Roentgenology and Radiology · MU Plovdiv

4. Principles of ultrasound examination and ultrasound applications

Inside: 7 sections · 1 table · 1 formula · 16 flashcards · 5 min

Topic 4 of 66 in the MU Plovdiv syllabus for Roentgenology and Radiology (Medicine, Year 3). The free AceMed note covers it in 7 sections, about 650 words (5 min of reading), with 16 flashcards.

In this note

  1. Introduction
  2. Physical principles
  3. Transducer and image formation
  4. Imaging modes
  5. Artefacts and limitations
  6. Safety and examination technique
  7. Clinical applications

Introduction

This topic covers the physical basis of diagnostic ultrasound (US), how an image is formed from reflected sound, the main imaging modes including Doppler, the artefacts and limits that every examiner must recognise, and the principal clinical applications of the method.

Physical principles

Diagnostic ultrasound uses mechanical longitudinal waves above the audible range, usually between 2 and 18 MHz. Sound is not ionising, which is its main advantage over X-ray based methods. Key concepts:

  • Frequency (f) and wavelength (λ) are linked by the speed of sound in the medium, c.
  • Average speed of sound in soft tissue is about 1540 m/s; it is much higher in bone and much lower in air and lung.
  • Acoustic impedance (Z = ρc) determines how much energy is reflected at a boundary between two tissues. The greater the impedance difference, the stronger the echo; a tissue-air interface reflects almost everything.
  • Attenuation (absorption, scattering, reflection) increases with frequency, so higher frequencies give better resolution but less penetration.

Depth of a reflector is calculated from the time of flight of the echo, with the division by two accounting for the round trip:

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