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Science to Practice |
Department of Radiology, Cincinnati Children’s Hospital, 3333 Burnet Ave, MLC-5031, Cincinnati, OH 45229-3039. alan.brody@cchmc.org
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When abdominal viscera pass from the abdomen into the chest during fetal development, the space available for lung growth is decreased; both lungs are affected, and in severe cases both can be markedly hypoplastic. One method of improving lung growth is to occlude the trachea so that continued production of fetal lung fluid causes an increase in the size of the lung. In this issue of Radiology, Wedegaertner and colleagues (1) evaluate the ability of ultrasonography (US) and magnetic resonance (MR) imaging to monitor lung volumes in fetal sheep treated with tracheal occlusion.
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US criteria have been developed to predict the outcome of fetuses with congenital diaphragmatic hernia (CDH) (2). A US measure of pulmonary hypoplasia, the lung-to-head ratio, has been shown to correlate with survival in fetuses with CDH (3). Fetal position, however, may limit the ability to obtain images in the necessary orientation. In addition, conventional two-dimensional US provides single images at varying orientations. This limits the use of these images for the acquisition of volumetric data.
MR imaging has also been used for fetal evaluation. MR images are less affected by fetal orientation, and image quality does not depend on the presence of amniotic fluid. Readily available measurement tools allow calculation of volumetric information. MR imaging is less available than US and more expensive. Unlike US, sedation may be required to decrease fetal motion.
In this study, US and MR imaging were performed in seven sheep with and without tracheal occlusion. True fast imaging with steady-state precession MR images of the lungs were obtained in coronal and transverse planes, and the transverse area of both lungs was measured with US at three levels. Lung growth was evaluated with serial studies, and final measurements were compared with lung volumes at autopsy. The use of an animal model allowed both frequent examinations and autopsy data on all subjects.
The authors found that increased lung growth following tracheal occlusion could be followed with both US and MR imaging. MR imaging findings correlated well with lung volume at autopsy. Good correlation was also seen between MR imaging and US measurements obtained on the same day.
The Practice
Clinical use.—By using measures that include high lung-to-head ratio, absence of liver herniation into the chest, and herniation of viscera after 25 weeks gestation, fetuses with CDH and a good prognosis can be identified. These fetuses do not benefit from prenatal intervention. Fetuses with liver herniation and a low lung-to-head ratio have a very poor prognosis (4). Fetal repair of the diaphragmatic defect has also been unsuccessful in these high-risk infants (5). For these infants, fetoscopic tracheal ligation has been used in a small number of infants to stimulate lung growth and improve outcome (4).
The need to monitor lung volumes after tracheal occlusion was demonstrated in this study, with an unintended incomplete tracheal occlusion in one of the experimental animals. This animal did not show the lung growth that was seen in the remaining treated animals. The authors also point out that overgrowth of the lung is another potential complication requiring lung volume monitoring.
Future opportunities and challenges.—This study was performed in sheep without CDH. These results will need to be confirmed in an animal model of CDH and in human subjects. In the study, general anesthesia was used prior to MR imaging. Fetal motion will likely be an issue in human studies. This may affect the choice of MR sequence and imaging plane. Normative data in humans will be necessary to best evaluate the expected increase in lung growth. The use of three-dimensional US should be evaluated in future studies to see if this modality will combine the greater availability of two-dimensional US with the volume measurement capability of MR imaging in a single examination. Continued investigation should help to clarify the best use of these different modalities in specific clinical situations.
Summary
The authors used a fetal sheep model to show that both US and MR imaging are useful tools to measure fetal lung volume following tracheal occlusion. The further development of these tools will determine the optimal means of monitoring the effectiveness of this potentially life-saving technique.
FOOTNOTES
See also the article by Wedegaertner et al in this issue.
REFERENCES
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