X-Ray Tube & Beam Fundamentals of Diagnostic Imaging

The x-ray tube is the primary unit where x-ray photons are created.
Three primary controlling factors that influence the production of x-ray images:
- Kilovoltage peak (kVp): Determines the energy of the x-ray photons produced.
- Milliamperage (mA): Controls the quantity of x-rays produced.
- Exposure Time (seconds): The duration for which x-rays are produced.
A radiographer can adjust all three factors on the control panel to produce a quality image.

The production of x-rays requires a stream of rapidly moving electrons that are suddenly decelerated or stopped.
The negative electrode (cathode) is heated, causing electrons to be emitted.
These emitted electrons are attracted to the positively charged electrode (anode).

The protective housing of the x-ray tube has several distinct features:
- Provides solid, stable mechanical support.
- Constructed from lead-lined metal to shield against leakage radiation.
- Acts as an electrical insulator and thermal cushion for the tube.
- Leakage radiation (x-rays other than the primary beam) must not exceed 100 mR/hr when measured at a distance of 1 meter from the source at maximum output.
Contains the following components:
- Insulating oil
- Cooling fan
- X-ray tube (glass envelope)

The x-ray tube is housed within a glass envelope, which may now be metal.
Main purpose: maintain a vacuum within the tube.
Contains the following key components:
- Cathode
- Anode
- Induction motor
- Target window

The cathode is the negatively charged electrode, responsible for providing the source of electrons for x-ray production.
Contains the following components:
- Focusing cup:
- Surrounds each filament on its back and sides; the front is open.
- Made of nickel and receives a strong negative charge that forces electrons together into a cloud through electrostatic repulsion.
- Filament(s):
- Typically consists of two filaments (one large, one small) known as a dual-focus tube.
- Filaments are made of tungsten with 1-2% thorium added, giving them a very high melting point and preventing easy vaporization.

The anode is the positively charged electrode, composed of molybdenum, copper, tungsten, and graphite.
Contains the following components:
- Target: The site where x-ray photons are produced.
- Stator: Responsible for the rotation of the anode.
- Rotor: The component that turns the anode during x-ray production.

The target of the anode is the place where x-ray photons are produced.
There are two types of targets:
- Stationary Targets: Fixed in position.
- Rotating Targets: Most commonly used; designed to spread the heat produced during x-ray production.
- Made of tungsten and rhenium alloy due to high melting point (3400°C or 6152°F).
- Typically angled between 5-20°, with an average angle of 12°.

When electrons strike the target, their kinetic energy is transferred to tungsten atoms in the anode, resulting in x-ray production.
Interactions occur within the top 0.5mm of the anode surface and include two types of interactions:
- Bremsstrahlung Radiation (Breakdown Radiation)
- Characteristic Radiation

Bremsstrahlung literally means "breaking" radiation.
In this process, an incident electron avoids the orbital electrons and interacts with the nucleus of target atoms.
The closer the incident electron passes to the nucleus, the more energy it loses, generating stronger photons.
This interaction results in a polyenergetic beam:
- The average energy equals one third of the kVp selected.
- Accounts for 100% of photons produced below 70 kVp and approximately 85% of the beam above 70 kVp.

Characteristic radiation occurs when an incident electron ejects an electron from the K-shell of a tungsten atom.
An outer-shell electron then drops into the open position, creating an energy difference.
The emitted energy difference produces an x-ray photon, characteristic of the element involved (in this case, tungsten).
The binding energy of the K-shell for tungsten is 69.5 keV.
This type of radiation accounts for approximately 15% of the x-ray beam above 70 kVp.

X-ray energy is measured in kiloelectron volts (keV).
The x-ray beam is polyenergetic, where the lowest energies are approximately 15-20 keV, and the highest energies equal the kVp set on the control panel.
The average energy of an x-ray beam is always lower than the maximum kVp.

A radiographic exposure is initiated by a radiographer using two switches located on the x-ray unit's control panel, specifically deadman switches that require positive pressure to be applied during the entire exposure process.

Filament current heats up the filament, causing thermionic emission (the boiling off of electrons).
Electrons gather in a cloud around the filament (space charge).
The focusing cup maintains this electron cloud by applying a strong negative charge.

The rotating target starts to turn rapidly, reaching top speed quickly, preparing for exposure.

The high negative charge repels electrons strongly, allowing them to stream away toward the anode (which constitutes the tube current).

The high positive charge strongly attracts electrons in the tube current.
These electrons strike the anode, resulting in the production of x-rays and heat (99% heat, 1% photons).

The line-focus principle explains the relationship between the actual focal spot and the effective focal spot:
- Actual Focal Spot: Refers to the physical size of the area on the anode target that is exposed to the electrons from the tube current, determined by filament size.
- Effective Focal Spot: The size of the focal spot as measured directly under the anode target.
- A smaller anode target size produces a smaller effective focal spot size.

The anode heel effect arises due to the angle of the target within the tube.
It causes the x-ray beam to have greater intensity on the cathode side and a lower intensity on the anode side.
The intensity difference can be as much as 45%.
Clinical implications include:
- Place thicker parts of the anatomy towards the cathode for optimal imaging.
- The effect is less noticeable with larger focal film distances (FFDs) and smaller films.

Filtration primarily aims to remove low-energy photons from the x-ray beam which could reach patients and be absorbed superficially, thus contributing to patient dose.
Effective filtration increases the average energy of the beam (beam hardening) and improves the quality, though it decreases quantity.
Low-energy photons lack the ability to penetrate body tissues.

Inherent Filtration: Permanently present in the path of the x-ray beam, includes components like the tube envelope, insulating oil, window of the tube housing, and mirror of the collimator.
Added Filtration: Filtration added to the port of the x-ray tube, primarily aluminum.
Total Filtration: The sum of added and inherent filtration.
- Current guidelines state that x-ray tubes operating above 70 kVp must have a minimum of 2.5 mm of aluminum or equivalent.
Compensating Filtration: Filtration placed between the collimator and the patient, including wedge filters, trough filters, and gonadal shielding.

Total filtration is computed as:
$Total\, Filtration = Inherent + Added$
It is crucial for ensuring the x-ray equipment meets safety and efficacy standards in clinical settings.