Ultrasound - Study Guide | Franklin University

University:
Franklin University
Course:
SCIE-100 | Physical Science
Academic year:

2020
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31

Pages:

95
Author:

GalacticGlider

Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport. This is what characterizes sound waves in air as longitudinal waves. Ultrasonic Waves and properties Mechanical waves are longitudinal compression waves • “Ultrasound” refers to frequencies greater than 20kHz, the limit of human hearing • For Medical imaging typically 100 Times higher frequency than audible by human typically 2 to 20 MHz Longitudinal Waves In a longitudinal wave, the motion of the medium is parallel to the direction of the wave . This means that the particles move left and right which in turn makes the other particles start to oscillate. Sound waves are longitudinal waves. Transverse wave A transverse wave is a wave in which the motion of the medium is a right angles to the direction of the wave. piezoelectric effect Piezoelectricity, also called the piezoelectric effect, is the ability of certain materials to generate an AC (alternating current) voltage when subjected to mechanical stress or vibration, or to vibrate when subjected to an AC voltage, or both. The most common piezoelectric material is quartz. Certain ceramics, Rochelle salts, and various other solids also exhibit this effect. Piezoelectric Crystal Random arrangement of dipoles (natural state) Alignment and excitation on the crystal Crystal response to voltage across the excitation electrodes Piezoelectric materials For diagnostic medical applications PZT: lead zirconate titanate Barium lead titanate Barium lead zirconate PVDF: polyvinylidene fluoride Lithium sulfate For therapeutic US transducers: Quartz Lead metaniobate Crystal Generating sound waves Plane Concave Convex Wave interference Wave interference is the phenomenon that occurs when two waves meet while traveling along the same medium. The interference of waves causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium Constructive Interference when waves are at the same place at the same time, the amplitudes of the waves simply add together Destructive interference If the two amplitudes have opposite signs, they will subtract to form a combined wave with a lower amplitude. This is called destructive interference. Transducer Wave Front Plane Crystal Last Axial Maximum Far field Near field Crystal x= θ X r 2 λ , 0.61λ sin θ = r Concave Crystal Focal point Plane Crystal Lenses Focal point The Speed of Sound The compressibility κ and density ρ of a material, combined with the laws of conservation of mass and momentum, directly imply the existence of acoustic waves Ultrasound waves travel at a speed of sound c, given by c= 1 ρk or C= B ρ Parameters effecting attenuation 1- Scattering 2- Refraction 3- Reflectivity 4- Absorption Attenuation Scattering An important interaction between ultrasound and tissue is scattering or nonspecular reflection, which is responsible for providing the internal texture of organs in the image. Refraction Interaction that occurs between ultrasound and tissue is refraction. If the ultrasound beam strikes an interface between two media at an angle other than 90 degrees, however , the transmitted part will be refracted or bent away from the straight-line path Reflectivity If the ultrasound beam strikes an interface between two media at an angle of 90 degrees, a percentage will be reflected back to the first medium and the rest will be transmitted in to the first medium and the rest will be transmitted in to the second medium without a change in direction. Transmition of ultrasound wave at an Interface Acoustic Impedance(Z) Z=ρ.C Reflected Ii It Ir Z1 Z2 Intensity reflection coefficient: Intensity reflection coefficient: Intensity transmission coefficient: Reflection of Sound : When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction, this phenomenon is called the reflection of sound Use of Ultrasound in Medicine 1- Therapeutic a- Physiotherapy b- Phonophoresis c- Hyperthermia 2- Diagnostic a- A-mode b- B-mode c- Doppler d- M-mode Physiotherapy Therapeutic ultrasound is in the frequency range of about 0.83.0 MHz This vibration can cause a deep heating locally though usually no sensation of heat will be felt by the patient Ultrasound can produce many effects other than just the potential heating effect. It has been shown to cause increases in tissue relaxation, local blood flow, and scar tissue breakdown. The effect of the increase in local blood flow can be used to help reduce local swelling and chronic inflammation, and, according to some studies, promote bone fracture healing Ultrasound can produce many effects other than just the potential heating effect. It has been shown to cause increases in tissue relaxation, local blood flow, and scar tissue breakdown. The effect of the increase in local blood flow can be used to help reduce local swelling and chronic inflammation, and, according to some studies, promote bone fracture healing Phonophoresis This is a non-invasive way of administering medications to tissues below the skin; perfect for patients who are uncomfortable with injections. With this technique, the ultrasonic energy forces the medication through the skin. Cortisone, used to reduce inflammation, is one of the more commonly used substances delivered in this way Hyperthermia Elevating tissue temperature to a above 42°C (107°F) and maintaining for several minutes can increase blood flow and drug absorption in the targeted region without causing permanent damage Diagnostic A-mode DIAGNOSTIC ULTRASOUND A one-dimensional presentation of a reflected sound wave in which echo amplitude (A) is displayed along the vertical axis and time of rebound (depth) along the horizontal axis; the echo information is presented from interfaces along a single line in the direction of the sound beam. Amplitude 1 2 depth 3 Time gain compensation (TGC) is a setting applied in diagnostic ultrasound imaging to account for tissue attenuation. By increasing the received signal intensity with depth Amplitude depth depth Amplitude L R L R Application of A-mode 1- Echoencephalography for the detection of midline 2- Foreign bodies in the eye 3- tissue characterization 4- Identification of cysts in the breast 5- NDT B-Mode is a two-dimensional ultrasound image display composed of bright dots representing the ultrasound echoes. The brightness of each dot is determined by the amplitude of the returned echo signal. These images allow for visualization and quantification of anatomical structures, as well as for the visualization of diagnostic and therapeutic procedures. Lesions such as tumors and atherosclerotic plaques can be identified and the extent of disease burden can be quantified. A M L1 L2 K1 K2 B M B L1 L2 C K1 K2 B ML1 L2 K1 K2 B Reference position Y X1 2 3 Y1 2 3 X 10 10 Time for an Ultrasound An ultrasound is generally performed for all pregnant women at 20 weeks of gestation. During this ultrasound, the doctor will confirm that the placenta is healthy and attached normally and that your baby is growing properly in the uterus. The baby's heartbeat and movement of its body, arms, and legs can also be seen on the ultrasound. The gender of the baby can usually be determined at 20 weeks. Shown here is a 2D ultrasound (inset) contrasted with a 4D ultrasound, both at 20 weeks. 1- Sequential Linear Arrays 2-Segmental Linear arrays 3-Sequential segmental Linear arrays 4-Linear Phased arrays Sequential linear array transducer 11 2 2 0-195 195-390 3 130 Segmental linear array transducer 1 23 4 0-195 64 Sequential segmental Linear arrays 1 2 3 4 5 6 Sequence 3 Sequence 2 Sequence 1 16 1 2 3 4 5 d1 d2 d3 d4 d5 d1 d2 d3 d4 d5 Linear Phased arrays Ultrasound beams may be by curving the focused piezoelectric plate or by interposing a lens or reflector between a flat plate and the target. A phased array of transducers is focused electronically ‫‪Annular Array‬‬ ‫آراﯾﺷﮭﺎي ﺣﻠﻘوي‬ ‫‪4‬‬ ‫‪3‬‬ ‫‪2‬‬ ‫‪1‬‬ ‫‪2‬‬ ‫‪4 3‬‬ ‫‪3 2 1‬‬ ‫‪4‬‬ Laminar Flow Parameters affecting velocity profile 1-plug flow 2-Narrowed lumen 3-Vessel curvature 4-Increase vessel diameter 5-Stenosis Plug flow Laminar flow is changed to plug flow when blood undergoes acceleration • Vessel curvature skews the blood flow velocities so higher velocity components are present at the outer edge of the vessels Narrowed Lumen Narrowed lumen alters laminar flow to a flat velocity profile Eddy Flow 3 2 1 3 2 Sudden increase in vessel diameter creates multiple flow patterns region (1) uniform high velocity flow, region (2) stagnant flow, region (3) eddy flow Turbulent Flow • 1 2 3 4 5 Disruption of flow caused by stenosis. Velocity profile includes (1) laminar, (2) jet, (3) flow reversal, (4) turbulence and (5) laminar Doppler Effect Doppler Ultrasound A Doppler ultrasound is a noninvasive test that can be used to estimate your blood flow through blood vessels by bouncing high-frequency sound waves (ultrasound) off circulating red blood cells • A Doppler ultrasound may help diagnose many conditions, including: • Blood clots • Poorly functioning valves in your leg veins, which can cause blood or other fluids to pool in your legs (venous insufficiency) • Heart valve defects and congenital heart disease • A blocked artery (arterial occlusion) • Decreased blood circulation into your legs (peripheral artery disease) • Bulging arteries (aneurysms) • Narrowing of an artery, such as in your neck (carotid artery stenosis) Doppler shift equation 2 F0V Fd = C 2 F0V cos θ Fd = C Transducer Frequency Velocity of blood Velocity of sound in tissue F0 V C Bio-effect of ultrasound • The resonance size of a bubble depends on the applied frequency of the sound field. When bubbles reach their resonance size due to growth by processes called rectified diffusion or coalescence, two possible events may occur. The bubble may become unstable and collapse, often violently, within a single acoustic cycle or over a small number of cycles. This is termed transient cavitation. The other possibility is that the bubble oscillates for many cycles at, or near, the linear resonance size. This is termed stable cavitation. The terms transient and stable cavitation are also used to define whether or not the bubbles are active in light emission (sonoluminescence) or chemical reactions Bio-effect of ultrasound • In acoustic cavitations, a sound wave imposes a sinusoidally varying pressure upon existing cavities in solution. During the negative pressure cycle, the liquid is pulled apart at sites containing such a gaseous impurity, which are known as “weak spots” in the fluid. The number of bubbles that are produced during this rarefaction cycle is proportional to the density of such weak spots present in the fluid Bio-effect of ultrasound • Gas bubbles in liquids under the influence of a sound field can do several things. A bubble can meet another bubble in solution, combining to form a larger bubble. This is termed coalescence. In a gas saturated solution such as water above a certain threshold pressure, individual bubbles can also grow with time over several acoustic cycles. This is termed rectified diffusion. Lifetime of an ‘Acoustic Transient Cavitation’ Bubble. If gas bubbles grow large enough, they can leave the system entirely due to buoyancy. This is termed degassing. Bubbles of a certain size can also become unstable and collapse, often violently. The range of bubble radii at which this occurs is very wide, and is usually much lower than the linear resonance radius. Bubble collapse can sometimes be accompanied by fragmentation into smaller bubbles. Under suitable conditions, light emission can be observed, and this is termed sonoluminescence.

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