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
Mammography
Digital subtraction angiography (DSA) (current module)
DSA systems are specifically designed to capture rapid sequences of high-contrast, high-resolution images while maintaining tight synchronisation with contrast injection.
This requires a dedicated imaging chain, capable of high frame rates, short X-ray pulses, and real-time digital processing.
System architecture
A modern angiography suite integrates four major components:
| Component | Purpose |
|---|---|
| High-performance X-ray generator and tube | Produces short, high-mA pulses at 60–80 kVp for crisp, low-blur frames. |
| Flat-panel detector (FPD) | Captures images at up to 30 frames per second with high DQE and low lag. |
| Digital acquisition system | Manages synchronisation, digitisation, and subtraction processing. |
| Automated injector and control interface | Coordinates contrast injection with image acquisition. |
All subsystems are synchronised by the system controller, ensuring precise timing between exposure, detector readout, and contrast delivery.
X-ray generator and exposure control
Pulsed X-ray output
- DSA requires short, high-intensity exposures (3–15 fps typical).
- Each frame is produced by a brief pulse (<10 ms) at high tube current (hundreds of mA).
- Short pulses freeze motion and reduce blur from cardiac or respiratory movement.
Automatic exposure control (AEC)
- Continuously adjusts tube current to maintain consistent detector signal.
- Must respond dynamically during the run as patient thickness and attenuation change with projection angle.
Beam quality
- Tube potential: typically 60–80 kVp, optimised for iodine contrast (K-edge = 33 keV).
- Added copper filtration (0.1–0.3 mm Cu) removes unnecessary low-energy photons, reducing skin dose.
Image acquisition and timing
Frame rate and pulse width
- Typical acquisition rates:
- 3–7.5 fps – abdominal or peripheral angiography
- 15–30 fps – neuroangiography or cardiac imaging
- Pulse width: 5–10 ms (shorter for higher temporal resolution).
- Shorter pulse → less motion blur but requires higher mA (↑ instantaneous tube load).
Timing synchronisation
- Precise coordination between exposure and contrast injection is critical.
- Automated injectors are synchronised electronically with the imaging system, triggering exposures at the onset of contrast arrival.
- In cardiac and neuro DSA, exposures can be ECG-gated to reduce pulsation artefacts.
Acquisition modes
| Mode | Description | Clinical use |
|---|---|---|
| Mask–run subtraction (standard DSA) | Acquires mask, then live series during contrast injection. | Standard for most vascular studies. |
| Fluoro-DSA / Road-map mode | Combines continuous fluoroscopy with a subtracted background image for catheter guidance. | Interventional procedures. |
| Bolus chase / stepping table DSA | Table moves sequentially during run to follow peripheral contrast bolus. | Lower limb or peripheral arterial studies. |
| Biplane acquisition | Two orthogonal imaging chains acquire simultaneously (e.g. neurointervention). | Reduces contrast load and acquisition time. |
| Rotational DSA (3D-RA) | Tube rotates around patient during single contrast injection; 3D volume reconstructed via cone-beam CT algorithms. | 3D vessel visualisation and stent planning. |
Image processing and data handling
- On-board digital processors perform real-time subtraction, pixel shift, and temporal filtering.
- Bit depth (14–16 bit) provides sufficient dynamic range to visualise both dense contrast and fine collateral flow.
- Temporal filters smooth noise while maintaining contrast edge integrity.
- Edge enhancement and dynamic range compression improve vessel visibility for display and recording.
Key points and exam tips:
- DSA systems use pulsed X-ray generators and high-frame-rate detectors for dynamic vascular imaging.
- Short pulse width minimises motion blur but increases instantaneous tube load.
- Detector requirements: high DQE, linear response, minimal lag, and wide dynamic range.
- Acquisition modes include standard subtraction, road-mapping, and 3D rotational angiography.
- Precise synchronisation with contrast injection is critical for optimal vessel opacification.
- Common exam question: “Describe the system design features of a modern DSA unit and explain how they enable high temporal and spatial resolution imaging.”
Up next
Next, we will move on to Contrast Media and Injection Dynamics, which will explain the physics of iodine contrast, injection parameters, and how flow dynamics, vessel size, and timing influence image quality in DSA.