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13.5 X-rays in Medicine

GCSE Physics · AQA · Chapter 13: Electromagnetic Waves

A free GCSE Physics lesson for AQA (8463) from OBA School: x-ray images, contrast media, CT scans, radiation dose and X-ray therapy. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain how an X-ray image is made and why bones appear white
  • Describe how contrast media and CT scans let doctors see soft tissues
  • Explain what is meant by radiation dose and convert between sieverts and millisieverts
  • Describe how patients and hospital staff are protected from X-rays
  • (Higher tier) explain why X-rays for therapy differ from X-rays for imaging

Key words

  • Radiograph: An X-ray image (photograph). Dense parts that absorb X-rays appear white.
  • Contrast medium: A substance that absorbs X-rays well, swallowed or injected so a soft-tissue organ shows up, e.g. a barium meal.
  • Flat-panel detector: An electronic detector that turns X-rays into light and then into electrical signals, so a computer can show the image.
  • CT scan: Computed tomography: X-ray images taken from many angles are combined by a computer into slices or a 3D image.
  • Radiation dose: A measure of the risk of harm to the body from ionising radiation. Unit: sievert (Sv).
  • Millisievert (mSv): One thousandth of a sievert: 1 Sv = 1000 mSv.

Taking an X-ray image

X-rays are made in an X-ray tube when fast-moving electrons hit a metal target. Their wavelength is about the size of an atom (around 10⁻¹⁰ m). To take a radiograph:

  • The part of the body is placed between the X-ray tube and a detector.
  • Lead plates with a gap between them let X-rays reach only the area being examined; lead is a very good absorber.
  • X-rays pass through soft tissue but are absorbed by bones, teeth and metal objects.
  • Where X-rays reach the detector it turns dark. Behind bones fewer X-rays arrive, so bones appear white. A break or a hole in a tooth lets X-rays through, so it shows as a dark line or area.

A photographic film must be kept in a lightproof cassette, because ordinary light would also darken it. Most hospitals now use a flat-panel detector: its sensors (a charge-coupled device, CCD) change the X-rays into light and then into electronic signals, and a computer displays a digital image straight away.

Seeing soft tissue: contrast media and CT scans

Soft organs such as the stomach and intestines absorb about as much as the tissue around them, so they barely show. The patient swallows a contrast medium – for example a barium meal – which is a good absorber of X-rays. It coats the inside of the organ, so its shape and any ulcer or blockage show up clearly.

In a CT scan the X-ray tube rotates around the patient and detectors on the other side record the X-rays from many directions. A computer combines these readings into cross-section “slices” or a 3D image, which can show soft tissues such as the brain in detail. A CT scan gives a much bigger dose than a single X-ray image.

Radiation dose

X-rays, gamma rays and the alpha and beta radiation from radioactive substances are all ionising. The radiation dose measures the risk of harm to the body. It depends on:

  • the type of radiation (inside the body, alpha does far more damage than the same energy of X-rays)
  • how long the body is exposed
  • the energy per second absorbed by the body.

Dose is measured in sieverts (Sv). Doses are usually tiny, so millisieverts (mSv) are used.

1 Sv = 1000 mSv sieverts → millisieverts: × 1000 millisieverts → sieverts: ÷ 1000

Worked example: comparing an X-ray with background radiation

A chest X-ray gives about 0.02 mSv. The UK average background dose is about 2.7 mSv per year. How many days of background radiation is this?
background per day = 2.7 ÷ 365 = 0.0074 mSv
days = 0.02 ÷ 0.0074 = 2.7 days

Answer: about 3 days of background radiation

High doses kill cells; low doses can cause mutations and cancer. There is no dose known to be completely safe, so doctors only use X-rays when the benefit of the diagnosis outweighs the small risk. Everyone also receives background radiation, for example from radon gas from the ground and cosmic rays from space.

Safety: Patients: lead plates or aprons shield the parts not being examined, and the exposure is kept as short as possible. Staff: the radiographer stands behind a lead screen or leaves the room while the tube is on and wears a film badge to record the dose received.

X-ray therapy (Higher tier)

X-rays can also destroy tumours. Thick lead plates with a gap between them let a beam through to the tumour only. X-rays for therapy have a shorter wavelength and carry much more energy than X-rays for imaging.

  • Imaging uses lower-energy X-rays: these are absorbed by bones and teeth but pass through soft tissue and through cracks, which gives a clear image. They do not carry enough energy to destroy a tumour.
  • Therapy uses higher-energy X-rays that deposit enough energy in the tumour to kill its cells. The beam is aimed from several directions so healthy tissue gets less.

Link: Ultrasound, not X-rays, is used to scan an unborn baby: ultrasound is not ionising, so it does not risk damaging the fast-dividing cells of the baby.

Common mistakes

  • Saying bones show up because they “reflect” X-rays. Bones absorb X-rays, so less reaches the detector behind them.
  • Writing that bones look dark. On a radiograph bones are white; the areas where X-rays got through are dark.
  • Getting the conversion upside down: 0.5 Sv is 500 mSv, not 0.0005 mSv.
  • Saying a contrast medium is radioactive. It just absorbs X-rays well.
  • Thinking very small doses are completely safe. There is no known safe limit, only a smaller risk.

Revision questions

Try these, then check your answers below each one.

Question 1. Why do bones appear white on an X-ray image? (Choose from: Bones absorb X-rays, so fewer reach the detector behind them, Bones reflect X-rays back to the tube, Bones emit light when X-rays hit them, X-rays travel faster through bones.)

Answer: Bones absorb X-rays, so fewer reach the detector behind them

Working: Bones absorb X-rays. The detector behind them receives few X-rays and stays light, while soft tissue lets X-rays through and appears dark.

Question 2. Convert 0.45 Sv into millisieverts.

Answer: 450 mSv

Working: 0.45 × 1000 = 450 mSv.

Question 3. A patient swallows a substance before an X-ray of the stomach. What is this substance? (Choose from: A barium compound, A lead compound, A radioactive tracer, Water.)

Answer: A barium compound

Working: A barium meal is a contrast medium: it absorbs X-rays well, so the stomach shows up on the image.

Related lessons

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This lesson is section 13.5 (AGCPH1305) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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