The chest radiograph (CXR) is typically the first imaging test performed in patients with potential pericardial disease. Within 10 years of the discovery of x-rays, publications highlighted their value in detecting pericardial disease. Current guidelines recognize the CXR has a role in identifying alternative diagnoses, complications and signs indicating the etiology. Inherent in guidelines recommending a CXR is the assumption that the clues to pericardial disease will be recognized and the limitations appreciated. Whilst further characterization requires echocardiography (computerized tomography (CT) or magnetic resonance imaging (MRI), this review highlights potential findings on CXR.

Pericardial effusion
Normal pericardial fluid volume is less than 50ml. Effusions of less than 200ml are typically not visible on a CXR.¹ With slow accumulation over 2 L of fluid may be present.² Importantly rapid accumulation causes tamponade at a smaller volume than a slowly accumulating effusion, so will be less readily appreciated.

Note that most of the research on CXR signs predates the widespread adoption of echocardiography and CT – the quoted statistics likely overestimate the value of CXR in current practice.

The most sensitive sign for a pericardial effusion on CXR is enlargement of the cardiac silhouette (cardiothoracic ratio (CTR) >50%) (Figure 1). This has reasonable sensitivity (71%), but low specificity (41%). Specificity increases as cardiomegaly increases (76% with CTR of 60%), but sensitivity falls.³

Figure 1

Specificity can be increased by comparing with previous CXRs. An increase in transverse cardiac diameter of greater than 1.5cm, particularly over a short interval (<30 days), is reported as being 80% specific, but only 46% sensitive.³

A pericardial effusion will generally symmetrically expand the pericardial contour. This gives rise to a "globular" configuration. Features on a frontal CXR include flattening of the upper heart borders followed by broad bulging. For those preferring pareidolic terms (pareidolia refers to the human ability to recognize shapes in an unrelated image) this is referred to as "flask-shaped" or the "water bottle sign" (presumably after the older style round leather canteen). An alternative term is the "chicken on a fence" sign (the chicken representing the cardiac silhouette sitting on the diaphragm).⁴

Posterior pericardial expansion produces another sign on the frontal radiograph: widening of the carinal angle⁵ (the angle between the left and right main bronchi being >90°). This sign is more commonly associated with left atrial enlargement but can result from any "mass" below the carina.

On the lateral radiograph expansion of the pericardium posteroinferiorly gives rise to the "posteroinferior bulge sign" (or "bulge sign") (Figure 2). Anterosuperior expansion causes "loss of the retrosternal space". Neither sign is specific.

Figure 2

Two signs derive from direct observation of the effusion. The "pericardial fat stripe" refers to visualizing fluid density between the epicardial and pericardial fat. As these fat layers are usually more prominent adjacent to the right ventricle, this sign is typically identified anterior to the heart on the lateral radiograph. Whilst very specific (94%), sensitivity is low (12%).³ Pareidolic descriptors for this appearance include the "oreo cookie sign" and, less commonly, the "sandwich sign".

In the "differential density sign" the pericardial fluid is distinguished adjacent to the myocardium. This requires a relatively large, lower density effusion (such as a chylous or transudative effusion) and is geometrically enhanced by the anatomy around the cardiac apex,⁶ so is best seen adjacent to the left heart border on the frontal CXR, but can also constitute part of the "bulge sign" (above).

Some describe a "well defined" cardiac silhouette due to dampening of cardiac motion artefact by the pericardial fluid. There are many other technical factors at play (including exposure time and post processing), making this very subjective.

One final "sign" in the literature is that predominantly left sided pleural effusions are associated with pericardial effusions. This was postulated to be due to the larger area of contact between the left pleura and pericardium (allowing spread of infection/ tumour, or even fluid to exude).³ More recent articles have not, however, confirmed this association.⁷

Pneumopericardium
Pneumopericardium implies loss of pericardial integrity, so is an important finding in settings such as trauma and radiofrequency ablation for atrial fibrillation. Small quantities of air in the pericardium will not be detected on CXR. With sufficient volume, air in the pericardium is apparent as a radiolucent zone, or "halo", around the heart (Figure 3). The term "halo" is also used for the radiodense line of the parietal pericardium around a pneumopericardium.

Figure 3

Radiologically the main challenge is differentiating pneumopericardium from the more common pneumomediastinum. The most specific feature is the "continuous (left) diaphragm sign". Visualization of the superior surface of the diaphragm across the midline occurs when there is air between the heart and the diaphragm – as the parietal pericardium is fused with the central diaphragm, this can only occur with pneumopericardium. A pneumopericardium is well defined because of its serosal lining – in contrast to pneumomediastinum where gas dissects in the tissue planes producing "stranding". The anatomy defines the distribution of a pneumopericardium – therefore air above the great vessels indicates pneumomediastinum. Unlike pneumomediastinum, air in the pericardium freely moves to a nondependent location in the pericardial space. Historically a decubitus radiograph could help differentiate. In current practice this effect might be encountered with sequential erect and supine CXRs.

Like pericardial effusions, a large pneumopericardium can cause tamponade. This produces the "small heart sign" – referring to the global reduction in heart size on the frontal radiograph. One criterion for this is a sudden decrease in the cardiac diameter of >2cm.⁸

Pericardial calcification
Pericardial calcification on a plain radiograph strongly suggests constrictive pericarditis in patients with heart failure. Conversely 27-50% of patients with constrictive pericarditis have pericardial calcification.^2,9 Calcification is more common in idiopathic constriction and typically occurs near the basal ventricles and atrioventricular grooves (right > left).⁹ Consequently calcification is most readily visualized in the pericardium anterior to the right ventricle on the lateral radiograph (Figure 4).

Figure 4a

Figure 4b

Calcification on a CXR has been described as the "armoured heart sign".

The key to distinguishing calcifications on a CXR is recognizing their anatomical location. Other common calcifications include myocardial infarction (typically have a shorter arc); left atrial calcification (can produce a rounded appearance, particularly in a dilated atrium); coronary artery calcification (tubular and curvilinear), valvular calcification and pleural calcification.

Pericardial defects
Pericardial defects can be congenital or acquired (including surgery, cardiopulmonary resuscitation and trauma – both blunt and penetrating).

One key CXR feature of large pericardial defects is cardiac malposition with a normally located (relatively "midline") trachea.

In congenital absence of the left pericardium (which represents 70% of all pericardial defects¹⁰) there is flattening and elongation of the left heart border (the "Snoopy sign"¹¹), loss of the right heart border (overlying the vertebra), prominence of the pulmonary artery, lung between the aortic knuckle and the pulmonary artery, and lung between the left hemidiaphragm and base of the heart.¹² Congenital complete absence has a similar appearance, although the lucency between the heart and diaphragm is more common in complete absence.¹³

Partial absence has a more varied appearance. Herniation of the left atrial appendage causes a bulge on the upper left heart border.¹⁰ Herniation of a ventricle can cause a "boot-shaped heart" – a pareidolic description for upturning of the cardiac apex (the term is typically used in tetralogy of Fallot). On the right examples include prominence of the right pulmonary artery (due to lung surrounding the vessel), prominence of the right heart border, or lucency (lung) between the right inferior heart border and diaphragm with inferior defects.¹⁴

These appearances highlight that herniation through defects can be of either cardiac structures out of the pericardium, or extracardiac structures (such as lung) into the pericardium. A diaphragmatic pericardial defect can result in herniation of abdominal structures – of which bowel is most visible on the CXR.

In trauma the CXR is particularly important. Unfortunately, diagnosis of pericardial injury is typically hampered due to being obscured by co-existent injuries and by limitations in technique (such as being supine). Signs of pneumopericardium or pericardial effusion should be sought but signs of effusion are frequently absent in trauma (in 80%) as tamponade occurs at a smaller volume when there is rapid accumulation.¹⁵

Pericardial tears are uncommon with blunt trauma, occurring in 0.3 to 0.5%.¹⁶ The left pleuropericardium is most often involved (50%), followed by the diaphragmatic pericardium (27%), the right pleuropericardium (17%) and superior mediastinal (4%). In two-thirds of cases tears will be complicated by cardiac herniation which can result in torsion and strangulation.¹⁶ Displacement of the heart without an obvious cause (such as haemopneumothorax or lung collapse) raises the possibility of cardiac herniation. In extreme cases there may be an empty, air filled pericardial sac.¹⁵ Other signs discussed under congenital pericardial defects may be present. Herniation can be delayed.¹⁷

Pericardial masses
There is a wide range of pericardial masses including cystic lesions, organized effusions, primary and secondary malignancies.² Radiographic findings include those of any associated pericardial effusion, an abnormal contour or a discrete mass.¹⁷

The most common lesion is a pericardial cyst. These are seen on a CXR as well defined, smooth lesions that are most commonly in the right anterior cardiophrenic angle (followed in frequency by the left anterior cardiophrenic angle).¹⁸ These are almost always congenital and/ or idiopathic. One study found 3% of patients had multiple cysts; while 11% had symptoms attributed to the cyst (most commonly pain). They can change in size over time, although even when they increase they remained asymptomatic.¹⁹

Some tumours contain fat or calcium, which may provide a radiographic clue to their nature.¹⁸ Beyond identification of an abnormality, the CXR has a limited role.

References

1. Co SJ, Yong-Hing CJ, Galea-Soler S, et al. Role of imaging in penetrating and blunt traumatic injury to the heart. Radiographics 2011;31:E101-15.
2. Bogaert J, Francone M. Pericardial disease: value of CT and MR imaging. Radiology 2013;267:340-56.
3. Eisenberg MJ, Dunn MM, Kanth N, Gamsu G, Schiller NB. Diagnostic value of chest radiography for pericardial effusion. J Am Coll Cardiol 1993;22:588-93.
4. Han J, Xiang H, Ridley WE, Ridley LJ. Chicken on a fence sign: pericardial effusion. J Med Imaging Radiat Oncol 2018;62:23.
5. Chen JT, Putman CE, Hedlund LW, Dahmash NS, Roberts L. Widening of the subcarinal angle by pericardial effusion. AJRAm J Roentgenol 1982;139:883-7.
6. Tehranzadeh J, Kelley MJ. The differential density sign of pericardial effusion. Radiology 1979;133:23-30.
7. Woodring JH. Distribution of pleural effusion in congestive heart failure: what is atypical? South Med J 2005;98:518-24.
8. Lamba A, Dutta R, Chand RK. Pneumopericardium after minimally invasive atrial septal defect closure. Ann Card Anaesth 2018;21:99.
9. Bogaert J, Meyns B, Dymarkowski S, Sinnaeve P, Meuris B. Calcified constrictive pericarditis: prevalence, distribution patterns, and relationship to the myocardium. JACC Cardiovasc Imaging 2016;9:1013-4.
10. Cuccuini M, Lisi F, Consoli A, et al. Congenital defects of pericardium: case reports and review of literature. Ital J Anat Embryol 2013;118:136-50.
11. Han J, Xiang H, Ridley WE, Ridley LJ. Snoopy sign: Congenital absence of the left pericardium. J Med Imaging Radiat Oncol. 2018;62:47.
12. Shah AB, Kronzon I. Congenital defects of the pericardium: a review. Eur Heart J Cardiovasc Imaging 2015;16:821-7.
13. Gatzoulis MA, Munk MD, Merchant N, Van Arsdell GS, McCrindle BW, Webb GD. Isolated congenital absence of the pericardium: clinical presentation, diagnosis, and management. Ann Thorac Surg 2000;69:1209-15.
14. Koo CW, Newburg A. Congenital absence of the right pericardium: embryology and imaging. J Clin Imaging Sci 2015;5:12
15. Adams A, Fotiadis N, Chin JY, Sapsford W, Brohi K. A pictorial review of traumatic pericardial injuries. Insights Imaging 2012;3:307-11.
16. Co SJ, Yong-Hing CJ, Galea-Soler S, et al. Role of imaging in penetrating and blunt traumatic injury to the heart. Radiographics 2011;31:E101-15.
17. Restrepo CS, Gutierrez FR, Marmol-Velez JA, Ocazionez D, Martinez-Jimenez S. Imaging patients with cardiac trauma. Radiographics 2012;32:633-49.
18. Restrepo CS, Vargas D, Ocazionez D, Martínez-Jiménez S, Betancourt Cuellar SL, Gutierrez FR. Primary pericardial tumors. Radiographics 2013;33:1613-30.
19. Alkharabsheh S, Gentry III JL, Khayata M, et al. Clinical features, natural history, and management of pericardial cysts. Am J Cardiol 2019;123:159-63.

Clinical Topics: Arrhythmias and Clinical EP, Congenital Heart Disease and Pediatric Cardiology, Heart Failure and Cardiomyopathies, Noninvasive Imaging, Pericardial Disease, SCD/Ventricular Arrhythmias, Atrial Fibrillation/Supraventricular Arrhythmias, Congenital Heart Disease, CHD and Pediatrics and Arrhythmias, CHD and Pediatrics and Imaging, Acute Heart Failure, Computed Tomography, Echocardiography/Ultrasound, Magnetic Resonance Imaging, Nuclear Imaging

Keywords: Atrial Appendage, Atrial Fibrillation, Artifacts, Calcium, Cardiopulmonary Resuscitation, Bronchi, Cardiomegaly, Coronary Vessels, Diaphragm, Constriction, Heart Failure, Echocardiography, Heart Ventricles, Hemopneumothorax, Magnetic Resonance Imaging, Mediastinal Cyst, Mediastinal Emphysema, Myocardial Infarction, Myocardium, Neoplasms, Pain, Pericardial Effusion, Pericarditis, Constrictive, Pericardium, Pleura, Pleural Effusion, Pneumopericardium, Prednisolone, Pulmonary Artery, Pulmonary Atelectasis, Tetralogy of Fallot, Tomography, Tomography, X-Ray Computed, Trachea, X-Rays

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