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

Abstract

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.

Diffusion-weighted magnetic resonance imaging of thymoma: ability of the Apparent Diffusion Coefficient in predicting the World Health Organization (WHO) classification and the Masaoka-Koga staging system and its prognostic significance on disease-free survival

“…For children and adolescents, this finding is consistent with a previous study that evaluated the fatty replacement of the normal thymus with chemical-shift MRI in 95 healthy volunteers. 20 Those investigators found that chemical-shift MRI can detect physiological fatty infiltration of the normal thymus in all subjects ≥16 years, in half of adolescents aged [10][11][12][13][14][15][16] years, but in none <10 years, although a microscopic and gradually increasing amount of fat in the normal thymus exists from birth. 20 In addition, a recent report described a case of non-suppressing normal thymus in a 21-year-old woman.…”

“…20 Those investigators found that chemical-shift MRI can detect physiological fatty infiltration of the normal thymus in all subjects ≥16 years, in half of adolescents aged [10][11][12][13][14][15][16] years, but in none <10 years, although a microscopic and gradually increasing amount of fat in the normal thymus exists from birth. 20 In addition, a recent report described a case of non-suppressing normal thymus in a 21-year-old woman. 23 Thus, similarly to the normal thymus, cases of nonsuppressing TLH at chemical-shift MRI may also occur in patients with MG, even in early adulthood, as seen in our study.…”

“…6,7 Thus, the in-phase and opposed-phase images were separately obtained at chemical-shift MRI, and the signal intensity of the thymic abnormality (TLH and THY) or normal thymus was compared with that of the paraspinal muscles or chest wall muscles by using the chemical shift ratio (CSR) to assess any drop-out in signal intensity of the thymus between the two images. 6,7,20 However, the CSR measurement, which is normalized to the paraspinal musculature, could be affected by age and body mass index, since subjects lose muscle mass as they age and the muscle has more fat interspersed between the muscle fiber bundles in older and/or overweight subjects. 6,24 For this reason, in the present study, we preferred the use of dual-echo chemical-shift MRI, which does not require any normalization for its calculation, as used in previous studies for differentiating adrenal adenomas from non-adenomas.…”

“…More recently, a study involving 30 patients with TETs has demonstrated the ability of ADC in differentiating low-risk from high-risk tumours, and early from advanced disease [19]. Despite such promising results, that study is influenced by a strong correlation between Jeong and Masaoka-Koga classifications and by the inclusion of thymic carcinomas in the advanced disease group which leads to further decrease of the mean ADC value of this group compared with the early disease group [19,20]. Moreover, perfusion-sensitive ADC values have been used for analyses by including the b value of 0 sec/mm2 for ADC computation, obtaining a measured signal which reflects not only pure molecular water diffusion, but also Bpseudo-diffusion^ resulting from fast-moving water molecules from blood flow in vessels and capillaries [21][22][23][24][25].…”

Computed tomography and magnetic resonance imaging of mediastinal tumors

“…It has been recently reported that chemical shift MRI is useful for characterizing the normal thymus by detecting the intravoxel fat tissue within the thymus. Chemical shift MRI can depict physiological fatty replacement of the normal thymus in nearly 50% of subjects age 11-15 years, and in nearly 100% of those over 15 years (5) (Fig. 1).…”