Application of Ultrasound in Medicine - PMC

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Ultrasound device, essentially, consists of a transducer, transmitter pulse generator, compensating amplifiers, the control unit for focusing, digital processors and systems for display. It is used in cases of: abdominal, cardiac, maternity, gynecological, urological and cerebrovascular examination, breast examination, and small pieces of tissue as well as in pediatric and operational review.

1.&#;INTRODUCTION

In physics the term &#;ultrasound&#; applies to all acoustic energy with a frequency above human hearing (20,000 hertz or 20 kilohertz). Typical diagnostic sonographic scanners operate in the frequency range of 2 to 18 megahertz, hundreds of times greater than the limit of human hearing. Higher frequencies have a correspondingly smaller wavelength, and can be used to make sonograms with smaller details. Diagnostic sonography (ultrasonography) is an ultrasound-based diagnostic imaging technique used to visualize subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions. Sonography is effective for imaging soft tissues of the body. Sonographers typically use a hand-held probe (called a transducer) that is placed directly on and moved over the patient. A water-based gel is used to couple the ultrasound between the transducer and patient (1, 2).

Although discovered 12 years before the X-ray ray (.), the ultrasound is a much later found application in medicine. The first practical application of ultrasound is recorded during the World War I in detecting of submarines. The application of ultrasound in medicine began in fifties of last century. First was introduced in the obstetrics, and after that in all the fields of the medicine (the general abdominal diagnostics, the diagnostics in the field of the pelvis, cardiology, ophthalmology and orthopedics and so on) (3). From the clinical aspect the ultrasound possesses the priceless significance because of its noninvasive, good visualization characteristics and relatively easy management (4,5). From the introducing of the processing of the signals of gray scale in B-mode of the sonography became the widely accepted method. The progress in the forming of the transducers has led to better space resolution and the imaging of very small structures in the abdomen (0.5-1 cm). The development of real-time system led to, even, to the possibility of the continued visualization or the ultrasound fluoroscopy (1). In the ultrasound diagnostics can be differed two techniques (2): transmission and reflection

Transmission technology is based on distinguishing the tissues with different absorbance of ultrasound. Due to uneven absorption of ultrasound images provides internal structure that consists of a mosaic of lighter and darker places. This technology is now abandoned (6,1).

Reflection technology (echo) registers the pulse is reflected from the boundary of two tissues with different acoustic resistance. The technique is based on principle of functioning sonar (&#;Sonar Navigation and Ranging&#;). A sound wave is typically produced by a piezoelectric transducer encased in a probe. Strong, short electrical pulses from the ultrasound machine make the transducer ring at the desired frequency. The frequencies can be anywhere between 2 and 18 MHz&#;s The sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner machine. This focusing produces an arc-shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth. Newer technology transducers use phased array techniques to enable the sonographic machine to change the direction and depth of focus. Almost all piezoelectric transducers are made of ceramic (1).

To generate a 2 D-image, the ultrasonic beam is swept. A transducer may be swept mechanically by rotating or swinging. Or a 1D phased array transducer may be use to sweep the beam electronically. The received data is processed and used to construct the image. The image is then a 2D representation of the slice into the body. 3D images can be generated by acquiring a series of adjacent 2D images. Commonly a specialized probe that mechanically scans a conventional 2Dimage transducer is used. However, since the mechanical scanning is slow, it is difficult to make 3D images of moving tissues. Recently, 2D phased array transducers that can sweep the beam in 3D have been developed. These can image faster and can even be used to make live 3D images of a beating heart.

Four different modes of ultrasound are used in medical imaging (1, 3).

These are:

• A-mode: A-mode is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy.

• B-mode: In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen.

• M-mode: M stands for motion. In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables doctors to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe.

Doppler mode: This mode makes use of the Doppler effect in measuring and visualizing blood flow. Doppler sonography play important role in medicine. Sonography can be enhanced with Doppler measurements, which employ the Doppler effect to assess whether structures (usually blood) are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example a jet of blood flow over a heart valve, its speed and direction can be determined and visualized. This is particularly useful in cardiovascular studies (sonography of the vasculature system and heart) and essential in many areas such as determining reverse blood flow in the liver vasculature in portal hypertension (6,7). The Doppler information is displayed graphically using spectral Doppler, or as an image using color Doppler (directional Doppler) or power Doppler (non directional Doppler). This Doppler shift falls in the audible range and is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, pulsing sound (8).

The transoesophageal echo cardiography (TEE) opened the window in the diagnostic imaging in the field of the cardiography, card surgery and anesthesia. Using TEE in 2-D mode, the anesthesiologist can monitor the heart movements, and cardiac surgeon will become the valuable information about the heart condition after the critical surgical procedure.

Method of cleaning using ultrasound

Sonorex ultrasonic cleaner from the s or s

Ultrasonic cleaning of a mobile

Ultrasonic cleaning is a process that uses ultrasound (usually from 20 to 40 kHz) to agitate a fluid, with a cleaning effect. Ultrasonic cleaners come in a variety of sizes, from small desktop units with an internal volume of less than 0.5 litres (0.13 US gal), to large industrial units with volumes approaching 1,000 litres (260 US gal).

The principle of the ultrasonic cleaning machine is to convert the sound energy of the ultrasonic frequency source into mechanical vibration through the transducer. The vibration generated by the ultrasonic wave is transmitted to the cleaning liquid through the cleaning tank wall so that the micro-bubbles in the liquid in the tank can keep vibrating under the action of the sound wave, destroying and separating the dirty adsorption on the surface of the object.

Depending on the object being cleaned, the process can be very rapid, completely cleaning a soiled item in minutes. In other instances, cleaning can be slower, and exceed 30 minutes.[1]

Ultrasonic cleaners are used to clean many different types of objects, including industrial parts, jewelry, scientific samples, lenses and other optical parts, watches, dental and surgical instruments, tools, coins, fountain pens, golf clubs, fishing reels, window blinds, firearm components, car fuel injectors, musical instruments, gramophone records, industrial machined parts, and electronic equipment, optical lenses, etc. They are used in many jewelry workshops, watchmakers' establishments, electronic repair workshops,[2] and scientific labs.

History

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Ultrasonic cleaning has been used industrially for decades,[when?] particularly to clean complex shape parts and/ or having small intricate holes/galleries, and to accelerate surface treatment processes.[3]

It appears that ultrasonic cleaners developed as a natural evolution of several earlier inventions that used vibrations to agitate and mix substances, and thus there is no clear "inventor" of ultrasonic cleaning. US patent , issued December  , is the earliest patent on record that specifically uses the term "Ultrasonic cleaning" although earlier patents refer to the use of ultrasound for "intense agitation," "treatment" and "polishing," e.g. US  .

By the mid-s there were at least three ultrasonic cleaner manufacturers established in the United States and two in the United Kingdom; and by the s ultrasonic cleaners were widely established for industrial and domestic use.[4][5]

Process characteristics

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Ultrasonic cleaning uses cavitation bubbles induced by high-frequency pressure (sound) waves to agitate a liquid. The agitation produces high forces on contaminants adhering to substrates like metals, plastics, glass, rubber, and ceramics. This action also penetrates blind holes, cracks, and recesses. The intention is to thoroughly remove all traces of contamination tightly adhering or embedded onto solid surfaces. Water or other solvents can be used, depending on the type of contamination and the workpiece. Contaminants can include dust, dirt, oil, pigments, rust, grease, algae, fungus, bacteria, lime scale, polishing compounds, flux agents, fingerprints, soot wax and mold release agents, biological soil like blood, and so on. Ultrasonic cleaning can be used for a wide range of workpiece shapes, sizes, and materials, and may not require the part to be disassembled prior to cleaning.[6]

Objects must not be allowed to rest on the bottom of the device during the cleaning process, because that will prevent cavitation from taking place on the part of the object not in contact with solvent.[2]

Design and operating principle

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In an ultrasonic cleaner, the object to be cleaned is placed in a chamber containing a suitable solution (in an aqueous or organic solvent, depending on the application). In aqueous cleaners, surfactants (e.g., laundry detergent) are often added to permit dissolution of non-polar compounds such as oils and greases. An ultrasound generating transducer built into the chamber, or lowered into the fluid, produces ultrasonic waves in the fluid by changing size in concert with an electrical signal oscillating at ultrasonic frequency. This creates compression waves in the liquid of the tank which 'tear' the liquid apart, leaving behind many millions of microscopic 'voids'/'partial vacuum bubbles' (cavitation). These bubbles collapse with enormous energy; temperatures and pressures on the order of 5,000 K and 135 MPa are achieved;[7][8] however, they are so small that they do no more than clean and remove surface dirt and contaminants. The higher the frequency, the smaller the nodes between the cavitation points, which allows for cleaning of more intricate detail.

Ultrasonic transducers showing ~20 kHz and ~40 kHz stacks. The active elements (near the top) are two rings of lead zirconate titanate, which are bolted to an aluminium coupling horn.

Transducers are usually piezoelectric (e.g. made with lead zirconate titanate (PZT), barium titanate, etc.), but are sometimes magnetostrictive. The often harsh chemicals used as cleaners in many industries are not needed, or used in much lower concentrations, with ultrasonic agitation. Ultrasonics are used for industrial cleaning and are also used in many medical and dental techniques and industrial processes.

Cleaning solution

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In some circumstances, ultrasonic cleaners can be used with plain water, but in most cases, a cleaning solution is used. This solution is designed to maximise the effectiveness of ultrasonic cleaning. The primary solvent may be water or a hydrocarbon (historically, toxic solvents such as carbon tetrachloride and 1,1,1-Trichloroethane were used industrially, but have been phased out[9][10]). There are several formulations, dependent on the item being cleaned and the type of contamination (e.g., degreasing of metal, cleaning of printed circuit boards, removing biological material, and so on).

Reduction of surface tension increases cavitation, so the solution usually contains a good wetting agent (surfactant). Aqueous cleaning solutions contain detergents, wetting agents and other components, which have a large influence on the cleaning process. The correct composition of the solution is very dependent upon the item cleaned. When working with metals, proteins, and greases, an alkaline detergent solution may be specifically recommended. Solutions are typically heated, often around 50&#;65 °C (122&#;149 °F), however, in medical applications, it is generally accepted that cleaning should be at temperatures below 45 °C (113 °F) to prevent protein coagulation that can complicate cleaning.

Some ultrasonic cleaners are integrated with vapour degreasing machines using hydrocarbon cleaning fluids: Three tanks are used in a cascade. The lower tank containing dirty fluid is heated causing the fluid to evaporate. At the top of the machine there is a refrigeration coil. Fluid condenses on the coil and descends into the upper tank. The upper tank eventually overflows and relatively clean fluid runs into the work tank where the cleaning takes place. The purchase price is higher than simpler machines, but such machines may be more economical in the long run. The same fluid can be reused many times, minimising wastage and pollution.

Uses

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Most hard, non-absorbent materials (metals, plastics, etc.) not chemically attacked by the cleaning fluid are suitable for ultrasonic cleaning. Ideal materials for ultrasonic cleaning include small electronic parts, cables, rods, wires, and detailed items, as well as objects made of glass, plastic, aluminium, or ceramic.[11]

Ultrasonic cleaning does not sterilize the objects being cleaned, because spores and viruses will remain on the objects after cleaning. In medical applications, sterilization normally follows ultrasonic cleaning as a separate step.[12]

Industrial ultrasonic cleaners are used in the automotive, sporting, printing, marine, medical, pharmaceutical, electroplating, disk drive components, engineering and weapons industries.

Ultrasonic cleaning is used to remove contamination from industrial process equipment such as pipes and heat exchangers.

Limitations

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Ultrasonic cleaning is used widely to remove flux residue from soldered circuit boards. However, some electronic components, notably MEMS devices such as gyroscopes, accelerometers and microphones can become damaged or destroyed by the high-intensity vibrations they are subjected to during cleaning. Piezoelectric buzzers can work in reverse and produce voltage, which may pose a danger to their drive circuits.

Safety

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• Ultrasonic cleaners can produce irritating, high-frequency noise and hearing protection may be needed in case of continuous exposure.
• It is recommended to avoid using flammable cleaning solutions because ultrasonic cleaners increase the temperature even when not equipped with a heater. (Some industrial units are specifically certified as explosion proof.)
• When the unit is running, contact with the cleaning solution could cause thermal or chemical injury; the ultrasonic action is relatively benign to living tissue but can cause discomfort and skin irritation.[13]
• Ultrasonic cleaners are electrically powered, meaning there is a risk of electric shock in case of malfunction, especially if the cleaning solution comes into contact with electrified components.

See also

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References

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