X-Ray Production and it's Properties

1. X-Ray Production and it's Properties

2. X-rays discovered in 1895 by Wilhem Roentgen Roentgenology – Branch of medicine dealing with x-ray or gamma rays in diagnosis and treatment. Radiology – Includes x-rays, but also: - Natural & artificial nuclides - CT - MRI - Ultrasound Radiography – The art and science of recording x-ray images on a medium Fluoroscopy – Observation of x-rays on a fluorescent screen to show anatomical function and structure

3. The Electromagnetic Spectrum All electromagnetic waves (including x-ray) travels at the speed of light 186,000 mi/sec (3 X 108 cm)

4. An X-Ray Tube Consists of: 1) Degassed Tube 2) Hot Filament (Negative) 3) Target (Positive) 4) High Voltage Across the Electrodes 5) Oil

5. Conditions Necessary for X-Ray Production 1) Separation of electrons Accomplished through: - Thermionic emission - Production of a space charge 2) Production of high speed electrons - Produced by applying high voltage from secondary of high voltage transformer - Current between electrodes measured in mA - Total current measured in mAS (mA X time) 3) Focusing of electron 4) Deceleration of electrons at the anode - 99.4% of electron energy given off as heat, .6% converted to x-ray

6. X-Ray Production

7. Electron Interaction at the Target Bremsstrahlung Radiation Electron

8. Electron Interaction at the Target Characteristic Radiation

9. Target Material Must meet 2 criteria: 1) High melting point 2) High atomic number - Higher atomic number produces more Brems radiation - Higher atomic number also produces more penetrating characteristic radiation Target material is primarily composed of tungsten (AN 74) - May be rotating or stationary - Molybdenum added in mammography targets & to dissipate heat in other targets - Rhenium added to increase elasticity of target Target efficiency = K X Z X kVp For tungsten – (1 X 10-4) X 74 X 80 - .6%

10. Properties of X-Rays 1) Highly penetrating and invisible 2) Electrically neutral 3) Polyenergetic 4) Liberates small amount of heat when passing through matter 5) Travel in straight lines 6) Ionize gases indirectly 7) Cause certain crystals to fluoresce 8) Cannot be focused by a lens

11. Properties of X-Rays 9) Travels at the speed of light 10) Affect photographic film 11) Produce chemical and biological changes 12) Produce scatter and secondary radiation

12. Specifications of the X-Ray Beam Quantity (Intensity) • Amount of radiant energy flowing per second • per unit area of surface • - Affected by both mA and time • Roentgen (R) – A given amount of ionization • of air particles produced by radiation • Quantity (output) is measured in R/min • Output = Exposure in R • (R/Min) Time in Min. • Total Exposure = R/Min (output) X Time

13. The Five Factors Controlling Output (Intensity) of the X-ray Beam 1. Target Material 2. Tube Current 3. Tube Potential 4. Distance 5. Filtration

14. Quality (Penetration) of the X-Ray Beam Planck’s Quantum (Corpuscular) Theory – X-ray consists of packets of energy called photons that are electrically neutral & travel at the speed of light. E = hf E = Photon energy h = Planck’s constant (4.15 X 10-15 eV-s) f = Photon frequency in Hz • Photon energy is directly proportional to • frequency and inversely proportional to • wavelength. • Highly penetrating x-rays have high frequency • and short wavelengths. • High kVp produces x-ray with higher frequency, • shorter wavelengths and provides increased • penetration for large or high density anatomy. • - However, this is not a direct relationship.

15. Polyenergetic Vs Monoenergetic Radiation • Polyenergetic radiation – • - Stated in kVp • Monoenergetic radiation – Stated in • electron volts (eV) • eV – The energy acquired by an electron when • it is accelerated through a potential • difference of one volt. • 100 kVp is approximately 30 – 50 kEv Causes of polyenergetic x-rays 1. Fluctuating kVp 2. Process of x-ray production 3. Numerous interactions with target atoms 4. Off-focus radiation

16. Spectral Distribution Curves – Graphic representations showing the range of intensities of x-ray photons at given kVp’s - Photon energy and intensity range increase with increasing kVp

17. Methods to Specify X-Ray Quality Can be done by: 1. Half-value layer – That thickness of material that will decrease radiation output by 1/2. 2. Applied kVp Hard Vs Soft X-Rays Hard – High energy, very Penetrating - Produced with high kVp & high atomic number filtration Soft – Low energy, low penetration - Produced with low kVp and low atomic number filtration - Grenz Rays – 10 -20 kVp (very damaging to patient)

18. Interactions of Ionizing Radiation and Matter Attenuation – A decrease in the number of radiation photons per unit of travel in matter. Attenuation occurs due to: 1) Absorption 2) Emission of scatter and secondary Scatter – Photons undergoing a change of direction Secondary – Emission of characteristic radiation after an x-ray is absorbed by an atom.

19. X-Ray Interactions With Matter • Some Basics: • X-rays are photons • Travel at the speed of light • Are electrically neutral • X-rays interact with electrons in atoms • The binding energy is greatest for inner • shell electrons • Outer shell electrons have lower binding • energies and can be removed more easily

20. Photoelectric Effect • 1.An x-ray gives up all of its energy to free • an inner shell electron (usually k shell) • 2. Inner shell hole is filled by a higher level • electron with emission of characteristic • radiation • 3. Holes in successively higher shells are filled • by electrons (cascading) • Key Points: • Occurs in absorbers of high atomic number • The major contributor to patient dose • The major contributor to image contrast • Characteristic radiation from photoelectric • is a form of secondary radiation • Predominates below 150 kVp

21. Coherent (Unmodified/Thomson) Scatter 1. A very low energy photon strikes an outer shell electron, but does not dislodge it. 2. The incident photon undergoes a change of direction without change in wavelength, frequency or energy. • Key Points: • Occurs below the useful range of x-ray • Produces scatter radiation

22. Compton Interaction (Modified Scatter) 1. X-ray photon dislodges an outer shell electron - This electron is now known as a Compton (recoil) electron 2. X-ray photon is scattered - Has decreased energy - Energy of photon depends on angle of scatter (greater angle = less energy) • Key Points: • The major contributor to film fog & • occupational personnel • Predominates above 150 kVp

23. Pair Production 1) A megavoltage photon interacts with the nucleus of an atom. 2) This gives rise to a negatron (electron) and positron (positive electron). 3) The negatron combines with other atoms in need of an electron. 4) The positron combines with and annihilates an electron. The energy of this annihilation effect is carried off by two, .51MeV photons. Points to Remember • Occurs above the diagnostic range of x-rays • Becomes the predominant reaction above • 24 MeV • Is used in PET to construct images • Positron decay from isotope causes pair • production which is detected by PET • detectors to form an image

24. Pair Production

25. Scatter and Secondary Radiation From Tissue Interactions Scatter Radiation – Occurs as a result of initial contact between radiation photon & atom. Includes • Primary electrons -Electrons that comes from inside • the atom after contact with a radiation photon. • 1) Photoelectrons • 2) Compton (recoil) Electrons • 3) Positrons/Negatrons • These account for the absorption of radiation. • Secondary Radiation – Radiation that occurs • secondary to an interaction of a radiation photon • and atom (i.e., characteristic radiation) • Exception - Characteristic produced at target • is considered primary radiation Tissue InteractionResulting Radiation 1) Characteristic (non-target) Secondary 2) Coherent Primary Scatter 3) Modified Scatter Primary Scatter 4) Annihilation Effect Secondary

26. Summary Photoelectric • Predominates in the 25 – 150 kVp range • Is the primary contributor to image contrast • and patient dose • Occurs in greater amounts in tissues with • higher atomic number (i.e., bone Vs soft tissue) Compton Effect • Degrades image quality through the production • of fog • Is the primary contributor to occupational personnel exposure • Predominates above 150 kVp • Comprises 80 – 90% of the beam in soft tissue Pair Production • Predominates at 24 mEv and above

27. Measuring Radiation Roentgen (R) – Quantity of x-ray or gamma ray causing a certain amount of ionization in air. - 1 R = 2.56 X 10-4 coulomb/kg air - Applies only to x-ray or gamma rays Linear Energy Transfer (LET) – Amount of ionizing energy transferred to tissue per unit of travel by radiation. RAD (Gy) – Amount of ionizing energy absorbed per gram of irradiated tissue. - Equivalent to 100 ergs per gram - 1 Gy = 100 RAD - Applies to measuring dose from any type of radiation Both LET and RAD effected by: 1) Quantity of radiation (effected by mA & time) 2) Quality (penetration) 3) Nature of tissue 4) Distance

28. Modification of the X-Ray Beam Using Filters 1) Inherent Filtration – Filtration in the tube housing. A) glass envelope B) oil C) Window (port) 2) Added Filtration – Filtration outside the tube housing A) Al/Cu filters B) Mirror C) Collimators Filtration Affects: 1) Exposure rate (quantity) – Increased filtration will decrease exposure and image density and vice versa - This is an inverse relationship 2) Quality (penetration) Increased filtration (HVL) or increased atomic number of filter will increase the average penetration of the beam & vice versa - Direct relationship - Increased filtration decreases contrast