Wk 3 Ranging

Learning Objectives

  • Understand the principles of ranging used in ultrasound

  • Calculate distance using range equations, considering round-trip time

  • Describe assumptions made in diagnostic ultrasound regarding echo range

  • Analyze consequences of different propagation speeds in mediums

Pulse-Echo Principle and Ranging

  • Echolocation: Uses sound speed to calculate distances between the transducer and tissues/interfaces.

    • Based on round-trip time of the echo.

    • Formula: v = d x 2 / t

Factors Affecting How Echolocation Works

  • Determined by:

    • Speed of sound in the medium

    • Time taken for sound to travel

    • Direction of the sound

Key Components of Sonographic Images

  • Composed of scan lines

  • Depth determination requires:

    1. Direction of echo

    2. Distance to reflector/scatterer (echo producer).

Speed of Sound in Soft Tissue

  • Typical assumed speed: 1540m/s1540 m/s or 1.54mm/μsec1.54 mm/\mu sec.

  • Not all tissues travel at 1540m/s1540 m/s.

  • Differences can affect distance determinations.

Distance Determination Process

  1. Time Tracking: The ultrasound system tracks echo return time.

  2. Range Equation:

    • Distance=Time×VelocityDistance = Time \times Velocity

    • Corrected for round-trip time to provide accurate measurements.

Complete Range Equation

  • Depth location based on round-trip travel time:

    • Pulse round trip time (µs)=2×Distance (mm)Propagation speed (mm/µs)\text{Pulse round trip time (µs)} = \frac{2 \times \text{Distance (mm)}}{\text{Propagation speed (mm/µs)}}

  • In soft tissue, the round trip time for 1 cm depth is 13 µs.

Round-Trip Travel Times for Depths

Depth (cm)

Travel Time (µs)

0.5

6.5

1

13

2

26

3

39

4

52

5

65

10

130

15

195

20

260

  • For every 1 mm increase in depth, there is approximately 1.3 µsec of roundtrip travel time.

Consequences of Variable Speed of Sound in Different Mediums

  • Certain mediums have different sound speeds that can affect ultrasound performance:

    • Air: 330m/s330 m/s (no transmission)

    • Bone: 4080m/s4080 m/s (no transmission)

    • Lung: 550m/s550 m/s (no transmission)

    • Muscle: 1570m/s1570 m/s (close to assumption)

    • Fat: 1440m/s1440 m/s (ideal scenario)

Table of Medium-Acoustic Values

Material

Density (Kg/m³)

Velocity (m/s)

Impedance (MRayls)

Attenuation Coefficient (u)

Air

1.2

330

0.0004

12

Lung

400

500

0.26

40

Soft Tissue

1050

1540

2.0

0.5

Fat

920

1440

1.35

0.63

Liver

1060

1560

1.66

1.0

Muscle

1060

1560

1.62

0.125

Blood

1500

4080

6.7

20

Bone

1000

1480

1.52

0.0022

Water

1000

1480

1.52

0.0022

Speed Error Artifacts

  • Effect: Slower mediums increase roundtrip time, making echoes appear as though they came from further away.

    • Example: A needle may appear bent, called the "broken needle" or "bayonet" sign, indicating a speed error artifact.

Recap of Ranging Principles

  • Understanding range definition: distance from transducer to reflector.

  • Importance of including roundtrip time in the range equation.

  • Assumptions of consistent propagation speed across tissues may lead to speed error artifacts and inaccurate readings.