Medical physics: A Level Physics questions with answers
Free Medical physics questions for A Level Physics (9702), part of Thermal & modern physics. Each question has the answer and the marking points an examiner would look for.
An X-ray beam passes through 4.0 cm of tissue with linear attenuation coefficient 0.20 cm−1.
(a)
Calculate the fraction of the intensity transmitted.
[2]
(b)
Calculate the thickness of tissue that halves the intensity.
[2]
(c)
Explain what is meant by contrast in an X-ray image and one way it can be improved.
[2]
(d)
In PET scanning, a positron annihilates with an electron. Calculate the energy of each gamma photon produced.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
0.449
I/I0 = e−μxC1 = 0.449 A1
2
(b)
3.47 cm
x½ = ln2/μ C1 = 3.47 cm A1
2
(c)
difference in intensity; contrast medium
contrast: difference in degree of blackening between areas B1 use a contrast medium with a high attenuation coefficient (e.g. barium) B1
2
(d)
8.2 × 10−14 J (0.511 MeV)
E = mec2C1 = 8.2 × 10−14 J A1
2
Question 2Structured · 4 marksA* challenge
PET scanning uses a tracer that emits positrons.
(a)
Explain how gamma photons are produced in PET scanning.
[2]
(b)
Calculate the energy of each gamma photon.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
positron–electron annihilation, two photons opposite
positron annihilates with an electron B1 two gamma photons emitted in opposite directions B1
2
(b)
8.2 × 10−14 J (0.511 MeV)
E = mc2 for one electron mass C1 8.2 × 10−14 J A1
2
Question 3Structured · 11 marks
This question is about producing and using X-rays in medical imaging.
(a)
Describe how X-rays are produced in an X-ray tube.
[3]
(b)
An X-ray tube operates at 80 kV. Calculate the minimum wavelength of the X-rays produced.
[2]
(c)
Explain why most of the energy of the electrons becomes thermal energy and how the target is kept cool.
[2]
(d)
Explain the purpose of an aluminium filter placed in the beam.
[2]
(e)
Explain why a contrast medium such as barium is used when imaging the intestines.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
electrons accelerated by high p.d.; hit metal target; decelerate, emit X-ray photons (braking radiation)
electrons from a heated cathode are accelerated through a high p.d. B1 they strike a metal (tungsten) target B1 they decelerate rapidly and emit X-ray photons B1
3
(b)
1.55 × 10⁻¹¹ m
eV = hc/λminC1 1.55 × 10−11 m A1
2
(c)
most collisions just heat target; rotating anode / cooling with oil
only about 1% of the energy becomes X-rays, the rest heats the target B1 the anode is rotated / cooled by oil circulation B1
2
(d)
absorbs low-energy (soft) X-rays that would be absorbed by the patient without contributing to the image
absorbs low-energy X-rays B1 these would be absorbed by the skin, giving the patient a dose without improving the image B1
2
(e)
soft tissues have similar attenuation; barium high Z absorbs strongly, giving contrast
soft tissues have similar attenuation coefficients so give little contrast B1 barium has a high attenuation coefficient so the intestines show up clearly B1
2
Question 4Structured · 9 marks
This question is about PET scanning.
(a)
Explain what is meant by a tracer in PET scanning.
[2]
(b)
Describe what happens when a positron meets an electron.
[2]
(c)
Calculate the minimum energy of each photon produced, in MeV.
[2]
(d)
Explain how the detectors locate the point where annihilation occurred.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
radioactive substance (positron emitter) introduced into body, absorbed by tissue of interest
a substance containing a radioactive (β+-emitting) isotope B1 which is absorbed by the tissue being studied (e.g. fluorine-18 in glucose) B1
2
(b)
annihilation; two gamma photons in opposite directions
annihilation: their mass is converted into energy B1 two gamma-ray photons are emitted in opposite directions B1
2
(c)
0.511 MeV
E = mec2 = 8.2 × 10−14 J C1 0.511 MeV A1
2
(d)
ring of detectors; two photons detected at same time define a line; time difference gives position along line
a ring of detectors around the patient B1 two photons detected simultaneously define a line on which annihilation occurred B1 the difference in arrival times gives the position along the line B1
3
Question 5Structured · 3 marks
X-ray images are formed by attenuation in tissue.
(a)
State the equation for the attenuation of a parallel X-ray beam.
[1]
(b)
Explain how contrast in an X-ray image arises between bone and soft tissue.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
I = I₀e−μx
I = I₀e−μxB1
1
(b)
different attenuation coefficients
bone has a much larger attenuation coefficient than soft tissue B1 so less intensity is transmitted through bone B1
2
Question 6Structured · 11 marks
This question is about medical imaging.
(a)
Explain how ultrasound is produced using the piezo-electric effect.
[2]
(b)
Define specific acoustic impedance.
[1]
(c)
Ultrasound passes from soft tissue (Z = 1.7 × 106 kg m−2 s−1) to bone (Z = 7.8 × 106 kg m−2 s−1). Calculate the fraction of the intensity reflected.
[2]
(d)
Explain why a gel is applied between the transducer and the skin.
[2]
(e)
An X-ray beam passes through 4.0 cm of tissue with linear attenuation coefficient 0.35 cm−1. Calculate the percentage of the intensity transmitted.
[2]
(f)
Explain why a CT scan gives more information than a single X-ray image, and why it involves a higher dose.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
alternating p.d. across crystal makes it vibrate at its resonant frequency
an alternating p.d. applied across a piezo-electric crystal makes it change shape / vibrate B1 at the resonant frequency of the crystal this produces ultrasound B1
2
(b)
Z = ρc
product of the density of the medium and the speed of sound in it B1
1
(c)
0.412
IR/I = (Z2 − Z1)2/(Z2 + Z1)2C1 0.412 A1
2
(d)
air has very different Z to skin; nearly all reflected; gel matches Z
air and skin have very different acoustic impedances so almost all ultrasound would be reflected B1 the gel has an impedance similar to skin, allowing transmission B1
2
(e)
24.7%
I = I0e−μxC1 24.7% A1
2
(f)
many images from different angles combined into 3D / slices; more exposures
images are taken from many angles round the patient and combined into cross-sections / a 3D image B1 the patient is exposed to many X-ray images, so a larger dose B1
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Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in