IB HL Medical Imaging – X-rays

X-rays (short wavelength, high energy EM radiation) are produced when charged particles like electrons are accelerated around nuclei. Since they are negatively charged, they are accelerated towards a positive nucleus, emitting as they curve around a ‘braking radiation’ or‘bremsstrahlung’ in German. The degree of ‘bend’ determines the energy of the X rays produced, so the radiation emitted is over a continuum of wavelengths .

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Bremsstrahlung

Additionally, the electrons may directly promote other electrons in the metal atoms of the target from its lower to higher energy levels- when these decay back  they emit characteristic X-photons. Here two characteristic jumps are superposed on the bremsstrahlung background. The relative intensity is a measure of how likely this event is.

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Characteristic X peaks for Mo. Notice the short wavelength cut-off corresponding to the electron delivering all its energy to forming an X-ray.

 

We won’t go into details here, except the only real difference between these and gamma rays is that gamma rays are emitted spontaneously from excited nuclei.

X-rays pass through human tissue, being selectively absorbed by denser material. In 1901, Wilhelm Roentgen was the first person ever to win the Nobel Prize for Physics and his discovery revolutionised the medical world.

Some schools have a small version of one of these.Screen Shot 2014-04-15 at 10.49.37 AM

Electrons are accelerated in a vacuum towards a metal target, W in this case.  X rays are produced – notice they are not subject to a ‘law of reflection’ – and pass through and out of a collection window for use. At diagnostic voltages (140KV for a chest X-ray, lower for dental use) the anode gets very hot and has to be cooled, the target is often rotated otherwise the heat generated would destroy it. The X rays are partially absorbed by the area of interest in the patient and the data is collected on photographic film as a negative image where high absorption is seen as a light area and vice versa.

The mechanism for energy loss in the body is photoelectric.  Since this effect is strongly dependent upon Z, there is substantial difference between Z(bone), about 14 and Z(soft tissue) about 7, so X rays are good at contrasting bone and soft tissue but not very good at contrasting between different soft tissues since their Z values are too similar.

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Increasing the sharpness of the shadow gives better diagnostic information. We can’t ‘focus’ X-rays  using a glass lens like we can with visible light so we have to arrange for rays normal to the film only to fall on its surface since scattered rays will blur the image. There is a section on p703-4  which explains how this is achieved by  filtering and lead grids – a possible exam question.

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