radiography	CR	X-rays → trapped electrons in photostimulable phosphor (BaFBr:Eu²⁺)	Laser scanning of image plate
Direct Digital Radiography	DR	X-rays → electrical charge This conversion can be either indirect or direct	Electronic readout from detector array

Let’s break these down further.

Computed Radiography (CR)

A. Image Plate Composition

• Photostimulable phosphor (PSP) plate made of barium fluorobromide doped with europium (BaFBr:Eu²⁺).
• The phosphor stores energy from absorbed X-rays in the form of trapped electrons.

B. Latent Image Storage

• When exposed to X-rays, electrons in the phosphor crystal are excited to higher energy levels and become trapped in electron traps (F-centres).
• The number of trapped electrons is proportional to local X-ray exposure. The trapped electrons form a latent image.

C. Image Readout

1. The plate is placed in a CR reader.
2. A red laser scans across its surface, releasing trapped electrons.
3. As electrons return to ground state, they emit blue light photons.
4. The emitted light is collected by a photomultiplier tube (PMT), converted to an electrical signal, and digitised.
5. The plate is then erased by bright white light to remove residual signal for reuse.

Direct Digital Radiography (DR)

Unlike CR, DR detectors provide instant electronic readout with no intermediate plate scanning.

There are two main detector types:

Detector Type	Conversion Mechanism	Example Material	Notes
Indirect conversion	X-rays → Light → Electrical signal	CsI, Gd₂O₂S	Used in most general DR systems and fluoroscopy
Direct conversion	X-rays → Electrical signal (no light)	Amorphous selenium (a-Se)	Used mainly in mammography for high spatial resolution

NB: These are both DIRECT digital radiography systems. Meaning they instantaneously create a digital image. Don’t get confused with the two direct digital detector types which can be either direct or indirect.

They work as follows (more detail in later lessons)

Indirect Conversion Systems

1. Scintillator layer (e.g. CsI:Ti or Gd₂O₂S:Tb) converts X-rays to visible light.
2. Photodiode layer (amorphous silicon) converts light to electrical charge.
3. Thin-Film Transistor (TFT) array collects and transfers the charge for digitisation.

Properties:

• High X-ray absorption efficiency (especially CsI, due to columnar crystal structure).
• Some light spread → slight reduction in spatial resolution.
• Used widely in general radiography, fluoroscopy, and portable systems.

Direct Conversion Systems

1. X-rays interact directly with amorphous selenium (a-Se).
2. Ionisation produces electron–hole pairs in proportion to X-ray intensity.
3. An applied electric field drives the charges toward collection electrodes.
4. Thin-Film Transistor (TFT) array reads out the charge signal and digitises it into an image.

Properties:

• No light diffusion → excellent spatial resolution.
• Best suited to low-energy imaging (e.g. mammography).
• Slightly lower absorption efficiency at high energies due to thin selenium layer.

Comparison of CR and DR

Feature	Computed Radiography (CR)	Digital Radiography (DR)
Detector	PSP plate (BaFBr:Eu²⁺)	Flat panel (CsI or a-Se)
Readout	Laser scanning	Direct electronic (TFT array)
Acquisition time	~1–2 min	Instant
Spatial resolution	~2.5–5 lp/mm	~3–7 lp/mm
DQE (Detective Quantum Efficiency)	Moderate	High
Workflow	Cassette-based	Cassette-less, integrated
Cost	Lower initial	Higher initial, lower operating

Key Takeaways and Exam Tips:

• Digital radiography replaces film with electronic detectors that convert X-rays into digital data.
• Two main systems: CR (indirect, laser readout) and DR (flat-panel detectors).
• Indirect detectors use scintillators (e.g. CsI, Gd₂O₂S).
• Direct detectors use amorphous selenium (a-Se).
• Advantages: wide dynamic range, lower dose, faster workflow, and post-processing flexibility.
• Limitations: high cost, scatter sensitivity, dose creep risk.
• Common exam question: “Compare computed radiography (CR) and digital radiography (DR) in terms of image formation and detector performance.”

Up Next

Next, we’ll cover Computed Radiography (CR) in detail, exploring the physics of photostimulable phosphors, image readout mechanisms, plate erasure, and image quality characteristics.