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Basic Ultrasound Physics

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  1. Section 1

    What is Ultrasound ?
  2. How Are Images Generated ?
  3. Ultrasound Physics Videos
  4. Section 2
    Artifacts
  5. Section 3
    Probe Characteristics
  6. Knobology
  7. Imaging Modes
  8. Section 4
    Scanning & Display Orientation

Quizzes

Basic Ultrasound Physics Imaging Modes
Lesson 7 of 8
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Imaging Modes

Introduction

Several imaging modes are available. Most hand held portable devices are limited to 2D imaging (B-Mode), Motion Mode (M-Mode) and Color Doppler.

B-Mode

Brightness Mode is also known as grey scale imaging. The scanned structures are displayed as a cross sectional image on the display monitor with pixels of differing brightness. Strong reflectors (bone) are displayed brighter. Poor reflectors (blood) are displayed darker. Knowledge of the speed of ultrasound in tissues and time taken for ultrasound waves to return to the transducer allows for the depth of the structure to be determined, and subsequently displayed.

B-Mode
Brightness mode and ultrasound scan lines. 2D imaging.
Figure 1. Multiple scan lines allow for generation of a cross sectional image.

In order to construct a cross-sectional image, the pulse-echo sequences from a multitude of neighboring scan lines are sequentially summated in real-time, generating a moving image.

M-Mode

M-Mode (Motion Mode) offers excellent temporal and axial resolution. A single piezoelectric crystals emitts and receives ultrasound pulse waves. The graphical representation consists of structures scanned within the single scan line (y-axis) plotted against time (x-axis).

M-Mode – Motion Mode
M-Mode of the LV in the parasternal short axis view.
Figure 2. M-Mode of the LV in the parasternal short axis view. The movement of the anterior and inferior walls of the LV against time is seen.

M-Mode is useful for investigating moving structures in realation to time. Examples include the following:

  1. Mitral valve leaflet motion during the cardiac cycle. Determining E-Point Septal Separation (EPSS).
  2. Left ventricular wall thickening and motion during thecardiac cycle. Determination of LV chamber size. Fractional shortening determination.
  3. Lung scanning – Presence or absence of lung sliding.

Color Doppler

We use color doppler to visually detect motion or blood flow. The principles of color doppler are similar to those of pulsed-wave Doppler. A larger region can however be interrogated. Blood flow is assigned a color, typically blue or red, depending on whether the flow is moving toward or away from the transducer.

Apical 4 Chamber view showing tricuspid regurgitation.
Figure 3. Apical 4 Chamber view showing tricuspid regurgitation. Flow away from the transducer is blue in colour.

Spectral Doppler

We use pulsed wave or continuous wave doppler in order to determine blood flow. Velocity is plotted on the y axis against time on the x axis. Pulsed wave doppler (PWD) uses a small sample volume or gate to detect blood flow at a specific location on the 2D image. In comparison, continuous wave doppler (CWD) measures all velocities in the path of the ultrasound beam.

Both PWD and CWD are based on the doppler shift principle. When an object is moving toward the emitter, there is an increase in frequency of the returning ultrasound waves. On the other hand, objects that are moving away from the emitter will return reflected ultrasound waves at a lower frequency. The velocity of the moving object is determined by this frequency shift. In the case of blood flow, the moving objects are red blood cells.

Doppler shift caused by a moving source of sound waves.
Figure 4. Doppler shift caused by a moving source of sound waves.