The pericardium (from the Greek περί, “around”, and κάρδιον, “heart”) is a double-walled sac containing the heart and the roots of the great vessels. The pericardium is the external layer of the heart providing protection and support to inner structures. It is composed by and external fibroserosal part (parietal pericardium) and an internal serosal part (visceral pericardium) (Fig. 1.1). The internal serosal is also named epicardium, which has direct connection with the myocardium. The parietal pericardium has an inner serosal part that is in continuity with the epicardium and an external fibrous part (Fig. 1.2).

Between the visceral and parietal pericardial layers, a virtual pericardial cavity is filled by 20–30 ml of plasma ultrafiltrate (pericardial fluid) that acts as a lubricant allowing myocardial contraction without attrition with the surrounding anatomical structures [1, 2]. The pericardial fluid is produced by the serosal part of the pericardium which is provided by mesothelial cells with microvilla and cilia that further expand the available pericardial surface. The reflection of the visceral pericardium into the parietal pericardium over the great vessels is responsible for the creation of spaces, where pericardial fluid can accumulate and they can be seen on imaging. Greater spaces are called sinuses, while smaller spaces between adjacent anatomic structures are called recesses. The main sinuses include the transverse sinus located between the aorta and pulmonary trunk anteriorly and the atria and veins posteriorly and the oblique sinus located behind the left atrium and between the pulmonary veins and the inferior vena cava (Fig. 1.3) [3, 4].

Both sinuses may be accessed for electrophysiology purposes for ablation of cardiac arrhythmias. Fat is present under the epicardium (epicardial fat) and in connection with the parietal pericardium. Fat tissue provides mechanical, immunological protection of the heart, as well as a source of fatty acids and thus energy and may also have endocrine functions by cytokines that act through paracrine mechanisms on myocardial and endocardial cells [5].

The pericardium is fixed to the surrounding anatomical structures by ligaments: anteriorly to sternum by the sternopericardial ligaments, posteriorly to the vertebral column and inferiorly to the diaphragm.

The normal thickness of the pericardium is less than 3 mm in imaging studies from clinical practice. In imaging the measure is overestimated and autoptic and surgical studies provide an anatomical size of less than 1–2 mm [4, 6].