SEMINAR 1. Fundamentals of radiologic PHYSICS.pptx

Fundamentals of Radiological Physics

Introduction
• Radiology: Medical speciality using imaging (X-rays, CT, MRI) to
diagnose and treat diseases.
• Importance of fundamentals:
• Understanding device functionality (e.g., X-ray tubes, detectors).
• Digital logic in modern imaging systems.
• For achieving Safe use of radiation

Electricit
y
• It is a form of energy resulting from the
existence of charged particles (such as
electrons or protons), either statically as an
accumulation of charge or dynamically as a
current .
• The flow of charge in a conductor is defined as
electricity.
There are two branches of electrical physics:
• Electrostatics : It is a branch of physics that
deal with the study of charge at rest.
Electrostatics deal with the study of forces,
fields and potential arise from static charge.
• Electrodynamic : Electrodynamics is the study
of electric charges in motion.

Electrostatic Charge
Definition: Electrostatic charge is
a buildup of electric charge that occurs
when electrons are transferred from
one material to another.
Coulomb’s Law: The force between two
charges is proportional to their product
and inversely proportional to the square
of the distance between them.
Applications in Radiology: Electrostatic
principles are used in image intensifiers
and digital detectors to manipulate
electron flow.

ELECTRICAL
CURRENT
• The flow of electric charge in a conductor is
called an electric current.
• It is equal to the quantity of charge passing
a given point in one second.
• Charge may flow through solid, liquid and
gas or vacuum.
• The unit of current is called ampere (A).
The electric current through a wire is
called one ampere, if one coulomb of charge
flows through the wire in one second.
• It is found that one ampere current consists
of 6.281 × 1018
electrons/sec.
• In practice, milliampere (mA) and
microampere (µA) are used as units, 1 mA
= 10–3
A, and 1 µA = 10–6
A.
5

Types of Electric
Current
• There are two kind of electrical current-
1. Direct current
2. Alternating current
• Direct current- DC current is defined as a uni-
directional flow of electric charge. In DC current,
the electrons move from an area of negative
charge to an area of positive charge without
changing direction. This is unlike alternating
current (AC) circuits, where current can flow in
both directions.
• Alternating current- An alternating current (AC)
is defined as an electric current that changes
direction and magnitude periodically. Unlike
direct current (DC), which flows in one direction,

Types of Alternating Current
Single phase A.C- Only one coil rotating
between two magnetic poles and giving
rise to one sine wave is called single
phase alternating current.
Polyphase A.C- Three coil rotating
between two magnetic poles and giving
rise to three sine wave is called polyphase
or three phase alternating current.
These three coil are displaced equally
from each other within 360.
7

Capacitors
• A capacitor is an electrical component that stores and releases
electrical energy in the form of an electric field. It consists of
two conductive plates separated by an insulating material
called a dielectric. When a voltage is applied, one plate
accumulates positive charge while the other accumulates
negative charge, creating an electric field between them.
• Dielectric Material: Increases capacitance.
• Applications:
• Used in X-ray machine circuits to store and release charge rapidly.
• Smoothing circuits in rectifiers.

•The capacitance (C) of a capacitor is given by:
: where:
• C = Capacitance (in Farads, F)
• ε = Permittivity of the dielectric material
• A = Area of the capacitor plates (m²)
• d = Distance between the plates (m)
•Another important formula is the charge-
voltage relationship:
Q=CV where:
• Q = Charge stored (Coulombs, C)
• V = Voltage applied (Volts, V)

Electrical Energy
• Definition: The ability to do work due to electric potential.
• Formula: W=VQ (Work = Voltage × Charge).
• UNITS :Joule (J) / Kilowatt-hour(kWh) / Electron-Volt(eV)
• ELECTRIC Power (P): It is the rate at which work is done or energy is
transformed in an electrical circuit. Simply put, it is a measure of how
much energy is used in a span of time.
• P=VI (Power = Voltage × Current). S.I. UNIT :Watt, joule per
second
• Applications:
• X-ray production relies on converting electrical energy into
electromagnetic radiation.
• Efficient power management in radiology equipment enhances
performance and safety.

Magnetic Field and Electric Charge
Magnetic Field (B): Region around a
magnet where magnetic forces act.
•Relation to Electricity: Moving electric
charges generate a magnetic field.
•Right-Hand Rule: Determines the
direction of the magnetic field around a
current-carrying conductor.
•Applications in Radiology:
•MRI scanners use strong magnetic fields
to align protons in the body.
•Magnetic fields influence electron
movement in cathode-ray tube.

Electromagnet
ic Induction
(EMI)
Faraday’s Law: A changing magnetic field induces an
electric current in a conductor.
Lenz’s Law: The induced current opposes the change that
caused it.
Mutual Induction: When a changing current in one coil
induces voltage in another.
Self-Induction: When a changing current in a coil induces
voltage within the same coil.
Applications in Radiology:
Transformers in X-ray machines use mutual induction to
step up/down voltage.
MRI uses electromagnetic induction to generate signals
from tissues

Electromagnetic Induction (EMI)
• Electromagnetic induction is the process of generating an electric
current in a conductor by changing the magnetic field around it.
This phenomenon is the basis for many electrical devices used in
radiology, including X-ray transformers, generators, and MRI
systems.

Faraday’s Law of Electromagnetic
Induction
Michael Faraday discovered that a changing magnetic field induces an
electromotive force (EMF) in a conductor.
• Faraday’s First Law : An EMF (Voltage) is induced in a conductor whenever
there is a change in the magnetic field surrounding it.
• Faraday’s Second Law : The magnitude of the induced EMF is directly
proportional to the rate of change of the magnetic flux.
• More flux change = Higher induced voltage.
• Application in Radiology
• X-ray generators use electromagnetic induction to transform voltage for
the X-ray tube.
• MRI scanners rely on changing magnetic fields to induce signals from
hydrogen nuclei.

Faraday’s Laws of Electromagnetic
Induction
• First Law: A changing magnetic field in a coil induces an
electromotive force (EMF) in it or a nearby coil.
• Second Law: The magnitude of induced EMF is proportional
to the rate of change of magnetic flux:
E = N*d
− Φ/dt; where:
• E = Induced EMF (volts)
• N = Number of turns in the coil
• dΦ = Rate of change of magnetic flux
• In transformers, AC voltage applied to the primary coil
creates a varying magnetic flux in the core, which induces a
voltage in the secondary coil.

Lenz’s Law
Lenz’s Law states that the induced EMF opposes the change
that caused it.
• In a transformer, this means that the induced current in the
secondary coil opposes the magnetic flux change in the
primary coil.
• This opposition ensures efficient energy transfer but also
leads to losses like eddy currents and hysteresis losses.

Mutual Induction
Definition: When a changing current in one coil induces a
voltage in a nearby coil.
• Requires two coils:
• Primary Coil: The one connected to
the power source.
• Secondary Coil: The one where EMF
is induced.
• Applications in Radiology
A. Step-up and step-down transformers
in X-ray machines use mutual
induction to modify voltage.
B. Autotransformers adjust kV settings
in radiographic equipment.

Self-Induction
• Definition: When a
changing current in a coil
induces an EMF in the
same coil.
Occurs due to the coil’s
own changing magnetic
field.

Vacuum
Tubes
• Vacuum tubes (also called electron
tubes or thermionic valves) are
electronic devices that control the
flow of electrons in a vacuum.
I. Vacuum Diodes
II. Triodes
III. Tetrodes and Pentodes

Gas-Filled
Thermionic
Tubes
•Structure: Contains a small amount of gas
(e.g., Argon, Mercury).
•Working:
•The presence of gas increases conduction
efficiency and reduces power loss.
•Used where higher current handling is needed.
•Application:
•Thyratrons (gas-filled tubes) were used in
early X-ray generators for high-voltage
switching.
•Some fluoroscopy circuits used them before
semiconductor advancements.

•Cold-cathode gas-filled diode, also
known as glow tube, is shown
schematically in Fig. (a). The dot
within the circle indicates the
presence of gas in the tube
envelope. Most of the cold-cathode
gas-filled tubes are used as diodes.
• Cold-cathode gas-filled triode, also
known as grid glow tube

Semiconductor Devices
Semiconductors replaced vacuum tubes due
to their compact size, efficiency, and reliability.
• Junction Diodes
• Barrier-Layer Rectifiers
• Transistors

Junction Diodes
• Structure: A PN-junction (combination of p-
type and n-type semiconductors).
• Working:
• Allows current only in one direction
(forward bias).
• Blocks current in reverse bias (acts as a
rectifier).
• Application:
• Used in rectifiers of modern X-ray
machines to convert AC to DC.
• Found in power supply units of imaging
systems.

Construction of Diode
• There are two types of semiconductor material;
• Intrinsic and Extrinsic semiconductor.
• An intrinsic semiconductor is a pure semiconductor in
which hole and electrons are available in equal numbers at
room temperature.
• In an extrinsic semiconductor, impurities are added to
increase the number of holes or the number of electrons.
• Impurities are tri-valent (boron, indium, aluminum) or
• Pentavalent (phosphorous, Arsenic, Antimony).

• A semiconductor diode has two layers. One layer is
made of a P-type semiconductor present, and the
second layer is made of an N-type semiconductor layer.
• If we add trivalent impurities in silicon or germanium, a
greater number of holes are present, and it is a positive
charge. Hence, this layer is known as the P-type layer.
• If we add pentavalent impurities in silicon or
germanium, a greater number of electrons are present,
and it is a negative change. Hence, this layer is known as
the N-type layer.
• The diode is formed by joining both N-type and P-type
semiconductors together. This device is a combination of
P-type and N-type semiconductor material hence it is
also known as PN Junction Diode.

• A junction is formed between the P-type and N-type
layers. This junction is known as PN junction.
• A diode has two terminals; one terminal is taken from the
P-type material, and it is known as Anode. The second
terminal is taken from the N-type material, and it is
known as Cathode.

Barrier-Layer Rectifiers
• Definition: Special diodes that efficiently rectify current
using a metal-semiconductor junction.
• Application:
• High-frequency rectification in X-ray power supplies.
• Used in detectors for signal processing in imaging
equipment.

Working of Diode
• In the N-type region, electrons are the majority charge carriers and holes are
minority charge carriers.
• In the P-type region, the holes are majority charge carrier, and the electrons
are negative charge carriers.
• Because of the concentration
difference, majority charge
carriers diffuse and recombine
with the opposite charge. It
makes a positive or negative ion.
These ions are collected at the
junction. And this region is known
as the depletion region.

• When anode terminal (p-type) of
diode is connected to a negative
terminal and cathode(n-type) is
connected to the positive terminal of
a battery, the diode is said to be
connected in reverse bias.
• Similarly, when anode terminal (p-
type) is connected with a positive
terminal and cathode (n-type) is
connected with the negative terminal
of the battery, the diode is said to be
connected in forward bias.

SEMINAR 1. Fundamentals of radiologic PHYSICS.pptx

Transistors
• Types:
• Bipolar Junction Transistors (BJT) – Uses electron and hole movement
for amplification.
• Field-Effect Transistors (FET) – Voltage-controlled, used in switching
applications.
• Function: Used as amplifiers, switches, and oscillators in electronic circuits.
• Application:
• Used in digital control circuits of X-ray machines.
• Found in MRI and CT scanners for signal amplification.

Photoelectric Devices
• These devices convert light energy into electrical signals,
essential in radiology detectors.

Vacuum Phototubes
• Structure: Similar to a diode but with a light-
sensitive cathode.
• Working:
When light falls on the photo-emissive
surface, electrons are released, initiating the
photoelectric effect. Electrons emitted from the
cathode move toward the positively charged
anode, generating an electric current.
• Application: Used in older X-ray film readers
for automatic exposure control.

Photomultipliers (PMT)
• Working:
• Light photons strike the
photocathode, releasing
electrons.
• Electrons are amplified
through a series of dynodes.
• Application: Used in
scintillation detectors for IITV,
CT and PET imaging.
•Structure: A vacuum tube with multiple electrodes (dynodes).

Semiconductor Photoelectric
Devices
• Types:
• Photodiodes: Convert light into electrical current.(solar cells)
• Phototransistors: Amplify the light signal.(similar to
photodiodes with amplification capability)
• CCD (Charge-Coupled Devices): Used in digital imaging sensors.
• Application in Radiology:
• Photodiodes are used in digital X-ray detectors.
• CCDs are used in modern digital imaging systems.

Amplifiers
•Definition: Devices that increase the strength of electrical signals.
•Types:
1.Operational Amplifiers (Op-Amps) – Used in signal
processing.
2.Power Amplifiers – Used in high-voltage control.
•Application:
•Used in MRI and CT systems to amplify weak signals.
•Helps in boosting detector signals in fluoroscopy and digital
radiography.

Oscillators
• Definition: Circuits that generate continuous AC waveforms.
It converts DC power into an AC signal of a specific frequency
• Types:
• Positive Feedback Oscillators – Sustain oscillations.
• Negative Feedback Oscillators – Used for signal stability.
• Application:
• Used in timing circuits of medical imaging devices.
• Found in radiofrequency (RF) systems of MRI.

TRANSFORMER
DEFINITION – A transformer is a
device that either increases or
decreases the voltage and current in a
circuit.
• The transformer assembly is a
grounded metal box filled with oil.
• It contains a low-voltage transformer
for the filament circuit and a high
voltage transformer and a group of
rectifiers for the high voltage circuit.
38

THE NEED OF TRANSFORMER FOR X-
RAY TUBE
The x-ray generator receives 220 V, 50 Hz Alternating current.
Filament heating requires a potential difference of approximately
10 V, whereas electron acceleration requires a potential difference
that can be varied between 40,000 and 150,000 V.
Transformers are used to change the potential difference of the
incoming electric energy to the appropriate level.
39

PRINCIPLE
Transformer is based on the
principle of Electromagnetic
Induction i.e., when current
flows through the primary
coil, it creates a magnetic
field within the core and this
magnetic field induces a
current in the secondary coil.
40

CONSTRUCTION OF
TRANSFORMER
CONSTRUCTION:-
A transformer consists of two wire coils wrapped
around a closed core. The core may be simple rectangle with the
winding wound around opposite sides of the rectangle. It contains
two circuits :
1. PRIMARY CIRCUIT:
The circuit containing the first coil which is
connected to the available electric energy source.
2. SECONDARY CIRCUIT:
The circuit containing the second coil from
which comes the modified electric energy.
41

WORKING OF A
TRANSFORMER
• STEP 1 – Primary coil of the
transformer is supplied with
Alternating Current.
• STEP 2 – As the current flows
through the primary coil, a
changing magnetic field is set
up around it in the iron core.
• STEP 3 – As per the principle of
Electromagnetic Induction,
current is induced in the
secondary coil.
42

The working of transformers is
governed by the following laws:
• Faraday’s Laws of Electromagnetic Induction
• Lenz’s Law
• Transformer Turns Ratio Law
• Power Conservation Law (Ideal Transformer)

Transformer Turns Ratio Law
• The voltage ratio between the
primary and secondary coils is
given by:
Vs/Vp=Ns/Np ;

where;
• Vp= Primary voltage
• Vs= Secondary voltage
• Np= Number of turns in the
primary winding
• Ns= Number of turns in the
secondary winding
44
Types Based on Turns Ratio:
•Step-Up Transformer:
Ns>Np
, so Vs>Vp
(increases voltage).
•Step-Down Transformer:
Ns<Np
, so Vs<Vp(decreases
voltage).

Power Conservation Law (Ideal
Transformer)
An ideal transformer has no losses, so power input equals power
output: 𝑃𝑝=𝑃𝑠
OR
𝑉𝑝𝐼𝑝=𝑉𝑠𝐼𝑠 ;where:
𝑃𝑝 = Power in primary coil
𝑃𝑠 = Power in secondary coil
𝐼𝑝 = Primary current
𝐼𝑠 = Secondary current
This means that if voltage is increased, current decreases, and vice
versa.

• For example, the voltage across the primary coil was 100 V,
and that across the secondary coil 30,000 V. If the current
in the primary coil is 30 A, then the current in the
secondary coil will be :
100 x 30 = 30,000 x Is
Is = 0.1 A (100 mA)
• In a step-up transformer, the current on the secondary side
(Is) is smaller than the current on the primary side (Ip).
• In a step-down transformer, the secondary current is
larger than the primary current
46

TRANSFORMER RATING
• Transformer rating is the statement described by the
manufacturers in regard to the limits of useful output given by the
transformer.
• It is important to know the rating and maximum limit under which
we can operate a transformer because if it is operated beyond
that limit, it may become too hot leading to damage.
47

TYPES OF TRANSFORMER
• There are two basic circuits in a
diagnostic x-ray unit.
• One circuit contains the step-
up transformer and supplies
the high voltage to the x-ray
tube.
• The other circuit contains a
step-down transformer and
supplies the power that heats
the filament of the x-ray tube.
• A transformer called the ‘Auto-
transformer’ supplies the primary
voltage for both these circuits. 48

1. STEP-UP TRANSFORMER
• A transformer with more turns in the secondary coils than in the
primary coil increases the voltage of the secondary circuit and is
called a Step-up transformer.
• Step-up transformer increases the voltage and decreases the
current.
• Step-up transformer is connected between main supply and x-ray
tube.
• When electrons are produced, they need to be accelerated from
cathode to anode for x-ray production.
• A very high voltage in the range of 150 kVp is required which is
provided by high voltage transformer.
49

•Purpose:-
•1.To provide very high voltage(of
the order of 40kV - 150 kV) to
produce x– rays.
•2. Primary connection from
autotransformer
•3. Secondary connection to
rectifier circuit.
•4. Provide attachment for mA
meter.
50

2. STEP-DOWN TRANSFORMER
• A transformer with fewer turns in the secondary coil than in the
primary coil decreases the voltage and is called a Step-down
transformer.
• It decreases the voltage and increases the current.
• Step-down transformer is connected directly to the filament of the
tube.
51

•Purpose:-
•Supply power to
filament circuit of xray
tube ( 6V -12V with high
current 3-5A)
•Primary connection
from autotransformer.
•Secondary connected to
filament.
52

3. AUTO-TRANSFORMER
• An auto-transformer is
designed to supply the
voltage of varying
magnitude to several
different circuits of the x-ray
machine including both the
filament circuit and high
voltage circuit.
• An auto-transformer
consists of a single winding
wound on a laminated
closed core. 53

AUTO-TRANSFORMER cont.
• Provides voltage for the x-ray tube filament circuit
• Provides voltage for the primary of the high voltage transformer
• Provides suitable voltage for subsidiary circuits
• Provides a convenient location for the kVp meter that indicates the
voltage to be applied across the x-ray tube.
• The autotransformer works on the principle of SELF INDUCTION.
• An Alternating current applied between the input points will
induce a flow of magnetic flux around the core.
• This magnetic flux will link with all the turns forming the coil,
inducing a voltage into each turn of the winding.
54

EFFICIENCY OF TRANSFORMER
It is the ratio of output power and input power in watts, expressed as
percentage(%).
EFFICIENCY= (OUTPUT POWER × 100) / INPUT POWER
• Ideally it should be 1 i.e., output = Input, but actually it is less than 1 i.e., output
< input.
• So, efficiency of transformer is never 100% and this is due to the losses of
transformer.
55

TRANSFORMER LOSSES
• Copper loss due to resistance in the coils:-
• when current flows through the wires, the material of wires
resist the current.
• Greater the current, greater is the resistance. So, some
power is lost in overcoming resistance which is called
copper loss.
• Copper losses are highest at full load.
• SOLUTION:-
• Material as low resistance as possible is selected eg., pure
56

IRON LOSSES (Losses in core windings)
1. HYSTERESIS LOSS:-
• Hysteresis loss is a heat loss caused by the magnetic properties of the core.
• The continuous movement of the magnetic particles, as they try to align
themselves with the magnetic field, produces molecular friction.
• This, in turn, produces heat. This heat is transmitted to the windings. The
heat causes windings resistances to increase.
Minimized by: Using special alloys(SILICON STEEL) for the coil.
2. EDDY CURRENT LOSS:-
• Eddy currents are also set up due to changing magnetic field and loss
appears as heat in the core.
Minimized by: Laminating the core. 57

LOCATION OF TRANSFORMERS IN CIRCUIT
58

• Mechanical auto-transformers are NOT PRESENT
NOWADAYS AS IT IS REPLACED WITH NEWER
TECHNOLOGIES such as solid state or electronic
autotransformers

RECTIFICATION
An x-ray tube to produce x-rays requires that its filament should
satisfy two conditions:-
1. That it should be heated so that the electrons are given off.
2. That it should be connected to a voltage source which makes it
negative in respect to anode so that electrons are attracted
towards anode.
• Current flows through the x-ray tube only in one direction i.e. from
cathode to anode while the voltage from the transformer is
alternating current.
• This conversion from alternating current to direct current
(unidirectional) is called RECTIFICATION.
60

RECTIFIER
• A rectifier is a device that allows an electrical current to flow in one
direction but doesn’t allow current to flow in other direction.
• Rectifiers are incorporated in the x-ray circuit in series with x-ray tube.
• Exactly the same current flows through the x-ray tube and the rectifier.
• High voltage rectifiers can be of vacuum tube type or solid state
composition.
• X-ray tube is a sort of valve type rectifier or diode rectifier as it has two
electrodes and allows the current to flow in one direction only.
61

TYPES OF RECTIFIER
62
THERMIONI
C DIODE
VALVES
SOLID
STATE
DEVICES

THERMIONIC DIODE VALVES
• An evacuated tube with two electrodes
in it is called THERMIONIC DIODE
VALVE. Hence it is like an x-ray tube
having:-
• A glass envelope enclosing a
vacuum.
• Two electrodes within the glass
envelope, one of which is a heated
filament.
• The filament of a valve is heated by a
step-down transformer and emits
electrons which are drawn across to the
anode.
• When a potential difference is applied
across both the electrodes and the 63

FUNCTIONING OF A DIODE VALVE:-
•If the valve is connected in a complete
circuit such that cathode is -ve with
respect to anode electrons are drawn
towards the anode and valve passes
current.
• If the cathode is +ve with respect to the
anode, no electrons will be drawn
across the valve and it blocks the
current thus the supply of current to x-
ray tube is unidirectional only. But these
diode valves which were used earlier
are replaced with solid state rectifiers.
• Hence, its function is to pass current in
one direction only and to block any
reverse flow.
64

SOLID STATE RECTIFIER
65
• As the name solid state implies, conduction takes
place by electron travel through solid materials.
• The material used are semiconductors. Their
characteristics come in between metals and non-
metals.
• Si(silicon) and Ge (germanium) are
semiconductors generally used in these.
• SELENIUM was the first material used for solid
state rectifiers.
• Today, most X-ray generators use silicon
rectifiers which are 20-30cm in length and 2cm in
diameter.

The Self Rectified HT Circuit
•It is also known as
single phase alternator,
in which as alternating
current electrical
generator that produces
a single, continuously
alternating voltage.
66

• In self rectified HT
circuit:- The X-ray tube
acts itself as a rectifier. It
is directly connected to
secondary windings of
transformer.
• Cathode connected to
secondary windings and
anode to other.
• Current only flows from
cathode to anode.
• Cathode is source of
free electrons.
67

APPLICATIONS
• PORTABLE AND LOW POWER MOBILE AND DENTAL UNIT.
ADVANTAGES:- DISADVANTAGE
• Small in size Hot anode can emit electrons
• Simple design
• Simple to operate
• Light in weight
• Less cost
68

Voltage applied to tube
When the two rectifiers are connected in
series with the x-ray tube and cathode is
negative terminal and anode is positive
terminal, the electrons will flow from
cathode to anode.
• When the voltage reverses during
inverse half of the alternating cycle,
the rectifiers stops current flow.
• When rectifiers are used in this
manner they produce half wave
rectification.
• The only advantage of rectifiers is that
they protect the x-ray tube from full
potential of inverse cycle.
Half wave rectified circuit (Single Pulse)

-
-
R1
R2
R1
R2
70
Half wave rectified
circuit
First Half Cycle:
Diodes closed
Tube current (mA) results
Second Half Cycle:
Diodes open
No voltage applied to tube
No tube current (mA)

LIMITATIONS
• 60 pulses per second
• Only positive half cycle of HT
transformer used
• Inefficient
• Negative half cycle wasted
71
Blocked (not used)
Applied to x-ray tube
Output of High Tension Transformer Applied to X-ray Tube

Single phase full wave
rectified circuits (Two Pulse)
• In this circuit both half cycles of
AC are used to produce X-rays
by employing a bridge of four
rectifiers.
• Both halves of alternating
voltage are used to produced X-
rays so the x-ray output per
unit time is twice as large as it
is with half wave rectification.
72

First Half Cycle Second Half Cycle
(also mA waveform)
R1
R2
R3
R4
R1
R2 R3
R4

74
• Higher output than
self or half wave
rectified circuits.
• Less strain on HT
cables and less
insulation cost.
Tube
Output of High Tension Transformer Applied to X-ray Tube

LIMITATIONS
• COSTLY
• MORE COMPLEX
• HEAVIER, NOT EASY TO TRANSPORT
• LARGE IN SIZE
• RIPPLE FACTOR IS 100% AS IT IS PULSATING X-RAY BEAM WITH
VOLTAGE VARIATION BETWEEN ZERO TO PEAK AND AGAIN ZERO
75

SEMICONDUCTORS
• The heart of solid state rectifier is a semiconductor.
• Semiconductors are material whose electrical properties lie between
CONDUCTORS and INSULATOR.
• Silicon and Germanium are the examples of semiconductors.
• Pure semiconductor have little conduction towards electricity, so to enhance the
conductivity impurities are added by a process called doping.
According to addition of impurities it can be classified into two types:-
 P-type semiconductor
 N-type semiconductor
76

P-TYPES CONDUCTOR
• It is an extrinsic semiconductor
which is obtained by doping trivalent
impurity atoms such as boron,
gallium, indium to the pure
germination or silicon
semiconductor.
• The impurity atoms added create
vacancies of electrons(holes)in
structure and are called “acceptor”
atoms.
• The holes are majority charge carries
and electrons are minority carriers.
77

N-TYPE SEMICONDUCTOR
• Materials with surplus free electrons can
be produced by putting minute amounts
of penta-valent impurities into
semiconductors and these free
electrons are able to move through the
substance.
• Such materials with many free electrons
are called N-type materials.
• Examples are:- silicon and germanium
with a minute amount of phosphorous
added impurity.
• Impurity atoms added provide extra
electrons in the structure and are called
“donor” atoms. 78

P-N JUNCTION
•It is formed by joining P-type and N-type
semiconductors together called PN junction.
•Thus, electron easily flow from the N-type
layer(electron rich) towards P-type layer(hole
rich) i.e. from “donor” towards “acceptor” but
not in opposite direction from P towards N
type. Hence unidirectional flow of current is
obtained.
•The block to the current in reverse direction
occurs at the junctions between the two
materials N type and P type i.e. the region
where the barrier exists is very thin.
79

•Connection of diode to potential
source is called BIASING.
•When higher potential of
sources is connected to P-side of
diode then it is FORWARD
BIASED.
•When higher potential of
sources is connected to N-side of
diode then it is REVERSE BIASED.
80

ADVANTAGES OF SOLID STATE
OVER DIODE VALVE RECTIFIER
• Longer life
• No filament heated
• More Robust
• Smaller in size
• More compact that is occupy less
space, better for mobile unit.
• More reliable
81

• The core of a transformer is laminated. It is made up of
thin sheets of special iron alloys separated from each
other by thin insulating layers.
• These sheets are clamped tightly together. The purpose
of the laminations is to reduce eddy currents, which
waste power and appear as heat in the transformer
core.
• Current only flows through the secondary coil when
magnetic field changes(either increases or decreases)
• No current flows during steady state of magnetic field.
82

EXPOSURE TIMERS
There are many ways to control the length of x-ray exposure. The
following are the types of the exposure timer:-
1) Mechanical timer (rarely used today)
2) Electronic timer
3) Automatic exposure control (photo-timer)
4) Ionization chamber autotimer
83

ELECTRONIC TIMER
• Principle:-It works on the principle that the length of exposure is
determine by the time required to charge a capacitor through a
selected resistance.
• When exposure button starts, it also starts charging the capacitor.
The exposure is terminated when the capacitor is charged to a
value necessary to turn on associate electronic circuit.
• Time can be varied by varying the value of the resistance in
charging the circuit.
84

AUTOMATIC EXPOSURE CONTROL
(PHOTOTIMER)
• Mechanical and electronic timer are subjected to human error. To
overcome the human error the automatic phototimer is used to
control the x-ray exposure.
• It can detect radiation which passing through different density of
tissue of human body and produce a small electric current which is
used to charge the capacitor.
• When the capacitor reaches the predetermined charge, it can used to
bias the gate of SCR in the x-ray circuit and cause the exposure to
terminate.
85

IONIZATION CHAMBER AUTOTIMER
• An ionisation chamber consists of
two thin parallel sheet of
aluminium or lead foil. The gas
usually room air present between
the plates.
• The gas become ionised when
struck by the radiation and
ionisation is directly proportional
to the amount of radiation.
• The capacitor starts charging,
when gas ionised by radiation.
• The electric circuit is activated and
will terminate the exposure.
86

Conclusion
• ELECTRICAL & Electronic components form the backbone of modern radiographic imaging
systems. Basic knowledge of these electronic component (vacuum diodes,
semiconductor diodes, generators, rectifiers, semiconductors, photodiodes, charge-
coupled devices (CCDs)), and other semiconductor devices are crucial for
comprehending how radiological equipments function efficiently and accurately.
• The evolution from vacuum tubes to semiconductor devices has revolutionized medical
imaging, leading to more compact, energy-efficient, and high-performance diagnostic
tools. Rectifiers and generators play a vital role in supplying and regulating power, while
photodiodes, CCDs with advanced FPD’s enhance image acquisition and processing.
These advancements have significantly improved image quality, patient safety, and
diagnostic precision.
• Knowledge of fundamental concepts is also important in maintaining dose to patient ,
their attendants and staff to minimum levels .
• By integrating fundamental physics with cutting-edge technology, radiology continues to
progress, enabling faster and more accurate diagnoses. A solid grasp of these
fundamental concepts empowers radiologic technologists and medical professionals to
operate imaging systems effectively, troubleshoot technical issues, and contribute to
further innovations in the field.

SEMINAR 1. Fundamentals of radiologic PHYSICS.pptx

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