Chapter 6: X-ray Production, Tubes, and Generators PDF

# Chapter 6: X-ray Production, Tubes, and Generators X-rays are produced when highly energetic electrons interact with matter, converting some or all of their kinetic energy into electromagnetic radiation. The x-ray tube insert contains: - an electron source - a vacuum environment - a target ele...

# Chapter 6: X-ray Production, Tubes, and Generators X-rays are produced when highly energetic electrons interact with matter, converting some or all of their kinetic energy into electromagnetic radiation. The x-ray tube insert contains: - an electron source - a vacuum environment - a target electrode An external power source provides high voltage to accelerate the electrons. The x-ray tube insert is mounted within a tube housing which includes: - a metal enclosure - protective radiation shielding - x-ray beam filters - collimators The x-ray generator supplies the tube potential to accelerate the electrons. This is done by: - a filament circuit to control tube current - an exposure timer These components work together to produce a beam of x-ray photons with controlled: - fluence (number of incident photons per unit area) - energy fluence (energy weighted number of photons per unit area) - collimated trajectory ## 6.1 Production of X-rays ### 6.1.1 Bremsstrahlung Spectrum X-rays are produced from the conversion of kinetic energy of electrons into electromagnetic radiation when they are decelerated by interaction within a target material. - An electrical potential difference of 20,000 to 150,000 V (20 to 150 kV) is applied between the electrodes. - The negative pole is applied to the **cathode**, which is the source of electrons. - The positive pole is applied to the **anode**, which is the target of electrons. Electrons are emitted by the cathode and accelerated by the tube potential through the vacuum to strike the anode. **1 eV** is the energy obtained by an electron after it is accelerated across a potential difference of 1 V. $1 \text{ eV} = 1.603 \times 10^{-19} \text{ joule (J)}$ An applied x-ray tube potential of 50 kV accelerates electrons to a kinetic energy of 50 keV. On impact with the target, the kinetic energy of the electrons is converted to other forms of energy. - Most interactions are collisional with other electrons and produce nothing but heat. - Electrons that reach the proximity of an atomic nucleus are decelerated by the positive charge of the nucleus. - The magnitude of energy lost by an electron is determined by the closest distance between the incident electron and the nucleus. The closer the interaction distance, the higher the x-ray energy. ### 6.1.2 Characteristic X-rays Electrons in an atom are distributed in orbital "shells" with specific electron binding energies. - The innermost shell, the **K shell**, has the highest electron binding energy. - The L, M, and N shells have progressively less binding energy. When the kinetic energy of an incident electron exceeds an electron's shell binding energy, an electron can be ejected from its shell, creating a vacancy. An outer shell electron with less binding energy immediately transitions to fill the vacancy, and a characteristic x-ray is emitted with an energy equal to the difference in the electron binding energies of the two shells. Characteristic x-rays are designated by the shell in which the electron vacancy is filled and a subscript of a or ẞ indicates the electron transition. - **Ka:** transition is from an adjacent shell. - **Kẞ:** transition is from a non-adjacent shell. ## 6.2 X-ray Tubes The x-ray tube provides an environment to produce bremsstrahlung and characteristic x-rays. It contains: - a cathode - an anode - a rotor/stator - a glass or metal envelope - tube ports - cable sockets - tube housing The x-ray generator controls the x-ray tube by altering the voltage, current, and exposure time. ### 6.2.1 Cathode The cathode contains: - an electron emitter - a focusing cup The emitter is usually a tungsten wire tightly coiled in a filament configuration (often called the filament), electrically connected to the filament circuit in the x-ray generator. Most x-ray tubes for diagnostic imaging have two filaments of different lengths, each positioned in a slot machined into the focusing cup. ### 6.2.2 Anode The anode is a metal target electrode that is maintained at a positive potential difference relative to the cathode. - Electrons are accelerated toward the anode and deposit most of their energy as heat. - Only a small fraction is emitted as x-rays. - The rate of x-ray production is limited by the amount of heat the anode can take. - The focal spot is the area impacted by the electrons, also limiting power density. Tungsten is the most widely used anode because of its high melting point (3,000°C) and high atomic number (Z = 74). ### 6.2.3 Anode Configurations: Stationary and Rotating A simple x-ray tube design has a stationary anode, consisting of a tungsten insert embedded in a copper block. The copper: - mechanically supports the insert - efficiently conducts heat from the tungsten target A rotating anode allows higher x-ray output by spreading the heat over a larger area as the anode surface rotates relative to the electron beam. The rotating anode is designed as a beveled disk mounted on a rotor assembly supported by bearings in the x-ray tube insert. For radiography and fluoroscopy applications, the bulk of the material is molybdenum, with a tungsten target blended with 3% to 10% of rhenium to enhance ductility of ~0.5 mm thickness sintered onto the focal track area. ## 6.2.4 Anode Angle, Field Coverage, and Focal Spot Size The actual focal spot size is the physical area on the anode that is struck by electrons and is primarily determined by the length of the cathode filament and the width of the focusing cup slot. The projected length of the focal spot area at the central ray in the x-ray field is smaller because of geometric foreshortening of the projected distribution from the anode surface. The effective focal spot is smaller, due to the anode angle, and larger towards the cathode side of the field. The width of the focal spot does not change appreciably with position in the image plane. ## 6.2.5 Heel Effect The heel effect is a reduction in the x-ray beam fluence on the anode side of the x-ray field. - X-rays directed toward the anode side have longer path lengths through the anode material than x-rays directed toward the cathode side. - This reduces x-ray fluence. - It is more prominent with a shorter source-to-image distance (SID). - Beam fluence is greater on the cathode side of the field. The orientation of the x-ray tube cathode over thicker parts of the patient can result in a better balance of transmitted x-ray photons through the patient and onto the image receptor. ## 6.2.6 Off-Focal Radiation Off-focal radiation results from electrons that elastically rebound from the anode, and accelerate back to the anode, outside of the focal spot. This produces low x-ray fluence over the entire anode surface with a fraction transmitted through the tube port. Off-focal radiation: - increases quantum noise - reduces contrast in the image - adds to patient dose Reducing off-focal radiation is achieved in x-ray tubes designed with a grounded anode, as electrons are just as likely to be attracted to other structures at ground potential. ## 6.2.7 X-ray Tube Insert The x-ray tube insert contains the cathode, anode, rotor assembly, and support structures sealed in a glass or metal enclosure under a high vacuum. - The high vacuum prevents electrons from colliding with gas molecules and is necessary for electron beam devices. - Molecules can outgas from tube structures and degrade the vacuum. A "getter" circuit is used to trap these molecules. - X-rays traverse the tube port, made of the same material as the tube enclosure. ## 6.2.8 X-ray Tube Housing The x-ray tube housing: - mechanically supports - electrically and thermally insulates - protects the x-ray tube insert from the environment Special oil in the space between the x-ray tube insert and tube housing provides heat conduction and electrical insulation to assist in cooling and protection. Most radiographic x-ray tubes have expansion bellows that accommodate oil expansion as it heats. ## 6.2.9 X-ray Tube Filtration Added filtration refers to sheets of metal intentionally placed in the beam typically at the tube port within the collimator assembly. - These filters selectively attenuate the low-energy x-rays in the spectrum that have little likelihood of transmission through the patient. - Radiation dose to the patient can be substantially reduced by eliminating low energy x-rays that do not contribute to the image formation process. ## 6.2.10 Collimators Collimators (x-ray beam apertures) are used to adjust the size and shape of the x-ray field that emerges from the tube port. - The collimator assembly typically is attached to the tube housing with a swivel joint. - Two pairs of adjustable parallel-opposed lead shutters define a rectangular x-ray field. - A light bulb can be activated to create a light field (reflected by an internal mirror) to keep the bulb out of the x-ray field, which defines the x-ray field. ## 6.2.11 X-ray Tube Designs Imaging systems have become specialized with requirements for x-ray sources. - These include CT, interventional cardiac and vascular x-ray, single exposure and fluoroscopic systems, mammography, and mobile x-ray sources. - Characteristics of operation determine the design requirements and power output of x-ray tubes. - The energy delivered is the product of tube voltage, tube current, and exposure time for the integration period in kilowatt-seconds (kWs). ## 6.2.12 Recommendations to Maximize X-ray Tube Life X-ray tubes eventually must be replaced. To maximize life, follow these recommendations: 1. Minimize filament boost "prep" time. 2. Use lower tube current with longer exposure times. 3. Avoid extended or repeated operation of the x-ray tube with high technique factors. 4. Use the manufacturer's recommended warm-up procedure. 5. Limit rotor start-and-stop operations. ## 6.3 X-ray Generators The principal function of the x-ray generator is to control tube current, high voltage, and exposure time to an x-ray tube. - The electrical transformer converts voltage through a process called electromagnetic induction. - *Electromagnetic induction* occurs when a changing magnetic field induces an electrical potential difference (voltage) in a nearby conductor, or similarly when a conductor is moving through a stationary magnetic field. ## 6.3.2 Transformers Transformers use the principle of electromagnetic induction to change the voltage of an electrical power source. They contain: - two distinct, electrically isolated wire coils wrapped about a common iron core. - An oscillating magnetic field is produced in the *primary winding*, where each turn of the wire adds to the magnetic field amplitude that uniformly permeates the iron core. The fluctuating magnetic field then induces a changing voltage on the *secondary winding*. The voltage induced on the secondary winding is proportional to the input voltage and the ratio of the number of turns on the primary winding to the number of turns on the secondary. ## 6.3.3 X-ray Generator Modules Modular components of the x-ray generator include: - the high-voltage power circuit - the stator circuit - the filament circuit - the focal spot selector - automatic exposure control (AEC) circuit. Generators have circuitry and microprocessors that monitor the selection of potentially damaging overload conditions to protect the x-ray tube. ## 6.3.4 Operator Console The parameters that are selected in the operation of a radiographic system include: - the tube voltage (kV) - the tube current (mA) - the exposure time (s) - the product of mA and time (mAs) - fixed technique versus AEC mode - the AEC sensor location to be used - the focal spot size ## 6.3.5 High-Frequency X-ray Generator Several x-ray generator circuit designs are in use, including single-phase, three-phase, constant potential, and high-frequency inverter generators. The high-frequency generator is now the contemporary choice for diagnostic x-ray systems. A high-frequency alternating waveform (up to 40,000 Hz) is generated and used for efficient conversion of low to high voltage by a step-up transformer. Subsequent rectification and voltage smoothing produce a nearly constant voltage between the cathode and anode. ## 6.3.6 Voltage Ripple Ideally, the voltage applied to an x-ray tube would be constant. - For older single-phase generators, there was a substantial difference in the average kV and the peak kV (kVp). - For high-frequency generators, which are now more prevalent, there is little difference between the average and peak kilovoltage. ## 6.3.7 Timers Digital timers have largely replaced electronic timers based on resistor-capacitor circuits and charge-discharge timers in older systems. Digital timer circuits: - have extremely high reproducibility - microsecond accuracy ## 6.3.8 Switches The high-frequency inverter generator typically uses electronic switching on the primary side of the high-voltage transformer to initiate and terminate the exposure. ## 6.3.9 Automatic Exposure Control The AEC system is used far more often than manual exposure time settings in radiography. - It measures the actual amount of radiation incident on the image receptor during the acquisition and terminates x-ray production when the optimal radiation levels are obtained. - It compensates for patient thickness and other variations in attenuation to produce more consistent exposures than manual techniques. The AEC system consists of: - one or more radiation detectors - an amplifier - digital signal-to-noise (SNR) selector - an integrator circuit - a comparator circuit - a termination switch - a backup timer safety shutoff switch. ## 6.4 Power Ratings, Anode Loading, and Cooling The power rating of an x-ray tube focal spot or x-ray generator is the maximal power that an x-ray tube focal spot can accept, or the generator can deliver under certain conditions. - The power, in **kW**, is calculated for a 0.1 s exposure time using the equation: $Power (kW) = 100 \text{ kV} \times I(A)$ ## 6.4.2 Historical Units: The Heat Unit and the Joule The heat unit (HU) is a traditional unit that provides a simple way of expressing x-ray tube anode energy deposition and dissipation for a single-phase generator. - The number of HU can be calculated from the parameters defining the radiographic technique: $Energy (HU) = a \times peak \text{ voltage (kVp)} \times tube \text{ current (mA)} \times exposure \text{ time (s)}$ ## 6.5 Factors Affecting X-ray Emission The output of an x-ray tube is often described by the terms quality, quantity, and exposure. - **Quality:** describes the penetrability of an x-ray beam. - **Quantity:** refers to the number of x-ray photons comprising the beam. - **Exposure:** is proportional to the energy fluence of the x-ray beam spectrum. ## 6.5.1 Anode Target Material The elemental composition of the target affects the efficiency of bremsstrahlung radiation production. - Incident electrons are more likely to have radiative interactions in higher-Z materials. - The energies of characteristic x-rays produced in the target depend on the elemental composition; this affects the quantity of bremsstrahlung photons and the quality of the characteristic radiation. ## 6.5.2 Tube Voltage (kV) The applied potential difference across the cathode and anode determines the peak energy in the bremsstrahlung spectrum, affecting the quality of the output spectrum. - The efficiency of x-ray production is related to tube voltage, approximately proportional to the square of the tube potential. - Increase in kV increases the efficiency of x-ray production and the quantity and quality of the x-ray beam. ## 6.5.3 Tube Current (mA) The number of x-ray photons produced is proportional to the number of electrons flowing from the cathode to the anode per unit time. - Exposure rate is proportional to the tube current. - X-ray quantity is directly proportional to the product of tube current and exposure time (mAs). ## 6.5.4 Beam Filtration Added metal filters modify both the quantity and quality of the x-ray beam by preferentially removing the low-energy photons in the spectrum due to their higher attenuation. - This results in a lower quantity of x-rays with a relatively greater number of higher energy photons. - Increasing beam filtration increases beam quality. ## 6.6 Summary In summary, x-rays are the basic radiologic tool for most medical diagnostic imaging procedures. Knowledge of x-ray production, x-ray generators, and x-ray beam control is important for further understanding of the image formation process and the need to obtain the highest image quality at the lowest possible radiation dose.