Ultrasound in Medical & Industrial Imaging

What is Ultrasound?

Ultrasound refers to sound waves with frequencies above 20 kHz, which is higher than the upper limit of human hearing. These waves are longitudinal waves, meaning the vibrations of particles in the medium are parallel to the direction the wave travels. Because ultrasound frequencies are too high for humans to hear, they are inaudible but can be used in various imaging applications. Typical ultrasound frequencies used in medical and industrial imaging range from about 1 MHz to 15 MHz.

Medical Imaging Uses

Ultrasound is widely used in medicine to create images of the inside of the body. It helps detect the structure of organs such as the heart, liver, and unborn babies in the womb. The process is non-invasive, meaning it does not require surgery or entering the body, and it is considered very safe since it does not use ionising radiation.

Ultrasound waves are sent into the body, and when they hit a boundary between different tissues (e.g., fluid and soft tissue, or soft tissue and bone), some waves are reflected back. These echoes are detected and used to form an image of the internal structures.

Industrial Imaging Uses

In industry, ultrasound is used for non-destructive testing to check the quality of materials and structures without damaging them. It can detect flaws such as cracks or voids inside metals, plastics, or composites.

Ultrasound is also used to measure the thickness of materials, especially where one side is inaccessible, such as pipes or aircraft parts. By sending pulses of ultrasound and measuring the time it takes for echoes to return from the far side, the thickness can be calculated accurately.

How Ultrasound Works

Ultrasound imaging relies on sending short pulses of ultrasound waves into a material or body. These pulses travel through the medium until they reach a boundary between two different materials or tissues. At this boundary, some of the ultrasound waves are reflected back as echoes.

By measuring the time delay between sending the pulse and receiving the echo, the distance to the boundary can be calculated using the formula:

$$\text{Distance} = \frac{\text{Speed of ultrasound} \times \text{Time delay}}{2}$$

The division by 2 accounts for the ultrasound wave travelling to the boundary and back.

Multiple echoes from different boundaries create a pattern that can be processed to form an image showing the internal structure of the object or body part.

For instance, if ultrasound travels at 1500 m/s in soft tissue and the time delay for an echo is 0.0004 s, the distance to the boundary is:

$$\text{Distance} = \frac{1500 \times 0.0004}{2} = 0.3 \text{ m}$$

This means the boundary is 0.3 metres away from the ultrasound probe.

Ultrasound Reflection and Image Formation

When ultrasound waves hit a boundary between two materials with different densities or elastic properties, part of the wave is reflected back, and part is transmitted through. The amount of reflection depends on the difference in acoustic impedance between the materials.

In medical imaging, these reflections create echoes that are detected by the probe and converted into images showing the shape and position of organs or tissues.

In industrial testing, echoes reveal internal flaws or measure thickness by timing how long it takes for echoes to return from the far side of the material.