Ultrasound technology uses sound waves beyond human hearing for various applications, from medical diagnostics to industrial inspection. Specialized components called ultrasound crystals are central to this technology. These crystals enable the generation and detection of high-frequency sound waves, forming the basis for modern ultrasound systems.
What Are Ultrasound Crystals?
Ultrasound crystals are functional components within an ultrasound transducer, also known as a probe. The transducer acts as both the “mouth” and “ears” of the ultrasound machine, sending out sound waves and receiving their echoes. These crystals convert electrical energy into mechanical vibrations, producing high-frequency ultrasound waves.
When echoes return and strike the crystals, they convert these vibrations back into electrical signals. This dual ability allows the same crystal to transmit and receive sound. Transducer probes contain one or more crystal elements, with multiple-element probes enabling ultrasonic beam steering.
The Science Behind Ultrasound Crystals
Ultrasound crystals convert energy forms due to the piezoelectric effect. This principle describes how certain materials generate an electrical charge under mechanical stress, like pressure or vibration. Conversely, these materials change shape when an electric field is applied.
In an ultrasound device, an alternating electric current causes the piezoelectric crystal to rapidly expand and contract. These vibrations produce ultrasonic sound waves that travel outward. When these sound waves encounter structures, some reflect as echoes back to the transducer. These returning echoes cause the crystal to vibrate, generating an electrical current that the ultrasound machine interprets to create an image.
The frequency of sound waves depends on the crystal’s thickness; thinner crystals generate higher frequencies, while thicker ones produce lower frequencies. Crystals are cut to a thickness that is half the desired wavelength for efficient sound production.
Key Materials for Ultrasound Crystals
An ultrasound crystal’s performance depends on its material composition. Lead zirconate titanate, or PZT, is the most widely used ceramic for transducers due to its piezoelectric properties. PZT efficiently converts electrical energy to sound and vice versa, offering a good balance of piezoelectric coefficient and electromechanical coupling. It is also used for high-power therapeutic applications due to its low mechanical and dielectric losses.
While PZT is common, other materials are used for specialized purposes. Early transducers used quartz crystals, which are naturally piezoelectric. More recently, single-crystal materials like lead-magnesium-niobate lead-titanate (PMN-PT) and lead-indium-niobate lead-magnesium-niobate lead-titanate (PIN-PMN-PT) have emerged. These single crystals offer higher electromechanical coupling factors and broader bandwidths, leading to improved image quality and lower noise, especially for advanced diagnostic and high-frequency applications.
Beyond Imaging: Diverse Applications
While medical diagnostic imaging, like monitoring pregnancies or examining internal organs, is a recognized use, ultrasound crystals enable a broader range of applications. Therapeutic ultrasound uses these crystals for high-intensity focused ultrasound (HIFU) in non-invasive surgical procedures, such as ablating kidney stones or treating early-stage tumors. Ultrasound also aids physical therapy for tissue heating and healing.
In industrial settings, ultrasound crystals are used in non-destructive testing (NDT) to detect flaws or measure material thickness without damage. They are also employed in ultrasonic cleaning, where high-frequency sound waves create cavitation bubbles to dislodge contaminants. These crystals are used in ultrasonic welding and wire bonding to join plastics and metals through vibrations. Their ability to generate and detect sound waves extends their utility to humidifiers, sonar, and drug delivery systems.