Unit 1 Physics - Semester 1 Andrew Simmons 11B
Medical Physics - X-Rays
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X-Rays in Medical Diagnosis

The nature of X-rays and the range of frequencies used

X-rays are a form of ionising electromagnetic radiation and have a very high frequency and a very short wavelength. Their wavelengths range between 0.001 to 10 nm.

The technology used in X-ray production

[Image credit: Jacaranda Physics 1 2nd Edition © John Wiley & Sons, Inc.]

An X-ray tube works as follows:

  • The heated filament is positively charged and the tungsten target is negative.
  • Electrons are emitted from the heated filament towards the tungsten target due to the very high potential difference between them.
  • The tungsten target absorbs the electrons and releases some of the energy in the form of X-rays.
This process is very inefficient however and a lot of energy is released in heat. For this reason the tungsten target has a copper mounting because it conducts heat and is cooled with by circulating oil through the mount. Spinning the tungsten target at high speed also helps to stop it overheating.

[Image credit: Jacaranda Physics 1 2nd Edition © John Wiley & Sons, Inc.]

Narrower beams of X-rays will produce a sharper image. The tungsten target is therefore angled so that a wide beam of electrons will produce a narrow beam of X-rays.

The clinical application of X-rays to form images

Hard and soft X-rays

Hard X-rays are X-rays with a higher frequency and are more penetrating than soft X-rays. Soft X-rays are usually filtered when doing a scan because they can't penetrate through a patient's body and add needless risk of radiation damage.


Attenuation is a measure of how much something absorbs X-rays. The amount of attenuation increases with atomic density (number of protons in the nuclei). For example, bones have a higher attenuation than soft tissue and therefore bones produce a dark shadow when X-rayed where as soft tissue appears much fainter.


When a patient has an X-ray, they are usually scanned at a frequency of approximately 7×108 Hz because body tissues absorb this frequency the best.

Use of X-rays in various parts of the body

X-rays are best suited to imaging bones and have a very high resolution. For imaging soft tissue however, there is very little contrast and so a contrast medium is needed. Contrast mediums are substances given to the patient that absorb X-rays and produce an image of the area under investigation when X-rayed. Usually CAT (Computer Axial Tomography) scans, in which a series of X-rays are taken from various angles and interpreted by a computer, are better for imaging soft tissue.

Examples of images attained using X-ray technology

This is an X-ray of a broken leg and shows that a bone has been broken.

This X-ray shows that the lung on the right is damaged by tuberculosis.

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This is a secondary school project (use with caution). See bibliography for references and image credits.