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A Visit to Ashford Hospital

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A VISIT TO ASHFORD HOSPITAL 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). ...read more.


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 from 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. \the 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. ...read more.


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 Insert Fluoroscopy: 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 increased 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. ...read more.

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