Ultrasonic Principles: An In-Depth Exploration from Basic Physics to Emerging Technologies

1. Review of the Basic Physics of Ultrasound

Ultrasound is a type of sound wave with a frequency higher than the human hearing range (typically above 20 kHz). Its physical properties make it widely applicable in medical, industrial, and scientific fields. Ultrasound waves can propagate as longitudinal or transverse waves. Longitudinal waves involve particle vibration in the same direction as wave propagation, commonly found in gases and liquids. Transverse waves involve particle vibration perpendicular to the direction of wave propagation and are more common in solids. These propagation characteristics determine how ultrasound behaves in different media.

Acoustic impedance is a key parameter describing how ultrasound propagates across different media. It is defined as the product of a medium's density and the speed of sound within it. When ultrasound passes from one medium to another with a large impedance difference, strong reflection occurs — this is the basis for ultrasound imaging. For example, in medical ultrasound scanning, when ultrasound travels from soft tissue to bone, the significant impedance difference creates a strong reflection signal, helping doctors identify tissue structures.

Ultrasound waves attenuate during propagation due to absorption, scattering, and reflection. Absorption refers to the conversion of ultrasound energy into heat by the medium; scattering occurs when ultrasound encounters microscopic inhomogeneities and spreads in various directions; reflection is the rebound of ultrasound at the interface of different media. These mechanisms limit the penetration depth of ultrasound but also provide contrast for imaging. Compared to X-ray imaging and MRI scanning , ultrasound scanning is radiation-free and cost-effective, making it especially important in obstetrics and cardiovascular examinations.

2. Advanced Applications of Ultrasound

High-Intensity Focused Ultrasound (HIFU) is a major breakthrough in medical technology. It uses the focusability of ultrasound to precisely target a specific area within the body, generating localized high temperatures to ablate tumor tissue while minimizing damage to surrounding healthy tissue. Several hospitals in Hong Kong have adopted HIFU for treating uterine fibroids and prostate cancer, with clinical data showing a success rate exceeding 80%.

Ultrasound transducers are the core components of ultrasound equipment. Their performance directly affects imaging quality and therapeutic outcomes. Piezoelectric materials are key to transducers, converting electrical energy into mechanical vibrations (for transmitting ultrasound) or converting mechanical vibrations into electrical signals (for receiving ultrasound). Advances in micromachining technology have enabled ultrasound transducers to become smaller and operate at higher frequencies, improving imaging resolution. For example, intravascular ultrasound (IVUS) probes are only about 1 mm in diameter yet provide high-definition images of coronary arteries.

Ultrasound imaging technologies continue to evolve. 3D ultrasound provides a stereoscopic anatomical view through multi-angle scanning and computer reconstruction, particularly useful in obstetric examinations. Elastography assesses tissue hardness by measuring tissue deformation under ultrasound, offering valuable insights for early diagnosis of liver fibrosis and breast cancer. These technologies complement traditional X-ray and MRI scanning, offering more diagnostic options.

3. Ultrasound in Emerging Technologies

In the energy sector, ultrasound applications are expanding. Ultrasound-enhanced heat transfer uses cavitation effects to disrupt thermal boundary layers, significantly improving heat transfer efficiency. Research by the Hong Kong Polytechnic University shows that applying ultrasound in heat exchangers can increase the heat transfer coefficient by over 30%. Ultrasound catalysis utilizes localized high-temperature and high-pressure environments generated by ultrasound to accelerate chemical reactions, showing great potential in biofuel production.

In materials science, ultrasound-assisted synthesis accelerates the preparation of nanomaterials. The cavitation effect produced by ultrasound creates extreme physical conditions that promote chemical reactions. A research team at the Hong Kong University of Science and Technology has successfully used ultrasound to synthesize high-quality graphene, increasing production efficiency fivefold. Ultrasound surface treatment improves the surface properties of metal materials through high-frequency vibration, enhancing wear resistance and fatigue strength.

In environmental protection, ultrasound degradation of pollutants is gaining attention. Free radicals generated by ultrasound effectively break down organic pollutants. Trials by the Hong Kong Environmental Protection Department show that combining ultrasound with ozone in treating industrial wastewater can increase COD (chemical oxygen demand) removal rates to over 90%. This method is more efficient and environmentally friendly compared to traditional treatment processes.

4. Challenges and Future Outlook for Ultrasound Technology

Despite rapid development, ultrasound technology still faces challenges. The trade-off between penetration depth and resolution is a major bottleneck: high-frequency ultrasound provides high-resolution images but limited penetration depth, while low-frequency ultrasound offers better penetration but lower resolution. This limits ultrasound scanning in obese patients and deep tissue examinations — areas where MRI scanning excels.

Future ultrasound technology will move toward miniaturization and intelligence. In miniaturization, MEMS (micro-electro-mechanical systems) technology will make ultrasound devices more portable and even implantable for long-term monitoring. Intelligence will be reflected in AI-assisted diagnostic systems. A research team at the Chinese University of Hong Kong has successfully developed an AI-based ultrasound thyroid nodule classification system with 95% accuracy. These innovations will further expand the applications of ultrasound.

5. Infinite Possibilities of Ultrasound Technology

From fundamental physics to cutting-edge technology, ultrasound demonstrates remarkable adaptability and innovation potential. In medical diagnostics, it forms one of the three pillars of modern imaging alongside X-ray and MRI scanning. In industrial and scientific fields, it drives technological innovation in materials, energy, and environmental protection. With continuous advancements in new materials and algorithms, ultrasound technology will undoubtedly shine in more areas and contribute significantly to human society.