Introduction to Diagnostic and Therapeutic Radiography (Module Code HS1934)

Welcome to the HS1934 lecture for students in the Diagnostic and Therapeutic Radiography program. It is crucial to confirm that you are attending the correct session. If you are in the wrong session, please consult the lecturer for assistance. This session is expected to be busy, so please fill all available seats without leaving gaps or placing bags on seats. To accommodate late arrivals, fill seats furthest from the entrance first.

Instructor Information

Dr. Benard Ohene-Botwe
Senior Lecturer, Diagnostic Radiography
Email: benard.ohene-botwe@city.ac.uk
Phone: +44 (0)20 7040 4387

Session Content Overview

Principles of X-ray Production (Bremsstrahlung and characteristic radiation)
Properties of X-rays/photons
Interactions of X-rays/photons with matter

Understanding Photons

A photon is defined as the smallest possible particle of light. X-rays are composed of photons and travel at the speed of light, approximately 3 \times 10^8 \text{ m/s}.

Discovery of X-rays

X-rays were discovered on November 8, 1895, by Wilhelm Conrad Röntgen, a German mechanical engineer and physicist.
Röntgen's discovery earned him the first Nobel Prize in Physics in 1901.
His initial experiment involved capturing an image of his wife Bertha's hand, showcasing the properties of X-rays.

Properties of X-rays

X-rays are invisible and have no mass.
They travel at the speed of light in a vacuum and in straight lines.
X-rays possess a very short wavelength and are unaffected by electric and magnetic fields.
X-rays can cause ionization, which involves the gain or loss of electrons by atoms or molecules.

Interaction with Matter

X-rays can pass through healthy body tissue but are absorbed by denser materials such as metal and bone.
They affect photographic film similarly to visible light, resulting in a blackened image.
X-rays can induce photoelectric emission, where electrons are removed from their atomic shells upon absorbing X-rays.

X-ray Production Process

X-rays are produced in an X-ray tube, which consists of various components designed to facilitate the generation of X-rays.

Components of an X-ray Machine

X-ray Tube
X-ray Display Monitor
Collimator
X-ray Table or Couch
Emergency Stop Button
Erect Bucky

X-ray Tube Structure

The X-ray tube is housed in a protective shield filled with pure oil, which acts as both an electrical insulator and a coolant. The tube features a radiolucent window that allows the useful X-ray beam to exit.

Cathode and Anode

Cathode (-): The negative side of the X-ray tube, responsible for emitting a focused electron beam toward the anode.
Anode (+): The positive side where electrons collide to produce X-rays and dissipate heat generated during the process.

Cathode Details

The cathode consists of a filament, focusing cup, connecting wires, and cathode support. The filament is heated to produce electrons through thermionic emission. The focusing cup directs the electrons toward the anode target.

Anode Details

The anode is composed of a tungsten target embedded in a copper stem. The tungsten target is where the electrons collide to produce X-rays, specifically at the focal spot. The copper helps dissipate heat generated during the process.

Types of Anodes

Stationary Anode: Remains fixed during operation.
Rotating Anode: Spins at high speeds, providing advantages such as effective heat distribution, increased lifespan, and higher energy output.

Target Material: Tungsten

Tungsten is used as the target material due to its high melting point (3,370°C), high atomic number (Z=74), good thermal conductivity, and low vapor pressure. These properties ensure efficient X-ray production and longevity of the X-ray tube.

Glass Envelope

The glass envelope of the X-ray tube is made of borosilicate glass, which supports a vacuum necessary for efficient electron movement. The vacuum prevents gas atoms from interfering with the electrons, allowing for optimal X-ray production.

Production of X-rays

X-rays are generated when fast-moving electrons strike the anode target. The electrons are accelerated by a potential difference (kilovoltage) and upon impact, about 1% of their kinetic energy is converted into X-rays, while the remainder is dissipated as heat.

Operational Parameters

Radiographers can adjust various parameters on the operating console, including:

Exposure time (milliseconds)
Tube current (milliamperes)
Tube voltage (kilovolts or kilovolt peak)

X-Ray Interactions and Their Implications

Impact of Scatter Radiation

Scatter Radiation: Reduces the quality of radiographic images by decreasing contrast.
Filtration: Low-energy scatter radiation is removed through filtration techniques, enhancing image quality.
Radiation Dose: Scatter contributes to the radiation dose received by both patients and staff.

Lower kVp settings result in scatter being absorbed closer to the area of interest.
Higher kVp settings allow more scatter to escape the patient, increasing exposure to remote areas.

Techniques to Reduce Scatter:
- Use of grids or air gap techniques.
- Compression of tissues to reduce volume and scatter production.
- Beam collimation and positioning of the image intensifier (II) closer to the patient.
- Implementation of lead barriers and protective clothing for staff.

X-Ray Photon Interactions

X-Ray Spectrum: At 150 kVp, both bremsstrahlung and characteristic radiation are present.
Photoelectric Effect:
- The incoming x-ray photon is absorbed by an atom, typically interacting with electrons in the K or L shell.
- The absorbed energy allows the electron to escape, creating a positive ion.
- Energy of the photoelectron is calculated as: KE_{photoelectron} = E_{photon} - Binding Energy_{electron}.
- Vacancies created in electron shells are filled by electrons from higher shells, emitting characteristic radiation.

Intensity and Quality of X-Ray Beams

Intensity: Defined as the total energy carried by the beam per second per unit area, influenced by:
- Number of x-ray photons (quantity).
- Energy of x-ray photons (quality).
- Tube current (mA) and kVp settings.
Quality: Refers to the penetrating power of the x-ray beam, determined by kVp settings. Higher quality x-rays penetrate tissues more effectively, affecting image contrast.

Beam Filtration

Purpose: Beam filters are used to remove low-energy (soft) x-rays that do not contribute to image formation, thus reducing patient skin dose.
Materials: Common materials for filtration include:
- Aluminum (Z=13) for diagnostic imaging.
- Copper (Z=29) for radiotherapy.
- Molybdenum and palladium are also used.

Interactions of X-Rays with Matter

Types of Interactions:
- Elastic Scattering: Also known as Rayleigh or coherent scattering, involves no energy loss and is predominant at low energies.
- Compton Scattering: Involves energy loss and change of direction of the incident x-ray photon, accounting for most scatter in diagnostic radiology.
- Photoelectric Absorption: Predominant at low energies, leading to emission of characteristic x-rays and Auger electrons.
- Pair Production: Occurs at high energies (≥1.022 MeV), resulting in the creation of an electron and positron pair.
Attenuation: Refers to the reduction in the intensity of x-rays as they pass through matter, influenced by the type of interaction.

Photoelectric Absorption

Mechanism:
- Occurs when the photon energy is equal to or greater than the binding energy of the electron.
- Probability of photoelectric absorption is highest when photon energy matches binding energy, decreasing rapidly with increasing energy.
- Characteristic x-rays are emitted when vacancies in electron shells are filled.
Factors Influencing Absorption:
- Proportional to the atomic number of the material (Z³) and inversely proportional to the energy of the incident photon (E³).
- Bone absorbs more x-rays than soft tissue due to its higher atomic number.

Compton Scattering

Process: Involves an incident x-ray photon losing energy and changing direction upon interaction with an electron.
Consequences:
- Results in scatter radiation, reducing image contrast.
- Increases radiation dose to both patients and staff.

Pair Production and Photodisintegration

Pair Production:
- Occurs when a high-energy photon interacts with a nucleus, producing an electron and positron.
- Positron eventually annihilates with an electron, producing gamma-ray photons.
Photodisintegration:
- Involves high-energy photons (≥10 MeV) striking a nucleus, causing it to disintegrate into nuclear fragments.

Conclusion

Understanding the interactions of x-rays with matter is crucial for optimizing radiographic techniques, minimizing patient and staff radiation exposure, and improving image quality.

Overview of X-ray Production

X-rays are generated when high-speed electrons collide with the target material of an anode. The process involves several key components and parameters:

Filament: A heated wire that emits electrons.
Target: The material (typically tungsten) that the electrons strike.
Evacuated Glass Tube: A sealed environment that prevents air from interfering with electron flow.
Potential Difference (Voltage): Measured in kilovolts (kV), it accelerates electrons from the filament to the target.

Mechanism of X-ray Production

When electrons from a heated filament are accelerated towards the target anode, they undergo two primary processes upon impact:

Bremsstrahlung (Braking Radiation): Occurs when electrons are decelerated by the nucleus of the target atoms, converting kinetic energy into X-rays.
Characteristic Radiation: Involves the ejection of inner-shell electrons by incident electrons, resulting in the emission of X-rays as outer-shell electrons fill the vacancies.

Parameters Affecting X-ray Production

The radiographer can adjust several parameters to optimize X-ray production for diagnostic purposes:

Exposure Time (ms): Duration for which X-rays are produced.
Tube Current (mA): Measures the number of electrons crossing from cathode to anode per second.
Tube Voltage (kVp): Controls the peak potential difference, affecting the quality and penetrating power of the X-ray beam.
mAs (milliampere-seconds): The product of exposure time and tube current, influencing the total amount of X-ray produced.

Energy Conversion in X-ray Production

Most of the energy from incident electrons is converted into heat rather than X-rays. The efficiency of X-ray production is low, with only about 1% of the kinetic energy resulting in X-ray photons.

Interaction of Photons with Matter

The interaction of X-ray photons with matter is characterized by a measure known as cross-section, which indicates the likelihood of photon interaction with a material. The attenuation of the X-ray beam depends on the material's atomic composition and density.

Elastic Scattering (Coherent Scattering): Photons are deflected without energy loss, occurring primarily at low energies.
Photoelectric Absorption: Photons are completely absorbed, transferring all energy to an electron.
Compton Scattering: Photons lose energy and change direction upon colliding with electrons, which is significant in diagnostic radiology.
Pair Production: Occurs at high energies (500 keV to 500 MeV), resulting in the creation of electron-positron pairs.

Bremsstrahlung Radiation

Bremsstrahlung occurs when electrons approach the nucleus of a tungsten atom, experiencing an attractive force that causes them to slow down and lose kinetic energy, which is then converted into X-rays. The energy spectrum of X-rays produced is continuous, with a peak energy (Emax) that varies with the selected kVp.

Characteristics of Bremsstrahlung

Most photons produced have intermediate energy levels.
Energy distribution is continuous, with fewer photons at the extremes of the energy range.
Coherent mass attenuation coefficients depend on the atomic number and the energy of the incident photon.

Characteristic Radiation

Characteristic radiation occurs when incident electrons displace inner-shell electrons in the target material. The resulting vacancy is filled by electrons from higher energy shells, emitting X-ray photons with energies specific to the target material.

K-alpha Spikes: Emission of X-rays when an electron falls from the L-shell to the K-shell, typically at 59 keV for tungsten.
K-beta Spikes: Emission when an M-shell electron fills a K-shell vacancy, typically at 67 keV.
Characteristic radiation contributes less to the overall X-ray production compared to Bremsstrahlung.

Scattering and Its Implications

Scattering of X-ray photons is a critical factor in diagnostic radiology:

Compton scattering accounts for most scattered radiation, affecting image quality and patient dose.
Higher kVp settings increase scatter radiation, which can lead to higher doses in areas not directly imaged.
Techniques such as grids, air gaps, and compression are employed to minimize scatter and improve image quality.

Radiation Dose Considerations

Both patients and radiography staff are exposed to radiation during X-ray procedures. Effective measures, including beam collimation and protective barriers, are essential to minimize unnecessary exposure.

Fundamentals of X-Ray Production and Interaction

Fundamentals of X-Ray Production and Interaction