Medical physics (A-level only) questions

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• AQA 7408 · June 2025 · Paper 3BB · Q1 · 9 marks

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  [1.4] It was relatively easy to score one mark in this question with over 50% of students doing so. The first mark was available to students who described hypermetropia or referred to d1’s variation or A and d2’s variation. © 2025 AQA To achieve two marks, the student had to explicitly describe how A and d1 were related and how this relationship related to hypermetropia. Lower achieving students mistook hypermetropia for myopia and discussed changes in far point with age.

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  This question is about accommodation in a typical eye with no sight defects. The accommodation amplitude in dioptres is A = 1/d1 - 1/d2, where d1 is the near-point distance and d2 the far-point distance, both in metres. A graph for a typical person shows A decreasing with age: approximately 13.4 D at age 10, 11.2 D at 20, 8.8 D at 30, 5.8 D at 40, 2.0 D at 50, 1.3 D at 60 and 1.1 D at 70.

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• AQA 7408 · June 2025 · Paper 3BB · Q3 · 7 marks

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  [3.3] Almost 33% of students scored both marks for their sketch of the equal loudness curve. Reasons for students losing marks typically included: • the minimum intensity level outside of the allowed range of greater than 2 kHz and less than 4 kHz. • mot passing through 1000 Hz, 100dB • where they had a second minimum, its value had a lower intensity level than the intensity level at 3kHz. • drawing a curve that intersected with the original curve.

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  A person with good hearing produces an equal-loudness curve. On logarithmic frequency axes, the curve is approximately: (10 Hz, 120 dB), (20 Hz, 100 dB), (50 Hz, 78 dB), (100 Hz, 65 dB), (200 Hz, 53 dB), (500 Hz, 43 dB), (1000 Hz, 40 dB), a small maximum near (1500 Hz, 42 dB), a minimum near (3500 Hz, 35.5 dB), and (10 000 Hz, 54 dB).

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• AQA 7408 · June 2025 · Paper 3BB · Q4 · 8 marks

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  [4.2] Approximately 30% of students did not score any marks in this question. The answers were very vague and lacked details that would demonstrate the knowledge expected in this area of the specification. Higher performing students typically gained at least two marks by being able to relate technical information to image production of the desired region without over exposure. © 2025 AQA

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  A rotating-anode X-ray tube is used to image a femur. X-rays originate at the focal spot on the rotating anode.

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• AQA 7408 · June 2024 · Paper 3BB · Q1 · 4 marks

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  [01.1] The majority of student were able to correctly identify the defect of vision, most of whom used the correct medical term, myopia.

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  A human eye has a far point of 6.0 m. An eye with astigmatism has the prescription -4.00 -0.75 x30.

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• AQA 7408 · June 2024 · Paper 3BB · Q2 · 6 marks

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  [02.2] This was poorly answered, with around 70% of students failing to gain either mark despite very similar questions having been asked in previous series. Many students gave very vague answers about hearing loss, or focussed on the anatomy of the ear, describing how that would be affected rather than describing the impact of that damage on the person’s hearing. A significant proportion of students did not appear to have a clear understanding of the difference between frequency and intensity level in this context. Comments about hearing loss being maximum at 110 dB, or sounds of 110 dB needing to now be louder were common, often even referring to this as the frequency of the sound. Many students also suggested that hearing loss would be at a maximum at the frequency of the sound that caused it, rather than at 4000 Hz.

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  A full stadium produces a peak sound-intensity level of 110 dB at its centre. On another occasion the number of spectators is reduced by 60%, so 40% remain. Assume each spectator produces the same sound intensity on both occasions and spectators are distributed evenly around the stadium.

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• AQA 7408 · June 2024 · Paper 3BB · Q3 · 10 marks

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  [03.1] The majority of students gained at least one mark here, identifying coherent and non-coherent bundles as the two types of bundles used in endoscopy. Many then went on to describe the fibre arrangement, which was not necessary in answering the question. Clear descriptions of the functions, however, were required for the subsequent marking points, and lots of students accessed a second mark by giving at least one of these clearly. For the coherent bundle, as in previous series, ‘transmission of light’ was considered an insufficient description of the function. Answers needed to make it clear that the light was being transmitted into the body for the purpose of illumination in order for a mark to be awarded. The discussion of the requirement for a cladding was less well answered, with just over a third of students gaining 3 or 4 marks. The common misconception highlighted by this question is that total internal reflection will not occur unless there is a cladding on a fibre, with many students arguing on this basis that both bundles would require cladding. It was clear from responses that students have learnt about several purposes of cladding on optical fibres, and the challenge of this question was to determine which purpose was relevant in this context. In the most successful answers students had recognised that the important issue was whether or not light would be able to pass between the individual fibres in a bundle.

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  An endoscope uses optical-fibre bundles. Modal and material dispersion can cause problems in long-distance, high-data-rate fibre-optic communication systems.

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• AQA 7408 · June 2024 · Paper 3BB · Q4 · 10 marks

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  [04.2] The vast majority of students gained at least one mark on this calculation, with over a third obtaining the full three marks. Students were successful with both routes shown in the mark scheme and neither one proved significantly more popular. The question provided data in mixed units, both minutes and hours. Students were generally very successful in using consistent units in their processing of the data; errors in this were rare. Many of the students who gained two marks did so through using only the physical half-life rather than calculating the effective half-life. This confirmed the poor understanding of the different half-lives shown in 04.1. Those students who were unable to secure the higher marks often tried to estimate a remaining percentage by interpolating between whole numbers of half -lives rather than calculating a percentage accurately by using relevant equations.

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  Fluorine-18 has a biological half-life of 6.0 hours and a physical half-life of 110 minutes. It is injected as a tracer for a PET scan.

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• AQA 7408 · June 2024 · Paper 3BB · Q5 · 5 marks

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  [05.2] The vast majority of students accessed at least one mark, usually by calculating the acoustic impedance of the muscle. Over three-quarters of students then used the correct equation to calculate the percentage of the ultrasound that would be reflected, but the majority omitted the final step of the calculation and did not find the percentage transmitted.

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  A medical ultrasound transducer contains a piezoelectric quartz crystal with electrodes at its front and back. A backing material lies immediately behind the crystal, an acoustic absorber surrounds the rear assembly, and a plastic membrane covers the front from which the ultrasound beam emerges.

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• AQA 7408 · June 2023 · Paper 3BB · Q1 · 2 marks

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  This question was well answered, with the vast majority of students being awarded at least one of the marks for relevant comments about the comparative properties of beta and gamma radiation. A two-mark answer required mention of ionisation at some point in the response, rather than less specific “damage” or “harm”. A substantial number of students forfeited their second mark for this omission. A small minority of students confused the properties of beta and gamma radiation, some stating that either one or the other of them was not ionising.

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  Brachytherapy treats a small tumour using a sealed radioactive source placed inside the patient's body next to the tumour.

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• AQA 7408 · June 2023 · Paper 3BB · Q2 · 10 marks

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  [02.2] More than half of the students gained some credit for this question. The majority of students who were awarded zero failed to link their answer to the eye at all, instead discussing the light levels on the pixels. Many students recognised that the key to this question was the eye’s use of cones in bright conditions and rods when the light level is reduced. Although the question referred only to “reduced” intensity rather than darkness, students were expected to infer from the stated decrease in resolution that cones were no longer in use and rods were exclusively in operation. Although students did not have to be explicit about this in their statement, they were penalised for suggesting that cones were still activated when the image could no longer be resolved. Many students also recognised the relevance of the fact that cones have their own unique neurone whereas rods share their neurones; however many then struggled or did not attempt to explain why this would lead to a reduction in resolution. Some excellent answers were seen, which could be expressed in a variety of ways, but these were not common. The inclusion of additional information about rods and cones, which had no relevance to the question, was not uncommon.

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  Electrophoretic screens are used in handheld electronic devices. The screen contains individual squares known as pixels. Pixels can be changed independently from light to dark to create the shapes of letters and numbers. An external light source is needed in order to read the screen. Figure 1 shows a letter formed by three electrophoretic screens that have different pixel line densities. Pixel line density is the number of pixels along a 1.0 cm length of the screen. Figure 1

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• AQA 7408 · June 2023 · Paper 3BB · Q3 · 4 marks

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  [03.3] This question was well answered with almost half of students gaining both marks and the majority gaining at least one mark. Of those who only obtained one mark, the most common errors were to ignore or to misuse the factor of 20. Full marks were also not awarded to those students who left their final answer in the form of a fraction. Of those students who failed to gain a mark, many incorrectly stated the relationship between pressure, force and area, using p F A = . This physics error did not receive credit.

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  This question concerns sound intensity and the pressure amplification of the middle ear.

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• AQA 7408 · June 2023 · Paper 3BB · Q4 · 11 marks

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  [04.3] Although very few students gained full marks on this question, the specific marks which were awarded varied, with all separate marking points being awarded many times. Many students were able to correctly state the units on both sets of axes, although some then lost those marks through the use of incorrect prefixes. Abbreviations such as “secs” were not accepted. Many students were unsure of the range of values for the y-axis, and a common error was to label the origin as zero rather than the start of the line being at zero. A label at this point was not insisted upon, but the values shown needed to be consistent with this point being zero. Of those students who knew the appropriate range of time values for the x-axis, many were not awarded the mark due to poor graphical work. Most commonly, students did not label the origin as zero. Some went up in uneven increments or used awkward scales, placing their number every third square. Others failed to put any vertical marks onto the axis leading to vaguely positioned, floating numbers.

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  Different ultrasound techniques are used to investigate the health of a patients kidneys.

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• AQA 7408 · June 2023 · Paper 3BB · Q5 · 8 marks

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  [05.2] Many students answered this well, gaining at least one mark for mention of a barium meal. For a two-mark answer, reference to higher atomic number of barium was considered inadequate; students were required to comment on the impact of barium on the X-rays’ absorption or image. Students who were awarded no marks had often failed to appreciate the stated context of an image being formed on photographic film, suggesting other image-forming techniques. Mention of radio tracers was fairly common and did not receive credit.

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  A chest X-ray on photographic film shows the lungs dark, the ribs intermediate and the heart light. Along the indicated horizontal section, a rib is very thin, the heart is much thicker, and the path through a lung is thickest.

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• AQA 7408 · June 2022 · Paper 3BB · Q1 · 8 marks

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  [1.1] Most students were able to identify that the eye defect was hypermetropia. Myopia was the most commonly chosen distractor, with only a very few choosing astigmatism.

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  An eye condition is corrected using a +4.0D lens.

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• AQA 7408 · June 2022 · Paper 3BB · Q2 · 11 marks

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  [2.3] This question proved difficult for most students with only a very small number of students scoring 2 marks. A large number of students were not able to score on this question. Some students were able to identify that X-rays absorbed by tissues before the bone reduce 𝐼𝐼0 and therefore reduce the intensity of X-rays absorbed. Credit was given for suggesting that not all of the reduction in intensity was caused by absorbed X-rays but some were also scattered. A lot of students incorrectly stated that X-rays were reflected, which was treated as neutral (i.e. ignored). Only a very few students could clearly state that the amount absorbed assumed that the X- rays all travelled through 0.04 m of bone which was not true due to the bone being a cylinder. Only the X-rays in the centre travelled through 0.04 m of bone, the rest travelled through less bone. Students had to make clear it was the 𝑥𝑥 in 𝐼𝐼= 𝐼𝐼0𝑒𝑒−𝜇𝜇𝜇𝜇 that was too large and not the diameter used to calculate the cross sectional area. Most students referred instead to the area calculation, which already took the variation in diameter into account by quoting a mean value. Some students also referred to a change in 𝑥𝑥 but along the length of the bone; this did not get the mark because this variation in 𝑥𝑥 is included in the mean value. It was also possible to gain this mark by using a mean value for 𝑥𝑥 in Question 02.2 or by suggesting that the cross sectional area of the bone was less than the area of the film, if the area of the film was used in 02.2, although very few students answered with either of these options.

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  An X-ray image of a bone uses mean bone thickness 0.040 m, incident intensity I_0=0.013 W m^-2, attenuation coefficient mu=58.3 m^-1 and exposure time 0.80 s. The projected bone length is sqrt(0.25^2+0.09^2) m and its mean width is 0.040 m.

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• AQA 7408 · June 2022 · Paper 3BB · Q3 · 7 marks

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  [3.2] A large number of students were not familiar with this procedure, with over half not scoring. Some students were familiar with this procedure and were able to describe it well. A majority of these students knew that the reference frequency was 1 kHz. To gain the first mark it had to be clear that the reference frequency of 1 kHz is compared to sounds at other frequencies. A mention of 1 kHz was not enough by itself. Some methods suggested lacked rigour because they involved either just stating the frequency should be altered or failing to explain that the intensity or volume must be changed until the two signals sound with the same loudness. For two marks it had to be clear that a second signal must be kept at constant frequency while the volume was changed until two signals were heard to be equally loud. A minority described the procedure for determining the threshold of hearing rather than equal loudness curves; this did not score any marks.

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  Question 3 from AQA A-level Physics Paper 3BB, June 2022.

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• AQA 7408 · June 2022 · Paper 3BB · Q4 · 6 marks

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  The levels-of-response question was more accessible than in previous years, with very few students scoring 0 or 1 mark. A mark of 4 was the most common score. Most students were familiar with the three types of scans and were able to list information about them, with only a minority mixing up the features of the three scans. However, this question required students to evaluate the scans’ suitability in a particular context, rather than list everything they knew about the scan. While MR scans do produce high-resolution images of soft tissue, the question asked about kidney stones, which were described in the question as looking like bone in the three scans. Students should have recognised that MR scans perform poorly when viewing bone and were therefore not suitable in this case, particularly when viewed against their high cost. The question then became a comparison between the higher quality image with a large exposure to radiation with the CT scanner weighed against the lower quality image but with a lack of ionising radiation of the ultrasound scan. Both scans are used in the NHS for identifying kidney stones and selecting either enabled the answer to access top marks. Some students did provide a very detailed description of how one or more of the scans worked. However, this did not address the question that was asked and therefore gained no marks, unless it touched on any of the relevant factors. While metal in the body is something which doctors have to consider when performing MR scans, this does not prevent MR scans taking place in a lot of cases and therefore any comments about metal in the body with reference to MR scans was treated as neutral. A significant number of students tried to focus on the 2-D or 3-D nature of MR scans and CT scans, with students stating either of the two scans being 2-D compared to the other which was 3-D. Both scans take multiple 2-D cross-sections which can be combined into a 3-D image by a computer. However, this is not usually necessary and is rarely done. Therefore this was also treated as neutral, as in previous questions on the topic. Some students incorrectly stated that a barium meal should be used or tried to suggest X-ray techniques that are not applicable to CT scans to minimise the dosage of radiation. A significant number of students stressed the real-time nature of an MR scan. While real-time MR scans have been developed in recent years, they are rarer and more expensive than a standard MR scan and are used for tracking moving objects in the body and would therefore not be relevant to a discussion about kidney stones; this was therefore treated as a neutral comment. For a higher band answer worth 5-6 marks, students were expected to recognise the trade-off between quality and safety. For 6 marks the quality comparison needed to use the appropriate terminology such as resolution or detail visible, while a 5-mark answer typically just referred to better quality images. At this level of comparison very few answers lacked the details required for the top band. For a middle band answer, typical answers would not compare the quality of ultrasound with the CT scan – just referring to both being able to image bone. There was also less detail about the scans, with most 4-mark answers typically only making one relevant comment about each scan. Often these answers did not address the quality issue at all beyond commenting that the MR scan did not produce a good image of bone or focussing entirely on the quality of the image without recognising any of the other relevant issues, such as safety. Alternatively, some answers did not adequately express the idea that an ultrasound is capable of imaging bone but it does so at low resolution. A typical 3-mark answer often only made a correct comment about two of the scans or had significant errors. In this band some errors were introduced, for example students thought that because ultrasound could not see through bone, it could not then image the bone itself. While the term non-invasive could be applied to all 3 scans, it was clear that a number of students were mixing up the term non-invasive with non-ionising when they used the term to contrast MR scans and ultrasounds with CT scans. For a lower band answer, students typically selected the MR scan focussing on its high quality images of soft tissue but not recognising that this was not relevant for imaging kidney stones. An MR scan focussed answer could gain 3 marks when it was sufficiently detailed and listed relevant factors in the other scans, but this was rare. Most answers in this band were able to list some relevant factors about the scans and therefore only a very few answers scored 1 mark. A very small number of students did not state a preference for the type of scan and therefore also scored in this band. These students however, did not usually provide a level of detail that would have moved them much higher even if they had stated a preference.

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  Question 4 from AQA A-level Physics Paper 3BB, June 2022.

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• AQA 7408 · June 2022 · Paper 3BB · Q5 · 3 marks

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  [5.2] This question was more tolerant of imprecise language than 5.1 so that students were not penalised twice for the same mistake. Consequently this was more accessible than 5.1. Most students had some idea of how this worked and were able to gain marks on this question. When describing the emission of radio-frequency photons, it had to be clear that the photons released were radio frequency or the same frequency as the incident pulses. It also had to be clear that it was de-excitation rather than excitation which released the photons. A distinction was drawn between the pulses causing the protons to excite and flip their spin and the pulses allowing the protons to excite or flip their spin; the latter did not gain credit.

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  Question 5 from AQA A-level Physics Paper 3BB, June 2022.

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• AQA 7408 · November 2021 · Paper 3BB · Q1 · 15 marks

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  A hospital produces technetium-99m on site for tracer imaging. Its half-life is about 6 h and it emits gamma radiation only.

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• AQA 7408 · November 2021 · Paper 3BB · Q2 · 4 marks

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  Three head scans are shown. Image A makes skull bone bright with little soft-tissue detail; B has high-resolution bright brain/eye soft-tissue detail and dim bone; C is low resolution, showing mainly external boundaries with little detail through the skull.

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• AQA 7408 · November 2021 · Paper 3BB · Q3 · 5 marks

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  A point sound source has power 17 W.

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• AQA 7408 · November 2021 · Paper 3BB · Q4 · 4 marks

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  An X-ray emission spot is 1.0 mm wide and 1.0 m above the photographic plate. An object a distance d above the plate produces geometric penumbra; similar triangles give penumbra/spot width=d/(1.0-d).

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• AQA 7408 · November 2021 · Paper 3BB · Q5 · 7 marks

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  This question concerns spectacle prescriptions and correction of myopia.

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• AQA 7408 · November 2020 · Paper 3BB · Q1 · 9 marks

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  This question concerns vision in bright and very dark conditions and the use of an eye patch to preserve dark vision.

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• AQA 7408 · November 2020 · Paper 3BB · Q2 · 5 marks

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  A sound wave causes a maximum pressure increase of 2.5 x 10^-3 Pa at the tympanic membrane and 5.0 x 10^-2 Pa in the fluid of the inner ear. The tympanic membrane is a circle of diameter 1.0 cm.

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• AQA 7408 · November 2020 · Paper 3BB · Q3 · 8 marks

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  X-ray photons are used to treat a brain tumour 11 cm below the surface. Brain-tissue attenuation coefficients are 0.15 cm^-1 at 100 keV and 0.087 cm^-1 at 500 keV.

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• AQA 7408 · November 2020 · Paper 3BB · Q4 · 13 marks

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  An ultrasound transducer contains a piezoelectric crystal, electrodes, damping/backing material and a matching layer. It is used for pulse-echo medical diagnosis.

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• AQA 7408 · June 2019 · Paper 3BB · Q2 · 9 marks

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  02.1 was answered well by more able students. There were two approaches possible: • a calculation of power using 𝑃𝑃= 𝐼𝐼𝐼𝐼 for a distance of 11 m, followed by a determination of the intensity for the same power at the new distance. Some students failed to gain a mark by using the wrong area, usually the area of a circle instead of the surface area of a sphere; • a proportion calculation 𝐼𝐼1𝑥𝑥1 2 = 𝐼𝐼2𝑥𝑥2 2. Less able students confused intensity with intensity level and scored 0. In 02.2, more able students answered well, converting the intensity levels in the question into intensities. A minority calculated the wrong ratio, but most were then able to gain two marks. Less able students calculated the ratio of the intensity levels instead of intensity. A minority expressed the ratio as 200:1 which was not penalised. 02.3 required students to discuss which measure, intensity level or intensity, was more appropriate to compare perceived loudness. Only the most able students were able to answer this question, stating that intensity level was more appropriate as it was a logarithmic scale which matched the ear’s response. This required students to evaluate their knowledge of intensity level and intensity in the context of perceived loudness. However, the answers given suggested that many students did not have a good understanding of either intensity level or intensity. Some students confused the dB scale (which measures intensity level) with the dBA scale (which is adjusted for frequency). Answers which referred to a frequency dependence of intensity level could not gain full marks on this question. Some students did not realise that the ratio of intensities (= 200 from question 02.2) was not a realistic increase in loudness when moving from 11 m to 7 m, and therefore the answer could not have been intensity. 02.4 was answered poorly by most students, with only a few able to score full marks. There was a general familiarity with the concept of hearing loss resulting from age deterioration or loud noise; however, detail was either missing or incorrect. Most students knew that hearing loss due to age was most significant at higher frequencies. Many students thought that hearing loss due to loud noises was either at 3 kHz or the frequency at which the noise occurred. Only a few were able to quote the correct frequency of 4 kHz for this damage. Most students did not state that hearing loss occurred at all frequencies in both cases.

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  Three customers listen to a loudspeaker. Customer P is initially 11 m from it, where the sound intensity is 3.4 x 10^-8 W m^-2.

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• AQA 7408 · June 2019 · Paper 3BB · Q3 · 7 marks

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  03.1 was about the production of X-rays in an X-ray tube. It was clear that around one-third of students were unfamiliar with this, although students who were familiar proved able to answer the question well. For the continuous spectrum, most students knew that decelerating electrons produced X-rays, but very few of them were able to go on to explain why this was continuous. A common error was to state that a variation in incident electron energy was responsible, rather than assign the change to varying decelerations of electrons all with the same initial energy. Some students wrote about the electrons accelerating around the nucleus, and this was tolerated. However, the idea of electrons slowing down and transferring their lost kinetic energy into X-rays is a much clearer way of expressing what is going on. A small number of students thought the continuous nature of the spectrum referred to a continual emission of X-rays rather than X-rays of all energies up to a maximum being emitted. For the characteristic spectrum, many students thought that electrons were excited by electron collisions with tungsten, rather than by the removal of inner electrons. Those students who had the right idea but did not refer to inner and outer electrons gained partial credit. Students had to mention electrons rather than merely exciting and de-exciting atoms to gain the mark here. A common mark awarded here was that it was the downward movement of electrons through the energy levels which led to the release of the photon. 03.2 showed a graph of photon energy against X-ray intensity and required students to sketch the curve produced when the anode pd was increased. Most students were able to gain one mark here, although only a minority were able to score both marks. Common mistakes were: curve not being larger at all frequencies; peak not shifting to the right; characteristic spikes not remaining in the same place, and maximum photon energy not increasing. 03.3 asked the students to sketch the graph produced when an aluminium filter was used. Many students seemed unfamiliar with the use of an aluminium filter and were not able to draw a sensible graph. Students were expected to draw a curve which removed some photons at all frequencies, but removed more at lower frequencies. This should have produced a curve which was lower at all frequencies and where the peak shifted to the right. Very few students were able to draw this correctly. Common mistakes were: peak not shifting to the right, and not taking sufficient care in drawing so that the line rose above the original intensity indicating an increase of intensity at this frequency.

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  Electrons strike a tungsten target in a medical X-ray tube. The initial spectrum is a continuous distribution with characteristic spikes.

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• AQA 7408 · June 2019 · Paper 3BB · Q4 · 9 marks

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  04.1 gave the students a simplified graph from an ultrasound A scan and asked them to calculate the diameter of an eye. Students had to identify the first pulse as the front of the eye and the last pulse as the back of the eye. Many students failed to do this, either ignoring the first pulse or choosing other pulses. A small minority of students did not read the scale correctly or did not know what the symbol 𝜇𝜇s meant; thus giving answers which were clearly ridiculous, giving the size of an eye ranging from several km down to 10−9 m. The second mark was more accessible, where students had to use the speed of sound with a time measurement from the graph to calculate a distance. The last mark was awarded for dividing the distance by two to account for ultrasound travelling in both directions. 04.2 was well answered, with most students able to describe A and B scans and state that a B scan was the more appropriate in this case. The majority of the students knew what an A and B scan was, with a small minority not realising that an A scan did not give a picture but a graph as in 04.1. Some students were also able to discuss how an A scan was generated from a single sensor, while a B scan was produced from multiple sensors. While most students knew that an A scan was more accurate for measuring distances, they were less well able to articulate the advantages of a B scan. Students struggled with applying their knowledge to amniocentesis and only a few wrote about the pictures of the B scan allowing the needle, foetus, uterus and placenta to be identified and their relative positions determined. In order to obtain marks in the top band the answers had to relate to amniocentesis and not just state why a B scan was better than an A scan in general terms. Students who stated that the A scan should be used were limited to 3 marks. A common misconception among lower scoring students was a confusion between what was measured and how it was interpreted. Both A and B scans measure the intensity of the reflected pulse; in an A scan this is represented by the amplitude of an oscilloscope trace and in a B scan by the brightness of a pixel. In neither case is amplitude (or ‘brightness’) measured. A small minority incorrectly stated that a B scan was in 3D, many of those going on to say that an A scan produced a 2D image rather than a graph. Examiners did not accept a ‘1D image’ as a synonym for ‘graph’. Some students described the oscilloscope electron beam moving horizontally in an A scan and vertically in a B scan. In reality it moves both horizontally and vertically in both scans. In an A scan the horizontal movement is related to time, while the vertical movement is related to the intensity. In a B scan the horizontal movement is related to horizontal position and the vertical movement is related to depth.

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  This question concerns medical ultrasound.

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• AQA 7408 · June 2018 · Paper 3BB · Q2 · 7 marks

  7 marks

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  The first part, 02.1, required the students to use the data provided to make a sensible comment on whether the eye could resolve two images on the retina. This could be done in many ways, but the final statement required a direct comparison between either two calculated values, or a comparison between a calculated value and data quoted in the question. In order to gain full marks, students needed to demonstrate that in order to ensure that two images could be resolved, the minimum distance between the two images was greater than two cell diameters. This point was missed by many students who either used one or three cell diameters as the limiting factor. Many other students could not use the data supplied in a meaningful way, with a surprising number trying to use the lens equation to work out a focal length. The mean mark on this question was 0.97 out of 4, with 8.7% of students scoring all four marks available. Part 02.2 required students to sketch and label the response curves for the three colour cones of the eye. Although most students gained a mark for drawing three curves and naming the curves blue, green and red with increasing wavelength, hardly any gained either of the other marks for relative heights or range of wavelengths.

  Text version

  Foveal cones average1.5 x10^-6 m diameter. Two point sources are12 mm apart and61 m from the eye lens; retina is21 mm behind lens.

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• AQA 7408 · June 2018 · Paper 3BB · Q3 · 6 marks

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  This question required the students to discuss the basic principles of a CT scanner, to explain why a CT scan was advantageous for assessing head injuries and to explain why a simple X-ray procedure was more suitable for assessing other injuries. Their answers highlighted the lack of clarity in many students’ written work. Many failed to address the advantages of the CT scan for assessing head injuries and thus limited the mark available. A noticeable number seemed to have little or no idea of what a CT scan is and either wrote about an MR scan, or about injecting radioisotopes and detecting the emitted radiation using gamma cameras. The mean mark on this question was 2.55 out of 6, with only 1.1% of students scoring all six marks available.

  Text version

  Doctors now prefer CT over a simple radiograph for assessing head injuries, though simple X-rays remain suitable elsewhere.

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• AQA 7408 · June 2018 · Paper 3BB · Q4 · 8 marks

  8 marks

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  Part 04.1 showed a suggested graph drawn by a student to show the response of a sound meter set to the dBA scale over a range of frequencies in relation to the curve for the dB setting, showing constant output power of the source. The students were asked to discuss whether the curve drawn by the student was correct. There were three main points which were wrong, the fact that the two curves did not cross at 1000 Hz, the fact that the peak of the dBA curve was not at 3000 Hz, and the fact that there was a 6 kHz plateau in the dBA curve. There were many good answers to individual errors, but few were able to put all the answers together and gain full marks. A noticeable number of students suggested that the dBA curve was upside down. The other noticeable error was the inability to read the logarithmic scale, suggesting that the peak of the dBA curve was 3100 or 3200 Hz. Unfortunately, just over 45% of students failed to score on this question. Part 04.2 was answered well by many students, with over a quarter of them gaining all four marks. There were many ways to attempt this question and the examiners were impressed with some of the approaches used by students. The marks were awarded for different calculations which involved specific equations and a final mark for an adequate discussion comparing either the student’s calculated answer with a value quoted in the question, or two calculated answers. The main source of error, which led to a two-mark penalty, was using the wrong formula for the area of a sphere when relating intensity and power. An error here was carried forward so that the remaining marks could still be gained.

  Text version

  A constant-power sound is measured over frequency. Its unweighted dB reading is a horizontal line. A proposed solid dBA curve starts well below that line at100 Hz, rises and crosses it at about750 Hz, reaches a maximum near4 kHz, then falls slightly and is almost level from about5 kHz to10 kHz. A7.8 W point-source drill has barriers2.0 m away; pedestrian limit110 dB.

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• AQA 7408 · June 2018 · Paper 3BB · Q5 · 8 marks

  8 marks

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  Part 05.1 required students to discuss why initial scans are important in deciding the best treatment of a patient with a tumour. This was a very open question, with many relevant points being allowed to gain credit. Over half of students did score all three marks, however many students were unable to write clear statements, often writing two opposite statements which cancelled each other. Part 05.2 was a simple multiple choice question on the meaning of half-value thickness which the majority of students (76.2%) answered correctly. Many students were able to do the calculation in part 05.3 (59.4% gained at least three marks). However, only a small number of students were able to state the correct unit for the mass attenuation coefficient.

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  High-energy X-rays treat an internal tumour after planning scans. Lead half-value thickness for500 keV X-rays is4.2 x10^-3 m; lead density1.1 x10^4 kg m^-3.

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• AQA 7408 · June 2017 · Paper 3BB · Q1 · 9 marks

  9 marks

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  This question was on the defect of the eye and its correction. In part 01.1 many students only provided one of the points needed. Students sometimes lost a mark by providing a correct answer and a wrong answer for the same point, for example ‘the eyeball was too short or the lens was too strong’. In part 01.2 it was pleasing to find few students who found ‘u’, the object distance, rather than ‘v’, the image distance. However, the units in the calculation proved to be challenging for many students. The power of lens in dioptres meant that object distance needed to be in metres in the calculation. The final answer then needed to be converted back to centimetres. The most common wrong answer was ‘0.51’ which only gained 1 mark for the correct number of significant figures. Part 01.3 proved to be difficult, with the majority of students thinking that it was ‘the aided near point’ rather than the correct answer of ‘the unaided near point’. Part 01.4 required a ray diagram to be drawn, which a significant number of students were unable to do. Students should be aware that when using a correcting lens, objects are always real and images are always virtual. The majority of diagrams showed the formation of a real image. Many students also failed to score the labelling mark for correct labelling of object, image and a correct principal focus. The students who were able to draw the correct ray diagram were, in the main, also able to correctly label the diagram.

  Text version

  A person has hypermetropia. A spectacle lens of power +2.0 D allows the person to just see clearly an object 24 cm from the eye.

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• AQA 7408 · June 2017 · Paper 3BB · Q2 · 12 marks

  12 marks

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  This question was on the dangers of X-rays and the use of ultrasound. Part 02.1 was an open text response part which considered the danger of X-rays and how the danger was managed for a simple X-ray of a suspected broken arm. Most students were aware of the danger and were able to explain the possible result of cell damage and the formation of cancerous tumours. However, the discussion of three methods for limiting the danger often lacked relevance and explanation. Irrelevant answers often seen included use of fluoroscopic imaging with a barium meal, and details of the rotating anode which allowed the anode to emit X-rays in only one direction. There were many who stated that the patient needed to be covered by lead sheets. Where students had an understanding of the situation, some good answers were seen, but these were few in number. Part 02.2 asked students to explain whether X-rays or ultrasound would be best to investigate a suspected fault in the aorta as it passed through the abdomen. Some students failed to read the question and wrote about imaging the heart, stating that ultrasound could not be used as it would have to pass through the ribs. When students were discussing the correct situation, many failed to gain the mark for stating that ultrasound gave better definition between different tissues and blood. Having pointed to the dangers of X-rays in the first part of the question, it was noticeable that many students failed to state that another reason for using ultrasound was the fact that it produced no known adverse effects on the patient. The last part, 02.3, was a calculation of acoustic impedance. The information given in the question required the student to use the negative value of a square-root in order to complete the calculation. This was rarely seen. Students who used the positive root and then completed the calculation correctly were given 3 out of the 4 marks available. Other errors were seen, examples of which were not converting the percentage to the correct number, not rooting the value and calculating the wrong acoustic impedance, Z1 rather than Z2.

  Text version

  A patient with a suspected broken arm may be X-rayed. A second patient has a suspected fault in the wall of the abdominal aorta, which may be examined using ultrasound or X-rays.

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• AQA 7408 · June 2017 · Paper 3BB · Q3 · 9 marks

  9 marks

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  This question was on a new part of the specification, PET scans. In part 03.1, a common answer was ‘a radioactive isotope’, which gave no additional information and could not be given the mark. Part 03.2 was well answered by many students. A common incorrect response was the complete line which started with 1 𝑇 , and then continued with a series of equal signs to 68.9 with no indication that this was in fact T. Part 03.3 asked for some discussion as to the suitable time the patient may be left to relax. As the question stated that the isotope was injected into the blood, credit was focussed on clear reference to the isotope being carried around the body in the blood stream. Very few students suggested that time may then be needed for the part of the body being investigated to take in the isotope. The most commonly awarded mark was for some sensible link between the time to relax and the effective half-life. This question revealed some fairly common misconceptions about the term half-life. Many stated or implied that a scan could not be performed after one half-life had passed, whilst others suggested that no decay would occur until this point. Some students used technetium-99 as the isotope, quoting a half-life of 6 hours, when they were told previously that the half-life of the isotope being used was 110 minutes. Part 03.4 on the process on annihilation was well answered with many students getting both marks. Part 03.5 on the time delay between photon detection provided many good answers. The maximum delay was worked out as 6.7 x 10-10s by most students, but some students were not able to suggest that the minimum time delay was in fact 0 for gamma photons produced in the centre of the head.

  Text version

  A PET scan uses an injected radionuclide with physical half-life 110 min and biological half-life 185 min. Gamma detectors are arranged around a head 0.20 m across.

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• AQA 7408 · June 2017 · Paper 3BB · Q4 · 5 marks

  5 marks

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  This question was based on ECG traces. Part 04.1 proved difficult. The labelling of the potential axis caused most problems with many students failing to put the 0 for the scale in the correct place. However, the scale on the time axis also caused problems. A common mistake was to place a number at the end of an axis without another number on the axis to provide a point of reference. For example, the sole number 0.8 at the right of the time axis did not gain credit. Part 04.2 required students to look at the noise on the trace and suggest reasons for that noise. Answers such as “the skin had not been cleaned”, or “dead skin and hair had not been removed”, were treated as neutral answers, but with the inclusion of “resulting in the electrodes not being securely connected” the correct point was made and a mark awarded. Another answer which was felt to be too vague for the award of a mark was “the electrodes weren’t properly attached”. Use of statistics Statistics used in this report may be taken from incomplete processing data. However, this data still gives a true account on how students have performed for each question.

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  An ECG trace from a healthy person has the normal P-QRS-T form. A second healthy person produces a noisy/faulty trace.

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• AQA 7408 · sample1 · Paper 3BB · Q4 · 10 marks

  10 marks

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  A radionuclide has physical half-life 20 days. Its corrected count rate in a patient falls from 2700 counts s^-1 to 1200 counts s^-1 in 5 days. Technetium-99m emits gamma radiation, has half-life 6.0 h and gamma energy 140 keV; iodine-131 emits beta-minus and gamma radiation, has half-life 190 h and gamma energy 610 keV.

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