Control of kV and mA in x-ray tube.pptx

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. CONTROL OF KV AND mA PRESENTED BY:- SAGAR CHAULAGAIN BSC.MIT 1 ST 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.

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

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

12. 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.

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

14. 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.

15. 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.

16. 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.

17. 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

18. 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

19. 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.

20. Control of KV Line Auto transformer KVp slector High voltage transformer Rectifier Circuit Anode of the X-ray tube

21. 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

22. 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.

23. 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.

24. 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.

25. 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 .

26. 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.

27. 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

28. 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 .

29. 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.

30. 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.

31. 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

32. 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

33. 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.

34. 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.

35. Difference between half wave and full wave rectification

36. 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.

37. 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:-

38. 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.

39. 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

40. 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%

41. Control of mA Line Auto transformer mA selector Filament transformer Filament of the cathode of X-ray tube

42. 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.

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

44. 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.

45. 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.

46. 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.

47. 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

48. 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.

49. 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.

50. 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).

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

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

53. Thank You