Composition of the X-ray spectrum

X-ray photons forming the X-ray beam that exits the tube towards the patient can be either Characteristic or Bremsstrahlung photons. The number and energy of individual photons differs. The X-ray spectrum represents the photon numbers and photon energies within the beam.

Continuous Component (Bremsstrahlung)

• Formed by photons emitted when electrons are decelerated in the nuclear field.
• Photon energies range continuously from near 0 up to a maximum equal to the kinetic energy of the incident electron (Emax = e·kVp).
• The distribution is skewed: most photons have energies around one-third of Emax.

Discrete Component (Characteristic Radiation)

• Appears as sharp spikes superimposed on the continuous spectrum.
• Produced when inner-shell vacancies in the target atom are filled by higher-shell electrons.
• Energies correspond exactly to differences in shell binding energies and are unique to the target material.

Characteristic radiation examples by target:

• Tungsten: K_α ≈ 59 keV, K_β ≈ 67 keV.
• Molybdenum: K_α ≈ 17.4 keV, K_β ≈ 19.6 keV.
• Rhodium: K_α ≈ 20.2 keV, K_β ≈ 22.7 keV.

 

Factors influencing the X-ray spectrum (SUMMARY)

Parameter	Effect on Spectrum	Explanation
kVp (tube voltage)	↑ kVp → increases maximum photon energy (Emax) and average photon energy; total intensity ↑ markedly.	Electrons have greater kinetic energy → produce more Bremsstrahlung photons at higher energies.
mA / exposure time (mAs)	↑ mAs → increases number of photons; shape unchanged.	More electrons striking the target → proportional increase in photon quantity.
Target material (atomic number, Z)	↑ Z → higher Bremsstrahlung output and higher-energy characteristic lines.	Stronger nuclear field → greater deceleration and higher photon energy.
Filtration	Removes low-energy photons; decreases total intensity; increases mean photon energy (beam hardening).	Low-energy photons absorbed by filters or tube housing.
Voltage ripple / generator type	↓ ripple → smoother, higher-mean-energy output.	High-frequency generators maintain nearly constant potential, producing a more efficient, harder beam.

Let’s discuss these in a little bit more detail. These are common themes that will appear throughout the course. You’ll notice we’ve already touched upon generator types and target materials previously. Never a bad idea to revisit concepts though!

Effects of tube voltage (kVp)

Increase in kVp:

• Raises the maximum photon energy (rightward shift of spectrum endpoint).
• Increases the average photon energy (beam becomes more penetrating).
• Increases total photon number (area under curve).
• Reduces image contrast due to more tissue penetration/ less attenuation.

Relationship:

Beam intensity ∝ (kVp)²

A 15% increase in kVp roughly doubles beam intensity (the 15% rule).

Effects of tube current (mA) and exposure time (s)

• Beam quantity is directly proportional to mAs.

Quantity ∝ mA × time

• Doubling mAs doubles the number of photons but does not change their energies.
• Spectrum shape remains the same; only amplitude changes.

Effects of filtration

Filtration modifies the emitted spectrum by preferentially removing low-energy photons, resulting in:

• Reduced total intensity (area under curve decreases).
• Increased mean photon energy (spectrum shifts right).
• Reduced patient skin dose.
• More uniform beam quality.

Half-Value Layer (HVL):
The thickness of material (usually aluminium) required to reduce beam intensity by half.

• HVL increases as beam quality (mean energy) increases.
• Diagnostic requirement: ≥ 2.5 mm Al equivalent for tube potentials above 70 kVp.

Effect of target material

Higher-Z targets → more efficient X-ray production and higher-energy photons.

Material	Z	Characteristic Energies (keV)	Beam Type / Use
Tungsten	74	Kα = 59, Kβ = 67	Broad spectrum for general imaging and CT
Molybdenum	42	Kα = 17.4	Low-energy beam for soft-tissue contrast (mammography)
Rhodium	45	Kα = 20.2	Slightly harder beam for thicker breasts

Effects of generator type (voltage ripple)

• Single-phase systems: high voltage ripple (100%) → photons produced in bursts → broader, less efficient spectrum.
• Three-phase (6- or 12-pulse): ripple reduced to 4–13% → more consistent beam output.
• High-frequency generators: ripple <1% → almost constant potential; produces a harder, more efficient spectrum with higher mean energy for the same kVp.

 

Key takeaways and exam tips:

• The diagnostic X-ray spectrum = continuous Bremsstrahlung + discrete characteristic lines.
• Emax = applied tube voltage (kVp).
• Increasing kVp → ↑ Emax, ↑ mean energy, ↑ output.
• Increasing mAs → ↑ output, same shape.
• Filtration → ↓ intensity, ↑ mean energy.
• Beam quality measured by half-value layer (HVL).
• High-frequency generators produce the most constant (efficient) spectrum.
• Common exam question: “Describe and explain the shape of the diagnostic X-ray spectrum and the factors that affect it.”

Up next:

In this section we’ve examined the factors that influence the X-ray spectrum. We described spectrum changes by talking about the intensity/output/number of photons in the beam – this is the beam quantity, and we’ve talked about the average energy of the beam – this is called beam quality. Let dive deeper into these two terms. I’ll leave a little summary here of the factors above and how they changed quantity and quality of the beam.

Property	Definition	Main Determinants	Measurement
Quality	Mean photon energy / penetrating power	kVp, filtration, generator type	HVL (mm Al)
Quantity	Total photon output	mAs, kVp², target Z	Exposure or air kerma (mGy)