Normal MRI findings
On axial T1-weighted images (WI), the unaltered pericardium is visualized as a thin dark line. This structure stands out with high contrast because it is located strictly between two zones of high MR signal intensity — the prepericardial and subepicardial adipose tissue.
A minimal, physiological amount of fluid may normally be detected between the outer fibrous and inner serous layers. It accumulates in natural depressions — recesses (recessus).
Topography and shape of recesses
Knowledge of the exact localization of pericardial recesses is critical for a practicing physician. There are two main spaces that require special attention:
- Superior pericardial recess. Originates from the transverse sinus (sinus transversus pericardii) and curves around the right wall of the ascending aorta. It has a complex three-dimensional structure:
- Anterior extension: wedges between the ascending aorta and the pulmonary trunk (truncus pulmonalis), acquiring a characteristic triangular shape on axial slices.
- Posterior extension: localized directly behind the ascending aorta and appears as a crescent.
- Right lateral part: descends between the ascending aorta and the superior vena cava.
- Oblique pericardial sinus of the pericardium (Oblique pericardial sinus). Represents the most dorsally located space of the cavity. It is localized posterior to the left atrial wall.
Imaging visualization and pitfalls
On computed tomography (CT) scans, recesses are discernible as sharply delineated structures in nearly 44% of cases. However, their presence frequently provokes overdiagnosis and medical errors.
- Due to its complex shape, the superior recess can mimic: aortic dissection, mediastinal tumors, thymic hyperplasia, or enlarged lymph nodes.
- Fluid accumulation in the oblique sinus (sinus obliquus pericardii) is misconstrued as a bronchogenic cyst or an esophageal pathological process.
The use of Cine-MRI allows resolving these diagnostic doubts, reliably differentiating normal anatomical spaces from true anomalies and pathological masses.
Assessment of pericardial thickening
The signal intensity of thickened leaflets in non-ischemic acquired heart diseases is highly variable and directly depends on the activity of the inflammatory process.
| Inflammation stage | MRI signal intensity |
|---|---|
| Chronic fibrous | Low (on T1- and T2-WI) |
| Subacute | Intermediate or high |
In the subacute phase, the high signal from inflamed tissues merges with the characteristics of liquid effusion. Special sequences are used for accurate evaluation:
- Late enhancement: used in acute conditions, clearly demarcates inflamed leaflets and fluid effusion.
- Fat saturation: indispensable in myopericarditis to separate the area of contrast enhancement in the myocardium from the hyperintense pericardium.
Clinical patterns: effusion and hematoma
Let us examine imaging semiotics using two classic examples.
1. Pericarditis with effusion On T1-WI in the short axis, fluid accumulation appears dark (no signal), while moderate leaflet thickening is noted. On Cine-MRI, this same fluid produces a hyperintense (bright) signal. This is explained by the features of the SSFP sequence (steady-state free precession), where brightness depends on the ratio of T2 to T1 relaxation times.
2. Pericardial hematoma (complication of endomyocardial biopsy) Fresh blood has specific patterns. On T1-WI (in the horizontal long-axis projection) near the right atrium, localized fluid with a high signal is visible. On fat-suppressed T2-WI (in the short axis), this focus also remains hyperintense, which is a reliable marker of acute hemorrhage.
Briefly: answers to topical questions
What anatomical layers and leaflets is the pericardium composed of?
The pericardium consists of two main layers: the outer fibrous and the inner serous layer.
- Fibrous pericardium (pericardium fibrosum) is the outer closed sac transitioning onto the walls of large vessels.
- Serous pericardium (pericardium serosum) is the inner layer divided into two plates:
- Parietal layer (lamina parietalis) lines the fibrous pericardium from the inside.
- Visceral layer (lamina visceralis), or epicardium, covers the heart and consists of mesothelium and a thin connective tissue plate.
What anatomical structures bound the transverse pericardial sinus anteriorly and posteriorly?
The transverse pericardial sinus is located posterior to the ascending aorta. The cited sources do not specify the structures bounding it anteriorly and posteriorly.
What is the normal thickness of the pericardium in millimeters when measured on a computed tomography scan?
The normal thickness of the pericardium measured on computed tomography (CT) and magnetic resonance imaging (MRI) is less than 2 millimeters. Normally, it is visualized as a thin line, and the fluid volume in the pericardial cavity does not exceed 50 ml. Pericardial thickening up to 4 mm or more is the primary diagnostic criterion for constrictive pericarditis.
Which arteries supply blood to the pericardium?
Blood supply to the pericardium is provided by branches of the internal thoracic and bronchial arteries.
- Pericardiacophrenic arteries (aa. pericardiacophrenicae) originate from the internal thoracic arteries (aa. thoracicae internae) at the level of the 1st rib, accompany the phrenic nerves, and provide blood flow to the pericardium.
- Bronchial arteries (arteriae bronchiales) — in addition to the bronchial tree and lung parenchyma, supply oxygen to part of the pericardium and visceral pleura.
Remember the shape of the superior recess extensions: Anterior is anterior to the trunk (triangular), while Posterior is behind the aorta (shaped like a crescent).
Class and exam questions
How to distinguish inflamed pericardium from subacute effusion on basic MRI sequences?
Visually doing this is practically impossible, as both effusion and subacute inflammation can produce a high signal. To accurately differentiate them, late contrast enhancement sequences must be used.
What pathologies are most frequently mistaken for the superior pericardial recess?
Due to its location, it can mimic ascending aortic dissection, thymic pathology, an enlarged lymph node, or another anterior mediastinal mass.
What does an acute pericardial hematoma look like on MRI scans?
Fresh hemorrhage demonstrates a high (bright) signal on both T1-weighted images and fat-suppressed T2-WIs.
What else to study on this topic
- Physical principles of signal formation in steady-state free precession (SSFP / Cine-MRI).
- Detailed anatomy of the transverse pericardial sinus and its connections with great vessels.
- Specific applications of the Fat saturation mode for differential diagnosis of myocarditis and pericarditis.
- MRI semiotics of chronic constrictive pericarditis (fibrotic changes).
Master the entire topic in the "Diagnostic Radiology" course
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- Complete study guide on "Radiological Anatomy of the Pericardium" and 2,199 more pages of the subject
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