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- Curriculum Content: A clear outline of the content covered by the delivery guide
- Thinking Conceptually: Expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject
- Thinking Contextually: A range of suggested teaching activities using a variety of themes so that different activities can be selected which best suit particular classes, learning styles or teaching approaches.
Content (from A level)
6.5 Medical imaging: This section provides knowledge and understanding of X-rays, CAT scans, PET scans and ultrasound scans. This section shows how the developments in medical imaging have led to a number of valuable non-invasive techniques used in hospitals. Not all hospitals in this country are equipped with complex scanners. Learners have the chance to discuss the ethical issues in the treatment of humans and the ways in which society uses science to inform decision making.
6.5.1 Using X-rays
(a) Basic structure of an X-ray tube; components – heater (cathode), anode, target metal and high voltage supply
(b) Production of X-ray photons from an X-ray tube
(c) X-ray attenuation mechanisms; simple scatter, photoelectric effect, Compton effect and pair production
(d) Attenuation of X-rays; I=I0 e-μx , where μ is the attenuation (absorption) coefficient
(e) X-ray imaging with contrast media; barium and iodine
(f) Computerised axial tomography (CAT) scanning; components – rotating X-tube producing a thin fan-shaped X-ray beam, ring of detectors, computer software and display
(g) Advantages of a CAT scan over an X-ray image.
6.5.2 Diagnostic methods in medicine
(a) Medical tracers; technetium–99m and fluorine–18
(b) Gamma camera; components – collimator, scintillator, photomultiplier tubes, computer and display; formation of image
(c) Diagnosis using gamma camera
(d) positron emission tomography (PET) scanner; annihilation of positron–electron pairs; formation of image
(e) Diagnosis using PET scanning.
6.5.3 Using ultrasound
(a) Ultrasound; longitudinal wave with frequency greater than 20 kHz
(b) Piezoelectric effect; ultrasound transducer as a device that emits and receives ultrasound
(c) Ultrasound A-scan and B-scan
(d) acoustic impedance of a medium; Z =ρc
(e) Reflection of ultrasound at a boundary; Ir/Io = (Z2 – Z1)/ (Z2 + Z1)
(f) Impedance (acoustic) matching; special gel used in ultrasound scanning
(g) Doppler effect in ultrasound; speed of blood in the patient; Δf/f = 2vcosθ/c for determining the speed v of blood.
Approaches to teaching the content
This topic can be approached from various starting points. Students will have a variety of prior experiences of the content depending on the GCSE science courses they studied. Teachers may choose differing paths between the topics or within the topics themselves.
Although students may have touched upon Medical Physics at GCSE there is very little depth covered in terms of how for example, X-rays or ultrasound waves are actually produced, instead relying on a more descriptive understanding of how they interact with tissue inside the body. Therefore, it is probably in the best interest of the learner that this topic is studied after they have studied electrons waves and photons. In that respect, they will have a much better qualitative and quantitative understanding of what is going on. Although there are a number of ways of progressing through this Topic, a simple way to approach the teaching of this topic is to consider the following questions for every one of the non-invasive techniques considered:
- What is the basic method being considered? This discuses some of the ideas from physics already considered in previous topics.
- How do we make the type of radiation used?
- How is this radiation used in the body i.e., what is the radiation used to image?
- How is an image formed?
- What are the factors that affect the image and so, how can the images be improved?
- What are the advantages and disadvantages of the technique over other possible methods (including invasive techniques)?
The topic of Medical Physics is one of the unusual topics in the specification because it is not really an area that could be of pure conceptual study but rather is a multi-topic area of study – making use of many different areas of physics. As such, it does not normally fall into the normal way of teaching.
It is suggested that the best way of teaching the Medical Physics topic is to leave it towards the end for the course and make use of the knowledge the students will have built up over the majority of their time studying A Level Physics. In this way, the early part of the topic can be used as a form of revision.
The topic is essentially a discussion of the uses of various types of wave (electromagnetic and sound) for diagnosis in medicine. It can be split into three main areas for teaching purposes:
Ultrasound (6.5.3). The use of high frequency sound waves is something that is part of GCSE so the students will be aware of some aspects of this part of the topic.
Ultrasonic waves are used in two areas:
- Ultrascans (A-scan and B-scan); used for imaging the internal structure of a body.
- Doppler scanning; used for measuring blood flow speed and heart rate.
X-rays (6.5.1). The use of these high frequency electromagnetic waves in to areas:
- Shadow images; used for producing a two dimensional image of the internal structure.
- Cat scans; multi-directional images that are used to produce a three dimensional image.
Nuclear radiation (6.5.2). The use of alpha, beta and (very high frequency) gamma radiation for two areas:
- PET scanning.
- Medical tracers.
Common misconceptions students may have
- Where do the X-rays come from when the electrons collide with the target?
When the electrons collide with the target, the majority of their kinetic energy is turned into internal energy of the target i.e. it gets hot. A small amount of the kinetic energy is turned into X-rays and this can be any value of electromagnetic energy up to the energy of the electron. This is why a spread of energies is produced. The lowest possible wavelength is equivalent to the highest possible frequency and therefore, the highest possible energy – when all of the electron’s kinetic energy has turned into electromagnetic energy.
- When considering the interactions of X-rays, when should the particle model be used and when should the wave model be used?
The wave model is used when electromagnetic energy propagates i.e. moves from one location to another (e.g. reflection, refraction, diffraction and interference). The particle model is used when the energy interacts (e.g. the photoelectric effect, the Compton effect, emission and absorption spectra).
- Why are there sharp lines in the x-ray spectrum?
The sharp lines are due to electron energy transitions in the target material. The electrons which are colliding with the target and transfer some of their energy to the target’s electrons. These electrons are stuck around the nucleus and they use the incident electron’s energy to jump to a higher energy level around the nucleus. Note that the target electrons do NOT jump out of the atom – this is not the photoelectric effect.
The excited electrons then fall down to a lower energy and when they do so, they emit a photon of energy equal to the transition energy – in the x-ray spectrum.
It should be noted that this would only happen if the nucleus of the target atom has a high Z number. If the atom is low in the periodic table, then such transitions will only give out a maximum of UV photons. When Z is bigger, the nucleus has a greater positive charge and the orbiting electrons are more tightly held. This greater attraction means that the inner electrons occupy a smaller space around the nucleus. This in turn means their energies and energy transitions are all larger (moving from UV into the x-ray spectra).
- Why is a voltage produced when a piezoelectric material is pressured?
In a piezoelectric material, the arrangement of the atoms is such that within the crystal structure, the centre of positive charge and the centre of negative charge is not at the same point. When pressure is applied to the material, the atoms move and the position of the centre of positive and negative charge moves, but in different ways. This causes a change in the positions of the two charges and this is seen as a voltage on the surface of the material.
Be prepared to describe the same process in reverse for the use of a piezoelectric crystal in detecting an ultrasonic pulse.
Students should be aware that reflected pulses result from any change in medium and they should be prepared to perform simple speed-distance-time calculations. Note that the relevant ‘time’ in these calculations may be half the time measured from the graph, if the graph is indicating a reflected pulse that has travelled to the boundary and back to the detector.
This is an almost irrelevant question for this particular topic of the specification since the topic is the context of the use of the physics. The beauty of Medical Physics is that it shows how the different areas of physics (often viewed by students as being different), are actually just aspects of a larger whole. This topic brings that out clearly, since it is important to be aware of many different pieces of information from different topics, in order to understand the context of Medical Physics.
It is strongly recommended that teachers take every opportunity (within reason!), to show the wide breadth of physics areas feeding into this topic and encourage students to carefully consider what they already know, in trying to form a good understanding of the work.
What is certainly true of Medical Physics, is that it is has been crucial to the development of mankind and remains so. Not only does this area of physics offer significant opportunities for career development, but for society, it opens the door to continued health and wellbeing. It is reasonably certain that in the near future, medicine will take further turns to develop itself into being more proactively preventative rather than reactive. So the ability of the medical profession to identify risk and possible illness either before it happens or before it becomes chronic will be crucial – it is highly likely that physicists will make that happen.
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