Normal Fetal Chest on MRI

T2-weighted images are the most useful for evaluating the lung anatomy. The trachea, bronchi, and lungs reveal homogeneous hyperintensity on T2-weighted images (Fig. 1) in comparison to chest wall muscles, because they contain a significant amount of amniotic and alveolar fluid. An increase in the signal intensity of the lungs can be observed as the lungs mature. In cases of lung compression, the amount of alveolar fluid is decreased, resulting in a more hypointense signal. Other structure that can be evaluated with T2-weighted MR imaging of the fetal chest is the thymus, which usually has intermediate signal intensity (Fig. 1).

T1-weighted images are useful in the identification the position of the liver and bowel loops containing meconium, which are visualized as hyperintense structures in patients with CDH.

Congenital Pulmonary Airway Malformation

CPAM is the most common congenital lung malformation and accounts for 30-40% of all congenital chest diseases (1, 2). Exact etiology of CPAM is not known, it is considered as hamartomatous malformation characterized by an abnormal branching of the immature bronchioles, with a lack of normal alveolar development (3).

CPAM is a histopathologically heterogenous group reclassified (types 0-IV) in 2002 by Stocker et al. (4). The term CPAM has been renamed as being preferable to the term congenital cystic adenomatoid malformation, since the lesions are cystic in only three of five types of these lesions and adenomatoid in only one type (Type III) (2).

CPAM Type 0-Acinar dysplasia/agenesis is rare malformation largely incompatible with life. Lungs are small, firm with diffusely granular surface.

CPAM Type I-It accounts for nearly 65% of cases. It is operable with good prognosis. It often appears as multilocular hyperintense lesions on T2-weighted images, with some cysts 2-10 cm in diameter; one cyst can be dominant in size and may have smaller peripheral cysts associated with it (Fig. 2, 3)

CPAM Type II-it accounts for 10%-15% of cases. It has poor prognosis because it is frequently associated with other congenital anomalies (bilateral renal agenesis, extralobar pulmonary sequestration, cardiovascular malformation). It usually appears as multilocular hyperintense lesions on T2-weighted images, with small uniform cysts 0.5-2 cm in diameter (Fig. 4, 5).

CPAM Type III- It is infrequent and accounts only about 5% of cases. It is small cystic or solid type, exclusively seen in first few days to months of life with characteristic male predominance. It is commonly associated with maternal polyhydramnios, fetal anasarca. So it has high mortality rate. Grossly cysts are small measuring less than 0.2 cm in diameter, manifest as homogeneously hyperintense solid mass involving an entire lobe or even an entire lung.

CPAM Type IV-It is hamartomatous malformation of the distal acinus and accounts for 10%-15% of cases with an age range of newborn to 4 years. This lesion involves a single lobe. Grossly large, thin walled solitary cyst. It can not be differentiated by imaging from type I.

The findings of MRI depend on the size of the cysts. Typically, the signal intensity of the cysts is higher than that of the surrounding normal lung parenchyma (5). Macrocystic CPAM (> 5 mm) appears as a lobulated mass with inhomogeneous hyperintensity (1, 5), and microcystic CPAM (< 5 mm) as a lobulated mass of homogeneous hyperintensity with arterial vascular architectural distortion without visible cysts.

The differential diagnosis for CPAM includes congenital bronchogenic cyst (single macrocystic CPAM), BPS, and CLFO (microcystic CPAM). Although bronchogenic cyst are typically isolated within the carinal region, in about 15% of cases the cyst is intraparenchymal (1). In these unusual cases, a specific diagnosis cannot be made prenatally. Differentiation between CPAM type III and BPS is challenging and is based on vascularization (1). Unlike BPS, CPAM communicates with the tracheobronchial tree and has an arterial blood supply and venous drainage from the pulmonary circulation. CLFO can be distinguished from CPAM by the presence of bronchovascular markings extending to the periphery of the involved lobe and by atelectasis of adjacent tissue (6).

More than 50% of CPAMs resolve spontaneously during pregnancy (7). Typically, CPAMs with a volume of less than 57% of total lung volume resolve completely, whereas CPAMs with a volume of more than 84% of total lung volume do not resolve completely (5). CPAMs require postnatal surgery after 1 month of life because of the risk for infection and a low risk for becoming malignant.

The association between CPAM and malignancy has been well documented. Type Ⅰ CPAM may involve malignant transformation of mucinous bronchioloalveolar carcinoma (8). Type Ⅱ CPAM may involve malignant transformation- Rhabdomyosarcoma. Type Ⅲ CPAM requires examination of the entire lesion to exclude pulmonary blastoma by confirming whether or not sarcomatous differentiation is present in the solid parts (2).

Bronchopulmonary Sequestration

BPS is the second most common lesion in prenatal diagnosis. It consists of nonfunctioning pulmonary tissue, which fails to connect to the normal tracheobronchial tree and receives systemic blood supply, via the thoracic or abdominal aorta (5).

The most frequent location is in the left lower lobe (more than two-thirds) (9); 90% are supradiaphragmatic (Fig. 6), and less than 10% are infradiaphragmatic (10). Extralobar BPS is the type that occurs in the fetus. The sequestered tissue has its own pleural lining and systemic venous drainage into the azygos vein or inferior vena cava (in 25% of cases to pulmonary veins) (11). Extralobar BPS may be associated with other malformations, such as CDH, pulmonary hypoplasia, intestinal duplication cysts, and cardiac anomalies (12). Intralobar BPS lacks its own pleura but is enveloped by the visceral lung pleura and drains into the pulmonary veins (13). Intralobar BPS is rarely diagnosed prenatally and is usually associated with type II CPAM (hybrid lesions) (14).

MRI findings of pure BPS include a well-defined triangular homogeneous hyperintense mass with a higher signal intensity than that of a normal lung, but with lower signal intensity than that of the amniotic fluid, as well as visualization of the systemic feeding artery (Fig. 6). The differential diagnosis includes microcystic CPAM and CLFO, and systemic arterial supply is the key finding for BPS.

Hybrid lesions exhibit radiologic and pathologic findings of BPS and CPAM (generally type II CPAM) (Fig. 7).

Congenital Lobar Fluid Overload

CLFO occurs because of alveolar fluid trapped in the affected lung segment or lobe due to transient mucus plug impaction or extrinsic compression by a mass lesion or cartilage abnormalities causes bronchial narrowing; it usually involves the left upper and right middle lobes (5). It is synonymous with congenital lobar emphysema seen postnatally.

Prenatal MRI demonstrates homogeneous increased T2 signal mass without pulmonary architectural distortion (Fig. 8). On MRI, CLFO can be differentiated from microcystic CPAM, BPS, and bronchial atresia due to its homogeneity and intact lung segment or lobe with stretched hilar vessels without pulmonary architectural distortion (5).

Congenital Diaphragmatic Hernia

CDH is the main indication for fetal thoracic MR imaging. It is caused by a defect in the diaphragmatic musculature, with herniation of the abdominal viscera into the thorax leading to deficiencies in lung development (1). 85% are left-sided (Bochdalek hernia) (Fig. 9), 13% right-sided, and 2% bilateral (15). The structures that most commonly herniate in left diaphragmatic hernias are omental fat, small intestine, left liver lobe, and stomach. Kidneys and pancreas rarely herniate. In right diaphragmatic hernias, the liver (right liver lobe) is the organ that herniates most frequently.

The degree of pulmonary hypoplasia and liver herniation are major prognostic factors. Liver position is an independent risk factor for fetal lung volume estimates, as liver herniation is associated with a poorer prognosis in fetal CDH. The supradiaphragmatic position of the liver is difficult to visualize with US. MR imaging can identify the diaphragmatic defect (discontinuity of the hypointense band on T2-weighted images) and the anomalous position of the liver, indicated by a hyperintense signal on T1-weighted images and hypointense signal on T2-weighted images, especially in the sagittal and coronal planes (16).

Thymic Cyst

Thymic cysts represent 3% of anterior mediastinal lesions and can be congenital or acquired. The first group are similar to the other congenital mediastinal cysts, with regular contour and homogeneous, T1 hypointense, T2 hyperintense contents. They are often unilocular (Fig. 10). Acquired lesions are frequently associated with inflammatory/infectious processes (HIV, lupus and Sjögren's disease, among others) and neoplastic processes (such as non-Hodgkin lymphoma and, less frequently, thymoma, thymic carcinoma and mediastinal seminoma) presenting like heterogeneous, multilocular formations with septations (17).