RAD 111: X-Ray Tube, X-Ray Production, and Heat Units Exhaustive Study Guide

Review of X-Ray Circuitry and Transformers

• Transformer Construction and Requirements     - Construction components include an iron core and primary and secondary coils.     - Transformers require alternating current (AC).     - Transformers operate on the principle of electromagnetic induction involving:         - Self-induction.         - Mutual induction.     - The system is divided into a primary side and a secondary side.

• X-Ray Circuitry Transformer Designs     - Air core.     - Open core.     - Closed core.     - Shell type.

• Transformer Efficiency     - Transformers are not 100% efficient due to various losses:         - Copper losses: Resulting from resistance in the wires ($I^2R$).         - Iron Losses: Caused by hysteresis and eddy current.

• Specific Transformer Components     - Autotransformer (kVp selector):         - Operates on the principle of self-induction.         - Features taps that permit small changes in the turns ratio.         - Characterized by a variable turns ratio.     - Step-Down Transformer (Filament transformer):         - Operates on the principle of mutual induction.         - Supplies a high current to the cathode filament wire.         - Characterized by a fixed turns ratio.     - Step-up Transformer (High-voltage Transformer):         - Operates on the principle of mutual induction.         - Changes volts to kilovolts.         - Characterized by a fixed turns ratio.

• Transformer Laws     - Voltage and Turns Ratio: Directly proportional ($\frac{V_s}{V_p} = \frac{N_s}{N_p}$).     - Voltage and Current: Inversely proportional ($\frac{I_p}{I_s} = \frac{V_s}{V_p}$).     - Current and Turns Ratio: Inversely proportional ($\frac{I_p}{I_s} = \frac{N_s}{N_p}$).

Rectification and Voltage Ripples

• Rectifier Function and Location     - A rectifier changes AC to pulsating direct current (DC).     - The X-ray tube requires pulsating direct current to function.     - The rectifier is located between the secondary side of the step-up (SU) transformer and the X-ray tube.

• Voltage Ripple Percentages by Circuit Type     - Half-wave: $100\%$ ripple.     - Full-wave: $100\%$ ripple.     - 3-phase, 6-pulse: $14\%$ ripple.     - 3-phase, 12-pulse: $4\%$ ripple.     - High Frequency: $1\%$ ripple.

The Physics of X-Rays and the Electromagnetic Spectrum

• General Properties of X-Rays     - classified as ionizing radiation.     - Possess no electrical charge.     - Possess no mass.     - Travel at the speed of light.

• Wave-Particle Duality     - Wave characteristics: Defined by wavelength, frequency, and amplitude.     - Particle characteristics: Behave as a photon (a bundle of energy).

• Intensity and Distance     - The intensity of the X-ray beam varies with the square of the distance, known as the Inverse Square Law.

The X-Ray Tube: Structural Components

• Three Main Components     1. Glass or metal envelope (evacuated to create a vacuum).     2. Cathode assembly (negative electrode).     3. Anode assembly (positive electrode).

The Cathode Assembly (-)

• Anatomy and Purpose     - Components: Focusing cup and filament wire(s).     - Purpose: Acts as a source of electrons and focuses the electron stream.

• Filament Wires     - Dual Focused Filament: Consists of one long filament wire and one short filament wire.     - Material: Thoriated tungsten.         - Chosen for its high melting point.         - Chosen for its low vaporization.     - Physical Attributes: Long, thin coil of wire.

• Laws of Resistance in Filament Wires     1. Conductor: Allows electrons to flow freely through it.     2. Cross-sectional area is inversely proportional to resistance.     3. Length is directly proportional to resistance.

• Cathode Operation     - Thermionic Emission: The process where the filament circuit produces high current to heat the wire, releasing electrons from its surface.     - Space Charge: The cloud of electrons surrounding the filament.     - Space Charge Effect: The limitation of further electron emission due to the repulsion of the existing electron cloud.

• The Focusing Cup     - Contains the filaments.     - Materials: Nickel, stainless steel, or molybdenum.     - Charge: Negatively charged to focus electrons into a narrow beam.

• Laws of Electrostatics Relevant to the Cathode     1. Like charges repel; unlike charges attract.     2. Electric charges reside on the external surface of conductors.     3. Electric charges concentrate at the regions of greatest curvature on a conductor.     4. Only negative charges (electrons) are free to move in solid conductors.

The Anode Assembly (+)

• Anatomy and Purpose     - Components: Rotating anode with stem, target (focal track), and induction motor.     - Purpose:         - Attracts electrons.         - Suddenly stops electrons.         - Acts as the site of X-ray production.         - Dissipates heat.

• Anode Target Materials     - General X-Ray Units (Target):         - Tungsten ($90\%$): High atomic number ($Z=74$), high melting point ($3370^\circ C$), good thermal conductor, resists vaporization, efficient for X-ray production, moldable, and low cost.         - Rhenium ($10\%$): Added to prevent pitting of the anode disk.     - Mammography Units: Molybdenum ($Z=42$).     - Anode Base Materials: Molybdenum ($Z=42$) and Graphite (carbon, $Z=6$).     - Anode Stem: Molybdenum ($Z=42$) with a small diameter.

• Focal Spot Size (FSS)     - Determined by the filament wire used.     - Long filament wire: Results in a large focal spot and is used for larger tube currents/mA stations.     - Short filament wire: Results in a small focal spot and is used for smaller tube currents/mA stations.     - Focal Spot Blooming: An increase in focal spot size due to electron repulsion in the tube current, typically caused by high mA and low kVp settings.

• Induction Motor     - Rotor:         - Shaft of copper bars with an iron core in the middle.         - Located inside the glass envelope.         - Contains silver-plated steel ball bearings.     - Stators:         - Electromagnets powered by AC current.         - Located outside the glass envelope.     - Principle: Operates on electromagnetic induction.

• Types of Anode Tubes     - Rotating Anode Tube: Rotates at $3,600$ to $10,000\,rpm$.     - Stationary Anode Tube: Uses a copper block with a tungsten target.

Geometric Principles: Line-Focus and Anode Heel Effect

• Line Focus Principle     - Relates the actual focal spot to the effective focal spot.     - A $20^\circ$ angle produces a larger effective focal spot than a $10^\circ$ angle.     - Diagnostic X-ray tube target angles typically vary from $7^\circ$ to $20^\circ$.

• Anode Heel Effect     - There is a variation in the intensity of the X-ray beam between the cathode and anode sides.     - Intensity Distribution:         - Cathode side: $120\%$         - Center: $100\%$         - Anode side: $75\%$     - The effect is more noticeable with:         - Small (steep) anode angles.         - Small Focal Spot Size (FSS).         - Shorter Source-to-Image Distances (SIDs).         - Larger field sizes.

Off-Focus (Extrafocal) Radiation

• Occurs when electrons strike areas other than the focal spot of the anode target.
• Results in low-intensity X-ray emission from the entire area of the anode.
• Degrades image quality.

External Tube Housing and Safety

• Lead-lined Metal Housing: Contains the x-ray tube and oil for insulation and cooling.
• Leakage Radiation Limits: The housing must allow no more than $100\,mR/hr$ of leakage radiation to escape when measured at $1\,meter$ from the source at maximum output.

Heat Units (HU) and Anode Cooling

• Calculation of Heat Units     - Formula: $HU = mA \times \text{time (sec)} \times kVp \times \text{generator factor} \times \text{number of exposures}$.     - Generator Factors:         - Single phase: $1.0$         - 3-phase, 6-pulse: $1.35$         - 3-phase, 12-pulse: $1.41$         - High frequency: $1.45$     - Example Calculation: For 2 exposures at $90\,kVp$, $400\,mA$, $0.125\,sec$ on a high frequency unit:         - $400 \times 0.125 \times 90 \times 1.45 \times 2 = 13,050\,Heat\,Units$.

• Heat Dissipation Methods     - Radiation.     - Convection.     - Conduction.

• Tube Rating Charts     - Instantaneous Load Chart: Indicates maximum kVp and mA for a specific time for a single exposure. Based on focal spot size, anode angle, anode diameter, and rotation speed.     - Anode Cooling Chart (Cooling Curve): Used to determine how long it takes the anode to cool.         - Example: If exposures result in $60,000\,HU$, and the chart shows $60,000\,HU$ at $4\,minutes$ and total cooling at $10\,minutes$, cooling time is $10 - 4 = 6\,minutes$.         - Example: To cool from max capacity down to $100,000\,HU$, find the time at $100,000\,HU$ (e.g., $3\,minutes$).

X-Ray Production: Mechanisms and Efficiency

• Requirements for Production     1. Source of electrons.     2. External energy source to accelerate electrons.     3. Target electrode to decelerate electrons.     4. Vacuum environment.

• Efficiency and Interaction Results     - X-ray production is highly inefficient.     - $99\%$ is converted to Heat (excitation of outer-shell electrons).     - $1\%$ is converted to X-ray photons.

• Types of Target Interactions     - Heat Production: Kinetic energy of projectile electrons excites outer-shell electrons of the target atoms.     - Bremsstrahlung (Brems) Radiation (White Radiation):         - Caused by the deceleration of an electron near the nucleus due to the strong nuclear force field.         - Produces a polyenergetic/heterogenous beam with a wide range of energies.         - The distance from the nucleus during deflection determines the resulting photon energy.     - Characteristic Radiation (Line Radiation):         - Caused by the ionization of an inner K-shell electron.         - An outer L-shell electron transitions to fill the inner K-shell vacancy (Characteristic Cascade).         - The energy of the photon is equal to the difference in binding energies between the orbits.         - Requires an incident electron energy of $70\,keV$ or higher.         - Only electrons dropping into the K-shell contribute significantly to the beam.

X-Ray Beam Quality and Quantity

• Definitions     - Quality: Refers to the energy and penetrating ability of X-ray photons.     - Quantity: Refers to the total amount or number of X-ray photons.

• Factors Affecting Quality     - kVp (increases quality).     - Filtration (increases quality by removing low-energy photons).     - Circuit waveform (High frequency/3-phase increase quality).

• Factors Affecting Quantity     - mAs ($mA \times \text{exposure time}$): Directly affects quantity.     - kVp: Increases quantity.     - Filtration: Decreases quantity.     - Circuit waveform: More constant voltage increases quantity.     - Anode Material: Higher atomic number (Z) increases both quality and quantity.

• X-Ray Emission Spectrum     - A graph comparing the amount of X-rays and the range of energy (keV).     - Includes the Bremsstrahlung curve and discrete Characteristic peaks.