AJR 2005; 184:339-342
© American Roentgen Ray Society
Lipomatous Hypertrophy of the Interatrial Septum: Increased Uptake on FDG PET
Chieh-Min Fan1,
Alan J. Fischman1,
Boon Han Kwek1,2,
Suhny Abbara1 and
Suzanne L. Aquino1
1 Department of Radiology, GRB-290, Massachusetts General Hospital, 55 Fruit
St., Boston, MA 02114.
2 Present address: Department of Radiology, Singapore General Hospital,
Singapore.
Received February 17, 2004;
accepted after revision May 14, 2004.
C-M Fan and S. L. Aquino have each made substantial contributions to the
design and analysis of this work and the writing of the article. Each of them
is equally responsible for the integrity of all data in the article and for
the correctness of the analysis and conclusions supported by those data.
Address correspondence to S. L. Aquino.
Abstract
OBJECTIVE. The objective of our study was to evaluate FDG uptake in
patients with cardiac lipomatous hypertrophy of the interatrial septum
(LHIS).
CONCLUSION. Increased FDG uptake occurs in LHIS, a benign fatty
infiltration of the interatrial septum. Increased uptake in the right heart on
FDG PET warrants correlation with additional imaging to assess for LHIS and
avoid false interpretation of malignancy.
Introduction
Lipomatous hypertrophy of the interatrial septum (LHIS) is a relatively
uncommon disorder of the heart characterized by benign fatty infiltration of
the interatrial septum that usually spares the fossa ovalis. The prevalence of
LHIS is estimated to be between 1% and 8%
[1]. Microscopically, LHIS is
characterized by fat infiltration between the myocardial fibers of the atrial
septum. Although histologically benign, LHIS has been associated with adverse
clinical sequelae including supraventricular arrhythmias, syncope, and sudden
death
[2–6].
LHIS also can create a masslike bulge mimicking fat-containing neoplasms that
can arise in the atrial septum, including rhabdomyomas, myxomas,
rhabdomyosarcomas, and liposarcomas. The CT, echocardiography, and MRI
characteristics of LHIS have been described in the literature
[7–9],
but to date no reports describe its appearance on FDG PET. We report increased
uptake of FDG within LHIS as an incidental finding in a series of patients
undergoing whole-body FDG PET over a 2-year period for cancer screening.
Materials and Methods
The study was approved by our institution's human research committee. Over
a 2-year period, we identified 11 patients with LHIS who had undergone FDG
PET. The study cohort consisted of 11 patients, eight women and three men,
with an average age of 67 years (range, 49–78 years). The indication for
FDG PET was a history of lung cancer in seven patients and one case each of
lymphoma, metastatic squamous cell carcinoma, metastatic colon carcinoma, and
polyneuropathy possibly paraneoplastic in the remaining four patients.
Patients were excluded if there was evidence for mediastinal or hilar
lymphadenopathy on PET and CT that would overlap the area of increased uptake
in the right heart.
In all patients, CT scans of the thorax were acquired within 30 days of the
FDG PET scans. Six patients had serial CT scans from 1- to 5-year intervals
that verified the stability of the LHIS. Two additional patients without prior
CT scans underwent cardiac MRI to evaluate the abnormal FDG PET uptake in the
right heart.
CT Protocol
CT was performed using a HiSpeed or Light-Speed scanner (GE Healthcare).
Scans were obtained at a 5-mm slice interval during a single breath-hold with
IV contrast material (100 mL of ioxilan, 300 mg I/mL) administered at an
injection rate of 2 mL/sec. The thoracic CT scans were reviewed by two
radiologists. The dimensions of the interatrial septum were measured anterior
and posterior to the fossa ovalis. The Hounsfield unit measurements of the
atrial septal tissue also were obtained to confirm the presence of fat
density.
MRI Protocol
Cardiac MR scans were obtained on a Sigma CVI 1.5-T clinical system (GE
Healthcare). The patients were scanned in the supine position with a cardiac
phased-array coil. ECG-gated and respiratorygated high-resolution axial
conventional spin-echo images were acquired with 7-mm slice thickness, 1-mm
gap, and matrix size of 256 (frequency-encoding steps) x 224
(phase-encoding steps). The TR was equal to one cardiac cycle length, and the
TE was set at 8 msec; the number of excitations was 4. The sequence was
repeated at identical spatial coordinates and time points in the cardiac cycle
using identical scanning parameters except that chemical fat saturation was
used.
FDG PET Protocol
FDG PET studies were performed using an ECAT-HR+ camera (Siemens/CTI). The
patients fasted for at least 6 hr before scanning, and blood glucose levels
were measured before injection of FDG. From 15 to 20 mCi (555–740 GBq)
of FDG was administered IV as a bolus, and static emission images were
obtained 60 min later in multiple bed positions, each 10 min in duration.
Transmission images, measured with rotating rod sources loaded with 68Ge, were
obtained in each bed position for attenuation correction. Image reconstruction
was performed with ordered subset expectation maximization algorithms. The
dose, time of injection, and the patient's body weight were used to calculate
the peak standardized uptake value (SUV). The SUV was calculated at the region
of increased uptake at the interatrial septum, the middle mediastinum (above
the left ventricle), and the liver.
FDG PET scans and CT scans were reviewed with and without fusion overlay
using the computer registration software and viewing station of REVEAL-MVS
(Mirada Solutions). The degree of FDG activity was graded visually as no
discernible uptake or as uptake less than, equal to, or greater than
background mediastinal activity.
Results
The mean CT Hounsfield unit density of the interatrial septa was
–35.3 H (range, –94.0 to 7.88 H). In all patients the interatrial
septum had a bilobed or dumbbell shape that is typical of LHIS (Figs.
1A,
1B,
1C,
1D, and
1E). The mean cross-sectional
thickness of the interatrial septum behind the fossa ovalis was 14.6 mm
(range, 9.1–22.6 mm) and was 10.3 mm (range, 6.6–15.9 mm) anterior
to the fossa ovalis. The CT findings are summarized in
Table 1.
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Fig. 1A. —71-year-old woman with squamous cell carcinoma of right lower
lobe. Axial CT scan (A), T1-weighted MR image (B), and
T1-weighted MR image with fat-saturation (C) show lipomatous
hypertrophy of the interatrial septum (LHIS) (arrows). Note fat
density of LHIS on CT scan and signal dropout within lesion on fat-suppressed
MR image.
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Fig. 1B. —71-year-old woman with squamous cell carcinoma of right lower
lobe. Axial CT scan (A), T1-weighted MR image (B), and
T1-weighted MR image with fat-saturation (C) show lipomatous
hypertrophy of the interatrial septum (LHIS) (arrows). Note fat
density of LHIS on CT scan and signal dropout within lesion on fat-suppressed
MR image.
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Fig. 1C. —71-year-old woman with squamous cell carcinoma of right lower
lobe. Axial CT scan (A), T1-weighted MR image (B), and
T1-weighted MR image with fat-saturation (C) show lipomatous
hypertrophy of the interatrial septum (LHIS) (arrows). Note fat
density of LHIS on CT scan and signal dropout within lesion on fat-suppressed
MR image.
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FDG PET showed focal increased FDG uptake in nine patients (82%),
corresponding to the regions of LHIS on CT (Figs.
1A,
1B,
1C,
1D,
1E,
2A,
2B,
2C,
2D,
2E, and
2F). In these nine patients,
the SUV of the interatrial septum was 1.6–6.1 times greater than the SUV
of the mediastinum blood pool. The mean SUV of the atrial septa was 5.6
(range, 1.8–13.4) compared with the mean mediastinal blood pool SUV of
1.8 (range, 1.3–2.3) and mean liver SUV of 2.1 (range, 1.6–2.5).
In two patients with LHIS on CT, there was no evidence of increased FDG uptake
in the interatrial septum on PET. Their mean septal SUV was 1.0, whereas it
was 2.0 and 2.2 in the mediastinum blood pool and liver, respectively. The FDG
PET findings are summarized in Table
2.
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Fig. 2A. —69-year-old woman with non-Hodgkin's lymphoma of left tibia.
Serial CT scans over 6-year period confirm stability of lipomatous hypertrophy
of the interatrial septum (LHIS). Note characteristic appearance of LHIS on
June 2003 examination image (B) and bilobed fatty expansion of
interatrial septum (white arrows) that spares fossa ovalis (black
arrow, B).
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Fig. 2B. —69-year-old woman with non-Hodgkin's lymphoma of left tibia.
Serial CT scans over 6-year period confirm stability of lipomatous hypertrophy
of the interatrial septum (LHIS). Note characteristic appearance of LHIS on
June 2003 examination image (B) and bilobed fatty expansion of
interatrial septum (white arrows) that spares fossa ovalis (black
arrow, B).
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Fig. 2C. —69-year-old woman with non-Hodgkin's lymphoma of left tibia.
Serial CT scans over 6-year period confirm stability of lipomatous hypertrophy
of the interatrial septum (LHIS). Note characteristic appearance of LHIS on
June 2003 examination image (B) and bilobed fatty expansion of
interatrial septum (white arrows) that spares fossa ovalis (black
arrow, B).
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MR images were obtained in two patients. In both patients the interatrial
septum displayed increased signal on T1-weighted images with signal dropout on
T1-weighted images acquired with fat-saturation, confirming fat density (Figs.
1A,
1B,
1C,
1D, and
1E).
Discussion
LHIS is a histologically benign process in which adipose tissue, a normal
component of the interatrial septum, increases and infiltrates between the
myocardial fibers. LHIS usually is detected as an incidental finding on
echocardiography, and the estimated prevalence is 1% at autopsy, 2–8% on
echocardiography [1], and 2.2%
on MDCT [10]. Although LHIS
can have a focal masslike appearance, it is distinguished from true
intracardiac lipomas by its lack of a discrete capsule and its composition of
both mature adipocytes and fetal fat cells or brown fat
[11].
The histologic features of LHIS were examined in detail by Page
[11], who analyzed the septal
fat deposition pattern in 50 unselected hearts compared with 10 selected cases
of extreme fatty enlargement of the interatrial septum. Page found that septal
fat deposits increase with patient age, are contiguous with epicardial fat
deposits, and correlate with increased epicardial fat in obese individuals.
With increased septal fat infiltration, there is progressive disruption and
disorganization of the myocardial fibers with development of fibrosis. These
changes are implicated in the pathogenesis of impaired contractility and
electrical conduction, which may underlie the infrequent association of LHIS
with supraventricular arrhythmias and sudden death.
In LHIS, the masslike fat deposits are not encapsulated but may appear
constrained at the margins by normal structures including the pericardium,
fossa ovalis, atrial walls, and atrioventricular groove. LHIS has a
characteristic bilobed appearance on cross-sectional imaging at the level of
the fossa ovalis due to fat accumulation in the anterior and posterior septum,
with sparing of the fossa ovalis itself. The diagnosis is confirmed by
detection of nonenhancing fat density on CT or fat signal intensity within the
characteristically shaped lesion on MRI
[1] (Figs.
1A,
1B,
1C,
1D, and
1E). In our series, we noted
marked preferential FDG uptake within the atrial septa in nine of 11 patients
with CT or MRI confirmation (or both) of LHIS. Preferential uptake of FDG by
brown fat, especially in the nonfasting state, has been reported in the rat
model [12]. Not all of our
patients with LHIS had increased uptake of FDG on PET, and we postulate that
variable presence of brown fat in LHIS may be responsible for this
finding.
It is important to recognize that increased FDG uptake in benign processes
potentially can mimic malignancy either in adjacent lymph nodes or in the
myocardium. False interpretation of increased FDG uptake as neoplasm in LHIS
would stage a patient with malignancy inappropriately as having a metastasis
and potentially result in suboptimal therapy. Fusion PET/CT helps clarify the
region of localized FDG uptake and confirm that it lies in the region of LHIS
rather than the adjacent right hilum, mediastinum, or pleura.
Our study is retrospective and therefore we cannot determine the prevalence
of LHIS in our population. Another major limitation of this study is the
absence of histologic confirmation of LHIS in our patients because none of
them underwent an intracardiac biopsy. However, we were able to confirm the
presence of fat in the LHIS on MRI in two patients and document stability of
the LHIS on serial CT (Figs.
2A,
2B,
2C,
2D,
2E, and
2F) in six.
In conclusion, LHIS is a relatively frequent and usually clinically
innocuous abnormality of the heart. We have found that LHIS can show increased
FDG uptake on PET. This finding may be related to the presence of brown fat,
which can be present in LHIS and has been reported to show increased FDG
uptake in the rat model. We recommend that when increased uptake is seen
within the right heart on FDG PET, correlation with CT and possibly MRI be
performed to assess for the presence of LHIS and to avoid the false-positive
diagnosis of malignancy.
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Abstract
Introduction
