X-Ray Production Emission & Filtration

X-ray ProductionX-rays are produced inside the x-ray

tube when high energy projectile electrons from the filament interact with the atoms of the anode

Conditions necessary: Source of electrons Target (anode) High potential difference Sudden deceleration of projectile

electrons

Bremsstrahlung Target InteractionsCreated when incident (filament) electron

interacts with the nucleus of an anode target atom.

Occurs at all kV settings– Electron approaches nucleus.– Nuclear force field is too strong for electron

to penetrate. Electron slows down (“brakes”) Braking causes a loss of energy Energy loss is released as a Brems x-ray

photon Electron changes course & keeps going in

new direction

Bremsstrahlung Target InteractionsThe closer the electron

gets to the nucleus, the more it brakes; resulting in a higher energy Brems photon.

Electron can collide with nucleus losing all of its energy, pass close to the nucleus and lose most of its energy, or pass at a distance and lose little of its energy.

Average energy of Brems is 1/3 of the maximum kV used.

Characteristics Target Interactions Occur when incident (filament)

electron interacts with an orbital electron of the anode target atom.

Incident (filament) electron has energy ≥ binding energy of orbiting electron.

Filament electron knocks inner-shell electron from orbit creating “hole”. The atom is now unstable.

Characteristics Target Interactions When outer shell

electrons fill inner shell vacancies, a characteristic cascade occurs. This produces several x-ray photons at different energies from each atom.

Photon is named for the “hole” filled (K-characteristic, L-characteristic, etc.)

X-ray Emission Spectrum

X-ray spectrum is the distribution of the energy of the x-ray photon emerging from an x-ray source.

General form of an x-ray emission spectrum divided into two:

Characteristic radiation

Bremsstrahlung radiation

Discrete X-ray Spectrum

~There are 15 different energies x-ray.

There are 5 vertical line representing K x-rays.

There are 4 representing L x-rays.

Remaining represent lower energy outer shell electron.

K x-rays are the only characteristic x-rays of tungsten that have sufficient energy to be of value in radiography.

Continuous X-ray Spectrum

The Bremsstrahlung x-ray energies range from zero to a peak and back to zero.

This is referred to as the Continuous X-ray Spectrum.

The majority of the useful x-rays are in the continuous spectrum.

The maximum energy will be equal to the kVp of operation.

This is why it is called kVp (peak).

Four Factors Influencing the X-ray Emission Spectrum1. The electrons accelerated from the

cathode do not all have the peak kinetic energy. Depending upon the type of rectification and high voltage circuits, many electrons will have very low energy that produces low energy x-rays.

2. The target is relatively thick. Many of the bremsstrahlung x-ray emitted result from multiple interactions of the projectile electrons.

Each successive interaction results in less energy.

Four Factors Influencing the X-ray Emission Spectrum3 . Low energy x-rays are more

likely absorbed by the target.

4 . External filtration is always added to the tube assembly. This added filtration serves to selectively remove the lower energy photon.

Filtration “Hardening” of beam Removes low energy

“soft” photons. Increases average beam

energy.Soft tissue penetration

requires approximately 30-40 kilo electron volt (keV) photons.

Low energy photons cannot penetrate the part

Only contribute to patient dose.

Filtration

Purpose of added filtration is to remove low energy, (long wavelength photons).

Amplitude and position of continuous spectrum is affected.

Amplitude of discrete spectrum is affected.

Types of Filtration

Inherent filtration

- 0.5 mm Al equivalent

- X-ray tube design.

Glass or metal envelope.

Dielectric oil bath.

Glass window of housing.

Types of FiltrationAdded Filtration -1.0 mm Al equivalent. - Any filtration outside x-ray tube and

housing.Silver on collimator mirror.Thin layers of aluminum or copper

permanently added between the collimator and protective housing.

Filters may be changed.

Types of FiltrationCompound filtration - K-edge filters Two or more materials. Each layer absorbs

characteristic photons created in previous layer.

Compensation Filtration

- Evens radiographic density with parts that have uneven tissue thickness or densities

- E.g : wedge for foot or T-spine trough for CXR.

Compensation Filtration Applications

X-ray beam Quality and QuantityThe quality of the x-ray beam is

controlled by the voltage and milliamperes reduces exposure time for the patient.

X-ray Beam Quality

X-ray Beam Quantity

The quality of the x-ray beam is controlled by the amount of voltage. Voltage provides contrast to the film.

The desired contrast appears as various shades of gray, black, white in the x-ray negative.

Increased voltage provides less contrast or more shades of gray.

The beam has more penetrating. Decreased voltage, on the other hand provides more contrast.

But less penetrating power in the low voltage exposure.

The quantity of the x-ray beam is controlled by the amount of milliamperes.

The more x-rays (photons) in the x-ray beam, the more dense (dark) the x-ray negative (radiograph) becomes.

By increasing the milliamperes, we increase the number of available electron at the cathode filament.

When voltage is applied to the x-ray tube, the electron cross the gap.

Thank You!