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1. Loosening, lengthening, or lessening of tension in a muscle. 2. In nuclear magnetic resonance, r. is the decay in magnetization of tissue after the direction of the surrounding magnetic field is changed; the different rates of r. for individual nuclei and tissues are used to provide contrast in imaging. [L. relaxatio (see relax)]
cardioesophageal r. r. of the lower esophageal sphincter which can allow reflux of acidic gastric contents into the lower esophagus, producing esophagitis.
isometric r. decrease in tension of a muscle while the length remains constant due to fixation of the ends.
isovolumetric r. isovolumic r
isovolumic r. that part of the cardiac cycle between the time of aortic valve closure and mitral opening, during which the ventricular muscle decreases its tension without lengthening so that ventricular volume remains unaltered; the heart is never precisely isovolumetric (vs. isovolumic) except during long diastoles with a midiastolic period of diastasis.isovolumetric r;
longitudinal r. in nuclear magnetic resonance, the return of the magnetic dipoles of the hydrogen nuclei (magnetization vector) to equilibrium parallel to the magnetic field, after they have been flipped 90°; varies in rate in different tissues, taking up to 15 seconds for water. See TI.spin-lattice r., spin-spin r;
spin-lattice r. longitudinal r
spin-spin r. longitudinal r
transverse r. in nuclear magnetic resonance, the rapid decay of the nuclear magnetization vector at right angles to the magnetic field after the 90° pulse is turned off; the signal is called free induction decay. See T2. Cf. longitudinal r.
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