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Ultrasound therapy uses high energy sound waves (those above the range where human can hear) to help ease painful joints and muscles. Human ear can hear sound in the range of 20 to 20000 Hz. However, ultrasound produced in diagnostic machine has frequency beyond the audible range of human ear, so we cannot hear the ultrasound waves when it passes through human tissue.
Ultrasound treatment is done by a physical therapist or occupational therapist who guides the waves into the body from the head of ultrasound machine. Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. The limits varies for different persons. It is approximately 20 kHz serves as a useful lower limit in describing ultrasound. The production of ultrasound is typically used to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium or organ. The most well known application of ultrasound is use in sonography to produce pictures of fetuses.
Ultrasound is a form of mechanical energy which passes in waveform, for example sound waves. However, unlike X-ray or other radiations ultrasound requires a medium for the wave to propagate like water or soft tissue. X-ray waves can pass through vacuum whereas ultrasound waves cannot.
The sound waves when strike a hard surface at a particular distance produces echoes. Some of the ultrasound waves or energy is absorbed in the media and some are reflected back as echoes. These echoes or reflected waves are again converted to light waves which are displayed over a screen. As the media may be, soft tissue, firm or hard the absorption of ultrasound wave varies and the echoes, so the visual display varies in blackness and whiteness and accordingly the normal or pathological anatomy is diagnosed.
The reflected sound waves are called echoes. The amount of echoes produced will depend on the structural property of the media or organs. This property of the media or organs is called Acoustic Impedance. It is a product of density and velocity of sound in the substance.
Attenuation means absorption. Attenuation can be explained as the reduction in intensity of sound beam by various methods like absorption, reflection, refraction and scattering. When attenuation is more than intensity decreases more because ultrasound passes a longer distance from source of origin(transducer). Accounting for attenuation effects in ultrasound is important because reduced signal amplitude can affect the quality of the image produced. Attenuation is more if the tissue or organ is large or of greater depth. Once the attenuation becomes more, intensity of beam decreases, so the reflection also decreases in quality, so less clear will be the picture display. This is the reason why a thin person can have a better image than an obese person. Approximately on an average tissue around 1dB/cm/MHz of human body give visible image of diagnostic quality on ultrasound. By knowing the attenuation that an ultrasound beam experiences traveling through a medium, one can adjust the input signal amplitude to compensate for any loss of energy at the desired imaging depth. From this property, the inference can be drawn why air or bone cannot display diagnostic ultrasound image because air and bone has very high absorption co-efficient.
Transducer is one instrument which produces ultrasound waves. In other way, it is said that "transducer is an instrument which converts one form of energy into another". In ultrasound the transducers converts electrical energy to mechanical energy to produce ultrasound waves and vice versa. The part of the transducer which does this work is a piezoelectric crystal. This crystal can be natural or synthetic. This type of crystals has an inherent property of vibrating when an electrical current is applied and conversely produce electrical impulse if vibrated. This is the piezoelectric effect. When the crystal in transducer is excited by an electric pulse, it vibrates and produces ultrasound. The crystal remains in quiet phase and when some echoes reflected from various organ tissue interfaces hit the crystal and conversely produces small electric signals which are recorded by ultrasound machine and displayed in visual form. So the crystal behaves both as a transmitter of ultrasound waves and receiver of echoes.
Besides frequency the focal zone of the transducer is also mentioned by the manufacturers. Focal zone or focal length of the transducer is the distance from the face of the transducer at which the ultrasound bean is narrowest. A high mHz transducer has wider near zone to penetrate all area of an organ whereas low frequency transducer has less area of encroachment in near zone. It can penetrate far organs better where a high mHz transducer does very less penetration.
Electrical connection in ultrasound machine
The electrical connection to the machine, it is always advisable to supply electric current to the machine through Servo stabilizer not directly to the machine. A Servo stabilizer will stabilize the current at a particular voltage as needed by the machine. Automatically it will bring the voltage to desired level if the supply voltage is low or high. Another main object of Servo stabilizer is that it will not allow spontaneous fluctuation in electric current to pass to the machine. It will cut off and put the machine off automatically thereby protect those costly machine. Sometimes very small flicker variation of the electric current may pass to the machine even through the Servo stabilizer. For more precise electrical protection, a "Line Isolator" another electrical precise protective device is sometimes put in parallel to Servo stabilizer. The electric supply first passes to Servo then to line isolator and then to ultrasound machine.
How do ultrasound machine function?
This circuit functions with inaudible (ultrasonic) sound. Sound of frequency up to 20 kHz is audible to human beings. The sound of frequency above 20 kHz is called ultrasonic sound. The circuit described generates (transmits) ultrasonic sound of frequency between 40 and 50 kHz. As with any other remote control system this circuit too comprises a mini transmitter and a receiver circuit. Transmitter generates ultrasonic sound and the receiver senses ultrasonic sound from the transmitter and switches on a relay. The ultrasonic transmitter uses a 555 based astable multivibrator. It oscillates at a frequency of 40-50 kHz. An ultrasonic transmitter transducer is used here to transmit ultrasonic sound very effectively. The transmitter is powered from a 9-volt single cell. The ultrasonic receiver circuit uses an ultrasonic receiver transducer to sense ultrasonic signals. It also uses a two-stage amplifier, a rectifier stage, and an operational amplifier in inverting mode. Output of op-amp is connected to a relay through a complimentary relay driver stage. A 9-volt battery eliminator can be used for receiver circuit, if required. When switch S1 of transmitter is pressed, it generates ultrasonic sound. The sound is received by ultrasonic receiver transducer. It converts it to electrical variations of the same frequency. These signals are amplified by transistors T3 and T4. The amplified signals are then rectified and filtered. The filtered DC voltage is given to inverting pin of op-amp IC2. The non- inverting pin of IC2 is connected to a variable DC voltage via preset VR2 which determines the threshold value of ultrasonic signal received by receiver for operation of relay RL1. The inverted output of IC2 is used to bias transistor T5. When transistor T5 conducts, it supplies base bias to transistor T6. When transistor T6 conducts, it actuates the relay. The relay can be used to control any electrical or electronic equipment.
How to generate ultrasound waves
Periodic motion causes pressure waves in surrounding physical media. In the diagram, when the piston is shoved forward it compresses the medium. The compression travels through the medium. As the piston moves back and forth, it creates more compressions that travel through the medium like cars down a highway. The more quickly the piston moves back and forth, the closer one compression is to the next one.
Sound waves are made of high pressure and low pressure pulses traveling through a medium. The high pressure areas (compression) are where the particles have been squeezed together; the low pressure areas (rarefaction) are where the particles have been spread apart. The wavelength of sound is the distance between two successive high pressure pulses or two successive low pressure pulses. Wavelength of sound decreases as frequency increases. The sound we normally hear is from 20 to 20 000 cycles per second. Ultrasound means sound that has a higher frequency than our normal hearing. Ultrasound used for medical purposes is from one MHz (one million cycles per second) to 20 MHz. Ultrasound imaging does not usually use higher than 10 Mhz. Higher frequency ultrasound waves can form sharper images, but the images are fainter because tissues absorb higher frequency energy more readily. Just like any other type of sound, the higher the frequency of ultrasound, the shorter the wavelength. Ultrasound has a wavelength of about 1.5 mm.
The speed of ultrasound does not depend on its frequency. The speed of ultrasound depends on what material or tissue it is traveling in. The mass and spacing of the molecules of the material and the attracting force between the particles of the material all have an effect on the speed of the ultrasound as it passes through. Ultrasound travels faster in dense materials and slower in compressible materials. In soft tissue sound travels at 1500 m/s, in bone about 3400 m/s, and in air 330 m/s.
Over the past two decades ultrasound has undergone numerous advances in technology such as gray-scale imaging, real-time sonography, high resolution 7.5-10 MHz transducers, and color-flow Doppler and more.
Ultrasound is reflected at the boundaries between different materials. Ultrasound reflects very well wherever soft tissue meets air, or soft tissue meets bone, or where bone meets air. Frequency is unchanged as sound travels through various tissues. That means that in tissues where sound travels more slowly, wavelength decreases. Just as the spacing between cars on a highway narrows when they slow down for construction, the compression areas of a wave get jammed together when sound slows down.
Biomedical ultrasonic applications
Ultrasound has therapeutic applications, which can be highly beneficial when used with dosage precautions. According to Radiology Info, ultrasounds are useful in the detection of pelvic abnormalities and can involve techniques known as abdominal (transabdominal) ultrasound, vaginal (transvaginal or endovaginal) ultrasound in women and also rectal (transrectal) ultrasound in men. Besides that, focused high- energy ultrasound pulses can be used to break calculi such as kidney stones and gallstones into fragments small enough to be passed formt he body without undue difficulty, a process known as lithotripsy. Treating benign and malignant tumors and other disorders via a process known as high intensity focused ultrasound(HIFU), also called focused ultrasound surgery (FUS). In this procedure, a generally lower frequencies than medical diagnostic ultrasound is used (250-2000 kHz), but significantly higher time averaged intensities. The treatment is often guided by magnetic resonance imaging(MRI). Delivering chemotheraphy to brain cancer cells and various drugs to other tissues is called acoustic targeted drug delivery. These procedure generally use high frequency ultrasound (1-10 MHz) and a range of intensities (0-20 watts/cm 2). The acoustic energy is focused on the tissue of interest to agitate its matrix and make it more permeable for therapeutic drugs.
Ultrasonic testing is a type of nondestructive testing commonly used to find flaws n materials and to measure the thickness. Frequencies of 2 to 10 MHz are common but for special purposes other frequencies are used. Inspection may be manual or automated and is an essential part of modern manufacturing processes.
Ultrasonic cleaners are used for jewellery, lenses and other optical parts, dental instrument, surgical instrument and industrial parts. An ultrasonic cleaner works mostly by energy released from the collapse of millions of microscopic cavitations near the dirty surface.
The ultrasonic disintegration similar to ultrasonic cleaning, biological cells including bacteria can be disintegrated. High power ultrasound produces cavitations that facilitates particle disintegration or reactions. This is used in science for chemical purpose in killing bacteria.
In ultrasonic welding of plastics, high frequency low amplitude vibration is used to create heat by way of friction between the materials to be joined. The interface of the two parts is specially designed to concentrate the energy for the maximum weld strength.