DOI:10.2214/AJR.05.1095
AJR 2007; 188:429-432
© American Roentgen Ray Society
Focal Fatty Replacement of the Pancreas: Usefulness of Chemical Shift MRI
Hye Jin Kim1,
Jae Ho Byun,
Seong Ho Park,
Yong Moon Shin,
Pyo Nyun Kim,
Hyun Kwon Ha and
Moon-Gyu Lee
1 All authors: Department of Radiology and Research Institute of Radiology,
University of Ulsan College of Medicine, Asan Medical Center, 388-1,
Poongnap-dong, Songpa-gu, Seoul 138-736, South Korea.
Received June 26, 2005;
accepted after revision August 31, 2005.
Address correspondence to J. H. Byun
(jhbyun{at}amc.seoul.kr).
Abstract
OBJECTIVE. The purpose of this study was to describe the typical CT
and chemical shift MRI findings and to evaluate their usefulness in the
diagnosis of focal fatty replacement of the pancreas in five patients.
CONCLUSION. The presence of a perceptible focal low-attenuation
lesion in the pancreas on CT and the reduction in signal intensity of the
lesion on the opposed phase of chemical shift MR images are useful for
differentiating focal fatty replacement of the pancreas from true pancreatic
neoplasm. Therefore, these imaging findings obviate invasive diagnostic
procedures and surgery in the care of patients with focal fatty replacement of
the pancreas.
Keywords: chemical shift MRI • CT • fat • MRI • pancreas
Introduction
Focal fatty replacement of the pancreas is a rare benign disease
[1,
2]. It has been associated with
a variety of diseases, including obesity, diabetes mellitus, chronic
pancreatitis, hereditary pancreatitis, obstruction of the pancreatic duct by
calculus or tumor, and cystic fibrosis
[1-4].
Although several reports have described the CT features of focal fatty
replacement of the pancreas, to our knowledge the chemical shift MRI findings
have not been reported. If the degree of focal fatty replacement is not
severe, it is difficult to differentiate focal fatty replacement of the
pancreas from true pancreatic neoplasm on CT. We describe the typical CT and
chemical shift MRI findings and evaluate their usefulness in the diagnosis of
focal fatty replacement of the pancreas in five patients.
Materials and Methods
Patients
We retrospectively reviewed the radiology records of 135 patients who had
undergone both MRI and CT of the pancreas between October 2004 and April 2005.
Among these 135 patients, five with focal fatty replacement of the pancreas
were identified. These patients included three women and two men with an age
range of 39-77 years (mean age, 62 years). In all cases, the indication for
MRI was characterization of a focal pancreatic lesion seen on CT. The
indications for the CT examination were as follows: suspected pancreatic tumor
(n = 3), right upper quadrant abdominal pain (n = 1), and
intermittent abdominal pain and jaundice (n = 1). The mean interval
between the two examinations was 11 days (range, 1-33 days). There were no
follow-up images for any of the study patients.
The medical records were reviewed to determine several clinical findings:
the presence of obesity, diabetes mellitus, and coexistent disease; and
laboratory findings of serum amylase, lipase, and total cholesterol levels.
Patients whose body mass index was greater than 30 were classified as obese.
Our institutional review board does not require its approval or patient
informed consent for this type of retrospective review.
Imaging Techniques
CT was performed with a 4-MDCT scanner (LightSpeed QX/i, GE Healthcare).
Scans were acquired in a craniocaudal direction with the following parameters:
section thickness, 5 mm; reconstruction interval, 5 mm; pitch, 6; tube
current, 210-270 mA at 120 kVp. Four patients underwent both unenhanced and
contrast-enhanced CT. One patient underwent only contrast-enhanced CT.
Nonionic contrast material (iopromide, [Ultravist 300, Schering]) was injected
IV with a power injector (LF CT 9000, Liebel-Flarsheim) at a rate of 3-4 mL/s
for a total dose of 150 mL. Contrast-enhanced CT scans were obtained during
the arterial phase with a bolus tracking technique and during the portal
venous phase with a 72-second delay after contrast injection in patients with
suspected pancreatic tumor (n = 3) and only during the portal venous
phase in patients with abdominal pain (n = 2).
MRI was performed on a 1.5-T unit (Magnetom Vision, Siemens Medical
Solutions) with a phasedarray body coil. In-phase and opposed-phase dualecho
chemical shift MR images were obtained with the following parameters in the
transverse plane with a 2D spoiled gradient-echo sequence: TR/TE, 152/5.3 and
152/2.7; section thickness, 6-8 mm; intersection gap, 1.6 mm; field of view,
35 cm; matrix size, 256 x 256; flip angle, 80°. Dual-echo
acquisition, whereby both the in-phase and opposed-phase MR images are
obtained during the same breathhold, was used for all five patients.
Image Interpretation
Two gastrointestinal radiologists with 9 and 5 years of experience analyzed
the CT images by consensus. The reviewers were aware of the presence of focal
fatty replacement of the pancreas. Location, shape, size, enhancement pattern,
presence of contour bulging and pancreatic duct dilatation, and attenuation
value of focal fatty replacement of the pancreas on unenhanced and
contrast-enhanced CT images were analyzed. We evaluated the signal intensity
of focal fatty replacement on both inphase and opposed-phase MR images. On the
opposed-phase MR images, reduction in signal intensity of the pancreatic
lesion was identified by comparison with in-phase MR images.
Results
None of the five patients presented with obesity or diabetes mellitus. One
patient was found to have hilar cholangiocarcinoma. The others had no
coexistent disease. The serum amylase and lipase levels were normal in all
five patients, and the serum total cholesterol level was normal in four
patients. Only one patient had a slightly high serum total cholesterol level
of 256 mg/dL (normal value, < 240 mg/dL).
The CT and chemical shift MRI findings in patients with focal fatty
replacement of the pancreas are summarized in
Table 1. The anterior aspect of
the head of the pancreas adjacent to the superior mesenteric vein exhibited
perceptible fatty replacement in all patients (Figs.
1A,
1B,
1C,
1D and
2A,
2B,
2C,
2D). One of the five study
patients had fatty replacement in the head and neck of the pancreas. The
largest diameter of the focal fatty replacement of the pancreas ranged from
1.2 to 3.5 cm, with a mean of 2.2 cm. All patients had heterogeneous
enhancement in the area of focal fatty replacement of the pancreas on
contrast-enhanced CT scans (Figs.
1A,
1B,
1C,
1D and
2A,
2B,
2C,
2D). The mean attenuation
values of this area were -28.0 and 27.9 H on unenhanced and contrast-enhanced
CT images, respectively. There was no dilatation of the pancreatic duct in any
of the five patients. On in-phase MR images, the signal intensity of the focal
fatty replacement was similar to that of the remnant pancreatic parenchyma in
four study patients (Figs. 1A,
1B,
1C,
1D and
2A,
2B,
2C,
2D) and higher in the fifth
study patient. Compared with in-phase MR images, opposed-phase MR images of
all patients showed prominent reduction in signal intensity of the focal fatty
replacement of the pancreas (Figs.
1A,
1B,
1C,
1D and
2A,
2B,
2C,
2D).
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Fig. 1A —60-year-old woman with abdominal pain of 2 months' duration and
suspected pancreatic mass as found on sonography performed at outside
hospital. Axial unenhanced CT scan shows ovoid, low-attenuation lesion with
bulging contour (arrows) in anterior aspect of head of pancreas.
Attenuation value of lesion is -17 H.
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Fig. 1B —60-year-old woman with abdominal pain of 2 months' duration and
suspected pancreatic mass as found on sonography performed at outside
hospital. Axial contrast-enhanced CT scan corresponding to A shows
heterogeneous enhancement of low-attenuation area (arrows) in
pancreas. Attenuation value of lesion is 37 H.
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Fig. 1C —60-year-old woman with abdominal pain of 2 months' duration and
suspected pancreatic mass as found on sonography performed at outside
hospital. Transverse in-phase MR image (TR/TE, 152/5.3) shows no abnormality
in head of pancreas.
|
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Fig. 1D —60-year-old woman with abdominal pain of 2 months' duration and
suspected pancreatic mass as found on sonography performed at outside
hospital. Transverse opposed-phase MR image (152/2.7) corresponding to
C shows hypointense mass (arrows) in head of pancreas at same
site as in A and B.
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Fig. 2A —39-year-old man with hilar cholangiocarcinoma. Axial unenhanced CT
scan shows ovoid, low-attenuation lesion (arrows) without bulging
contour in anterior aspect of head of pancreas and abutting superior
mesenteric vein. Attenuation value of lesion is -33 H.
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Fig. 2B —39-year-old man with hilar cholangiocarcinoma. Axial
contrast-enhanced CT scan corresponding to A shows heterogeneous
enhancement (arrows) of low-attenuation area in head of pancreas.
Attenuation value of lesion is 4 H.
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Discussion
Pancreatic lipomatosis or fatty replacement of the pancreas is the most
frequent pathologic finding in the adult pancreas
[5,
6]. Fatty replacement is
distributed as patch reticular or multiple tiny nodules throughout the
pancreas; however, some investigators have found uneven fatty replacement of
the pancreas, which is seen as a focal fatty mass
[2,
7]. This appearance may
simulate a pancreatic mass such as a cystic neoplasm or other pancreatic
tumor. Although unenhanced CT has an essential diagnostic role in depicting
the same density as adjacent retroperitoneal fat with negative attenuation
values, a mild degree of focal fatty replacement of the pancreas cannot be
diagnosed with CT alone. Moreover, contrast-enhanced CT cannot show a negative
attenuation value of this focal fatty replacement because normal pancreatic
parenchyma entrapped between fatty replacement areas exhibits contrast
enhancement. MRI may be helpful for confirming the presence of this type of
focal fatty replacement of the pancreas, and in several studies focal
pancreatic fatty replacement has been diagnosed with MRI
[3,
8,
9]. However, visualization of
focal or uneven pancreatic fatty replacement with chemical shift MRI has to
our knowledge not been reported in the literature.
In our five study patients, unenhanced CT scans showed perceptible negative
attenuation values of focal fatty replacement, but contrast-enhanced CT scans
showed positive attenuation values simulating a true mass. Unlike CT scans and
in-phase MR images, chemical shift opposed-phase MR images revealed definite
reduction in signal intensity in all five patients. Chemical shift MRI has the
advantage that the reduction in signal intensity of focal fatty replacement on
opposed-phase images differentiates focal fatty replacement of the pancreas
from true pancreatic tumors, which in general do not contain lipid
[10]. On in-phase MR images,
only one of our patients had higher signal intensity of focal fatty
replacement than that of remnant pancreatic parenchyma. In the four patients
who had signal intensity of focal fatty replacement similar to that of remnant
pancreatic parenchyma on inphase MR images, we assumed that these patients had
minimal focal fatty replacement of the pancreas. In these four patients, the
opposed-phase MR images showed a distinct reduction in signal intensity,
indicating that opposed-phase MRI is helpful for establishing the correct
diagnosis of focal fatty replacement of the pancreas. Five of 135 patients had
carcinoma of the head of the pancreas, but pancreatic carcinoma showed no
decrease in signal intensity on chemical shift MRI. Pancreatic carcinoma
frequently had ancillary CT findings, such as dilatation of the main
pancreatic duct, direct vascular invasion, lymphadenopathy, secondary
pancreatitis, or positive attenuation value on unenhanced CT, which were
differentiated from focal fatty replacement of the pancreas.
In our study, focal fatty replacement of the pancreas was present in the
anterior aspect of the head of the pancreas in all patients. Matsumoto et al.
[7] reported that although
uneven fatty replacement of the pancreas can occur in any portion of the
pancreas, fatty replacement was more severe in the anterior aspect of the head
of the pancreas. Although the reason for the more severe fatty replacement in
the anterior aspect of the pancreatic head was unclear, those authors'
findings correlate well with ours. Dhillon et al. found an embryologic ventral
pancreas composed of smaller exocrine cells and scanty or no interacinar fat
and more abundant interlobular fibrous tissue than its embryologic dorsal
counterpart [7]. Therefore,
previous reports [9,
11] showed that on CT and
sonography, focal fatty sparing of the pancreas was present within the dorsal
aspect of the pancreatic head or the uncinate process.
Our study was limited by several factors, including the small number of
patients, lack of histopathologic confirmation, and lack of follow-up images.
Focal fatty replacement of the pancreas, however, is difficult to confirm
because it does not necessitate surgery or biopsy. Therefore, further
prospective study and follow-up examinations are necessary.
In conclusion, despite the rarity of focal fatty replacement of the
pancreas, chemical shift MRI has an essential role in the correct diagnosis
and avoidance of unnecessary invasive diagnostic procedures and surgery in
cases of the suspected finding of focal fatty replacement on CT scans.
Knowledge of the typical ventral aspect of the pancreatic head is also helpful
for determining the correct diagnosis of focal fatty replacement of the
pancreas.
Acknowledgments
We thank Bonnie Hami, department of radiology, University Hospitals Health
System, Cleveland, Ohio, for editorial assistance in preparing the
manuscript.
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Abstract
Introduction
