medical x ray production basics

1. PRODUCTION OF
X-RAYS
Vineeth C
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2. Atomic Structure
• An atom is composed of
1.electrons (with a negative charge),
2.protons (with a positive charge) and
3.neutrons (no charge).
•The protons and neutrons are found in the nucleus of
the atom and the electrons rotate (orbit) around the
nucleus.The number of electrons equals the number of
protons in an atom so that the atom has no net charge
(electrically neutral).
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3. Electromagnetic Radiation
• An x-ray is one type of electromagnetic radiation.
• Electromagnetic radiation represents the movement of
energy through space as a combination of electric and
magnetic fields.
• All types of electromagnetic radiation, which also
includes radiowaves, tv waves, visible light, microwaves
and gamma rays, travel at the speed of light (186,000
miles per second).They travel through space in wave
form.
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4. E.M. Spectrum
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5. X-ray Characteristics
• X-rays are high energy waves, with very short
wavelengths, and travel at the speed of light.
• X-rays have no mass and no charge
• x-rays are invisible.
• X-rays travel in straight lines.
• An x-ray beam cannot be focused to a point; the x-ray
beam diverges as it travels toward and through the
patient.
• X-rays are differentially absorbed by the materials they
pass through. More dense materials will absorb more x-
rays than less dense material .This characteristic allows
us to see images on an x-ray film.
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6. X-ray production
• X-rays produced when highly energetic electrons
interact with matter and convert their kinetic energy
into electromagnetic radiation
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7. Figure shows the main components of a
modern x-ray tube
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8. 8
anode
Cathode

9. CATHODE STRUCTURE
• Cathode includes filament(s) and associated circuitry
tungsten material : preferred because of
1. its high melting point (3370°C)
2. slow filament evaporation
3. no arcing
4. minimum deposit ofW on glass envelope
• To reduce evaporation the emission temperature of the
cathode is reached just before the exposure in stand-by,
temperature is kept at ±1500°C so that 2700°C emission
temperature can be reached within a second
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10. Modern tubes have two filaments (called a dual-focus tube)
a long one :higher current/lower resolution
a short one : lower current/higher resolution
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11. Heating a filament to release electrons is called
thermionic emission or the
Edison effect
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12. The focal spot is the volume of target within which
electrons are absorbed and x rays are produced.
For radiographs of highest clarity, electrons should
be absorbed within a small focal spot.To achieve a
small focal spot, the electrons should be emitted
from a small or “fine” filament
• The focusing cup has a negative charge, like the electrons 12

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14. ANODE
• The anode is a metal target electrode that is maintained at a
positive potential difference relative to the cathode
• Electrons striking the anode deposit the most of their energy
as heat, with a small fraction emitted as x-rays
• Tungsten is the most widely used anode material because of its
high melting point and high atomic number (Z=74)
• Tungsten anode can handle substantial heat deposition
without cracking or pitting of its surface
• Molybdenum (Mo, Z = 42) and rhodium (Rh, Z = 45) are used as
anode materials in mammographic x-ray tubes.These
materials provide useful characteristic x-rays for breast
imaging
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15. Anode Design
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Simplest type of x-ray
tube has a stationary
(fixed) anode
Rotating anodes used for
most diagnostic x-ray
applications
Greater heat loading and
consequent higher x-ray
output capabilities
Electrons impart energy
to a continuously rotating
target, spreading thermal
energy over a large area
and mass

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17. Line Focus Principle
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18. • With the target at this angle, x rays appear to originate
within a focal spot much smaller than the volume of the
target absorbing energy from the impinging electrons.
• This apparent reduction in size of the focal spot is
termed the line-focus principle. Most diagnostic x-ray
tubes use a target angle between 6 and 17 degrees.
• In the illustration, side a of the projected or apparent
focal spot may be calculated by
a = A sin θ
• where A is the corresponding dimension of the true focal
spot and θ is the target angle.
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19. Heel Effect
• The heel effect refers to a reduction in the x-ray beam
intensity toward the anode side of the x-ray field
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20. • A large voltage is applied between two electrodes (the cathode
and the anode) in an evacuated envelope.
• The cathode is negatively charged and is the source of
electrons;
The anode is positively charged and is the target of electrons.
• As electrons from the cathode travel to the anode, they are
accelerated by the electrical potential difference between
these electrodes and attain kinetic energy.
• The electric potential difference, also called the voltage for
electric potential difference is the volt (V).
• The kinetic energy gained by an electron is proportional to the
potential difference between the cathode and the anode.
• On impact with the target, the kinetic energy of the electrons is
converted to other forms of energy.The vast majority of
interactions produce unwanted heat by small collisional energy
exchanges with electrons in the target.
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21. • Occasionally (about 0.5% of the time), an electron comes
within the proximity of a positively charged nucleus in
the target electrode.
• Coulombic forces attract and deceleratethe
electron, causing a significant loss of kinetic energy and
a change in the electron's trajectory.
• An x-ray photon with energy equal to the kinetic energy
lost by the electron is produced (conservation of energy).
• This radiation is termed bremsstrahlung, a German word
meaning "braking radiation."
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23. Characteristic Radiation
• Characteristic x-rays are produced when a high-speed
electron from the filament collides with an electron in
one of the orbits of a target atom; the electron is
knocked out of its orbit, creating a void
• This space is immediately filled by an electron from an
outer orbit, energy is released in the form of a
characteristic x-ray.
• Characteristic x-rays have energies “characteristic” of the
target material
• For example, if a K-shell electron is ejected and an L-shell
electron drops into the space, the energy of the x-ray will
be equal to the difference in binding energies between
the K-and L-shells 23

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25. X-ray Spectrum
• An x-ray beam have a wide range of x-ray energies; this
is called an x-ray spectrum.
• The average energy of the beam will be approximately
1/3of the maximum energy.
• The maximum energy is determined by the kVp setting.
If the kVp is 90, the maximum energy is 90 keV
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27. FILTRATION
• An x-ray beam traverses several attenuating materials before
it reaches the patient, including the glass envelope of the x-ray
tube, the oil surrounding the tube, and the exit window in the
tube housing.These attenuators are referred to collectively as
the inherent filtration of the x-ray tube
• The second component is the addition of aluminum disks
placed in the path of the x-ray beam - added filtration.
• Total filtration - 2.5 mm aluminum equivalent ≤ 70 kVp
1.5 mm aluminum equivalent < 70 kVp
• (An x-ray beam of higher average energy is said to be “harder”
because it is able to penetrate more dense (i.e., harder)
substances such as bone. An x-ray beam of lower average
energy is said to be “softer” because it can penetrate only less
dense (i.e., softer) substances such as fat and muscle) 27

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29. Exposure Factors
• kVp - energy or penetrating quality
• mA - heating of the filament / greater the number of x-
rays
• s – exposure time
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30. Factors affecting X-ray Emission
• described by the terms quality, quantity, and exposure.
• Quality - the penetrability of an x-ray beam, with higher
energy x-ray photons having a larger HVL and higher
"quality.“
• Quantity refers to the number of photons comprising the
beam.
• Exposure-proportional to the energy fluence of the x-ray
beam and therefore has quality and quantity associated
characteristics.
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31. Determined by six major factors
1. X-rayTubeTarget Material,
2. Voltage,
3. Current,
4. ExposureTime,
5. Beam Filtration,
6. and GeneratorWaveform
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32. RADIOGRAPHIC GRIDS
• Invented By Dr. Gustav Bucky
• Devices that reduce the amount of scattered radiation
reaching the image receptor
• Grids strips are made of Lead (Pb)-Absorbs scatter
radiation through Photoelectric Interaction
• Grid radiography is recommended for:
• Anatomical parts > 10 cm
• With high kVp ( not always—mammo)
• Soft tissue structures to increase contrast
• Structures affected by pathological condition that would
increase scatter production 32

33. Grid Ratio; G.R.= H/D
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35. Reference
• The Essential Physics of Medical Imaging – Second Edition- JerroldT
Bushberg
• Medical Imaging Physics - Fourth Edition -William R. Hendee
• The Physics Of RadiationTherapy- Fifth edition – Faiz M Khan
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