3. • Born in Lennep, Germany in 1845
• Studied in Zurich, Switzerland
• Appointed as professor of physics at Giessen(1879)
and at Wurttemberg(1888)
• In 1895, while investigating cathode rays, he noted a
new ray with greater penetrating power coming
from cathode of a vacuum tube, not deflected by
magnetic fields, and named it as X-rays(unknown
rays)
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4. • Announced his finding about X-rays before the
Wurttemberg society(1895)
• Received Nobel prize in 1901
• Died at Munich on February 10, 1923, from
carcinoma of the intestine.
4
6. The very first X-ray image of human body taken which was
of Wilhelm Conrad von Roentgen’s wife’s hand with ring.
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7. Dr. Charles L. Leonard(1903)
• Demonstrated the power of Roentgen rays to alter
the characters of malignant cells , prevent spread
and development
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8. X-ray Tube
Consists of:
•Glass envelope which is
evacuated to high vacuum
•Cathode(-) with tungsten
filament and focusing cup
•Anode(+) with tungsten
target
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9. The Cathode
• Tungsten filament (atomic number 74 and high
melting point of 3370oC) when heated emits
electrons(thermionic emission)
• Focusing cup directs electrons towards the anode so
that they strike the focal spot
• Diagnostic tubes have dual focus filaments
separately for small and large focal spots
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10. The Anode
• Tungsten target with high melting point to withstand
intense heat produced by electronic bombardment
• Copper anode for removing the heat from target where
it is cooled by oil , water or air (rotating anodes to reduce
the temperature in diagnostic x-rays)
• Copper hood for preventing the electrons from striking
the walls
• Focal Spot : target from where x-rays are emitted (
should be smaller for diagnostic purpose and larger
acceptable for therapeutic purpose)
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11. Principle of Line focus: helps in adjusting the size of focal spot
•A is the side of the actual focal
spot
• a is the apparent side of focal
spot, then
•a=A SinӨ
•Hence making the target angle Ө
small, we can reduce the size of
apparent focal spot
•Ө is 6 -17o for diagnostic tube and
about 30o for therapeutic tube
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13. Heel effect
• X-rays produced at various depth in the target vary in
amount of attenuation , greater attenuation for
those coming from depth
• Intensity of x-ray beam decreases from cathode to
anode direction
• These variations can be minimized by using
compensating filters
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14. Basic X-ray Circuit
• Divided into:
– the high-voltage circuit to provide the accelerating
potential for the electrons
– the low-voltage circuit to supply heating current to the
filament
• the filament temperature controls the current in the
circuit due to the flow of electrons across the tube
and hence the x-ray intensity
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15. High voltage circuit
• X-ray tube with cathode and anode
• Step-up transformer to supply high voltage and low
current to x-ray tube
• Autotransformer for stepwise adjustment in voltage
• Voltage selector switch for selecting the turn of a
coil in autotransformer
• Rheostat to introduce desired resistance in the
circuit and vary the voltage in continuous manner
• Voltmeter for recording voltage to the x-ray tube
• Milliammeter for recording the tube current
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16. Low voltage circuit
• X-ray tube with cathode and anode
• Step down transformer to supply low voltage high
current to the filament for electron emission
• Choke coil filament control to control the current
from the main power line to the filament
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18. • The tube current and the x-rays are generated only
during the half-cycle when the anode is positive
• A machine operating in this manner is called the
self-rectified unit.
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19. Voltage Rectification
• To solve the problem during inverse voltage cycle:
– when anode is negative relative to cathode, no x-rays
produced
– When target gets hot and emits electrons which will flow
from anode to cathode, bombard cathode filament and
destroy it
• Can be done in two ways:
– Half wave rectification
– Full wave rectification
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20. Half wave rectification
•Rectifiers placed in series in
the high voltage part of the
circuit prevents conduction
during inverse voltage cycle
•Two types of high voltage
rectifiers:
•Valve state type
•Solid state type
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21. Full wave rectification
•Four rectifiers arranged in
the high voltage part of the
circuit so that cathode is
negative and anode is positive
in both cycles
•Electrons flow from filament
to target in both cycles
•In fig, electronic current
flows through the tube via
ABCDEFGH when transformer
end A is –ve and via
HGCDEFBA when A is +ve
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22. Physics of X-ray Production
• Produced by two different mechanisms and give rise
to:
– Bremsstrahlung x-rays
– Characteristic x-rays
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23. Bremsstrahlung X-rays
• Results from collision of high speed electron over a
nucleus
• Electron with its electromagnetic radiation when
passes in the vicinity of nucleus , it suffers a sudden
deflection (Coulomb forces of attraction) and
acceleration
• Part or all of the energy of the electron is dissipated
from it and propagates in space as electromagnetic
radiation
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24. • In megavoltage x-ray tubes , the electrons bombard
the target from one side and the x-ray beam is
obtained on the other side.
• In the low voltage x-ray tubes, it is advantageous to
obtain the x-ray beam on the same side of the target.
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25. Characteristic X-rays
• An electron with kinetic energy interacts with the
atom of the target by ejecting an orbital electron
• Vacancy is filled by an outer electron with emission
of electromagnetic radiation
• Eo(kinetic energy of incident electron), ∆E(energy
given to the orbital electron a part of which is spent
in overcoming the binding energy) and Eo-E∆ is the
energy of incident electron after collision
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26. Critical Absorption Energy
It is the threshold energy that an incident electron must
possess in order to first strip an electron from the atom
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27. X-ray Energy Spectra
• X-ray photons produced by the x-ray machine are
heterogenous in energy
• Spectrum shows a continuous distribution of energy
(bremsstrahlung) superimposed by discrete energies
(characteristic)
• With no filtration, inherent or added, the calculated
energy will be a straight line and given by
• IE=KZ(Em-E) where IE is the intensity of photons with
energy E, Z is the atomic number of target, Em is the
maximum photon energy and K is the constant
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28. Spectral distribution of x-rays for tungsten target
with no filtration and with filtration of 1mm
aluminum
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29. Factors Affecting X-ray Production
• The output of an x-ray tube is often described by the
terms:
– Quality : the penetrability of an x-ray beam
– Quantity : the number of photons comprising the beam
– Efficiency : the ratio of output energy as x-rays to input
energy deposited by electrons
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30. • The factors affecting those characteristics of x-rays
are:
–
–
–
–
Anode target material
Tube voltage
Tube current
Beam filtration
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31. • Anode target material:
– Incident electrons are more likely to have radiative
interactions in higher-Z materials
– The energies of characteristic x-rays produced in the
target depend on the target material
– The efficiency of bremsstrahlung radiation production is
roughly proportional to atomic number
– Thus the target material affects the quantity and
efficiency of bremsstrahlung photons and the quality of
the characteristic radiation
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32. • Tube voltage (kV)
– determines the maximum energy in the bremsstrahlung
spectrum and affects the quality of the output spectrum
– also affects the efficiency of bremsstralung x-rays
(Efficiency=9x10-10ZV)
• Tube current (mA)
– proportional to the number of electrons flowing from the
cathode to the anode per unit time
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33. • Beam filtration:
– modifies the quantity and quality of the x-ray beam by
absorbing the low-energy photons in the spectrum
– reduces the number of photons (quantity) and increases
the average energy, also increasing the quality
33
