Can cardiac magnetic resonance imaging reliably ... - Springer Link

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benign and neoplastic fat? ... interatrial septum from other cardiac lesions involving abnormal fatty tissue ... nonencapsulated mature adipose tissue and en-.
The International Journal of Cardiovascular Imaging 18: 227–230, 2002. Ó 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Can cardiac magnetic resonance imaging reliably differentiate between benign and neoplastic fat? Yang Faridah A. Aziz & Paul R. Julsrud Department of Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, USA Received 3 July 2001; accepted in revised form 3 November 2001

Key words: interatrial septum, lipoma, lipomatous hypertrophy, liposarcoma, magnetic resonance imaging

Abstract Magnetic resonance imaging is currently frequently used to differentiate lipomatous hypertrophy of the interatrial septum from other cardiac lesions involving abnormal fatty tissue including lipomatous neoplasms. This report discusses potential problems with this approach by emphasizing the variable appearance of lipomatous hypertrophy of the interatrial septum on magnetic resonance imaging.

Introduction Lipomatous hypertrophy of the interatrial septum (LHIS) is characterized by an accumulation of nonencapsulated mature adipose tissue and enlarged cardiac myocytes. First described by Prior in 1964 [1], this entity was initially diagnosed primarily at autopsy as an incidental finding. Due to advances in echocardiography, computed tomography (CT) and cardiac magnetic resonance imaging (MRI), it is purported that LHIS can now be diagnosed noninvasively [2, 3]. However, the appearance of this entity can vary greatly which in turn can create difficulties for these imaging techniques. Examples of the different appearance of LHIS are presented to demonstrate the difficulty of tissue characterization in certain instances by cardiac MRI.

Magnetic resonance images In a random series of 50 hearts, the normal fat thickness in the interatrial septum ranged from 2 to 7 mm depending on age [4] (Figure 1). Some consider a thickness of the atrial septum cephalic

to the fossa ovalis measuring more than 2 cm to be diagnostic of lipomatous hypertrophy [5]. Echocardiography, CT and MRI have all been purported as capable of making the diagnosis of LHIS. The fatty nature of LHIS is demonstrated as an echo-dense mass on echocardiography. On CT, LHIS is described as a nonenhancing smoothly marginated homogenous mass of fat attenuation confined to the interatrial septum [6]. On MRI, LHIS appears as a thickening in the interatrial septum with signal characteristics similar to subcutaneous fat [2, 3, 7]. Sparing of the fossa ovalis, as described by Page in pathological specimen [4], is well depicted by MRI [7] (Figures 2 and 3). Due to sparing of the fossa ovalis, a bilobed or ‘camelhumped’ appearance is described in severe cases of LHIS [3] (Figure 4). Fat-suppression MRI imaging has been described as being helpful since it permits differentiation of LHIS from other high signal intensity cardiac lesions in this region; such as atrial myxomas, thrombi and hemangioma [3, 7] (Figure 5). LHIS has been reported to be as large as 6 cm in size [5]. Difficulty may arise in differentiating LHIS from lipomatous neoplasms with similar MRI characteristics, such as lipoma and liposarcoma

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Figure 1. Axial T1-weighted spin-echo image of the interatrial septum. Note the normal thickness of interatrial septum (arrowheads).

Figure 3. Axial T1-weighted spin-echo image of the interatrial septum at the level of fossa ovalis demostrating marked LHIS with sparing of the fossa ovalis. Note the increased pericardial fat also seen in this patient.

Figure 2. Axial T1-weighted spin-echo image of the interatrial septum. Note the high signal intensity ‘mass’ (arrowheads) in the interatrial septum which has similar signal intensity to subcutaneous fat. Sparing of the fossa ovalis (short arrow) is characteristic of LHIS. Note increased pericardial and epicardial fat in this patient (long arrow).

Figure 4. Axial T1-weighted spin-echo image of the interatrial septum at the level of fossa ovalis in a patient with LHIS demonstrating ‘camel-humped’ appearance which is characteristic of this entity (arrow).

(Figure 6). Figure 7 illustrates the MRI findings in a patient with an extensive amount of fat in the interatrial septum. The inhomogenous appearance raised the question of a neoplastic process. Due to this concern the patient underwent surgery with excision of the fatty mass. On initial gross inspection the mass was thought to be a lipoma. However on histologic examination the diagnosis of LHIS was established due to absence of a cap-

sule, the presence of cardiac myocytes and the lack of mitotic activity.

Discussion The term ‘lipomatous hypertrophy of the interatrial septum’ does not describe the true pathological process of this entity. Microscopically, the characteristic feature is a mixture of multi-vacuolated fat cells and cardiac myocytes. In most cases,

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Figure 5. Fat-suppression image at the similar location in the same patient illustrated in Figure 3 showing complete signal suppression of this lesion and the surrounding subcutaneous fat.

Figure 6. Axial T1-weighted spin-echo image through the midportion of the heart demonstrating a large lipoma (arrow) of the interatrial septum. Note that the mass has the same signal intensity as the subcutaneous fat.

the interspersed cardiac myocytes are greatly enlarged. The increase in mass in the atrial septum is due to an increased number of fat cells rather than hypertrophy of individual cells [8, 9]. Other terms such as ‘massive fatty deposits of the atrial septum’ have been suggested [5]. The cause of LHIS is unknown. There is no sex predilection and associations with advanced age, obesity and cardiomegaly have been documented [5]. The amount of fat in the interatrial septum correlates closely with the amount of epicardial fat

Figure 7. Axial T1-weighted spin-echo image of the interatrial septum in a patient with LHIS (arrow). Note the inhomogeneity within this lesion which is attributed to the extensive presence of enlarged myocytes which was found on histologic examination

present [4]. In one CT series, increased epicardial fat was seen in 83% of patients with LHIS [6]. LHIS, although considered a benign process may be associated with atrial arrhythmia, coronary artery artherosclerosis, congestive heart failure and sudden death [5, 9]. The incidence of atrial arrhythmia increases with increasing thickness of fat in the septum [5]. Since it is crucial to distinguish LHIS from other neoplastic processes to avoid unnecessary surgery, imaging modalities such as MRI, are being used to noninvasively diagnose this condition [10]. However, histopathological diagnosis via needle or incisional biopsy may be necessary to achieve a definitive diagnosis. LHIS, unlike lipoma, lacks a capsule and although entrapped myocytes may be present in a lipoma these are usually seen at the base of the tumor [8]. The presence of multi-vacuolated fat cells in LHIS and enlarged hypertrophied myocytes may be confused with liposarcoma. However lack of mitosis and absence of other features of neoplasm establishes the diagnosis of LHIS [9].

Conclusion LHIS is characterized by accumulation of adipocytes and cardiac myocytes within the interatrial septum and typically has a homogenous appearance

230 on MRI. However, depending on the amount and distribution of myocytes present, certain cases of LHIS may appear inhomogenous. An inhomogenous fatty mass raises the concern of lipomatous neoplasm. We suggest that in this situation MRI cannot reliably exclude a neoplastic process.

References 1. Prior JT. Lipomatous hypertrophy of the cardiac interatrial septum: a lesion resembling hibernoma, lipo-blastomatosis and infiltrating lipoma. Arch Pathol 1964; 78: 11–15. 2. Applegate PM, Tajik AJ, Ehman RL, Julsrud PR, Miller FA. Two-dimensional echocardiographic and magnetic resonance imaging observations in massive lipomatous hypertrophy of the atrial septum. Am J Cardiol 1987; 59: 489– 491. 3. Fisher MS, Edmonds PR. Lipomatous hrpertrophy of the interatrial septum: diagnosis by magnetic resonance imaging. J Comput Tomogr 1988; 12: 267–269. 4. Page DL. Lipomatous hypertrophy of the cardiac interatrial septum: its development and probable clinical significance. Hum Pathol 1970; 1: 151–163.

5. Shirani J, Roberts WC. Clinical, electrocardiographic and morphologic features of massive fatty deposits (lipomatous hypertrophy) in the atrial septum. J Am Coll Cardiol 1993; 22: 226–238. 6. Meaney JF, Kazerooni EA, Jamadar DA, Korobkin M. CT appearance of lipomatous hypertrophy of the interatrial septum. AJR 1997; 168: 1081–1084. 7. Mortele KJ, Mergo PJ, Williams WF. Lipomatous hypertrophy of the atrial septum: diagnosis with fat suppressed MR imaging. J Reson Imaging 1998; 8: 1172–1174. 8. Burke A, Virmani R. Tumors of the heart and great vessels. In: Atlas of Tumor Pathology. Fasc 16, Ser 3. Washington DC: Armed Forces Institute of Pathology, 1996; 91–98. 9. Burke A, Litovsky S, Virmani R. Lipomatous hypertrophy of the atrial septum presenting as a right atrial mass. Am J Surg Pathol 1996; 20(6): 678–685. 10. Zeebregts CJ, Hensens AG, Timmermans J, Pruszczynski MS, Lacquet LK. Lipomatous hypertrophy of the interatrial septum: indication for surgery? Eur J Cardiothorac Surg 1997; 11: 785–787. Address for correspondence: Paul R. Julsrud, Department of Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, USA Tel.: þ507-2552505; Fax: þ507-2557872; E-mail: [email protected]