Thoracic Imaging 2003 - Society of Thoracic Radiology

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rare, and consist largely of sarcomatous lesions, particularly
angiosarcoma.
2) Pericardial Disease
The pericardium is well-visualized on spin-echo images as a
dark-signal intensity structure that is highlighted by the bright
signal intensity of the surrounding epicardial and mediastinal
fat. The normal pericardium measures no more than 3 mm in
width. A simple (transudative) pericardial effusion appears as
widening of the pericardium with maintenance of the low signal
intensity on T1-weighted images. It is particularly well-seen
anterior to the heart and lateral to the left ventricle. Pericardial
fluid usually appears bright on T2-weighted gradient-echo
images. Hemorrhagic or exudative fluid may show medium or
high signal intensity on T1-weighted images.
Echocardiography is the primary technique used to detect
pericardial effusions but is less optimal in defining pericardial
thickening due to constriction. Pericardial constriction has a
similar clinical and hemodynamic profile to restrictive cardiomyopathy.
Moreover, therapeutically there is an important
distinction between restrictive cardiomyopathy, in which medical
treatment is directed at the underlying cause, and pericardial
constriction, in which surgical stripping is often performed.
MR imaging is the technique of choice to make this distinction.
The finding of a thickened pericardium confirms the diagnosis
of pericardial constriction. MR imaging is also useful to diagnose
congenital absence of the pericardium.
3) Valvular and Ischemic Heart Disease
Echocardiography remains the principal technique to evaluate
valvular disease. Both the valve diameter and estimate of
the gradient across the valve can be measured with echocardiography.
On MRI, it is also possible to detect valvular stenosis or
regurgitation on gated gradient-echo images (cine MRI). Using
this bright blood sequence, stenosis or regurgitation is identified
as a plume of low-signal intensity that emanates from the valve
in the direction of blood flow during the appropriate part of the
cardiac cycle. The low-signal intensity is caused by dephasing
that occurs in the turbulent jet. Some physiologic low signal
intensity may be observed in otherwise normal patients but it is
usually less extensive and of shorter duration. Valvular vegetations
are difficult to identify but perivalvular abscess or septic
pseudoaneurysm is more readily shown on MRI.
Most ischemic heart disease is evaluated by echocardiography,
angiocardiography and nuclear cardiography, although the
role of MRI is increasing rapidly. From the standpoint of morphology,
MRI is occasionally used to distinguish between true
and false aneurysms. True aneurysms involve all three layers of
the heart, and are typically apicolateral. False aneurysms traverse
all three layers with containment by the pericardium or
mediastinal structures. A posteroinferior location is most common.
A false aneurysm requires urgent surgical repair to prevent
free rupture. The key to differentiation of the two entities on
MRI is evaluation of the neck of the aneurysm. True aneurysms
are wide-mouthed whereas false aneurysms are narrowmouthed.
4) Cardiomyopathies and Dysplasias
Three major types of cardiomyopathy are recognized: hypertrophic,
dilated and restrictive. Most patients with cardiomyopathy
undergo echocardiography and/or angiocardiography and
MR imaging is used in a secondary role. The most common
type of hypertrophic cardiomyopathy is characterized by asymmetric
thickening of the septum in comparison with the lateral
wall of the left ventricle. There is (paradoxical) systolic anterior
motion (SAM) of the mitral valve that may contribute to the
outflow obstruction in the subaortic region. MRI findings show
striking thickening of the septum and, often, lateral wall of the
left ventricle with nearly complete obliteration of the ventricular
cavity during systole. MRI can also demonstrate the turbulent
flow in left ventricular outflow track and SAM of the mitral
valve.
Dilated cardiomyopathy is caused by ischemia and a variety
of toxic and infectious agents that lead to severe compromise of
the left ventricular ejection fraction. MRI typically shows a
markedly dilated left ventricle that is hypokinetic. Restrictive
cardiomyopathy is due to several infiltrative diseases (amyloidosis,
sarcoidosis, hemochromatosis) that impair diastolic filling.
On MRI, it appears as slight myocardial thickening with relatively
preserved myocardial function. The main role of MRI is
to distinguish it from pericardial constriction.
A condition for which MR imaging has proved to be a primary
technique is arrhythmogenic right ventricular dysplasia
(ARVD). ARVD is an abnormality that occurs predominantly in
adolescents and young and may cause ventricular tachycardia or
sudden death. Pathologically, it caused by replacement of the
right ventricular free wall myocardium by fat or fibrosis. MRI
findings in this condition include areas of fatty replacement and
marked thinning of the free wall. Cine imaging may reveal
abnormalities of wall motion.
5) Functional Cardiac Imaging
A further indication of the versatility of cardiac MR imaging
is its ability to provide functional information about cardiac status.
Using cine imaging, wall motion abnormalities due to
ischemia or primary myocardial disorders can be assessed. Left
ventricular ejection fraction (end diastolic volume - end systolic
volume/end diastolic volume) can be calculated by acquiring
multiphase images contiguously through the heart along its
short axis. The end-diastolic and end-systolic volumes can be
determined for each section allowing determination of a global
left ventricular ejection. Alternatively, the ejection fraction can
be estimated from a single long axis image. As noted above,
valvular disease can be detected with gradient-echo imaging.
The severity of the lesion correlates qualitatively with the extent
of loss of signal that emanates from the valve. Another capability
of MR imaging is to calculate shunting in patients with septal
lesions. This measurement is obtained using a phase encoded
gradient echo sequence to measure blood flow in the aorta
and pulmonary artery. The proportion of flow in the two vessels
indicates the amount of shunting (i.e. a left-to-right shunt
demonstrates more flow in the pulmonary artery).
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