Medical Equipment Technology
Department
Introduction to Biomed. Imaging Systems
Dr. Yousif Mohamed Y. Abdallah
Lecture No.5-6
1
X-Ray
Production
Objectives:
 X-ray tube interactions
 Characteristic and Bremsstrahlung X-rays
 X-ray emission spectrum
X-ray Imaging System
• PRINCIPAL PARTS
– Operating Console
– High-voltage generator
– X-ray tube
• PRIMARY FUNCTION
– The system is designed to provide a large number of
e- at cathode with high kinetic energy focused to a
small target at anode.
X-ray Tube Construction
A
B
C
F
D
E
G
Radiographic Equipment
How “X-rays” are created
 Power is sent to x-ray tube via cables
 mA (milliamperage) is sent to filament on
cathode side.
 Filament heats up – electrons are produced
 Negative charge
How “X-rays” are created
• Positive voltage (kVp) is applied to anode
• Negative electrons are attracted across the tube
to the positive anode.
• Electrons slow down and finally come to rest
• Electron beam is focused from the cathode to
the anode target by the focusing cup
 The distance between filament and the x-ray
tube target is 1 cm.
 Velocity of electron is raised from
zero............half the speed of light
E- traveling from cathode to anode
 Projectile electron interacts with the orbital
electron of the target atom.
 This interaction results in the conversion of
electron kinetic energy into thermal energy
(heat) and electromagnetic energy in the form of
infrared radiation (also heat) and x-rays.
Tube Interactions
 Heat (99%)
 x-rays (1%)
 X-rays = Characteristic
Bremsstrahlung
Heat
• Most kinetic energy of projectile e- is converted
into heat – 99%
• Projectile e- interact with the outer-shell e- of the
target atoms but do not transfer enough energy
to the outer-shell e- to ionize
• Outer shell electrons are simply raised to an
excited/ higher energy level.
Heat production
 Outer shell electrons immediately drop back to
their normal energy level with the emission of
infrared radiation.
 The constant excitation and return of outer shell
electrons are responsible for most of the heat
generation
Heat is an excitation
rather than an ionization
Heat production
 Production of heat in the anode increases
directly with increasing x-ray tube current
 Doubling the x-ray tube current doubles the heat
produced
 Increasing kVp will also increase heat
production
 Efficiency of x-ray production is independent of
the tube current
 Efficiency of x-ray production increases with
increasing kVp.
 At 60 kvp.........0.5%
 At 100 kVp.......1%
 At 20 MV..........70%
Characteristic Radiation
• Projectile electron interact with inner shell
electron
• Projectile e- with energy high enough to totally
remove an inner-shell electron of the target atom
e.g. tungsten
• Characteristic x-rays are produced when outershell e- fills an inner-shell
Only K-characteristic x-rays of tungsten
are useful for imaging
Bremsstrahlung Radiation
 Bremsstrahlung is produced by projectile einteracting with the nucleus of a target atom
Bremsstrahlung Radiation
• A projectile e- that completely avoids the orbital
e- as it passes through a target atom may come
close enough to the nucleus of the atom to come
under the influence of its electric field
• projectile e- kinetic energy to EM energy
• electrostatic force
Bremsstrahlung Radiations
 As the projectile electro passes by the nucleus, it
is slowed down and changes its course, leaving
with reduced kinetic energy in a different
direction .
 This loss of kinetic energy reappears as an xray.
Bremsstrahlung
is a German
word meaning
“slowed-down
Radiation”
X-ray energy
 Characteristic x-rays have very specific
energies. K-characteristic x-rays require a tube
potential of a least 70 kVp
 Bremsstrahlung x-rays that are produced can
have any energy level up to the set kVp value.
Brems can be produced at any projectile evalue
Discrete spectrum
 Contains only specific values
Characteristic X-ray Spectrum
 Characteristic has discrete energies based on
the e- binding energies of tungsten
 Characteristic x-ray photons can have 1 of 15
different energies and no others
Characteristic x-ray emission spectrum
Continuous Spectrum
 Contains all possible values
Bremsstrahlung X-ray Spectrum
 Brems x-rays have a range of energies and form
a continuous emission spectrum
Factors Affecting
the x-ray emission spectrum
•
•
•
•
•
Tube current,
Tube voltage,
Added filtration,
Target material,
Voltage waveform
• The general shape of an emission spectrum is
always the same, but the position along the
energy axis can change
Quality
 The farther to the right the higher the effective
energy or quality
Quantity
 The more values in the curve, the higher the xray intensity or quantity
mAs
 A change in mA results in the amplitude change
of the x-ray emission spectrum at all energies
 The shape of the curve will remain the same
mA increase from 200 to 400
kVp
 A change in voltage peak affects both the
amplitude and the position of the x-ray emission
spectrum
Filtration
• Adding filtration is called hardening the x-ray
beam because of the increase in average
energy
• Filtration more effectively absorb low-energy xrays than high energy x-rays
• Characteristic spectrum is not affected & the
maximum energy of x-ray emission is not
affected
Filtration
 Adding filtration to the useful beam reduces the
x-ray beam intensity while increasing the
average energy (higher quality)
 Lowering the amplitude and shifting to the right
What A does this graph indicate?
Target Material
 The atomic number of the target affects both the
quantity and quality of x-rays
 Increasing the target atomic number increases
the efficiency of x-ray production and the energy
of characteristic and bremsstrhlung x-rays
Target material
Voltage Waveform
 5 voltage waveforms: half-wave rectification, fullwave rectification, 3-phase/6-pulse, 3-phase/12pulse, and high-frequency.
 Maintaining high voltage potential
Voltage generators
Factors affecting X-Ray beam quality and quantity
An increase in
Results in
Current(mAs)
An increase in quantity; no change in quality
Voltage (kVp)
An increase in quantity and quality
Added filtration
A decease in quantity and an increase in
quality
Target atomic number(Z) An increase in quantity and quality
Voltage ripple
A decrease in quantity and quality
kVp
KINETIC ENERGY OF ELECTRONS
CATHODE --------
MADE OF TUNGSTEN + 1%-3% THORIUM
TUNGSTEN
Z # 74
MELTING POINT- 3,410 DEG. CELSIUS
THORIUM
Z # 90
THERMIONIC EMISSION
CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS
ANODE +++++
TUNGSTEN
TARGET
TUNGTEN AS TARGET
HIGH Z# - 74------EFFICIENCY OF X-RAY PRODUCTION
HIGH MELTING POINT –3,410 ° C– TARGET HEATED TO 2,000 ° C
X-RAY PRODUCTION
 BREMSSTRAHLUNG RADIATION
 CHARACTERISTIC RADIATION
BREMSSTRAHLUNG RADIATION
 If an incoming free electron gets close to the nucleus of a
target atom, the strong electric field of the nucleus will
attract the electron, thus changing direction and speed of the
electron.
 The Electron looses energy which will be emitted as an Xray photon. The energy of this photon will depend on the
degree of interaction between nucleus and electron, i.e. the
passing distance.
 Several subsequent interactions between one and the same
electron and different nuclei are possible. X-rays originating
from this process are called bremsstrahlung.
 Bemsstrahlung is a German word directly describing the
process: "Strahlung" means "radiation", and "Bremse"
means "brake
Bremsstrahlung Radiation
BREMS
DIFFERENT DEGREES OF DECCELERATION
X-RAYS
HEAT
BREMS RADIATION IS:
POLYENERGETIC !
90% OF X-RAYS ARE PRODUCED THROUGH BREMS
INTERACTIONS WHEN 80-100 KVP APPLIED
BREMS EMISSIONCONTINUOUS
Characteristic X-rays
 The high energy electron can also cause an
electron close to the nucleus in a metal atom to be
knocked out from its place.
 This vacancy is filled by an electron further out from
the nucleus.
 The well defined difference in binding energy,
characteristic of the material, is emitted as a
monoenergetic photon.
 When detected this X-ray photon gives rise to a
characteristic X-ray line in the energy spectrum.
Characteristic Radiation
KE OF PROJECTILE ELECTRON > BINDING ENERGYORBITAL ELECTRON
CHARACTERISTIC CASCADE
TUNGSTEN-74
BINDING ENERGIES
OF DIFFERENT SHELL ELECTRONS
K-70 KEV
L-12 KEV
M-2.8 KEV
CHARACTERISTIC X-RAYS
L
M
K
70-12 = 58 keV
K
70-3 = 67 keV
L
12-3 = 9 keV
M
TRANSFER OF ELECTRONS BETWEEN
OUTER SHELLS RESULTS IN:
HEAT PRODUCTION !
EACH CHARACTERISTIC RADIATION
( ex. K TO L TRANSFER) IS:
MONOENERGETIC !
THERE ARE MANY
CHARACTERISTIC RADIATION
PRODUCED IN ONE ATOM
THEREFORE CHARACTERISTIC
RADIATION
IS ALSO POLYENERGETIC !
CHARACTERISTIC EMISSIONLESS POLYENERGETIC!
X-RAYS PRODUCTION AN INEFFICIENT PROCESS
80 KVP
99.4% HEAT 0.6% X-RAYS
% EFFICIENCY OF X-RAY PRODUCTION
% EFFICIENCY= K * Z * kVp
K = 1 * 10-4
X-RAY EMISSION
 ISOTROPICAL- IN EVERY DIRECTION
USEFUL RADIATION – PROJECTED
TOWARD THE PATIENT
LEAKAGE RADIATION
HOUSING
OFF-FOCUS RADIATION
USEFUL
OFF-FOCUS
73