This presentation is about control of kv and ma in x-ray circuit, it includes different components of X-ray imaging system (operating console , high voltage generator and X-ray tube ). high voltage generator and high frequency generator is well explained along with step up ,step down and an auto transformer .

1. PRESENTED BY:- SAGAR CHAULAGAIN
BSC.MIT 1ST YEAR
ROLL NO. :-157
MAHARAJGUNJ MEDICAL CAMPUS
(IOM)
This Photo by Unknown Author is licensed under CC BY-SA

2. Contents:-
Introduction
X-ray Imaging System
Components of X-ray imaging system
Operating console
X-ray tube
High voltage generator
Autotransformer
Control of KV
 KV indication
 High voltage transformer
 Rectification
 Ripple
 Control of mA
 Filament transformer
 mA selection
 mA indication
 mAs meter

3. Introduction
Controlling the KV and mA is an important aspect of medical imaging
technology.
KV represents voltage applied to the X-ray tube while mA represents the
tube current.
Adjusting these parameters can help to optimize image quality and reduce
patient exposure to radiation.

4.  The three main component of x-ray imaging system are:-
Operating console
X-ray tube
High voltage generator
THE X-RAY IMAGING SYSTEM

5. THE X-RAY IMAGING SYSTEM
 the primary function of the x-ray imaging system is to convert electric energy
into electromagnetic energy.
 This system provides a controlled flow of electrons intense enough to produce
an x-ray beam appropriate for imaging.
 These systems are usually operated at voltages of 25 to 150 kVp and at tube
currents of 100 to 1200 mA

6. In some types of x-ray imaging systems, such as dental and portable machines,
these three components are housed compactly.
But with most systems, the x-ray tube is located in the examination room, and
the operating console is located in an adjoining room with a protective barrier
separating the two.
The protective barrier must have a window for viewing the patient during the
examination.
And the high voltage generator is always close to the x-ray tube, usually in the
examination room, housed in an equipment cabinet positioned against a wall.
CONT:-

7. Operating console:-
More familiar to radiological technologist
At the operating console a technologist can control tube voltage (KV), tube
current(mA), and exposer time (s), so that the useful X-ray beam of proper quality
and quantity can be obtained.
Operating consoles are based on computer technology. Controls and meters are
digital, and techniques are selected with a touch screen.
Numeric technique selection is often replaced by icons indicating the body part,
size, and shape.

8. CONT:-
Most x-ray imaging systems are designed to operate on 220 V power,
Unfortunately, electric power companies are not capable of providing 220 V
accurately and continuously
Because of variations in power distribution to the hospital and in power
consumption by various sections of the hospital, the voltage provided to an x-ray
unit easily may vary by as much as 5%.
Such variation in supply voltage results in a large variation in the x-ray beam,
which is inconsistent with production of high quality image

9. CONT:-
The operating console usually provides for control of line compensation.
The line compensator measures the voltage provided to the x-ray imaging
system and adjusts that voltage to precisely 220 V.
Older units required technologists to adjust the supply voltage while observing
a line voltage meter.
Today’s x-ray imaging systems have automatic line compensation and hence
have no meter.

12. X-ray tube :-
Provides an environment for the production of bremsstrahlung and
characteristic X-ray.
The basic component of X-ray tube are :-
o Cathode
o Anode
o Rotor/stator
o Glass envelop
o Tube port
o Cable socket
o Tube housing

13. This Photo by Unknown Author is licensed under CC BY-SA-NC

14. High voltage generator:-
Principle function of the X-ray generator is to provide current at a high voltage to
X-ray tube.
It receives the electrical energy from the electrical power system and converts it
into DC form to supply the X-ray tube
The KV, mA and exposer time are the three major parameters which can adjust by
the generator which controls the X-ray production.

15. CONT:-
 Contains 3-primary parts:
- The high voltage Transformer
- The filament Transformer
- The Rectifiers

17. Control of KV:-
Radiation quality refers to the penetrability of the x-ray beam and is expressed in
kilovolt peak (kVp)
Range from 25kv (for mammography ) up to 150kv (in high kv technique)
 The tube kv during the exposure is the output voltage of the secondary winding
of the high tension transformer, to which the x-tube is connected either directly
or through a system of high tension rectifier.

18. CONT:-
This output voltage may be controlled by changing the input voltage
Since the high tension transformer is of a fixed turns ratio. When its primary
voltage changes its secondary voltage(the tube kv) changes also.
The variable input voltage for the high tension transformer is obtained from
autotransformer which is connected across the primary winding of the high
tension transformer.
Kv change is achieved by varying the output from the Autotransformer and this
is applied to the high tension transformer as its primary (input) voltage.

19. Autotransformer:-
Woks on the principle of self induction.
It’s consists of an single iron core with only one winding of wire about it;
This single winding has a number of connections along its length.
And this single winding acts as both primary and secondary winding
It’s uses are generally restricted to cases in which only a small step-up or
step-down in voltage is required.

20. CONT:-
Thus an autotransformer would not be suitable for use as high voltage
transformer in an X-ray imaging system .
The power supplied to the X-ray imaging system is delivered first to the
autotransformer.
Supplies precise voltage to the high voltage circuit and to filament circuit.
Autotransformer Law is same as transformer law, i.e Voltage is directly
proportional to the number of turns ratio and inversely proportional to current

21. CONT:-
Vs/Vp=Ns/Np=Ip/Is
where Vp = the primary voltage
VS = the secondary voltage
NP = the number of windings enclosed by primary connections
NS = the number of windings enclosed by secondary connections
Ip = the primary current
Is = the secondary current
 The ability to provide an adjustable secondary voltage make an
Autotransformer used in controlling kilovoltage

22. CONT:-
A and A’ are primary connections that conduct input
power to the autotransformer.
Connections. C: increases voltage due to proximity
to end and number of turns encased by the
connections.
D and E : decreases voltage.

23. Control of KV

24. KV Indication
 Two meters are incorporated into the high voltage circuit one to measure KVp
and another to measure mA. The meters themselves are located in the control
panel but their connections are in the high voltage circuit.
KV selector varies the output voltage from autotransformer.
To know KV which is being selected, there must be indication on control panel.
I. Calibrated autotransformer
II. Pre-reading kilovolt meter

25. Calibrated autotransformer
Each setting of the selector is marked with kV value in gradations or steps.
Secondary voltage from high tension transformer= primary voltage from
autotransformer x turns ratio of high tension transformer
 Peak kilovoltage= secondary voltage x 1.41
Irregularities in this transformer results kilovoltage drop. Load current through
windings of high tension transformer must flow against the resistance of
windings.

26. KV drop
Voltage absorbed in overcoming resistance is lost to x-ray tube called kilovoltage
drop.
kV drop increases with inc. in mA (V=IR)
Also add kV drop in case of use of rectifiers
Total kV drop= kV drop in transformer + kV drop in rectifiers
Actual kV available for x-ray tube= kVp - total kV drop
Therefore KV available for x-ray tube also vary with mA, low at high mA & high at
mA near 0.

27. Modern calibration
Modern x-ray set give KV indication by means of selector position marked truthfully.
It is made possible by
• kilovoltage compensation
• Extra voltage equal to KV drop is supplied to primary circuit of high tension
transformer from autotransformer when tube current is selected.

28. Pre-reading kilovolt meter
That indicates the kilovoltage that will be applied to the x-ray tube once the x-ray
exposure starts ( the potential difference between cathode and anode).
An AC instrument connected across output terminals of autotransformer so actually
reads voltage applied to primary windings of high tension transformer when exposure
begins.
Gives information before kV is actually applied to x-ray tube & therefore actually
reads voltage, not kVp.

29. Difference from actual kVp
Meter can have a scale which is calibrated to kVp on secondary side of high tension
transformer= primary voltage x turns ratio x 1.41
As we know, actual kV applied to tube= kVp – kV drop
so, reading & actual kVp differ & the difference depends on selected tube current.
Manufacturer determine the actual tube kVp at each of tube current & compare with
the meter indication. Then meter reading is brought down by reducing the voltage
across the meter through meter-reading compensator.

30. High Voltage Generator
 Responsible for increasing the output voltage from the Autotransformer to
the KVp necessary for X-ray production
 High voltage generator has three component,
 High voltage transformer
 Rectification circuit
 Filament transformer

31. High voltage transformer
Step up transformer i.e number of secondary winding are greater in
number than primary.
Works on the principle of mutual induction.
Primary winding is connected to autotransformer and seconday winding
with anode of the X-ray tube.
Converts voltage to kilo voltage peak which is used to accelerate
electrons fastly.

32. CONT:-
Follows transformer law:
Vs = Ns = Ip
Vp Np Is
Increase in V is directly proportional to turns ratio (Ns/Np) & the
current is reduced proportionally
The turns ratio of a high voltage transformer is usually between 500:1
and 1000:1 because transformer operate only ac.
Oil immersed in earthed metal tank to insulate & cool it.

33. Rectification circuit
Rectification is the process of converting AC to DC.
Although transformers operate with alternating current, x-ray tubes must be
provided with direct current.
X-rays are produced by the accelerating of the electrons from the cathode to the
anode and can not produced by electrons flowing in the reverse direction
Reversal of electron flow would be disastrous for the x-ray tube.
Voltage rectification is required to ensure that electrons flow from x-ray tube
cathode to anode only.

34. For the electron flow is to be only in the cathode to anode direction, the sec.
voltage of the high voltage transformer must be rectified
 Rectification is accomplished with diodes.
 A diode is an electronic device that contains two electrodes.
 Originally, all diode rectifiers were vacuum tubes called valve tubes; these
have been replaced by solid-state rectifiers made of silicon
 Rectification can be done by two ways :-
 Half wave rectification
 Full wave rectification

35. Half wave rectifier
 Typically in half wave rectification, two
rectifier are connected in series with the
X-ray tube, with one on each side
 As a result electrons can flow from cathode
to anode during the first half of each AC
cycle but are blocked during the second
half cycle

36. Full wave rectification
 In full wave rectification, two pairs of
rectifiers are configured to operate
alternatively and electron flow from cathode
to anode during both half of AC cycle in a
pulsating current.
 Voltage across a full-wave–rectified circuit is
always positive.

37. Difference between half wave and full wave rectification
Half wave rectification Full wave rectification
 Contain one or two diode.  Contains at least four diode
 Rectifiers conduct only on positive half
cycle.
 Rectifiers conducts alternatively on both
positive and negative half cycle.
 produces 60 X-ray pulses each second  Produces 120 X-ray pulses each seconds
 only one half of the AC waveform appears
in the output.
 All of the input waveform is rectified into
usable output.
 It wastes half the supply of power  There is no waste of power supply.
 It also requires twice the exposure time.  It requires half the exposure time than half
wave rectifier.

38. Difference between half wave and full wave rectification

39. Three phase power
 Single-phase power results in a pulsating x-ray beam.
 Disadvantages of this is that, intensity only significant when voltage is near peak
and low voltage produces low-energy photons.
 Three-phase power is a more efficient way to produce x-rays than is single-phase.
 Commercial electric power is usually produced and delivered by three-phase
alternating-current generators
 With three-phase power, multiple voltage waveforms are superimposed on one
another, resulting in a waveform that maintains nearly constant voltage.

40.  This kilovoltage waveform instead of pulsation is called as rippling.
 Half wave rectification of 3 phase input gives 6 pulse output
 Full wave rectification of 3 phase input gives 12 pulse output.
 Operation in 3 phase power is equivalent to 12% increase in KV or almost
doubling of mAs over single phase power(15% rule)
CONT:-

42. Ripple
 The ripple factor is the variation in the voltage across the x-ray tube expressed
as a percentage of the maximum value.
 Single-phase power has 100% voltage ripple because the voltage goes from zero
to a maximum value with each cycle.
 Three-phase, six-pulse power produces voltage with only approximately 14%
ripple
 Three-phase, 12-pulse power results in only 4% ripple
 High-frequency generators have approximately 1% ripple and therefore greater
x-ray quantity and quality.

44. HIGH FREQUENCY GENERATOR
 Newest development in high voltage generator, smaller than conventional
generators (80% reduction in size)
Converts standard mains voltage i.e. 50 Hz to higher frequency usually 500-
25000 Hz.
Uses inverter circuit.
Voltage ripple less than 1%.
Can be placed within x-ray tube housing .So used in portable machines.
Can utilize single or three phase mains supply

46. Advantage of using high frequency generator
 They are very much smaller than 60-Hz high voltage generators, easy to used
in portable machine.
 Produces high output and accuracy.
 produce a nearly constant potential voltage waveform, improving image
quality at lower patient radiation dose.
 Ripple is less than 1%

47. Control of mA

48. Control of mA
 X-ray tube current is controlled through a separate circuit called filament
circuit.
 Filament circuit regulates current flow through the filament of the x-ray tube.
 Tube current is altered by altering the number of electrons which are emitted
from its heated filament, which can be achieved by changing the temperature of
filament.
 The filament is heated by electric power which a step down transformer
provides, the filament is directly connected to the secondary winding of this
transformer.

49.  The x-ray tube current, crossing from cathode to anode, is measured in
milliamperes (mA).

51. Filament Transformer
 Step-down transformer.
 It steps down the voltage to approximately 12 V and provides the current of about
3 to 6A to heat the filament.
 The primary winding of the filament transformer obtains its voltage by tapping
off an appropriate number of turns from the autotransformer
 And the secondary winding of this transformer is connected directly to the
filament.

52. mA selector
 Precise control of filament heating is critical, because a small variation in
filament current results in a large variation in x-ray tube current
 A change in filament voltage of about 5% will result in a 20- to 30-% change
in x-ray tube current.
 x-ray filament current may be controlled by altering the voltage to the primary
of the step-down transformer
 It is done by the addition of resistors connected in series in the circuit leading
from the autotransformer.

53.  Several other components in the filament circuit are used to stabilize the voltage
to the filament transformer, including a voltage stabilizer and a frequency
stabilizer
 There is also a circuit that automatically compensates for the space charge effect.

54. mA indication
 Should have readable indication of the tube current while the exposure lasts. It is
done by means of mA meter.
 Connected at center of secondary windings of high tension transformer &
measures current flowing in secondary circuit of high tension transformer
 Secondary winding of high tension transformer is wound in 2 halves & inner end
of halves connected to earth forming earthed mid junction or grounded center. It
is the only part of high tension circuit which is virtually at 0 V. In this way, no
part of meter is in contact with high voltage & meter may be safely put on
operating consoles

56. mAs Meter
 Records milliampereseconds i.e. product of tube current & time for which it flows
(electric charge).
 When exposure time is very short, an ordinary ma meter doesn't have enough time
to give accurate reading.
 If exposure time is <1 s, before needle of mA meter can reach right place on scale,
tube current is cut off at end of exposure & needle falls back to zero.
 mAs meter is often provided with double scales just as in mA meter.

57. Summary
 The x-ray imaging system has three principal sections: the x-ray tube, operating
console, and the high-voltage generator.
 The operating console usually provides for control of line compensation, kVp, mA,
and exposure time.
 The autotransformer has a single winding and is designed to supply a precise voltage
to the filament circuit and to the high-voltage circuit of the x-ray imaging system.
 The high-voltage generator contains three primary parts: the high-voltage
transformer, the filament transformer, and rectifiers.

58.  Voltage rectification is required to ensure that electrons flow from x-ray tube cathode to
anode only.
 Three-phase power is a more efficient way to produce x-rays than single-phase power
as, Single-phase power results in a pulsating x-ray beam.
 Filament transformer steps down the voltage to approximately 12 V and provides the 3
to 6A current to heat the filament.
 The x-ray tube current, crossing from cathode to anode, is measured in milliamperes
(mA).

59. Questions ?????
1) What is the role of an autotransformer in controlling the voltage supply to the X-ray tube
during an exposure?
2) What are the percentage of voltage ripple for various power source?
3) Why does the x-ray circuit require rectification?
4) Location of mA meter and KVp meter?
5) What does KVp meter measures actually?
6) What are the advantages of using high frequency generators in place of high voltage
generators?
7) How do we control mA?
8) Difference between high voltage transformer and filament transformer?
9) Difference between tube current and filament current?

60. Reference :-
Radiologic science for technologist by SC Bushong 11th edition.
Christensen's Physics of Diagnostic Radiology 4th edition.
Chesney’s equipment for student radiographers 4th edition.
The Essential Physics of Medical Imaging by Bushberg.
Various online source.