Link
Medical physics topic overview
Use the topic page to revisit the lesson sequence and linked syllabus points.
Open resourceLesson 04
Revision
Consolidate ultrasound, X-ray, CT, and PET imaging before moving on from Side A.
Objectives
Lesson outcomes
- Retrieve the key definitions, equations, and imaging sequences across A4 medical physics.
- Practise mixed questions involving ultrasound, X-rays, CT scanning, and PET scanning.
- Compare how different medical imaging methods use reflection, attenuation, absorption, and radioactive tracers.
- Identify which A4 subtopics need targeted follow-up practice.
Syllabus
CIE 9702 syllabus points
16 linked
- 24.1.1 understand that a piezo-electric crystal changes shape when a p.d. is applied across it and that the crystal generates an e.m.f. when its shape changes
- 24.1.2 understand how ultrasound waves are generated and detected by a piezoelectric transducer
- 24.1.3 understand how the reflection of pulses of ultrasound at boundaries between tissues can be used to obtain diagnostic information about internal structures
- 24.1.4 define the specific acoustic impedance of a medium as Z = ρc, where c is the speed of sound in the medium
- 24.1.5 use IR / I0 = (Z1 – Z2)2 / (Z1 + Z2)2 for the intensity reflection coefficient of a boundary between two media
- 24.1.6 recall and use I = I0e –µx for the attenuation of ultrasound in matter
- 24.2.1 explain that X-rays are produced by electron bombardment of a metal target and calculate the minimum wavelength of X-rays produced from the accelerating p.d.
- 24.2.2 understand the use of X-rays in imaging internal body structures, including an understanding of the term contrast in X-ray imaging
- 24.2.3 recall and use I = I0e –µx for the attenuation of X-rays in matter
- 24.2.4 understand that computed tomography (CT) scanning produces a 3D image of an internal structure by first combining multiple X-ray images taken in the same section from different angles to obtain a 2D image of the section, then repeating this process along an axis and combining 2D images of multiple sections
- 24.3.1 understand that a tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied
- 24.3.2 recall that a tracer that decays by β+ decay is used in positron emission tomography (PET scanning)
- 24.3.3 understand that annihilation occurs when a particle interacts with its antiparticle and that mass–energy and momentum are conserved in the process
- 24.3.4 explain that, in PET scanning, positrons emitted by the decay of the tracer annihilate when they interact with electrons in the tissue, producing a pair of gamma-ray photons travelling in opposite directions
- 24.3.5 calculate the energy of the gamma-ray photons emitted during the annihilation of an electron-positron pair
- 24.3.6 understand that the gamma-ray photons from an annihilation event travel outside the body and can be detected, and an image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons
Definitions
Required definitions
Specific acoustic impedance
the product of density and speed of sound in a medium,
Z = rho c.
Lesson Notes
Student guidance and lesson notes
Overview
This lesson brings the A4 medical physics topic together. You should be able to compare the physical principles behind ultrasound, X-ray imaging, CT scanning, and PET scanning, and choose the right equation or explanation for each context.
What You Need to Know
- Ultrasound questions often combine piezoelectric transducers, echo timing, acoustic impedance, reflection, and attenuation.
- X-ray questions may test production, minimum wavelength, attenuation, contrast, and CT image reconstruction.
- PET questions require the sequence from tracer to beta-plus decay, annihilation, gamma photons, detector timing, and image formation.
- Medical physics explanations need clear links between the physics interaction and the diagnostic information gained.
How to Work Through It
- Start with retrieval of the key equations, definitions, and imaging sequences from chapter 24.
- Work through mixed questions without sorting them by imaging method first.
- Mark each response for physics explanation, equation choice, units, and sequence accuracy.
- Finish by writing a short target list for the weakest A4 subtopic.
Check Your Understanding
- Can you explain the role of a piezoelectric transducer in ultrasound?
- Can you calculate minimum X-ray wavelength from accelerating p.d.?
- Can you distinguish X-ray contrast, CT reconstruction, and PET tracer concentration?
- Can you explain how annihilation photons allow a PET scanner to locate tissue activity?
Common Mistakes
- Revising the imaging methods as descriptions only, without practising the equations.
- Mixing up attenuation of ultrasound and attenuation of X-rays without checking the context.
- Saying CT and PET both make 3D images without explaining the different physics behind each one.
- Forgetting the full PET sequence from tracer uptake to gamma-ray detection.
Next Steps
- Revisit the weakest imaging method using the lesson slides or targeted questions.
- Keep A4 examples available for later synoptic questions involving waves, radiation, and nuclear physics.
Lesson Resources
Materials for this lesson
Use these videos, slide decks, documents, or links to work through the lesson.