Radiology Physics X-ray Production

RADIOLOGY PHYSICS X-RAY PRODUCTION

Reported by MARIA THERESA M. NAVARRO, M.D.

Part I

X-RAY MACHINE • X-ray tube • Operating console • High-voltage generator

Operating Console • Apparatus that allows the radiographer to control the x-ray tube current and voltage so that the useful beam is of proper quantity and quality

High Voltage Generator • Responsible for converting the low voltage from the electric power company into a kilovoltage of the proper waveform

X-RAY TUBE • 1. SUPPORT STRUCTURE • 2. PROTECTIVE HOUSING • 3. GLASS OR METAL ENVELOPE

SUPPORT STRUCTURE a. Ceiling support - allows the greatest case of movement and range of position b. Floor-to-ceiling support c. Floor-mount system d. Fluoroscopy tube e. C-arm – mobile fluoroscopy unit

PROTECTIVE HOUSING a. Reduces leakage radiation to 100mR per hour at 1 meter b. Provides mechanical support protecting the tube from damage c. To conduct heat away from the x-ray target

GLASS OR METAL ENVELOPE • Surrounds the cathode (-) and anode (+), which are the electrodes at the ends of the vacuum tube (Coolidge tube). • made of Pyrex glass to withstand the tremendous heat generated. • maintains a vacuum or empty space

CATHODE • negative side of the x-ray tube • TWO PRIMARY PARTS – a. Filament – b. Focusing cup

CATHODE • A. FILAMENT • THERMIONIC EMISSION At 4 amperes and above, the outer-shell electrons are literally boiled off and ejected from the filament

CATHODE • B. FOCUSING CUP -where the filament is embedded - negatively charged

FILAMENT CURRENT • SPACE CHARGE • SPACE CHARGE EFFECT

ANODE • positive side of the x-ray tube • Function: – electrical conductor – mechanical support – thermal conductor

• Two Types: – Stationary anode – Rotating anode

TARGET • •

area of the anode struck by the electrons from the cathode TUNGSTEN a. High atomic number (74) – high efficiency in x-ray production and in high energy x-rays b. Thermal conductivity – nearly equal to that of copper ; efficient for dissipating heat c. High melting point – 3400 C ; can stand up under high tube current without pitting or bubbling.

Part II

Two Types of X-rays • CHARACTERIC X-RAYS - produced by transitions of orbital electrons from outer to inner shells • BREMSSTRAHLUNG X-RAYS - braking of projectile electrons by the nucleus - most x-rays in the diagnostic range

1. mA - a change in mA results in a directly proportional change in the amplitude of the xray emission spectrum at all energies. 2. KVP - when kVP is increased, the relative distribution of emitted x-rays shifts to the right to higher energies

3. Added Filtration - increase in the effective energy of the x-ray beam (higher quality) with an accompanying reduction in x-ray quantity 4. Target Material - as the atomic number of the target material increases, the efficiency of the Bremsstrahlung radiation increases and the high energy x-rays increase in number more than the low-energy x-rays

Part III

X-RAY QUANTITY • Number of x-rays in the useful beam, which is the beam forming the radiographic image • INTENSITY of radiation, measured in mR.

X-RAY QUANTITY FACTORS AFFECTING X-RAY QUANTITY

1. 2. 3. 4.

mAs kVp SID filtration

X-RAY QUALITY • PENETRABILITY or PENETRATING POWER of an x-ray beam

X-RAY QUALITY •

FACTORS AFFECTING X-RAY QUALITY

1. Kilovoltage 2. Filtration

Types of Filtration 1. Inherent Filtration - built into the glass or metal envelope 2. Added Filtration - in the form of aluminum sheets 3. Compensating Filters - provided variation in beam quality, depending on thickness

Part IV

Types of X-ray Interaction with Matter 1. 2. 3. 4. 5.

Classical Scattering Comptom Effect Photoelectric Effect Pair Production Photodisintegration

CLASSICAL SCATTERING • COHERENT or THOMPSON SCATTERING • interaction between low energy x-rays and atoms • x-ray loses NO energy but changes direction slightly

COMPTON EFFECT •

in moderate energy x-rays and outershell electrons • source of most of the OCCUPATIONAL radiation • ionization of the target atom change in the photon direction -> reduction in photon energy

PHOTOELECTRIC EFFECT • photon absorption interaction causes electron removal from the atom (photoelectron) • Characteristic x-rays are produced • also a secondary radiation and behave as scatter radiation

PAIR PRODUCTION • Occurs with x-rays that have energies greater than 1.02 meV. • The photon interacts with nuclear force field, and 2 electrons that have opposite electrostatic charges are created.

PHOTODISINTEGRATION • interaction between high energy photons and nucleus. • photon is absorbed by the nucleus ->nuclear fragment is emitted.

•At low energies, the majority of x-ray interactions are photoelectric, whereas at high energies Compton scattering predominates