Explain the principles of ultrasound imaging and its obstetric applications.

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Accepted Answer

## Ultrasound Imaging: Principles and Obstetric Applications

Ultrasound uses high-frequency sound waves (2 to 18 MHz), which are non-ionizing.

### Principle
A piezoelectric transducer converts electrical pulses into ultrasound pulses and converts returning echoes back into electrical signals (pulse-echo principle). Echoes arise at tissue boundaries with different acoustic impedance ($Z = ho c$). Depth is found from time of flight: $d = c\,t/2$ with $c \approx 1540$ m/s in soft tissue.

```mermaid
flowchart LR
  A[Pulser] --> B[Piezoelectric transducer]
  B --> C[Sound pulse into tissue]
  C --> D[Echo from boundaries]
  D --> B
  B --> E[Amplifier and TGC]
  E --> F[Scan converter]
  F --> G[Display]
```

### Display modes
| Mode | Shows |
|---|---|
| A-mode | Amplitude vs depth |
| B-mode | 2D brightness image |
| M-mode | Motion vs time (heart valves) |
| Doppler | Blood flow velocity: $f_d = 2 f v \cos	heta / c$ |

### Obstetric applications
- Confirm pregnancy and gestational age, count foetuses.
- Measure biparietal diameter, femur length and abdominal circumference to estimate foetal growth.
- Detect foetal heartbeat and malformations (anomaly scan).
- Locate placenta (placenta previa) and assess amniotic fluid.
- Doppler for foetal and umbilical blood flow; 3D/4D scans for the face and structure.
- Guide procedures such as amniocentesis.

### Advantages
Safe (no radiation), real-time, portable and low cost.

### Exam tip
Write the pulse-echo principle, $d = ct/2$, the block diagram and four obstetric uses.

Ultrasound Imaging: Principles and Obstetric Applications

Principle

Display modes

Obstetric applications

Advantages

Exam tip

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