Fluoroscopy and real-time imaging – Learn Radiology Physics

This lesson will serve as an overview of fluoroscopy and real-time imaging. These concepts will be discussed in more depth later (in the fluoroscopy module).

Fluoroscopy provides real-time X-ray imaging, allowing dynamic visualisation of moving structures such as the gastrointestinal tract, vascular system, or guidewires during interventional procedures.

Unlike static radiography, fluoroscopy delivers a continuous or pulsed X-ray beam, with images displayed and refreshed in real time.

Principles of Fluoroscopy

Purpose

To produce a live X-ray image that shows anatomy and motion in real time, enabling diagnostic assessment and procedural guidance.

Basic Components of fluoroscopy systems

X-ray tube – operated at lower mA but similar kVp to radiography.
Image receptor – traditionally an image intensifier (II), now largely replaced by flat-panel detectors (FPDs).
Display system – real-time monitor display (25–30 frames per second).
Automatic brightness control (ABC) – adjusts tube output to maintain consistent image brightness as patient thickness changes.

As mentioned above there are two different image receptor systems.

Let’s start by looking at image intensifier systems (which are largely being phased out).

Image Intensifier Systems (also known as conventional fluoroscopy)

Although now being phased out, image intensifiers remain a cornerstone concept for understanding fluoroscopic imaging physics. You’ll see the initial process is similar to indirect digital detection. But instead of electrons being stored in a TFT, they are focused and accelerated towards an output phosphor which produces light that is used to create a real time image.

A. Structure

Input phosphor (CsI) – converts X-rays to visible light.
Photocathode – emits electrons when stimulated by light photons.
Electrostatic focusing lenses – accelerate and focus electrons toward the output.
Anode – maintains potential difference (~25–35 kV) to accelerate electrons.
Output phosphor (ZnCdS) – converts electron energy back into visible light, producing a bright, minified image.

B. Mechanism of Image Formation

X-rays exiting the patient strike the input phosphor, producing light photons.
These light photons cause the photocathode to emit electrons proportional to the light intensity.
The electron beam is accelerated and focused onto the smaller output phosphor, producing a minified image (∼1–2 cm diameter).
The light emitted by the output phosphor is optically coupled to a video camera or CCD sensor, which digitises the signal for display.

Image Intensifier Gain

The II increases image brightness by combining two mechanisms:

Type of Gain	Definition	Typical Value
Flux gain	More light photons produced per X-ray photon	~50–70
Minification gain	Same number of electrons focused onto smaller output area	(Input diameter / Output diameter)² ≈ 100–300
Total brightness gain	Product of flux and minification gain	3,000–10,000×

As I’ve said though, image intensifiers are being phased out. Why is this?

Limitations of Image Intensifiers

Geometric distortion: pincushion or S-shaped due to curved input phosphor.
Vignetting: reduced brightness at edges.
Veiling glare: internal light scatter reducing contrast.
Lag: residual image after rapid motion.
Bulky design and limited field of view.

These drawbacks led to the development of modern flat-panel fluoroscopy detectors.

Let’s look at the second image receptor system, flat-panel detectors.

Flat-Panel Detectors (Modern Fluoroscopy)

Luckily for us, these detectors are very similar to the detectors we’ve already looked at in direct digital radiography.

These detectors also utilise either indirect or direct conversion.

Type	Conversion Mechanism	Common Material	Advantages
Indirect FPD	X-rays → Light → Charge	CsI (scintillator)	High efficiency, real-time response
Direct FPD	X-rays → Charge	a-Se	Excellent resolution, used in mammographic or high-detail systems

Each pixel contains a photodiode and a TFT switch, allowing electronic readout at high frame rates (typically 15–30 fps).

Advantages Over Image Intensifiers

No geometric distortion or vignetting.
Compact, flat design with wide dynamic range.
Better spatial and contrast resolution.
Digital integration with PACS and dose monitoring.
No degradation with age or magnetic interference.

So those are the two predominant systems for detecting and displaying X-rays in fluoroscopy.

I want to touch on a few more points regarding fluoroscopy. Bearing in mind this is still only an introduction.

Pulsed Fluoroscopy

Rather than a continuous beam, modern fluoroscopy systems use pulsed X-ray emission. This is typically 3–15 pulses per second.

Each pulse produces a single frame, allowing high-quality imaging with reduced dose.

Benefits:

Up to 50–80% reduction in patient dose.
Reduced motion blur.
Easier image storage (discrete frames).
Enables dose-rate tailoring to procedure type.

Automatic Brightness Control (ABC)

ABC maintains consistent image brightness by automatically adjusting exposure factors:

If image brightness falls → system increases mA, kVp, or pulse width.
If image brightness rises → system reduces output accordingly.

Different control modes prioritise:

Low noise (higher dose).
Low dose (more noise).
Constant image brightness for visual consistency.

Where fluoroscopy differs greatly from traditional radiography is the exposure times, and ultimately, patient dose. As a result, we need to implement dose saving mechanisms to reduce the total patient dose. I’ll summarise these in the following table.

Dose Reduction Techniques

Technique	Principle
Pulsed fluoroscopy	Reduces total X-ray on-time.
Last-image hold (LIH)	Allows viewing of final frame without additional exposure.
Tight collimation	Reduces irradiated field and scatter.
Low-dose mode	Lowers mA and pulse rate.
Remove grid (for small patients)	Reduces dose without major loss of contrast.
Use of copper filtration	Removes low-energy photons that add dose but not image quality.
Positioning (source below table)	Reduces operator dose from scatter.

Key Takeaways and Exam Tips:

Fluoroscopy provides real-time X-ray imaging, traditionally using image intensifiers, now replaced by flat-panel detectors.
Image intensifiers amplify brightness via flux and minification gain.
Flat-panel systems eliminate distortion, offer better resolution, and integrate digitally.
Pulsed fluoroscopy, LIH, and collimation are key dose-saving techniques.
Automatic brightness control (ABC) maintains image brightness dynamically.
Common exam question: “Outline methods used to reduce patient dose in fluoroscopy.” “Compare and contrast image intensifier and flat-panel detectors.”