Physical Properties of Ultrasound: A Board Review for ARDMS and FRCR Exams

Overview

Understanding the physical properties of ultrasound is fundamental for both the ARDMS Sonography Principles and Instrumentation (SPI) exam and the FRCR Part 1 Physics module. This topic forms the foundation upon which all ultrasound imaging, interpretation, and artifact analysis is built. Mastery of concepts like frequency, wavelength, attenuation, and interactions with tissue (reflection, refraction, scattering) is essential for optimizing image quality, understanding artifacts, and performing basic physics calculations on the exam. — — —

High-Yield Key Points

• Ultrasound is sound with a frequency above the human audible range (>20,000 Hz or 20 kHz). Medical ultrasound typically uses frequencies between 2 and 18 MHz.
• Frequency (f): The number of cycles per second (Hz). Higher frequency = better resolution, but less penetration. Set by the transducer crystal.
• Wavelength (λ): The distance over one complete cycle. λ = c / f, where c is the speed of sound in soft tissue (~1540 m/s). Wavelength determines the axial resolution (ability to distinguish two objects along the beam).
• Period: The time for one complete cycle. Period = 1 / f.
• Amplitude: The height of the wave from baseline to peak. Related to the strength or intensity of the sound wave. Decreases as the beam is attenuated.
• Attenuation: The loss of ultrasound energy as it travels through tissue. It is caused by absorption (conversion to heat, the primary factor), reflection, and scattering.
  • Attenuation Coefficient (α): Expressed in dB/cm/MHz. In soft tissue, it averages 0.5 dB/cm/MHz.
  • Penetration: Directly related to attenuation. Lower frequencies attenuate less, providing greater penetration.
• Reflection (Specular): Occurs when the ultrasound beam hits a smooth interface larger than the wavelength (e.g., diaphragm, vessel wall). Strong echoes return to the transducer. Angle of incidence = Angle of reflection.
• Refraction: The bending of the ultrasound beam as it passes from one medium to another with different propagation speeds. Follows Snell’s Law. Can cause misregistration artifacts (e.g., edge shadowing).
• Scattering (Non-specular): Occurs when the ultrasound beam hits structures smaller than the wavelength or rough interfaces (e.g., red blood cells, liver parenchyma). Creates the “speckle” or background echotexture. Rayleigh scattering occurs when the structure is much smaller than λ.
• Acoustic Impedance (Z): The resistance of a medium to sound passage. Z = density (ρ) × speed of sound (c). The unit is kg/m²/s or Rayls. Reflection occurs at interfaces where there is a change in acoustic impedance. No reflection occurs if Z is the same in both media. — — —

Mnemonics

• Frequency vs. Penetration: “Hi-Fry, Lo-Flow” (High Frequency = high resolution but shallow; Low Frequency = low resolution but deep penetration).
• Attenuation Order: “R-A-S” – Reflection, Absorption, and Scattering are the three causes of attenuation. Absorption is the Most Significant.
• Reflection Conditions: “S.M.A.S.H.” – Smooth interface, Medium size (larger than λ), Angle-dependent, Strong echoes, High impedance difference. — — —

Comparison Tables

Table 1: Frequency, Resolution, and Penetration

Table 2: Types of Ultrasound Interaction with Tissue

Board-Style Questions

1. A sonographer uses a 7 MHz linear transducer to scan the thyroid. She then switches to a 3 MHz curved array to scan the liver. Compared to the thyroid exam, the liver exam will most likely demonstrate: A. Better axial resolution and greater penetration B. Worsened axial resolution and greater penetration C. Improved lateral resolution and less penetration D. Increased attenuation and worsened lateral resolution Answer: B Explanation: Penetration is inversely related to frequency (and attenuation). A lower frequency (3 MHz) provides greater penetration for deeper structures like the liver but at the cost of lower spatial resolution (specifically axial resolution, which is determined by wavelength/frequency). Option B correctly identifies this trade-off.

2. The loss of ultrasound energy as the beam travels through tissue is called attenuation. The primary cause of attenuation in soft tissue is: A. Refraction B. Reflection C. Absorption D. Scattering Answer: C Explanation: Attenuation is the sum of energy losses from absorption, reflection, and scattering. Absorption (conversion of acoustic energy to heat) is the dominant mechanism in soft tissue, accounting for most of the attenuation. This is why the attenuation coefficient is often given as a function of frequency (dB/cm/MHz).

3. An ultrasound image shows a bright, curved echo superficial to a cyst, but the cyst itself is not well visualized at its edges, creating a “shadow.” The bright echo is most likely due to specular reflection, while the shadow is most likely due to: A. Increased scattering B. Refraction C. High attenuation from a calcified plaque D. Frequency-dependent absorption Answer: B Explanation: This describes a classic edge artifact or refraction shadow. As the beam hits the curved, highly reflective interface (like the wall of a cyst or calcification) at an oblique angle, part of the beam is reflected (creating the bright echo) and part is refracted (bent). The refracted beam may not return to the transducer, creating a posterior acoustic shadow at the edges of the structure. This is distinct from true posterior acoustic shadowing, which is caused by complete reflection or high attenuation (e.g., from a gallstone).

4. The acoustic impedance (Z) of a medium is defined as: A. Density × Wavelength B. Frequency × Speed of Sound C. Density × Speed of Sound D. Wavelength × Period Answer: C Explanation: Acoustic impedance is a fundamental property of a medium defined by the formula Z = ρ × c, where ρ is the density (kg/m³) and c is the propagation speed (m/s). The unit is Rayls (kg/m²/s). This property is crucial because a change in acoustic impedance at an interface is what produces an echo.

5. Which interaction with tissue is independent of the angle of the ultrasound beam and is responsible for the echogenicity of the liver parenchyma? A. Specular Reflection B. Refraction C. Non-specular Scattering D. Absorption Answer: C Explanation: Scattering (non-specular reflection) occurs when the beam hits structures much smaller than the wavelength (like collagen fibers in liver parenchyma or red blood cells). Because these interfaces are randomly oriented and small, the returned echoes are not dependent on the transducer’s angle, producing a uniform, homogeneous “speckle” or background echotexture. This is essential for Doppler imaging. — — —

Summary

For board exams, remember the core physical properties: Frequency determines resolution vs. penetration. Attenuation (~0.5 dB/cm/MHz) is primarily due to absorption. Ultrasound interacts with tissue via reflection, refraction, and scattering. Specular reflection is angle-dependent and creates strong boundaries; scattering is angle-independent and creates parenchymal echotexture. Acoustic impedance (Z = ρ × c) differences drive reflection. Understanding these principles is key to optimizing images, recognizing artifacts, and answering physics questions correctly.