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echo (ek´o)

1. A reverberating sound sometimes heard during auscultation of the chest. 2. In ultrasonography, the acoustic signal received from scattering or reflecting structures or the corresponding pattern of light on a CRT or ultrasonogram. 3. In magnetic resonance imaging, the signal detected following an inverting pulse. [G.]
atrial e. electrical reactivation of the atrium by a retrograde impulse returning from the A-V node while the antegrade impulse continues to the ventricle; characterized electrocardiographically, by a pair of P waves enclosing a QRS complex, the second P wave being inverted, indicating that it is the reverse (the retrograde pathway) of the pathway of the first P wave (the antegrade pathway).
nodus sinuatrialis e. , NS e. a postectopic sinus beat occurring earlier than would be expected from the preceding sinus node discharge interval; i.e., the interval following a premature beat of supraventricular origin is less than the ordinary cycle length between sinus beats, whereas ordinarily the interval would be expected to exceed cycle length.
e. planar a method of magnetic resonance imaging that allows rapid image acquisition during free induction decay, using technically difficult rapidly oscillating radiofrequency gradients.
spin e. a commonly used technique to recover T2 relaxation signals in magnetic resonance imaging, by using a 180° inverting pulse in the pulse sequence to compensate for loss of transverse magnetization caused by magnetic field inhomogeneities.


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