Introduction to M-mode Echocardiography

Echocardiography is a visualization technique that uses ultrasound as the information carrier. Ultrasound is an acoustic wave with a frequency higher than 20 kHz. However, for diagnostics, the ultrasound used is typically in a higher frequency range of about 2 MHz, which allows improved accuracy with somewhat reduced penetration.

As ultrasound penetrates through a nonhomogeneous media, it reflects from the boundaries of the regions with different acoustic impedance. An echocardiogram is a recording of the reflected ultrasonic beam.

There are two major types of echocardiography: M-mode (below left) and two-dimensional (below right) (Fig. 2, [3]).

[Graphics:Images/index_gr_2.gif]

Figure 2. Graphics courtesy of College of Veterinary Medicine, the University of Tennessee.
http://www.vet.utk.edu

In the M-mode a single beam of ultrasound is used (Fig. 3, [3]).

[Graphics:Images/index_gr_3.gif]

Figure 3. Graphics courtesy of College of Veterinary Medicine, the University of Tennessee.
http://www.vet.utk.edu

The reflections of the signal are recorded and displayed as monochronic dots. The location is proportional to the distance from the reflective region, and the intensity contains the information about acoustic impedance of the region. The M-mode has excellent axial resolution, but it carries essentially one-dimensional information. The two-dimensional echocardiograph typically uses a multielement transducer, which generates a single ultrasonic beam with changing direction. This technique allows one to create a two-dimensional image, which has good lateral resolution but lacks accuracy in the axial dimension.