A Level Physics (A2) chapter 13: Medical physics MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
76 exam-style questions on Medical physics, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. Medium A has twice the density of medium B. The speed of sound in A is 0.75 times the speed of sound in B. What is (specific acoustic impedance of A) ÷ (specific acoustic impedance of B)?
A
0.375
B
0.67
C
1.5
D
2.7
Show answer
Answer: C. Z = ρc, so ZA/ZB = 2 × 0.75 = 1.5. Dividing the density ratio by the speed ratio gives 2.7.
2. A 2.5 mm thickness of aluminium halves the intensity of a parallel beam of X-rays. What total thickness of aluminium reduces the intensity of the beam to 12.5% of its initial value?
A
7.5 mm
B
20 mm
C
10 mm
D
5.0 mm
Show answer
Answer: A. 12.5% = 1/8 = (½)3. Since I = I0e−μx, each extra 2.5 mm halves the intensity again, so 3 × 2.5 = 7.5 mm is needed. Attenuation is exponential, so the thickness is not proportional to the fraction removed.
3. Fluorine-18 (proton number 9) is a PET tracer. Which equation describes its decay?
A
189F → 1810Ne + 0−1e + antineutrino
B
189F → 188O + 0+1e + neutrino
C
189F → 147N + 42He
D
189F → 188O + 0−1e + neutrino
Show answer
Answer: B. A PET tracer decays by β+ emission: a proton becomes a neutron, so the proton number falls from 9 to 8 (oxygen), and a positron and a neutrino are emitted. The first option is β− decay; the last does not conserve charge.
4. How does computed tomography (CT) scanning produce a three-dimensional image?
A
a single X-ray image is taken with a very high accelerating p.d., so that the image shows the depth of each structure
B
a radioactive tracer inside the body emits X-rays, which are detected by a ring of detectors from all directions
C
X-ray images of one section from many angles are combined into a 2D image, and 2D images of many sections along an axis are then combined
D
pulses of ultrasound are sent in from many angles, and the echoes from each section are combined into one image
Show answer
Answer: C. The X-ray tube and detectors rotate around the patient, recording one slice from many directions; a computer builds a 2D image of that slice. Repeating this for slices along the body gives a 3D image.
5. The intensity of an ultrasound beam falls from 50 mW cm−2 to 20 mW cm−2 as it travels 3.0 cm through a tissue. What is the attenuation coefficient of the tissue?
A
0.31 m−1
B
13 m−1
C
20 m−1
D
31 m−1
Show answer
Answer: D. I = I0e−μx gives μ = ln(50/20) ÷ 0.030 m = 0.916 ÷ 0.030 = 31 m−1. Using lg instead of ln gives 13 m−1, and leaving x in cm gives 0.31.
6. Fluorine-18 has a half-life of 110 minutes. A dose must have an activity of 400 MBq when it is injected, 220 minutes after it was prepared. What activity must it have when it is prepared?
A
200 MBq
B
800 MBq
C
1600 MBq
D
3200 MBq
Show answer
Answer: C. 220 minutes is two half-lives, so the activity falls to a quarter: the initial activity must be 4 × 400 = 1600 MBq. 800 MBq allows for only one half-life.
7. What does a PET image show?
A
the density of the tissue, in the same way as an X-ray image
B
the positions of the boundaries between tissues with different acoustic impedances
C
the concentration of the tracer in the tissue, and so where the tissue is most active
D
the positions of the detectors around the patient
Show answer
Answer: C. The image is built from the positions of many annihilation events, so it maps the concentration of the tracer and hence the activity of the tissue, rather than its structure or density.
8. The two gamma-ray photons from an annihilation are detected by detectors on opposite sides of a patient. One detector records its photon 0.40 ns before the other. Where did the annihilation take place?
A
12 cm from the midpoint between the detectors, nearer the detector that records first
B
6.0 cm from the midpoint between the detectors, nearer the detector that records second
C
6.0 cm from the midpoint between the detectors, nearer the detector that records first
D
12 cm from the midpoint between the detectors, nearer the detector that records second
Show answer
Answer: C. If the annihilation is a distance x from the midpoint, one photon travels x less and the other x more, a path difference of 2x = cΔt. So x = 3.00 × 108 × 0.40 × 10−9 ÷ 2 = 0.060 m, on the side of the detector reached first.
9. The minimum wavelength of the X-rays from an X-ray tube is 2.5 × 10−11 m. What is the accelerating p.d. across the tube?
A
8.0 × 10−15 V
B
2.5 × 104 V
C
5.0 × 104 V
D
1.0 × 105 V
Show answer
Answer: C. eV = hc/λmin, so V = (6.63 × 10−34 × 3.00 × 108) ÷ (1.60 × 10−19 × 2.5 × 10−11) = 5.0 × 104 V. 8.0 × 10−15 is the photon energy in joules, not yet divided by e.
10. What is the SI unit of specific acoustic impedance?
A
kg m−4 s
B
kg m−3 s−1
C
kg m−1 s−2
D
kg m−2 s−1
Show answer
Answer: D. Z = ρc: (kg m−3) × (m s−1) = kg m−2 s−1. Dividing density by speed instead gives kg m−4 s.
76, each with the answer and a short explanation, across 3 topics: Production and use of ultrasound, Production and use of X-rays, PET scanning. Every chapter test draws ten of them, so you can take it many times.
Is the A Level Physics (A2) chapter 13 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
