Ultrasound Physics

Part 1. Questions 1 - 25

Ultrasound has several characteristics that contribute to its diagnostic utility. First, ultrasound can be directed as a beam and focused. Second, as ultrasound passes through a medium, it obeys the laws of reflection and refraction. Finally, targets of relatively small size reflect ultrasound and can therefore be detected and characterized. A major disadvantage of ultrasound is that it is poorly transmitted through a gaseous medium and attenuation occurs rapidly, especially at higher frequencies. As a wave of ultrasound propagates through a medium, the particles of the medium vibrate parallel to the line of propagation, producing longitudinal waves. Thus, a sound wave is characterized by areas of more densely packed particles within the medium (an area of compression) alternating with regions of less densely packed particles (an area of rarefaction).
compression and rarefaction.

The amount of reflection, refraction, and attenuation depends on the acoustic properties of the various media through which an ultrasound beam passes. Tissues composed of solid material interfaced with gas (such as the lung) will reflect most of the ultrasound energy, resulting in poor penetration. Very dense media also reflect a high percentage of the ultrasound energy. Soft tissues and blood allow relatively more ultrasound energy to be propagated, thereby increasing penetration and improving diagnostic utility. Bone also reflects most ultrasound energy, not because it is dense but because it contains so many interfaces. The ultrasound wave is often graphically depicted as a sine wave in which the peaks and troughs represent the areas of compression and rarefaction, respectively. Small pressure changes occur within the medium, corresponding to these areas, and result in tiny oscillations of particles, although no actual particle motion occurs.

If the tissue boundary width is less than the wavelength of the ultrasound wave, the ultrasound wave will not be reflected. Tissue boundaries that are smooth and have a width greater than the ultrasound wave act as a mirror or a specular reflector which results in a significant amount of reflection of the signal. The angle of incidence is the angle of the ultrasound beam and the tissue plane. For reflection, the angle of incidence is less than 90 degrees. Reflection occurs at tissue boundaries and tissue interfaces. Tissue boundaries that are perpendicular to the ultrasound wave's path act as excellent reflectors, whereas, tissue boundaries that are parallel to the ultrasound wave's path act as poor reflectors.

When the ultrasound wave is not reflected, either due to a parallel tissue boundary or high acoustic impedance, the ultrasound signal is not recorded and the display shows a lack of signal or dropout of the ultrasound picture. Myocardial walls that are perpendicular to the ultrasound beam are easily visualized. The myocardial walls that are perpendicular to the ultrasound beam act as excellent reflectors of the ultrasound signal. The walls parallel to the beam are not easily visualized. Walls parallel to the ultrasound signal result in poor signal reflection. In fact, the part of the parallel wall is not seen because the signal has been lost, which is commonly called dropout.
As the ultrasound wave travels through one medium or tissue into another medium or tissue, a change in acoustic impedance occurs. The amount of change of acoustic impedance will determine the amount of reflection. Acoustic impedance is determined by the density of the tissue. Large changes in density between two tissues will result in a large change of acoustic impedance. The change in impedance between two structures is call acoustic impedance mismatch. The difference in acoustic impedance between two tissues accounts for the amount of reflection that will occur at the tissue border. As the ultrasound wave passes through each tissue boundary, it loses some energy or amplitude while some of the wave is reflected.

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