X-RAY TUBES CONTRIBUTING TO GLOBAL WARMING

Authors

Adarsh Sharma, Nidhi Kapur
Department of Radio diagnosis & Imaging
PGIMER, Chandigarh. India

INTRODUCTION :

The beginning of 21st century brought us with many new technologies that made human life easier & faster, but it also projected us towards some life threatening factors which caused a vigorous change in global temperature introducing earthquakes and tsunamis to earth. An average rise in global temperature because of anthropogenic factors is called global warming & is a major topic to bother us. Causes for global warming vary from traditional (house hold bulbs, deforestation, etc.) to industrial (burning polythene). Properties of x-rays drew our attention to use them in non-medical fields too like security checks on air ports, finding faults in metallurgy, pipe lines transporting water, oil & gas but as radiologic students the point of focus in our article will only be restricted to factors contributed by radiology department.
A radiology department contains X- ray equipments (X-ray generators & X-ray tubes), computers and air conditioners. All these components contribute to the heat production & hence to global warming to some extent but the major contribution are from our X-ray tubes. For last pasting decades, we only remained concerned to loading tubes with greater efficiencies assuming that the only involved risk in using X-ray tubes are X-rays itself that we accepted on “Risk versus Benefit” criteria. X- ray tubes made no remarkable, visible harm to the environment like we see in laboratories (disposable equipment like syringes or chemicals), nor we need to bother about disposing off the source (i.e. tubes) after a long era of use like in Radiotherapy. But what about that harm which can neither be seen by eyes nor can be felt immediately by human senses, but increases with the pace of time.
Now we will consider an example to show relation between exposures made by X-ray tubes & global heating. Let us assume that among 6 billion people residing on planet Earth 10% of the population undergo radiological investigations annually. As we know an exposure for few milliseconds can heat the filament 40,000 - 51,000 Joule. So the heat when produced by 600 million (60 crore) will result in heat production of:

50,000 x 60, 00, 00,000 = 30 TJ of heat (TJ – Terra joules)
Now this much amount of heat is directly thrown to the environment. Repeated exposures can pull this count to more numeral heights. This becomes a point of attraction affecting global temperature increase.

X-Ray tube as a source: A household bulb running for several hours become a source of heat & this heat is continuously being added to surrounding environment directly. This made a fluorescent bulb a threatening cause for global warming. To tackle this problem our govt. replaced indecent bulbs with CFL’s. But what steps about X-ray tubes which can’t be replaced. An X-ray tube running for a couple of hours adds thousands times heat as much as an electric bulb for the same time. A single exposure lasting for few milliseconds adds thousands of heat units to surrounding, then the heat added by those hot vacuum tubes serving emergencies, fluoroscopy, CT & Angiography are beyond imagination. As we know 99% of the energy we provide to an X-ray tube is converted into heat & this heat is directly spooned to the surrounding environment. The problem is enhanced in dealing with therapeutic tubes where electron energy is in MeV. During an exposure temperature of focal spot may rise to 2500°C & that of anode upto 1500°C. Hence the X-ray tubes can be an important hidden source of global warming.


Mechanism of heat production :

When X-ray tubes are run at high potential, electrons boil from cathode to anode because of thermionic emission. These electrons before interacting with its respective atom, to produce X-rays, come in the vicinity of other atoms too. These atoms make the electrons to loose its energy in the form of heat (fig. A) & then during actual interaction heat & X-ray photons are produced.

Mechanism of heat dissipation:
The electrons bombarding the focal spot and heat produced are now accepted by anode & this heat is now dissipated in following manner (fig. B, C):-

FIG. B- Diagram showing heat dissipation	FIG. C- Block diagram for heat dissipation

1. Radiation : As there is no matter between glass & the anode hence 80-85% of heat is dissipated to the casing as radiation.
2. Convection : 10-15% only by convection.
3. Conduction : 3-5% as conduction back to anode stem & rotor

Casing heat dissipation :  An exposure lasting for few milliseconds may add 40,000-60,000 HU to anode this results to be 54,000-81,000 J of heat energy. To ready the anode for next rounds of exposure, this heat is given off to surrounding oil pool. This switches on a process of exchange of heat by oil & produce convection currents. Sometimes some air circulating fan is added to the system to exchange heat with the environment. Now note that no condenser or sink is there for this exchange and the heat is directly feed to the surrounding assuming nature as the only sink.

Heat from x-ray tubes is directly fed to surrounding oil pool and from there to the environment.

How to reduce the amount of heat produced :


Continuous heat dissipation to the environment might some day become a of bothering about. But no other safer substitutes of X-ray production do we have. Addition of condensers to the assembly will make it more complicated and uneconomic. Recent advancements of producing X-rays without employing thermionic emission as in cold-cathode tubes is not that practical and beyond the scope of this article. Hence the immediate and important steps we can take to reduce the amount of heat produced are listed below:


1. Anode condition: - A careless & long use of anode for a decade causes it to be pitted. Pitting is roughening of anode resulting dig impressions on it hence more heat will be produced due to poor interaction of electrons with anode. Hence we must use a smooth surfaced anode.

2. Optimum exposure factors: - As kinetic energy of electron associated with it is directly proportional to KV applied. More kinetic energy means more heat produced. Also the time for which an exposure lasts is a determining factor for heat produced. Hence use of optimum exposure for optimum time will reduce the heat output to environment.

3. Avoid repeated exposures: - By repeating the same examination for same patient unnecessarily will lead to increase in following:-

• The amount of heat added to the surrounding 
• Patient radiation dose 
• Wear & tear of the equipment

To overcome these factors no unnecessary exposures should be given to the patient.

4. Generator quality: - A good conditioned generator will provide suffice power to an X-ray tube hence reducing the energy loss. A three phase generator produces more heat in comparison to a single phase generator but the constant kilo voltage provided by the former overweighs this limit. So a generator producing less eddy current with high kilo voltage output will result in less heat loss to the surrounding  & can be employed as a safe measure.

5. Using Metal/Ceramic tubes: - Use of metal ceramic tubes ensures no off-focus radiation. Metal  casing of a metal ceramic tubes attracts back scattered electrons from the anode disk towards itself as a result no electron strikes anode at point other than focal spot hence no off-focus radiation is produced. It serves three benefits;

• less amount of heat produced
• no unnecessary radiation dose to patients
• less wear & tear of machine

6. Reducing coast time: - Coast time is the time taken by anode disc of a rotating anode to stop after end of an exposure. Rotation of anode results in friction between anode stator & ball bearings resulting into a significant increase in the amount of heat produced.  Hence subsequent reduction in coast time may significantly reduce the amount of heat produced. It can be avoided by using magnetic brakes between stator & windings of anode assembly.

CONCLUSION: -  

The unavoidable need of X-rays has placed it in a crucial category in medical diagnosis. Properties of X-rays lead us to use it in non-medical field too. New designs for x-ray tubes were introduced in past century & newer modalities increased their efficiencies in terms of loading. But e never turned our attention towards the fate of heat produced as a by product of energy conversion. This heat produced is almost 99% of the initial kinetic of the electrons & is directly spooned to the surrounding environment. Moreover the temperature of cathode also contributes to this rise. The heat dissipated by CT & Fluoroscopic tubes complicated the situation. The continuous addition of heat to nature made the hot vacuum tube a remarkable source of heat enhancing a world wide problem called Global Warming. The situation becomes tough to deal with if we use rough, pitted anode, unreasonable exposure factors & inaccurate generators. Soon some steps must be taken to attend this issue otherwise carelessness would ultimately lead us to immerse into water.