Ultrasound Interaction With Tissue PDF

Ultrasound Interaction with Tissue Attenuation in Ultrasound Attenuation refers to the progressive reduction in amplitude or intensity of ultrasound waves as they travel through a medium. This effect is due to the absorption of sound energy, the reflection and scattering of sound waves. Fa...

Ultrasound Interaction with Tissue Attenuation in Ultrasound Attenuation refers to the progressive reduction in amplitude or intensity of ultrasound waves as they travel through a medium. This effect is due to the absorption of sound energy, the reflection and scattering of sound waves. Factors Influencing Attenuation: 1. Path Length: Longer travel distances result in more attenuation 2. Frequency: Higher frequencies undergo more attenuation Attenuation in Ultrasound Attenuation is measured in decibels (dB). Attenuation Coefficient (dB/cm) = Half the frequency (MHz) An average loss of 0.5 dB/cm per MHz frequency. Equation: Total attenuation (dB) = Attenuation Coefficient (dB/cm) x Path length (cm) Factors Contributing to Attenuation Attenuation of ultrasound waves is influenced by three primary processes: 1. Absorption: The predominant mechanism of attenuation where ultrasound energy is converted to heat within the tissue. 2. Reflection: The redirection of part of the ultrasound wave back towards the transducer when it encounters a boundary between different media. 3. Scattering: The spreading out of the sound wave in various directions, often occurring with rough or irregular surfaces. Levels of Ultrasound Attenuation High Attenuation (Materials like bone, air, scars, fibrous tissue, fat, stones, calcifications, metal, and plastic.) Moderate Attenuation (Typical of organ parenchyma.) Low Attenuation (Fluids exhibit less attenuation compared to other tissues.) Acoustic impedance mismatch Acoustic impedance (Z) quantifies tissue stiffness and elasticity, determined by the product of tissue density (ρ) and the speed of sound (C). Formula: Z = ρ x C, where ρ is measured in kg/m³ and C in m/s. Units for Z are expressed in rayls (1 rayl = 1 kg/(m²·s)). The transmission and reflection of ultrasound at tissue interfaces are influenced by acoustic impedance disparities. Similar impedance values across tissues result in greater transmission; substantial differences cause increased reflection Acoustic impedance Using the values for density and Density Speed of the speed of ultrasound given in Medium Ultrasound (kg/m3) the table, calculate the acoustic (m/s) Air 1.3 330 impedance/ calculate the acoustic Water 1000 1500 impedance mismatch of muscle Blood 1060 1570 and bone. Fat 925 1450 Muscle 1075 1590 Bone 1400 4080 Barium titanate 5600 5500 (transducer material)  Acoustic impedance Calculate the intensity reflection coefficient Speed of of ultrasound when transitioning from Density Ultrasound Medium (kg/m3) muscle to bone, and provide an (m/s) Air 1.3 330 interpretation of your findings Water 1000 1500 a = (Z2−Z1)2 ÷ (Z1+Z2)2 Blood 1060 1570 = (5712000 – 1709250)2 ÷ (1709250 + 5712000)2 Fat 925 1450 = (1.6022008 × 10^13) ÷ (5.5074952 × 10^13) Muscle 1075 1590 Bone 1400 4080 = 0.029 = 2.9% Barium titanate 5600 5500 (transducer material) Incident Sound in Ultrasound Incident sound refers to the ultrasound beam before it interacts with a boundary between two different media. Types of Incidence: Normal (perpendicular) and oblique. Normal Incidence occurs when the ultrasound path is perpendicular to the boundary. Incident Sound in Ultrasound Oblique Incidence occurs when the ultrasound beam strikes the interface between two media at an angle other than 90°. Incident Angle: The angle at which the ultrasound enters the medium. The degree of reflection and refraction is influenced by the acoustic impedance differences of the two media Factors Contributing to Attenuation: Absorption Absorption is conversion of sound energy into heat within a medium. Factors Affecting Absorption: 1. Relaxation Time: Slower molecular relaxation requires more energy, leading to increased absorption. 2. Frequency: Higher frequencies produce more heat due to friction and molecular motion, enhancing absorption Factors Contributing to Attenuation: Reflection Process Reflection is the redirection of part of the ultrasound wave back towards the source. More likely occurs when the boundary's dimension is significant relative to the wavelength (large, flat, and smooth boundary). The degree of reflection is influenced by the acoustic impedance difference between the two media. Types of Ultrasound Reflection Occurs at large and smooth interfaces. 1. Specular When the incident angle is perpendicular, and the Reflection interface is larger than the beam's width, partial reflection back to the source occurs. 2. Non-Specular Happens with rough or irregular surfaces. (Diffuse) The ultrasound beam is scattered in multiple directions, leading to a range of reflection angles Reflection (A) Illustration of specular versus diffuse reflection. The smooth surface in specular reflection results in more return of the reflected sound waves to the transducer (green arrows) creating a more hyperechoic (brighter) image. The less uniform tissue in diffuse reflection results in less return of the reflected sound waves and a more hypoechoic (darker) image. (B) Sonogram showing the appearance of specular reflection. Note that the large smooth surface of the bone (yellow arrows) leads to a bright signal due to the significant impedance difference between it and the surrounding tissue. (C) Sonogram showing the appearance of more diffuse reflection in muscle tissue. Note that the smaller differences in acoustic impedance reflect various shades of gray rather than the bright signal noted with the interface of bone. Factors Contributing to Attenuation: scattering Process Scattering refers to the way sound waves spread out in different directions when they encounter tissues with irregular surfaces or interfaces similar in size to the wavelength of the sound. This phenomenon leads to the reflection of ultrasound waves back to the transducer, which is known as backscatter. The image artifacts produced by backscatter are called 'speckle.’ Scattering intensity increases when the tissue interface size is comparable to or smaller than the wavelength of the incident ultrasound wave. Factors Contributing to Attenuation: Refraction Process Refraction is the change in direction of a sound wave as it passes across a boundary between two different media at an oblique angle, not perpendicular to the boundary. Refraction only occurs if the sound wave hits the boundary at an angle that is not 90 degrees (Oblique Incidence). The speed of sound must differ between the two media; if the speeds are the same, no refraction will occur. Refraction occurs due to the difference in propagation speeds of sound in different media, as defined by Snell's Law. Factors Contributing to Attenuation: Refraction Process Factors Contributing to Attenuation: Refraction Process The direction of the refracted beam depends on the speed of sound in the second medium relative to the first medium.