INTRODUCTION — Magnetic resonance imaging (MRI) is an important tool in assessment of diseases of the heart, mediastinum, pleura, and chest wall [1,2]. Strengths of MRI include excellent tissue contrast, multiplanar imaging capability, sensitivity to blood flow, and lack of ionizing radiation. The lack of ionizing radiation make MR an excellent imaging choice for pediatric examinations, pregnant women and patients requiring serial and longitudinal follow-up . Application of MRI in intrinsic lung disease has been limited by signal loss from physiologic lung motion, low signal intensity due to paucity of protons, and magnetic field inhomogeneities induced by the air/tissue interfaces in lung, problems that may be overcome with improvements in imaging hardware and pulse sequences, like ultrashort echo time or zero echo time, multicoil parallel acquisitions and acceleration methods [3,4]. Prospective studies have shown that nonvascular thoracic MR has the potential to reduce the number of planned surgical interventions, modify the surgical approach, increase surgeon comfort with the patient management plan, increase diagnostic certainty, and avoid unnecessary follow-up or clinical care . The noncardiac clinical indications for thoracic MRI will be presented here; technical aspects of thoracic and cardiac MRI are reviewed separately. (See "Principles of magnetic resonance imaging" and "Clinical utility of cardiovascular magnetic resonance imaging".)
Chest wall and diaphragm — MRI is an excellent imaging modality for assessment of primary chest wall tumors , chest wall phlegmons or abscesses, and chest wall or diaphragmatic extension of intrathoracic masses. On T1-weighted images, particularly with contrast material enhancement, the extent of invasion of normal tissues can usually be established (image 1A-C and image 2A-D and image 3A-B and image 4A-C) [7-10]. (See "Principles of magnetic resonance imaging".)
Vascular encasement or invasion is frequently well visualized on T1-weighted images. GRE (bright blood) sequences can sometimes demonstrate vascular invasion more clearly, and can regularly demonstrate the vascular supply of tumors. T2-weighted sequences play a secondary role in evaluation of the chest wall. Their primary use is in demonstrating areas of cystic degeneration, inflammation, and edema.
The ability of MRI to display arbitrary planes of section used to be an advantage over computed tomography (CT) in assessment of the lung apices, diaphragm, and spinal column. However, the newer generation of multislice CT scanners, by enabling rapid near-isotropic imaging of the entire thorax at submillimeter resolution, has considerably narrowed this advantage. The ability to manipulate the relative signal intensities of normal and abnormal tissue through appropriate use of MRI pulse sequences remains a relative advantage of MRI over CT and achieves superior contrast resolution and more accurate tissue characterization. Although cortical bone destruction is better demonstrated by CT, bone marrow involvement by tumor is better visualized on MRI.
Pleura — MRI is comparable to CT in evaluating pleural disease [11-13]. MRI may also be better at demonstrating extension of pleural lesions into the chest wall, mediastinum, and diaphragm. Imaging in sagittal and coronal planes is especially helpful in assessing the extent of malignant tumors, such as mesothelioma (image 4A-C) [14-16]. MRI can potentially characterize pleural effusions, and differentiate between exudates, transudates, and hemothoraces . Chylous pleural effusions can be assessed with MRI multipoint Dixon fat quantification . However, CT remains advantageous for visualizing calcification within pleural lesions and displaying the "split pleura sign" seen in exudative pleural effusions or empyemas . Magnetic resonance imaging has proved advantageous in elucidating the source of a chylothorax by analyzing the morphology of the thoracic duct and detecting accessory lymphatic channels in patients with chylous pleural effusions . Nonenhanced MR lymphography has proven advantageous: it is a noninvasive technique based on heavily T2-weighted sequences, enabling the display of the lymphatic circulation .
Paraspinal masses — MRI is well suited for evaluation of paraspinal masses because of its multiplanar imaging capability. It can clearly demonstrate the craniocaudal extent of disease, involvement of the spinal column, and/or extension into the spinal canal (image 5A-B and image 6 and image 7). CT evaluation of intraspinal extent of paraspinal masses can suffer from beam-hardening artifact produced by the high attenuation spinal column; MRI has no similar limitation. Paraspinal schwannomas display a target appearance with high signal in the periphery and lower signal centrally. In rare instances, paraspinal desmoid tumors can be encountered and mimic ganglioneuromas.
Mediastinum and hila — MRI is useful in evaluating mediastinal and hilar disease, though it cannot match the superior spatial resolution of CT (image 8A-B and image 9A-B and image 10 and image 11) [21,22]. An advantage of MRI includes its multiplanar capability, which enhances:
●Mediastinal tumor staging. Coronal and sagittal sections reliably detect subcarinal and aorticopulmonary lymph node masses.
●Assessment of local tumor extension. Lack of signal from flowing blood in spin echo images permits distinction of solid tissue (such as lymph nodes) from blood vessels.
●Tissue characterization. Double inversion recovery T2 spin echo sequences allows for the diagnosis of thymic cysts and bronchogenic cysts with high attenuation on CT scanning. Low T2 signal due to fibrous tissue results in “dark lymph nodes”; it facilitates the diagnosis of treated lymphoma, sarcoidosis, or sclerosing mediastinitis. Chemical-shift MRI can be useful in the diagnosis of thymic hyperplasia, dermoid cysts, and hamartomas (all these lesions contain microscopic fatty tissue with loss of signal on double echo, opposed phase gradient echo imaging) . Iron deposition in tissue can be identified with loss of signal on double echo, in-phase imaging due to the T2 star effect, eg, in sickle cell disease or after multiple blood transfusions .
The use of MRI in lymphoma staging is well established [25,26], although the generally greater spatial resolution of CT makes it superior to MRI for this indication . MRI can also be valuable in lesion characterization, as with distinguishing hematoma or fibrosis from neoplasm.
Diffusion-weighted MR imaging has been utilized in characterizing and staging thymic epithelial tumors .
Both MRI and CT have been partially supplanted by positron emission tomography (PET) with fluorodeoxyglucose (FDG) in mediastinal tumor staging . Unlike MRI and CT, which are primarily anatomic imaging techniques, FDG-PET can identify malignant tumors based on their high glucose utilization and metabolic characteristics, in addition to their appearance. Integrated PET-CT scanners provide additional advantages by combining functional information from PET with anatomic information from CT in a single examination. PET/MR imaging has been introduced for use in patients and may have diagnostic performance comparable to PET/CT , but PET/MRI is not widely available.
Short inversion time, inversion-recovery, fast spin-echo MRI and diffusion-weighted MRI show future potential and may even have greater sensitivity and accuracy in evaluating lymph node involvement in non-small cell carcinoma of the lungs .
Lung parenchyma — MRI currently plays only a limited role in evaluation of the lung (image 12). This is the result of many factors, including poorer spatial resolution than CT, low proton density, long imaging times compared with CT causing physiologic motion artifact, and magnetic susceptibility-induced signal loss at the border between the alveolar wall and alveolar gas. In interstitial lung disease, MRI may find a niche in distinguishing active disease (high signal on T2-weighted images) from inactive disease/fibrosis (low T2 signal intensity) .
Novel lung techniques are being investigated . Ultrashort echo time and zero echo time MR imaging have been used to image the lung parenchyma [3,32] and to quantify emphysema severity . Elastic registration during inspiration and expiration with quantification of voxel deformity can be used to facilitate early diagnosis of pulmonary fibrosis .
Imaging of inhaled hyperpolarized helium or xenon is being studied as a way to determine the apparent diffusion coefficient of helium or xenon and regional lung ventilation [35-38]. For instance, exercise-induced bronchoconstriction can be evaluated reproducibly with hyperpolarized HE3 MRI or hyperpolarized Xenon 129 imaging [3,39]. Blood tagging techniques with gadolinium or arterial spin labeling are being studied as a way to evaluate regional lung perfusion and in specialized centers, high-altitude pulmonary edema . Assessment of lung morphology with balanced steady-state free precession MR imaging has been shown to be inferior to CT in imaging lung parenchymal disease, yet it represents an alternative radiation-free imaging modality in patients with pulmonary fibrosis . These techniques are not widely available in clinical practice at this time. Simultaneous imaging of lung structure and function with triple-nuclear hybrid MR imaging has been demonstrated in a research setting, utilizing single breath hold technique with two hyperpolarized gasses, Helium and Xenon 129 with simultaneous registered anatomic proton MR images of lung structure .
Evaluation of cystic fibrosis with proton MRI has been introduced in selected centers for routine clinical management and long-term monitoring of these patients. Proton MRI provides morphologic information concerning mucus distribution, bronchiectasis, inflammatory airways wall thickening, consolidation and atelectasis of lung parenchyma [3,43,44]. Air-trapping and perfusion abnormalities due to hypoxic vasoconstriction (von Euler-Liljestrand reflex) can be revealed with three-dimensional gradient-echo imaging, balanced steady-state free precession, arterial spin-labeling and hyperpolarized gases . Allergic bronchopulmonary aspergillosis in cystic fibrosis can be imaged with T1-and T2-weighted spin-echo sequences; mucoid impaction with T1 high signal and T2 low signal can lead to inversion of the expected signal owing to an increase in calcium salts, iron or manganese contents. This inverted mucoid impaction signal can help in the diagnosis of allergic bronchopulmonary aspergillosis (ABPA) in cystic fibrosis with bronchiectases .
In patients with chronic obstructive pulmonary disease (COPD), traditional gas-based ventilation imaging methods with Helium-3 or hyperpolarized Xenon 129 could be replaced in the future with two-dimensional or three-dimensional proton-based regional ventilation MRI techniques [46,47].
Lung cancer screening — CT is well-established as a screening tool for early detection of lung carcinoma in high-risk patients, now endorsed by the US Preventive Services Task Force . MRI is a potential alternative to CT for lung cancer screening, the primary advantage of which is absence of ionizing radiation. Many technical challenges to detection of small pulmonary nodules by MRI persist, including lower spatial resolution compared with CT, breathing and cardiac motion artifacts, and magnetic susceptibility artifacts. However, many of these limitations are yielding to hardware and pulse sequence improvements, such that lung cancer screening by MRI is now feasible, although not widely available . In particular, imaging at higher field strength and using ultrashort echo times has been shown to increase MRI sensitivity for lung nodule detection .
Bronchogenic carcinoma — MRI is comparable to CT in staging lung cancer complicated by chest wall, diaphragm, pleura, or mediastinal involvement (image 1A-C and image 2A-D and image 3A-B and image 9A-B) [29,51-54]. MRI may have greater specificity in the diagnosis of adrenal masses found on staging examinations in patients with lung cancer. However, MRI currently plays only an adjunctive role to CT for staging of lung cancer, because of higher cost, poorer spatial resolution, longer examination duration, less availability, and because CT can diagnose and stage lung cancer in a single examination. The use of MRI is reserved for those cases in which the tumor is suspected to be unresectable, but CT findings are not definitive (as in Pancoast [superior sulcus] tumors) [55,56] or cases in which the patient has a history of reaction to iodinated contrast agents (see "Superior pulmonary sulcus (Pancoast) tumors"). Whole-body hybrid PET/MR imaging may play a future role in staging of bronchogenic carcinoma [57,58] and in characterizing pulmonary nodules .
Aortic disease — MRI is an excellent imaging tool for the diagnosis of aneurysm and aortic dissection. Spin echo sequences and other "black blood" techniques, can detect the medio-intimal flap and determine the type and extent of dissection. Gradient echo sequences can differentiate the true lumen (with higher flow and higher signal intensity) from the false lumen (with lower flow and lower signal intensity). Contrast material-enhanced MR angiography is widely used for evaluation of aortic disease as technical capabilities of scanners have improved. In the past, transesophageal echocardiography and MRI have been considered the two methods of choice for diagnosing dissections. However, advances in CT technology and the greater availability of multislice CT have caused this latter modality to assume a primary role in imaging of aortic disease. (See "Clinical features and diagnosis of acute aortic dissection".)
Pulmonary vascular disease — Pulmonary artery dilation, a finding suggestive of pulmonary artery hypertension, can be imaged well by MRI . MR angiography is able to detect vascular intraluminal filling defects (eg, emboli), but pulmonary emboli are currently better visualized using CT [61,62]. New MR sequences may further facilitate visualization of pulmonary emboli . Dynamic contrast-enhanced lung perfusion MRI facilitates screening for chronic thromboembolic pulmonary hypertension . The use of intravenous gadolinium-based contrast material (as a first pass perfusion agent) and arterial spin labeling (endogenous perfusion assessment without injection) may expand the role of MRI in the diagnosis of pulmonary vascular disease [4,64-68]. (See "Clinical presentation, evaluation, and diagnosis of the nonpregnant adult with suspected acute pulmonary embolism" and "Epidemiology, pathogenesis, clinical manifestations and diagnosis of chronic thromboembolic pulmonary hypertension".)
Future applications of MR in the chest include MR imaging evaluation of cardiovascular risk in metabolic syndrome and molecular body imaging as well as whole-body population-based MR imaging [69,70].
Functional, dynamic MR imaging of the chest is currently being studied in the evaluation of diaphragmatic motion , in the analysis of lung tidal volumes, and in the assessment of postoperative effects on the chest wall and diaphragm dynamics of breathing .
SUMMARY AND RECOMMENDATIONS
●Indications for magnetic resonance imaging (MRI) of the chest - MRI is an important tool in the evaluation of chest structures. Although computed tomography (CT) plays a primary role in noncardiac chest imaging, the multiplanar capabilities and excellent tissue contrast resolution and tissue characterization of MRI make it equal or superior to CT in several areas including:
•Assessment of the lung apices, diaphragm, and spinal column
•Evaluation of pleural disease
•Evaluation of mediastinal cysts with high attenuation on CT scanning
•Evaluation of paraspinal masses
•Assessment of local tumor extension, particularly chest wall invasion, and delineation of blood vessel invasion
•Metastatic invasion of bone marrow
•Certain aspects of staging of bronchogenic carcinoma; however, MRI still plays an adjunctive role to CT in this setting
•Serial and long-term follow-up of patients with cystic fibrosis
●Indications of CT chest - CT remains the preferred technique in the following settings:
•Evaluation of lung parenchymal disease
•Evaluation of mediastinal lymph nodes and lymphoma staging
•Assessment of cortical bone metastases
•Staging of bronchogenic carcinoma (in most cases)
•Aortic aneurysms and dissection
•Detection of pulmonary embolism
•Lung cancer screening
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