The Fetal Spleen in Low-Risk Pregnancies and prior to Preterm Birth: Observational Study of the Role of Anatomical and Functional Magnetic Resonance Imaging

Abstract

Introduction: Spontaneous preterm birth complicates ∼7% of pregnancies and causes morbidity and mortality. Although infection is a common etiology, our understanding of the fetal immune system in vivo is limited. This study aimed to utilize T2-weighted imaging and T2* relaxometry (which is a proxy of tissue oxygenation) of the fetal spleen in uncomplicated pregnancies and in fetuses that were subsequently delivered spontaneously prior to 32 weeks.

Methods: Women underwent imaging including T2-weighted fetal body images and multi-eco gradient echo single-shot echo planar sequences on a Phillips Achieva 3T system. Previously described postprocessing techniques were applied to obtain T2- and T2*-weighted imaging of the fetal spleen and T2-weighted fetal body volumes.

Results: Among 55 women with uncomplicated pregnancies, an increase in fetal splenic volume, splenic:body volume, and a decrease in splenic T2* signal intensity was demonstrated across gestation. Compared to controls, fetuses who were subsequently delivered prior to 32 weeks' gestation (n = 19) had a larger spleen when controlled for the overall size of the fetus (p = 0.027), but T2* was consistent (p = 0.76).

Conclusion: These findings provide evidence of a replicable method of studying the fetal immune system and give novel results on the impact of impending preterm birth on the spleen. While T2* decreases prior to preterm birth in other organs, preservation demonstrated here suggests preferential sparing of the spleen.

Keywords: Chorioamnionitis; Fetal immunity; Infection; Magnetic resonance imaging; Preterm delivery; Preterm rupture of membranes.

Fig. 1.

MR images of the spleen acquired in a control fetus at 31+2 weeks’ gestation. Top row: axial images; bottom row: coronal images. a T2-weighted images following reconstruction using deformable slice to volume registration (spleen in red). b T2 multi-echo prior to reconstruction (second echo time shown). c T2^* map generated from T2ME images and reconstructed in T2 space (spleen in red).

Fig. 2.

Summary of recruitment of women at high risk of spontaneous preterm birth prior to 32 gestational weeks.

Fig. 3.

Fetal splenic volume in control fetuses and fetuses who were subsequently delivered prior to 32 gestational weeks. Yellow dots: control fetuses (n = 55); blue triangles: fetuses who were subsequently delivered prior to 32 weeks (n = 19) (p = 0.146). Small dashed blue line: 5th centile; solid orange line: 50th centile; long dashed gray line: 95th centile. Formulas for generating centiles where GA = gestational age at MRI: 50th centile: y = (6438.557745157574) + (−655.102252428557 × GA) + (17.27958002820324 × GA × GA); 5th and 95th centiles: y = (6438.557745157574) + (−655.102252428557 × GA) + (17.27958002820324 × GA × GA) ± (1.645 × [0 + −1.673780165632131 × GA] + [0.5140457330570599 × GA × GA]).

Fig. 4.

Fetal spleen volume:body volume ratio in control fetuses and fetuses who were subsequently delivered prior to 32 gestational weeks. Yellow dots: control fetuses (n = 55); blue triangles: fetuses who were subsequently delivered prior to 32 weeks (n = 19) (p = 0.027). Small dashed blue line: 5th centile; solid orange line: 50th centile; long dashed gray line: 95th centile. Formulas for generating centiles where GA = gestational age at MRI: 50th centile: y = (8.080781009961062e−05 × GA) + (−0.000720353421025719); 5th and 95th centiles: y = (8.080781009961062e−05 × GA) + (−0.000720353421025719) ± (1.645 × [7.041853238979974e−06 × GA + 0.0002079991357377875]).

Fig. 5.

Fetal spleen volume:body volume ratio demonstrating the impact of preterm prelabor rupture of the membranes. Control fetuses: yellow dots (n = 55); green squares: fetuses who were subsequently delivered prior to 32 gestational weeks following preterm prelabor rupture of the membranes (n = 11) (p = 0.026); blue diamonds: fetuses who were subsequently delivered prior to 32 gestational weeks without preterm prelabor rupture of the membranes (n = 9) (p = 0.283). Small dashed blue line: 5th centile; solid orange line: 50th centile; long dashed gray line: 95th centile.

Fig. 6.

Fetal splenic mean T2^* in control fetuses and fetuses who were subsequently delivered prior to 32 gestational weeks. Yellow dots: control fetuses (n = 55); blue triangles: fetuses who were subsequently delivered prior to 32 weeks (n = 19) (p = 0.755).