Composition and relaxation of the proton magnetization of human enamel and its contribution to the tooth NMR image

Abstract

The spin-spin, T2, and spin-lattice, T1, relaxation times and the magnetization of protons were measured in human enamel. The proton free induction decay was analyzed into solid-like interstitial water, enamel apatite, and semiliquid-like water components. The solid-like interstitial water was evaluated to be approximately 5 wt% and the semiliquid-like water to be approximately 1 to 2%. Neither in wet nor in dry natural enamel does the solid water exchange upon deuteration nor can it be extracted in vacuum. The semiliquid natural water, which is in the closed pores of the structure in the two samples above also remains unexchanged upon an 8 hr deuteration and cannot be extracted in a vacuum. With the lineshape-relaxation correlation NMR the free induction decays from heads and tails of the enamel rods were resolved. The solid-like water with T2 of approximately 14 microseconds and the apatite with a T2 of approximately 61 microseconds have T2's too short to be observed in an NMR zeugmatogram. Therefore only the semiliquid water component having an apparent T*2 of about 240 microseconds would contribute to the NMR image of human enamel. Since the relative intensity of this proton magnetization component in tooth is quite small the NMR image of tooth would show primarily the dentin and liquids within the tooth and on its surface.

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“…The time constant describing the signal’s free induction decay (FID) is known as the transverse relaxation time ( T 2 ). The FID of mineralized dental tissue has multiple components, with a mean T 2 of about 200 μs (7) for the dentin and 60 μs (8) for the enamel. These time intervals are less than those needed for conventional MRI pulse sequences to accomplish spatial encoding with pulsed magnetic field gradients, which typically requires more than 1 ms.…”