20.2 Diagnostic Use of Ultrasound in Medical Imaging

20.2 Diagnostic Use of Ultrasound in Medical Imaging

Ultrasound is a widely used medical imaging technique that allows doctors to view internal body structures in real time. The labelled diagram of ultrasound machine is shown in Fig. 20.2. It operates by sending high frequency sound waves, typically in the range of $1\,\mathrm{MHz}$–$15\,\mathrm{MHz}$, into the body using a device called a transducer. These waves travel through the body and reflect off boundaries between different tissues. The reflected echoes are then collected and analyzed to create an image. One of the key advantages of ultrasound is that it does not use ionizing radiation and making it a safe, non-invasive, and repeatable imaging method. The functioning of ultrasound imaging is based on several physical principles. First, the piezoelectric transducer emits pulses of ultrasound waves, which travel at different speeds depending on the type of tissue they pass through. When these waves encounter a boundary between two tissues with different acoustic impedances (determined by tissue density and sound speed), part of the wave is reflected. The transducer then switches to receive mode and detects these reflected waves using the direct piezoelectric effect. These echoes are converted into electrical signals and processed by the machine. Finally, a computer calculates the time delay and intensity of the returning echoes to construct a visual image. Areas with stronger reflections appear brighter in the image, helping to reveal the internal structures of organs and tissues.

(Fig. 20.2: Ultrasound machine)

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The crystals in an ultrasound probe are smaller than a grain of rice, but they can both send and detect sound waves.