X-ray physics notes curriculum
Fundamentals of radiation
The X-ray machine
Production of X-rays
Interaction of radiation with matter
X-ray detection and image formation
Image quality
Radiation safety in X-ray imaging
Fluoroscopy (current module)
Mammography
The purpose of fluoroscopy is to display anatomical motion and procedural detail in real time.
Achieving this requires balancing image quality with radiation dose.
Fluoroscopic image quality is determined by three core parameters, spatial resolution, contrast resolution, and temporal resolution, each influenced by system design and exposure settings.
These are summarised below:
| Parameter | Definition | Main Influencing Factors |
|---|---|---|
| Spatial resolution | Ability to distinguish small, high-contrast structures | Detector pixel size, focal spot, magnification, motion blur |
| Contrast resolution | Ability to differentiate tissues of similar attenuation | Beam energy (kVp), scatter, detector DQE, noise |
| Temporal resolution | Ability to visualise motion without blur or lag | Frame rate, pulse width, detector readout speed |
Optimisation involves adjusting these parameters to meet the clinical task while keeping dose as low as reasonably achievable.
Spatial Resolution
Definition
Spatial resolution defines the smallest object size that can be visualised as distinct from its surroundings.
Factors affecting spatial resolution
| Factor | Effect | Notes |
|---|---|---|
| Focal spot size | Smaller focal spot → reduced geometric unsharpness | Limited by tube heat loading |
| Magnification mode | Smaller input field → improved geometric detail | Increases dose due to higher ABC output |
| Detector pixel size | Smaller pixels → higher limiting resolution | Determined by TFT matrix design |
| Motion blur | Longer pulse width → increased blur | Use short pulse duration for moving anatomy |
Typical limiting spatial resolution:
- Image intensifier: 2–3 lp/mm
- Flat-panel detector: 3–4 lp/mm
Contrast Resolution
Definition
Contrast resolution represents the ability to detect small differences in X-ray attenuation within the image.
Influencing factors
- kVp: higher kVp reduces subject contrast but improves dose efficiency.
- Scatter: reduces image contrast; minimised by collimation and filtration.
- Detector DQE: high-efficiency detectors preserve signal differences at lower doses.
- Image processing: contrast enhancement and windowing optimise visible range.
- Noise: quantum noise limits low-contrast detectability; increasing SNR improves contrast resolution.
For fluoroscopy, acceptable contrast resolution typically allows detection of 1–2 % differences in attenuation at standard dose rates.
Temporal Resolution
Definition
Temporal resolution determines how well motion can be followed over time.
Determinants
| Parameter | Effect on Temporal Resolution | Trade-off |
|---|---|---|
| Frame rate (fps) | Higher frame rate → smoother motion | Increases dose |
| Pulse width | Shorter pulse → less motion blur | Lower photon flux → higher noise |
| Frame averaging | Reduces noise but causes lag and blurring of motion | Adjust to procedure type |
Typical frame rates:
- 3–7.5 fps: low-dose guidance
- 15 fps: general dynamic imaging
- 30 fps: high-speed angiography or cardiac studies
Noise and Signal-to-Noise Ratio (SNR)
Let’s quickly review this concept that we’ve covered previously.
Image noise in fluoroscopy arises primarily from quantum noise (random statistical variation) in photon detection. SNR increases with the square root of detected photon number:
SNR ∝ √N
Since photon number N is proportional to dose, SNR ∝ √dose.
Improving image quality by doubling SNR therefore requires a fourfold increase in dose.
Optimisation involves finding the lowest dose that provides sufficient SNR for the clinical task.
Modulation Transfer Function (MTF) and Detective Quantum Efficiency (DQE)
MTF
Describes how image contrast is transferred at different spatial frequencies.
High MTF at fine frequencies indicates good sharpness and resolution.
DQE
Measures the efficiency with which the detector converts incident X-ray signal into useful image information.
Image Processing in Fluoroscopy
Modern systems apply real-time digital processing to enhance image perception while controlling noise.
| Technique | Purpose |
|---|---|
| Edge enhancement | Improves visibility of boundaries; may increase apparent noise |
| Temporal filtering / frame averaging | Smooths random noise across frames |
| Dynamic range compression | Preserves detail in bright and dark regions |
| Noise reduction algorithms | Maintain contrast at low dose levels |
Optimisation of Image Quality vs Dose
Fluoroscopic image optimisation balances quality and dose through parameter adjustment. Essentially adjustments can be summarised as follows.
| Adjustment | Effect on Image Quality | Effect on Dose |
|---|---|---|
| ↑ kVp | ↓ contrast, ↑ penetration | ↓ dose |
| ↑ mA | ↑ SNR, ↓ noise | ↑ dose |
| ↓ Pulse rate | ↓ temporal resolution | ↓ dose |
| ↑ Filtration | ↓ low-energy photons | ↓ skin dose |
| ↑ Collimation | ↓ scatter, ↑ contrast | ↓ DAP |
| Magnification mode | ↑ spatial resolution | ↑ dose |
Key Points and Exam Tips:
- Image quality in fluoroscopy is governed by spatial, contrast, and temporal resolution.
- Spatial resolution depends on focal spot size, magnification, and pixel size.
- Contrast resolution is limited by scatter and noise; improved by high DQE and tight collimation.
- Temporal resolution increases with frame rate but raises dose.
- SNR ∝ √dose, doubling SNR requires quadrupling dose.
- Optimisation requires balancing image requirements with ALARA principles.
- Common exam question: “Describe the factors that influence image quality in fluoroscopy and explain how they are balanced against radiation dose.”
Up next
Next, we will complete the Fluoroscopy module with Radiation Protection in Fluoroscopy Suites, describing scatter distribution, operator exposure, and the protection measures required to minimise occupational and patient dose during fluoroscopic procedures.