Pediatric Pulmonology, Supplement 23:118-119 (2001)

Imaging of Congenital Lower Respiratory Tract Malformations: Prenatal Diagnosis by

Magnetic Resonance Imaging

Anne M. Hubbard

An important determinant of fetal survival is whether there is adequate development of the lungs. The bronchi and bronchioles are developed by 16 to 20 weeks gestational age, and a significant number of alveolar ducts and blood vessels appear by 16 to 24 weeks gestation age. The normal fetal lung on T2-weighted images is homogeneous and of moderate signal intensity. With progressive maturation of the lungs, there is in- creased signal intensity within the lungs due to increased production of alveolar fluid.' On ultrasound (US), the appearance of the normal fetal lung is homogeneous and moderate in echogenicity. With increasing maturation of the fetus, there is increased echogenicity of the lung relative to the liver. The best sonographic predictor of increasing lung maturity has been relating the measure- ment of the circumference of the fetal chest to gestational age and the femur length.* These measurements, however, are inaccurate if there is an intrathoracic mass. Normal lung volumes have been documented by magnetic resonance imaging (MRI).3 Exponential growth of the lungs with increasing gestational age has been shown. Changes in the physical properties of the fetal lungs have also been demon~trated.~ There is an increase in T1 and T2 relaxation times of the lung with increasing gestational age. With increased experience, the relaxation time measurements may provide additional information about the normal and abnormal physiologic development of the lungs in utero.

The most common masses within the fetal chest are congenital cystic adenomatoid malformation (CCAM), bronchopulmonary sequestration (BPS), fetal hydro- thorax (FHT) and congenital diaphragmatic hernia (CDH). CCAM of the lung is an uncommon lesion characterized by a multicystic mass of pulmonary tissue with an abnormal proliferation of bronchiolar structures with a connection to the normal bronchial tree.

Vascular supply is from the pulmonary artery and pulmonary veins. On prenatal MRI, the appearance of these tumors is variable depending upon whether they are micro- or macrocystic.' Type 1 or microcystic lesions are high in signal intensity compared to the normal lung. With increasing numbers of micro- or macrocysts, discrete cysts may be seen within the tumor. On MRI, it 0 2001 Wiley-Liss, Inc.

is easier to demonstrate surrounding normal compressed lung tissue than it is with US. These tumors may arise from any lobe of the lung. Hydrops may be seen with large CCAMs due to obstruction of venous return. A BPS is a mass of nonfunctioning pulmonary tissue that lacks a bronchial connection to the tracheal tree. Extralobar sequestrations are the most common form seen prena- tally. They frequently receive their blood supply from a systemic artery. Sequestrations are most common in the lower lobes. On MRI, there is a wedge-shaped area of very high, homogeneous signal intensity.' US better demonstrates anomalous systemic vessels because of the ability to do real time imaging with color flow. These lesions may occur in the upper abdomen and be confused with adrenal tumors.

MRI is most useful for evaluating atypical chest masses. Foregut cysts can be demonstrated with connec- tion to the vertebral bodies. These are usually variable- sized, simple cysts.5 Laryngeal or tracheal atresia can cause enlargement of both lungs, which may present as bilateral echogenic lungs on US. This may be confused with bilateral cystic adenomatoid malformations. On MRI, the dilated trachea and bronchi can be visualized filled with fluid, as well as massive homogeneous enlargement of the lungs with high signal intensity.'

Congenital diaphragmatic hernia (CDH) represents a failure of formation of the diaphragmatic leaflets. It most commonly occurs in the posterior aspect of the left hemidiaphragm. Survival rates in fetuses with CDH range from 4 1%-90% with no significant improvement over the last 20 years.6 Herniation of liver into the chest has been shown to be associated with a worse prognosis. On US, CDH may be diagnosed by demonstrating a shift of the heart away from the midline and an area of increased

From the University of Pennsylvania School of Medicine, The Children's Hospital of Philadelphia, Philadelphia, USA.

Address correspondence and reprint requests to Dr. Anne M. Hubbard, The University of Pennsylvania School of Medicine, Chief, Body MRI, The Children's Hospital of Philadelphia, Department of Radiology, 34th and Civic Center Boulevard, Philadelphia, Pennsylvania I9 104. E-mail: hubbard@email.chop.edu

echogenicity in the base of the lung.' Evaluating the position of the liver on US may be difficult, as the echo texture of the liver and lung may be similar. US depends on the depiction of the position of the portal veins and hepatic veins above or below the diaphragm to predict herniation of the liver into the chest.* This gives indirect information about the liver position. With MRI, there is direct visualization of the position of the liver.' On T1 weighted images, the liver is high in signal intensity and is very conspicuous. Consequently, the position of the liver above or below the diaphragm is easily determined. MRI has been found to be more sensitive than US in detection of liver herniation into the chest." Also on T1 weighted images, the meconium-filled bowel is very high in signal intensity, making the position of the bowel above or below the diaphragm easy to evaluate. On T2 weighted images, the liver is lower in signal intensity than muscle. Both the right and left lungs can be visualized with MRI, and lung volumes can be obtained. Currently, intrauterine therapy is being performed at some institutions with tracheal occlusion to promote lung growth." Criteria for being included in this therapy are herniation of liver into the chest and lung volume that falls in a poor prognostic category. Consequently, accurate knowledge of the position of the liver, and the amount of lung tissue is necessary.

Prenatal MRI enhances the evaluation of fetal chestmasses for prenatal intervention, delivery planning, and surgery. With increasing use of MRI in evaluation of the fetal lungs, there is the possibility to learn more about the physiologic changes in the normal and abnormal fetal lungs.

Lower Respiratory Tract Malformations 11 9

REFERENCES 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

Hubbard AM, Adzick NS, Crombleholme TM, Coleman BG, Howell LJ, Haselgrove JC, Mahboubi S. Congenital chest lesions: diagnosis and characterization with prenatal MR imaging. Radiology 1999;212:43-48. Ohlsson A, Fong K, Rose T, Hannah M, Black D, Heyman Z, Gonen R. Prenatal ultrasonic prediction of autopsy-proven pulmonary hypoplasia. Am J Perinatol 1992;9:33&337. Duncan K, Gowland P, Moore R, Baker P, Johnson I. Assessment of fetal lung growth in utero with echo planar MR imaging. Radiology 1999;210: 197-200. Duncan K, Gowland P, Freeman A, Moore R, Baker P, Johnson I. The changes in magnetic resonance properties of the fetal lungs: a first result and a potential tool for the non-invasive in utero demonstration of fetal lung maturation. Br J Obstet Gynecol 1999;

Gulrajani M, David K, Sy W, Braithwaite A. Prenatal diagnosis of a neuroenteric cyst by magnetic resonance imaging. Am J Perinat 1993;18:3O4-306. Harrison M, Adzick N, Estes J, Howell L. A prospective study of the outcome of fetuses with diaphragmatic hernia. JAMA 1994;

Chinn D, Filly R, Callen P, Nakayama D, Harrison M. Congenital diaphragmatic hernia diagnosed prenatally by ultrasound. Radi-

Bootstaylor B, Filly R, Harrison M, Adzick N. Prenatal sonographic predictors of liver herniation in congenital diaphrag- matic hernia. J Ultrasound Med 1995;14:515-520. Hubbard A, Adzick N, Crombleholme T, Haselgrove J. Left sided congenital diaphragmatic hernia: value of prenatal MR imaging in preparation for fetal surgery. Radiologv 1997;203:636-640. Hubbard AM, Crombleholme TM, Adzick NS, Coleman BG, Howell LJ, Meyer JS, Flake AW. Prenatal MRI of congenital diaphragmatic hernia. Am J Perinatol 1999;16:407-413. Hedrick MH, Estes JM, Sullivan KM, Bealer JF, Kitterman JA, Flake AW, Adzick NS, Hamson MR. Plug the lung until it grows (PLUG): a new method to treat congenital diaphragmatic hernia in utero. J Pediatr Surg 1994;29:612-617.

106:122-125.

271 ~382-384.

ology 1983;148:119-123.