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Chapter 3 Production of X-RaysX-rays were discovered by Roentgen in 1895 while studying the cathode rays (streams of electrons) in a tube.

This new radiation could penetrate opaque substance, produce flourescence, blacken a photographic plate, and ionize gas. It is a form of electromagnetic radiation.

Wilhelm Conrad Roentgen (1845-1923) the radiograph of Mrs. Roentgen's hand

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3.1 The X-Ray Tube

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3.1 The X-Ray Tube (cont’d)

The main components of an x-ray tube are the cathode and the anode, sealed opposite to each other in a highly evacuated vacuum tube.

The cathode is a tungsten filament which when heated emits electrons.

The anode consists of a copper rod and a piece of tungsten target (for producing the x-rays).

The x-ray beam emerges through a thin glass window in the tube envelope.

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3.1 The X-Ray Tube (the anode)

Tungsten is used as target because of its high Z (74) and high melting point (3370C).

Copper is used for its high heat conduction, heat is removed by oil (which also serves as an insulator to the tube housing from high voltage applied to the tube), water or air from outside the tube. Sometimes, a rotating anode is used to reduce the temperature of the target at any one spot.

Anode hood is used to shield unwanted stray radiation.

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3.1 The X-Ray Tube (the anode, cont’d)

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3.1 The X-Ray Tube (the anode, cont’d)

The focal spot is the target area from which the x-rays are emitted. It should be as small as possible for producing sharp radiographic images. For therapy tubes, relatively larger focal spots are acceptable since the image quality is not a primary concern.

The apparent size of the focal spot is related to the apparent side = A sin.

In diagnostic radiology, the angle is small (6 -17), producing apparent focal spot sizes from 0.10.1 to 2.02.0 mm. For therapy tubes, the angle is larger (30), producing areas from 55 to 77 mm.

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Variation of X-Ray Intensity and Maximum X-ray Beam Size Because of the Anode Heel Effect

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3.1 The X-Ray Tube (the cathode)

The cathode consists of a filament (to emit electrons), a circuit (to provide filament current), and a negatively charged focusing cup (to direct electrons towards the anode).

The size of the focal spot depends on the filament size. On a diagnostic tube, there are usually 2 filaments to provide “dual-focus”.

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3.2 Basic X-Ray Circuit

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3.2 Basic X-Ray Circuit (cont’d)

The circuit can be divided into two parts:

The high voltage (~100 kV) circuit to provide the accelerating potential for the electrons. The low voltage (~10V) circuit to supply heating current to the filament.

The filament current can be adjusted to control the x-ray output (intensity).

The high-voltage can be adjusted by an autotransformer (rheostat, a variable resistor).

The alternating voltage is characterized by its peak voltage = line voltage2

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3.2 Basic X-Ray Circuit (cont’d)

For example, a 220V line voltage, after being stepped up by an x-ray transformer of turn ratio 500:1, the resultant peak voltage applied to the x-ray tube is:

kVV 6.155564,1555002220

The anode is positive relative to the cathode only through half of the cycle, during which the current flows through. During the other half, the voltage is reversed and the current cannot flow through. A machine operated this way is called self-rectified.

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3.3 Voltage Rectification (cont’d)

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3.3 Voltage Rectification (cont’d) electron flow

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A(+) HGCDEFBA

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3.3 Voltage Rectification (cont’d)

Rectifier, valve or solid state (semiconductor), is used to prevent the current to flow in the wrong direction.

Half-wave rectification allows the current to flow in the right direction (only half of the cycle).

Full-wave rectification allows the current to flow throughout the cycle in the right direction.

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3.4 Physics of X-Ray Production

There are two mechanisms by which x-rays are produced:

1. Bremsstrahlung: incoming electron interacts with the nucleus, giving out photons.

2. Characteristic x-rays: incoming electron knocks out one of the atomic electrons, creating an hole in the shell; characteristic x-ray emitted as a result of the hole being refilled by an outer-shell electron.

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3.4 Physics of X-Ray Production (Bremsstrahlung)

When an electron passes near a nucleus, its path is deflected and loses energy as bremsstrahlung photons.

0 < h < E (electron energy)

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3.4 Physics of X-Ray Production (Bremsstrahlung, cont’d)

For high-energy electrons (MeV), bremsstrahlung photons emitted mostly in forward direction, for low energy electrons (100 keV), they are more isotropic (equal in all directions).

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3.4 Physics of X-Ray Production (Bremsstrahlung, cont’d)

The probability of bremsstrahlung production Z2.

The efficiency = 9 10-10 ZV

For example, with tungsten target (Z=74), the efficiency for electrons accelerated through 100 kV is :

9 10-10 74 105 = 0.67%

The rest of the energy (>99%) dissipated as heat.

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3.4 Physics of X-Ray Production (characteristic x-rays)

The incoming electron, with energy E0, knocks out an electron, leaving the atom ionized.

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When an outer shell electron falls to an inner shell to fill the vacancy, a characteristic x-ray is produced, with the energy equal to the difference of the binding energies of the 2 shells involved. For example, if an L-shell electron fills a vacancy on the K-shell, the characteristic x-ray has an energy = EK – EL. This energy is discrete, and characteristic to a particular element.

Recall that the energy spectrum of bremsstrahlung photons is continuous.

3.4 Physics of X-Ray Production (characteristic x-rays, cont’d)

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3.5 X-Ray Energy Spectra

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3.5 X-Ray Energy Spectra (cont’d)

If no filtration, the theoretical energy spectrum is a straight line. However, due to inherent and additional filtration, the low-energy portion of the spectrum is reduced, making the beam ‘harder’, that is, more penetrating.

The maximum energy on the spectrum is the energy of the incoming electron, which equals to the value of the applied voltage peak (e.g. 100 keV electrons from 100 kVp). The average energy is approx. 1/3 of the maximum energy.

The quality of the x-ray beam is specified by a quantity half-value layer.

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3.6 Operating Characteristics

filament current

tube current

tube voltage

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3.6 Operating Characteristics (cont’d)

The relationship between x-ray output and filament current, tube current, and tube voltage.

The x-ray output is measured by the quantity, exposure, defined as the amount of ionization produced per mass of air.

The x-ray output is very sensitive to the filament current, thus it is important to keep it stable for constant output.

The x-ray output is linearly proportional to the tube current, and varies approximately as squares of the tube voltage (kilovoltage).

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