24B08: Exam Report

Outline the following with respect to ultrasound:

  1. The physical principles when used for tissue imaging using the following headings:

    1.  generation and detection of the ultrasound;

    2.  reflection and scattering;

    3. attenuation;

    4. refraction. (35% of marks)

  2. The relationship between transducer properties, image resolution and tissue penetration. (35% of marks).

  3. The principles of the doppler effect and its applications. (30% of marks).

75% of candidates passed this question.

In this context an “outline” question requires a definition of the principle followed by a brief description or overview.

Good answers used the structure of the question to provide a definition of the concepts of generation / detection / reflection / scattering / attenuation and refraction and then used this to describe how the ultrasound image is produced or disrupted due to these factors.

The discussion around resolution required candidates to link how the transducer and its frequency affects axial, linear, contrast and temporal resolution.

The principle underpinning this is the relationship between frequency / wavelength, tissue penetration and image resolution.

The doppler effect component of this question required an explanation of the doppler effect and how it is used in medical ultrasound for example measurement of blood flow and direction.

The Doppler equation was not required for full marks though did help some candidates explain the doppler effect.

Wiii / 24B08: Outline the following with respect to ultrasound:

A. The physical principles

Definitions

US Waves- frequencies- 2-15 MHz

Generation and detection of the ultrasound via

Peizoelectric effect

  • Transducer probe contains layers of peizolelectric crystals
  • Upon incidence of electric energy>the crystals vibrate at various frequencies/converted to mechanical energy and
  • Vice versa- reflected US waves are converted to electric energy
  • The reflected waves are analyzed for
  • amplitude (determines echogenicity) and
  • time (determines depth)
  • Ultrasound beam has 3 dimensions — Axial, Elevation and Lateral
  • Modes of Display (A- Amplitude, B- Brightness, M- Time Motion)

Reflection

Occurs at tissue interfaces- proportional to difference in impedance

Refraction

Refers the bending of sound waves as they pass through different mediums or layers of a medium at different temperatures

Scattering

Refers to the reflected waves, not received by the probe

Attenuation

Refers to loss of US waves with increase in the depth due to all of the above phenomena

B. Relationship between frequency / image resolution / penetration:

Higher the frequency of US waves

  • Shorter the wavelength – reflected effectively by smaller superficial structures
  • Narrow beam width results in higher resolution of image
  • More readily attenuated by tissue, limiting their penetration depth

Lower the frequency of US waves

  • The opposite occurs > preferred for deeper structures

C. The principles of the doppler effect and its applications

Definition

The change in observed frequency of a sound wave reflected by a moving target relative to the position of observer / receiver.

Equation

Not necessary as per the college
\( \Large \nu = \frac{\delta \times \mathit{F}{s}}{2 \times \mathit{F}_{0} \times \cos \Theta} \)
The reflected Frequency is Higher Towards- red and Lower Away – blue

Applications

  • Cardiology – Blood flow direction and velocity
    • Eg: valvular stenosis, regurgitation
    • Abnormal connections- ASD, VSD
    • Cardiac output- VTI
    • Diastolic dysfunction- E/A ratio
  • Vascular
    • Eg: venous insuffiency, thrombosis, stenosis, aneurysms

Author: Nazma Navilehal Rajasab