Medical Ultrasonography - Modes of Sonography

Modes of Sonography

Several modes of ultrasound are used in medical imaging. These are:

• A-mode: A-mode (amplitude mode) is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy.
• B-mode or 2D mode: In B-mode (brightness mode) ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen. More commonly known as 2D mode now.
• C-mode: A C-mode image is formed in a plane normal to a B-mode image. A gate that selects data from a specific depth from an A-mode line is used; then the transducer is moved in the 2D plane to sample the entire region at this fixed depth. When the transducer traverses the area in a spiral, an area of 100 cm2 can be scanned in around 10 seconds.
• M-mode: In M-mode (motion mode) ultrasound, pulses are emitted in quick succession – each time, either an A-mode or B-mode image is taken. Over time, this is analogous to recording a video in ultrasound. As the organ boundaries that produce reflections move relative to the probe, this can be used to determine the velocity of specific organ structures.
• Doppler mode: This mode makes use of the Doppler effect in measuring and visualizing blood flow
  • Color Doppler: Velocity information is presented as a color coded overlay on top of a B-mode image
  • Continuous Doppler: Doppler information is sampled along a line through the body, and all velocities detected at each time point is presented (on a time line)
  • Pulsed wave (PW) Doppler: Doppler information is sampled from only a small sample volume (defined in 2D image), and presented on a timeline
  • Duplex: a common name for the simultaneous presentation of 2D and (usually) PW Doppler information. (Using modern ultrasound machines color Doppler is almost always also used, hence the alternative name Triplex.)
• Pulse inversion mode: In this mode two successive pulses with opposite sign are emitted and then subtracted from each other. This implies that any linearly responding constituent will disappear while gases with non-linear compressibility stands out.
• Harmonic mode: In this mode a deep penetrating fundamental frequency is emitted into the body and a harmonic overtone is detected. In this way depth penetration can be gained with improved lateral resolution.