Sound: The effects of ultrasound on different tissue types

In ultrasound imaging, high-frequency vibrational waves are generated by a tool applied to the surface of a patient’s body. The waves produced by this source propagate through the soft tissues in the patient’s body, scattering and reflecting from the different tissues that they encounter as they travel. The waves are recorded again as they exit the patient’s body in order to determine the relative amounts of absorption and scattering that the waves experienced. This information may be used to infer the presence and distribution of tissues with which the waves interacted as they passed through the body.

In the simplest ultrasound experiments, a pulse of fixed-frequency sonic waves is applied to a specific part of the body. The frequency and phase of the sound waves that are reflected back to the emitter after the cessation of the pulse are measured and compared to the known properties of the pulse. The change in signal strength between the transmitted waves and the received waves gives the total loss, or attenuation--- defined as the logarithmic decrease in amplitude (in dB) for each centimeter of thickness of tissue through which the sound waves travelled. By scanning the combined emitter/detector over an area of the patient’s body, a two-dimensional image can be formed based on the attenuation at each point in the patient’s body. The image shows dark areas where waves were not reflected by the body, and light areas where the waves were strongly reflected back towards the sensor.

In Figure 1, data from a series of ultrasound experiments was compiled and analyzed in order to infer the attenuation of various common tissues. This data is plotted against the frequency of sound waves used for the experiment. The slope of each trend line on the plot is indicated beside it.

Figure 1: A plot of the attenuation for various body tissues as a function of the frequency used in the the ultrasound apparatus. The slopes of the trendlines are inset (Jiménez et al. 2014).

Which of the following is NOT a commonality between the sound waves used in ultrasound and the light waves used in conventional microscopes?