# Basic Ultrasound Wave Properties

Basic Ultrasound Wave Properties

Michael J. Lanspa

1. Which of the following will increase the speed of sound transmitting through a medium?

A. Increasing the frequency of the sound

B. Increasing the density of the medium

C. Increasing the stiffness of the medium

D. Increasing the wavelength of the sound

1. Correct Answer: C. Increasing the stiffness of the medium

Rationale: The propagation speed is determined by the medium. The speed of sound increases with lower density and higher stiffness media. All sound, regardless of the frequency, travel at the same speed through any specific medium. This means that sound with a frequency of 5 MHz and sound with a frequency of 3 MHz travel at the same propagation speed if they are traveling through the same medium. Increasing the wavelength will decrease the frequency of sound, but since the speed of sound is determined by the medium, it will not affect the propagation speed.

Selected References

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

2. Ziskin MC. Fundamental physics of ultrasound and its propagation in tissue. Radiographics. 1993;13:705-709.

2. Which of the following is true regarding sound?

A. Sound is a transverse wave.

B. Sound is a longitudinal wave.

C. Sound waves transfer mass, not energy, from place to place.

D. Sound waves propagate at a constant velocity in a vacuum.

2. Correct Answer: B. Sound is a longitudinal wave.

Rationale: Sound waves are mechanical, longitudinal waves. In longitudinal waves, the particles move along the same axis as the direction of propagation (Figure 1.2).

In transverse waves (e.g., ocean waves), the particles move in a perpendicular direction (orthogonal) to the direction of the wave (Figure 1.3).

Traditionally speaking, sound waves transfer vibration energy, not matter or mass, although a new theoretical study suggests ordinary sound waves carry a small amount of negative mass in Newtonian conditions. Lastly, sound must travel through a medium. Sound cannot propagate in a vacuum.

Selected References

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

2. Nicolis A, Penco R. Mutual interactions of phonons, rotons, and gravity. Phys Rev B. 2018;97:134516.

3. Ziskin MC. Fundamental physics of ultrasound and its propagation in tissue. Radiographics. 1993;13:705-709.

3. Which of the following remains constant with increasing depth?

A. Frequency

B. Power

C. Amplitude

D. Intensity

3. Correct Answer: A. Frequency

Rationale: Frequency, pulse duration, period, and axial resolution do not change with depth. If the medium is homogeneous, velocity and wavelength do not change either, although they can change when traveling from one medium to another. Amplitude (defined by the maximum variation of a sound wave from its mean) attenuates (decreases) with depth. Power is the amount of energy transfer, measured in Watts. Power is proportional to the square of the amplitude. Therefore, power will decrease with depth because the amplitude decreases. Intensity is the power divided by the cross-sectional area of the beam. As the beam diameter increases (past the focal point) and the power decreases with depth, both cause the intensity to decrease as well.

Selected References

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

2. Ziskin MC. Fundamental physics of ultrasound and its propagation in tissue. Radiographics. 1993;13:705-709.

4. You perform a chest ultrasound on a patient with a pneumothorax. What is true regarding the ultrasound beam as it travels from soft tissue to air?

A. The velocity increases

B. The pulse duration increases

C. The wavelength decreases

D. The frequency decreases

4. Correct Answer: C. The wavelength decreases

Rationale: The velocity of sound through soft tissue is 1540 m/s, while it is 330 m/s in air, thus the velocity will decrease when traveling from soft tissue to air. Frequency and pulse duration remain unchanged with change in medium. Wavelength will decrease proportional to the decrease in velocity (speed = wavelength × frequency), making option C the correct choice. Although not mentioned, the acoustic impedance of air is much higher than soft tissue, which would result in an increase in amplitude and power as the ultrasound travels from soft tissue to air.

Selected References

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

2. Ziskin MC. Fundamental physics of ultrasound and its propagation in tissue. Radiographics. 1993;13:705-709.

5. What is the wavelength of a 5 MHz ultrasound traveling through soft tissue?

A. 3.25 mm

B. 0.308 mm

C. 0.154 mm

D. 0.616 mm

5. Correct Answer: B. 0.308 mm

Rationale: Wavelength multiplied by frequency equals velocity. The velocity of sound in soft tissue is 1540 m/s (1.54 mm/µs). One can calculate the wavelength in mm by taking 1.54 mm/µs and dividing by frequency in MHz. 1.54/5 = 0.308 mm.

Selected References

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

2. Ziskin MC. Fundamental physics of ultrasound and its propagation in tissue. Radiographics. 1993;13:705-709.

6. The probe sends out a sound wave and records an echo 0.1 ms later. How deep is the structure that reflected an echo, assuming the medium is soft tissue?

A. 1.54 cm

B. 3.08 cm

C. 7.7 cm

D. 15.4 cm

6. Correct Answer: C. 7.7 cm

Rationale/Critique: Ultrasound calculates the depth of an object by measuring the time it takes the signal to return to the transducer and assumes the wave velocity is 1540 m/s. The distance of a reflector is the time in flight divided by 2 (to account for travel toward and back from the reflector), multiplied by velocity.

The above equation yields an answer of 7.7 cm for 0.1 ms. The other choices are incorrect.

An abbreviated method to calculate the distance for sound traveling through soft tissue is to simply multiply the time in flight by 77 cm/ms (1540/2 m/s). 77 cm/ms × 0.1 ms = 7.7 cm.

Selected References

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

2. Ziskin MC. Fundamental physics of ultrasound and its propagation in tissue. Radiographics. 1993;13:705-709.

7. What is true about the pulse repetition period (PRP) in ultrasound?

A. The sonographer can change the PRP by increasing the pulse duration.

B. As the PRP increases, the imaging depth increases.

C. The PRP is the inverse of the sound frequency.

D. The sonographer cannot change the PRP.