Normal anatomy of the fetal thorax

At ultrasonography, the fetal lungs normally appear homogenous and are slightly more echogenic than the liver. The echogenicity of the lung increases as gestation advances. The presence of cysts or focal increased echogenicity of the lung parenchyma indicates a mass. On the four-chamber view, the heart occupies 25-30% of the thoracic volume and is positioned in the left anterior quadrant, just to the left of midline. The axis of the heart is determined relative to  the interventricular septum, which makes an angle of   45⁰ with the midline. Cardiomediastinal shift may often be the first clue to the presence of unilateral chest mass or diaphragmatic hernia. Hence, the four chamber view of the heart is an important landmark in the fetal chest and should be visualized in all fetuses during 2nd and 3rd trimesters as part of routine obstetric imaging. Fetal lung volumes can be measured with three and four dimensional ultrasonography and should be calculated in fetuses with lung abnormalities for the estimation of residual lung volume.


At magnetic resonance imaging, the trachea, bronchi and lung demonstrate high T2 signal intensity relative to the chest wall muscles, since they contain a significance amount of fluid. As the lungs mature, there is increasing production of alveolar fluid, thereby increasing production of alveolar fluid, thereby increasing the signal intensity of the lungs relative to the liver.  Normal  lung volumes can be calculated with magnetic resonance imaging as well and have been shown to increase with gestational age.