A visit to Ashford hospital Essay Example

A visit to Ashford hospital Essay Example A visit to Ashford hospital Essay A visit to Ashford hospital Essay Essay Topic: The Visit On the 11th March 2002 we visited the Ashford hospital where we were shown the x-ray department and a radiographer who gave us a talk for about an hour and a half. In my report I will include most of the aspects in x-rays and at last two in detail, starting from the basic definition of an x-ray to how and why we use them to the effects of x-rays.First it is important to know what an x-ray is: x-rays are high photon energy (that is to say short-wavelength) electromagnetic radiation. They are used in medicine both for diagnosis (radiography) and for therapy (radiotherapy). It is not possible either to reflect or to refract x-rays and therefore x-rays cannot be focused.There are three types of radiation from radioactive materials: Alpha (?), Beta (?), Gamma (?).The term radiotherapy refers to the treatment of a medical condition (usually cancer) by means of X-ray, Y-rays or beams of energetic electrons. X-ray radiotherapy falls in to two main classes-superficial therapy and megavoltage (or MV) therapy.Superficial therapy is used to treat conditions of the skin and surface tissues. The tube voltages employed are such that the X-rays have low penetrating power and therefore cause littlie damage to the healthy tissue beneath the area being treated.Megavoltage therapy is used to treat the condition inside your body and has almost completely replaced the lower voltage techniques once used for this purpose. The electrons used to create the X-ray are accelerated to the enormous energies required in a linear accelerator (LINAC).Advantages of MV Therapy- It decreases the damage sustained by the patients skin. The beam is so penetrating that hardly any of its energy is absorbed by the skin and surface tissues.- It reduces damage to the bone.Rotating BeamsThe purpose of radiotherapy is to destroy malignant (i.e Cancer) cells whilst doing as little damage as possible to the healthy tissue and bone around them. One way of achieving this is to aim the beam at the tumor from a n umber of different directions, i.e. to rotate it about the tumor. This technique is known as multiple beam or rotating-beam therapy and produces a considerable cumulative effect at the tumor but a much-reduced effect everywhere else.Treatment PlanningAny amount of radiation is potentially harmful to the person being exposed to it and therefore it is important that the following considerations are taken into account.- The likely benefits of exposure to radiation must outweigh the risks involved.- The radiation does must be the minimum consistent with obtaining good quality images or destroying malignant cells.- It must not be possible to obtain equally useful information by less risky methods.- The beam must be collimated so that only part of the body that needs to be exposed is exposed.- Increasing the PD across an X-ray tube increases the penetrating power of the X-rays produced but it decreases the proportion of the beam that is attenuated by photoelectric absorption, and therefor e decreases the contrast of the X-ray image. The PD should be high enough to produce the required degree of penetration but not so that there is insufficient contrast. Another consequence of increasing the tube voltage is that it increases the energy, and therefore the penetrating power, of the scattered radiation. This increases the likelihood of the patient receiving a significant radiation dose in parts of the body some distance away from the part being exposed intentionally. It also increases the chance of scattered radiation escaping from the patient a potential hazard to hospital staff in the vicinity.- The radiographer must employ good techniques so that repeat exposures are not required. (This is an important consideration when assessing just what is the minimum dose an underestimating would necessitate a second exposure and therefore an overall increase in dose.)A metal filament (usually tungsten) is heated and some of the electrons acquire sufficient thermal energy to es cape from the surface. The higher the temperature the filament is heated to, the greater the number of electrons that are in effect boiled off. This filament forms the negative cathode. These electrons are accelerated across an evacuated X-ray tube towards the positive anode by a large voltage that is applied across the tube.These electrons strike the anode where 99 % of their energy is converted to heat with less than 1 % resulting in X-radiation. One theory, which is very important to the theory behind X-ray, is the Band theory.The continuous background (or bremsstrahlung radiation) is produced by electrons colliding with the target and being decelerated. The energy of the emitted X-ray quantum is equal to the energy lost in the deceleration. An electron may lose any fraction of its energy in this process. The most energetic X-rays (ie. Those whose wavelength is ?min). X-rays with longer wavelengths are the result of electrons losing less than their total energy.The line spectrum is the result of electron transitions within the atoms of the target material. The electrons that bombard the target are very energetic (100keV) and are capable of knocking electrons out of deep-lying energy levels of the target atoms. (this corresponds to removing an electron from an inner orbit on the Bohr model). If an outer electron then falls into one of these vacancies, an X-ray photon is emitted. The wavelength of the X-ray is given by E = hc/?, where E is the difference in energy of the levels involved, c is the speed of the light and h is Plancks constant. Since the energy levels are characteristic of the target atoms so too are the wavelengths of the X-rays produced in this way.For example, when calculating the wavelength of the most energetic X-rays produced by a tube operating at 1.0 * 10^? V (h = 6.6 * 10^-34 Js, e =1.6 * 10^-19 C, c =3.0 * 10^8 ms-1.)The most energetic X-ray are those produced by electrons which lose all their kinetic energy on impact.KE on impact = wo rk done by accelerating PD= 1.6 * 10^-19 * 1.0 * 10^5Maximum KE lost = 1.6 * 10^-14 joulesThe energy of the corresponding X-ray quantum is hc/?min and thereforehc/?min = 1.6 * 10^-14i.e. ?min = (6.6 * 10^-34 * 3.0*10^8)/1.6 * 10^-14= 1.24 * 10^-11 mJust clarify:(max. photon energy) Emax = eV and ?min = hc/eV(where V is the Potential difference)Band theoryIn an isolated atom, the energy of an electron depends mainly on its distance from the nucleus. An electron has a negative charge and a nucleus has a positive charge, and as an electron falls towards a nucleus it loses energy. The energy of one electron in an atom is also affected by the presence of all the other electrons within that atom, since they all have negative charge and so repel one another.If atoms are very close together, as in a solid, then the energy of each electron is affected by the nuclei and electrons of many nearby atoms. This has the effect of smearing out the energy levels into broad bands. The electrons are no t longer restricted to certain well-defined energies; instead, there are broad ranges of allowed energy, and higher energy band is shared between atoms. If an electron has enough energy to be in this band, then it can break free of its parent atom and move through the solid, i,e it can take part in conduction, so this upper band is called the conduction band. If, however, an electron is still bound to its parent atom then it cannot be moved around freely and it is said to be in the valence band. Between the two bands is a range of energies known as the forbidden gap. As shown in the diagram below.The size of the forbidden gap determines whether a given material is a conductor or an insulator. In metals, the conduction and valence bands overlap, so the conduction band always contains electrons, and so metals are good conductors. In insulators, the conduction and valence bands are separated by a large forbidden gap, and the conduction band is virtually empty. To promote an electron fr om the valence band would require a large amount of energy. In a semiconductor there is still a gap, but the range of the forbidden gap is much smaller. If the energy supplied by heating or by allowing the material to absorb photons, then some electrons gain enough energy to cross the gap and enter the conduction band. The more energy supplied the more electrons are promoted, the resistance of many semiconductors fall with increasing temperature.The X-rays emitted from an X-ray tube have a range of energies, which is called the X-ray spectrum. These X-rays are produced by two different mechanisms, which are distinctive in the resulting spectrum.Continuous spectrum (approx 80% of the output)The electrons pass close to the positive nuclei of the target atoms and are slowed down. he kinetic energy that they lose is converted into photons of electromagnetic radiation which have a continuous range of energies up to a maximum value equal to the tube voltage applied.Continuous spectrum A continuous range of photon energies (up to a maximum) is produced as electrons are decelerated in the target.Characteristic spectrumSome of the electrons penetrate deep into the target atoms, ejecting orbital electrons from the innermost shells near the nuclei. Orbiting electrons from outer orbits fill gaps in the inner shells and emit photons that are characteristic of the target atom. As long as the target has a high enough atomic number, the resulting photon will be the X-ray range.Tungsten is used as the target material for nearly all X-rays tubes because it has a high atomic number and so yields high X-ray outputs and a high melting point to withstand the large amount of heat produced. The characteristic K line spectrum produced bt Rungsten is about 709 keV, which is a tube voltage used for many exposures.Line SpectrumA limited of precise characteristic photon energies is generated through electron transitions to the K and L shells.Focal Spot sizeIdeally the X-rays produced wo uld originate from a point source, which would result in a clear shadow being formed with sharp edges (like a light image formed in a pin hole camera). This is not practical because of the great amount of heat being produced by the X-ray tube. If all the electrons were directed at one tiny point, the target would melt. The target is therefore designed to increase the actual target focal spot while keeping the projected focal spot as small as possible, thus keeping the amount of geometric blur to a minimum. This is achieved by using a disc shaped anode with the target area shapely angled and spinning throughout the exposure.Positioning the film as close as possible to the patient also reduces geometric blur.So how do we select exposure?To see why different exposures are selected for different areas, the methods by which an X-ray beam is attenuated must be studied.Simple scatterThe incident photon energy does not have sufficient energy to remove an electron from its atom so it is simp ly deflected from its course without loss of energy. This scatter is proportional to the square of the atomic number.Photoelectric EffectThe incident photon gives up all of its energy to an inner orbital electron, ejecting the electron from its atom. Some of the energy of the electron is used in overcoming the binding energy of the electron. The vacancy in the shell will be filled by electrons in orbital further from the nucleus, producing characteristic radiation, which in the case of the X-ray photons interacting with the body tissue are small and in the infer-red part of the EM spectrum. Photoelectric attenuation is most useful to diagnostic imaging because the photoelectric absorption is proportional to the cube of the atomic number.Compton scatterThe energy of the photon is much greater than the binding energy of the electron. Only part of its energy is given during the interaction with the outer electron. The photon continues in a different direction with reduced energy and th e electron dissipates its energy through ionization. It is independent of the atomic number.Simple is a problem as it reduces image sharpness and increases low-energy absorption in the patients skin. Photoelectric absorption is useful because, as it is proportional to the cube of the atomic number of the absorbing material, it produces contrast on the image. Material of high atomic number absorb more X-rays by this method and those of low atomic number absorb less. As the tube voltage is increased photoelectric attenuation tends to fall and Compton scatter to rise in importance, and as Compton scatter is not proportional to the atomic number of the absorbing material, the contrast is reduced.How do we reduce scatter?- Reduce the beam size by using an adjustable diaphragm, this reduces the field size and thus reduces the random scatter produced.- Use filtration on the X-ray beam to reduce the lower energy photons that result in simple scatter, allowing photoelectric attenuation to do minate.- Use of compression to make the part X-rayed thinner.So what is the radiographer doing?When taking a simple X-ray, the radiographer must:- Choose the tube voltage- this is usually in the range 60-120kVMore kV increases maximum photon energyIncreases average photon energyIncreases total intensityEffect of changing tube voltage on resulting X-ray spectrum.* Choose the tube current this depends on the number of electrons crossing the tube per second. This is controlled by the filament current, which determines the rate at which electrons are emitted from the cathode.Increasing tube current increases the overall intensity but does not increase the maximum photon energy or change the shape of the spectrum.Increases the blackening of the filmIncreases the absorbed dose by the patient.Increases the heating of the target.Effect of the tube current on X-ray spectrum* Choose the expose timeIncreasing the exposure timeIncreases the blackening of the filmIncreases the patient absorbed doseIncreases the chances of movement blur* Choose the focal spot sizeIncreasing the focal spot size increases the tube current and the voltage that can be applied but reduces geometric unsharpness.How do we stop scatter from getting to the film?We use a grid formed by rows of lead strips stops the scattered radiation from reaching the film. To get rid of the grid lines on the image, the grid may be moved during the exposure.We can also use an air gap to reduce the scatter reaching the film.And what about the film?If we were just to use film just to record the X-ray that were transmitted through the patient we would have to use very large exposures because up to 97% of X-rays pass straight through the film without affecting it at all. We therefore use special cassettes that contain fluorescent screens. These fluorescent screens absorb the X-radiation and re-emit visible radiation in a pattern that is the same as that of the original X-ray beam.Construction of the Rotating Anode Tube InsertFluoroscopy:In X-ray fluoroscopy, X-rays are passed through the patient and onto a fluorescent screen to produce an immediate visible image. This has the advantage over photographic film in that it allows dynamic processes. Sadly, unacceptably high X-ray intensities would be needed to produce image that could be viewed directly. A device known as an image intensifier can increase the brightness by a factor of over a thousand and allows the radiation dose to be cut by up to 90% of the unintensified level. The intensifier has a fluorescent screen in contact with a photocathode. This combination converts the X-rays first to visible photons and then to electrons. The number of electrons at any point on the photocathode is directly proportional to the X-ray intensity transmitted by the patient.The electrons produced by the cathode are then accelerated through a potential difference of about 20 kV, using a series of focusing anodes, towards a second fluorescent screen. The increase d energy and concentration of the electrons creates an image that is very brighter than that on the first screen, and which is usually picked up by a TV camera and fed to a TV monitor or video recorder.Fluoroscopy is a technique that is used sparingly, for despite image intensification, the radiation dose to the patient is still significantly higher than that I a standard radiographic examination. Dose savings can be made by using short bursts of X-rays rather than a truly continuous exposure, but even so the examinations remain relatively dose-intensive.Bibliography:For my report the sources which I used were; the textbook Physics A-level by Roger Muncaster, also the textbook Salters Horners Advanced Physics, a leaflet given to me by the Ashford hospital and finally my own knowledge.Conclusion:You can obviously tell that the visit to the hospital was very informative and helpful. Funnily enough at the time I was writing this report I had broken my wrist and was treated at the Ealin g hospital. On my last visit, to take the full plaster cast off I asked the radiographer if she could give me copy of my wrist, but unfortunately she said that it had to go in report. Instead she let me get my hands on some unwanted X-rays, which was the next best thing, so included them in my report.