X-ray physics notes curriculum
Fundamentals of radiation
The X-ray machine
Production of X-rays (current module)
Interaction of radiation with matter
X-ray detection and image formation
Image quality
Radiation safety in X-ray imaging
Fluoroscopy
Mammography
The diagnostic X-ray beam can be described by two key physical properties: quality and quantity. These determine the penetrating power, intensity, and clinical performance of the beam.
Optimising both is essential for balancing image contrast and radiation dose.
Beam quantity
Beam quantity refers to the total number of X-ray photons emitted per unit time or per exposure.
It determines the intensity or output of the beam (measured in mGy at a specific distance).
Beam Quantity ∝ mAs × (kVp)2
- mAs (milliampere-seconds) = tube current × exposure time.
- Doubling mAs doubles the number of photons produced.
- Increasing kVp increases both the number and energy of photons, roughly following a squared relationship.
Factors affecting beam quantity
| Parameter | Effect on Quantity | Explanation |
|---|---|---|
| mA (tube current) | ↑ mA → ↑ number of photons (linear relationship). | More electrons accelerated from cathode → more photon-producing collisions. |
| Exposure time (s) | ↑ time → ↑ total photon output (linear). | Longer exposure duration allows more electrons to strike target. |
| kVp (tube voltage) | ↑ kVp → large increase in photon number (∝ kVp²). | Higher energy electrons produce more Bremsstrahlung photons. |
| Filtration | ↑ filtration → ↓ total photons (low-energy photons removed). | Beam hardening reduces photon count but improves efficiency. |
| Target material (Z) | ↑ Z → ↑ photon production efficiency. | Stronger nuclear field enhances Bremsstrahlung probability. |
| Generator type | High-frequency → ↑ output (constant potential). | Reduced voltage ripple increases mean photon energy and photon yield. |
Clinical Implications of Beam Quantity
- Image brightness/detector exposure is directly related to photon quantity.
- Patient dose increases proportionally with mAs (assuming all other factors constant).
- Automatic Exposure Control (AEC) adjusts mAs automatically to achieve consistent image receptor exposure across varying patient thicknesses.
Beam quality
Beam quality refers to the mean energy and penetrating ability of the X-ray beam.
It is determined mainly by tube voltage (kVp) and filtration.
- High-quality beam → more penetrating → lower subject contrast.
- Low-quality beam → less penetrating → higher contrast, higher dose per photon.
Measured by:
- Half-Value Layer (HVL): thickness of a specified material (usually aluminium) that reduces beam intensity by 50%.
- Higher HVL = harder (higher-quality) beam.
Factors affecting beam quality
| Parameter | Effect on Quality | Explanation |
|---|---|---|
| kVp | ↑ kVp → ↑ mean photon energy (beam becomes harder). | Electrons strike target with more energy → produce higher-energy photons. |
| Filtration | ↑ filtration → ↑ mean energy (beam hardening). | Low-energy photons removed from spectrum. |
| Target material (Z) | ↑ Z → ↑ mean photon energy. | Higher binding energies and stronger nuclear fields shift spectrum to higher energies. |
| Voltage ripple | ↓ ripple → ↑ mean energy. | Constant potential output ensures electrons always accelerated at high energy. |
What does changing the quality and quantity of the beam do to the image and patient dose?
| Parameter | Image Effect | Dose Effect |
|---|---|---|
| ↑ kVp | ↓ Image contrast (more penetration) | ↓ Dose per photon (more efficient beam) |
| ↓ kVp | ↑ Image contrast | ↑ Patient dose per photon (more absorption) |
| ↑ mAs | ↑ Image brightness (film), ↑ detector exposure (digital systems) | ↑ Dose (directly proportional) |
| ↑ Filtration | ↓ Contrast (removes soft photons) | ↓ Dose (less skin exposure) |
Summary:
- kVp → controls beam quality and intensity (non-linear).
- mAs → controls beam quantity (linear).
- Filtration → removes low-energy photons → reduces dose, increases beam hardness.
The 15% rule
A useful rule of thumb in practice is the 15% rule.
Increasing kVp by 15% approximately doubles the beam intensity (quantity).
To maintain the same receptor exposure, mAs must be halved.
| Change | Effect |
|---|---|
| +15% kVp | ×2 beam intensity |
| −15% kVp | ÷2 beam intensity |
This allows the operator to control contrast and dose whilst maintaining a very similar detector exposure.
Key takeaways and exam tips:
- Beam quantity = total photon number → ∝ mAs × (kVp)².
- Beam quality = mean photon energy → determined by kVp and filtration.
- 15% rule: ↑ kVp by 15% ≈ doubling of intensity → halve mAs to compensate.
- Half-Value Layer (HVL) measures beam hardness.
- Filtration reduces skin dose and removes non-diagnostic photons.
- Common exam question: “Explain how beam quality and quantity affect image contrast and patient dose.” “How does changing the kVp alter the X-ray spectrum?”
Up next:
Next, we’ll move on to Filtration and Beam Hardening. We’ll explore how filtration shapes the X-ray spectrum, alters beam quality, and affects patient dose and image quality.