DRAD

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Define ‘Radiography’

The techniques involved in producing various radiographic images

Define ‘Radiology’

The interpretation of various radiographic images

Define ‘Attenuation’

Reduction in the intensity of the main X-ray beam caused by absorption and scattering

Define ‘Scattering’

Change in direction of a photon with or without a loss of energy

Define ‘Absorption’

Deposition of energy, i.e. removal of energy from the beam

Define ‘Ionisation’

Removal of an electron from a neutral atom producing a negative ion (the electron) and a positive ion (the remaining atom)

Define ‘Radiolucent’

Less dense materials that permit X-ray beams to pass through, hitting the receptor

Define ‘Radiopaque’

Dense materials that absorb or stop X-ray penetration

What affects the shadow density of an object?

• specific type of material of which the object is made

• thickness or density of the material

• shape

• intensity of xray beam

• position of object in relation to the xray beam and image receptor

• sensitivity and type of image receptor

What is superimposition?

The overlapping of anatomical structures on a radiograph due to the projection of three-dimensional structures onto a two-dimensional image

Key limitations of a 2D radiographic image

• superimposition

• loss of depth information - cannot tell true spatial relationships

• distortion - can be stretched or compressed

• magnification - objects can appear larger

• geometric inaccuracies - shapes and lengths may not be true to reality

• limited detail of complex anatomy - 3D structures are flattened

What factors affect overall image quality of a 2D radiograph?

• contrast

• image geometry

• characteristics of the xray beam

• image sharpness and resolution

Define ‘Contrast’

The visual difference between the various black, white, and grey shadows

Define ‘Image geometry’

The relative positions of the image receptor, object, and xray tubehead

Describe the positioning of the Image Receptor, Object, and X-ray Beam

• the object and the image receptor should be in contact or as close together as possible

• the object and the image receptor should be parallel to one another

• the X-ray tubehead should be positioned so that the beam meets both the object and the image receptor at right angles

Ideal characteristics for an X-ray beam

• sufficiently penetrating, to pass through the patient and react with the film emulsion or digital sensor and produce good contrast between the different shadows

• parallel, i.e. non-diverging, to prevent magnification of the image

• produced from a point source, to reduce blurring of the edges of the image (penumbra effect)

Two main groups of traditional dental radiographs

• intraoral

• extraoral

How are X-rays described in terms of energy?

X-rays consist of wave packets of energy called photons, with each photon representing one quantum of energy

What is a photon?

A single packet (quantum) of X-ray energy

Define ‘Atomic number’

The number of protons in the nucleus of an atom

Define ‘Neutron number’

The number of neutrons in the nucleus of an atom

Define ‘Atomic mass number’

The sum of the protons and neutrons in an atom

Define ‘Isotope’

An atom with the same atomic number but different neutron number so therefore a different atomic mass

Define ‘Radioisotope’

An isotope with unstable nuclei which undergo radioactive disintegration

What determines the chemical behaviour of an atom?

The number of electrons determines the chemical behaviour of the element and forms the basis of the periodic table

Why are atoms in the ground state electrically neutral?

Number of positive charges (protons) is balanced by the number of negative charges (electrons), resulting in no overall charge

What is ‘excitation’ in an atom?

Excitation occurs when an electron moves from an inner shell to a higher energy outer shell. The atom remains neutral but is in an excited state

Main features of an X-ray Tube

• cathode (negative)

• anode (positive)

• tungsten target

• focusing device

• glass vacuum tube

• high voltage supply

• protective casing (lead-lined)

• x-ray window

Purpose of the Cathode in an X-ray tube

Consists of a heated filament of tungsten that provides the source of electrons

Purpose of the Anode in an X-ray tube

Consists of a tungsten target for electrons set into the angled face of a large copper block to allow efficient removal of heat

Purpose of the Focusing Device in an X-ray Tube

Aims the stream of electrons at the focal spot on the target

Why is the target in an X-ray tube made of tungsten?

• high atomic number - high number of electrons, efficient production of x-ray

• high melting point - withstand heating

Purpose of the Glass Vacuum Tube in an X-ray Tube

Maintains a vacuum so that electrons can travel freely and prevents collisions with air molecules

Purpose of the High Voltage Supply in an X-ray Tube

Creates a potential difference between the cathode and anode so the electrons can be accelerated at a high speed towards the target

Purpose of the Protective Housing of an X-ray Tube

• Lined with lead which absorbs unwanted x-rays because x-rays are emitted in all directions

• Contains insulating oil which facilitates the removal of heat

Describe how x-rays are produced in an x-ray tube

1. The filament is electrically heated and a cloud of electrons is produced around the filament.

2. The high-voltage (potential difference) across the tube accelerates the electrons at very high speed towards the anode.

3. The focusing device aims the electron stream at the focal spot on the target.

4. The electrons bombard the target and are brought suddenly to rest.

5. The energy lost by the electrons is transferred into either heat (about 99%) or X-rays (about 1%).

6. The heat produced is removed and dissipated by the copper block and the surrounding oil.

7. The X-rays are emitted in all directions from the target. Those emitted through the small window in the lead casing constitute the beam used for diagnostic purposes

What happens when high-speed electrons strike the tungsten target in an X-ray tube?

Two main types of collisions:

• heat-producing collisions

• x-ray-producing collisions

Describe a Heat-producing Collision

• The incoming electron is deflected by the cloud of outer-shell tungsten electrons, with a small loss of energy, in the form of heat

• The incoming electron collides with an outer-shell tungsten electron, displacing it to an even more peripheral shell (excitation) or displacing it from the atom (ionisation), again with a small loss of energy in the form of heat

Why are heat-producing interactions more common than x-ray-producing interactions?

• There are millions of incoming electrons and many outer-shell tungsten electrons with which to interact

• Each individual bombarding electron can undergo many heat-producing collisions resulting in a considerable amount of heat at the target

Why must heat be removed from the X-ray tube target, and how is this achieved?

• Heat must be removed quickly to prevent damage to the tungsten target

• Achieved by embedding the tungsten target in a copper block, which has high thermal capacity and good thermal conductivity

Describe an X-ray-producing Collision

• The incoming electron penetrates the outer electron shells and passes close to the nucleus of the tungsten atom - it is dramatically slowed down and deflected by the nucleus with a large loss of energy, which is emitted in the form of x-rays

• the incoming electron collides with an inner-shell tungsten electron, displacing it to an outer shell (excitation) or displacing it from the atom (ionisation),with a large loss of energy and subsequent emission of x-rays

What are the two types of X-ray spectra produced by X-ray-producing collisions?

• Continuous spectrum (Bremsstrahlung radiation)

• Characteristic spectrum

Describe the Continuous Spectrum

The continuous spectrum is a range of X-ray energies produced when high-speed electrons are slowed down (decelerated) near the nucleus of tungsten atoms - the amount of deceleration and degree of deflection determine the amount of energy lost by the bombarding electron/resultant emitted photon

• Far from nucleus → small energy loss → low-energy X-rays

• Very close → large energy loss → high-energy X-rays

Why is the Continuous Spectrum described as ‘continuous’?

• electrons can lose any amount of energy

• a wide range or spectrum of photon energies is therefore possible

What type of electron deflection is most common in continuous X-ray production and what does it produce?

Small deflections are most common and produce low-energy photons

Why do low-energy photons not contribute to the useful X-ray beam?

They have low penetrating power and are absorbed within the X-ray tube or patient, so they do not reach the detector - their removal is called filtration

What type of deflection produces high-energy photons and how common is it?

Large deflections (close interaction with the nucleus) produce high-energy photons, but these events are rare

What determines the maximum photon energy (Emax) in an X-ray tube?

The potential difference (kV) across the X-ray tube

Describe the Characteristic Spectrum

The characteristic spectrum consists of X-rays with specific (discrete) energies that are unique to the target material (e.g. tungsten)

1. High-energy electron hits inner-shell electron

   • A bombarding electron ejects an inner-shell (e.g. K-shell) electron from a tungsten atom

2. Vacancy is created

   • This leaves a gap in the inner shell

3. Electron from outer shell drops down

   • An electron from a higher energy shell moves down to fill the vacancy

4. Energy is released

   • The difference in energy between the two shells is emitted as an X-ray photon

Why is the Characteristic Spectrum described as ‘characteristic’?

• The energy of the emitted X-ray depends on:

  • The difference between electron shell energy levels

• These energy differences are unique to each element

➡ So the X-rays are characteristic of tungsten

What do the K and L lines represent in a Line Spectrum?

They are named according to the electron shell involved in the transition:

• K lines → transitions to the K shell

• L lines → transitions to the L shell

Which characteristic X-ray lines are diagnostically important and why?

Only the K lines are diagnostically important because L lines have too little energy to be useful

What is required to produce a characteristic K-line in tungsten?

The bombarding electron must have ≥ 69.5 kV to eject a K-shell electron

What is meant by the “critical voltage” in X-ray production?

It is the minimum voltage (69.5 kV for tungsten) required to produce characteristic K-line photons

What happens if the X-ray tube operates below the critical voltage?

No characteristic K-line photons are produced

What is the typical operating voltage range of dental X-ray machines?

Approximately 60–90 kV

What is meant by intensity and quality of an X-ray beam?

• Intensity = number (quantity) of photons

• Quality = energy of photons (penetrating power)

What factors affect X-ray beam intensity and quality?

• Tube voltage (kV)

• Tube current (mA)

• Distance (d)

• Time (t)

• Filtration

• Target material

• Voltage waveform

How do X-rays travel in free space?

• Travel in straight lines

• Speed = 3 × 10⁸ m/s

• Do not require a medium

What is the inverse square law?

Intensity is inversely proportional to the square of the distance:
👉 Doubling distance → intensity becomes ¼

How does wavelength relate to X-ray energy?

• Short wavelength → high energy → high penetration

• Long wavelength (soft X-rays) → low energy → low penetration

What happens when X-rays interact with matter?

They are attenuated by:

• Absorption

• Scattering

Why are X-rays considered hazardous?

They are ionising radiation, capable of causing biological damage

Can humans detect X-rays?

No — X-rays are undetectable by human senses

How are X-rays used to produce images?

1. Affect film emulsion → radiographic image

• When X-rays pass through the body, different tissues absorb them differently.

• The remaining X-rays hit the film (or sensor).

• On traditional film, X-rays change the film emulsion chemically, creating a visible image after processing.

• Areas that get more X-rays appear darker, and areas that absorb more X-rays (like bone) appear lighter.

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2. Cause fluorescence → used in screens and digital sensors

• X-rays can make certain materials glow (fluoresce).

• This glow is used in:

  • Intensifying screens (to expose film more efficiently)

  • Digital sensors (which convert X-rays into light, then into an electronic image)

What are the four possible fates of X-ray photons when they strike matter?

• Completely scattered with no loss of energy

• Completely absorbed (total loss of energy)

• Scattered with partial absorption (loss of energy)

• Transmitted unchanged

What are the four main interactions of X-rays with matter at the atomic level?

• Unmodified (Rayleigh) scattering – pure scatter

• Photoelectric effect – pure absorption

• Compton effect – scatter + partial absorption

• Pair production – pure absorption

What is Rayleigh scattering?

A low-energy interaction where the photon is scattered without loss of energy (no ionisation)

What is the photoelectric effect?

The photon is completely absorbed, ejecting an inner-shell electron → ionisation

What is the Compton effect?

The photon is partially absorbed and scattered, losing some energy and ejecting an outer-shell electron.

What is pair production?

A high-energy interaction where the photon is absorbed and produces an electron–positron pair (not relevant in dental X-rays)

Which two X-ray interactions are important in the dental energy range?

• Photoelectric effect

• Compton effect

Why are only the photoelectric and Compton effects important in dentistry?

Because dental X-ray energies are such that:

• Rayleigh scattering is minimal

• Pair production requires much higher energy and does not occur

Summary of the Stages in the Photoelectric Effect

1. The incoming X-ray photon interacts with a bound inner-shell electron of the tissue atom.

2. The inner-shell electron is ejected with considerable energy (now called a photoelectron ) into the tissues and will undergo further interactions.

3. The X-ray photon disappears having deposited all its energy; the process is therefore one of pure absorption.

4. The vacancy that now exists in the inner electron shell is filled by outer-shell electrons dropping from one shell to another.

5. This cascade of electrons to new energy levels results in the formation of very low energy radiation (e.g. light), which is quickly absorbed.

6. Atomic stability is finally achieved by the capture of a free electron to return the atom to its neutral state.

7. The high-energy ejected photoelectron behaves like the original high-energy X-ray photon, undergoing many similar interactions and ejecting other electrons as it passes through the tissues. It is these ejected high-energy electrons that are responsible for the majority of the ionization interactions within tissue, and the possible resulting damage attributable to X-rays.

What energy condition is required for the photoelectric effect to occur?

The X-ray photon energy must be equal to or slightly greater than the binding energy of the inner-shell electron

How does atomic number (Z) affect the photoelectric effect?

The probability is proportional to Z³
➡ Higher Z = more photoelectric interactions

Why is lead effective in radiation protection?

Lead has a high atomic number (Z = 82), making it a strong absorber of X-rays via the photoelectric effect

Why do bone and soft tissue appear different on radiographs?

• Bone (Z ≈ 12) → higher photoelectric absorption

• Soft tissue (Z ≈ 7) → lower absorption

➡ Creates radiographic contrast

How does X-ray energy (kV) affect the photoelectric effect?

Probability is proportional to 1 / kV³
➡ Lower kV = more photoelectric absorption

What is the effect of low kV on radiographs?

• ✅ Higher contrast

• ❌ Increased patient dose

What is the overall result of the photoelectric effect?

Ionisation of tissues

How do intensifying screens use the photoelectric effect?

They absorb X-rays and emit light, which then exposes the film

Summary of the Stages in the Compton Effect

1. The incoming X-ray photon interacts with a free or loosely bound outer-shell electron of the tissue atom.

2. The outer-shell electron is ejected (now called the Compton recoil electron ) with loss of some of the energy of the incoming photon, i.e. there is some absorption. The ejected electron then undergoes further ionizing interactions within the tissues (as before).

3. The remainder of the incoming photon energy is deflected or scattered from its original path as a scattered photon.

4. The scattered photon may then:
   – Undergo further Compton interactions within the tissues
   – Undergo photoelectric interactions within the tissues – Escape from the tissues – it is these photons that form the scatter radiation of concern in the clinical environment.

5. Atomic stability is again achieved by the capture of another free electron.

What is the energy relationship in the Compton effect?

The incoming X-ray photon has much greater energy than the binding energy of the outer-shell electron

Is the Compton effect dependent on atomic number (Z)?

No — it is independent of atomic number, as the photon interacts with free or loosely bound electrons

Why does the Compton effect provide poor diagnostic information?

Because it is not dependent on Z, there is little differentiation between tissues, resulting in low image contrast

At what energies does the Compton effect predominate?

At higher X-ray photon energies (high kV)

How does high kV affect radiographic image quality?

• ❌ Reduced contrast

• ❌ More scatter (Compton effect)

What happens to the energy of the photon in the Compton effect?

The scattered photon has less energy than the incoming photon, as some energy is transferred to the recoil electron

How does scatter direction relate to photon energy?

• High-energy photons → forward scatter

• Low-energy photons → back scatter

Why is forward scatter important in radiography?

It can reach the image receptor and degrade image quality

What is the overall result of the Compton effect?

Ionisation of tissues

3 main components of Dental X-ray Equipment

• Tubehead

• Positioning arms

• Control panel and circuitry

Ideal Requirements of Dental X-ray Equipment

• Safe and accurate

• Capable of generating X-rays in the desired energy range and with adequate mechanisms for heat removal • Small

• Easy to manoeuvre and position

• Stable, balanced and steady once the tubehead has been positioned

• Easily folded and stored

• Simple to operate and capable of both film and digital imaging

• Robust

What are the main components of a dental X-ray tubehead?

• Glass X-ray tube (filament, copper block, target)

• Step-up transformer

• Step-down transformer

• Lead shielding

• Oil (for cooling)

• Aluminium filtration

• Collimator

• Spacer cone / beam-indicating device

What is the function of the step-up transformer?

Increases mains voltage (~240 V) to high kV needed to accelerate electrons

What is the function of the step-down transformer?

Reduces voltage to provide low voltage, high current to heat the filament

Why is lead shielding used in the tubehead?

To minimise leakage radiation

What does the collimator do?

Shapes and restricts the beam:

• Rectangular (image receptor size) OR

• Round (max 6 cm diameter)

What is the function of the spacer cone (beam-indicating device)?

• Directs the X-ray beam

• Maintains correct focus-to-skin distance (FSD)