X-ray Production (Topic 6)

X-ray Production

Wilhelm Conrad Roentgen

Discoverer of X-rays

X-ray Discovery Date

November 8, 1895

X-ray Discovery Location

Wurzburg University in Germany

Anna Bertha Ludwig

First radiograph (hand radiograph)

Substance that Glowed during Discovery

Glowing Barium platinocyanide

First clinical application of X-rays

February 1896, Darmouth College

X-ray tube window

Is the area of the glass or metal enclosure, approximately 5 cm2, that is thin and through which the useful beam of x-rays is emitted

X-ray tube is made up of?

Pyrex glass to withstand heat

X-ray tube life span

atleast 10 years

X-ray tube average cost

P600k-P800k

Regulates the devices for x-ray production

Center for Device Regulation, Radiation Health and Research

Useful beam

X-rays emitted through the window

Leakage radiation

X-rays that escaped through the tube housing

Leakage radiation maximum limit

Should not exceed 100 mR/hr at 1 m when operated at maximum condition

Protective housing oil function

Serves as both an insulator against electric shock and thermal cushion to dissipate heat

Cathode

The negative side of the x-ray tube and has two primary parts: a filament and a focusing cup

Filament size

Usually approximately 2 mm in diameter and 1 or 2 cm long

Filament types

small and large filament

Filaments are usually made of

thoriated tungsten

Tungsten melting point

3410 oC

Tungsten characteristic

Does not vaporize easily

Addition of thorium to tungsten

Increases the efficiency of thermionic emission (1% to 2% thorium is added)

Thermionic emission

When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are “boiled off” and ejected from the filament

Focusing Cup

Focuses the electrons towards the target

Space charge

A concept in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges

Anode

The positive side of the x-ray tube; it conducts electricity and radiates heat and contains the target

Functions of anode assembly

Electrical conductor, Mechanical support, Thermal dissipator

Target

Area of the anode struck by the electrons from the cathode

Focal Spot

Is the actual x-ray source

Small focal spot use

Used when better spatial resolution is required

Small focal spot range

Ranges from 0.1 to 1 mm

Large focal spot use

Used when large body parts are imaged and when other techniques that produce high are required

Types of Anode

Stationary anode, Rotating anode

Stationary anode use

Are used in dental x-ray imaging systems, some portable imaging systems, and other-purpose units in which high tube current and power are not required

Rotating anode characteristic

Capable of producing high intensity x-ray beam

Rotating anode heat capacity improvement

Can be further improved by increasing the speed of anode rotation

Rotating anode speeds

Rotates 3400 rpm and 10,000 rpm

Stationary anode target description

Consists of tungsten alloy embedded in a copper anode (About 4 mm2)

Rotating anode target description

The entire rotating disc is the target (About 3159 mm2)

Electromagnetic Induction Motor

The rotating anode is powered by an electromagnetic induction motor

Induction Motor parts

The stator and the rotor

Reasons Tungsten is material of choice for the target for general radiography

Atomic number, Thermal conductivity, High melting point

Mammographic X-ray Tubes targets

Have molybdenum or rhodium targets principally because of their low atomic number and low K characteristics x-ray energy

Tungsten alloying

Alloying tungsten (usually with rhenium) gives it added mechanical strength to withstand the stresses of high speed rotation

Molybdenum and graphite property for rotation

Have lower mass density than tungsten, thus allowing the target easier to rotate

Molybdenum (Mo) Atomic Number

42

Molybdenum (Mo) K-shell electron binding energy

19 keV

Rhodium (Rh) Atomic number

45

Rhodium (Rh) K-shell electron binding energy

23 keV

Line-Focus Principle

Results in an effective focal spot size much less than the actual focal spot size

Target angles in diagnostic x-ray tubes

Vary from approximately 5 to 20 degrees

Line-Focus Principle (other name)

“GOETZE PRINCIPLE”

Line-Focus Principle result

Allows high anode heating with small effective focal spots

Relationship between target angle and focal spot size

As the target decreases, so does the focal spot size

Heel Effect

Radiation intensity is greater on the cathode side of the x-ray field than that on the anode side

Heel Effect relationship to anode angle

The smaller the anode angle, the larger is the heel effect

Off Focus Radiation

Electrons bounce off the focal spot and then land on other areas of the target, causing x-rays to be produced from outside of the focal spot

Kinetic Energy

The energy of motion

KE formula

½ mv2

KE in the formula

Kinetic energy in joules

m in the formula

mass in kilograms

v in the formula

velocity in meters per second

Projectile Electrons

Electrons travelling from cathode to anode

Projectile electron interaction

Interacts with orbital electron of the target atom, resulting in the conversion of electron kinetic energy into thermal energy (heat) and electromagnetic energy in the form of infrared radiation (also heat) and x-ray

Kinetic energy converted to heat

Approximately 99% of kinetic energy of projectile electrons is converted to heat

Kinetic energy converted to x-ray

Only approximately 1% of projectile electrons is converted to x-ray

Efficiency of x-ray production and tube current

Independent of the tube current

X-ray conversion efficiency at 60 kVp

0.5% conversion

X-ray conversion efficiency at 100 kVp

1% conversion

X-ray conversion efficiency at 20 MV

70% conversion

Efficiency of x-ray production and kVp

Increases with increasing kVp

Characteristic Radiation

Emitted when an outer-shell electron fills an inner-shell void

Characteristic Radiation property

Characteristic of the target material

Characteristic Radiation percentage at 100 kVp

Approximately 15% of the x-ray beam is characteristic

Bremsstrahlung Radiation (production)

Produced when a projectile electron is slowed by the electric field of a target atom nucleus

Bremsstrahlung Radiation (nickname)

“Slowed down radiation”

Bremsstrahlung

German word

“bremsen” translation

to brake

“Strahlung” translation

radiation

Bremsstrahlung Radiation (literal translation)

“braking radiation” or “deceleration radiation