Reflection/deflection/absorption/scatter

Reflection/deflection/absorption/scatter

When sound waves move on the boundary surface between two media with different densities, part of the beam is reflected to the transducer. This phenomenon is called reflection. The remainder of the beam continues on into the tissue, but under a different angle. This is called deflection. As sound waves penetrate the tissue, part of the energy is converted into heat. This energy loss is called absorption. Finally, part of the sound waves are lost in scatter. This takes place when sound waves move through inhomogeneous tissue or in a 'hard’ boundary surface (= large density difference between two media). Part of the sound waves are reflected in random directions, a small part of which towards the transducer. For a summary see figure 10.

Color Doppler

Blood stream patterns may be evaluated using echo Doppler. One of the applications of echo Doppler is color Doppler. This technique can be used to evaluate the presence of flow and flow direction in a blood vessel.
Sound reflections of a moving object undergo frequency changes. During the examination the difference between the emitted and received frequencies is measured; the frequency shift/Doppler shift (fig. 11).

he Doppler shift and Doppler angle (fig. 12) are then calculated, allowing determination of blood circulation patterns. Note: the specific Doppler calculation will not be explained further in this course.

As explained above, moving objects undergo a change in frequency. In color Doppler, frequency changes are converted into color on screen. Blue means the blood is moving away from the transducer; red means the blood is moving towards the transducer (note: blue and red does not necessarily mean low-oxygen and high-oxygen blood respectively). Explanation: when blood moves towards the transducer, the wave length of the sound wave shortens, the sound frequency increases (positive Doppler shift). The opposite happens in blood moving away from the transducer (= negative Doppler shift). See also figure 13.

Duplex Doppler

The flow signal of a blood vessel can also be represented in a spectrum.
The Doppler shift is shown on the vertical line, time on the horizontal line (fig. 14/15). Blood flowing towards the transducer has a positive Doppler shift and is shown above the line.  Flow under the line has a negative Doppler shift (= flow away from the transducer).

Artifacts

Ultrasound examinations are associated with a diversity of ultrasound artifacts and can be encountered during the examination. Unfortunately, these artifacts cannot all be discussed in this course.
Two important artifacts are explained here: acoustic shadow and posterior sound transmission. Even though these are artifacts, they are valuable in practice. 

Acoustic shadow

Acoustic shadowing is caused by two different phenomena, total reflection or absorption. Total reflection occurs on the boundary surface between gas/tissue because of the large difference in density between gas and tissue. Total absorption occurs when the sound waves are absorbed by calcareous structures (= including stones, bone). Sound waves are (virtually) all reflected/absorbed; no sound waves reach the area behind these structures, making this part of the ultrasound image entirely anechogenic (= black). This is termed acoustic shadow (fig. 16).

Acoustic shadowing is important in detecting disorders including tendon calcifications, stones or free air. The artifact is also used to differentiate solid and calcified masses, e.g. gallbladder polyp (fig. 17) from bile stones.

Posterior sound transmission 

In order to differentiate a cyst from a solid lesion, two artifacts are use: posterior wall amplification and increased sound transmission.  These phenomena occur when sound waves move through an anechogenic structure, usually a cyst. The sound wave loses little energy as it passes through the fluid in the cyst. That is why there is more energy left in the sound wave in the posterior wall and behind the structure than at the same level in the surrounding area (note: the surrounding tissue is more solid). More energy will therefore be left to reflect to the transducer. This results in a echogenic posterior wall and echogenic area behind the cyst (fig. 18).