| T1 | spin-lattice or longitudinal relaxation time; tricuspid first sound |
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| MLF | medial longitudinal fascicle |
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| OPLL | of the Posterior longitudinal ligament |
| PLL | posterior longitudinal ligament |
| riMLF | rostral intersititial nucleus of the medial longitudinal fasciculus |
| longitudinal study | <epidemiology> A study taking place over time. If individuals are followed, this is a longitudinal cohort study. If individuals are not followed, but classes (usually age classes) are restudied, this is a longitudinal cross-sectional study. The converse of a horizontal study. (05 Dec 1998) |
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| longitudinal sulcus of heart | See: anterior interventricular groove, posterior interventricular groove. (05 Mar 2000) |
| longitudinal wave | <physics> Waves where the variation of the field is partially or totally in the direction of propagation (parallel to wavennumber, k [a vector]). Examples include sound waves and Langmuir waves. Contrasted with transverse waves, where the variation is perpendicular to the direction of propagation, such as light waves. (13 Nov 1997) |