Effect of immersion medium on ¹H-NMR spatially-resolved T₂-relaxation in teeth

Objective: To test the hypothesis that the aqueous storage solution does not affect the distribution of spatially-resolved ¹H spin-spin relaxation times (T₂) in teeth, This has significance for quantitative NMR micro-imaging, e.g. in forensic applications, caries studies, etc. Methods: Intact teeth a)freshly-extracted and b)water-stored for >2 years (both n=10) were placed in 5ml of different solutions for at least 3 weeks. Solutions: Strawberry Ribena^R, Toothkind Ribena^R (GlaxoSmithKline), high-purity water (Merck Ltd.), Dundee tap water, 0.75M saline, fluoride mouthwash (Tesco Value^R), Coca-Cola^R. Teeth were imaged in air. Images were acquired using a Bruker DPX300 AVANCE FT-NMR system with micro-imaging accessory. The position of the tooth was obtained by a tripilot pulse sequence, then an image of a selected 2mm-thick centrally-located vertical slice was obtained using the multi-slice-multi-echo (MSME) spin-echo sequence [90-tau-(180-Acquire)_n where n=32 or 64]. From these data, a 200μm² pixel MSME image and a pure ¹H-T₂ map were produced. Results: Within each freshly-extracted tooth, T₂ varies from 16ms up to 101ms after Coca-Cola immersion and from 155ms up to 381ms after high-purity water immersion (the extreme cases). Corresponding T₂ responses occur for the minimum and maximum with other solutions used. Older water-stored teeth follow a similar pattern to that of freshly-extracted teeth. Time between extraction and imaging has an effect far less than exposure medium. Conclusion: T₂ gives a measure of ¹H mobility; greater mobility results in a higher value. T₂ is affected by the composition of the dentally relevant aqueous solution to which the tooth is exposed before NMR micro-imaging. The pre-imaging aqueous environment has to be documented and considered when quantitative NMR micro-imaging of teeth is undertaken. Further investigation is required to discover whether immersion in pure water before imaging removes this effect.