Lecture 19 – Beam Intensity & Penetrability Final

X-ray photon duality

X-ray photons behave as both waves and particles: as waves they have wavelength and frequency; as particles they are discrete bundles of energy (photons) that interact with matter in collisions.

X-ray photon

A massless, chargeless bundle of electromagnetic energy that travels at the speed of light and carries energy proportional to its frequency (E = h·f).

Beam quantity (intensity)

The number of x-ray photons in the beam per unit area at a given distance. It is related to patient/IR exposure and is measured in units like mGy or mR.

Beam quality (penetrability)

The ability of the x-ray beam to penetrate tissue; depends on the average photon energy. Higher quality = more penetrating beam.

Primary controller of beam quantity

mAs (milliampere-seconds). Doubling mAs doubles quantity (intensity) of photons at a given distance.

Primary controllers of beam quality

kVp and filtration. Increasing kVp or adding filtration increases beam quality (penetrability).

Effect of increasing kVp on quantity and quality

Increasing kVp increases both beam quantity (more photons) and beam quality (higher average energy).

Effect of increasing mAs on the beam

Increases beam quantity (more photons) but does not change beam quality (photon energies stay the same).

Bremsstrahlung radiation (Brems)

X-ray photons produced when a high-speed projectile electron is decelerated or ‘braked’ as it passes near the nucleus of a target atom, losing kinetic energy that is emitted as a photon with a random energy up to the set kVp (continuous spectrum).

Characteristic radiation

X-ray photons produced when a projectile electron ejects an inner-shell (usually K-shell) electron from a target atom, and an outer-shell electron drops down to fill the vacancy, emitting a photon with a discrete energy equal to the binding-energy difference between the shells (line spectrum).

Relative contributions of Brems vs characteristic (tungsten target, diagnostic kVp)

In the diagnostic kVp range above about 70 kVp, most photons are Bremsstrahlung (roughly 80–90%), while a smaller fraction (roughly 10–20%) are characteristic; below the K-shell binding energy, essentially all photons are Bremsstrahlung.

Energy levels of characteristic photons in tungsten

Characteristic x-ray photons from tungsten most importantly come from K-shell transitions and have discrete energies around 57–69 keV (differences between K and L/M/N binding energies).

X-ray emission spectrum

A graph that shows the number of x-ray photons (y-axis) at each energy level (x-axis). It has a continuous Bremsstrahlung portion plus sharp spikes at the characteristic energies of the target material.

Effect of increasing mAs on the emission spectrum

The entire emission spectrum gets taller (more photons at every energy), but the shape and average energy stay the same.

Effect of increasing kVp on the emission spectrum

The spectrum shifts to the right (higher maximum and average photon energy) and generally gets taller (more photons) – higher quantity and higher quality.

Effect of adding filtration on the emission spectrum

Filtration removes more low-energy photons, reducing overall quantity (spectrum gets shorter) but increasing average photon energy (spectrum becomes ‘harder’).

Tube output (beam intensity, course definition)

The radiation exposure produced by the x-ray tube per unit time at a given distance (often expressed as air kerma per minute, e.g., Gy/min). It depends on mA, time, kVp, distance, and filtration.

Filtration (general definition)

The use of absorbing material (usually aluminum) placed in the beam to remove low-energy photons, reducing patient skin dose and increasing average beam energy.

Inherent filtration

Filtration built into the tube and housing: the glass envelope, oil, and window; typically about 0.5 mm Al equivalent.

Added filtration

Sheets of metal (usually aluminum) and the collimator mirror added to the inherent filtration to achieve the required total filtration.

Required total filtration for diagnostic x-ray tubes

At least 2.5 mm aluminum equivalent of inherent + added filtration for general radiographic equipment operating above 70 kVp (per standard regulations).

Filter units (Al/eq)

The amount of filtration is measured in aluminum equivalence (mm Al/eq), meaning the thickness of aluminum that would provide the same attenuation as the material or combination used.

Effect of filtration on beam quality and quantity

Filtration removes low-energy photons, decreasing beam quantity (intensity) but increasing average photon energy (quality) and reducing patient skin dose.

Half-value layer (HVL)

The thickness of a specified absorber (often aluminum) required to reduce the beam intensity to one-half of its original value; a measure of beam quality/penetrability.

Effect of beam hardening on HVL

As beam quality (average energy) increases, the HVL increases because a thicker absorber is needed to cut the beam intensity in half.

Beam hardening

An increase in the average energy (quality) of the x-ray beam as it passes through a filter or patient because lower-energy photons are preferentially removed. The beam becomes more penetrating but less intense overall.

Polyenergetic x-ray beam

A beam that contains photons with a range of energies from very low up to the set kVp; diagnostic x-ray beams are polyenergetic due to Bremsstrahlung production, characteristic spikes, and filtration effects.

Monoenergetic beam (example)

A beam in which all photons have the same energy, like many gamma-ray sources from radioisotopes; not typical of diagnostic x-ray tubes.

Average photon energy in a diagnostic beam

For a given kVp, the average photon energy is roughly one-third to one-half of the peak kVp (e.g., a 120 kVp beam has an average energy of about 40–60 keV).

Beam quality units of measure

Beam quality (penetrability) can be described by average photon energy (keV), average frequency, average wavelength, half-value layer (HVL), and sometimes linear energy transfer (LET). Higher average energy, frequency, and HVL mean higher quality.

Beam intensity

The rate of x-ray photon flow per unit area at a given distance from the source; often used interchangeably with exposure or output.

Inverse square law (intensity)

Beam intensity is inversely proportional to the square of the distance from a point source: I₂ = I₁ × (D₁² ÷ D₂²). Doubling distance reduces intensity to one-fourth.

mAs–distance (exposure maintenance) formula

To maintain the same exposure when changing SID: mAs₂ = mAs₁ × (D₂² ÷ D₁²). As distance increases, mAs must increase by the square of the distance ratio.