Attenuation coefficients for body tissues tissue

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Attenuation coefficients for body tissues Tissue Attenuation coefficient Ultrasound characterisitics Air 4500 Poor propagation, sound waves often scattered Bone 870 Very echogenic (reflects most back, high attenuation) Muscle 350 Echogenic (bright echo) Liver/kidney 90 Echogenic (less bright) Fat 60 Hypoechoic (dark echo) Blood 9 Hypoechoic (very dark echo) Fluid 6 Hypoechoic (very dark echo, low attenuation) 4 Fundamentals Fundamentals
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is automatically set over the mid-range of the screen. However, some machines have a button that allows you to shift that narrow part of the beam up and down. Finally, acoustic power refers to the amount of energy leaving the transducer. It is set to a default in most diagnostic ultrasound machines to prevent adverse biologic effects, such as tissue heating or cell destruction. This is to adhere to the ALARA or “as low as reasonably acceptable” principle – meaning the lowest amount of energy is used to obtain the information clinically needed to care for the patient. Therapeutic ultrasound operates differently from the diagnostic ultrasound discussed so far in that it purposely uses the heating properties of ultrasound to affect tissue. Often, therapeutic ultrasound is used in physical therapy or rehabilitation after orthopedic injuries to help mobilize tissue that has been scarred. Basic instrumentation Ultrasound devices all use the same basic principle for generating ultra- sound waves and receiving the reflected echoes. This principle is made possible by a property that quartz and some other compounds, both natural and synthetic, possess called the piezoelectric effect . The piezoelectric effect refers to the production of a pressure wave when an applied voltage deforms a crystal element. Moreover, the crystal can also be deformed by returning pressure waves reflected from within tissue. This reflected deformation generates an electric current that the machine translates into a pixel. As mentioned, this pixel’s gray shade depends on the strength or amplitude of the returning echo and thus the strength of the electric current it generates. Many different arrangements of this basic piezoelectric transducer/probe have been developed ( Figure 1.6 ). For example, a convex probe has crystals embedded in a curved, convex array. The further the beams have to travel, the more the ultrasound beams fan out. This reduces lateral resolution in deeper tissue. It also produces a sector- or pie-shaped image. A linear array probe ( Figure 1.7 ) has crystals embedded in a flat head. As a result, the Figure 1.5 Axial resolution ( left ) improves with higher frequency. Lateral resolution ( right ) improves with narrow bandwidth (focal zone). Fundamentals 5 Fundamentals
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ultrasound beams travel in a straight line. Because the ultrasound beams are directed straight ahead, a rectangular image is produced. Probes come in different sizes or “footprints” because sometimes smaller probes are needed to sneak through ribs or other structures that are not ultrasound- friendly. The phased-array probe ( Figure 1.8
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