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 x-ray 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

1. mAs

2. kVp

3. SID

4. filtration

FACTORS AFFECTING X-RAY QUANTITY

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. Classical Scattering

2. Comptom Effect

3. Photoelectric Effect

4. Pair Production

5. 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 outer-shell 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 lowlow energies, the majority of x-ray interactions are

photoelectric, photoelectric, whereas at high high energies

Compton scattering Compton scattering predominates