Introduction to image detection and formation

Once X-rays have passed through the patient, the pattern of transmitted photons must be captured and converted into a visible image.

This process, transforming an invisible distribution of X-ray intensities into a measurable signal, lies at the heart of diagnostic radiology.

The Imaging Chain

Every radiographic system follows the same essential sequence, often referred to as the imaging chain:

1. X-ray beam generation (X-ray tube)
2. Patient interaction (attenuation via photoelectric and Compton effects)
3. Detection (capture of transmitted photons)
4. Signal conversion (X-rays → light or charge)
5. Image formation (analogue or digital signal recorded)
6. Display and storage (viewed on film or monitor, archived digitally)

The detection and signal conversion stages determine how efficiently the system converts X-rays into image data.

Purpose of the Detector

The image receptor (detector) serves two fundamental roles:

• Capture: detect the intensity pattern of transmitted X-rays emerging from the patient.
• Convert: transform that invisible photon distribution into a visible image or electronic signal.

Ideal detector characteristics:

• High detection efficiency (absorbs most incident photons).
• High spatial resolution (preserves fine detail).
• Low noise (minimal signal variability).
• Linear and predictable response to exposure.
• Wide dynamic range (detects both low and high exposures without saturation).

Analogue vs Digital Detection

We will discuss these in greater depth in the following sections, but here is a birds-eye-overview:

Feature	Film–Screen (Analogue)	Digital (CR/DR)
Signal type	Continuous optical density	Discrete digital values
Detector	Film + intensifying screen	Photostimulable plate (CR) or flat panel (DR)
Dynamic range	Narrow (limited latitude)	Wide (exposure-tolerant)
Response	Non-linear (H&D curve)	Linear (proportional to exposure)
Post-processing	None (fixed image)	Extensive (windowing, enhancement)
Image access	Delayed (chemical processing)	Immediate (digital display)

Analogue systems form a latent image on film, later developed chemically. Digital systems record a numerical signal that can be processed, displayed, and transmitted electronically.

Pathways of X-ray Detection

There are two fundamental detection pathways, depending on whether the X-rays are first converted to light or directly to electrical charge:

1. Indirect detection:
   • X-rays → visible light (scintillator) → electrical charge.
   • Used in film, digital radiography and fluoroscopy systems.
2. Direct detection:
   • X-rays → electrical charge (no light stage).
   • Used in amorphous selenium detectors, used in digital radiography, particularly in mammography.

Conversion Efficiency and Image Quality

Three key parameters describe how efficiently a detector converts X-rays into image information:

Parameter	Definition	Clinical Relevance
Quantum Detection Efficiency (QDE)	Fraction of incident photons absorbed	Higher QDE → better dose efficiency
Modulation Transfer Function (MTF)	Ability to preserve contrast at different spatial frequencies	Higher MTF → better fine detail
Detective Quantum Efficiency (DQE)	Combined measure of efficiency and noise performance	Higher DQE → better image quality at lower dose

These parameters will be covered in more detail later in this section.

Overview of system type

Again, this is a basic summary. More on these systems in the next few lessons!

System Type	Detection Mechanism	Signal Output
Film–Screen Radiography	X-rays → Light (screen) → Film density	Optical image
Computed Radiography (CR)	X-rays → Trapped electrons → Laser-stimulated light	Digital image
Direct Digital Radiography (DR)	X-rays → Charge (a-Se or CsI + photodiodes)	Digital image
Fluoroscopy	Continuous X-rays → Light → Electronic signal (real-time)	Dynamic digital display

Key Takeaways and Exam Tips

• Image formation begins when transmitted X-rays are detected and converted into a measurable signal.
• The detector partially determines image sharpness, contrast, noise, and dose efficiency.
• Two main detection pathways: indirect (via light) and direct (via charge).
• Film–screen systems are analogue; CR and DR systems are digital.
• Digital detectors offer wide latitude and lower dose for similar image quality.
• Common exam question: “Outline the steps in the X-ray imaging chain and describe the function of the image receptor.” “Name 5 desirable characteristics of a detector system.”

Up Next

Next, we’ll explore Film–Screen Radiography, where we’ll look at how X-rays were historically detected using intensifying screens and film, including latent image formation, characteristic curves, and the limitations that led to digital imaging. This is mentioned in most radiology physics syllabi but is rarely tested in great depth. Commonly one is asked to compare film to digital systems.