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Obstetric Imaging |
1 From the Departments of Radiology (A.M.H., J.C.H., S.M.) and Surgery (N.S.A., T.M.C., L.J.H.), Children's Hospital of Philadelphia, 34th and Civic Center Blvd, Philadelphia, PA 19104, and the Department of Radiology, University of Pennsylvania Medical Center, Philadelphia (B.G.C.). Received March 25, 1998; revision requested June 18; revision received September 8; accepted December 21. Address reprint requests to A.M.H. (e-mail: Hubbard@email.chop.edu).
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Abstract |
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 TOP Abstract IntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: Eighteen pregnant women were referred for MR imaging of possible fetal chest tumors seen at US (16 congenital cystic adenomatoid malformation [CCAM], two bronchopulmonary sequestration [BPS]). The presence, position, size, and characteristics of masses were determined and correlated with postnatal results.
RESULTS: The MR imaging diagnoses were three cases of congenital diaphragmatic hernia, nine of CCAM, two of BPS, and one each of foregut cyst, lung atresia, tracheal atresia, and bronchial stenosis. MR imaging results were in agreement with US results in nine fetuses and in disagreement in nine. MR imaging diagnoses were confirmed at surgery or autopsy in 17 fetuses. MR imaging results led to an error in diagnosis in one fetus with BPS.
CONCLUSION: Fetal chest masses had characteristic MR imaging appearances. MR imaging was accurate for distinguishing congenital diaphragmatic hernia from CCAM and was useful for less common diagnoses and determination of the origin of very large chest tumors. Prenatal diagnosis was changed in some patients owing to MR results and affected treatment and counseling of parents. MR imaging is a valuable adjunct to US for prenatal diagnosis of fetal chest masses.
Index terms: Fetus, MR, 856.121411, 856.121416 • Fetus, neoplasms, 856.872, 856.875 • Fetus, respiratory system, 856.8754, 875.8755, 875.8756, 875.8759 • Fetus, US, 856.12981 • Magnetic resonance (MR), comparative studies
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Magnetic resonance (MR) imaging has been used to evaluate uterine and fetal anatomy. Until the advent of fast MR imaging, however, the images were severely degraded owing to fetal motion. Maternal sedation or in utero paralysis of the fetus by means of intramuscular injection of pancuronium into the fetus was frequently needed (4). With the advent of fast MR imaging, prenatal imaging has entered a new age. Prenatal MR imaging has been successfully performed by using echo-planar imaging, as well as rapid acquisition with relaxation enhancement (RARE) imaging (5–8).
The purpose of this study was to compare the diagnostic utility of prenatal MR imaging and US for evaluation of chest tumors and to determine if MR imaging could provide additional valuable information.
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MATERIALS AND METHODS |
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MR imaging was performed with a 1.5-T magnet (Vision; Siemens Medical Systems, Iselin, NJ) equipped with a phased-array body coil. The nonsedated mother was positioned either supine or in a partial left lateral decubitus position. The following imaging sequences were performed: RARE (HASTE; Siemens Medical Systems) imaging (4.4/64 [repetition time msec/effective echo time msec]; flip angle, 120°; section thickness, 6 mm) in the sagittal, coronal, and axial planes relative to the fetal chest; two-dimensional fast low-angle shot imaging (174/4.1 [repetition time msec/echo time msec]; flip angle, 80°; section thickness, 6 mm) in the sagittal and coronal planes relative to the fetus; turbo fast low-angle shot imaging (11/5.3 [effective]; flip angle, 20°; section thickness, 5 mm) in the axial plane relative to the fetus; and echo-planar free induction decay imaging (0.8/56; flip angle, 90°; section thickness, 5 mm) in the axial plane. For each sequence, 4–20 seconds were needed to acquire 20 anatomic images.
All MR studies were interpreted by one radiologist (A.M.H.), who also knew the results of US. MR imaging results were compared with the US report; US images were not available for review. The presence, position, size, and signal intensity characteristics of the masses were determined and correlated with findings from postnatal studies, including postmortem, surgical, or pathologic results.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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At follow-up, the MR imaging diagnoses were confirmed with either surgical or histopathologic results in 17 fetuses. In one case, a large CCAM lesion was predicted on the basis of both US and MR imaging results, but at pathologic examination the lesion was diagnosed as BPS. One lesion had been diagnosed as BPS at MR imaging and as mixed BPS and CCAM at pathologic examination. MR imaging substantially augmented US in terms of localization of masses relative to the lobar anatomy of the lung and other chest structures, particularly in very large lesions. With MR imaging, we were able to assess the volume of normal ipsilateral and contralateral lung.
The imaging characteristics of the chest lesions were variable. Normal lung tissue was homogeneous and of moderately high signal intensity, with markedly higher signal intensity than that of chest wall muscle (Fig 1). The signal intensity of compressed but otherwise normal lung was intermediate and lower than that of noncompressed normal lung (Fig 2).
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The foregut cyst manifested as a single large homogeneous cyst of high signal intensity, with a small tail of cystic tissue that extended into the mediastinum, posterior to the carina and anterior to the spine (Fig 6). Vertebral body anomalies were not demonstrated, but severe kyphosis of the upper thorax was present. One fetus was imaged twice at 3-week intervals after in utero removal of a CCAM lesion. MR imaging demonstrated lung growth and allowed differentiation of effusion and hemorrhage from normal lung tissue (Fig 3c).
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The prenatal US studies had been performed at several institutions. This made comparison of the US and MR images somewhat difficult. The quality of the US images generally was good. Most incorrect diagnoses were due to a misinterpretation of the US findings. The lesions that were misdiagnosed on the basis of US results were three congenital diaphragmatic hernias (two on the right side, one on the left side), which were diagnosed as CCAM; one tracheal atresia, diagnosed as bilateral CCAM; one pulmonary agenesis, diagnosed as CCAM; one neurenteric cyst, diagnosed as CCAM; one bronchial stenosis, diagnosed as CCAM; one very large CCAM, diagnosed as bilateral CCAM; and one BPS, diagnosed as CCAM.
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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At US, the normal fetal lung has a homogeneous and moderately hyperechoic appearance (1). The degree of echogenicity relative to that of the liver increases throughout gestation. The normal fetal lung on T2-weighted MR images has moderately high signal intensity (9) (Fig 1), with higher signal intensity than chest wall muscle but lower signal intensity than amniotic fluid. The compressed normal lung was not as high in signal intensity as was the normal noncompressed lung. This finding is probably secondary to decreased production of alveolar fluid in the compressed lung.
To date, to our knowledge US has not been useful for accurate predictions about fetal lung maturity (1). Preliminary MR imaging studies have been performed to evaluate changes in T1 and T2 signal intensity with increasing gestational age, but this approach has not yet proved to be a reliable method for assessment of fetal lung maturity (9). MR imaging can be used for accurate measurement of lung volumes and may prove to be helpful for prediction of whether sufficient lung tissue is present to sustain life after birth (10). The conspicuity of the signal intensity in fetal lung is high enough that, even in very large chest masses, small amounts of compressed normal lung can be visualized.
Accurate prenatal diagnosis of chest masses is important because the natural history of these lesions and their treatment vary substantially (11–13). We have previously shown (7) that MR imaging results can be used to easily diagnose congenital diaphragmatic hernia and differentiate it from primary chest tumors.
Because MR imaging relies on differences in tissue characteristics, a combination of fast imaging sequences with T1 and T2 weighting can be used to easily differentiate bowel and liver from lung. On two-dimensional fast low-angle shot MR images, which are T1-weighted images, the liver has very high signal intensity, and its position above or below the diaphragm can easily be determined (Fig 7). The MR signal intensity characteristics of the liver are very different from those of the lung. The US differentiation of liver from lung can be difficult. Meconium-filled distal bowel also has very high signal intensity on T1-weighted images. This may be helpful because, on US images, bowel loops that do not exhibit peristalsis may appear the same as a hyperechoic lung tumor.
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The natural history and prognosis of CCAM are variable (15). The prognosis associated with this lesion is dependent on the size, rather than the histologic type, of the lesion. Larger lesions are associated with a higher frequency of mediastinal shift, pulmonary hypoplasia, vascular compromise, and hydrops. Large lesions may also manifest with polyhydramnios, due probably to compression of the fetal esophagus by the mass and consequent interference with fetal swallowing of amniotic fluid. However, some CCAM lesions have been shown to involute in utero (15,16).
Hydrops is a harbinger of impending fetal death. Two of the fetuses with CCAM had hydrops at the time of imaging (Fig 3). One fetus had a large CCAM lesion associated with hydrops, abdominal ascites, and polyhydramnios. The ascites and hydrops resolved without intervention; however, the fetus died at birth because of severe pulmonary hypoplasia. The second fetus had hydrops and a large CCAM lesion at the time of the mother's presentation; in utero removal of the CCAM was performed at 29 weeks gestation. The hydrops resolved in utero, and MR imaging demonstrated growth of both lungs. The fetus was born with mild respiratory distress syndrome.
It has been shown (2) that prenatal resection of a CCAM lesion may result in the survival of 60% of fetuses who develop hydrops. MR imaging findings were important for the planning of in utero removal of the tumor. MR imaging provided a large field of view and the ability to depict all of the fetal chest anatomy, including the size of the tumor, the remaining normal lung tissue, and the relationship to other chest structures, in a way that the pediatric surgeon could easily visualize.
BPS is a mass of nonfunctional lung tissue that does not communicate with the normal bronchial tree and receives its vascular supply from systemic arteries (1,17). BPS lesions may be extralobar or intralobar. The extralobar form is most commonly diagnosed in the prenatal and neonatal periods, whereas the intralobar form is more usually diagnosed in childhood. These lesions may involute in utero.
The prenatal US appearance of BPS is of a homogeneously hyperechoic mass in the lower lobe of a lung. MR imaging in the one fetus with BPS in our series demonstrated the lesion to be well defined with straight margins and very high signal intensity (Fig 4). In the one fetus with mixed BPS and CCAM, the lesion could no longer be demonstrated at US and was believed to have involuted. However, a mass of moderate size and very high signal intensity could still be demonstrated at MR imaging. US had the advantage of being able to demonstrate anomalous vessels in one of these cases and helped confirm the prenatal diagnosis of BPS. Although there are MR sequences capable of demonstrating blood flow, we could not demonstrate the anomalous vessels in two fetuses.
Although tracheal atresia is rare, this diagnosis should be considered when bilateral hyperechoic lung lesions are encountered at prenatal US. Although bilateral CCAM has been described (17), it is extremely uncommon, and it is more likely that uniformly echogenic lungs represent some form of occlusion of the trachea or larynx. When the trachea becomes obstructed, fetal lung fluid created by the alveoli becomes trapped, which causes hyperplasia of lung tissue and tracheal dilatation. Prenatal MR imaging demonstrated the lungs to have homogeneous high signal intensity, with obvious dilatation of the fluid-filled tracheobronchial tree. Such fetuses are at serious risk for development of hydrops due to cardiac compression and obstructed venous return. It is hoped that, in the future, the level of airway occlusion may be accurately identified at both US and MR imaging, so that in utero treatment or preparation for delivery with placental support (the ex utero intrapartum treatment, or EXIT, procedure) can be performed (18). The fetus in our series who had tracheal atresia was delivered with placental support and underwent tracheostomy in the delivery room.
Prenatal MR imaging was helpful for definition of the anatomy of very large chest masses and lesions that had an atypical US appearance. MR imaging clearly demonstrated a large foregut cyst and allowed differentiation of the cyst from a CCAM lesion. In another fetus, what appeared at US to be bilateral masses was seen at MR imaging to be a solitary CCAM that crossed the midline anterior to the heart (Fig 6).
Even at institutions where fetal surgery is not performed, MR imaging may be helpful for further definition of a chest mass and confirmation of the prenatal diagnosis. The accuracy of the prenatal diagnosis and the knowledge of the prenatal natural history of different chest masses can affect parental counseling. In many of the patients in our series, the wrong diagnosis had been established on the basis of the US findings, which would affect the counseling that a parent would receive. Several parents referred to our institution had been counseled that their fetus had a lethal chest lesion, even though the lesion was not causing substantial mass effect. These parents continued their pregnancies and delivered babies who required surgery, from which the babies quickly recovered.
Although a poor outcome for most fetal chest masses was predicted in the past, the results in more recent reports (11,15) have shown that most such lesions are not lethal. Counseling must include not only the diagnosis, natural history, and possible prenatal interventions, such as cyst aspiration or fetal surgery, but also planned delivery at an institution that is prepared to resuscitate and immediately perform surgery in a neonate who may be in severe respiratory distress. The surgeons at our institution found that MR images helped them mentally visualize a large fetal mass prior to delivery. MR imaging provides an image of the fetus that is easier to understand than the US image for physicians and patients not familiar with the performance and interpretation of the results of US.
In this study, we characterized the MR imaging appearance of common and uncommon congenital masses. With the use of US, more fetal chest lesions are being recognized, but they are not always accurately diagnosed. In this series, MR imaging helped further characterize the fetal chest lesions and confirm or change the prenatal diagnosis. MR imaging allowed clear differentiation of congenital diaphragmatic hernia from primary chest tumors. MR imaging was useful for defining tissue and anatomic effects of large and atypical chest masses. This information had an important effect on prenatal counseling and therapeutic planning.
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Footnotes |
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Author contributions: Guarantor of integrity of entire study, A.M.H.; study concepts and design, A.M.H.; definition of intellectual content, A.M.H.; literature research, all authors; clinical studies, A.M.H., S.M.; data acquisition, A.M.H., L.J.H.; data analysis, A.M.H., S.M.; manuscript preparation and editing, A.M.H.; manuscript review, all authors.
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References |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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