Discuss the necessary characteristics of filament metals & construction.
Describe the control of thermionic emission from the filament.
Explain the function & design of the focusing cup.
Draw a complete rotating anode assembly.
Discuss the characteristics of anode targets.
Discuss the materials that make up each component of the x-ray tube and why those materials are significant.
X-Ray Production Requirements
Requirements for x-ray production include:
Source of Electrons: Necessary to initiate x-ray production.
Appropriate Target Material: Material must effectively convert the kinetic energy into x-ray photons.
High Voltage: Required to accelerate electrons from cathode to anode.
Vacuum: Important to prevent interference from gas or air molecules, enhancing efficiency.
The X-ray Tube
Parts of the Tube:
Glass envelope
Protective housing
Filament
Dual focused
Focusing cup
Anode
Target
Target angle
Stator
Rotor
Ball bearings
Structure of the Tube
Cathode: The negative charge component of the tube that generates electrons.
Anode: Positively charged component serving several functions.
Protective Housing: Ensures safety and functionality, enclosing all components.
Glass Envelope:
Made from Pyrex glass to endure high temperatures.
Contains high vacuum, preventing electron collision with gas molecules.
A window is present for the emission of x-rays, reducing absorption/scatter.
The Vacuum
The glass envelope's primary role is to maintain the vacuum between the cathode and anode:
Vacuum Maintenance: Involves the complete removal of air and sealing the tube.
Efficiency Increase: Without air, electrons can flow unimpeded from cathode to anode, enhancing operation efficiency.
The Cathode
Definition: The cathode is the negatively charged part of the x-ray tube.
Functions:
Produces thermionic emission (producing a cloud of electrons).
Conducts high voltage across the cathode-anode gap.
Focuses the electron stream to optimize x-ray production.
Cathode Assembly Components:
Filament
Focusing cup
Filament
Function: Provides resistance to the flow of electrons, causing sufficient heat for thermionic emission.
Specifications:
Dual-Focus: Comprised of 2 filaments.
Material: Thin thoriated tungsten wire, measuring 0.1-0.2 mm thick, coiled to 1-2 mm wide, and 7-15 mm long.
Operation: High current is applied during exposure, forming a thermionic cloud based on selected milliampere (mA) value.
Note: Prolonged rotor operation can reduce tube life, and the exposure button should be pressed in one smooth motion.
Dual-Focus (2 Filament)
Functionality: Generates a thermionic cloud of electrons that are directed towards the anode target.
Thermionic Emission Process: Electrons leave the filament surface, forming a cloud; during exposure, the cloud moves towards the anode to create x-ray photons.
Inefficiency: A small percentage of electrons are vaporized from the filament, contributing to vacuum degradation (gassy tube).
Focusing Cup
Structure: A nickel-made depression in the cathode housing the filament.
Purpose: Narrows the thermionic cloud as it travels to the anode.
Since electrons repel each other due to their negative charge, the focusing cup applies a low negative potential to gather them into a concentrated stream.
Space Charge Effect
Result of filament operation, where electron buildup causes emission opposition.
Limitations: Restricts x-ray tube operation to a maximum range of mA from 1,000 to 1,200.
Occurrence: Activated by engaging the rotor (boost mode).
The Anode
Definition: The positively charged component of the tube with three main functions:
Serves as a target for high-voltage electrons, producing x-ray photons.
Conducts high voltage back into x-ray generator circuitry.
Acts as the primary thermal conductor to dissipate heat.
Anode Assembly Components:
Anode target
Stator
Rotor
Anode (Target)
Types:
Stationary Anodes: Constructed from rhenium-alloyed tungsten.
Rotating Anodes:
Introduced in 1936, allowing for a larger target area.
Enhances efficiency, increasing target area up to 300 times that of stationary anodes.
Operates with high-speed bombardments of only 7-50 microseconds.
Faster rotation improves thermal energy dissipation.
Diameter: Rotating anode disks typically measure between 5-13 cm.
Material: Composed of rhenium-alloyed tungsten, backed by layers of molybdenum and graphite for heat management.
Tungsten
Ideal Choice for X-ray Production:
High atomic number (74), enhancing production of diagnostic-range photons.
Melting Point: Capable of withstanding temperatures between 1,000 - 2,000 degrees Celsius.
Excellent heat-conducting ability, important for managing thermal loads.
Rhenium provides elasticity to accommodate rapid thermal expansion in the focal track.
Molybdenum backing helps with effective heat dissipation.
Warm-Up Procedures: Required for cold anodes, involving average kVp and mA settings followed by specified exposure durations.
The Target Area
Definition: The designated section of the anode that the high-voltage electron stream impacts. This includes:
Target
Focus
Focal Point
Focal Spot
Focal Track: Circular path impacted by the electron beam, where x-ray photons are generated.
Measurement: Point source of x-ray photons from which all distances to the object and image receptor are measured, identified as Point 0.
A tape measure is attached to the collimator, beginning at 12 cm from the target.
Line-Focus Principle
Definitions:
Actual Focal Spot: The physical area of the focal track that is hit.
Effective Focal Spot: The area projected out of the tube towards the radiographed object.
Purpose: The line-focus principle enhances detail resolution while maximizing the area of the actual focal spot for better thermal conductivity.
The effective focal-spot size is determined by the actual focal-spot size, which is influenced by filament length and target angle.
Larger actual focal spots yield larger effective focal spots.
Target Angle Effects:
When the angle is less than 45-degrees, the effective focal spot remains smaller than the actual focal spot.
Common Target Angle: 12 degrees is typical for diagnostic radiography; angles range from 7 to 17 degrees.
Implication: Smaller target angles create smaller focal spots but may limit the primary beam size at short source-to-image distances (SID) – a 12-degree angle is necessary for 14 x 17 field-of-view at a 40” SID.
Focal Spot Size
Diagnostic X-ray Tube FSS Range: Between 0.1 mm to 3 mm in effective focal spot size.
Smaller Focal Spot Sizes: Allow for finer detail, ideal for lower mA stations (0.5 – 0.6 mA).
Larger Focal Spot Sizes: Suitable for heavy tube loads, optimal for higher mA stations (1 – 1.5 mA).
Anode Heel Effect
Definition: An inherent phenomenon of the x-ray tube related to the angled anode, resulting in varying radiation intensity.
More radiation is emitted on the cathode side, with photons directed towards the anode being absorbed by the target material.
Larger objects should be placed under the cathode side for effective imaging (referred to as the FAT CAT principle).
Quantitative Effects:
Approximately 20% more photons are present at the cathode end.
Approximately 25% fewer photons are present at the anode end.
Visible exposure differences can arise in x-ray exams due to receptor size and short SIDs, with the head of the table denoting the anode end.
Stator
Definition: The induction motor component that drives the anode.
Composed of electromagnets that function to rotate the anode.
Notably, the stator is the only aspect of the cathode assembly positioned outside of the tube envelope.
Rotor
Functionality:
The rotor is activated pre-exposure to initialize the anode rotation, involving direct current to the stator for rotation initiation.
Constructed from a hollow copper cylinder affixed to the anode disk through a molybdenum shaft.
The stator generates an electromagnetic field that propels the rotor at 3,200 – 3,600 revolutions per minute (rpm), with high-speed models achieving 10,000 – 12,000 rpm which significantly increases heat dissipation.
Inside the rotor, silver-plated steel ball bearings are utilized to aid the rotor's rotation.
Protective Housing
Material Composition: Metal encasing all tube components.
Functions:
Prevents electrical shock.
Facilitates tube cooling through thermal conduction.
Limits leakage radiation to below 1 mGy/hr (100 mR/hr).
Tube Housing - Control of Leakage Radiation & Scatter
Photon Emission: X-ray photons produced at the anode are emitted isotropically.
The primary beam consists solely of photons emitted through the window.
Remaining photons, which are extraneous, are absorbed by the metal housing to prevent leakage radiation.
The housing is made from cast steel lined with lead at the cathode end to capture unwanted photons.
All photons escaping the housing, except through the window, constitute leakage radiation, which must not exceed 100 mR/hr or 1 mGy/hr at a distance of 1 meter.
Additionally, housing provides cushioning against rough handling.
Off-Focus (Extrafocal) Radiation
Definition: Photons produced outside the designated focal spot.
Occurs when high-voltage electrons during strikes at the focal spot result in scattered electrons or photons.
Such radiation is of lower energy compared to the primary beam and can account for 25-30% of the overall primary beam.
Ghosting: Refers to unwanted diagnostic imaging showing anatomy due to off-focus radiation.