Ultrasound for medical diagnostics has changed over the years, which has significantly promoted patient care upgrading. The invention of ultrasound transducers has highly influenced the development of ultrasound. This technology has offered spectacular contributions to ultrasound tomography by permitting advancement from two-dimension to synchronous third-spatial to four-dimensional imaging. There has been a rapid transformation from ordinary to tailor-made elements, which have also progressed from one to several in a single ultrasound transducer. The transition has also facilitated the availability of a widespread collection of at least six ultrasound transducers to sonographers, unlike in the past, where one probe was provided. Therefore, transducer evolution has reformed patient care management, the extent of anatomy visualization, and dissection perception. This essay delineates the history of ultrasound, classification, and elements of ultrasound transducers. The paper will also cover aspects of ultrasound transducer ergonomics as it entails the weight, grip, and size, which are vital considerations during ultrasound transducer design. The article will finally discuss the bioeffect and precautions of ultrasound transducer application and future development.
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History of Ultrasound and Physics
Physics and ultrasound history cannot be comprehensively addressed without discussing the transducers, which are vital in ultrasound transmission. Transducers are generally defined as devices that transform one type of energy into another kind. The appliances have been in use for centuries and are acknowledged by famous scientists. One such scholar was Pythagoras, who discovered the link between frequency and pitch (Hirao & Ogi, 2017). This knowledge necessitated the making of the musical sonometer. Other key contributors to ultrasound transducer (UT) development during the nineteenth century included Sir Isaac Newton, Leonardo Da Vinci, and Aristotle (Hirao & Ogi, 2017). The primary physics invention that led to UT production was witnessed in the 1880s when two brothers, Pierre Curie and Jacque, established the piezoelectric effect.
However, not until the mid-1940s that Dr. Karl Dussik, an Austrian physician, became the first specialist to employ ultrasound technology in therapeutic diagnosis. In 1951, Howry used an immersion chamber ultrasound system apparatus to illustrate the first medical two-dimensional B-system photograph. Gerald Posakony, Joseph Holmes, Richard Cushman, and Douglas Howry later developed the Pan-scanner in 1957 (Hirao & Ogi, 2017). The system was utilized in the demonstration of a more clinically stable B-mode practical sonography. During imaging, the patient rested in a personalized dental seat mounted to a flexible opening of a semi-circular basin containing saline. The rotation of the UT was automatically performed within that briny fluid through the same configuration.
Other prominent ultrasound pioneers were MacVicar, Dr. Ian Donald, and Brown, who published their milestone experimental paper on the visualized abdominal tumors and masses in 1958. They also became the forerunners of obstetric sonography as they were able to utilize ultrasound to describe gestational sac images (Hirao & Ogi, 2017). The initial real-time and smart B-scanner (Vidoson system), with the capability of taking fifteen frames every second, was created in Germany by Siemens Medical Systems in 1965 (Hirao & Ogi, 2017). The modeled apparatus ensured fast, timely, convenient, and high-quality imaging.
Most of the earlier-developed instruments made use of either mobile or stationary mono-element ultrasound transducers (UTs). Nevertheless, the beginning of the digital electronic stimulation era from the early 1970s to the late 1980s facilitated the development of multi-element UTs and faster processing of image information. The advancement of transducer and ultrasound technology has been increasing at a constant rate since the start of 1970 (Hirao & Ogi, 2017). This improvement has led to the actualization of various technologies such as the generation of gray-scale pictures and increased frequencies of frames through the use of scan converters and mechanical field scanners. The two highlighted techniques were integrated with some three-dimensional imaging devices before the initiation of multiplex UT elements in intracavitary equipment. Further, the advent was characterized by miniaturization, Doppler radiology, Doppler spectral assessment, and countless other technical evolutions.
The emergence of digital technologies also led to the invention and advancement of ultrasound transducers’ non-diagnostic applications. The medical UTs employ sound waves of high magnitude to provide a marginal source of heating the tissue. The application is imperative for pressure surge absorption, which is regularly incorporated in physical therapy sections (Hirao & Ogi, 2017). UTs with high-intensity concentration are clinically employed to impair tissue by heating. They also damage tissue by direct mechanical impacts of the pressure spasm, as exhibited in kidney stones lithotripsy and extreme-ferocity steadfast ultrasound systems.
Ultrasound Transducer Elements
UTs make use of either artificial or natural dynamic components to generate the piezoelectric effect required for sonography functionalities. The ancient UTs that employed piezoelectric reverberation used quartz as the operational medium (Gougheri, Dangi, Kothapalli & Kiani, 2019). Quartz is a naturally existing element and moderately abundant in the environment, endowed with crystalline features that make it a compelling option. Large amounts of quartz crystals were first discovered in the United States (US) and Switzerland.
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Another naturally occurring element that is utilized in UTs is known as tourmaline. The material was located in Brazil, Southern California, and other sites in the world. Additional components which have been found in industrial piezoelectric items include barium lead zirconate, polyvinyl fluoride, barium titanate, and lead meta niobate. Lead zirconate titanate (PZT) has also been ascertained to be the most preferred for manufactured Doppler UTs (Gougheri et al., 2019). The aforementioned elements exhibit characteristics such as powerful natural frequency resonance, a high bridging coefficient, and exceptional repeatability attributes, making them more desirable for UT utilization.
Piezoelectricity is the representation of the aftermath reaction of particular materials that produce voltage when distorted by strains or pressure. Conversely, a substance warps in a reproducible manner when subjected to an applied voltage, often referred to as the inverse piezoelectric effect. Subsequently, the manufacturing of piezoelectric items requires placing raw material in a curie point (a rugged magnetic field at extreme temperatures). The process enables appropriate alignment of the material’s basic structural constituents to induce the piezoelectric power (Gougheri et al., 2019). Notably, the UTs are not sterilized in autoclaves or other raised temperature sources. This is because treated UTs lose piezoelectric potency when the curie point temperature is exceeded.
More recent composite elements of UTs are produced by slicing the piezoelectric solid in a predetermined design. The ensuing gaps between the formed miniatured individual components are infused with special epoxy resin. The shape and size of the pieces obtained are then infiltrated with final application specifications. The step involves determining and incorporating focusing properties, resonant frequency, and the acoustic impedance of the element. The element’s frequency resonance is computed as a function of the material’s propagation velocity and its thickness (Gougheri et al., 2019). The element thickness for PZT deployed in diagnostic clinical ultrasound systems for frequency ranging between 2 MHz and 15 MHz is usually a fragment of a millimeter.
The effective coupling of pressure ripples caused by the piezoelectric effect to the element’s tissue after the formation and molding is facilitated by the guilting of the identical layers. These undistinguishable films, such as acoustic gel, systematically conform to the active element-tissue resistance. Matching prevents immense backscattering at the tissue interface as well as reception and transmission mislaying. A damping or cushioning substance is attached at the rear of the element to avoid continuous ringing of the reactive component when triggered to generate a quick pulsation. The pounding is essential for optimal horizontal resolution during imaging (Gougheri et al., 2019). One of the distinctive buffing materials is the epoxy resin type containing tungsten filament.
The subsequent coverings of the UTs comprise electric and acoustic insulators. Acoustic resonators inhibit peripheral vibrations from producing a voltage in the functional elements (Gougheri et al., 2019). Electric lagging is important for averting any electric seepage from the elements to the sonographer and also armoring the inner components from any external electromagnetic disturbance. Finally, the entire coatings are retained in a precast plastic vessel to let the handler firmly hold the transducer during a checkup.
Classification of Ultrasound Transducers
Over the years, the design of UTs used for imaging instruments has endured substantial progression. The construction has evolved from uncomplicated stationary sole crystals to automatically screened elements. The advancement has also stretched to the current type of multi-element configurations such as phased, curvilinear, linear, two-dimension, and annular arrays (Gougheri et al., 2019). The solitary classical element and instinctively scanned UTs had an anchored focal profundity and were intuited by the UT plan.
Internally absorbed UTs depended on either a curved interior reflection surface or a bowed reactive element to establish a pivotal point. The outward concentration could be attained by encompassing an aural lens into the UT’s blueprint (Gougheri et al., 2019). Though UTs have a relative benefit of a slight trail and lack of manufacturing overheads, they are hardly utilized for analytical sonography. The move is occasioned by the fact that they have a static lens, and the failure of one crystal component causes the crashing of the entire UT.
Multiplex component arrangement in UTs offers higher flexibility in utilization since every element can be launched autonomously. Further, the resultant beam-shaped ultrasound can be directed and pointed electronically to accomplish the required outcome. If needed, several pieces can be stimulated as a collection to deliver the anticipated beam properties. However, the annular patterned UT that comprises numerous elements arrayed as concentric loops cannot be activated. This is because these UTs’ concentration is performed electronically, and routing is undertaken mechanically (Gougheri et al., 2019). Another exceptional case is the incessant-wave Doppler UT made up of two live elements without imaging ability, where one is tasked with constant transmission and the other unremitting reception.
Three-dimensional (3D) tomography is currently feasible, though when the consequential mounting frequency is high enough to be regarded as near real-time, it assumes four-dimensional (4D) imaging. The initial 3D transducers merged a manual scan for the third facet with rectilinear multi-element ranges for two extents, permitting capturing a considerable number of image data for dispensation (Gougheri et al., 2019). The voluminous dataset emanates from the grouping of the two-dimensional (2D) photographs prearranged on the third side like the pages in a booklet.
Physical unguided scanning in the third facet can be utilized to generate a 3D picture, which relies on the qualitative dependence of the sonographer. For quantifiable atomic sizes, a manual reproducing system requires some form of an electromagnetic UT monitoring system to render locational information. In mechanically screened diverse elements, the positional data is detected electronically. In recent times, manufacturers have succeeded in producing 2D aggregations of piezoelectric modules or composite configurations which can generate 3D images (Gougheri et al., 2019). The process entirely adopts the elements’ electronic utilization and eliminates the requirement for any mechanical appliances or other surveillance devices. Notably, these mechanically scanned or composite collection transducers can repress most of the sonographer dependency limitations, eradicate numerous mobility artifacts, and avail close to contemporaneously perceptible images. However, they are very expensive, and their routine use is only limited to the specialty of obstetric scanning.
Despite the above discussion on cloning UTs with multiple elements, the prevailing research focuses on developing a large enough, unitary, and stable piezoelectric quartz for any envisioned application usage. A small kernel granule is utilized to initiate the process, which is then propagated under very regulated conditions and elongated in veneers before polarization and cutting into preferred elements. Several benefits emanate from this technique, as the consequential UT will have significantly even characteristics and features (Gougheri et al., 2019). The formed UT will also have efficiency during electric pulsations to pressure torrents conversion and pressure curls to electric impulses transformation. The effect causes the formation and buckling of beams more accurately while raising the total UT sensitivity and bandwidth.
Ergonomics and Sonography
The two terminologies have been frequently employed in similar sentences synonymously. Several kinds of research have illustrated that about eighty to ninety percent of sonographers experience stress during scanning, and eventually, approximately twenty percent acquire a profession-culminating injury (Hirao & Ogi, 2017). According to the Agency of Labor and Statistics, primary and secondary overheads from musculoskeletal syndromes cost around sixty billion US dollars every year (Hirao & Ogi, 2017). Consequently, manufacturers have acknowledged the gap and have employed multiple ergonomic design factors for current UTs. The housings of the UTs are persistently redesigned to fit an impartial wrist site, with the center of gravity confined at the middle of the boundary.
Modern UTs are engineered to be smaller and narrower such that they are relatively lightweight compared to the preceding corresponding devices. They are also designed to harbor better hands of operators, whether right or left, long or short (Hirao & Ogi, 2017). The most recurrent injuries linked to UTs with poor ergonomic designs are tenosynovitis, carpal tunnel disorder, tendonitis, trigger finger, and de Quervain syndrome. The critical measure to avert such afflictions is ensuring that the UTs are well designed and aptly coupled with superior sonographer knowledge.
Another commonest mistake during scanning is applying a firm grip on the UT, more so when only a little pressure is required. Innovative designs of the UT aid in preventing undue pinch seizing or overstraining of the fingers. Light-weighted UT cables are also being developed, and assorted strain-discharging arm struts are accessible to lessen the stress on the forearm, wrist, and hand (Hirao & Ogi, 2017). Though gloves and acoustic gel are used as part of general precautions during imaging, improper sizing may pose challenges when trying to tightly and securely clamp UTs without extreme grip force.
If possible, a glove should adjust comfortably and accord a nonslip touch to match a similar texture on the UT casing. Abstraction in its early stages of development is the finger-initiated transducer which is mounted to the finger of the sonographer (Hirao & Ogi, 2017). The UT’s cable is fixed to the arm of the user to quash torque and other pressures on the sonographer’s wrist. The concept aims at optimizing the control of analysis such as peripheral vascular examinations and guided biopsies.
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Bioeffect and Precautions of Ultrasound Transducer Applications
The tissues of grownups tolerate the increase in temperature more than the neonatal and fetal tissues. A modern ultrasound system shows both the mechanical and thermal indices (Blackmore, Shrivastava, Sallet, Butler & Cleveland, 2019). The thermal indicator (TI) is demonstrated by the division of the UT acoustic throughput force by the projected energy necessary to increase tissue temperature by one degree Celsius. On the contrary, mechanical index (MI) is equivalent to the optimum rarefactional compression divided by the central iteration pulse bandwidth’s square root. Subsequently, MI and TI denote the comparative probability of mechanical and thermal risk, respectively, and a value exceeding one in either is dangerous.
The exposure period of the tissue also contributes to the biological impact resulting from the ultrasound. The scientists manipulate pregnant rodents exposed to ultrasound with an intensity of at least 1 W/cm2 to examine the time-variant adverse effects occurring in mice fetuses (Blackmore et al., 2019). Ultrasound-steered nerve needs the application of only diminutive proportions on the patient for a transient period. However, Vivo and Vitro experiments have not delineated any proof linking diagnostic ultrasound use in regular medical checkups with biological hazards.
The imminent progress of ultrasound transducer is one of the ongoing evolutions and improvements. One can now link a UT to a smartphone to utilize it as an ultrasound apparatus while upholding the initial cell phone’s entire communication functionalities (Hirao & Ogi, 2017). The attachment facilitates faster dissemination of pictures to other locations for more in-depth visualization or in hostile and traumatic settings to permit prior grooming for patient guidance. Miniaturization of such instruments raises ultrasound transferability beyond the sphere of computer-sized gadgets and hypothetically renders ultrasound pervasive in smartphones.
Another critical area that is attracting attention in this field is the development of wireless UTs. However, the venture faces challenges such as the need to transmit huge datasets to the main ultrasound machine swiftly. An additional limitation is the requirement to incorporate a power source in the UT, necessitating an increase in their weights and sizes (Hirao & Ogi, 2017). Nevertheless, enhanced portability and the presence of lightweight cable offer motivation for further advancement in the field. A tremendous milestone may be experienced in the UT element technology by developing fully advanced capacitive ultrasound transducers, which are micromachined (Hirao & Ogi, 2017). These devices assume integrated circuits and are built on a silicon planar. Theoretically, they permit the assignment of expansive orders to running elements in 2D and 3D arrangements within the UT frame.
The universe has had an experience of rapid advancement in ultrasound transducer technology. This speedy development has been facilitated by continued progress in computing and circuitry fields. Admittedly, UT mechanization evolutions have profoundly influenced image superiority and the snowballing contribution of sonography to therapeutic diagnosis and patient care. A widespread range of dissections can be scanned, and timely treatment can be done with certainty. This eliminates the need to schedule supplementary and more expensive clinical examinations.
Blackmore, J., Shrivastava, S., Sallet, J., Butler, C., & Cleveland, R. (2019). Ultrasound neuromodulation: A review of results, mechanisms, and safety. Ultrasound in Medicine & Biology, 45(7), 1509-1536. doi: 10.1016/j.ultrasmedbio.2018.12.015
Gougheri, H., Dangi, A., Kothapalli, S., & Kiani, M. (2019). A comprehensive study of ultrasound transducer characteristics in microscopic ultrasound neuromodulation. IEEE Transactions on Biomedical Circuits and Systems, 13(5), 835-847. doi: 10.1109/tbcas.2019.2922027
Hirao, M., & Ogi, H. (2017). Electromagnetic Acoustic Transducers (2nd ed.). Tokyo: Springer.