Ultrasonics and Its Use in Medicine Abstract Introduction Ultrasonics is defined as sound waves with a frequency above20kHz up to around 1GHz, above which is the hypersonic regime. (A) Ultrasonicwaves are above the audible range of hearing and their high frequencies and relativelyshort wavelengths give them a number of properties that are useful both innature and everyday life. Ultrasonics has profoundly impacted technology givingus many applications, as shown by figure 1: Acoustics Ultrasonic waves are mechanical and so like all sound wavesrequire a medium for propagation.
As they are longitudinal the displacements ofthe waves are parallel to the direction of travel. For a sinusoidal wavepropagating in the positive direction the wave function isgiven by equation 1: (L) Equation 1 Where A is the displacement amplitude, kis the wave number and wis the angular frequency. The speed of a longitudinal wave travelling through afluid (n)is given by equation 2: Equation 2 Where B is the bulk modulus of the fluidand ris the density of the fluid.
(L) Ultrasonics in nature Many animals are capable of ultrasonic communication,including some mammals and birds. This sensory function is not only used forcommunication but for navigation and many survival techniques such as detectingprey. The importance of ultrasonics to the animal depends on factors such as’attenuation, scattering’ and ‘audible noise’. (A) However, this sensingmechanism is often only used where normal mechanisms are less effective, forinstance when audible background noise is high. (A) Detection There are various methods of detection ultrasonic waves: Ultrasonic waves with a wavelength of only a few millimetrescan be detected using the Kundt’s tube method. A long glass tube filled withlycopodium powder is suspended horizontally; when ultrasonic waves pass throughthe superposition of incident and reflected waves cause a stationary wave.
Heaps of the powder form at the nodes allowing the wavelength of the ultrasonicwave to be calculated. (J) (K) Ultrasonic waves can also be detected using thermaldetectors. If platinum wire detectors are placed in the region of ultrasonicwaved the wire vibrates rapidly.
Stationary waves form and a cooling andheating effect results from the pressure varying alternatively at the nodes ofthe wave. The resistance changes accordingly and so the ultrasonic wave can bedetected. (J)(K) Flames are also used as a method of detection. If a narrowflame is moved along the medium in which the ultrasonic wave is travelling inthe nodes of the wave cause the flame to flicker. Determining the distancebetween the nodes allows the wavelength, frequency and velocity of theultrasonic wave through the medium to be calculated. (J) There are other ways in which ultrasonic waves can bedetected, some more appropriate than others depending on the situation.
One ofthe most common methods of detection in medicine uses the principles of thepiezoelectric effect, which is also used to artificially produce ultrasonicwaves. Ultrasonics inMedicine Ultrasonics have provided many useful applications inmedicine that aid in both the diagnosis and treatment of various conditions. Ithas allowed ‘affordable and effective imaging tools’ to be developed which,unlike some diagnostic techniques, are safe and non-invasive. Researchcontinues to find other ways in which ultrasound can be used by clinicians tofurther benefit the healthcare of patients.
(F) The piezoelectric effect is anintegral to many of the uses of ultrasonics in medicine: The PiezoelectricEffect Transducers contain piezoelectric materials which allowultrasonic waves to be both produced and detected. The piezoelectric effectexplains why this is possible. Many simple transducers consist of apiezoelectric ceramic connected to electrodes, often consisting of a thin metalfilm, such as silver, which are then connected to electrical wires. Differentshaped ceramics are used, the most common being square and round. The two mainclassifications of transducers are narrow-band and broad-band.
Narrow-bandtransducers are frequently used for high-intensity applications where lowfrequencies of 20-100 kHz are used; whereas, broad-band transducers aregenerally used for non-destructive testing and imaging with typical frequenciesbeing 0.5-50 MHz. (D) The piezoelectric material within a transducer has theability to produce an electric charge when a mechanical stress is applied; as wellas deform mechanically under the application of an electric field.
Piezoelectricity was first discovered in the1880s when quartz crystal was found to have this property, enabling ‘atransducer to transmit ultrasound and, reciprocally, to generate electricalsignals from received ultrasound waves.’ (B) The application of an electricfield to the piezoelectric material causes a variation in the shape of the dipolesof the material, which causes a slight change in the materials dimensions, thisproduces the ultrasonic waves. The reverse of this effect allows waves to bedetected; when ultrasonic waves reach the transducer they apply mechanicalstress to the piezoelectric material and so ‘the molecular dipole momentsre-orient themselves and thus cause a variation in surface charge density andthus a voltage.’ (X) This effect is illustrated by figure 2: Quartz is an example of piezoelectric single crystal.
Otherexamples of piezoelectric materials, more commonly used today, include leadzirconate titanate, lead titanate and lead metaniobate (these materials are allpiezoceramics). Piezoelectric ceramics are widely used due to their highcoupling capability and low dielectric loss (V), compared to single crystalsthey have a higher piezoelectric performance. (W) The properties ofpiezoelectric materials vary and so different materials are used depending onthe intended application of the transducer. (c) When deciding which piezoelectric material to use there aremany parameters to consider, the most important parameters being: theelectromechanical coupling constant (keff), the dielectricpermittivity (er),and the acoustic impedance (Z).
These factors all ‘determine the of ultrasonictransducers.'(R) (R)(D) (S) The electromechanical coupling constant can be defined as,’the square root of the ratio of energy available in electrical (mechanical)form under ideal conditions to the total energy stored from a mechanical(electrical) source.’ This can be calculated using the equation 3: Equation 3 Where fs is the frequency of the maximumconductance and fp is the frequency of the maximum transducer. (S) Theefficiency of emitters and sensitivity of receivers are both dependent on this’in such a way that a high k factor is always desirable.'(D) The dielectric permittivity is the ability of the materialto store charge.
(T) ‘The normal acoustic impedance of an absorbing material isthe complex ratio of the sound pressure at the surface of the material to theresulting volume current crossing the surface along a normal direction.’ (E)For imaging acoustic impedance ,Z, is an important physical property of tissuethat depends on the density of the tissue, r,and the speed of the wave in the medium, c, as shown by equation 4: Equation 4 This is particularly important factor to consider when thewave is passing from one tissue type to another. The acoustic impedance of thedifferent materials affects how much is transmitted between them and how muchis reflected back. If the difference in acoustic impedance between the tissuesis large then the reflection is high. When waves are incident n the boundary between two media ofacoustic impedance Z1 and Z2 the ratio of reflectedintensity Ir and incident intensity Ii is given byequation 5(M): Equation 5 Lead BasesPiezoelectric Ceramics Lead based piezoelectric ceramics have been widely used formany decades due to ‘remarkable properties and relatively low cost ofprocessing.’ (A1) However, it has become apparent more recently that they are’serious environmental concerns regarding the manufacture, use and disposal,'(X) of them. Therefore, the development of lead-free ceramics with similarproperties has become necessary.
In 2007 an article was published out-liningresearch in the development of lead-free piezoelectric ceramics. The results ofwhich are shown in figures 3 and 4: From figures 3 and 4 it can be seen that the dielectricpermittivity and piezoelectric coefficients of lead-free materials are lessthan that of the PZTs materials. It was also found that for lead basedmaterials the electromechanical coupling factor was 50% higher and the ‘highclamped permittivity for electrical impedance matching of smallelements in high frequency arrays,’ was nearly three times larger thanlead-free materials.(B1) From these results it is clear that current lead-freepiezoelectric ceramics are not as effective and that further investigation isneeded. Ultrasound Imaging One of the most widely used applications of ultrasonics inmedicine is ultrasound imaging, which can assist in diagnostics. Ultrasoundscans, sonograms, are used for many reasons including: the monitoring of adeveloping fetus, the studying of abdominal and pelvic organs to diagnose acondition and to guide surgeons during some surgical procedures. Ultrasoundimages are ‘visual representations of the interaction between sound waves andthe medium of wave propagation.
‘(F). Transducers are used to transmit acousticpulses; the incident waves travel into the tissue and when they reach theboundary between different tissue types some of the energy is reflected back andreceived by the transducer which then converts the image into signals that are amplifiedand processed into an image. There are a few modes of ultrasound scanning whichall have different uses: · Amplitude-modedisplay (A-mode): a single transducer is fixed and sends signals along aone dimensional line and the echoes can be plotted as a function of depth. · Brightness-modedisplay (B-mode): a linear array of transducers is moved to scan a planethrough the body allowing a two-dimensional image to be produced. · Time-motionmode (T.
M.-mode/C-mode): A rapid display of successive B-mode images allowsthe motion of internal organs to be see. This is because the reflectionsproduced by the boundaries of the organ move relative to the probe. (C1D1) There is a large difference in the acoustic impedance of airand skin and so the transmission of ultrasonic waves into the bodies tissues islow. For successful imaging ‘liquid coupling agents are required to transmitultrasonic waves effectively from the transducer face to the tissues.
‘(G) As ultrasound imaging requires contact between thetransducer and the skin of the patient. As the skin has some resistance, it maybe irritated by the current generated by the electrodes. Therefore, a cover isrequired on the electrodes ‘particularly if the galvanic action is intended forthe deeper tissues,’ (J1) to reduce this irritation. This makes imaging morecomfortable for the patient and makes it possible to keep the transducer incontact with the skin for a longer period of time, allowing the best possibleimages to be taken.
(J1) Enhanced Imaging Researchers are continually trying to find ways of improvingthe quality of images produced by ultrasound scans. Improvements in imagequality allow more detail to be seen and increase the accuracy of diagnostics. One way in which the quality of imaging has been improves isthe development of microbubble contrast agents, liquids containing microbubblesof gas. These agents are injected into the blood stream with the aim ofenhancing ultrasonic images. The agents are ‘intense sound wave reflectorsbecause of the acoustic differences between the liquid and the gasmicrobubbles,’ (U) and so this development allows blood flow to bedistinguished from surrounding tissue. When an ultrasonic wave propagates through a microbubble, itcauses it to oscillate producing waves with a harmonic content. The harmoniccontent can be increased by increasing the amplitude of the ultrasonic wave orby reaching frequencies close to the harmonic frequency of the microbubble. (F) Despite being strong scatterers at the fundamental frequencyit is difficult to separate the energy from the microbubbles with the energyfrom surrounding tissues.
(F) It is predicted that the intensity of ultrasoundbackscatter can be increased with the size of the microbubble. The ultrasoundbackscatter intensity ()is given by equation 6(G1): Equation 6