Ultrasound and Its Medical and Industrial Applications

1. Introduction

Ultrasound refers to sound waves with frequencies higher than the approximate upper limit of human hearing, which is around 20,000 Hz or 20 kHz.

Although humans cannot normally hear ultrasound, it can be generated, transmitted, detected and analysed using specialised electronic and mechanical equipment.

Ultrasound has major applications in medical diagnosis, pregnancy monitoring, industrial inspection, cleaning, distance measurement, navigation and scientific research.

Its ability to travel through materials and reflect from boundaries makes it especially useful for examining internal structures without destructive procedures.

2. Definition and Characteristics of Ultrasound

Ultrasound consists of mechanical sound waves with frequencies above approximately 20 kHz.

Its principal characteristics include:

1. It is a mechanical wave.
2. It requires a material medium for propagation.
3. It cannot travel through a vacuum.
4. It can be reflected, refracted, absorbed and transmitted.
5. It can travel through gases, liquids and solids.
6. Its wavelength is generally shorter than that of audible sound at the same speed.
7. It can provide detailed information about small structures when suitable frequencies and equipment are used.

Ultrasound is not a separate form of energy from sound. It is sound in a frequency range above ordinary human hearing.

3. Frequency and Wavelength

The relationship between wave speed, frequency and wavelength is:

[
v=f\lambda
]

Therefore:

[
\lambda=\frac{v}{f}
]

For a fixed medium, increasing frequency reduces wavelength.

This is important because shorter wavelengths can improve the ability of an ultrasound system to distinguish closely spaced structures, although penetration depth and attenuation must also be considered.

3.1 Medical Ultrasound Frequencies

Diagnostic medical ultrasound commonly uses frequencies in the approximate range of 2 to 15 MHz, although the actual range varies with the application.

Lower frequencies generally penetrate deeper into the body but provide lower spatial resolution.

Higher frequencies generally provide better spatial resolution but have less penetration depth.

3.2 Industrial Ultrasound Frequencies

Industrial ultrasonic systems use different frequencies according to their purpose.

Applications include:

• Material testing
• Thickness measurement
• Cleaning
• Welding
• Flow measurement
• Level sensing

4. Production of Ultrasound

Ultrasound is commonly produced using piezoelectric materials.

The piezoelectric effect is the ability of certain materials to generate an electrical potential when mechanically stressed and to undergo mechanical deformation when an electric field is applied.

4.1 Piezoelectric Effect

A piezoelectric transducer converts electrical energy into mechanical vibrations and mechanical vibrations into electrical signals.

When an alternating electrical voltage is applied, the piezoelectric material vibrates at the driving frequency.

These vibrations generate ultrasonic waves.

4.2 Common Piezoelectric Materials

Examples include:

• Quartz
• Lead zirconate titanate (PZT)
• Certain engineered piezoelectric ceramics
• Other specialised piezoelectric materials

4.3 Ultrasonic Transducers

An ultrasonic transducer is a device that converts electrical energy into ultrasonic waves or converts returning ultrasonic waves into electrical signals.

In many systems, the same transducer performs both transmission and reception.

5. Detection of Ultrasound

Ultrasound cannot normally be detected by human hearing. It requires electronic sensors and transducers.

The process commonly involves:

1. Generating an electrical signal.
2. Converting the signal into ultrasonic vibrations.
3. Sending the waves through a material or medium.
4. Receiving reflected or transmitted waves.
5. Converting the received waves into electrical signals.
6. Processing the signals to calculate distance, thickness, flow or internal structure.

The result may be displayed as an image, measurement, waveform or digital signal.

6. Medical Applications of Ultrasound

Medical ultrasound is widely used because it can produce images of internal structures without using ionising radiation.

6.1 Diagnostic Ultrasound Imaging

Diagnostic ultrasound uses high-frequency sound waves to create images of organs, tissues and other structures.

The system transmits pulses into the body and analyses returning echoes.

Different tissues reflect sound differently because of differences in acoustic impedance and tissue structure.

6.2 Pregnancy and Fetal Monitoring

Ultrasound is commonly used during pregnancy to examine fetal development and assess:

• Fetal position
• Growth
• Gestational age estimates
• Placental location
• Amniotic fluid
• Certain structural features

It is an important tool in prenatal care, although it does not replace all other medical examinations.

6.3 Abdominal Imaging

Ultrasound can examine organs such as:

• Liver
• Gallbladder
• Kidneys
• Spleen
• Pancreas
• Urinary bladder

It is frequently used to investigate abdominal pain and certain organ abnormalities.

6.4 Cardiac Ultrasound

Echocardiography uses ultrasound to examine the structure and function of the heart.

It can help assess:

• Heart chambers
• Heart valves
• Pumping function
• Blood flow
• Certain structural abnormalities

6.5 Doppler Ultrasound

Doppler ultrasound measures changes in the frequency of reflected ultrasound waves caused by moving blood cells.

It is used to evaluate blood flow in arteries and veins.

Applications include:

• Detecting narrowed blood vessels
• Assessing blood circulation
• Evaluating certain vascular conditions
• Monitoring blood flow in selected organs

6.6 Musculoskeletal Ultrasound

Musculoskeletal ultrasound can examine:

• Tendons
• Ligaments
• Muscles
• Joints
• Superficial soft tissues

It can assist in diagnosing injuries and guiding certain medical procedures.

6.7 Therapeutic Ultrasound

Therapeutic ultrasound uses acoustic energy to produce effects in tissues.

Depending on the equipment and treatment, applications include:

• Physiotherapy
• Tissue heating
• Certain specialised medical procedures
• High-intensity focused ultrasound (HIFU)

HIFU concentrates acoustic energy in a targeted region to produce controlled tissue effects.

Therapeutic ultrasound applications differ from diagnostic imaging and require appropriate clinical supervision.

7. Principle of Medical Ultrasound Imaging

Medical ultrasound imaging is based largely on the reflection of sound waves at boundaries between tissues with different acoustic impedances.

7.1 Acoustic Impedance

Acoustic impedance is the product of a medium’s density and the speed of sound in that medium.

[
Z=\rho v
]

Where:

• (Z) = acoustic impedance
• (\rho) = density
• (v) = sound speed

The difference in acoustic impedance between two tissues affects the amount of sound reflected at their boundary.

7.2 Reflection of Ultrasound

When ultrasound reaches a boundary between tissues, part of the energy may be reflected and part transmitted.

The returning echoes are detected by the transducer and used to construct an image.

7.3 Time-of-Flight Principle

The depth of a reflecting structure can be calculated from the time taken by an ultrasound pulse to travel to the structure and return.

[
d=\frac{vt}{2}
]

Where:

• (d) = depth of the reflecting structure
• (v) = speed of sound in the tissue
• (t) = round-trip time

Medical scanners use calibrated assumptions and signal-processing methods to convert echo timing into image depth.

7.4 Ultrasound Gel

A coupling gel is applied between the transducer and skin.

It removes the air gap and improves transmission of ultrasound energy because air causes strong reflection at the skin interface.

8. Types of Ultrasound Imaging

8.1 A-Mode

A-mode, or amplitude mode, displays returning echoes as peaks whose amplitudes represent echo strength.

It is used in specialised measurement applications.

8.2 B-Mode

B-mode, or brightness mode, displays echo strength as image brightness.

It is the basis of common two-dimensional diagnostic ultrasound imaging.

8.3 M-Mode

M-mode, or motion mode, records the movement of structures over time.

It is useful in selected cardiac examinations.

8.4 Doppler Imaging

Doppler imaging uses frequency shifts in reflected ultrasound to evaluate movement, particularly blood flow.

Colour Doppler displays flow information using colour coding, while spectral Doppler displays flow velocity over time.

9. Industrial Applications of Ultrasound

9.1 Non-Destructive Testing

Ultrasonic testing is a non-destructive testing method used to detect internal defects without destroying the material.

It is widely used for:

• Weld inspection
• Metal testing
• Pipeline examination
• Railway track inspection
• Aircraft component inspection
• Structural engineering assessment

The method detects reflected signals from cracks, voids, inclusions and other discontinuities.

9.2 Thickness Measurement

Ultrasonic thickness gauges measure the thickness of materials by sending sound pulses through them and analysing the returning reflections.

They are useful for monitoring corrosion and material wear in:

• Pipelines
• Storage tanks
• Pressure vessels
• Ship structures
• Industrial equipment

9.3 Ultrasonic Cleaning

Ultrasonic cleaning uses high-frequency sound waves in a liquid to produce cavitation.

Cavitation involves the formation, growth and collapse of microscopic bubbles.

The resulting localised effects help remove dirt and contaminants from complex surfaces.

Applications include cleaning:

• Jewellery
• Surgical instruments
• Electronic components
• Precision tools
• Laboratory equipment

9.4 Ultrasonic Welding

Ultrasonic welding uses high-frequency mechanical vibrations and pressure to join materials.

It is used particularly for certain plastics and thin metal components.

The vibrations generate heat at the joining interface, enabling the materials to bond.

9.5 Distance and Level Measurement

Ultrasonic sensors emit pulses and measure the time taken for echoes to return.

They can be used to measure:

• Distance to an object
• Liquid levels in tanks
• Object presence
• Position in automated systems

9.6 Flow Measurement

Certain ultrasonic flow meters measure fluid flow by analysing the transmission time or Doppler shift of ultrasonic signals.

These instruments are used in water distribution, industrial processing and energy systems.

10. Ultrasound in Navigation and Biology

10.1 SONAR

SONAR uses sound waves to detect and locate underwater objects.

Ultrasound may be used in some sonar-related systems, especially for short-range sensing and measurement.

10.2 Echolocation

Bats produce ultrasonic sounds and interpret returning echoes to navigate and detect prey.

Dolphins also use high-frequency sound signals for navigation and prey detection.

These biological systems demonstrate how reflected sound can provide information about the environment.

11. Advantages of Ultrasound

Important advantages include:

1. It does not use ionising radiation in diagnostic imaging.
2. It can produce real-time images.
3. It is useful for examining soft tissues.
4. Equipment can be portable.
5. It is generally non-invasive.
6. It can measure distances and thicknesses.
7. It can detect internal material defects without destroying the object.
8. It can be used for flow measurement.
9. It can clean complex components.
10. It can guide certain medical procedures.

12. Limitations of Ultrasound

Ultrasound also has limitations:

1. Sound waves are strongly reflected by air and bone interfaces.
2. Image quality depends on the operator and equipment.
3. High frequencies are attenuated more rapidly in many tissues.
4. Some structures are difficult to image because of their location.
5. Ultrasound may not provide sufficient information for every medical condition.
6. Industrial testing requires suitable calibration and skilled interpretation.
7. Ultrasound cannot penetrate every material equally effectively.
8. Therapeutic applications require appropriate safety controls.

13. Ultrasound Safety

Diagnostic ultrasound is generally regarded as a safe imaging technique when used appropriately by trained professionals.

However, ultrasound energy can produce thermal and mechanical effects in tissues.

The two important safety concepts are:

• Thermal effects: heating caused by absorption of acoustic energy.
• Mechanical effects: effects associated with pressure variations and phenomena such as cavitation.

The thermal index and mechanical index are displayed by many diagnostic ultrasound systems as indicators relevant to potential bioeffects.

The principle of prudent use is to employ the lowest acoustic output and shortest examination time consistent with obtaining the required diagnostic information.

14. Worked Numerical Examples

Example 1: Wavelength of Ultrasound

An ultrasound wave has a frequency of 2 MHz and travels through a medium at 1,500 m/s. Calculate its wavelength.

Given:

[
f=2\times10^6\text{ Hz}
]

[
v=1500\text{ m/s}
]

Using:

[
\lambda=\frac{v}{f}
]

[
\lambda=\frac{1500}{2\times10^6}
]

[
\lambda=0.00075\text{ m}
]

Answer: 0.75 mm.

Example 2: Depth Measurement

An ultrasound pulse returns from a tissue boundary after 100 microseconds. If sound speed in the tissue is 1,540 m/s, calculate the depth.

[
t=100\times10^{-6}\text{ s}
]

[
d=\frac{vt}{2}
]

[
d=\frac{1540\times100\times10^{-6}}{2}
]

[
d=0.077\text{ m}
]

Answer: 7.7 cm.

Example 3: Frequency and Wavelength

An ultrasound wave travels at 3,000 m/s and has a wavelength of 0.003 m. Calculate its frequency.

[
f=\frac{v}{\lambda}
]

[
f=\frac{3000}{0.003}
]

[
f=1,000,000\text{ Hz}
]

Answer: 1 MHz.

Example 4: Thickness Measurement

An ultrasonic pulse travels through a metal plate and returns from the back surface after 20 microseconds. If sound speed in the metal is 5,000 m/s, calculate the plate thickness.

[
d=\frac{vt}{2}
]

[
d=\frac{5000\times20\times10^{-6}}{2}
]

[
d=0.05\text{ m}
]

Answer: 50 mm.

Example 5: Doppler Shift

A diagnostic ultrasound system emits a frequency of 5 MHz. A reflected signal is shifted by 1,000 Hz due to the movement of blood cells. Calculate the frequency of the returned signal if the shift is positive.

[
f’=f+\Delta f
]

[
f’=5,000,000+1,000
]

[
f’=5,001,000\text{ Hz}
]

Answer: 5.001 MHz.

15. Common Examination Distinctions

• Ultrasound has a frequency above approximately 20 kHz.
• Ultrasound is a mechanical wave, not an electromagnetic wave.
• Piezoelectric transducers are widely used to generate and detect ultrasound.
• Medical ultrasound uses reflected sound waves to produce images.
• Acoustic impedance is the product of density and sound speed.
• The time-of-flight principle is used to calculate depth and distance.
• Higher frequency generally improves spatial resolution but reduces penetration.
• Doppler ultrasound measures frequency changes caused by motion.
• Ultrasonic testing is a non-destructive testing method.
• Ultrasonic cleaning uses cavitation.
• Diagnostic ultrasound does not use ionising radiation.
• Therapeutic ultrasound can produce heating and other tissue effects.

36 Key Takeaways for Exams

1. Ultrasound refers to sound waves with frequencies above approximately 20 kHz.
2. Ultrasound is a mechanical wave.
3. Ultrasound requires a material medium for propagation.
4. Ultrasound cannot travel through a vacuum.
5. Ultrasound can travel through gases, liquids and solids.
6. The wave equation is (v=f\lambda).
7. Increasing frequency reduces wavelength when sound speed remains constant.
8. Diagnostic medical ultrasound commonly uses frequencies in the megahertz range.
9. Lower ultrasound frequencies generally penetrate deeper.
10. Higher ultrasound frequencies generally provide better spatial resolution.
11. Piezoelectric materials are commonly used in ultrasonic transducers.
12. The piezoelectric effect enables conversion between electrical and mechanical energy.
13. Quartz and PZT are examples of piezoelectric materials.
14. An ultrasonic transducer can transmit and receive sound waves.
15. Medical ultrasound imaging relies on reflected sound waves.
16. Acoustic impedance is given by (Z=\rho v).
17. Differences in acoustic impedance influence reflection at tissue boundaries.
18. The depth of a reflector can be calculated using (d=vt/2).
19. Ultrasound gel improves acoustic coupling between the transducer and skin.
20. B-mode imaging displays echoes as brightness in a two-dimensional image.
21. M-mode imaging records movement over time.
22. Doppler ultrasound evaluates motion, particularly blood flow.
23. Echocardiography uses ultrasound to examine the heart.
24. Pregnancy monitoring is an important application of diagnostic ultrasound.
25. Diagnostic ultrasound does not use ionising radiation.
26. Ultrasonic testing is a non-destructive method of detecting internal material defects.
27. Ultrasonic thickness gauges measure material thickness using pulse timing.
28. Ultrasonic cleaning uses cavitation in a liquid.
29. Ultrasonic welding uses high-frequency mechanical vibrations to join materials.
30. Ultrasonic sensors can measure distance and liquid levels.
31. Some ultrasonic flow meters measure fluid movement.
32. Bats use ultrasound for echolocation.
33. Dolphins use high-frequency sound for navigation and prey detection.
34. Ultrasound can produce thermal and mechanical effects in tissues.
35. The thermal index and mechanical index are relevant ultrasound safety indicators.
36. Ultrasound has major applications in medicine, industry, engineering and scientific research.