| Abstract|| |
Diseases of the pericardium are an uncommon but important group of diseases in cardiology with several different pathologies. The current mainstay of cardiac imaging, echocardiography has some limitations in visualizing pericardial and mediastinal disease. Recent improvement in cardiac MRI allows new insight into imaging the pericardium and the pericardial space. This article describes different pathologies, including pericardial constriction, masses and congenital absence of the pericardium. CMR protocols and techniques are discussed including the use of different sequences, along with the use of contrast media and real time imaging for ventricular:ventricular interaction. The role of CMR is placed in context with other imaging modalities.
Keywords: Cardiac, MRI, pericardium
|How to cite this article:|
Westwood MA, Moon JC. Cardiovascular magnetic resonance for pericardial disease. Indian J Radiol Imaging 2007;17:133-6
| Introduction|| |
Recent advances in cardiovascular MRI (CMR) including the use of contrast have advanced our understanding and management of cardiology. CMR combines three key features: good visualisation of anatomy/morphology, high quality functional assessment (static, during dynamic manoeuvres or with stress) and the ability to distinguish normal and abnormal tissue, according to different magnetic properties, either before or after contrast. These qualities make CMR particularly suited for diseases of the myocardium; CMR also provides insight into diseases of the pericardium.  In particular, CMR can help with the diagnosis of constrictive pericarditis (CP) and its differentiation from restrictive cardiomyopathy (RCMP), which can at times be particularly difficult. There are a variety of diseases of the pericardium, which will be explored below:
Congenital absence/partial absence of pericardium
The pericardium plays an important role in thoracic development. It is thought that premature atrophy of the left duct of Cuvier (one of the common cardinal veins) leading to hypoplasia of the left pleuropericardial membrane results in complete or, more commonly, partial absence of the pericardium.  The condition is typically sporadic but may occasionally be familial.  If isolated rather than in association with other abnormalities, the majority of patients are asymptomatic. However the result is displacement of the heart into the left hemithorax. Typically, symptoms are minimal, often with non-specific chest pain, possibly due to the greater mobility of the heart. There is a possibility of strangulation of the heart through defects in partial absence. Investigations in these patients may result in poor echocardiographic windows, electrocardiographic (ECG) and chest radiographic (CXR) abnormalities, which may lead to erroneous diagnoses in some cases.
CMR easily detects abnormalities of cardiac position in the chest, [Figure - 1] and will detect pericardial defects, particularly if the pericardium is well outlined by layers of pericardial fat.
Pericardial thickening and constriction
Constrictive pericarditis was initially described by White in 1935  with surgical treatment emerging later in the 20 th century. At the time, the predominant cause of CP was tuberculosis but other causes are now prevalent [Table - 1]. Several investigative modalities can be used to try to distinguish RCMP from CP but none of them is sufficiently reliable to be used in isolation [Table - 2]. Even classical hemodynamic findings such as a raised RVEPD and diastolic pressure equalization have a predictive accuracy of only 85% only,  which may not give sufficient confidence to embark on high mortality surgical treatment.
Pericardial anatomy and thickening are well identified by CMR. Thickening may not be uniform and sometimes may be at the upper limit of normal of the reference range (3 mm), in which case, the signal intensity may aid in the diagnosis of a pericardial abnormality, [Figure - 2]. Pericardial calcification may reduce signal returned from the pericardium; pericardial calcification is better visualised on CT scans or even CXR. Multiple orthogonal planes may help define the exact extent of the pericardial disease. CMR may aid in surgical planning and also allow an estimate of the risk, the presence of a fat plane between the pericardium and myocardium indicating an easier dissection in that area, [Figure - 3]. Myocardial tagging may demonstrate the lack of pericardial 'slippage' over the underlying myocardium. Other scan findings may also contribute to diagnostic accuracy. The presence of pleural effusions and dilated venous structures (SVC and IVC) support constriction, whilst findings such as enlarged pulmonary arteries or left ventricular hypertrophy may suggest different causes. In particular, certain restrictive cardiomyopathies such as amyloidosis  and inherited cardiomyopathies with restriction, may be well seen. 
CMR may also document the functional changes of constriction. On a rest scan, a typical double bounce of the ventricular:ventricular interaction may be seen. This appearance can also be seen with echocardiography. Whilst other causes may give superficially similar appearances, such as bundle branch block and pulmonary hypertension, particular care must be taken in CMR when prospective gating is used because if the whole cardiac cycle is not covered, diastolic movement may give rise to artifactual appearances similar to ventricular:ventricular interaction. Retrospective cardiac gating is thus preferable. The high blood:myocardial contrast of the images is an advantage with CMR, but CMR cannot match echocardiography for some diastolic phenomena, particularly short time interval ones within mitral inflow patterns, iso-volumetric intervals  or tissue Doppler.  Similarly, hemodynamic changes are simply better discerned using cardiac catheterization rather than with the noninvasive modalities, but echo/CMR combinations make angiography as the hemodynamic gold standard less important in the work-up of some patients. CMR may have an advantage in assessing dynamic ventricular:ventricular interaction with respiration because there are no problems with imaging windows. A short axis, mid-myocardial real time acquisition is obtained over about six seconds, as the patient is instructed to slowly breathe deeply in and out. In inspiration, lowered thoracic pressure encourages venous return preferentially to the right side of the heart and RV filling increases. If there is pericardial constriction, this inevitably results in reduced left sided ventricular filling and the LV septum flattens, the LV becoming D-shaped. This abnormality reverses on expiration, [Figure - 4]. This technique is now recommended within a standardised CMR protocol for the pericardium. 
Pericardial cysts/masses and thrombus
CMR provides high spatial resolution with tissue characterisation yielding valuable information for distinguishing benign from malignant pericardial lesions. Masses, typically detected on standard views have their impact on normal cardiac function and are then interrogated with multiple different sequences and views encompassing the bulk of the mass. Sequences may include cine SSFP imaging, T1W turbo spin-echo, with and without fat suppression (fat saturation) and T2 weighting. Additional contrast may be added and perfusion, early and late enhancement assessed. Using these techniques, the presence in the lesion of fat, fluid or cystic components and vascularity can usually be determined [Table - 3]. The properties of the lesion can be compared to the myocardium, pericardial fat, the pericardium or any other mediastinal or parenchymal lung lesions that may be present. Malignant behavior such as breaching natural tissue planes, tracking along the pericardium or satellite lesions may be detected [Figure - 5]. Common nonmalignant pericardial masses where CMR characterisation aids clinical management include localised pericardial fat, hiatus hernias, old ventricular rupture (pseudo-aneurysm), thrombosed coronary artery vein graft aneurysms, postsurgical pericardial thrombus and pericardial cysts.
Echocardiography is of course, excellent at detecting pericardial effusions. CMR may complement it, when it is not clear whether there is a large amount of fat, which may occur with steroid therapy or in lipomatous hypertrophy of the interatrial septum rather than fluid or a mass. Fibrous or loculated appearances are easily detected and features of tamponade similar to those of constriction may be detected. However, trans-mitral flow variation with respiration is better assessed by echocardiography.
| Conclusions|| |
CMR is an important tool in the work-up of patients with suspected pericardial disease. With a variety of different techniques used within one scan to interrogate structure, function and composition of the pericardium and associated structures, it helps delineate the cause, define the extent of the pericardial disease and may assist in the timing and necessity of surgical intervention for pericardial disease. This role of CMR is likely to mature as further assessment techniques for CMR become available and access becomes more widespread.
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James C Moon
Department of Cardiology, The Heart Hospital, 16-18 Westmoreland Street, London W1G 8PH.
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5]
[Table - 1], [Table - 2], [Table - 3]