In the M-mode technique, all the ultrasonic pulses are propagated
along the same axis and different parts of the heart are studied
by changing the direction of the beam manually. An M- mode echocardiogram
is not a "picture" of the heart, but rather a diagram that shows
how the positions of its structures change during the course of
the cardiac cycle. It is an admirable method for studying a structure
like a heart valve, but it does not provide information about the
spatial relationships of different parts of the heart to each other.
However, this can be accomplished by scanning the ultrasound beam
rapidly back and forth across a section of the heart.
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| Fig. 1 |
Because access to the heart afforded by the ribs and lungs is very
limited, almost all cardiac scanners are of the sector type. A mechanical
sector scanner can use either an oscillating or rotating scan head.
In the rotating type (Click here to enlarge Figure
1), several transducers spin inside a small dome filled with
liquid. As each one passes over the heart, it transmits pulses and
receives echoes. The next element then takes over, like a succession
of beams from a lighthouse scanning over the sea. The echo signals
are displayed in B-mode form. Signals from the scan head are used
to steer the oscilloscope beam in the same manner as the ultrasound
beam. The result is a tomographic image of the heart, showing the
structures in the selected scan plane and their motion patterns.
2004