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PRODUCTION OF X-RAYS

By Dr. Deepa Gautam1st yr Resident ,Radiotherapy

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History• Wilhelm Conrad von Roentgen

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• 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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• 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.

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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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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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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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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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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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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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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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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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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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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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• 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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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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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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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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Physics of X-ray Production

• Produced by two different mechanisms and give rise to:– Bremsstrahlung x-rays– Characteristic x-rays

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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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• 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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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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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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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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Spectral distribution of x-rays for tungsten target with no filtration and with filtration of 1mm

aluminum

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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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• The factors affecting those characteristics of x-rays are: – Anode target material– Tube voltage – Tube current– Beam filtration

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• 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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• 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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• 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

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Thank you