X-Ray Production & Emission

Objectives: Review x-ray production requirements X-ray tube interactions X-ray emission spectrum

Principle Parts of the X-ray Imaging System Operating Console High-voltage generator X-ray tube The system is designed to provide a large number of e- with high kinetic energy focused to a small target

E- traveling from cathode to anode Projectile e- interacts with the orbital e- of the target atom. This interaction results in the conversion of e- _______ energy into ________ energy and ________ energy.

Tube Interactions 3 possible tube interactions Tube interactions are generated from _____ slamming into ________? Heat (99%), EM energy as infrared radiation (heat) & x-rays (1%) X-rays = Characteristic (20%) or Bremsstrahlung (80%)

Heat Most kinetic energy of projectile e- is converted into heat – 99% Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize

Heat is an excitation rather than an ionization

Heat production Production of heat in the anode increases directly with increasing x-ray tube current & kVp Doubling the x-ray tube current doubles the heat produced Increasing kVp will also increase heat production

Characteristic Radiation – 2 steps Projectile e- with high enough energy to totally remove an inner-shell electron of the tungsten target Characteristic x-rays are produced when outer-shell e- fills an inner-shell void All tube interactions result in a loss of kinetic energy from the projectile e-

It is called characteristic because it is characteristic of the target element in the energy of the photon produced

Only K-characteristic x-rays of tungsten are useful for imaging

Bremsstrahlung Radiation Heat & Characteristic produces EM energy by e- interacting with tungsten atoms e- of the target material Bremsstrahlung is produced by e- interacting with the nucleus of a target tungsten atom

Bremsstrahlung Radiation A projectile e- that completely avoids the orbital e- as it passes through a target atom may pass close enough to the nucleus of the atom to convert some of the projectile e- kinetic energy to EM energy Because of the electrostatic force?

Bremsstrahlung is a german word meaning slowed-down radiation

X-ray energy Characteristic x-rays have very specific energies. K-characteristic x-rays require a tube potential of a least 70 kVp Bremsstrahlung x-rays that are produced can have any energy level up to the set kVp value. Brems can be produced at any projectile e- value

Discrete spectrum Contains only specific values

Continuous Spectrum Contains all possible values

Characteristic X-ray Spectrum Characteristic has discrete energies based on the e- binding energies of tungsten Characteristic x-ray photons can have 1 of 15 different energies and no others

Characteristic x-ray emission spectrum

Bremsstrahlung X-ray Spectrum Brems x-rays have a range of energies and form a continuous emission spectrum

Factors Affecting the x-ray emission spectrum Tube current, Tube voltage, Added filtration, Target material, Voltage waveform The general shape of an emission spectrum is always the same, but the position along the energy axis can change

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

Quantity The more values in the curve, the higher the x-ray intensity or quantity

mAs A change in mA or s or both results in the amplitude change of the x-ray emission spectrum at all energies The shape of the curve will remain the same

mA increase from 200 to 400

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

Filtration Adding filtration is called hardening the x-ray beam because of the increase in average energy Characteristic spectrum is not affected & the maximum energy of x-ray emission is not affected

Filtration Adding filtration to the useful beam reduces the x-ray beam intensity while increasing the average energy Added filtration is an increase in the average energy of the x-ray beam (higher quality) with a reduction in x-ray quantity Lowering the amplitude and shifting to the right

What kVp does this graph indicate?

Target Material The 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 characteristic and bremsstrahlung x-rays

Target material

Voltage Waveform 5 voltage waveforms: half-wave rectification, full-wave rectification, 3-phase/6-pulse, 3-phase/12-pulse, and high-frequency. Maintaining high voltage potential

Voltage generators

X-ray Quantity or Intensity What units of measurement is used for radiation exposure or exposure in air? Milliampere-seconds (mAs) – x-ray quantity is proportional to mAs Kilovolt Peak (kVp) – If kVp were doubled the x-ray intensity would increase by a factor of four or kVp2

X-ray Quantity or Intensity Distance – x-ray intensity varies inversely with the square of the distance from the x-ray target When SID is increased, mAs must be increased by SID2 to maintain constant OD

Filtration 1 to 3 mm of aluminum (Al) added to the primary beam to reduce the number of low-energy x-rays that reach the patient, reducing patient dose Filtration reduces the quantity of x-rays in the low-energy range

Reducing low-energy photons

X-ray Quality or Penetrability As the energy of an x-ray beam is increased, the penetrability is also increased High-energy photons are able to penetrate tissue farther than low-energy photons High-quality = high-penetrability Low-quality = low-penetrability

HVL = Half-Value Layer What is the HVL HVL is affected by the kVp and added filtration in the useful beam Photon quality is also influenced by kVp & filtration HVL is affected by kVp

HVL In radiography, the quality of the x-rays is measured by the HVL The HVL is a characteristic of the useful x-ray beam A diagnostic x-ray beam usually has an HVL of 3 to 5 mm Al

HVL 3 to 5 mm Al = to 3 to 6 cm of soft tissue HVL is determined experimentally and a design specification of the equipment

X-ray Quality Kilovolt Peak (kVp) = increasing the kVp increased photon quality and the HVL

Types of Filtration Diagnostic x-ray beams have two filtration components – inherent filtration and added filtration Inherent filtration – The glass enclosure of the tube (the window) – approximately 0.5 mm Al equivalent

Added Filtration 1 or 2 mm sheet of aluminum between the tube housing and the collimator The collimator contributes an additional 1mm Al equivalent added filtration

Compensating filter A filter usually made of Al, but plastic can be used to maintain OD when patient anatomy varies greatly in thickness Are useful in maintaining image quality. They are not radiation protection devices

Wedge filter

Compensating Filter What is an aspect of the tube design that works as a compensating filter? What causes this?

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