Page 523 - Physics Coursebook 2015 (A level)

Chapter 32: Medical imaging
Improving X-ray images
The X-ray systems in use in hospitals and clinics today are highly developed pieces of technology. They do not simply show bones against a background of soft tissue. They can also show very fine detail in the soft tissue, including the arrangement of blood vessels.
Radiographers (the people in charge of X-ray systems) have three main aims:
■■ to reduce as much as possible the patient’s exposure to harmful X-rays
■■ to improve the sharpness of the images, so that finer details can be resolved
■■ to improve the contrast of the image, so that the different tissues under investigation show up clearly in the image.
Reducing dosage
X-rays, like all ionising radiation, can damage living tissue, causing mutations which can lead to the growth of cancerous tissue. It is therefore important that the dosage is kept to a minimum.
A radiographer may choose to record the X-ray image on film or digitally. X-rays are only weakly absorbed
by photographic film, so, historically, patients had to
be exposed to long and intense doses of X-rays. Today, intensifier screens are used. These are sheets of a
material that contains a phosphor, a substance that emits visible light when it absorbs X-ray photons. The film is sandwiched between two intensifier screens. Each X-ray photon absorbed results in several thousand light photons, which then blacken the film. This reduces the patient’s exposure by a factor of 100–500.
In digital systems, image intensifiers are used (Figure 32.7). The incoming X-rays strike a phosphor screen, producing visible light photons. These then release electrons (by the photoelectric effect) from the photocathode. The electrons are accelerated and focused by the positively charged anode so that they strike a screen, which then gives out visible light. The image on
this screen can be viewed via a television camera. At the same time, the image can be stored electronically. Digital systems have the advantage that images can be easily stored, shared and viewed.
Image intensifiers are particularly useful in a technique called fluoroscopy. A continuous X-ray beam is passed through the patient onto a fluorescent screen where a real- time image is formed. Using an image intensifier ensures that the patient is not exposed to dangerous levels of X-rays over a long period.
Improving sharpness
Figure 32.8 shows a remarkably sharp X-ray image of blood vessels in the human abdomen. The sharpness of the image is determined by the width of the X-ray beam. You will remember that the shadow of an object is much sharper if it is illuminated by a small lamp, rather than
a large lamp (Figure 32.9). So a good X-ray source must produce a narrow beam of parallel X-rays, as if they were coming from a distant point source.
Figure 32.8 An X-ray image of blood vessels branching out from an artery carrying oxygenated blood to the intestines.
Three factors determine the width of the X-ray beam:
■■ the width of the electron beam and the target it strikes – as shown in Figure 32.10, the wider the electron beam, the wider the X-ray beam
■■ the size of the aperture at the exit window – this can be reduced using adjustable lead plates (Figure 32.11)
■■ collimation of the beam – the beam is passed through lead slits (Figure 32.12), ensuring that it is an approximately parallel-sided beam and does not fan out.
photocathode
X-rays
input phosphor
Figure 32.7 An X-ray image intensifier.
focusing anode
output phosphor
electrons
vacuum
electrodes
511