Evaluation of normal fatty replacement of the thymus with chemical-shift MR imaging for identification of the normal thymus

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 = 15.6 years, range = 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 = 0.750, P < 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 age-related findings with clinical cases in order to avoid misinterpretation. J. Magn. Reson. Imaging 2005. © 2005 Wiley-Liss, Inc.