XRay Production Emission Objectives p Review xray production

X-Ray Production & Emission

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

Principle Parts of the X-ray Imaging System p Operating Console p High-voltage generator p X-ray tube p 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 p 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 p 3 possible tube interactions p Tube interactions are generated from _____ slamming into ____? Heat (99%), EM energy as infrared radiation (heat) & x-rays (1%) n X-rays = Characteristic (20%) or Bremsstrahlung (80%) p

Heat p Most kinetic energy of projectile e- is converted into heat – 99% p 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 p Production of heat in the anode increases directly with increasing x-ray tube current & k. Vp Doubling the x-ray tube current doubles the heat produced p Increasing k. Vp will also increase heat production p

Characteristic Radiation – 2 steps p Projectile e- with high enough energy to totally remove an inner-shell electron of the tungsten target p Characteristic x-rays are produced when outer-shell e- fills an inner-shell void p 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 p

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

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

Bremsstrahlung Radiation p 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 p Because of the electrostatic force?

Bremsstrahlung is a german word meaning slowed-down radiation

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

Discrete spectrum p Contains only specific values

Continuous Spectrum p Contains all possible values

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

Characteristic x-ray emission spectrum

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

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

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

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

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

m. A increase from 200 to 400

k. Vp p A change in voltage peak affects both the amplitude and the position of the x-ray emission spectrum

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

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

What k. Vp does this graph indicate?

Target Material p The atomic number of the target affects both the quantity and quality of x-rays p 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 p 5 voltage waveforms: half-wave rectification, full-wave rectification, 3 phase/6 -pulse, 3 -phase/12 -pulse, and high-frequency. p Maintaining high voltage potential

Voltage generators

X-ray Quantity or Intensity p What units of measurement is used for radiation exposure or exposure in air? p Milliampere-seconds (m. As) – x-ray quantity is proportional to m. As p Kilovolt Peak (k. Vp) – If k. Vp were doubled the x-ray intensity would increase by a factor of four or k. Vp 2

X-ray Quantity or Intensity p Distance – x-ray intensity varies inversely with the square of the distance from the xray target p When SID is increased, m. As must be increased by SID 2 to maintain constant OD

Filtration p 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 p 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 p High-energy photons are able to penetrate tissue farther than low-energy photons p High-quality = high-penetrability p Low-quality = low-penetrability p

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

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

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

X-ray Quality p Kilovolt Peak (k. Vp) = increasing the k. Vp increased photon quality and the HVL

Types of Filtration p Diagnostic x-ray beams have two filtration components – inherent filtration and added filtration p Inherent filtration – The glass enclosure of the tube (the window) – approximately 0. 5 mm Al equivalent

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

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

Wedge filter

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

Avez-vous des questions? ¿Tiene alguna pregunta?