AQA A-Level Physics: Medical Physics -- Complete Revision Guide
AQA A-Level Physics: Medical Physics -- Complete Revision Guide
Medical Physics is one of five optional units in AQA A-Level Physics (§3.10, Option B). Your school will have selected exactly one of the five options -- A (Astrophysics), B (Medical Physics), C (Engineering Physics), D (Turning Points in Physics), or E (Electronics) -- so you only need to study the option your centre has entered you for. Medical Physics appears in Paper 3, Section B, and is worth roughly a quarter of the Paper 3 marks.
This option is an unusually applied unit. It revisits ideas from across the compulsory specification -- waves, refraction, attenuation, charge and capacitance, magnetic fields, nuclear decay -- and bolts them onto the structure of a real diagnostic department. If you have ever wondered why a hospital owns a 1.5 T superconducting magnet, why ultrasound gel matters, or why a chest X-ray uses different settings to a mammogram, this is the unit that answers those questions. This guide walks through each of the AQA sub-topics in turn.
Physics of the Eye
The eye is treated as a two-lens optical instrument. Most of the converging power -- typically around 40 dioptres in a relaxed adult eye -- is supplied by the cornea, because it sits at the air-tissue interface where the refractive index change is largest. The crystalline lens contributes the remaining power (around 20 D in the relaxed state) and provides accommodation by changing shape under the action of the ciliary muscle.
Power is defined as the reciprocal of focal length in metres: P = 1/f. Powers of thin lenses in contact add, which is why the corneal and lens powers can be summed to give the total power of the eye (typically in the region of 60 D when the eye is relaxed and viewing a distant object). The near point and far point define the range of accommodation: in a healthy young adult these are conventionally taken as 25 cm and infinity.
You should be confident with the four standard refractive defects and their corrections:
- Myopia (short sight): the eye is too long, or its optical power too high. Distant objects focus in front of the retina. Corrected with a diverging (negative power) lens. The required power is calculated from the far point: P = -1/(far point in metres).
- Hyperopia (long sight): the eye is too short, or its optical power too low. The near point is further than 25 cm. Corrected with a converging (positive power) lens.
- Presbyopia: the age-related loss of accommodation as the lens stiffens. Reading glasses or varifocals supply the missing accommodation.
- Astigmatism: the cornea is not rotationally symmetric, so horizontal and vertical lines focus at different distances. Corrected with a cylindrical lens.
Exam tip: when calculating spectacle power for myopia, the spectacle lens must form a virtual image at the patient's far point of an object at infinity. The image distance is therefore the far point taken as negative, and the lens power follows immediately.
You should also understand spatial resolution of the eye in terms of cone spacing in the fovea and the Rayleigh criterion for diffraction at the pupil, and sensitivity in terms of rods versus cones, dark adaptation, and the spectral response curve.
Physics of the Ear
The ear is treated as a transducer that converts pressure variations in air into action potentials in the auditory nerve. The outer ear funnels sound onto the tympanic membrane. The middle ear contains the three ossicles (malleus, incus, stapes), which perform impedance matching between the low-impedance air column and the high-impedance fluid in the cochlea. Without this matching, almost all of the incident acoustic power would be reflected at the air-fluid boundary; the ossicles raise the transmitted fraction substantially, partly by lever action and partly by the area ratio between the tympanic membrane and the oval window.
The cochlea performs frequency analysis. The basilar membrane is stiffer and narrower at the base and floppier and wider at the apex, so different positions resonate at different frequencies -- high frequencies near the oval window, low frequencies deeper in. Hair cells transduce the mechanical motion into nerve signals.
Loudness is measured on a logarithmic decibel scale because the ear's response is approximately logarithmic over six orders of magnitude of intensity. The reference intensity is the threshold of hearing at 1 kHz, taken as 10^-12 W/m^2. Intensity level in decibels is then IL = 10 log10(I/I_0), and the loudness-weighted dBA scale corrects for the ear's non-flat frequency response.
Audiograms plot hearing threshold (in dB above the standard reference) against frequency, typically from 250 Hz to 8 kHz. A flat audiogram near 0 dB indicates normal hearing. A characteristic dip around 4 kHz is typical of noise-induced hearing loss. You should be able to distinguish:
- Conductive hearing loss: outer or middle ear pathology (wax, otitis media, otosclerosis). Air-conduction thresholds are raised but bone-conduction thresholds are normal.
- Sensorineural hearing loss: cochlear or neural damage. Both air- and bone-conduction thresholds are raised, and the loss is often worse at higher frequencies.
Biological Measurement and the ECG
The action potential is a transient depolarisation of an excitable cell membrane. At rest, the inside of a cardiac muscle cell is at roughly -90 mV with respect to the extracellular fluid, maintained by the sodium-potassium pump. A stimulus opens fast sodium channels, sodium flows in, and the membrane potential swings positive. Calcium channels then sustain a plateau, before potassium efflux returns the cell to its resting potential.
When a wave of depolarisation sweeps across the heart, the moving boundary between depolarised and polarised tissue produces a time-varying electric dipole. Electrodes placed on the skin pick up the projection of this dipole onto each lead axis, giving the electrocardiogram (ECG).
The classic three-letter labels are:
- P wave -- atrial depolarisation.
- QRS complex -- ventricular depolarisation (large because the ventricles have the most muscle mass).
- T wave -- ventricular repolarisation.
The 12-lead ECG uses ten physical electrodes (four limb leads and six chest leads) to produce twelve different projections of the cardiac dipole. Different leads give complementary views of the heart and help localise ischaemia or conduction defects. You will not be expected to interpret arrhythmias clinically, but you should understand why multiple leads are recorded and what each part of the waveform represents.
Signal amplitudes at the skin are small (on the order of a millivolt), so high-gain differential amplifiers with strong common-mode rejection are required, together with band-pass filtering to remove mains pickup and baseline drift.
Ultrasound, Fibre Optics, and Endoscopy
Diagnostic ultrasound uses longitudinal pressure waves in the range typically 1-15 MHz. Lower frequencies (around 2-5 MHz) penetrate deeper and are used for abdominal and obstetric scanning; higher frequencies (10 MHz and above) give better resolution but penetrate only a few centimetres, and are used for vascular and superficial imaging.
The transducer is a slab of piezoelectric material (commonly lead zirconate titanate) which expands and contracts when a high-frequency voltage is applied. The same element receives the returning echoes by the reverse piezoelectric effect.
Reflections at tissue boundaries are governed by the acoustic impedance Z = density x speed of sound. The intensity reflection coefficient at a boundary between media of impedances Z1 and Z2 is R = ((Z2 - Z1)/(Z2 + Z1))^2. Because the impedance mismatch between air and skin is very large, almost all of the ultrasound would be reflected at the skin surface without a coupling medium. Gel with an impedance close to that of soft tissue is used to exclude air.
Distance to a reflector is determined from the echo time t and the speed of sound c by d = ct/2. A-scans display amplitude against depth along a single line; B-scans build a two-dimensional image by sweeping the beam.
Fibre optics and endoscopy: Optical fibres rely on total internal reflection at the core-cladding boundary. An endoscope typically contains two coherent fibre bundles -- an illumination bundle that carries light from an external source to the tissue, and an imaging bundle that returns a coherent image to a camera or eyepiece. Numerical aperture, the critical angle, and modal dispersion are all relevant concepts here.
Magnetic Resonance Imaging (MRI)
MRI exploits the nuclear magnetic moment of the hydrogen nucleus (the proton). In an external static magnetic field B_0, proton spins precess at the Larmor frequency f = gamma B_0 / (2 pi), where gamma is the gyromagnetic ratio of the proton. At a clinical field strength of 1.5 T, the Larmor frequency is in the radiofrequency range (roughly 64 MHz).
A short radiofrequency pulse at the Larmor frequency tips the net magnetisation away from the longitudinal axis. After the pulse, the magnetisation relaxes back to equilibrium in two characteristic ways:
- T1 (longitudinal) relaxation: recovery of the magnetisation along B_0, on the order of hundreds of milliseconds to a few seconds in soft tissue.
- T2 (transverse) relaxation: dephasing of the rotating component, typically tens to hundreds of milliseconds.
By varying the timing of the radiofrequency pulses, an image can be made T1-weighted, T2-weighted, or proton-density weighted, giving very different soft-tissue contrast from the same anatomy. Spatial localisation is achieved by superimposing gradient fields on B_0 so that the Larmor frequency varies linearly across the body. Frequency and phase encoding then allow a two-dimensional image to be reconstructed by inverse Fourier transform.
MRI is non-ionising, which is a major safety advantage over X-rays and radionuclide imaging. The principal hazards are projectile risk from ferromagnetic objects in the strong field, radiofrequency heating, and acoustic noise from the gradient coils.
X-Ray Imaging
X-rays are produced in a vacuum tube by accelerating electrons through a potential difference of typically 50-150 kV onto a tungsten target. The resulting spectrum is a continuous bremsstrahlung background with characteristic line emissions superimposed where electrons knock out inner-shell electrons of the target atoms.
The intensity I transmitted through tissue of thickness x follows an exponential attenuation law: I = I_0 exp(-mu x), where mu is the linear attenuation coefficient. The half-value thickness is x_(1/2) = ln 2 / mu. The attenuation coefficient depends strongly on photon energy and on the effective atomic number of the tissue, which is why bone (high in calcium) attenuates X-rays much more strongly than soft tissue.
Image quality is determined by sharpness, contrast, and noise. Contrast can be enhanced with contrast media (barium for the gastrointestinal tract, iodine for blood vessels). Computed tomography (CT) acquires many projections from different angles and reconstructs a three-dimensional dataset, giving much better soft-tissue contrast than plane radiography but at a higher radiation dose.
You should know typical effective doses in millisieverts for common examinations only to the level of relative magnitude -- a chest X-ray is a fraction of a millisievert, a CT scan substantially more, and natural background dose is roughly a couple of millisieverts per year in the UK.
Radionuclide Imaging and Therapy
Radionuclide imaging is functional rather than purely anatomical: the distribution of a radioactive tracer reveals physiology. The workhorse isotope is technetium-99m, which decays to its ground state with the emission of a 140 keV gamma ray and has a half-life of approximately 6 hours -- long enough for an examination, short enough that the patient's residual activity falls quickly. Technetium-99m can be chelated to a wide range of pharmaceuticals to target different organs.
The gamma camera consists of a lead collimator (which restricts the accepted directions of incoming gammas), a large NaI(Tl) scintillator crystal, an array of photomultiplier tubes, and position and energy circuitry. Position is reconstructed by comparing the signals from the photomultipliers, and an energy window rejects scattered photons.
Positron emission tomography (PET) uses positron-emitting isotopes such as fluorine-18. Each emitted positron annihilates with a nearby electron and produces two 511 keV gamma rays travelling in opposite directions. Coincidence detection by a ring of detectors localises the annihilation event along a line, and many such events are reconstructed into a three-dimensional image. The most widely used PET tracer is fluorodeoxyglucose (FDG), which highlights tissues with high glucose uptake -- often used in oncology and cardiology.
Therapy uses higher-activity sources to deliver a targeted radiation dose. External beam radiotherapy uses high-energy X-rays from a linear accelerator. Brachytherapy places sealed sources close to or inside a tumour. Targeted radionuclide therapy uses unsealed sources of beta-emitters (such as iodine-131 for thyroid disease).
How to Study This Topic
Medical Physics rewards a structured approach because the unit deliberately spans several different physics topics.
- Build a one-page modality summary. For each imaging technique (X-ray, CT, ultrasound, MRI, gamma camera, PET) note: the physical interaction used, typical energies or frequencies, spatial resolution, contrast mechanism, dose (if ionising), and a clinical example. This gives you the comparative material the examiner often asks for.
- Drill the equations. I = I_0 exp(-mu x); R = ((Z2 - Z1)/(Z2 + Z1))^2; IL = 10 log10(I/I_0); P = 1/f for lenses; d = ct/2 for ultrasound; and the half-life relations from the compulsory specification.
- Common pitfalls: confusing intensity ratio (raw) with intensity level (decibels); mixing up T1 and T2 weighting; forgetting that the ultrasound round-trip time involves the factor of two; treating the eye as a single thin lens rather than a two-element system.
- Past-paper practice. Optional units re-use a small bank of structured questions, so working through several years' Paper 3 Section B questions on Medical Physics is the highest-yield single revision activity.
Related LearningBro Courses
The dedicated course pages on LearningBro give you full lessons, worked examples, and practice questions for each of the AQA A-Level Physics units:
- AQA A-Level Physics: Medical Physics -- the course that maps directly onto this guide.
- AQA A-Level Physics: Mechanics and Electricity
- AQA A-Level Physics: Waves and Particles
- AQA A-Level Physics: Thermal Physics and Fields
- AQA A-Level Physics: Nuclear Physics and Astrophysics
Remember that Medical Physics is one of five optional units. If your school has entered you for a different option, see our companion guides for Engineering Physics, Turning Points in Physics, and Electronics.
Common Exam Pitfalls
Medical Physics rewards a comparative, modality-by-modality discipline. The standard mistakes are predictable.
- Decibels treated as a linear scale. Intensity level IL = 10 log10(I / I_0) is logarithmic: an increase of 10 dB is a factor-of-10 increase in intensity, not "10 times louder" in the colloquial sense. Doubling the intensity gives only a 3 dB increase; doubling the perceived loudness corresponds to roughly 10 dB.
- Ultrasound round-trip arithmetic. The reflector distance is d = c t / 2, where t is the total go-and-return time. Forgetting the factor of 2 doubles the depth estimate. Conversely, when working from depth to expected echo time, multiply rather than divide by 2.
- Acoustic impedance units mishandled. Z = density x speed of sound has units of kg m^-2 s^-1, often written as Pa s m^-1. The reflection coefficient R = ((Z_2 - Z_1) / (Z_2 + Z_1))^2 is dimensionless, but the algebra is easier if you keep both impedances in the same units throughout.
- MRI T1 versus T2 weighting. T1 relaxation is the recovery of longitudinal magnetisation; T2 is the decay of transverse magnetisation. T1-weighted images show fluid as dark; T2-weighted images show fluid as bright. Memorising "T2 = wet bright" prevents the most common error.
- X-ray attenuation linearised wrongly. The exponential law I = I_0 exp(-mu x) is solved by taking natural logs, not common logs. ln(I_0 / I) = mu x; the half-value thickness is x_(1/2) = ln 2 / mu, not log 2 / mu.
- Lens power signs. Power P = 1/f in dioptres, with f in metres. Converging (corrective) lenses for hyperopia have positive power; diverging (corrective) lenses for myopia have negative power. A "calculate the spectacle power for a myope with far point 50 cm" question expects P = -1 / 0.50 = -2.0 D.
- Half-life of technetium-99m used in dose calculations. The physical half-life is about 6 hours; the biological half-life (rate of excretion) depends on the chelate; the effective half-life is the harmonic combination 1/T_eff = 1/T_phys + 1/T_bio. A question that quotes "the half-life of the tracer in the body" is the effective half-life, not the physical one.
Recommended Three-Week Revision Schedule
The Medical Physics option is best revised as six self-contained modules. A three-week plan splits naturally as follows.
- Week 1: Imaging by light and sound. Days 1-2 on the eye (lens power, defects, corrections). Days 3-4 on the ear (impedance matching, decibels, audiograms). Days 5-7 on ultrasound (transducer, reflection coefficient, A- and B-scans, gel) and endoscopy (total internal reflection, fibre bundles).
- Week 2: Imaging by EM radiation and magnetic resonance. Days 1-3 on X-ray imaging (bremsstrahlung, attenuation, CT). Days 4-7 on MRI (Larmor frequency, T1 and T2, gradient fields, safety).
- Week 3: Radionuclide imaging and exam practice. Days 1-3 on gamma cameras, PET, and therapy. Days 4-7 on Paper 3 Section B past papers, marked against AQA mark schemes, with a focus on the comparative six-mark questions that ask you to contrast two modalities.
Keep a single A4 modality summary sheet at the centre of your revision (physical interaction, typical energies, spatial resolution, dose, contrast mechanism, clinical example for each of the six imaging methods). It is the single most useful revision artefact for this unit.
A small but high-yield habit is to revise the unit in matched pairs. The eye and the ear share a comparative structure (each is a transducer with an impedance-matching front end and a frequency-resolving back end). Ultrasound and X-ray imaging share the same underlying inverse-problem structure (you have detected signal and want to reconstruct an internal anatomical map). MRI and PET are both based on resonance phenomena (the Larmor precession for MRI; the annihilation coincidence energy of 511 keV for PET). Pairing the topics rather than treating each modality as an isolated silo makes the comparative six-mark questions much easier to answer in the exam.