Wiii / 22A17: Write notes on: 1. The principles of ultrasound, 2. Transducer properties and image resolution, 3. The Doppler effect

22A17: Exam Report

Write notes on:

  • The principles of ultrasound

  • Transducer properties and image resolution

  • The Doppler effect

23% of candidates passed this question.

This question was taken from core syllabus that requires level one (L1) understanding.

Physical principles of ultrasound can be illustrated by outlining how ultrasound waves are generated from piezoelectric crystals, how they travel through the tissues, how they interact with different tissue planes and how the reflected waves return to the transducer and create images.

Properties of ultrasound transducers include different geometric configurations of transducer probes and frequency-wavelength- bandwidth properties of the crystals used in diagnostic ultrasound.

Understanding of physical concepts of image resolution including its various aspects (e.g., spatial, temporal, contrast resolution) is required to address the next portion of the question. “Doppler effect” can be illustrated by a definition and equation along with some practical implications.

This question was not answered well by majority of the candidates. Lack of knowledge and limited understanding resulted in poor average mark.

Wiii / 22A17: Write notes on:

 

  • The principles of ultrasound

  • Transducer properties and image resolution

  • The Doppler effect

1. The principles of ultrasound

Definitions

Sound:  A form of mechanical energy that travels in a longitudinal wave in a series of compression (high P) & rarefractions (low P).

Ultrasound images are produced using the generation of ultrasound waves from a transducer, and receiving reflected sound waves back at the transducer

Frequencies used in medical imaging (~1-18 MHz or similar

Reflection

Include the amplitude of the reflected wave is proportional to the acoustic mismatch between the tissues and the angle of incidence

Refraction

Refraction occurs at an acoustic interface when the angle of incidence is not 90 degrees

Scattering

Occurs when the sound wave interacts with small acoustic interfaces and scatters the beam

Attenuation

Decrease in amplitude and power of waves as they pass through tissue (e.g. large acoustic differences – air, bone)

Speed of sound in tissue – 1540m/s 

Distance from the transducer is calculated by the amount of time it takes for the sound wave to return to the transducer after being generated

2. Transducer properties and image resolution

Transducer Properties

  • Piezoelectric crystals and the piezoelectric effect – a change in shape when an electric current is passed through the crystals. 
  • The alternating current produces vibration -> ultrasound waves
  • When a mechanical wave distorts a PZ material -> produces a current, this is how the reflected sound wave is detected
  • Most of the time is spent in the “receiving” mode – produces a pulsed wave, and then “listens”. Pulses repetition frequency is 1000 – 10,000 Hz, and must decrease as the depth of the image increases

Image Resolution​

Ultrasound beam resolution and factors affecting resolution:

Spatial

  • The ability to display 2 distinct points as separate in the ultrasound image.

Axial

  • Depth from the transducer.
  • Affected by frequency (higher = better resolution, but at the cost of penetration)

Elevation

  • Thickness of the beam.

  • Affected by the design of the transducer.

  • Artefact occurs if too thick.

Azimuth

  • The width of the beam.
  • Affected by line density or lines / frame.
  • Resolution is best at the focal region of the beam. 

Temporal

  • Refers to the ability of the image to refresh in real time.

  • High depth and many lines / frame will reduce the frame rate.

Modes of display

Ultrasound beam resolution and factors affecting resolution:

A

  • The amplitude of recorded echoes is displayed along a line that represents depth.

B

  • The amplitude of recorded echoes are represented as brightness

2D

  • Modified B-mode that creates a 2D image by sweeping a beam through the array

M / Time-motion

  • A single B-mode line is selected, and is displayed with time as the x-axis.
  • Allows assessment of movement

3. The Doppler Effect

Definition

A change in frequency of a sound wave reflected by a moving target

Correct relationship with movement (higher if object moving towards the probe)

Equation

\( \Large ν = \frac{(c \times [ \mathit{f}_{s}-\mathit{f}_{0} ])}{2 \times \mathit{f}_{0} \times cos \Theta}  \)

Where

  • c = speed of sound in blood (1540m/s)
  • fs = frequency of reflected ultrasound
  • f0 = frequency of transmitted ultrasound
  • Θ = the intercept angle between the ultrasound beam and the moving target (E.G. Blood flow)
  • cos(0) = 1, so the beam is parallel, the angle can be ignored

For Optimal Doppler image

  • Lower frequencies
  • The beam should be as close to parallel to the target as possible

Doppler images can be used for

  • Colour doppler – shows the direction of flow
  • Pulsed-wave doppler – good axial resolution, not as good at detecting high-frequency changes as CWD
  • Continuous wave doppler – poor axial resolution as continuously transmitting

Author: Joshua Mclarty