T1

In magnetic resonance, the time for 63% of longitudinal relaxation to occur; the value is a function of magnetic field strength and the chemical environment of the hydrogen nucleus; for protons in fat and in water, in a 1.5T magnet, about 250 msec and 3000 msec, respectively. A T.-weighted image will have a bright fat signal.

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breast tumor smaller than 2 cm [TNM (tumor-node-metastasis) classification]; longitudinal relaxation time

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tricuspid valve closure; see first heart sound, under sound.