CHAPTER FIVE

X-RayRobert Eshun

S.L.T. Dept.Accra Polytechnic

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IntroductionE. m. Radiation.

Wavelengths in the range of 10 to 0.01 nm [frequencies 30 petahertz (+15) to 30 exahertz (+18)].

Also known as Rontgen rays.

Energies ranging from 120eV to 120keV.

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Bounded by gamma rays on the left and U.V. on the right.

Shorter in wavelength than U.V. rays.

Longer in wavelength than gamma rays.

X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

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X-rays penetrate easily through materials such as fat and muscle but not bones.

This makes them useful in medicine.

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Ionizing radiation and as such can be dangerous.

X-rays collide with atoms and knock out electrons.

Ionizing radiation is radiation with enough energy to remove tightly bound electrons from the orbit of an atom, causing the atom to become ionized.

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Exposure is the measure of X-rays ionizing ability.

The SI unit is coulomb per kilogram (C/kg).

It measures the amount of radiation required to create 1 coulomb of charge of each polarity in 1 kilogram of matter.

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Absorbed dose measures more appropriately the effect of ionizing radiation on matter .

This is the amount of energy deposited rather than the charge created.

The Gray (Gy) which has units of (J/kg), is the SI unit of absorbed dose.

This is the amount of radiation required to deposit 1 Joule of energy in 1 kilogram of any kind of matter.

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Equivalent dose is the measure of the biological effect of radiation on human tissue.

The sievert (Sv) is the SI unit of equivalent dose, which for X-rays is equal to the gray (Gy)

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INSTRUMENTATION of an x-ray equipment

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Electrons ejected from filament (cathode) through thermionic emission.

Released electrons accelerated by H.T. voltage towards the target (anode).

These high-energy electrons are sharply decelerated as they collide with a metal target, usually tungsten.

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Deceleration of charged particles (electrons) produces a radiation (X-rays).

Only about 1% of the electrons energy is converted to x-ray production.

The rest is heat in the anode.

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X-rays created by two different atomic processes.

X-ray fluorescence:

Electron knocks an orbital electron out of the inner shell of a metal atom.

Vacancy filled by electrons from higher energy levels emitting X-ray photons.

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Produces discrete spectrum of X-ray frequencies (spectral lines).

Lines generated are characteristic of the target element (anode).

Transitions from upper shells into K shells (producing K lines), into L shells (producing L lines) etc.

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Bremsstrahlung:

Given off by electrons decelerated by strong electric fields.

Continuous spectrum X-rays.

Increase linearly with decreasing frequency.

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Radiation protectionThree accepted standards. Controls the amount (or dose) received.

Time: Reduction in duration of exposure reduces the effective dose proportionally.

Distance: Increasing distance reduces effective dose. (Inverse square law)

Shielding: Reduces radiations to safe levels.

Radiation falls exponentially with thickness of shield.

Operators of x-ray eqmnt stay behind leaded glass screens and wear lead aprons.

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National regulatory authorities set the ff. requirements based on international recommendations for ionizing radiation:

Justification: Unnecessary use not permitted advantages must outweigh disadvantages.

Limitation: Individuals must be protected through individual radiation dose limits.

Optimization: Radiation doses should be kept as low as possible. Actual radiation dose should be much lower than the permitted limit.

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DetectorsPhotographic plate were used to produce

radiographic images. The images were produced right on the glass plates.

Film replaced the plates.

Now computed and digital radiography is replacing film in medicine.

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Photo-Stimulable Phosphors (PSP)

Excited electrons in phosphor material remain trapped in colour centres in the crystal lattice.

Stimulation by laser beam.

Light given off during laser stimulation is collected by a PMT.

Resulting signal is converted into a digital image.