Radiation physics-by <MUHAMMED ASLAM BS It is easy to Medical imaging technology students

1. MUHAMMEDASLAM BS
BSC MIT

2. “An X ray generator is the device
that supplies electric power to the X
ray tube”
An x ray generator begins with a
source of electric energy

3.  Xray tube requires electrical energy
to:
i. Boil electrons from the filament
ii. Accelerate these electrons from
cathode to anode

4. 1. main breaker - this is where the alternating current comes from to power the circuit.
2. exposure switch - when you push the button to start an exposure this switch closes to start the exposure.
3. autotransformer - this is where you adjust the kVp for the exposure.
4. timer circuit - this part of the circuit stops the exposure.
5. high-voltage step-up transformer - this transformer bumps the voltage up so that the x-ray tube has very high
voltage to make the electrons have enough energy to form x-rays.
6. four-diode rectification circuit - this makes the current only go in one direction through the x-ray tube.
7. filament circuit variable resistor - this variable resistor adjusts the current going to the filament.
8. filament step-down transformer - this transformer steps the voltage down and therefore the current up.
9. x-ray tube - this is where the x-rays are created.
10. rotor stator - this rotates the anode.

5.  It can be explained in two separate headings
 1)CONTROL PANELOR CONSOLE
 2)TRANSFORMERASSEMBLY

6.  Meters measure the actual mA & kVp during
exposure.
One exposure button readies X ray tube –heats
filament & rotates anode
Other button starts exposure
Timing mechanism terminates exposure

7.  TRANSFORMER - increases/decreases voltage in a
circuit
 RECTIFIER-changes alternating current to direct
current

8.  Grounded metal box filled with oil.
 Contains :
 low voltage transformer ;
 high voltage transformer & a group of rectifiers for the
high voltage circuit
 Oil serves as insulator

9.  Has 2 wire coils wrapped around a closed core
 Core- simple rectangle with windings wound around its opposite
sides
 PRIMARY CIRCUIT: connected to the available electrical
energy source
 SECONDARY CIRCUIT: from which the modified electrical
energy comes
 The core of the transformer is laminated to reduce the wastage
of power which appear as heat in the transformer core

10. Current flows through the
primary coil
Magnetic field is created in
the core
Magnetic field induces
current in secondary coil

11. 1st law
The voltage in the two circuits is proportional to the
number of turns in the two coils

12. 2nd law
A transformer cannot create energy.An increase in
voltage must be accompanied by a corresponding
decrease in current.The product of the voltage and
current in the two circuits must be equal

13.  single winding wound on a laminated closed core.
 “ self induction”
 An alternating current applied between input points
magnetic flux around the core
 This magnetic flux will link with all turns forming coil
inducing a voltage into each turn of the winding

14.  Functions:
1. Provides voltage for the X ray tube filament circuit
2. Provides voltage for the primary of the high
voltage transformer
3. Provides suitable voltage for subsidiary circuits
4. Provide a convenient location for the KVp meter
that indicates the voltage to be applied across the
x-ray tube

15.  Regulates current flow through filament.
 Power to heat filament is provided by small step
down transformer - Filament transformer

17.  The circuit has :
1. 2 transformers
2. An auto tansformer
3. A step up transformer

18.  Autotransformer is the kVp selector -located in the
control panel
 Voltage across primary coil of step up transformer
can be varied by selecting the appropriate number of
turns in the autotransformer
 The kVp can be adjusted in steps from
approximately 40 to 150 kVp

19.  It increases the voltage by a factor of approximately
600
 Potential difference across the secondary coil may
be as high as 1,50,000 V
 Hence immersed in oil in the transformer assembly

21.  Rectification is the process of changing
alternating current into direct current
The device that produces this change is called
RECTIFIER

22.  The high voltage transformer provides an alternating voltage to X
ray tube
 X ray tube connected to the secondary windings of the step up
transformer with one side of the transformer connected to
cathode and the other to the anode of the tube.
 e- flow at high speed from cathode to anode , when there is a
potential difference & produce X rays

23.  In next half of electrical cycle, anode is negative
& filament is positive so e- would flow from
target toward filament
 By blocking current flows in the inverse half of
the cycle, the X ray tube changes an alternating
current into direct current

24.  Because only half of the electrical wave is used to
produce X rays, the wave form is called HALF
WAVE RECTIFICATION
INPUTWAVE FORM FULLWAVE
RECTIFICATION
HALFWAVE
RECTIFICATION

25.  X ray tube itself acts as a rectifier, the circuit is
called “ SELF RECTIFIED ”.
 Disadvantages:
i. Half of the available electrical cycle is not used
to produce X rays.
ii. Due to prolonged exposures, anode gets
heated , to produce a current during the inverse
half cycle

26. “ A rectifier is a device that allows an electrical current
to flow in one direction but does not allow current to
flow in the other direction ”
 These are incorporated within the x ray circuit, and
same current flows through x ray tube and rectifier
 It can be vacuum-tube type or solid state composition
 The heart of a solid state rectifier is a semi conductor ,
usually a piece of crystalline silicon.

27.  In any atom , there are
energies between the
allowed energy levels
that are not available
for electrons to reside
in.
 When numerous atoms
together form solids,
these unallowable
energies form the
FORBIDDEN GAP.

28. No forbidden gap
at normal temp
and pressure
Forbidden gap is
of order of an
Electron volt
Forbidden gap is
of order 10 electron
volts

29. “ A material that at low temperatures acts as an
insulator but at normal room temperature acts as a
conductor ”
Semiconductors
N type
semiconductors
P type
semiconductors

30.  Silicon contains 4 valence electron ,
 One of the five electron of the impurity is
not utilised in the bonding called as covalent
bond
 Impurities are known as donor, since it
donates an extra electron
 Crystal resulting from the addition of the
donor is called N-type, N derived from
negative charge of the surplus electron
 Arsenic,Antimony etc..

31. If an impurity with 3 valence electron is added to
silicon,.
One silicon now has an electron that is looking for
another with which to form a covalent bond
Absence of this electron is called as HOLE.
Since hole is a positive particle the material is
called as a P-type semiconductor
acceptors used are Gallium,Indium,Aluminium..etc

33.  formed by a complex process in which P and N
materials are diffused into a single crystal.
 N-types is rich in electron and P-type is rich in holes
 Electron diffuse across the junction, N-type acquires a
net positive charge and P-type negative charge.
 This creates a DEPLETION LAYER.
 The device formed by P-N junction is called as DIODE

35.  If polarity of applied voltage is opposite to that
of junction, electron will flow from n-type to p-
type.
 When negative pole of a battery is connected to
N-type and positive pole to p-type, current will
flow.This is called FORWARD BIAS.
 If polarity is reversed,negative pole connected to
p-type, the junction potential will be
augmented, and no current would flow.This is
called REVERSE BIAS

37. DIRECTION OF
CURRENT FLOW
DIRECTION OF
ELECTRON FLOW

38.  When two rectifiers are connected in series with the X
ray tube
 Electrons flow through the tube from the cathode to
the anode
 When voltage reverses during the inverse half of the
cycle, the rectifier stops current flow
 This produces HALFWAVE RECTIFICATION
 Advantage is X Ray tube is protected from full
potential of the inverse cycle

39.  Full wave rectification employs full potential
of the electrical supply
 Both halves of the alternating voltage are
used to produce x rays
 So the X ray output per unit time is twice as
large as with half wave rectification.
 Voltage across the circuit is supplied by the
step up transformer

41.  In this manner 4 rectifier produce pulsating
direct current through x ray tube,even though
transformer supplied an alternating input
current
 Disadvantage is considerable portion of the
exposure time is lost.

42.  Commercial electric power is usually
produced and delivered by three phase
alternating current generators
 Three basic types:
1. Six pulse, six rectifer
2. Six pulse, twelve rectifier
3. Twelve pulse

43.  A 3 phase transformer
has 3 sets of primary
and secondary
windings
 The three sections of
copper windings in the
primary or secondary
are connected in one
of two configurations
termed delta and wye
.
DELTA
WYE OR STAR

45.  This design employs a delta wound primary
transformer with wye wound secondary
transformer
 Output of the secondary windings is rectified
with six solid state rectifiers

46.  There are three maximum and three
minimum voltages in one complete cycle
 When rectified there will be six positive
maximum voltages per cycle. Hence the term
“SIX PULSE”

47. “The variation in the voltage across the x ray
tube expressed as a percentage of the maximum
value”

49.  This circuit has a six pulse circuit with a 13.5%
theoretical ripple factor
 It has a fixed potential to ground
 Insulating requirements simplified

50.  Looks similar to six pulse, twelve rectifier
transformer
 Difference : the secondary is NOT a double
wye connection. It is a wye and delta
connection
 Hence, output of one winding will fill in the
ripple of the other . So a twelve pulse output

51.  Nearly constant potential
 Produce X rays efficiently throughout the
exposure
 Average X ray energy is higher
 Higher tube rating for extremely short X ray
exposures

52. “Means of supplying power for the X ray tube
independent of an external power supply”
TWOTYPES:
1. Capacitor discharge generators
2. Battery powered generators

53.  “a capacitor is an electrical device for storing
charge or electrons”
 110V or 220V power
 Output of high voltage transformers is
rectified and used to charge capacitor or a
bank of capacitors
 Uses a grid controlled X ray tube to start and
stop the X ray exposure

54. Small and easy
to move
Each exposure
starts at the
same kVp
Drop of about 1
kV for 1 mAs
Limited usage
in radiography
of thick body
parts like
abdomen
Has to be
charged
immediately
prior to use
ADVANTAGES
DISADVANTAGES

55.  Standard power supply is used to charge
large capacity nickel cadmium batteries
 Output from batteries is fed into a DC
chopper which interrupts the current many
times each second
 Typical value 500 times per second

56. Stores considerable
energy to generate X
rays
exposures independent
of a power supply
Supplies constant
output of Kv and mA
Heavy
Requires regular battery
maintenance

57.  Principle : “ in a transformer , the voltage
induced in the secondary coil is proportional
to the rate of change of current in the primary
coil”
 60 Hz power line frequency is converted to
6500 Hz

58. ADVANTAGES Supplies a constant nearly ripple
free voltage to the X ray tube
regardless of the input power
No special power supply or voltage
regulators required
Very small size

59. “ a switch is a device that turns the high
voltage applied to the X ray tube on and
off”

60.  If currents switched off rapidly,
 Energy stored in voltage system is removed
 high voltage spikes may damage the system
 If the voltage in the circuit are not removed,
the voltage across the X ray tube cannot go
to zero
SOLUTION –SWITCHING CIRCUITS

61. PRIMARY SWITCHING
 Switching takes place
in the primary circuit
 High currents and low
voltage
SECONDARY SWITCHING
 Switching takes place
in the secondary circuit
 Low currents and high
voltage

62. PRIMARY SWITCHING
 In general purpose
three phase units
SECONDARY SWITCHING
 In units designed for
rapid repetitive
exposures
 When extremely short
exposure times needed
 Angiography,
cineflurograhy

63. PRIMARY SWITCHING
1. Electromechanical
contractors
2. Thyatrons
3. Solid state silicon
controlled rectifiers
SECONDARY SWITCHING
1. Triode vacuum tubes
2. Grid controlled X ray
tubes

64. PRIMARY SWITCHING
 Easier and cheaper
 Can produce exposures
as short as 1 or 2
milliseconds
 Cannot produce
exposures at high
repetitive rate
SECONDARY SWITCHING
 Sharp crisp exposures
 Rapid on and off rates
with many repeated
exposures

65. Four basic types:
1. Mechanical timers
2. Electronic timers
3. Automatic exposure control
4. Pulse counting timers

66. The exposure button
starts exposure and also
starts charging the
capacitor
Exposure terminated
when the capacitor is
charged to a value
needed to turn on
associated electronic
currents
PRINCIPLE: The length of the X ray exposure is
determined by the time required to charge a capacitor
through a selected resistance

67.  They measure the amount of radiation
required to produce the correct exposure for
a radiographic examination
 Two types:
1. Entrance types: located in front of the
cassette
2. Exit types: located behind the cassette

68.  The essential element is a device that can
detect radiation and in response to this
radiation, produce a small current
a) Photomultiplier detectors
b) Ionization chambers
c) Solid state detectors

69.  Most commonly used
 Detector made of
Lucite covered with
phosphorus in areas :
Lucite Paddles
LIGHT GATE
PHOTO
MULTIPLIER
TUBE
LUCITE
PADDLE FLUORES-
CENT
SCREEN
EXPOSURE
TIMER
X RAY
GENERATOR

70. THANK U...