Introduction

Cardiac masses are characterized as either primary or secondary tumors. Primary cardiac tumors are rare, with a 0.001% to 0.03% incidence found in autopsies while secondary tumors have an incidence of 1.7% to 14%.¹ Pericardial tumors are even less common with primary pericardial tumors accounting for only 6.7%- 12.8% of all primary cardiac tumors.^2,3 Clinical diagnosis of pericardial masses is very difficult since patients may present with diverse, nonspecific symptoms depending on whether there is associated pericardial effusion, pericarditis, or invasion of adjacent structures with resultant hemodynamic effects. Thus, imaging plays an important role in the evaluation of such patients. Although the diagnostic workup often starts with chest roentgenography or transthoracic echocardiography, further investigation is often warranted in order to visualize the entire pericardium and more specifically characterize the lesions in question.

Cardiovascular Magnetic Resonance

Cardiac magnetic resonance (CMR) is currently the ideal diagnostic imaging test for assessment of the pericardium and pericardial masses. The normal pericardium appears as a less than 2mm dark band sandwiched between the bright epicardial and mediastinal fat layers.^4,5 CMR has been shown to improve detection of cardiac masses, particularly paracardiac masses, which may not be well visualized on routine transthoracic echocardiography (TTE).^6,7 Studies have demonstrated that 10-20% of cardiac tumors detected by CMR are missed on routine TTE,^6,7 with most of them located in the pericardium or adjacent mediastinum.⁶ CMR provides multiplanar imaging with a wide field of view, high spatial and temporal resolutions, and high intrinsic soft tissue contrast without need for ionizing radiation or iodinated contrast. Specific sequences allow for different tissue weightings and intravenous contrast can be applied for further insight into the internal composition of the mass⁸ (Table 1). Both still and cine images are often acquired to further characterize pericardial masses and evaluate for pericardial effusion, myocardial invasion, myocardial infarction, involvement of coronary arteries and secondary functional and hemodynamic effects on the heart such as compression, diastolic dysfunction and/or constrictive physiology. CMR also provides additional information on resectability of masses as well as associated complications such as invasion of mediastinal structures, regional or distant metastases, and encasement of vital structures.

Table 1: MRI Sequences Used to Evaluate the Pericardium

Sequence	Suggested Planes	Information
Scouts	Axial, sagittal, coronal	Localizing
HASTE FSE/TSE	Axial, sagittal, coronal	Define anatomy and plan  subsequent views
Cine SSFP	Long axis, short axis	Evaluate function, volumes, presence of masses
Myocardial tagging	Long axis, short axis, targeted views of mass	Evaluate pericardial movement and assess for local invasion of the mass
T1 and T2 FSE/TSE	Long axis, short axis, targeted views of mass	Assess pericardial morphology and tissue characteristics of mass
T2 FSE/TSE STIR	Long axis, short axis, targeted views of mass	Evaluate for pericardial edema due to inflammation and tissue characteristics of mass
Early contrast-enhanced T1-weighted FSE/TSE	Long axis, short axis, targeted views of mass	Evaluate for inflammation and tissue characteristics of mass
Delayed enhancement	Long axis, short axis, targeted views of mass	Evaluate for pericardial and myocardial inflammation and fibrosis, and tissue characteristics of mass
Real-time imaging	Short axis	Evaluate for ventricular interdependence
Velocity-encoded phase-contrast	Aorta, PA, systemic and pulmonary veins	Assess vascular flow patterns for possible secondary hemodynamic effects of mass

Pericardial Masses

Masses of the heart and pericardium are classified as neoplastic, both primary and secondary, non-neoplastic, and non-tumoral. A classification scheme of the most common pericardial masses encountered is shown in Table 2.

Table 2: Pericardial Masses

Neoplastic
Primary		Secondary	Non-neoplastic	Other
Benign	Malignant	Metastatic	Cyst*	Hematoma
Lipoma*	Mesothelioma*	Breast*	Pericardial diverticulum	Thrombus
Hemangioma	Sarcoma	Lung	Inflammatory pseudotumor	Loculated fluid
Fibroma	Lymphoma	Renal cell carcinoma		Pseudoaneurysm
Angioma		Lymphoma
Teratoma	Malignant teratoma	Melanoma		Enlarged lymph nodes
Paraganglioma	Hemangioendothelioma	Thymoma		Gossypiboma
Lymphangioma	Neuroectodermal tumor	Mediastinal tumor
Neurofibroma		Multiple myeloma
Lipoblastoma		Esophagus
Granular cell myoblastoma		Leukemia

The most common primary pericardial masses are benign pericardial cysts. These fluid- containing structures are thought to be formed from sections of the pericardium that pinch off during embryonic development.⁸ Although they can occur anywhere along the pericardium, they are most commonly seen in the right cardiophrenic angle.^8,5,9 Pericardial cysts are well-defined, homogenous structures on CMR,⁶ characterized by low signal intensity on T1 and high signal intensity on T2 images without contrast enhancement.^5,9 (Figure 1)

Figure 1: Pericardial Cyst

Primary pericardial tumors are rare and occur much less frequently than secondary pericardial metastasis.⁹ Primary tumors can be benign, including lipoma, hemangioma, teratoma or malignant, including mesothelioma, sarcoma and lymphoma (Movies 1, 2).¹⁰ Metastatic tumors are usually of breast, lung and bone marrow origin.¹⁰ CMR can provide information on tumor location, site of insertion as well as relationship to adjacent structures, allowing for evaluation of feasibility for surgical resection.¹⁰ (Figure 2) CMR can provide crucial information on the histopathology of cardiac masses,^6,9,11 although tissue biopsy is often still required for diagnosis. CMR can accurately detect the high fatty content of lipoma and liposarcomas noted by homogenous high T1 and T2 weighted signal intensity.^6,11 Compared to benign tumors, malignant tumors often demonstrate additional features of nonmobility, associated pericardial effusion, and myocardial invasion.⁶ (Figure 3) The presence of disrupted pericardium, hemorrhagic effusion, invasion into the epicardial fat, myocardium, or cardiac chambers and associated mediastinal or pericardial lymphadenopathy are additional signs of aggressive disease.¹²

Figure 2: Fibrosarcoma without invasion into myocarium

Figure 3: Cardiac lymphoma with invasion into myocardium

Pericardial hematomas usually form after surgery or trauma,⁴ and may be accurately assessed by CMR (Movie 3). CMR has been shown to have excellent accuracy in the differentiation of cardiac thrombi or hematoma from tumors.^6,9,11 In general, thrombi tend to be smaller, more homogeneous and less mobile compared to tumors⁹ while tumors often demonstrate increased hyperintensity on T2 weighted images and contrast enhancement.⁶ The age of the hematoma determines CMR appearances.^4,6 During the subacute phase, hematomas appear as a fluid collection with heterogenous intermediate to high signal on T1 and T2 weighted sequences while in the chronic phase, the hematoma displays low signal with a dark rim.⁴ (Figure 4)

Figure 4: Pericardial hematoma

Conclusion

CMR is a useful imaging tool to evaluate the pericardium and diagnose pericardial masses. With the use of various imaging sequences and wide field of views, CMR can noninvasively provide information on the cardiac structure, morphology, function and associated complications that are important to the diagnosis and management of a variety of pericardial masses.

Movie 1 – Lymphoma

Movie 2 – Fibrosarcoma

Movie 3 – Hematoma

References

1. Mann, D. L., Zipes, D. P., Libby, P., Bonow, R. O. & Braunwald, E. Braunwald's Heart Disease. (Elsevier Saunders, 2015).
2. Meng, Q. et al. Echocardiographic and pathologic characteristics of primary cardiac tumors: a study of 149 cases. Int. J. Cardiol. 84, 69–75 (2016).
3. Patel, J. & Sheppard, M. N. Pathological study of primary cardiac and pericardial tumours in a specialist UK Centre: surgical and autopsy series. Cardiovasc. Pathol. 19, 343–352 (2016).
4. Axel, L. Assessment of pericardial disease by magnetic resonance and computed tomography. J. Magn. Reson. Imaging 19, 816–826 (2004).
5. Rajiah, P. Cardiac MRI: Part 2, pericardial diseases. Am. J. Roentgenol. 197, (2011).
6. Patel, R. et al. Diagnostic Performance of Cardiac Magnetic Resonance Imaging and Echocardiography in Evaluation of Cardiac and Paracardiac Masses. Am. J. Cardiol. 117, 135–140 (2016).
7. Staab, W. et al. Detection of intracardiac masses in patients with coronary artery disease using cardiac magnetic resonance imaging: a comparison with transthoracic echocardiography. Int. J. Cardiovasc. Imaging 647–657 (2014). doi:10.1007/s10554-013-0357-9
8. Grizzard, J. D. Magnetic Resonance Imaging of Pericardial Disease and Intracardiac Thrombus. Heart Fail. Clin. 5, 401–419 (2009).
9. Pazos-Lopez, P. et al. Value of CMR for the differential diagnosis of cardiac masses. JACC Cardiovasc. Imaging 7, 896–905 (2014).
10. Bogaert, J. & Francone, M. Cardiovascular magnetic resonance in pericardial diseases. J. Cardiovasc. Magn. Reson. 11, 14 (2009).
11. Hong, Y. J. et al. The usefulness of delayed contrast-enhanced cardiovascular magnetic resonance imaging in differentiating cardiac tumors from thrombi in stroke patients. Int. J. Cardiovasc. Imaging 1–7 (2011). doi:10.1007/s10554-011-9961-8
12. Verhaert, D. et al. The role of multimodality imaging in the management of pericardial disease. Circ. Cardiovasc. Imaging 3, 333–343 (2010).

Keywords: Aneurysm, False, Aorta, Aortic Aneurysm, Bone Marrow, Breast Neoplasms, Carcinoma, Renal Cell, Coronary Vessels, Diverticulum, Echocardiography, Edema, Esophagus, Fibroma, Fibrosarcoma, Granuloma, Plasma Cell, Heart Atria, Heart Neoplasms, Hemangioendothelioma, Hemangioma, Hemodynamics, Inflammation, Leukemia, Lipoblastoma, Liposarcoma, Lymph Nodes, Lymphangioma, Lymphoma, Magnetic Resonance Spectroscopy, Mediastinal Cyst, Mediastinum, Mesothelioma, Multiple Myeloma, Myocardial Infarction, Myocardium, Neoplasms, Muscle Tissue, Neuroectodermal Tumors, Neurofibroma, Paraganglioma, Pericardial Effusion, Pericarditis, Pericardium, Pulmonary Artery, Pulmonary Veins, Sagittaria, Sarcoma, Teratoma, Thrombosis, Thymoma, Tricuspid Valve, Gamma Rays

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