understand how the reflection of pulses of ultrasound at boundaries between tissues can be used to obtain diagnostic information about internal structures

Production and Use of Ultrasound

Learning Objective

Understand how the reflection of pulses of ultrasound at boundaries between tissues can be used to obtain diagnostic information about internal structures.

1. What is Ultrasound?

Ultrasound refers to sound waves with frequencies above the upper limit of human hearing (typically >20 kHz). In medical diagnostics the frequencies used are usually between 2 MHz and 15 MHz.

2. Generation of Ultrasound Pulses

• Piezoelectric transducers: A crystal (e.g., quartz or PZT) deforms when an alternating voltage is applied, producing a short burst of sound.
• Pulse‑echo mode: The same transducer acts as a receiver after the pulse is emitted, allowing detection of reflected echoes.
• Key parameters:

  • Frequency (\$f\$) – determines resolution and penetration depth.
  • Pulse duration – short pulses give better axial resolution.
  • Peak‑to‑peak voltage – controls acoustic intensity.

3. Propagation of Ultrasound in Tissue

In a homogeneous medium the speed of sound \$c\$ is given by

\$c = \sqrt{\frac{K}{\rho}}\$

where \$K\$ is the bulk modulus and \$\rho\$ is the density of the tissue.

4. Acoustic Impedance and Reflection

The acoustic impedance \$Z\$ of a medium is defined as

\$Z = \rho c\$

When an ultrasound pulse encounters a boundary between two tissues with impedances \$Z1\$ and \$Z2\$, part of the wave is reflected and part is transmitted. The intensity reflection coefficient \$R\$ is

\$R = \left(\frac{Z2 - Z1}{Z2 + Z1}\right)^2\$

The transmitted intensity coefficient \$T\$ satisfies \$R + T = 1\$ (neglecting absorption at the interface).

5. Typical Acoustic Impedances of Human Tissues

6. How Reflections Provide Diagnostic Information

1. Time‑of‑flight measurement: The interval \$\Delta t\$ between pulse emission and echo reception gives the distance \$d\$ to the reflecting interface:

   \$d = \frac{c\,\Delta t}{2}\$

   The factor ½ accounts for the round‑trip travel.

2. Amplitude of the echo: The strength of the returned signal depends on \$R\$, which varies with the difference in acoustic impedance. Large impedance mismatches (e.g., soft tissue–bone) produce strong echoes; small mismatches produce weak echoes.
3. Image formation: By scanning the transducer across the body and recording echo amplitudes at many depths, a two‑dimensional map (B‑mode image) is constructed, where brightness corresponds to echo strength.
4. Interpretation of patterns:

   • Bright (hyperechoic) lines often indicate interfaces such as organ capsules or calcifications.
   • Dark (anechoic) regions suggest fluid‑filled spaces (e.g., cysts, bladder).
   • Intermediate (isoechoic) textures represent homogeneous soft tissue.

7. Clinical Applications of Pulse‑Echo Ultrasound

• Abdominal imaging: Detect liver lesions, gallstones, kidney stones.
• Cardiac ultrasound (echocardiography): Visualise heart chambers, valve motion, and blood flow (with Doppler).
• Obstetrics: Monitor fetal development, placenta position.
• Musculoskeletal: Assess tendon tears, muscle injuries, joint effusions.
• Vascular studies: Measure vessel diameter, plaque, and flow velocity.

8. Limitations and Sources of Error

• Attenuation increases with frequency; high‑frequency probes have limited penetration.
• Acoustic shadowing behind highly reflective structures (e.g., bone, calcifications) obscures deeper tissues.
• Angle dependence: Accurate Doppler measurements require the ultrasound beam to be aligned within \overline{60}° of flow direction.
• Operator dependence – image quality relies on correct probe placement and settings.

9. Summary Checklist

1. Identify the transducer type and its operating frequency.
2. Recall the definition of acoustic impedance \$Z = \rho c\$.
3. Calculate the reflection coefficient \$R\$ for a given tissue pair.
4. Use the time‑of‑flight formula \$d = c\Delta t/2\$ to locate structures.
5. Interpret echo brightness in terms of tissue composition.