CIE A Level Physics
CIE A Level Physics Medical Physics Exam Questions
Free CIE A Level Physics exam questions for Medical Physics, with a mark scheme for every pack. Covers ultrasound and acoustic impedance, attenuation, X-ray production and imaging, CT scanning and PET scanning with annihilation.
Medical Physics exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
StemUp mark scheme flashcards
A-Level Physics Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £10.99View flashcards
About CIE A Level Physics Medical Physics
Medical physics is Topic 24 of the Cambridge International AS & A Level Physics (9702) syllabus. It applies wave, nuclear and particle physics to diagnosis. You study the generation and detection of ultrasound with piezoelectric transducers, reflection at boundaries using specific acoustic impedance, and exponential attenuation. You also cover the production of X-rays, image sharpness and contrast, CT scanning, and PET scanning using tracers and electron-positron annihilation. Examiners often ask for intensity reflection coefficient calculations, why a coupling gel is needed, and how a PET scanner locates a tracer.
What you need to know
- Ultrasound production Ultrasound has frequency above 20 kHz. A piezoelectric crystal vibrates when an alternating p.d. is applied and produces an e.m.f. when it receives ultrasound, so it acts as both transmitter and receiver.
- Acoustic impedance Specific acoustic impedance Z = ρc. The intensity reflection coefficient is IR/I = (Z1 - Z2)^2/(Z1 + Z2)^2, so a large difference in Z causes strong reflection; gel is used to remove air between transducer and skin.
- Attenuation The intensity of a parallel beam falls exponentially with thickness: I = I0 e^(-μx), where μ is the linear attenuation coefficient.
- X-rays X-rays are produced when high-speed electrons hit a metal target. Image sharpness depends on a small source area; contrast depends on differences in attenuation and can be improved with a contrast medium.
- CT scanning Many X-ray images are taken from different angles in thin sections, and a computer combines them to build a three-dimensional image.
- PET scanning A tracer emits positrons that annihilate with electrons, producing two gamma photons of equal energy travelling in opposite directions. Detection times locate the tracer. Each photon energy equals the rest mass energy of an electron, mc^2.
Common mistakes in Medical Physics
- Saying gel is used to reduce friction or to make the probe slide. It matches acoustic impedance to prevent reflection at an air layer.
- Using a linear decrease for attenuation. Intensity decreases exponentially with thickness.
- Confusing sharpness and contrast in X-ray images.
- Saying PET produces a single gamma photon. Two photons are produced, moving in opposite directions to conserve momentum.
- Forgetting to square the bracket in the intensity reflection coefficient.
Try three Medical Physics exam-style questions
1. Soft tissue has acoustic impedance 1.63 × 10^6 kg m^-2 s^-1 and bone has 7.80 × 10^6 kg m^-2 s^-1. Calculate the intensity reflection coefficient at the boundary. (2 marks)
Show the mark scheme answer
IR/I = (7.80 - 1.63)^2/(7.80 + 1.63)^2 = 6.17^2/9.43^2; IR/I = 0.43.
2. A parallel X-ray beam passes through 3.0 cm of material with linear attenuation coefficient 0.50 cm^-1. Calculate the fraction of intensity transmitted. (2 marks)
Show the mark scheme answer
I/I0 = e^(-μx) = e^(-0.50 × 3.0); I/I0 = 0.22.
3. Explain how a PET scanner detects the location of a tracer in the body. (4 marks)
Show the mark scheme answer
Tracer emits positrons; positron annihilates with an electron; two gamma photons are emitted in opposite directions; detectors around the patient record the arrival times of both photons, so the point of annihilation along the line between detectors is found.
Medical Physics FAQs
What does Cambridge AS & A Level Physics Medical physics cover?
It covers ultrasound production and use, acoustic impedance and reflection, attenuation, X-ray production and imaging, CT scanning and PET scanning.
Which paper is Medical physics examined in?
It is an A Level topic assessed in Paper 4 (A Level Structured Questions).
What equations do I need for Medical physics?
Learn Z = ρc, IR/I = (Z1 - Z2)^2/(Z1 + Z2)^2, I = I0 e^(-μx) and E = mc^2 for the energy of each annihilation photon.
