Thoracic Magnetic Resonance Imaging Applications in Children

Lung pathologies in the pediatric population can usually be detected using chest radiography. Multidetector computed tomography (MDCT) imaging is often used as a supplementary method in the evaluation of lung diseases. Recently, magnetic resonance imaging (MRI) techniques were found to be reliable in the evaluation of pulmonary diseases in the pediatric population. This review study describes the routine application of MRI examinations and the use of thoracic MRI with a particular focus in pediatric patients.


Introduction
Lung pathologies in the pediatric population can usually be detected with the use of chest radiography. Multidetector computed tomography (MDCT) imaging is often used as a supplementary method in the evaluation of lung diseases. Furthermore, MDCT imaging is the preferred initial imaging modality for a detailed evaluation of lung diseases in children [1][2][3].
Traditionally, assessment of the thorax using magnetic resonance imaging (MRI) was limited due to low proton density, which yields weak signals [4][5][6]. Recently, nevertheless, MRI has been recognized as having utility in the evaluation of lung diseases in the pediatric population. Owing to the inherent soft tissue contrast, thoracic MRI can be performed without contrast media administration [7][8][9]. This development has also been supported by the option of MRI, under free-breathing conditions, which is very important in the pediatric population [7,10]. Therefore, the morphological and functional evaluation of lung diseases in children by MRI may provide an alternative to MDCT imaging. This study aims to provide a brief overview of the uses and effectiveness of thoracic MRI in the pediatric population.
General Principles, Artifacts, and Sequence Protocols Thoracic MRI has long been challenging due to the magnetically heterogeneous environment of the thorax. The lung parenchyma is a low proton-density structure and has a low signal-to-noise ratio. Furthermore, cardiac and respiratory motion artifacts can lead to difficulty in obtaining any reproducible thoracic MRI data, which is a critical issue [11][12][13][14][15]. Thus, T2-weighted sequences are essential in thoracic MRI. Recent studies usually refer to triggered T2-weighted turbo spin echo (TSE) sequences with a long echo train and T2-weighted half-Fourier acquisition single-shot turbo spin-echo sequences, which are usually emphasized ( Figure 1). Axial T1-weighted 3D-GRE (VIBE) sequence with high sensitivity for small lesions is acquired in a single-breath hold, whereas axial T2-weighted short tau inversion recovery with a multibreath hold method is used to detect enlarged lymph nodes and skeletal lesions [7,16].

Indications for Thoracic MRI in Children
Lung Diseases with Alveolar Involvement Pneumonia is a typical example of lung parenchymal disease with alveolar involvement. Its pathologies can be dependably detected using T2-weighted sequences. Using MRI, all processes that appear as opacification on X-ray images, such as alveolar infiltration and exudation patterns, can usually be evaluated more definitively and without summation effects. Therefore, thoracic MRI exhibits a remarkable diagnostic advantage over radiographs. MRI is also suitable for detecting complications of lung parenchymal infiltrations [14][15][16][17][18].

Lung Diseases with Interstitial Involvement
Interstitial parenchymal pathologies can often be detected using MDCT; however, MRI results of these pathologies sometimes differ remarkably from those of high-resolution MDCT. Acute interstitial lung diseases that involve infections and edema with interstitial infiltration can be recognized as a signal increase on T2-weighted MRI ( Figure 2). When these cellular and capillary structures decrease, the MRI of pulmonary fibrosis will reveal low signal intensity on T2weighted images [19,20].

Lung Neoplasia
Primary thoracic neoplasias are rare in the pediatric population, so there have been no clinical studies on the use of MRI in this context. In addition, most of the lung tumors are metastatic (Figures 3, 4). Benign lesions, such as thymoma or hemangioma, are sometimes detected (Figures 5, 6). MDCT is generally preferred as the gold standard in parenchymal metastasis detection. Thus, thoracic MRI has so far been used in the pediatric population solely to monitor the regression of known lung metastatic tumors. MRI can detect parenchymal nodules larger than 5 mm [3,9,14].

Lung Malformations
Pulmonary sequestrations are rare congenital lung malformations. Pulmonary sequestration with aberrant feeding vessel can be accurately diagnosed by magnetic resonance angiography (MRA). The main differential diagnosis is cystic adenomatoid malformation, which does not have an atypical vessel. MDCT has no diagnos-   tic advantage over MRI in this context. Thoracic MRI is recommended for the diagnosis of type III cystic adenomatoid malformations that involve small and solid cysts [3,14].

Cystic Fibrosis
MRI can be performed to detect the typical findings of cystic fibrosis in children, such as bronchial wall thickening, bronchiectasis, mucus plugging, and consolidations. T2-weighted sequences with respiratory gating are recommended to identify mucus-filled airways and enflamed bronchial walls [20][21][22].

Pulmonary Hypoplasia and Aplasia
Pulmonary hypoplasia or aplasia is a congenital disorder that involves incomplete development of the lung parenchyma. In pulmonary hypoplasia, the pulmonary artery is congenitally small and can be detected by MRA. Respiratory and cardiac-gated bright-blood imaging methods are also suitable for identifying small bronchial structures [3,23,24].

Pectus Excavatum
Assessment of pectus excavatum, a congenital concavity of the sternum and cartilages, ideally b includes a careful anatomical description using imaging modalities to detect the depth of depression and extent of cardiac compression ( Figure  7). The pectus severity index, described in 1973, is obtained by dividing the internal transverse distance of the thorax by the vertebral-sternal distance at the point of most significant depression, measured by computed tomography imaging. Nevertheless, thoracic MRI is also a good option for evaluating this deformity [25,26].

Conclusion
In children, MRI of the thorax provides more useful data for diagnosis than radiography. In many cases, thoracic MRI can replace radiography and MDCT, thereby reducing the children' s exposure to ionizing radiation. T2-weighted TSE sequences and respiratory gating procedures, in particular, make MRI an essential imaging modality with broad applicability in pediatric thoracic imaging. Therefore, MRI should be considered as an alternative to MDCT in the evaluation of lung diseases in children.
Ethics Committee Approval: Ethics committee approval was received for this study from the ethics committee of the Ethics Committee of Erzurum Regional Training and Research Hospital.

Conflict of Interest:
The author has no conflicts of interest to declare.
Financial Disclosure: The author declared that this study has received no financial support.