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).

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 Beams

The 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 number 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 Planning

Any 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 therefore 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.
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- 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 escape 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 ...

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