Radiology Physics: X-Ray Production and Properties

Core Review

The X-Ray Tube

The dental X-ray tube is a vacuum-sealed glass housing containing a cathode and an anode. Understanding each component is essential:

Cathode (Negative Electrode) — The source of electrons:

• Filament: A coiled tungsten wire that, when heated by the filament circuit, emits electrons through thermionic emission. Tungsten is used because of its high melting point (3,422°C), high atomic number (74), and good electron emission properties.
• Focusing cup: A molybdenum cup surrounding the filament that electrostatically focuses the electron beam into a narrow stream directed at the anode target.
• Filament circuit: Low-voltage (3-5 V), high-amperage circuit that heats the filament.

Anode (Positive Electrode) — The target where X-rays are produced:

• Target: A small tungsten plate embedded in a copper stem. Tungsten is used for its high atomic number (efficient X-ray production), high melting point (withstands heat), and good thermal conductivity.
• Copper stem: Conducts heat away from the target (approximately 99% of the kinetic energy of electrons is converted to heat; only about 1% becomes X-rays).
• Focal spot: The small area of the target where electrons strike. Smaller focal spot = sharper image but more heat concentration. Dental X-ray focal spots are typically 0.6-1.0 mm.

Additional Components:

• Glass housing: Maintains the vacuum necessary for electron flow without collisions with air molecules.
• Insulating oil: Surrounds the tube to absorb heat and provide electrical insulation.
• Lead casing: Prevents leakage radiation except through the window (port).
• Aluminum filter: At the tube window, removes low-energy (soft) X-rays that would increase patient dose without contributing to image formation.
• Collimator: Restricts the beam to the size needed for the image receptor, reducing patient dose and scatter radiation.

The Tube Circuit

The dental X-ray circuit consists of two interdependent circuits:

1. Filament Circuit (Low-Voltage Circuit)

• Voltage: 3-5 V
• Current: 3-5 A (amperes)
• Function: Heats the tungsten filament to produce electrons via thermionic emission
• mA control: Adjusts the filament current, which controls the number of electrons available

2. High-Voltage Circuit (Tube Circuit)

• Voltage: 60,000-70,000 V (60-70 kVp) for intraoral radiography; up to 90 kVp for panoramic
• Function: Creates the potential difference between cathode and anode that accelerates electrons across the tube
• kVp control: Adjusts the peak voltage, which controls the energy (speed) of electrons

X-Ray Production Mechanisms

When high-speed electrons from the cathode strike the tungsten target at the anode, X-rays are produced through two distinct mechanisms:

1. Bremsstrahlung Radiation ("Braking Radiation")

• The incident electron passes near the nucleus of a tungsten atom
• The positive charge of the nucleus decelerates (brakes) the electron
• The lost kinetic energy is emitted as an X-ray photon
• The energy of the resulting X-ray can range from near zero to the full kinetic energy of the incident electron (equals kVp)
• Bremsstrahlung produces a continuous spectrum of X-ray energies
• Accounts for approximately 70-80% of X-rays produced in diagnostic radiology

2. Characteristic Radiation

• The incident electron directly ejects an inner-shell (K-shell) electron from a tungsten atom
• An outer-shell electron drops down to fill the vacancy
• The energy difference between the shells is emitted as an X-ray photon
• The energy is specific (characteristic) to the element — tungsten K-shell characteristic X-rays have energies of approximately 59 and 67 keV
• Characteristic radiation produces discrete energy peaks, not a continuous spectrum
• Accounts for approximately 20-30% of X-rays produced

Important: For characteristic radiation to occur, the incident electron must have energy greater than the binding energy of the K-shell electron in tungsten (69.5 keV). Thus, characteristic radiation only occurs at kVp settings of approximately 70 kVp and above.

Technical Parameters

kVp (Kilovoltage Peak) — Controls X-ray ENERGY (penetrating power/quality):

• Higher kVp = higher energy X-rays = more penetration = more shades of gray (longer scale/lower contrast)
• Lower kVp = lower energy X-rays = less penetration = more black and white (shorter scale/higher contrast)
• Typical intraoral kVp: 60-70 kVp
• Primary effect: Contrast (gray scale)
• Secondary effect: Density (higher kVp also slightly increases density)
• Rule of thumb for contrast adjustment: increase kVp by 10-15 to produce a visible change

mA (Milliamperage) — Controls X-ray QUANTITY (number of X-rays):

• Higher mA = more electrons = more X-rays = increased density (darker image)
• Lower mA = fewer electrons = fewer X-rays = decreased density (lighter image)
• Typical intraoral mA: 6-8 mA for standard units; 2-4 mA for high-frequency units
• Primary effect: Density (overall darkness)
• mA DOES NOT affect contrast

Exposure Time — Controls the duration of X-ray production:

• Longer time = more X-rays = increased density
• Shorter time = fewer X-rays = decreased density
• Measured in impulses (1 impulse = 1/60 second) or seconds
• mA × time = mAs (milliampere-seconds), which is the primary determinant of density
• Example: 7 mA × 0.2 seconds = 1.4 mAs
• Modern DC and high-frequency units use shorter exposure times than older AC units

The Inverse Square Law

The inverse square law states that the intensity of radiation is inversely proportional to the square of the distance from the source:

I₁/I₂ = D₂²/D₁²

Where:

• I₁ = intensity at distance D₁
• I₂ = intensity at distance D₂

Clinical applications:

• Doubling the distance from the X-ray source reduces intensity to 1/4 (not 1/2)
• Tripling the distance reduces intensity to 1/9
• To maintain the same image density when changing distance, adjust mAs using the inverse square law
• The operator should stand at least 6 feet from the X-ray source, behind a protective barrier or outside the operatory

Example: If the radiation intensity at 16 inches from the source is 100 mR, what is the intensity at 32 inches?

• I₁/I₂ = D₂²/D₁² → 100/I₂ = 32²/16² → 100/I₂ = 1024/256 → I₂ = 100 × 256/1024 → I₂ = 25 mR
• The intensity is reduced to 1/4 when the distance doubles.

Properties of X-Rays

X-rays are a form of electromagnetic radiation with the following properties:

• Travel at the speed of light (3 × 10⁸ m/s)
• Travel in straight lines
• No mass, no charge
• Invisible
• Can penetrate materials that absorb or reflect visible light
• Can cause ionization (remove electrons from atoms)
• Can cause biological damage through ionization
• Produce latent image on photographic emulsion
• Produce fluorescence in certain materials (intensifying screens)
• Cannot be focused by lenses

The electromagnetic spectrum, arranged from lowest to highest energy: Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays

X-rays used in diagnostic dentistry have wavelengths of approximately 0.01-0.5 nm.

Primary, Secondary, and Scatter Radiation

• Primary radiation: The useful beam as it exits the tube head. Also called the central ray.
• Secondary radiation: Radiation produced when the primary beam interacts with matter (the patient). Includes scatter radiation.
• Scatter radiation: A type of secondary radiation that is deflected from its original path. Scatter degrades image quality and increases occupational exposure.
• Leakage radiation: Radiation that escapes through the tube housing, other than through the window. Must not exceed 100 mR/hour at 1 meter.

Clinical Application

When exposing a bitewing radiograph on a large adult patient, you may need to increase exposure factors to compensate for increased tissue thickness. Increasing kVp by 5-10 increases penetration, while increasing mAs increases density. If you take a radiograph at 70 kVp, 7 mA, and 0.20 seconds (1.4 mAs) and the image is too light, you could increase the exposure time to 0.30 seconds (2.1 mAs) — a 50% increase in mAs produces a visibly denser image without changing contrast.

Common Traps

• TRAP: Confusing kVp and mA effects. kVp = contrast; mA = density. The NBDHE loves to ask: "To produce a darker radiograph, which parameter should be increased?" The answer is usually mA (or exposure time), not kVp.
• TRAP: Thinking that doubling the distance halves the intensity. It reduces it to 1/4. This is a frequent calculation error.
• TRAP: Not knowing that characteristic radiation only occurs above 70 kVp. Below 70 kVp, only Bremsstrahlung radiation is produced.
• TRAP: Confusing the filament circuit (low voltage, 3-5 V) with the tube circuit (high voltage, 60-70 kVp).