Chapter 8: X-Ray Production

Projectile electron

An electron accelerated from the cathode to the anode that interacts with target atoms.

Kinetic energy (KE)

Energy of motion. For electrons in an x-ray tube, KE increases as kVp increases.

X-ray tube current (mA)

The flow of electrons from cathode → anode per second.

Electron–target interactions

Interactions that convert electron kinetic energy into heat and x-rays.

Anode heat

Heat produced when projectile electrons interact primarily with outer-shell electrons of target atoms.

99% rule

Approximately 99% of electron kinetic energy becomes heat given 100 kVp

1% rule

Approximately 1% becomes x-radiation in the diagnostic range given 100 kVp

X-ray production efficiency

The percentage of the electron's kinetic energy that is converted into x-ray energy rather than heat. It increases with kVp and is independent of mA.

X-ray quantity

The number of x-ray photons produced. It increases with mA/mAs and also increases with kVp.

Characteristic x-ray

X-ray produced when an inner-shell electron is removed and an outer-shell electron fills the vacancy.

Electron vacancy

Empty space created when an orbital electron is removed.

Binding energy

Energy required to remove an electron from its shell.

Characteristic x-ray energy

higher binding energy − lower binding energy

Effect of target atomic number on characteristic x-rays

Higher atomic number → higher binding energies → higher characteristic x-ray energies.

Useful tungsten characteristic x-rays

Primarily K-characteristic x-rays.

Bremsstrahlung x-ray

Produced when a projectile electron is slowed/deflected by the nuclear field of a target atom. Most diagnostic x-rays

What happens to Bremsstrahlungs’ electron’s KE

it is converted into x-ray photon

Bremsstrahlung energy range

Can have any energy from nearly 0 up to the kVp

Discrete spectrum

Bar graph contains only specific energy values. Characteristic x-rays produce this.

Amplitude/height of spectrum

Represents x-ray quantity/intensity.

Area under spectrum

Represents the total number/quantity of x-rays emitted

Shift to the right

Indicates increased average energy/quality of the x-ray beam.

X-ray quantity

Number of x-ray photons produced.

X-ray quality

Energy/penetrating ability of the x-ray beam.

Maximum x-ray energy

Equal to the kVp.

Minimum wavelength

Corresponds to maximum x-ray energy.

Continuous spectrum

Contains a range of possible energy values. Bremsstrahlung produces this.

What does increasing tube current (mA/mAs) do to the x-ray emission spectrum?

Increases amplitude/quantity of the spectrum.

What does increasing tube voltage (kVp) do to the x-ray emission spectrum?

Increases amplitude AND shifts the spectrum to higher energies.
→ Quantity ↑ and quality ↑.

What does increasing added filtration do to the x-ray emission spectrum?

Decreases amplitude while increasing average energy, especially at low energies, because filtration removes low-energy photons.

What does changing the target material do to the x-ray emission spectrum?

Changes amplitude and the position/energy of characteristic radiation lines.

What does the voltage waveform affect?

Changes amplitude, with the effect being most noticeable at high energies.

Increase target Z

Increases x-ray intensity and characteristic x-ray energy because of greater electron binding energies

Mammography target materials

Commonly molybdenum and rhodium.

Unique effect of target material

It changes the position of the discrete characteristic spectrum.

15% rule

Approximately 15% increase in kVp ≈ doubling mAs for similar receptor exposure.

Lower ripple

Produces more intense, higher-energy x-ray output for the same kVp/mAs.