Applications of Ultrasound Imaging Revision Notes | A Level AQA Physics

1. Why Ultrasound? The Case for Sound Over Radiation

Picture a clinician needing to image a developing foetus. X-rays are out — ionising radiation poses real risk to rapidly dividing cells. MRI is slow and expensive. Ultrasound, however, uses high-frequency sound waves that reflect harmlessly off tissue boundaries and return in microseconds. That reflected signal carries precise positional and structural information, and it costs a fraction of alternative methods.

The physics that makes this work is straightforward: whenever a sound wave crosses a boundary between two materials with different acoustic impedances, part of the wave reflects and part transmits. The greater the mismatch in impedance, the stronger the reflection. Medical ultrasound exploits this at every tissue interface inside the body.

This reflection principle underpins every application you need to know. Whether the probe is scanning a foetus, measuring the depth of a retinal detachment, or checking a weld in a steel pipe, the same physics applies: send a pulse, time the echo, calculate the distance using $d = \frac{v \Delta t}{2}$.

2. A-Scans and B-Scans: Two Ways to Read an Echo

Before examining specific applications, you need to be clear on the two scan modes, because exam questions frequently ask you to distinguish them or justify which is appropriate for a given clinical task.

An A-scan (amplitude scan) fires a single ultrasound pulse along one line and displays the returning echoes as peaks on a voltage–time graph. Each peak represents a boundary, and the time between the transmitted pulse and the echo tells you the depth of that boundary. The height of the peak encodes the strength of the reflection, which depends on the impedance mismatch at that boundary.

A B-scan (brightness scan) builds a two-dimensional cross-sectional image by sweeping the beam across many angles or positions. Each A-scan line contributes one column of pixels to the image, with echo amplitude converted to pixel brightness. The result is the familiar grey-scale image seen in obstetric scanning.

3. Clinical Applications: Where Ultrasound Is Used

Ultrasound imaging has a wide range of clinical applications, each justified by the physical properties of the technique. The most important ones for AQA A Level are obstetrics, cardiology (echocardiography), ophthalmology, and abdominal organ imaging. Each application exploits the same underlying physics but in a different anatomical context.

3a. Obstetrics: Foetal Scanning

Foetal scanning is the most widely recognised application of ultrasound. A B-scan probe is moved across the mother's abdomen, building a real-time cross-sectional image of the uterus and foetus. Because ultrasound is non-ionising, it carries no known risk of radiation damage to the rapidly dividing cells of the developing foetus — a critical advantage over X-ray imaging.

The scan can confirm gestational age by measuring foetal dimensions (such as crown-rump length or femur length), detect multiple pregnancies, assess placental position, and identify structural abnormalities. Real-time imaging is possible because modern probes fire and receive pulses thousands of times per second, updating the image continuously.

A coupling gel is applied between the probe and the skin to eliminate the air gap. Air has a very low acoustic impedance compared with skin, so without gel, almost all the ultrasound would reflect at the skin surface and none would enter the body. The gel fills the air gap, matching the acoustic impedance between the probe face and the skin surface, reducing the impedance mismatch and maximising ultrasound transmission into the tissue.

3b. Echocardiography: Imaging the Heart

Echocardiography uses ultrasound to image the heart's chambers, valves, and walls in real time. Because the heart is in continuous motion, the high temporal resolution of ultrasound — its ability to update images rapidly — makes it far better suited to cardiac imaging than MRI or CT for many routine assessments.

Doppler ultrasound is also used in cardiology to measure blood flow velocity. When ultrasound reflects from moving red blood cells, the frequency of the returning echo is shifted relative to the transmitted frequency — the Doppler effect. The magnitude of this frequency shift is proportional to the velocity of the blood, allowing clinicians to detect abnormal flow patterns such as those caused by valve stenosis or regurgitation.

3c. Ophthalmology: Measuring Eye Dimensions

In ophthalmology, A-scans are used to measure the axial length of the eye — the distance from the cornea to the retina. This measurement is essential for calculating the correct power of an intraocular lens implant before cataract surgery. The ultrasound pulse travels through the eye, reflecting at each tissue boundary (cornea, lens surfaces, retina), and the time between echoes gives the distance between structures.

B-scans are used in ophthalmology when the clinician needs a two-dimensional view — for example, to locate a retinal detachment or a tumour within the eye when the view through the pupil is obscured by a dense cataract or vitreous haemorrhage.

3d. Abdominal Imaging: Liver, Kidneys, and Gallbladder

Ultrasound is the first-line imaging investigation for many abdominal conditions. It can identify gallstones (which produce strong reflections due to their high impedance relative to surrounding bile), assess kidney size and structure, detect liver abnormalities such as cysts or tumours, and guide needle placement during biopsies.

One important limitation here is that ultrasound does not pass well through gas or bone. The bowel contains gas, which causes almost total reflection of the ultrasound beam, creating acoustic shadows that obscure structures behind. This is why ultrasound is less useful for imaging gas-filled structures such as the stomach or lungs, and why it cannot image structures behind the skull or ribs without specialist approaches.

4. Worked Example: Calculating Depth from an Echo

The distance calculation is a core exam skill. The key insight is that the ultrasound pulse travels to the boundary and back, so the total path length is twice the depth. You must halve the product of speed and time to get the one-way distance.

Worked example: Calculating the Depth of a Tissue Boundary from an Ultrasound Echo

An ultrasound pulse is transmitted into soft tissue. An echo is detected 80 μs after the pulse is sent. The speed of ultrasound in soft tissue is 1500 m s$^{-1}$. Calculate the depth of the reflecting boundary.

1. $d = \frac{v \Delta t}{2}$ — The factor of 2 accounts for the pulse travelling to the boundary and back. This is the standard equation for pulse-echo depth measurement. [M1]
2. $\Delta t = 80 \times 10^{-6} \text{ s} = 8.0 \times 10^{-5} \text{ s}$ — Always convert microseconds to seconds before substituting. 1 μs = 1 × 10$^{-6}$ s, so 80 μs = 80 × 10$^{-6}$ s.
3. $d = \frac{1500 \times 8.0 \times 10^{-5}}{2}$ — Speed = 1500 m s$^{-1}$, time = 8.0 × 10$^{-5}$ s. [M1]
4. $1500 \times 8.0 \times 10^{-5} = 0.12 \text{ m}$ — 1500 × 8.0 = 12000, then 12000 × 10$^{-5}$ = 0.12. This is the total path length (to boundary and back).
5. $d = \frac{0.12}{2} = 0.060 \text{ m}$ — Halving gives the one-way distance from probe to boundary. [A1]
6. $d = 0.060 \text{ m} = 6.0 \text{ cm}$ — Always include units. Converting to cm is acceptable and often more intuitive for biological depths.

Answer: $d = 0.060$ m (6.0 cm)

5. Comparing Ultrasound with Other Imaging Techniques

Exam questions often ask you to compare ultrasound with X-ray, CT, or MRI. The comparison must go beyond listing features — you need to link each advantage or disadvantage to an underlying physical or clinical reason.

The non-ionising nature of ultrasound is its single most important advantage in obstetrics and paediatrics, where repeated exposure to ionising radiation carries cumulative risk. Its real-time capability makes it uniquely suited to dynamic assessments such as cardiac valve function or guided needle procedures.

Where ultrasound falls short is in imaging structures behind bone (such as the adult brain) or within gas-filled cavities. For these, CT or MRI is preferred despite the higher cost or radiation dose. Recognising these trade-offs is exactly what an 'evaluate' or 'suggest' question is testing.

6. Non-Medical Applications of Ultrasound

The same pulse-echo principle used in medical scanning has important industrial applications. Non-destructive testing (NDT) uses ultrasound to detect internal flaws — cracks, voids, or inclusions — in materials such as steel, concrete, and composites, without cutting or damaging the material being tested.

In NDT, a transducer is coupled to the surface of the material (again using a coupling gel or fluid to eliminate air gaps). The pulse travels through the material and reflects from any internal boundary — including a crack or void — as well as from the far surface. The time and amplitude of each echo reveal the depth and size of any defect. This is directly analogous to the A-scan used in ophthalmology.

Sonar (Sound Navigation And Ranging) is another application: ships and submarines use ultrasound pulses to measure water depth and detect underwater objects. The pulse travels through water, reflects from the seabed or an object, and the echo time gives the distance. The physics is identical to medical pulse-echo imaging — only the medium and frequency range differ.

7. Exam Technique: Scoring Full Marks on Ultrasound Application Questions

Ultrasound application questions appear in several formats at A Level. 'Describe' questions want a clear sequence of events. 'Explain' questions demand a physical reason linked to a named principle. 'Evaluate' questions require you to weigh advantages against limitations and reach a justified conclusion. 'Suggest' questions expect you to apply your knowledge to an unfamiliar context — there may be more than one acceptable answer.

For any question asking why ultrasound is used in a specific context, structure your answer around three pillars: the physical property that makes it work (impedance mismatch, reflection, pulse-echo timing), the clinical or practical advantage (non-ionising, real-time, portable, low cost), and any relevant limitation (cannot penetrate bone or gas, operator-dependent image quality).