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The thymic tissue revealed homogeneous decrease in intensity on opposed-phase MR images relative to that seen on in-phase images in 15 healthy volunteers and two patients with hyperplastic thymus. Chemical-shift MR imaging may be useful in identifying normal thymic tissue and the hyperplastic thymus in early adulthood.

Purpose: To clarify the influences of age and gender on normal fatty replacement of the thymus in childhood, adolescence, and early adulthood using chemical-shift MRI. Materials and Methods:A total of 95 normal subjects (52 males and 43 females, mean age ϭ 15.6 years, range ϭ 7-25 years) who underwent chemical-shift MRI of the thymus were assessed prospectively. Signal intensity loss (SIL) of the thymus was determined by dividing the thymus/ muscle ratio on the opposed-phase image by that on the in-phase image. We evaluated SIL for its correlation with age and gender, and assessed SIL of the thymus with uncommon morphological features.Results: A significant correlation was found between SIL and age (r ϭ 0.750, P Ͻ 0.001). There was no significant difference in SIL between the genders. No significant SIL was identified in any of the subjects aged 10 years or less. However, significant SIL was found in 70.8% of those aged 11-20 years, 100% of those aged 21 years or more, and 46.2% of subjects with uncommon morphological features of the thymus. Conclusion:Chemical-shift MRI can depict physiologic fatty infiltration within the normal thymus in subjects over 11 years of age. It is crucial to correlate these normal agerelated findings with clinical cases in order to avoid misinterpretation.

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Alzheimer's disease amyloid fibrils are composed of the selfassembled 40-43 residue peptide Aβ 1 (Figure 1). Solving the atomic-level structure of these fibrils represents a key step in the study of biochemical processes related to Alzheimer's Disease. Unfortunately, progress toward this goal has been slow because conventional X-ray and NMR structural methods cannot be applied to fibrous protein samples. In developing any structural model of fibrils the space groups and symmetry operators for Aβ monomers are important considerations. In this communication, data is presented which demonstrates that when Aβ is free in solution, the amino terminus and the side chains of both lysine residues (16 and 28) are equally accessible for reductive alkylation. In contrast, when Aβ is self-assembled into fibrils, chemical access to lysines 16 and 28 can be differentiated. Furthermore, fractional alkylation of lysine 28 supports the argument that the "one-dimensional unit-cell" of an amyloid fibril contains at least two non-equivalent Aβ molecules. Together these data offer powerful new constraints for future models of fibril architecture.