Pericardial cysts are congenital mesothelial cysts arising from incomplete closure of the pleuropericardial membrane during embryological development. Their incidence is approximately 1 per 100,000, comprising 6-7% of mediastinal masses. Seventy percent are located at the right cardiophrenic angle — a pathognomonic location for this entity; 22% occur at the left cardiophrenic angle and 8% at atypical locations such as the superior or posterior mediastinum. The cyst has a thin, smooth wall lined by a single layer of mesothelial epithelium and contains clear, serous 'spring water' fluid — a feature pathognomonic at aspiration. They are typically discovered incidentally, are asymptomatic, and follow a benign course; rarely, enlargement may cause cardiac compression, dyspnea, or chest pain. On CT, they present as a thin-walled, unilocular, non-enhancing cystic lesion with water density (0-20 HU).
Age Range
20-70
Peak Age
40
Gender
Equal
Prevalence
Uncommon
Pericardial cysts arise from a developmental failure of closure of the pleuropericardial membrane during embryological development. In normal embryogenesis, the parietal pericardium forms from mesodermal folds in the primitive coelomic cavity — lateral pleuropericardial membranes grow medially and fuse to separate pleural and pericardial spaces. During this closure, if mesothelial tissue deposits become isolated from the main closure, they become sequestered and form the nidus of the cyst. The cyst wall is lined by a single layer of mesothelial epithelium that actively secretes serous fluid — this is why the cyst may slowly enlarge over time. The cyst content consists of clear, serous 'spring water' fluid; it has low protein content, contains no cells, and harbors no infectious agents. The water density (0-20 HU) appearance on CT occurs because the cyst content is low-protein serous fluid that is nearly pure water, absorbing X-rays similarly to water. On MRI, the low T1 signal and markedly bright T2 signal occur because free water protons have long T1 and very long T2 relaxation times — this confirms simple fluid content and excludes the T1 shortening caused by protein or cellular content in complex fluid. The cyst wall is avascular and lacks neovascularization, so no enhancement is seen — this feature distinguishes it from solid or inflammatory lesions.
The characteristic feature of pericardial cysts is the aspiration of crystal-clear, colorless, odorless serous fluid — this fluid is described as 'spring water' and is pathognomonic for pericardial cyst. On imaging, this feature manifests as pure water density (0-20 HU) on CT and simple fluid signal equivalent to CSF (T1 hypointense, T2 markedly hyperintense) on MRI. The 'spring water' description is used both clinically (aspiration finding) and radiologically (water density, simple fluid signal) and summarizes the congenital mesothelial origin, low-protein serous secretion characteristic, and benign nature of pericardial cysts. This sign definitively distinguishes pericardial cysts from other mediastinal cysts (bronchogenic cyst — mucinous/opaque fluid, thymic cyst — cholesterol crystals, dermoid — sebaceous content).
On non-contrast CT, a water-density (0-20 HU), thin-walled (<2 mm), unilocular, well-circumscribed, round or oval cystic lesion is seen at the right cardiophrenic angle (70% of cases). The cyst wall has smooth contours with no lobulation or wall irregularity. No internal septation, solid component, calcification, or debris is present. Cyst size typically ranges from 2-15 cm; most cases fall within the 3-8 cm range. The pericardial cyst is located immediately adjacent to the pericardium and shows broad-based contact with the pericardium — this feature distinguishes it from pleural or pulmonary lesions. At atypical locations (8% — superior or posterior mediastinum), diagnosis becomes more challenging. Associated pericardial effusion or pericardial thickening is typically absent.
Report Sentence
On CT, a ___ x ___ mm thin-walled, unilocular, non-enhancing cystic lesion of water density (___ HU) is seen at the right/left cardiophrenic angle; consistent with pericardial cyst.
On contrast-enhanced CT, the pericardial cyst shows no enhancement — neither in the cyst wall nor in the cyst content is contrast uptake seen. The density of the cyst content remains unchanged compared to non-contrast values (stays within 0-20 HU range). This finding confirms that the cyst wall is avascular and lacks neovascularization. The absence of enhancement reliably distinguishes the pericardial cyst from solid mediastinal masses (thymoma, lymphoma), vascular lesions (aortic aneurysm segment), and inflammatory collections (abscess, organized effusion). In the contrast-enhanced phase, while surrounding mediastinal structures (heart, great vessels, pericardium) enhance, the unchanged water density of the cyst creates a conspicuous contrast difference and more clearly delineates the lesion boundaries. Rarely, in complicated (infected or hemorrhagic) pericardial cysts, wall enhancement and increased content density may be seen — this situation challenges the simple cyst diagnosis.
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On contrast-enhanced CT, the described cystic lesion shows no wall or content enhancement (non-contrast: ___ HU, contrast-enhanced: ___ HU); simple cystic nature is confirmed.
On T2-weighted MRI, the pericardial cyst shows markedly hyperintense signal — signal intensity is equivalent to CSF and gallbladder fluid. This bright T2 signal is the characteristic finding of simple, low-protein serous fluid. The cyst content is homogeneous; no fluid-fluid level, debris, vegetation, or solid component is present. The cyst wall is thin and of low signal — wall thickening is a warning sign favoring malignancy or infection. No perilesional edema is seen around the lesion. Markedly hyperintense signal is also seen on STIR sequences — not affected by fat suppression because the cyst contains fluid, not fat. The degree of T2 hyperintensity (as bright as CSF) is important in differential diagnosis: bronchogenic cysts typically show intermediate-to-high T2 signal because mucinous/proteinaceous content shortens T2 relaxation time.
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On T2-weighted MRI, a ___ x ___ mm markedly hyperintense homogeneous cystic lesion with signal equivalent to CSF is seen at the right/left cardiophrenic angle; consistent with simple fluid-containing pericardial cyst.
On T1-weighted MRI, the pericardial cyst shows low signal intensity — lower than muscle signal, close to or equivalent to CSF signal. This low T1 signal reflects that the cyst content is simple serous fluid (low protein, free water). Homogeneous low signal is important: areas showing high signal on T1 are concerning for hemorrhagic content (met-hemoglobin, T1 shortening), proteinaceous content (>3 g/dL protein), or fat content, and exclude a simple cyst diagnosis. Wall visualization may be difficult on non-contrast T1 because contrast between the thin wall and surrounding mediastinal fat is limited. On fat-suppressed T1 sequences (T1 fat-sat), the pericardium and surrounding fat tissue lose signal, allowing better assessment of cyst margins. No wall thickening or irregularity is identified.
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On T1-weighted MRI, the cystic lesion shows homogeneous hypointense signal with no evidence of hemorrhagic or proteinaceous content; consistent with simple serous fluid content.
On diffusion-weighted imaging (DWI), the pericardial cyst shows no signal at high b-values (b=800-1000) — T2 shine-through effect may be seen but ADC map values are high (>2.0 × 10⁻³ mm²/s). This finding indicates that water molecules within the cyst content have free diffusion and excludes true diffusion restriction. The absence of diffusion restriction distinguishes the pericardial cyst from epidermoid cyst (true diffusion restriction — keratin debris), abscess (diffusion restriction — viscous purulent content), and solid tumors (diffusion restriction — high cellularity). High ADC map values confirm that fluid molecules move freely with crystal clarity — this feature is consistent with the 'spring water' fluid nature.
Report Sentence
On diffusion-weighted imaging, no true diffusion restriction is identified in the cystic lesion (ADC: ___ × 10⁻³ mm²/s); simple fluid content is confirmed.
On gadolinium-enhanced MRI, no enhancement is seen in the pericardial cyst wall or content. Comparison of pre-contrast and post-contrast T1 sequences shows no significant change in signal intensity. Subtraction images (post-contrast T1 − pre-contrast T1) demonstrate no signal increase in the cyst area — this definitively confirms the avascular nature. Even in delayed phase (5-10 minutes), no enhancement is seen — this excludes the late enhancement that can be seen in fibrous-capsule lesions (such as schwannoma). The absence of enhancement aids in distinguishing pericardial cysts from thymic cysts (which may show thin wall enhancement), bronchogenic cysts (which may demonstrate wall enhancement when complicated), and inflammatory/neoplastic lesions of the pericardium. The pericardium itself may normally show mild enhancement — this should not be confused with the non-enhancement of the pericardial cyst wall.
Report Sentence
On gadolinium-enhanced MRI, no enhancement is identified in the cyst wall or content, with no signal increase demonstrated on subtraction images; avascular cystic nature is confirmed.
On transthoracic ultrasonography, the pericardial cyst is seen as an anechoic (echo-free), thin-walled, well-circumscribed cystic lesion. Posterior acoustic enhancement (through transmission) is classically seen — the passage of the sound beam through the low-attenuation fluid within the cyst creates increased echo intensity in distal structures. The cyst wall is smooth, thin, and echogenic; no wall irregularity or thickening is seen. No internal echoes, debris, septation, or solid component is present. The lesion is located adjacent to the pericardium and may show minimal movement with cardiac motion. US is ideal for real-time evaluation of pericardial cysts: monitoring compressibility and morphology changes during the cardiac cycle provides additional information not possible with CT/MRI. On echocardiography, it may be recognized as an anechoic structure adjacent to the right atrium or ventricle. A limitation of US is reduced image quality when deep mediastinal locations are masked by air-containing lung tissue.
Report Sentence
On US, a ___ x ___ mm anechoic, thin-walled cystic lesion with posterior acoustic enhancement is seen adjacent to the pericardium; consistent with pericardial cyst.
On color Doppler ultrasonography, no flow signal is detected within the pericardial cyst — the lesion is completely avascular. No Doppler signal is seen in the cyst wall either; this finding confirms the wall is avascular, thin fibrous/mesothelial tissue. Power Doppler examination also demonstrates no vascularity within or in the wall of the cyst — since power Doppler is more sensitive than color Doppler for low-flow vessels, this negative finding strongly supports avascularity. Normal flow signals are seen in adjacent cardiac structures (coronary arteries, the pericardium itself); the contrast between these normal vascular signals and the cyst's avascularity supports the diagnosis. Absence of internal Doppler signal is critically important in distinguishing pericardial cysts from solid mediastinal tumors (thymoma — vascularized) and vascular anomalies (coronary artery aneurysm).
Report Sentence
On color and power Doppler examination, no vascularity is identified within or in the wall of the cystic lesion; avascular cystic nature is confirmed.
On CT, no calcification, septation, or solid component is seen in the pericardial cyst — these 'negative findings' have diagnostic value. The absence of calcification distinguishes the lesion from mature cystic teratoma (calcification common), calcified granuloma, and pericardial calcification in constrictive pericarditis. The absence of septation (unilocular structure) is important: multilocular cystic lesions create a different differential diagnosis group (lymphatic malformation, cystic thymoma). The absence of solid component is critical in excluding neoplastic lesions (thymoma, teratoma, lymphoma). The combination of these three negative findings — no calcification + no septation + no solid component — strongly supports the diagnosis of simple mesothelial cyst. The cyst wall is thin (<2 mm) with smooth contours; focal wall thickening or nodularity is concerning for malignancy.
Report Sentence
On CT, no calcification, septation, or solid component is seen in the described cystic lesion; simple unilocular cyst nature is supported.
Criteria
Located at right cardiophrenic angle (70% of cases), typical round/oval morphology, water density (0-20 HU), thin wall (<2 mm), unilocular, no enhancement, broad-based contact with pericardium
Distinct Features
Most common form; may be seen on PA chest radiograph as a smooth, well-circumscribed round opacity at the right cardiophrenic angle — frequently confused with prominent epicardial fat pad or Morgagni hernia. Diagnosis is typically confirmed on incidental CT. Size usually ranges 3-8 cm and remains stable on annual follow-up.
Criteria
Located at left cardiophrenic angle (22% of cases), imaging features identical to right side (water density, thin wall, no enhancement), consider epicardial fat pad and pericardial recess in differential diagnosis on the left side
Distinct Features
Less common on the left side; consider left ventricular aneurysm, pericardial fat tissue, and left superior pericardial recess (especially on MRI) in differential diagnosis. Left pericardial cysts may sometimes be confused with left atrial appendage or left pleural fluid — coronal and sagittal reformats clarify this distinction.
Criteria
Located outside the cardiophrenic angle (8% of cases) — superior mediastinum, posterior mediastinum, or paracardiac region; imaging features are typical (water density, thin wall, no enhancement) but differential diagnosis broadens due to atypical location
Distinct Features
Most challenging form to diagnose; due to atypical location, bronchogenic cyst, enteric cyst, lymphatic malformation, thymic cyst, or neurenteric cyst should be considered in differential diagnosis. MRI simple fluid signal (T1 low, T2 markedly bright) and absence of enhancement support the diagnosis, but aspiration (spring water fluid) or surgical excision may be needed for definitive distinction. In the superior mediastinum, ectopic thyroid tissue and parathyroid adenoma should also be excluded.
Criteria
Change in cyst content density/signal (CT >20 HU, MRI T1 high signal), wall thickening (>2 mm), wall enhancement may be present; hemorrhagic cyst T1 hyperintense (met-hemoglobin), infected cyst may show diffusion restriction on DWI
Distinct Features
Rarely encountered; when complications develop, simple cyst criteria are not met and differential diagnosis becomes challenging — neoplastic lesions must be excluded. Hemorrhagic cyst may result from spontaneous bleeding or trauma; T1 hyperintense signal reflects met-hemoglobin accumulation. Infected cyst may present with fever and chest pain; diffusion restriction on DWI and wall enhancement may be seen. Management of complicated cysts may range from conservative to surgical — aspiration or excision may be needed.
Distinguishing Feature
Bronchogenic cyst may be T1 hyperintense on MRI (proteinaceous/mucinous content — T1 shortening), heterogeneous or moderately hyperintense on T2; pericardial cyst is T1 hypointense, T2 markedly hyperintense (simple fluid signal). On CT, bronchogenic cyst may show >20 HU density (proteinaceous content). Bronchogenic cysts are typically subcarinal or paratracheal in location — rare at the cardiophrenic angle.
Distinguishing Feature
Thymic cyst is located in the thymic bed in the anterior mediastinum and typically seen in young adults; pericardial cyst is located at the cardiophrenic angle. Thymic cyst may show thin wall enhancement; pericardial cyst shows no wall enhancement. Thymic cysts may be multilocular; pericardial cysts are unilocular. At aspiration, thymic cyst may contain cholesterol crystals; pericardial cyst contains clear spring water fluid.
Distinguishing Feature
Mediastinal mature teratoma (dermoid) on CT shows heterogeneous composition with fat density (-20 to -120 HU), calcification, soft tissue components, and fluid; pericardial cyst is a uniform cystic lesion with homogeneous water density (0-20 HU). Teratoma is typically located in the anterior mediastinum and may show fat-fluid level. On MRI, teratoma shows fat signal on T1 (signal drop on fat suppression), while pericardial cyst shows no fat signal.
Distinguishing Feature
Constrictive pericarditis is characterized by pericardial thickening (>4 mm) and calcification; pericardial cyst is a thin-walled cystic lesion. In constrictive pericarditis, the pericardium is diffusely thickened and rigid, while in pericardial cyst, the pericardium is normal and a focal cystic formation is seen. Heart failure findings (IVC dilation, ascites, hepatomegaly) accompany constrictive pericarditis; pericardial cyst is usually asymptomatic.
Distinguishing Feature
Pericardial fat pad (epicardial fat pad) shows fat density (-50 to -120 HU) on CT; pericardial cyst shows water density (0-20 HU) — definitive distinction is made by density measurement. On MRI, fat tissue is T1 hyperintense and loses signal on fat suppression; pericardial cyst is T1 hypointense and unaffected by fat suppression. Morgagni hernia may show omental fat or bowel content on CT; coronal reformat demonstrates the diaphragmatic defect.
Urgency
surveillanceManagement
conservativeBiopsy
Not NeededFollow-up
12-monthPericardial cysts are benign congenital lesions with no risk of malignant transformation. In asymptomatic cases with typical location (right cardiophrenic angle) and typical imaging features (water density, thin wall, no enhancement), no treatment is needed — conservative follow-up is sufficient. Follow-up CT or MRI at 12 months after initial diagnosis is recommended; after size stability is confirmed, longer intervals or cessation of follow-up may be appropriate. Biopsy/aspiration indications: diagnostic uncertainty (atypical location or atypical imaging features) or symptomatic cases (cardiac compression, dyspnea, chest pain). Spring water fluid obtained at aspiration confirms the diagnosis but recurrence risk is high (25-35%). Surgical excision (VATS — video-assisted thoracoscopic surgery) is curative for symptomatic or enlarging cysts. Rarely, sudden death cases have been reported — due to large cysts causing cardiac tamponade-like compression.
Pericardial cyst is a benign lesion that usually requires no treatment. Typical imaging findings (right cardiophrenic angle, water density, no enhancement) are diagnostic and biopsy is not needed. Periodic follow-up (annual CT or echocardiography) is recommended for large cysts. Percutaneous aspiration or surgical resection may be performed for symptomatic cysts (chest pain, dyspnea, compression). Recurrence rate after aspiration is high; definitive treatment is surgical excision. If atypical location or complex features are present, biopsy should be performed to exclude malignancy.