Fetal thymus in the middle and late trimesters: Morphometry and development using post-mortem 3.0T MRI

Abstract

The present study aimed to investigate the anatomical microstructure, features and signals of the fetal thymus by 3.0T FS-T2 weighted turbo spin echo sequences, which could provide imaging evidence for the evaluation of early-stage development of fetal thymus. In addition, the T2-weighted three-dimensional (3D) sequences and the 3D processing may contribute to the establishment of reference ranges for the fetal thymus. A total of 64 specimens obtained from the fetuses of 16-39 weeks of gestational age (GA) were scanned by 3.0T MRI. Morphological changes and quantitative measurements of the fetal thymus were assessed, including the anteroposterior diameter, width, height, surface area and volume. The shape of fetal thymus varied and the majority were X-shaped, with a narrow top and wide bottom. Morphology measurements demonstrated gradual growth with increasing GA. There were high linear correlations between width, height, surface area and volume and GA. No significant differences were observed between the sexes. Post-mortem 3.0T MRI clearly demonstrated changes in external contours and internal structure with GA. The images and data obtained reflect normal development of the fetal thymus and enrich the imaging data of fetal morphometry.

Keywords: morphological measurements; normal fetal thymus; post-mortem 3.0T MRI; prenatal diagnosis.

Figure 1

The 3D T2-weighted MRI images of the fetal thymus at 25 weeks GA using Amira 5.4 software. (A-C) The silhouette of the fetal thymus was artificial marked with purple lines on each image in the transverse, sagittal and coronal planes. (D-F) After tracing, the 3D software system automatically fills the selected area red for easy correction. 3D, three-dimensional; MRI, magnetic resonance imaging; GA, gestational age.

Figure 2

Transverse and sagittal sections of 25 week GA in FS-T2 TSE sequence. The shape of the thymus is irregular and mostly divided into two lobes. The axial contour appears roughly square and the sagittal appears cuneiform. (C and D) enlarged images of A and B, respectively. Heterogeneous moderate signal intensity (long arrow) were noticed in the thymus. Signals of lower intensity (two short arrows) were noticed in the surrounding tissues. GA, gestational age; TSE, turbo spin echo.

Figure 3

The 3D visualization model and linear measurements of the fetal thymus at 25 week GA. The 3D visualization model (A-C) of the fetal thymus of 25 weeks GA using Amira 5.4 for 3D data visualization, analysis and modeling. (D) Different display modes can be chosen according to preference. (E) TA, TW and TH are the longest length of the three axes of fetal thymus displayed automatically (red arrow). 3D, three-dimensional; GA, gestational age; TA, anteroposterior diameter; TH, thymus height; TW, thymus width.

Figure 4

The statistical results between all the measurements and GA. The scattergrams, best-fit equations and correlation coefficients (R2) of (A) TA, (B) TW, (C) TH, (D) TS, (E) TV and GA. All the measurements linearly increase with GA. Each symbol represents a single fetus. GA, gestational age; TA, anteroposterior diameter; TW, thymus width; TH, thymus height; TS, thymus.