Incremental Value of Magnetic Resonance Imaging in Further Characterizing Hypodense Mediastinal and Paracardiac Lesions Identified on Computed Tomography

Figure 1. Illustration demonstrating the normal anatomy of the mediastinum, pericardium, and adjacent paracardiac region.

Figure 2. Flow chart illustrating an algorithmic approach for imaging the mediastinal and paracardiac lesions identified on computed tomography which may need further evaluation with magnetic resonance imaging. Contrast-enhanced magnetic resonance imaging with subtraction images are useful to identify subtle nodular or septal enhancement. In patients with acute or chronic renal failure gadolinium-based contrast agents should not be used due to the increased risk of nephrogenic systemic fibrosis.

Figure 3. A 55-year-old with past medical history of the upper extremity melanoma 12 years back, now presenting with a newly discovered mediastinal mass on annual surveillance radiograph. Contrast-enhanced computed tomography (a) demonstrates an anterior mediastinal mass (white arrow). Is this a new metastasis or a primary mediastinal malignancy? Thoracic magnetic resonance imaging obtained for further characterization. T1 image (b) shows high- signal intensity. Increased signal intensity on T1-weighted images can be seen with melanin, blood and protein. T2 image (c) shows increased signal within the lesion. High T1 can be seen with melanoma. The lesion demonstrates contrast enhancement (d), excluding a hematoma or a proteinaceous cyst, and thus confirming metastatic melanoma. Another T1 hyperintense lesion was identified in the right major fissure (not shown here) consistent with metastasis. Biopsy was performed only of one lesion that confirmed melanoma.

Figure 4. A 55-year-old with a history of factor X deficiency, chronic obstructive pulmonary disease, and cirrhosis being evaluated with ultrasound as part of pre-liver transplant workup, which demonstrated an incidental mediastinal mass. Contrast-enhanced computed tomography (a) obtained for further evaluation demonstrates an anterior mediastinal/pericardial lesion (white arrow). Is this benign or malignant? magnetic resonance imaging obtained for further characterization demonstrates increased signal intensity on T1 (b) and pericardial constriction on steady-state free precession (c) images. Postcontrast images (white arrows) at a more inferior level demonstrate no contrast enhancement (d: postcontrast venous phase; e: subtraction image). This is consistent with pericardial hematoma with constrictive pericarditis and this is the likely the cause of the cirrhosis. Patient underwent excision of this lesion. The properties of hematoma evolve as the lesion moves through acute, subacute, and chronic phases. See also Video 1.

Video 1. Electrocardiogram gated four-chamber cine steady-state free precession image demonstrates pericardial thickening with an organized hematoma. There is restricted motion of right atrium at right ventricle with paradoxical bowing of the ventricle septum.

Figure 5. Noncontrast-enhanced computed tomography (a) being performed as part of metastasis work up demonstrates a nonfluid attenuating anterior mediastinal lesion (32 HU). Noncontrast magnetic resonance can be useful to differentiate benign condition such as thymic hyperplasia from thymic tumors. A chemical shift ratio is calculated by comparing the signal intensity of the thymus gland with the paraspinal muscle. b: In phase sequence, c: out of phase sequence. Postcontrast-enhanced subtraction images demonstrate no contrast enhancement (d). In this case, the chemical shift ratio = 0.60, with no contrast enhancement seen. This is consistent with thymic hyperplasia.

Figure 6. Mature Teratoma. Contrast-enhanced computed tomography (a) being performed to evaluate a mediastinal lesion identified on chest radiograph in a 50 y/o patient presenting to the emergency department with chest pain demonstrates a complex calcified fat-containing anterior mediastinal lesion suggestive of a teratoma. Fat planes with pulmonary artery were effaced raising the suspicion of adherence (white arrow). This finding is important to assess preoperatively as the surgical approach may change. If the pulmonary artery is indeed adherent, the patient may need a cardiopulmonary bypass at the time of the resection. T2-weighted axial magnetic resonance imaging (b) demonstrates heterogeneous fat suppression. What is the cause? The shim volume at the time of acquisition was too large thus only the postfat is suppressed. Precontrast T1-weighted magnetic resonance imaging (c) obtained with corrected shim volume for the acquisition demonstrates adequate fat suppression with focal nodular enhancement (d). Cine steady state-free precession demonstrates the lesion adherence to the pulmonary artery. Pericardial adherence was also confirmed at surgery and required small pericardiectomy.

Figure 7. A 60-year-old patient with newly diagnosed lung cancer. Contrast-enhanced computed tomography (a) demonstrates a right hilar mass with computed tomography findings suggestive of pericardial invasion. In addition is there cardiac invasion also? T2 image without fat suppression (b) demonstrates loss of fat plane between the lesion and pericardium and let atrium (white arrow). Steady state-free precession image (c) demonstrates direct invasion of the left atria, which was not very obvious on computed tomography. Cine steady state-free precession demonstrated the absence of the sliding sign confirming left atrial invasion.

Figure 8. Chest computed tomography being performed as part of metastatic work up in a patient with cirrhosis and hepatocellular carcinoma. Axial computed tomography (a) image demonstrates a nondependent fluid collection in the right cardiophrenic angle abutting the right atrium. Could this be a necrotic lymph node, loculated pleural effusion or a pericardial cyst? Fortunately, same day abdomen magnetic resonance imaging was also being performed. Addition of a T2 (b) image demonstrates this to be hyperintense lesion at the cardiophrenic angle. A heavily-weighted T2 weighted (c) coronal image (similar to a magnetic resonance cholangiopancreatography) confirms fluid signal intensity, a finding consistent with a pericardial cyst.

Figure 9. Axial contrast-enhanced computed tomography (a) in a patient being staged for a metastatic cancer demonstrates a hypodense lesion posterior to the superior vena cava but abutting the ascending aorta. Is this a cyst, fluid collection or a necrotic lymph node? A heavily T2-weighted sequence (b) demonstrates fluid signal intensity, which along with the location is consistent with fluid in the periaortic pericardial recess (white arrow). Less likely differential may include an atypical location for a pericardial cyst. No further follow-up or imaging is recommended.

Figure 10. Low-dose noncontrast lung cancer screening computed tomography (a) in a patient with more than 25 pack years of smoking history demonstrates an anterior mediastinal lesion measuring 28 HU (white arrow). Is this a lymph node or a benign lesion such as a mediastinal cyst? T2-weighted images (b) show that this lesion has fluid signal. Fat-suppressed T1 image showingthat the lesion does not contain fat or septae (c). Postcontrast images (d) demonstrate no enhancement confirming this to be a thymic cyst. No further follow-up was needed for this.

Figure 11. Axial computed tomography (a) in a patient with chest and abdomen pain demonstrates a cystic lesion in the posterior mediastinum; this can be a lymph node or an esophageal duplication cyst (white arrow). Bright blood, steady state-free precession sequence (b) more definitively demonstrates this to be attached to the esophagus. This finding is consistent with an esophageal duplication cyst. These cysts can be lined with gastric mucosa which is prone to infection, perforation and hemorrhage.