Advances in technology coupled with optimized protocols now permit evaluation of the lungs with magnetic resonance (MR) imaging in the pediatric population. Although computed tomography remains the preferred imaging modality for this purpose, MR imaging provides a radiation-free alternative that can answer many important clinical questions and provide additional data. In addition, the use of newer techniques and equipment such as MR-imaging-compatible spirometers allows for functional assessment of the pediatric airways. This article reviews the up-to-date MR imaging techniques as well as imaging findings of selected clinically important disorders that affect the lungs and airways in the pediatric population.
Key points
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Magnetic resonance (MR) imaging can reliably identify lung nodules larger than 5 mm in children.
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MR imaging can permit accurate and dynamic evaluation of large airways.
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MR imaging is a valuable imaging modality to assess the progression of chronic lung diseases such as cystic fibrosis.
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Future chest MR imaging techniques have a great promise for functional imaging of the lungs in pediatric patients.
Introduction
In recent years, MR imaging with advanced imaging techniques has been receiving a lot of attention mainly because of its ability to assess lungs and airways in the pediatric population. Although computed tomography (CT), which is regarded as the gold standard imaging modality, provides exquisite resolution of the anatomic structures of the lungs and airways, it exposes the pediatric patient to ionizing radiation. MR imaging has been advocated as an adjunctive tool, particularly in pediatric patients, for the evaluation of chest pathology. In the past, the proton-poor environment, rapid signal dephasing, and respiratory motion presented significant obstacles for widespread adoption and clinical use of MR imaging lung studies. Nevertheless, by optimizing protocols and tailoring them to the individual pediatric patient and with the clinical question at hand, MR imaging can now provide excellent visualization of the relevant anatomy and pertinent abnormalities. The future of chest MR imaging includes a greater emphasis on functional information. The use of hyperpolarized gases, where available, provides excellent imaging of lung ventilation. Upcoming technologies, such as Fourier decomposition, promise the ability to provide functional perfusion and ventilation data without the use of intravenous or inhaled contrast agents. The overarching goal of this article is to provide up-to-date information regarding MR imaging techniques for practical assessment of lungs and airways in the pediatric population. Furthermore, several pediatric thoracic disorders involving the lungs and airways that can be evaluated with advanced MR imaging techniques are highlighted.
Introduction
In recent years, MR imaging with advanced imaging techniques has been receiving a lot of attention mainly because of its ability to assess lungs and airways in the pediatric population. Although computed tomography (CT), which is regarded as the gold standard imaging modality, provides exquisite resolution of the anatomic structures of the lungs and airways, it exposes the pediatric patient to ionizing radiation. MR imaging has been advocated as an adjunctive tool, particularly in pediatric patients, for the evaluation of chest pathology. In the past, the proton-poor environment, rapid signal dephasing, and respiratory motion presented significant obstacles for widespread adoption and clinical use of MR imaging lung studies. Nevertheless, by optimizing protocols and tailoring them to the individual pediatric patient and with the clinical question at hand, MR imaging can now provide excellent visualization of the relevant anatomy and pertinent abnormalities. The future of chest MR imaging includes a greater emphasis on functional information. The use of hyperpolarized gases, where available, provides excellent imaging of lung ventilation. Upcoming technologies, such as Fourier decomposition, promise the ability to provide functional perfusion and ventilation data without the use of intravenous or inhaled contrast agents. The overarching goal of this article is to provide up-to-date information regarding MR imaging techniques for practical assessment of lungs and airways in the pediatric population. Furthermore, several pediatric thoracic disorders involving the lungs and airways that can be evaluated with advanced MR imaging techniques are highlighted.
Evaluation of lung parenchymal abnormalities
MR Imaging Protocol
A fundamental MR imaging protocol evaluating the lung parenchyma includes a gradient recalled echo (GRE) multiplanar localizer, coronal T2 single-shot half Fourier turbo spin echo (HASTE), axial 3-dimensional (3D) GRE T1, coronal balanced steady-state free precession (true fast imaging with steady-state precession), and axial short tau inversion recovery. One can complete this practical MR imaging examination in less than 25 minutes. If necessary, postcontrast imaging with a 3D GRE T1-weighted sequence with fat saturation can provide information regarding enhancement characteristics. Pediatric patients with difficulty after breathing instructions because of their young age or critical condition often benefit from a sequence that does not rely on breath holds such as an axial T2 periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER/BLADE) performed with the patient free breathing.
Consolidation and Infection
Although CT remains the gold standard for evaluation of parenchymal lung abnormalities, the ability to characterize lung abnormality without exposing the child to ionizing radiation has propelled research into the use of alternative technology. Several studies have shown that ultrasonography can diagnose peripherally located lung consolidation as well as or better than radiography. However, chest ultrasonography becomes more difficult with increasing age because the acoustic windows become more limited with increasing ossification of the skeletal structures. Furthermore, deep parenchymal abnormalities surrounded by aerated lung go undetected by ultrasonography because of dissipation of the ultrasound beam by the air interface. For these reasons, the use of MR imaging to aid in the diagnosis of lung abnormalities has been evaluated by multiple investigators. Although CT provides greater spatial resolution than MR imaging, the use of multiple sequences offers characterization of tissue beyond the limits of CT.
Studies have shown that MR imaging can detect pneumonia and other consolidative processes in the lungs ( Figs. 1 and 2 ). A prospective study comparing 1.5T MR imaging using fast T1 and T2 imaging sequences with radiography for the detection of pneumonia proved that the 2 modalities were comparable. A comparison of different MR pulse sequences showed that HASTE was the best sequence for the detection of lung consolidation. In addition to the consolidation, MR imaging can also detect complications of pneumonia such as necrosis/abscess and pneumonia.
Other studies have compared MR imaging with CT, the current gold standard, and shown that the former is a high-sensitivity examination for evaluation of lung abnormalities. In one recent prospective trial of 71 pediatric patients in which patients underwent both CT and MR imaging evaluation within 24 hours, diagnostic accuracy of MR imaging was 97% compared with that of CT. The only undiagnosed lung findings were a single case of mild bronchiectasis and another case with a pulmonary nodule measuring 3 mm that went undetected. In addition, this study demonstrated excellent interobserver reliability between 2 readers, suggesting the robustness of this technique.
Although MR imaging is unlikely to define the causative agent in infection, clues gleaned from the imaging may help narrow the differential diagnosis in cases in which additional imaging is performed for problem solving. Lung necrosis in tuberculosis, for example, can appear low in signal intensity on fluid-sensitive sequences likely because of the underlying caseating necrosis associated with Mycobacterium . Iron deposition within foci of Aspergillus infection has also been associated with low signal on MR imaging. Certain infections such as those caused by Echinococcus result in a characteristic appearance. The resultant hydatid cysts present as cystic masses, often with smaller daughter cysts. A T2-hypointense rim has been described in these cystic masses and should at least raise the possibility of this diagnosis ( Fig. 3 ).
Lung Masses
Congenital lung masses constitute a group of developmental disorders affecting lung parenchyma in vascular and airway development. The diagnosis of congenital lung masses including sequestration and congenital pulmonary airway malformations (CPAMs) is increasingly being made prenatally with the use of fetal ultrasonography as a screening tool. Also, fetal MR imaging can provide additional information in the evaluation of these abnormalities. In the neonatal period, thoracic MR imaging further aids in characterization of these masses and confirms prenatal diagnoses. The use of T2-weighted sequences often permits reliable differentiation between normal and abnormal lung masses. In addition, T2-weighted imaging can help in identifying cystic components within these masses. However, large air-filled cysts may go unrecognized on MR imaging because of the lack of signal.
MR imaging may also identify feeding vessels and draining veins associated with congenital lung malformation such as pulmonary sequestration. Although one can identify flow voids on multiple MR sequences, MR angiography with postprocessed 3D images best illustrates feeding vessels, as in the case of a pulmonary sequestration wherein the anomalous arterial vessels arise from the aorta. Recent research has suggested that bronchial atresia lies on the same spectrum as these other types of congenital lung masses. The atretic bronchus is a mucous-filled tubular structure, which is both T1 and T2 hyperintense. Other signs of bronchial atresia such as subtle air trapping, however, likely go unnoticed on MR imaging using standard protocols. The improved spatial resolution of CT compared with that of MR imaging and the ability to visualize air-filled structures and associated complications such as trapped air are a large part of the reason that CT remains the preferred modality for imaging diagnosis and surgical planning. Nevertheless, the use of a proton-density-weighted GRE sequence with short repetition time(TR) and echo time (TE) and slice thickness between 5 to 8 mm can often allow detection of trapped air.
Primary lung neoplasms occur far less frequently in the pediatric population than in adults and are less common than metastatic disease. A list of relatively rare diagnoses in this category includes entities such as papillomas, myofibromas, hemangiomas, and hamartomas. Mesenchymal hamartomas are masses that arise from the chest wall, but on presentation may seem as if they arise from the lung ( Fig. 4 ). These masses constitute no risk of metastatic or recurrent disease and therefore require no treatment unless symptomatic. Characteristic MR imaging features of the mesenchymal hamartomas include hemorrhagic fluid levels within secondary aneurysmal bone cysts and calcification seen as hypointense signal on MR imaging.
Pathologists often classify inflammatory myofibroblastic tumors as benign pulmonary neoplasms, although these lesions sometimes recur or act aggressively. These masses often constitute a diagnostic and clinical dilemma because of their behavior. The other name for this lesion, inflammatory pseudotumor, also indicates that not all pathologists are convinced that these masses are neoplastic. MR imaging features of inflammatory myofibroblastic tumors include low signal on T1, high signal on T2, and homogeneous enhancement ( Fig. 5 ).
Malignant primary lung neoplasms also present in a variety of forms. Pleuropulmonary blastomas are uncommon malignancies with both mesenchymal and epithelial components that can present like CPAMs. Numerous other pediatric primary lung malignancies can, although rarely, arise de novo, with a full discussion of these cancers lying beyond the scope of this article ( Figs. 6 and 7 ).
Metastatic Pulmonary Nodules
Use of MR imaging for the detection of pulmonary nodules remains an alluring goal because many pediatric patients with cancer require routine surveillance for detection of lung recurrence or metastases. In these pediatric patients, the cumulative radiation dose of chest imaging may prove significant. Furthermore, these patients may possess increased sensitivity to the damaging effects of radiation with predisposition to developing new malignancies because of prior therapy or congenital sensitivity, such as in pediatric patients with ataxia telangiectasia. Consequently, replacement of routine surveillance CT scans with MR imaging would allow continued evaluation without exposing the child to additional risks.
MR imaging can reliably detect lung nodules larger than 5 mm ( Fig. 8 ). Other studies suggest that MR imaging becomes less sensitive for detection of lung nodules smaller than 3 mm (sensitivity of 73%) ( Fig. 9 ). A known limitation in MR imaging evaluation concerns calcified lung nodules. Calcification results in susceptibility-related loss of MR imaging signal intensity. Consequently, calcified nodules, which would be easily identified on CT, become occult on MR imaging. The improved tissue contrast, however, may add additional information regarding the cause of the nodule not evident on other imaging. The use of diffusion-weighted MR imaging has been suggested to also aid in both detection and characterization of malignant nodules.
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