1. Procedure details and patient preparation (Figure 1)
For lung MRI, the patient should be positioned supine. While a body coil may be used, a local receive coil array optimized for thoracic imaging significantly improves signal-to-noise ratio (SNR). Thoracic MRI coils typically include a flexible anterior and an embedded posterior part [4].
Children over 5 years can often perform coached breathing maneuvers during the scan. For younger children, moderate sedation or general anesthesia is used with free-breathing techniques. Infants are commonly scanned using the "feed and wrap" method, where they are fed and swaddled for minimal movement [5].
2. Physical Characteristics of the Lung Relevant to MRI and Strategies for Obtaining High-Quality Images (Figure 2)
The physical properties of lung parenchyma differ significantly from those of other tissues, such as the liver or brain, posing unique challenges for MRI. Two key factors are particularly relevant: the low proton density and the pronounced susceptibility differences at the air-tissue interfaces.
Since the MR signal is directly proportional to tissue proton density, the signal intensity of lung parenchyma is inherently low—approximately ten times weaker than that of adjacent tissues, even under ideal imaging conditions that disregard relaxation effects [4].
Additionally, imaging the lung is further complicated by the continuous motion caused by cardiac pulsation and respiratory activity. These dynamic movements introduce motion artifacts, which can severely impact image quality and diagnostic accuracy [6].
To address challenges related to cardiac motion, cardiac gating can minimize artifacts. However, in thoracic MRI of non-cardiac regions, respiratory motion is a more significant concern. Ideally, breath-holding techniques yield the highest image quality, but these are often impractical in pediatric populations. Breath-holding is reserved for older children (>5 years) or intubated patients and requires rapid pulse sequences.
An alternative approach involves acquiring data over an extended period while synchronizing acquisitions to the same respiratory phase using respiratory gating, ensuring images are less affected by motion artifacts.
Additionally, non–breath-hold imaging can be performed using advanced sequences that mitigate motion artifacts. Techniques such as undersampling k-space or employing specialized k-space sampling strategies, including periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER or BLADE), are particularly effective in minimizing respiratory motion artifacts and improving image quality in challenging cases [5].
To address the unique physical properties of the lung and optimize MRI, the following techniques are employed (Figure 3):
Short TE sequences: These reduce signal loss caused by the rapid decay of lung parenchymal signals.
Lower field strength (1.5T): Compared to 3T, 1.5T systems produce fewer susceptibility artifacts and motion-related distortions.
Respiratory gating: This technique synchronizes image acquisition with the respiratory cycle to reduce motion artifacts (Figure 4).
Navigator echoes: Enable dynamic tracking and correction of respiratory motion during the scan (Figure 5).
Parallel imaging: Accelerates data acquisition, minimizing motion-related distortions (Figure 6).
PROPELLER: Radial k-space sampling reduces motion artifacts and improves image quality in free-breathing patients (Figure 6) [2, 4].
3. Lung MRI Protocol
- Standard Fast Imaging Sequences [1]:
T2 SSFSE (Single Shot Fast Spin Echo): Provides rapid, high-resolution images with minimal motion artifacts.
3D T1 Spoiled Gradient Echo (GRE): Ideal for pre- and post-contrast imaging, allowing dynamic evaluations.
Short Tau Inversion Recovery (STIR): Fat-saturated sequence for enhanced lesion and tissue contrast.
Steady-State Free Precession (SSFP): Delivers balanced high signal intensity and good tissue contrast.
PROPELLER Sequences: Faster imaging with reduced motion artifacts.
- Advanced Sequences [1, 7, 8]:
Ultrashort TE and Zero Echo Time (ZTE): Effective for imaging lung parenchyma, reducing susceptibility artifacts caused by air-tissue interfaces.
Contrast-Enhanced MR Angiography (CEMRA):
Techniques: Uses "keyhole imaging" for high temporal resolution, with commercial implementations like TRICKS (GE Healthcare), TWIST (Siemens Healthcare), and 4D-TRACK (Philips Healthcare).
Allows thoracic MRA to be performed in 1-5 seconds, often during free breathing or with reduced anesthesia in pediatric patients.
Applications include detecting vascular anomalies, arteriovenous malformations (AVMs), and pulmonary embolisms.
MR Perfusion Imaging: Utilizes TWIST (low spatial resolution, high temporal resolution) for assessing pulmonary embolism, hypoxic ventilation conditions, and lung perfusion abnormalities.
Dynamic Functional Imaging: Low-resolution dynamic sequences are useful for evaluating tumor motion, lung wall dynamics, and diaphragmatic movement in radiotherapy planning.
Diffusion-Weighted Imaging (DWI)
DWI is employed for lesion characterization to differentiate between benign and malignant tissue.
Serai et al. proposed a set of lung MRI protocol parameters for currently available techniques. At our institution, we have adopted a protocol based on these recommendations (figure 7, 8) [1].
4. Indications of Lung MRI (figure 9)
Thoracic MRI has become a viable alternative to CT for many pathologies, offering high sensitivity and specificity in several conditions. The indications can be broadly divided into lung parenchymal, mediastinal, and chest wall pathologies, with its diagnostic utility being particularly significant in pediatric populations due to its radiation-free nature (figure 10) [1, 2, 3, 9].
- Lung Parenchymal Pathologies [2, 3, 5, 9, 10]
Infective Causes: MRI demonstrates high sensitivity for detecting pneumonia, its complications (e.g., necrosis, abscess), and other lung abnormalities. It also aids in narrowing differential diagnoses by identifying specific features such as necrosis and cavitation in tuberculosis or the characteristic hydatid cysts in Echinococcus infections. In cystic fibrosis, MRI detects active inflammation through bronchial wall enhancement and distinguishes mucus plugs from thickened walls. It also assesses lung perfusion changes due to hypoxic vasoconstriction or tissue damage, with perfusion alterations (figure 11, 12) [5, 2, 3, 5].
Congenital Causes: Congenital anomalies and malformations, including pulmonary sequestration, congenital pulmonary airway malformation, congenital diaphragmatic hernia, and congenital lobar emphysema (figure 13, 14) [10].
Interstitial Lung Diseases: Rare in childhood. MRI has limited utility compared to CT due to lower signal resolution, but recent studies suggest it may be useful in differentiating between areas of active inflammation and fibrosis [9, 11].
Airway Diseases: Effective for evaluating large airway pathologies, including masses and structural abnormalities (figure 15) [5].
Pleural Diseases: Both isolated pleural conditions and those associated with lung parenchymal disease (figure 16) [2, 3].
Systemic Pathologies: Useful for assessing pulmonary involvement in systemic conditions such as vasculitis, connective tissue diseases, and hematological disorders. MRI effectively demonstrates nodular and cavitary lesions in the lung parenchyma (figure 17) [12].
- Mediastinal Pathologies [2, 4, 7, 13, 14]
Lymph Nodes: High sensitivity for mediastinal lymphadenopathy [2].
Masses and Cysts: Comparable to CT in diagnosing mediastinal masses and cystic lesions. MRI effectively visualizes lesion contents, such as fat, distinguishes vascular structures, and assesses pericardial and chest wall invasion (figure 18, 19, 20, 21, 22, 23, 24) [13].
Vascular Pathologies: Effective in detecting embolism in larger pulmonary arteries, thoracic aorta anomalies, anomalous pulmonary venous return (APVR), pulmonary artery aneurysms, AVMs, and vasculitis. Advanced MRI angiography techniques enable the acquisition of both arterial and venous phase images with a single contrast injection, without the use of ionizing radiation (figure 25) [4, 7, 14].
- Chest Wall Pathologies [2, 3, 15, 16]
Tumors: Excellent for assessing chest wall tumors and their invasion. MRI provides a detailed evaluation of lesion contours and components, such as septations, T2 hypointense fibrous components, and hypointense foci associated with phleboliths (figure 26, 27, 28) [2, 3, 15].
Abscesses/Infections: Useful in identifying infections and abscess formations (figure 29) [2, 3].
Deformities: Pectus carinatum, pectus excavatum, and pectoralis muscle anomalies (Poland Syndrome). MRI offers significant advantages, including the absence of ionizing radiation, excellent soft tissue contrast, and multiplanar imaging capabilities (figure 30) [16].
