That X-Rays Can Damage Cells Or Cause Cancer by Ionisation Processes
Students should learn:
- that X-rays are used to produce images of fractured bones or, in conjunction with contrast media, organs
- that X-rays can damage cells or cause cancer by ionisation processes
- that the denser a material is, the greater the absorption of X-rays as they pass through it
- that a CT scanner uses a series of X-ray images to construct a three dimensional image of the body.
- describe how X-rays pass through some materials but not others and this can be used to see the internal structure of some objects
- describe the effects of X-rays on living cells
- describe the differences between a CT scan and an X-ray photograph.
- explain how X-rays damage cells.
X-rays can be used to diagnose and treat some medical conditions. [P3.1.1 b)]
Precautions to be taken when X-ray machines and CT scanners are in use. [P3.1.1 c)]
Evaluate the advantages and disadvantages of using … X-rays and computerised tomography (CT) scans. [P3.1] / Chapter map: Medical applications of physics
Bump up your grade: The X-Ray factor!
Lesson structure / Support, Extend and Practical notes
Starters
What’s up Doc? – Show the students a range of X-ray photographs and ask them to describe the problems they see. If you don’t have originals, then you can find some obvious ones on the internet. Start with obvious fractures; the students should already know that this is what an X-ray is typically for, but then move on to dental X-rays (wired jaws are good) and even contrast medium based images such as a barium meal. (5 minutes)
Definitions – Show the students the key words for this spread, ‘absorb, reflect, emit’, and ask them to write down their own definitions. These can include diagrams. Students can be extended by insisting on clear descriptions and demonstrating that they understand that electromagnetic radiation can be partially reflected by surfaces or partially absorbed as they pass through materials; a vital concept for this topic. You may support students by providing completed, or partially completed, diagrams reminding them of the processes. (10 minutes)
Main
- Start by discussing X-rays using as many images as you can. The images are negative; the areas on the film image that are black have been exposed to X-rays while the white areas have not. This shows that the X-rays have penetrated the soft tissues but have been absorbed by bones.
- This can lead to a discussion of why X-rays are harmful; the energy is absorbed in the body and damages cells, particularly cells in bones. You can also discuss the reason X-rays are absorbed by bone; it contains materials that are denser, particularly calcium. This absorption by metals can be explained by using X-rays of fillings where the metal absorbs virtually all of the X-rays. X-rays of plates and screws in legs always fascinate.
- You can now discuss why simple X-rays are not useful in studying organs; the X-rays will pass through and there will be no contrast. This leads directly to the idea that you can add a ‘dense’ material that passes through one organ and so you can image the organ. Discuss the properties of the contrast medium; it must pass into the correct organ, must not be too poisonous and must be opaque to X-rays. When talking about the safety precautions, you might ask the students what a radiographer, dentist or doctor does when taking an X-ray. They stand behind shields of lead or even move out of the room. You can discuss why a radiologist has to do this but the patient does not; the radiologist operates the machine many times each day and the dose would be large without the protection.
- In X-ray therapy you can describe the destruction of cells by higher intensity beams of X-rays. The energy is absorbed more easily because the X-rays are lower frequency (lower energy) instead of mostly passing through the body. This links back to the energy of electromagnetic waves.
- Students may have seen CT scanners in hospitals or at least on television. The function is fairly straightforward; a model is built up from dozens of images taken from different angles. The dose given by a CT scanner is therefore much larger than a single X-ray. This is one of the reasons why they are not used routinely to scan for simple injuries such as a broken arm.
Radiation danger – Give the students the hazard symbol for ionising radiation and explain what it is supposed to represent. The students should add a list of safety precautions that might be taken in a radiology department to the symbol; perhaps as icons below it. (5 minutes)
X-ray safety – Ask the students to describe safety procedures, such as film badges and lead-lined garments, that need to be used by patients, dentists or doctors to reduce or monitor exposure to X-rays or gamma rays. Extend students by asking them to discuss the reasons in details and the consequences of failing to follow the instructions. These should include the reasons the precautions have to be taken and why it is safe for a patient to receive a dose but not for the doctor. You can also support students by getting them to match up the precaution with the reason why it is taken. (10 minutes) / Support
Present each student with a photocopy of an X-ray photograph (or a simplified one) so that they can label the areas where the X-rays pass through or are absorbed.
Extend
In order to get a clear image while reducing the exposure to X-rays, image enhancements techniques are used. You could discuss the reasons behind these techniques along with the techniques themselves, including the design of the cassette and the shielding used on the machines, patients and doctors.
Course / Subject / Topic / Pages
Physics / Physics / P3 1.1 X-rays / Pages 208–209
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
- that ultrasound is sound with a frequency above 20 000 Hz and cannot be heard by humans
- that ultrasonic waves are reflected by the different layers of tissue and fluid in a body and so can be used to make measurements or produce images of internal organs
- that ultrasound does not cause ionisation and so it much safer than X-rays.
- compare ultrasound to audible sound waves
- explain how ultrasound can be used for medical scanning and the advantages of ultrasound over X-ray techniques
- work out the distance between interfaces.
- rearrange the s = v × t equation to solve problems. [HT only]
Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is. [P3.1.2 b)]
Calculation of the distance between interfaces in various media. s = v × t . [P3.1.2 c)]
Ultrasound waves can be used in medicine. [P3.1.2 d)]
Evaluate the advantages and disadvantages of using ultrasound, X-rays … [P3.1]
Compare the medical use of ultrasound and X-rays. [P3.1] / WebQuest: Ultrasound or X-Rays?
Viewpoint: Should the government invest in better prenatal scanning devices?
Animation: Ultrasound imaging
Support: Ultrasound
Lesson structure / Support, Extend and Practical notes
Starters
Mystery scans – Show the students some ultrasound scans of things other than a foetus, and see if they can identify the organs involved. Some students will not know the shapes of even the most major organs as they seldom look like the simple diagrams used in many books. (5 minutes)
Echo – Ask the students to work out how far away a cliff face is if, when you shout, the echo takes 4 seconds to arrive and the speed of sound is 330 m/s. Extend the students by asking them to perform the same type of calculation for a sound wave travelling in water where the speed of sound is 1000 m/s; this should help them to understand that the ultrasound pulses in the lesson travel at different speeds in different materials. To support students, you can provide a calculation frame including a diagram. (10 minutes)
Main
- Start with a basic recap of what an echo is, then lead on to the idea that if we time the echo, we can often work out how far away the reflecting surface is. You should be able to find suitable video clips of an ultrasound scan taking place. Most often these are B-scans that actually create images but it is useful to show that the transducer has to be placed in close contact with the skin or organ using a gel to improve the contact.
- When discussing ultrasonic scanners, the students need to know that certain tissues reflect different amounts of the signal and this is how they can be told apart. The boundaries between the materials are important; when there is a big change of material such as from muscle to bone, there is a larger reflection. The size of the reflections can be used to work out what the materials are; e.g. fat, muscle, bone.
- The thickness of materials is judged from the time it takes the sound wave to pass through them. You might want to extend students into performing calculations based on timing traces; see the extension material.
- Try to find some images other than the traditional baby scans, e.g. kidney and heart scans, to show that there is more than one use for the technique.
- The students may have to be reminded what ionisation is and why it is harmful when discussing the relative safety of ultrasound compared to X-rays. The example of foetal ultrasound scans is the easiest one to use.
- When covering ultrasound therapy, you can demonstrate sound waves being used to vibrate objects by placing a loudspeaker next to a glass of water and using the vibrations to create patterns on the surface. Adjusting the frequency might even lead to resonance, making larger waves. Explain that these tiny vibrations can be used to ‘shatter’ an object when the frequency is matched to the properties of the object.
Comparing light and sound – Make a detailed comparison of light and sound waves. This should include the nature of the waves (mechanical, electromagnetic) and their interaction with matter. Extend students by asking detailed examples and comparisons such as why X-rays are ionising while sound waves are not. Provide partially completed mind maps to support students.
(10 minutes) / Support
Students can be given a cloze activity to summarise the uses of ultrasound in medical physics. This should include details of the frequency ranges used and the techniques.
Extend
Some of the students can be stretched by analysing an oscilloscope trace to determine distances. They can be given the speed of sound in materials and then read the time taken for sound to pass through the material from the trace and so determine the thickness. They will need to realise that the sound passes through the material twice (there and back again) and take this into account when working out distance travelled.
Practical support
Demonstrating the range of hearing
Using a signal generator, you can demonstrate the range of human hearing. Simply connect it to a suitable loudspeaker and gradually increase the frequency until the students cannot hear the sound anymore. Connect the signal generator to a CRO to show the changes in the waveform as the frequency is increased. The best way to measure the upper limit of the student’s hearing is to get them to keep silent, raise their hand and then put it down again when they can no longer hear the sound and write down the frequency. You can also decrease the frequency slowly and ask the students to raise their hands when they can hear the sound again. You can then improve the precision of the measurements by calculating the mean of these two values and perhaps finding an overall class mean.
Demonstrating the range of hearing
Using a signal generator, you can demonstrate the range of human hearing. Simply connect it to a suitable loudspeaker and gradually increase the frequency until the students cannot hear the sound anymore. Connect the signal generator to a CRO to show the changes in the waveform as the frequency is increased. The best way to measure the upper limit of the student’s hearing is to get them to keep silent, raise their hand and then put it down again when they can no longer hear the sound and write down the frequency. You can also decrease the frequency slowly and ask the students to raise their hands when they can hear the sound again. You can then improve the precision of the measurements by calculating the mean of these two values and perhaps finding an overall class mean.
Course / Subject / Topic / Pages
Physics / Physics / P3 1.2 Ultrasound / Pages 210–211
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
- that the refractive index of a substance is the ratio of the sine of the angle of incidence to the sine of the angle of refraction as a ray enters the material
- how to measure the refractive index of a material.
- describe an experiment demonstrating refraction of light in a glass block
- construct accurate ray diagrams showing refraction in a range of situations
- calculate the refractive index of a material from the angles of incidence and refraction.
- calculate a second angle from one angle and refractive index [HT only].
… Refractive index =
[P3.1.3 c)]
Controlled Assessment: P4.3 Collect primary and secondary data. [P4.3.2 a) b) c) d)]; P4.4 Select and process primary and secondary data. [P4.4.2 a) b) c)] / Data handling skills: Investigating refraction using light
Practical: Measurement of refractive index
Extension: Refraction. Why a pool never looks as deep as it really is!
Maths skills: Refraction
How science works: Spectacle lens options
Lesson structure / Support, Extend and Practical notes
Starters
Refraction demonstrations – Start by demonstrating some effects of refraction to recap the behaviour of light. Place a pencil in a glass of water and note the apparent ‘breaking’. Place a coin at the bottom of a cup so it can’t be seen and then add water until the coin appears. You can use a video camera to show these things to a whole class more clearly. (5 minutes)
Optical preparation – Let the students demonstrate their skills using ray boxes, rulers and mirrors. They should set up the equipment for the later practical along with some mirrors to demonstrate the law of reflection. The students should show how to set up the equipment and record the data clearly. Support students by providing some of the instructions or extend them by asking them to collect enough evidence to establish the precision and certainty of measurements. (10 minutes)
Main
- The students should have encountered refraction in KS3, but the initial experiment can be used to revisit the basics and to go much further into Snell’s law.
- Start with the practical task giving the students as much instruction as they need. The focus should be on collecting accurate evidence as you are attempting to verify an established law of physics using the data. Demonstrate the technique and insist that the students try to measure the angle as accurately as possible.
- To measure the angle, the students need to draw the ray lines with a ruler; use the cross-drawing technique but make sure that the students are drawing the crosses far enough apart to ensure that lines are straight and reach the point of refraction. They should number each incident line and pair it up with a refracted line so that they do not get confused with the large number of lines.
- The results should be straightforward for the basic experiment; students then move on to verifying Snell’s law and this involves finding the sine of the measured angles. Take time in showing these calculations and be methodical in completing the table. The results of the calculation will show some variation, so you can discuss the causes of this: experimental errors (mainly inaccuracy in measuring angles) and some rounding errors during calculations. There should be a fairly close fit if the students have been careful.
- You can now discuss the measured property; the refractive index; most groups should have the same value (or near enough). If you provide one group with a different material (Perspex instead of glass) then they will have a different answer at this stage, giving you the opportunity to discuss the fact that different materials have different refractive indices.
- Calculations are essential here, so use the worked example and a couple of others. For some students, it may be difficult to perform calculations including sines; check with the mathematics department to see how they are used there.
- If you did not let the students explore the effect when the ray leaves the block, then demonstrate that refraction also occurs when the ray leaves the block.
Comparing light and sound – The students should make a comparison of light and sound waves. (5 minutes)
Maths error – Calculations are essential in this topic. Use the last part of the lesson to support students by going through a range of examples of refraction calculations (calculations of n from two angles are needed by all students; Higher Tier students should be able to rearrange the equation). Extend the Higher Tier students with this calculation: A ray of light in glass (refractive index 1.52) hits the back surface at an angle of 60°, what is the angle of refraction as it leaves the block? They should find that the calculation gives a maths error on their calculators, so demonstrate what happens in reality; they should see that the ray does not refract; it reflects inside the glass block. This will be a good starting point for the next lesson. (10 minutes) / Support