AQA Physics Unit 3 notes P3

PHYSICS UNIT 3

Medical Applications of Physics

X-rays

X-rays are part of the electromagnetic spectrum. They have a very short wavelength which are about the same as the diameter of an atom.

They are produced in an X-ray tube when fast moving electrons hit a target. They can be used to make a radiograph, X-ray photograph, in a hospital by directing X-rays at a part of a patient’s body under investigation. A photographic film or flat-panel detector is placed on the other side of the patient. They affect photographic film in the same way as light.

A flat-panel detector is a screen that contains a charge-coupled device (CCD). The sensors in the CCD are covered by a layer of a substance that converts X-rays to light. The light rays then create electronic signals in the sensors that are sent to a computer to display a digital radiograph.

X-rays are transmitted, pass through, soft tissue but are absorbed by bones, teeth and thick metal. The part of the film that X-rays reach become dark, so the bones appear white. This shows a ‘negative image’ of the bones. A hole or cavity in a tooth shows up as a dark area in a bright image of a tooth.

An organ that consists of soft tissue, can be seen by being filled with a substance called a contrast medium which absorbs X-rays easily.

Lead plates between the tube and the patient stop X-rays reaching other parts of the body. Lead is a good absorber of X-rays and so rays pass through the gaps between the plates.

X-rays can cause ionisation, which is dangerous. High doses kill living cells whereas low doses can cause cell mutation and cancerous growth. There is no evidence of a safe limit where living cells won’t be damaged. Because of this, precautions must be taken when working with X-radiation. They wear lead aprons, stand behind a lead screen, or leave the room while scans are being done.

As well as diagnose medical conditions like bone fractures and dental problems, X-rays can be used as treatment to destroy cancerous tumours. Thick body plates between X-ray tubes and the body stop X-rays reaching healthy body tissues. A gap between the tubes allows X-rays through to reach the tumour. These X-rays have a shorter wavelength than those used for radiographs. http://www.fda.gov/ucm/groups/fdagov-public/documents/image/ucm115328.gif

X-rays are also found in a computerised tomography scanner (CT scanner). It can be used to produce a digital, high-resolution 2D or 3D image of the body. CT scans can distinguish the different types of tissues unlike an ordinary X-ray machine. However, they do cost more and give a much higher radiation dose than an ordinary X-ray machine.

Ultrasound

The range of a human hearing is about 20Hz to 20,000Hz. Sound waves exceeding the highest frequency that humans can hear are called ultrasound waves. Electronic systems can be used to produce ultrasound waves as a deterrent for animals.

Ultrasound waves can be used in medicine for prenatal scans of a baby in

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