Thoracic Imaging 2003 - Society of Thoracic Radiology

Imaging of the Pericardium
Paul L. Molina, M.D.
Professor of Radiology, Director, Radiology Residency Training Program, Vice Chairman of Education, University of
North Carolina, Chapel Hill, North Carolina
Objectives
1. Review the normal appearance of the pericardium, including
the pericardial recesses, on both CT and MRI.
2. Learn the major clinical applications of CT and MRI in the
evaluation of patients with suspected pericardial disease.
3. Review the CT and MRI findings in a variety of pericardial
disease processes.
Introduction
Computed tomography (CT) is an established technique for
evaluation of the pericardium and is generally considered complimentary
to echocardiography in the assessment of complicated
pericardial effusions, pericardial thickening, calcific pericarditis,
pericardial masses and congenital anomalies. Magnetic
resonance imaging (MRI) is also useful in the evaluation of
pericardial disease. Advantages of MRI include its potential for
tissue characterization, absence of ionizing radiation and need
for intravenous contrast medium, and its ability to scan in any
plane. These potential advantages, however, are at times outweighed
by the disadvantages of higher cost, longer examination
time, and inability to accurately identify calcification. Also,
it may be difficult to adequately examine patients with arrhythmias
because of the need for cardiac gating of MR studies of
the pericardium.
Normal Anatomy
At least a portion of the normal pericardium, a double-layered
fibroserous sac enveloping the heart and origin of the great
vessels, is visible on CT and MRI in almost all patients. The
normal thickness of the pericardium on CT or MRI is 1-2 mm.
On CT it appears as a thin curvilinear density and on MR as a
low signal intensity line. The decreased signal intensity of the
pericardium is probably due to fluid flow during the cardiac
cycle. The caudal half of the pericardium is commonly
imaged, especially those portions overlying the anterolateral
cardiac surface where the pericardium is surrounded by fat in
the mediastinum and in the subepicardial region of the heart.
The most distal portion of the pericardium, just before its insertion
into the central tendon of the diaphragm and in front of the
inferior surface of the right ventricle, may measure up to 3-4
mm in thickness due to accumulation of small physiological
amounts of pericardial fluid. The dorsal aspect and the cephalad
portion of the pericardium are infrequently seen because of the
lack of sufficient surrounding fat in these areas.
The pericardial cavity contains several recesses around the
heart where normally small amounts of fluid can collect. An
understanding of these recesses is important in order not to confuse
fluid within them with mediastinal masses or enlarged
lymph nodes.
Pericardial Effusion
Echocardiography currently remains the primary means of
evaluating a patient for pericardial effusion. Its major advantages
include the ease of the examination and the portability of
the equipment, along with the absence of exposure to ionizing
radiation. Both CT and MRI are also useful for establishing the
diagnosis of pericardial effusion and are usually reserved for
those patients who have technically inadequate sonographic
studies or in whom there is a discrepancy between clinical and
echocardiographic findings.
A pericardial effusion generally is recognizable on CT as an
increase in thickness of the normal band-like pericardium. Most
commonly, the fluid has a near-water density value and represents
a transudate. Near soft-tissue density collections may
occur with an exudative or purulent effusion or a hemopericardium.
Blood in the pericardium may be isodense with the cardiac
muscle and therefore administration of intravenous contrast
media may be required for diagnosis, demonstrating enhancement
of the myocardium with no alteration of the surrounding
hemopericardium. On MRI a simple pericardial effusion usually
is characterized by low signal intensity on short TR/TE
images; its signal intensity increases on longer TR or longer TE
images. Hemorrhagic pericardial effusions often contain areas
of mixed, medium and high signal intensity that may vary with
the age of the effusion.
On supine CT and MRI scans, small pericardial effusions
usually collect dorsal to the left ventricle and behind the left lateral
aspect of the left atrium. Larger effusions extend ventrally
in front of the right ventricle and right atrium. In massive pericardial
effusions, the heart appears to float within the distended
pericardial sac and fluid extends cephalad to surround the origin
of the great vessels. The over-distended pericardium may project
caudally and compress the diaphragm and upper abdominal
organs. Encapsulated pericardial effusions can occur when
fibrous adhesions seal off portions of the pericardial space; dorsal
and right anterolateral loculations are most common.
Occasionally the encapsulated fluid will bulge toward the heart
and can result in the hemodynamics of cardiac tamponade or
constrictive pericarditis. With inflammatory pericarditis and
effusion, the pericardium itself may enhance following the intravenous
administration of contrast media.
Pericardial Thickening
The pericardium can respond to injury by fibrin production
and cellular proliferation in addition to fluid output. All three
mechanisms can occur concomitantly or independently.
Pericardial thickening may result from proliferation of fibrin
deposits or organized blood products or through neoplastic invasion.
Pericardial thickening from 0.5 to 2.0 cm or greater can
occur and may be focal or involve the entire pericardium.
Generally, the maximal thickening occurs ventrally. The thickened
pericardium usually is smooth but can be nodular in neoplastic
disease. Distinction from a small exudative or bloody
pericardial effusion may be difficult.
Constrictive Pericarditis Versus Restrictive
Cardiomyopathy
CT and MRI have shown value in differentiating constrictive
pericarditis from restrictive or infiltrative cardiomyopathy (e.g.,
amyloidosis). This distinction is often quite difficult clinically,
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