Radiology Physics and X-ray Production Flashcards

Fundamental Matter and Atomic Structure

Forms of Matter: Matter exists in three primary states: solid, liquid, and gas.
Atomic Composition: Atoms are the basic building blocks of matter and consist of three subatomic particles: - Protons: Positively charged particles located in the nucleus. - Neutrons: Neutrally charged particles located in the nucleus. - Electrons: Negatively charged particles that reside in orbits or shells around the nucleus.
The Nucleus: Made up of protons and neutrons.
Electron Orbitals/Shells: Electrons orbit the nucleus at specific energy levels.
Ionization: - This occurs when an atom has a greater or lesser number of electrons than protons, resulting in a net charge. - Radiological Significance: Ionization is a critical concept in radiology because X-rays cause ionization in human body tissues, which leads to biological effects.
Atomic Properties of Tungsten: Tungsten is used in X-ray production because it has a high melting point, allowing it to withstand the intense heat generated during the process.

Electromagnetic Energy and X-Ray Photons

Photons: X-ray photons are the discrete packets of energy that make up the X-ray beam.
Types of X-ray Production: - Bremsstrahlung Radiation - Characteristic Radiation
K-Shell Physics: - The removal of an electron from the K-shell is a primary method by which characteristic X-rays are created. - The binding energy for the K-shell in tungsten is approximately $69.5\,kV$ .
The Electromagnetic Spectrum: - X-rays: Characterized by high frequency and short wavelengths. - Radio Waves: Characterized by the longest wavelengths and the lowest energy. - Other components: Includes visible light, microwaves, and radio waves.
Wave Characteristics: - Sine Waves: Electromagnetic radiation travels in sine waves. - Amplitude: The height of the wave crest or the distance between the crest and the trough. - Wavelength: The distance from one crest to the next consecutive crest. - Frequency: The number of times per second the wave hits the crest (measured in cycles per second or Hertz). - Velocity: The speed at which the wave travels (speed of light).

Factors Affecting the X-Ray Beam (mAs and kVp)

Quantity (mAs): Refers to the number or total quantity of photons in the beam.
Quality (kVp): Refers to the energy and penetrability of the X-ray photons.
Image Characteristics: - Density/Brightness: Relates to the overall blackness or lightness of the image (controlled by $mAs$ ). - Contrast: Calculated by the variations of grays or the difference between black and white (controlled by $kVp$ ). - High Contrast: Results in fewer shades of gray (more black/white); often achieved with lower $kVp$ .
Exposure Levels: - Overexposure: The image is too black. - Underexposure: The image is too white.
Quantum Mottle: A grainy appearance on the image caused by insufficient photons hitting the image receptor (IR).
Scatter Radiation (Fog): Unwanted radiation that reduces image contrast.
Speed and Energy: An increase in the speed or velocity of electrons leads to an increase in the energy of the resulting X-rays ( $\uparrow \text{speed} = \uparrow \text{energy}$ ).

Electricity and Circuitry in Radiography

Electric Current: The flow of negative electrons toward a positive charge.
Potential Difference: The power or speed of electron flow, measured in volts ( $V$ ).
Electrical Resistance: Often referred to as "drag," it is anything that hinders the flow of current.
Types of Current: - Direct Current (DC): Current flows at a constant rate in one direction from the anode to the cathode. - Alternating Current (AC): Current flow changes or alternates directions at a rate of $60\,cycles/second$ or $60\,Hz$ .
Transformers: Devices used to vary voltage and amperage through electromagnetic induction. - Step-Up Transformer: Increases voltage while decreasing amperage (found in the high voltage secondary circuit). - Step-Down Transformer: Decreases voltage while increasing amperage (used to heat the filament). - Autotransformer: A variable transformer that serves as the $kVp$ selector (adjusts major and minor $kVp$ steps).
Rectification: The process of changing Alternating Current (AC) to Direct Current (DC). This occurs via rectifiers.
Operating Voltages: The X-ray circuit typically varies between $40,000\,V$ and $125,000\,V$ .

The X-Ray Tube and Production Process

Primary Components: - Cathode (-): The negative electrode; contains the filament and focusing cup. - Anode (+): The positive electrode; contains the tungsten target.
Thermionic Emission: The process of heating the filament to "boil off" electrons, creating an electron cloud.
Space Charge: The electron cloud formed at the entrance to the cathode around the filament.
Focusing Cup: A component on the cathode that keeps the electron cloud tightly packed; it typically requires DC power.
Target (Actual Focal Spot): The specific area on the anode where electrons strike to produce X-rays.
Prime Factors for Production: 1. Source of electrons (Filament heating). 2. Vacuum (Glass envelope to remove air). 3. High potential difference (Applied voltage/ $kVp$ to accelerate electrons). 4. Target material (Tungsten).

Radiation Units and History

Discovery: X-rays were discovered by Wilhelm Roentgen on November 8, 1895.
Radiation Measurement Units: - Exposure: Measured in Roentgen (Conventional) or Air kerma / Gray_a (SI). Measured as Intensity in Air ( $Air\,Ker/kg$ or $Coulombs/kg$ ). - Absorbed Dose: Measured in Rad (Conventional) or Gray (Gy) (SI). This measures the radiation absorbed by any material or patient. - Dose Equivalent: Measured in Rem (Conventional) or Sievert (Sv) (SI). Used to express the biological effect of dose on workers (occupational dose). - Effective Dose: Specifically refers to the absorbed effect on the patient and is also measured in Sieverts (Sv).
X-ray Weighting Factor: The weighting factor for X-rays is $1$ .
Dosimetry: A dosimeter is used to measure the amount of radiation dose a worker is receiving. Readings are typically provided in millisieverts ( $mSv$ ).
Entrance Skin Exposure (ESE): The patient dose in radiography is always measured at the skin level.

Biological Effects and Dose Limits

Cellular Level: Radiation can cause cell death, cell malfunction, and cell abnormalities.
Biological Thresholds: - Blood Changes: Observed at $250\,mSv$ ( $0.25\,Sv$ ). - Erythema (Skin Reddening): Observed at $2000\,mSv$ ( $2\,Sv$ ). - Lethal Dose: Occurs between $4000\,mSv$ ( $4\,Sv$ ) and $5000\,mSv$ ( $5\,Sv$ ).
Pregnancy Limits: - Monthly limit for pregnant workers: $0.5\,mSv$ . - Annual/Total gestation limit for pregnant workers: $5\,mSv$ .

Geometric Factors and Filtration

Filament Size: - Small Filament: Produces a smaller stream of electrons, resulting in fewer X-rays but higher detail. - Large Filament: Produces a larger stream of electrons, resulting in more X-rays.
Line Focus Principle: The relationship between the actual focal spot and the effective focal spot. - Actual Focal Spot: The area on the target bombarded by electrons. - Effective Focal Spot: The area of the target from which X-ray photons hit the image receptor (IR). - Anode Angle: Because of the angle of the anode, the effective focal spot is always smaller than the actual focal spot. - Standard Focal Spots: Small = $0.6\,mm$ ; Large = $1.2\,mm$ .
Anode Heel Effect: The absorption of X-rays by the heel of the anode. This causes the X-ray intensity to be greater at the cathode side of the beam than at the anode side, leading to an uneven distribution within the radiation field.
Filtration: - Primary Purpose: To remove low-energy, long-wavelength X-rays, thereby reducing patient skin dose. - Inherent Filtration: Built into the tube (glass, oil, mirror). - Added Filtration: Sheets of metal added to the tube port (typically aluminum). - Total Filtration Requirement: Must be a minimum of $2.5\,mm$ of Aluminum equivalent ( $Al\,eq$ ).