Multidetector Computed Tomographic Study Of Normal Thymus

Objective:Thymus size, shape, and tissue composition change with individual and age. Due toits complication, there is a lack of recognizing computed tomography (CT) manifestationsof thymus and causing unnecessary biopsy or surgery. Thus, a good understanding of theimaging manifestations of normal thymus is fundamental to diagnose thymic disorders.The purpose of this study is to systematically analyze multidetector computed tomography(MDCT) manifestations of normal thymus, and the results can be taken as a reference toidentify normal thymus.Methods:This study investigated characteristics of normal thymus CT manifestations inthe terms of fat percentage, age and gender. As subjects for this study, the697cases ofcontrast-enhanced chest CT images from Picture Archiving and Communication System(PACS) between Jan.1,2012and June30,2013were randomly selected, in which215normal thymus (124males and91women females aging from6to78) remain afterconsidering relevant factors. The morphological characteristics and the internal structurewere mainly studied with the110subjects from1~3grades of fat percentage. Researchparameters include: shape, thickness of left and right lobes, morphological characteristics ofmargin, anteroposterior (AP) dimension and internal CT value. Statistical methods adoptedinclude: kappa test, student t test, variance analysis, independent sample rank sumtest,correlation analysis, linear regression analysis, chi-square test of four fold table,contingency table analysis, and covariance analysis.Results:The result shows that it is more common in triangle shape than in quadrilateralfor normal thymus. The shape has an effect on the thickness of left and right thymus lobes(with larger thickness in quadrilateral than in triangle), but not on AP. In addition, asignificant difference presents with margin morphological distribution between left and rightthymus lobes. The most common type of morphology with thymus margin is straight.Besides that, convex left lobe is more common than concave, and opposite situation with theright lobe. Furthermore, for thymuses from subjects older than54, no noticeable soft tissue isfound and the fat percentage is always in grade4. For normal thymus of fat percentage in grades1~3, there is no significant difference inshape, left and right lobes thickness, margin morphological characteristics, and APdimension, but not in internal CT value. The internal CT value shows a negative correlationwith fat content: it decreases as fat content increases.For normal thymus of four age groups,there is no significant difference in shape, margin morphological characteristics, and APdimension, but not in thickness of left and right lobes and internal CT value. With theincrease of age, thickness of left and right lobes first decreases and then increases, andinternal CT value shows a strong linear (negative correlation) relationship with age (p﹤0.001).For normal thymus of two genders, there is no significant difference in shape, left andright lobes thickness, and margin morphological characteristics but not in AP dimension. APdimension for male is larger than for female. Furthermore, as for internal CT value, there isno significant difference between genders without considering the impact from age.For either gender, internal CT value and age show a strong linear relationship (p﹤0.001).The age impact on internal CT value does not change with gender (p=0.854﹥0.05),expressed as no statistical significance between the slopes of two regression equations.Conclusion:This study provides valuable statistical results for normal thymus in the aspects ofshape, thickness of left and right lobes, margin morphological characteristics, APdimension and internal CT value. These results can be used for accurately identifyingnormal thymus with MDCT.