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Non-Thermal Effects of Diagnostic Ultrasound
Radiation Force and its Possible Biological Effects
Outline:
1. Ultrasound ba...
Transducers: Convert one form of energy to another. Ultrasound transducers convert
electric energy into ultrasound energy ...
Attenuation: The ultrasound beam undergoes a progressive weakening as it penetrates the
body due to absorption, scattering...
gray. Typically the brighter gray shades represent echoes with greater intensity levels.
This mode allows you to scan.
M-M...
Acoustic Intensity: Is a physical parameter that describes the amount of energy flowing
through a unit cross-sectional are...
Heart: radiation force of ultrasound can reduce the strength of contraction of the heart in
a small animal
Human Perceptio...
Heart: radiation force of ultrasound can reduce the strength of contraction of the heart in
a small animal
Human Perceptio...
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  1. 1. Non-Thermal Effects of Diagnostic Ultrasound Radiation Force and its Possible Biological Effects Outline: 1. Ultrasound basics, diagnostics, history, physics -Sources: • Diagnostic Ultrasound Imaging: Inside and Out – Thomas L. Szabo • Musculoskeletal Sonography Technique, Anatomy, Semeiotics and Pathological Findings in Rheumatic Diseases - Fabio Martino, Enzo Silvestri, Walter Grassi, Giacomo Garlaschi • Basics of Ultrasound Imaging -Vincent Chan and Anahi Perlas 2. Biological Effects of Ultrasound -Sources: • The Safe Use of Ultrasound in Medical Diagnostics - Edited by Gail ter Haar (Chapter 5: Non-thermal effects of diagnostic ultrasound -J. Brian Fowlkes. & Chapter 6: Radiation force and its possible biological effects -Hazel C. Starritt) • Effects of Ultrasound on Transforming Growth Factor-B Genes in Bone Cells - J. Harle, F. Mayia , I. Olsen and V. Salih • Biological Effects of Low Intensity Ultrasound The Mechanism Involved, and its Implications on Therapy and on Biosafety of Ultrasound – Loreto B. Feril Jr. and Takashi Kondo • ISUOG statement on the non-medical use of ultrasound, 2009 - J. Abramowicz, C. Brezinka, K. Salvesen and G. Ter Haar • Fetal Thermal Effects of Diagnostic Ultrasound - Jacques S. Abramowicz, MD, Stanley B. Barnett, MSc, PhD, Francis A. Duck, PhD, Peter D. Edmonds, PhD, Kullervo H. Hynynen, MSc, PhD, Marvin C. Ziskin, MD Terms and Definitions: 1. Basic Ultrasound Terminology Ultrasound: Utilizes sound waves of very high frequency (2MHz or greater). It is propagated (To cause (a wave, for example) to move in some direction or through a medium; transmit.) via waves of compression and rarefaction, and requires a medium (tissue) for travel. The higher the frequency, the less depth penetration. However, the resolution is improved. Resolution: Is the parameter of an ultrasound imaging system that characterizes its ability to detect closely spaced interfaces and displays the echoes from those interfaces as distinct and separate objects. The better the resolution, the greater the clarity of an ultrasound image.
  2. 2. Transducers: Convert one form of energy to another. Ultrasound transducers convert electric energy into ultrasound energy and vice versa. Transducers operate on piezoelectricity meaning that some Materials (ceramics, quartz) produce a voltage when deformed by an applied pressure, and reversely results in a production of pressure when these materials are deformed by an applied voltage. Pulsed Transducers: Consists of one transducer element which functions as both the source and receiving transducers. Mechanical Probes: Allows the sweeping of the ultrasound beam through the tissues rapidly and repeatedly. This is accomplished by oscillating a transducer. The oscillating component is immersed in a coupling liquid within the transducer assembly. In our case the coupling fluid is deionized water. It is important that the fluid is bubble free, so that your image is not compromised. Check the water level in the transducer assembly before scanning and if you see air bubbles, make sure you fill it with the deionized water. Attenuation: A decrease in amplitude and intensity, as sound travels through a medium. Attenuation occurs with absorption (conversion of sound to heat), reflection (portion of sound returned from the boundary of a medium, and scattering (diffusion or redirection of sound in several directions when encountering a particle suspension or a rough surface). These different forms of attenuation are responsible for artifacts that may be in your image. Some of these artifacts are useful and some are not. Some artifacts are produced by improper transducer location or machine settings. Sound Waves: Audible sound waves lie within the range of 20 to 20,000 Hz. Clinical ultrasound systems use transducers of between 2 and 17 MHz. Sound waves do not exist in a vacuum, and propagation in gases is poor because the molecules are too widely spaced which is why lung does not image well with ultrasound. A gel couplant is used between the skin of the subject and the transducer face otherwise the sound would not be transmitted across the air-filled gap. The strength of the returning echo is directly related to the angle at which the beam strikes the acoustic interface. The more nearly perpendicular the beam is the stronger the returning echo; smooth interfaces at right angles are known as specular reflectors. This is best seen in the walls of a large blood vessel such as the aorta or the carotid artery. Transducers: The choice of which transducer should be used depends on the depth of the structure being imaged. The higher the frequency of the transducer crystal, the less penetration it has but the better the resolution. So if more penetration is required you need to use a lower frequency transducer with the sacrifice of some resolution. The shape of the beam is varied and is different for each transducer frequency. There is a fixed focused region of the ultrasound beam which is indicated on the system with a small triangle to the right of the image. This indicates the focal zone of that transducer and is where the best resolution can be achieved with that particular transducer. Effort should be taken to position the object of interest in the subject to within that focused area to obtain the best detail. This can be achieved with the use of more or less ultrasound gel and moving the transducer closer to or farther away from the subject. A-Mode Amplitude modulation: A single dimension display consisting of a horizontal baseline. This baseline represents time and or distance with upward (vertical) deflections spikes depicting the acoustic interfaces)
  3. 3. Attenuation: The ultrasound beam undergoes a progressive weakening as it penetrates the body due to absorption, scattering and beam spread. The amount of weakening is dependent on frequency, tissue density, and the number and types of interfaces B-Mode Brightness modulation: A two-dimensional display of ultrasound. The Amode spikes are electronically converted into dots and displayed at the correct depth from the transducer Complex: A mass that has both fluid-filed and solid areas within it Cystic: This term is used to describe any fluid-filled structure, for example, the urinary bladder Enhancement (acoustic): Sound is not weakened (attenuated) as it passes through a fluidfilled structure and therefore the structure behind appears to have more echoes than the same tissue beside it Frequency: The number of complete cycles per second (Hertz) Gain: Refers to the amount of amplification of the returning echoes Gel Couplant: A trans-sonic material which eliminates the air interface between the transducer and the animal’s skin Homogenous: Of uniform appearance and texture Hypo-echoic: A relative term used to describe an area that has decreased brightness of its echoes relative to an adjacent structure. Also a relative term used to describe a structure which has increased brightness of its echoes relative to an adjacent structure Interface: Strong echoes that delineate the boundary of organs, caused by the difference between the acoustic impedance of the two adjacent structures; an interface that is usually more pronounced when the transducer is perpendicular to it M-Mode: is the motion mode displaying moving structures along a single line in the ultrasound beam Noise: An artifact that is usually due to the gain control being too high Reverberation: An artifact that results from a strong echo returning from a large acoustic interface to the transducer. This echo returns to the tissues again, causing additional echoes parallel and equidistant to the first echo Shadowing: Failure of the sound beam to pass through an object, e.g. a bone does not allow any sound to pass through it and there is only shadowing seen behind it Time-Gain Compensation: Compensation for attenuation is accomplished by amplifying echoes in the near field slightly and progressively increasing amplification as echoes return from greater depths Velocity (of sound): Is the speed at which a sound wave is traveling. In soft tissue at 37 degrees C. sound travels at 1540 m/second Time Gain Compensation (TGC): Equalizes differences in received reflection amplitudes because of the reflector depth. Reflectors with equal reflector coefficients will not result in equal amplitude reflections arriving at the transducer if their travel distances are different. TGC allow you to adjust the amplitude to compensate for the path length differences. The longer the path length the higher the amplitude. The TGC is located on the right upper hand corner of the monitor, and is displayed graphically. B-MODE (brightness mode): The mode that is used for the display of echoes that return to the transducer. There is a change in spot brightness for each echo that is received by the transducer. The returning echoes are displayed on a television monitor as shades of
  4. 4. gray. Typically the brighter gray shades represent echoes with greater intensity levels. This mode allows you to scan. M-MODE (motion mode): Is a graphic B-mode pattern that is a single line time display that represents the motion of structures along the ultrasound beam, 1000fps. This mode allows you to trace motion i.e. heart wall motion, vessel wall motion. PW MODE (pulsed-wave mode): Frequency change of reflected sound waves as a result of reflection motion relative to the transducer used to detect the velocity and direction of blood flow. This reflection shift can be displayed graphically, as well as audibly. During Doppler operation the reflected sound has the same frequency as the transmitted sound if the blood is stationary ( we know that blood is not stationary it moves) therefore if the blood is moving away from the transducer a lower frequency is detected (negative shift) the spectrum appears below the baseline. If the blood is moving toward the transducer a higher frequency (positive shift) is detected and the spectral displays above the baseline. 2. Effects of Diagnostic Ultrasound Mechanical Effects: Effects related to cavitation or other interactions with ultrasound with tissues without resulting in heating. Thermal Effects: Effects of ultrasound related to temperature increases in tissue and the absorption of ultrasound energy in tissue Non-Thermal Effects: effects not related to temperature increases in tissue, has a variety of source mechanisms Cavitation: the variation of pressure in the ultrasound waves activates small pockets of gas or vapor, either naturally occurring within the tissue or can be exogenous. Inertial Cavitation: occurs when surrounding medium inertia controls the bubble motion, the bubble collapse can be rapid with large increases in the temperature inside and around the bubble causing mechanical stress to the area. Cavitational Nuclei: initial gas bodies Acoustic Radiation Force Impulse (ARFI): An ultrasound imaging mode that uses acoustic radiation force to generate images of the mechanical properties of soft tissue. Ultrasound Contrast Agents: gas filled microbubbles administered intravenously. Microbubbles have a high degree of echogenicity, which is the ability of an object to reflect the ultrasound waves. This produces a contrast between the microbubbles and the soft tissue surrounding it. Safety Profiles: measurement of how safe the contrast material is Radiation Force: A force generated in a material in an acoustic field. Radiation force exerted on tissue is related to the amount of energy absorbed by the tissue. The formula is Fv=2αI/c. α is the absorption coefficient of the tissue, I is the acoustic intensity, and c is the speed of sound. Another formula is Fr=W/c, where W is the total power absorbed from the ultrasound beam. Absorption coefficient: a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter. Acoustic Impedance: (Z) is a measure of the resistance to sound passing through a medium
  5. 5. Acoustic Intensity: Is a physical parameter that describes the amount of energy flowing through a unit cross-sectional area of a beam each second or the rate at which the wave transmits the energy over a small area Acoustic Radiation Force: a physical phenomenon resulting from the interaction of an acoustic wave with an obstacle placed along its path. Acoustic Streaming: An effect related to radiation force where liquid can be forced to flow. It has been used in diagnostics to differentiate fluid filled cysts from solid lesions. It results from the generation of a force field in a liquid in the direction of wave propagation. The movement is away from the transducer and is observable to the naked eye. It occurs as a result of the absorption of the acoustic energy from the ultrasound. Acoustic Streaming in vitro: speed of streaming is greater in amniotic fluid than in water because of the difference in absorption coefficient. Acoustic streaming In vivo: fluid movement reported in breast cysts, proposed diagnostic tool to differentiate between solid and fluid filled cysts. Streaming can alter the thickness of unstirred boundary layers. Non-Linear Propagation: propagation of high amplitude pulses can lead to enhanced absorption of ultrasound energy resulting in increased radiation force and streaming. Attenuation coefficient: the difference between the energy that enters a body part and the energy that is not detected. The difference is caused by the absorption and scattering of energy within the body tissues. Shear Viscosity: The shear viscosity of a fluid expresses its resistance to shearing flows, where adjacent layers move parallel to each other with different speeds Bulk Viscosity: When a compressible fluid is compressed or expanded evenly, without shear, it may still exhibit a form of internal friction that resists its flow. The bulk viscosity is important only when the fluid is being rapidly compressed or expanded, such as in sound and shock waves. Bulk viscosity explains the loss of energy in those waves. Mechanical Index: a real time output display to estimate the potential for inertial cavitation in vivo. MI=Pr.3/√fc. Pr.3 is the rarefactional pressure of the acoustic field, fc is the centre frequency. The index is based on the examination of the temperatures of the bubbles when they collapse. This temperature can reach 5000 K, where free radicals can be created. The mechanical index is roughly proportional to the mechanical work that can be performed in a bubble in the rarfactional phase of the acoustic field. Rarefaction: The instantaneous, local reduction in density of a gas resulting from passage of a sound wave, or the region in which the density is reduced at some instant. Rarefactional Pressure: the amplitude of a negative instantaneous sound pressure in an ultrasound beam. Rarefaction is the reduction in pressure of the medium during the acoustic cycle. 3. Observations of Effects Bone: pulsed ultrasound, not diagnostic. Pulsed-ultrasound is used to heal fractures. It accelerates the formation of fracture callus in humans. Lung: diagnostic ultrasound exposure can cause localized lung hemorrhage in animals, experiment. Neurological Development: handedness. Neuronal migration changes in animals.
  6. 6. Heart: radiation force of ultrasound can reduce the strength of contraction of the heart in a small animal Human Perception: we are able to perceive radiation force, fetus will respond to ultrasound during examination. Contrast: microbubble contrast agents can result in biological effects depending on the mechanical index. Fluids: movement as a result of acoustic streaming Cell Suspensions: thickness of unstirred layer changed with ultrasound Soft tissue: -physical effects: compression of blood vessels, accelerated healing of bone fractures in vivo, alteration in gene expression, enhancement of soft tissue regeneration not due to heat -sensory effects: possible to feel radiation forces from an ultrasound beam on the skin, decrease in aortic pressure of frogs, auditory nerve stimulated directly by ultrasound -developmental effects: Partial inhibition of the neural migration in the embryonic cerebral cortex of mice was found. This was most likely due to radiation force.
  7. 7. Heart: radiation force of ultrasound can reduce the strength of contraction of the heart in a small animal Human Perception: we are able to perceive radiation force, fetus will respond to ultrasound during examination. Contrast: microbubble contrast agents can result in biological effects depending on the mechanical index. Fluids: movement as a result of acoustic streaming Cell Suspensions: thickness of unstirred layer changed with ultrasound Soft tissue: -physical effects: compression of blood vessels, accelerated healing of bone fractures in vivo, alteration in gene expression, enhancement of soft tissue regeneration not due to heat -sensory effects: possible to feel radiation forces from an ultrasound beam on the skin, decrease in aortic pressure of frogs, auditory nerve stimulated directly by ultrasound -developmental effects: Partial inhibition of the neural migration in the embryonic cerebral cortex of mice was found. This was most likely due to radiation force.

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