X-ray Spectra/ Factors Affecting the X-ray Emission Spectrum/ X-ray Emission

Topic 7-8

Spectrum

Graphic representation of the range over which a quantity extends

Discrete Spectrum

It contains only specific values

Continuous Spectrum

It contains all possible values

X-ray Emission Spectrum

It can be graphed as the number of x-rays for each increment of energy in the keV

Characteristic X-ray Spectrum

• A plot of the frequency with which characteristic x-rays are emitted as a function of energy

• Characteristic x-rays have precisely fixed (discrete) energies and form a discrete emission spectrum

Bremsstrahlung X-ray Spectrum

• It extends from zero to maximum projectile electron energy

• greatest number of x-rays emitted (about 1/3 of maximum energy) 

• Maximum energy is equal to kVp operation

• Number of x-rays decreases rapidly at very low energies

Radiation Quantity

• X-ray quantity is the number of x-rays in the useful beam.

• The more values in the curve the higher the x-ray quantity

Radiation exposure

Units: R and Gy

X-ray Quantity and Distance (Inverse Square Law)

I₁/I₂ = d2²/d1²

X-ray Quantity and kVp Formula

I₁/I₂ = (kVp₁/kVp₂)²

Tube current or mAs (Exposure Maintenance Formula)

I₁/I₂ = mAs^₁/mAs₂

Direct Square Law

mAs₁/mAs₂= (SID1/SID2)^₂

Radiation Quality

The farther to the right the higher the effective energy or quality

Half-Value Layer

• The thickness of absorbing material/absorber necessary to reduce the x-ray intensity to half of its original value

• Characteristic of the useful beam - is the best method for specifying x-ray quality

Tube voltage

• A change in kVp affects both the amplitude and the position of the x-ray emission spectrum

• A change in kVp has no effect on the position of the discrete x-ray emission

Added filtration

• is called beam hardening because of the increase in average energy 

• effectively absorbed low energy x-rays than high energy x-rays 

• Characteristics spectrum is not affected and maximum energy of x-ray emission is not affected

Filtration

Adding (_) to the useful beam reduces the x-ray beam intensity while increasing the average energy (higher quality)

Inherent Filtration

• X-ray Tube glass or metal enclosure

• Equivalent: 0.5 mm Al

Added Filtration (Type of filtration)

• X-ray tube housing and x-ray beam collimators 

• Equivalent: 1 mm Al (port of x-ray tube) and 1 mm Al (collimators)

Total Filtration

2.5 mm Al (IF: 0.5 mm Al; AF: 2 mm Al- Collimator and x-ray tube port)

Compensating Filter

• Material inserted between an x-ray source and a patient to shape the intensity of the x-ray beam

• It is designed to make the remnant beam more uniform in intensity

Target material

• Atomic number of the target affects both the quantity and quality of x-rays

• Increasing the target atomic number increases the efficiency of x-ray production and the energy of characteristics and bremsstrahlung x-rays

Voltage Ripple / Waveform

1. Half wave

2. Full wave

3. Three phase, six pulse

4. Three phase, twelve pulse

5. High frequency

Factors Affecting X-ray Emission Spectrum (Tube current)

Increase in Quantity, No change in quality

Factors Affecting X-ray Emission Spectrum (Tube voltage)

Increase in both quantity and quality (directly proportional)

Factors Affecting X-ray Emission Spectrum (Added filtration)

Decrease in quantity, increase in quality

Factors Affecting X-ray Emission Spectrum (Target material)

Increase in both quantity and quality (directly proportional)

Factors Affecting X-ray Emission Spectrum (Voltage ripple/ waveform)

decrease in both quantity and quality (inversely proportional)

X-ray Quantity

Refers to the number of x-rays or intensity of the beam usually expressed in mR or mR per mAs

mAs

• a measure of the total number of electrons that travel from cathode to anode

• equal to mC (microcoloumb)

1 Coloumb is equal to?

6.25 x 10^18 electrons

X-ray Quality

Refers to the penetrating qualities of the x-ray beam and is expressed by kVp or HVL