DOI:10.2214/AJR.05.0809
AJR 2006; 187:W472-W480
© American Roentgen Ray Society
MRI of Islet Cell Tumors of the Pancreas
Steven Herwick1,
Frank H. Miller1 and
Ana L. Keppke1
1 All authors: Department of Radiology, Northwestern Memorial Hospital,
Northwestern University, The Feinberg School of Medicine, 676 N St. Clair St.,
Ste. 800, Chicago, IL 60611.
Received May 12, 2005;
accepted after revision August 2, 2005.
Address correspondence to F. H. Miller
(fmiller{at}northwestern.edu).
WEB
This is a Web exclusive article.
Abstract
OBJECTIVE. CT is the most widely used imaging technique for the
diagnosis of islet cell tumors, but MRI may be better for detecting small
lesions and metastases because of its optimal contrast resolution and ability
to easily perform dynamic imaging. The purpose of this pictorial essay is to
highlight the MRI features of these tumors and underscore potential
pitfalls.
CONCLUSION. Although classically considered well-defined, arterially
enhancing lesions that are bright on T2-weighted sequences, pancreatic islet
cell tumors have quite a broad spectrum of appearances. MRI is well suited for
detecting and characterizing pancreatic islet cell tumors as well as their
local effects and metastases.
Keywords: abdominal imaging • MRI • pancreas • pancreatic neoplasms • pancreaticobiliary imaging
Introduction
Islet cell tumors are neoplasms arising from neuroendocrine cells within
the pancreas or within the gastrinoma triangle, which is bordered by the
junction of the cystic and common bile ducts superiorly, the second and third
portions of the duodenum inferiorly, and the neck and body of the pancreas
medially. Because of the pluripotent nature of the cells of origin, they may
express any of a number of different polypeptide hormones. Hyperfunctioning
tumors tend to present when the lesion is still small because of symptoms
related to the secreted hormone. Nonhyperfunctioning tumors, on the other
hand, more often present after the lesion has reached a significant size, with
symptoms related to mass effect or metastases. The majority of
nonhyperfunctioning tumors are malignant, although the proportion of malignant
tumors varies greatly among hyperfunctioning subtypes. Islet cell tumors have
an increased prevalence in patients with von Hippel-Lindau disease and
multiple endocrine neoplasia type I, in whom multiple lesions and
extrapancreatic location are frequent.
Islet cell tumors are often difficult to detect and localize on imaging
studies due to their small size and variable imaging features. Surgery is the
treatment of choice, ideally with lesion enucleation and sparing of the
pancreas. Determination of the precise location and number of lesions and
identification of findings such as local spread and metastases are important
for surgical planning. Numerous authors have evaluated different imaging
techniques for maximizing the conspicuity of these lesions. MRI is a
noninvasive technique that has the ability to detect and localize pancreatic
and extrapancreatic lesions with high accuracy and to identify benign or
malignant characteristics of these lesions. CT is the most widely used imaging
technique for diagnosis of islet cell tumors, but MRI may be better for
detecting small lesions and metastases because of its optimal contrast
resolution and the ability to easily perform dynamic imaging. The aim of this
pictorial essay is to highlight MRI features of these tumors and underscore
potential pitfalls.
MRI Technique and Imaging Appearance
MRI Technique
Pancreatic imaging with MRI has made tremendous advances recently. In the
past, MRI was limited due to respiratory motion and cardiac pulsation
artifacts, bowel peristalsis, and long acquisition times for T1-weighted and
T2-weighted spin-echo and turbo spin-echo sequences. Recent advances allow
faster imaging acquisition with higher signal-to-noise ratios and breath-hold
images free of artifacts. The total MRI examination can be performed in less
than 30 minutes of scanner table time and 5 minutes of total acquisition time.
The MR protocol for imaging the pancreas at our institution includes axial and
coronal breath-hold T2-weighted HASTE sequences, axial in-phase and
opposed-phase images, and breath-hold T1-weighted fat-suppressed spoiled
gradient-echo shared prepulse (SHARP) sequences acquired before contrast
administration and during the arterial phase (15-20 seconds) and multiple
venous and delayed phases (60, 90, 120, and 180 seconds) after contrast
administration.
The examinations were performed on 1.5-T Magnetom Sonata (40 mT/m
amplitude, 200 mT/m/ms slew rate) and Symphony (30 mT/m amplitude, 100 mT/m/ms
slew rate) scanners (Siemens Medical Solutions) with high-performance gradient
systems using a phased-array body coil. The parameters for our HASTE sequence
were TR/TE, 1,000/63; slice thickness, 3-5 mm; matrix, 197 x 256; and
flip angle, 160°. Our T1-weighted fat-suppressed SHARP sequences used
210/1.94; slice thickness, 6 mm; flip angle, 70°; matrix, 132 x 256;
with 24 slices in a 16-second breath-hold with an integrated parallel
acquisition technique (iPAT) with a factor of 2. A fluoroscopy preparation
timing bolus technique, which allows direct monitoring of the arrival of
contrast material in the aorta, was performed to obtain images during the
arterial phase. The chemical shift in-phase and out-of-phase sequences were
obtained as a combined sequence with the following parameters: 190/4.76
(in-phase); TE, 2.38 (opposed-phase); flip angle, 70°; and matrix, 135
x 320.
The rapid acquisition of this T1-weighted sequence during a breath-hold
makes it ideal for dynamic imaging after IV gadolinium administration. Only
T2-weighted HASTE images and T1-weighted fat-suppressed spoiled gradient-echo
images are shown in this pictorial essay.
Signal Intensity
Islet cell tumors are usually bright on T2-weighted sequences, and some
institutions use T2-weighted sequences with fat suppression in an effort to
increase the conspicuity of lesions
[1]. However, lesions with
intermediate or low T2-weighted signal intensity may be seen
[1] (Figs.
1,
2A, and
2B). The unenhanced T1-weighted
fat-suppressed sequence provides excellent contrast between the
low-signal-intensity tumor and the normal pancreas, which is bright secondary
to the abundance of acinar proteins (Figs.
2A and
2B). This may be the best
sequence to detect subtle tumors
[1].
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Fig. 2A —63-year-old man with hypoglycemia and biochemical evidence of
insulinoma. Axial T1-weighted fat-suppressed spoiled gradient-echo MR image
shows 1-cm hypointense lesion (arrow) in pancreatic head, which was
surgically confirmed to be insulinoma. Note contrast of lesion relative to
normal high-signal-intensity pancreas.
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Enhancement
Hypervascular enhancement is one of the typical imaging features of islet
cell tumors and often helps distinguish them from the much more common
pancreatic adenocarcinomas, which tend to be hypovascular and desmoplastic.
Although islet cell tumors are classically considered hypervascular in the
arterial phase (Figs. 3A,
3B, and
3C), the degree, uniformity,
and timing of enhancement can be highly variable. As a result, the ability to
distinguish primary or secondary lesions from the surrounding organ may be
present on only one contrast-enhanced phase
[2]. In fact, some islet cell
tumors may be seen best on venous phase images
[3] or can be masked in
different perfusion stages and show isointense signal and enhancement
characteristics similar to those of the normal pancreas after contrast
administration, being nearly invisible on all but unenhanced images (Figs.
4A,
4B, and
4C).
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Fig. 3A —35-year-old woman with liver and pancreatic lesions
incidentally discovered at sonography. Histologic examination revealed hepatic
focal nodular hyperplasia and benign pancreatic islet cell tumor. Axial
T1-weighted fat-suppressed spoiled gradient-echo MR image shows
low-signal-intensity lesion (arrow) in pancreatic body, which is well
seen relative to normal hyperintense pancreas due to excellent soft-tissue
contrast resolution of MRI. Lesion appearance is nonspecific and can be seen
with adenocarcinoma as well.
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Fig. 3B —35-year-old woman with liver and pancreatic lesions
incidentally discovered at sonography. Histologic examination revealed hepatic
focal nodular hyperplasia and benign pancreatic islet cell tumor. Axial
arterial phase T1-weighted fat-suppressed spoiled gradient-echo MR image shows
lesion has marked enhancement (arrow) unlike adenocarcinoma, which
tends to be hypovascular.
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Fig. 3C —35-year-old woman with liver and pancreatic lesions
incidentally discovered at sonography. Histologic examination revealed hepatic
focal nodular hyperplasia and benign pancreatic islet cell tumor. Axial venous
phase gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR
image shows lesion (arrow) is essentially isointense to surrounding
pancreas, emphasizing importance of arterial phase and optimal timing.
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Fig. 4A —52-year-old woman with history of hypoglycemia and clinical
diagnosis of insulinoma, which could not be detected on multiple MDCT scans
over preceding 3 years. Axial T1-weighted fat-suppressed spoiled gradient-echo
MR image shows 1.4-cm lesion (arrow), which is hypointense relative
to surrounding normal pancreas.
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Fig. 4B —52-year-old woman with history of hypoglycemia and clinical
diagnosis of insulinoma, which could not be detected on multiple MDCT scans
over preceding 3 years. Axial arterial phase gadolinium-enhanced T1-weighted
fat-suppressed spoiled gradient-echo MR image shows lesion (arrow) is
now difficult to identify because it enhances to same degree as surrounding
pancreas.
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Fig. 4C —52-year-old woman with history of hypoglycemia and clinical
diagnosis of insulinoma, which could not be detected on multiple MDCT scans
over preceding 3 years. Axial venous phase gadolinium-enhanced T1-weighted
fat-suppressed spoiled gradient-echo MR image also shows lesion
(arrow) enhancing to same extent as surrounding pancreas.
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Fig. 5A —57-year-old man with history of gallstone pancreatitis and
cystic lesion of pancreas thought to represent pseudocyst or cystic neoplasm
at MDCT. Axial T1-weighted fat-suppressed spoiled gradient-echo MR image shows
hypointense lesion (arrow) in tail of pancreas.
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Fig. 5B —57-year-old man with history of gallstone pancreatitis and
cystic lesion of pancreas thought to represent pseudocyst or cystic neoplasm
at MDCT. Axial gadolinium-enhanced T1-weighted fat-suppressed spoiled
gradient-echo MR image shows ring-enhancing lesion (arrow).
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Fig. 5C —57-year-old man with history of gallstone pancreatitis and
cystic lesion of pancreas thought to represent pseudocyst or cystic neoplasm
at MDCT. Coronal gadolinium-enhanced T1-weighted fat-suppressed spoiled
gradient-echo MR image shows ring-enhancing lesion (arrow) due to
pancreatic islet cell tumor.
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Fig. 6 —52-year-old man with von Hippel-Lindau disease and pancreatic
mass diagnosed as serous cystadenoma on basis of findings at previous
fine-needle aspiration. Axial gadolinium-enhanced T1-weighted fat-suppressed
spoiled gradient-echo MR image shows 5-cm poorly marginated heterogeneously
enhancing mass of pancreatic head (long arrows), which proved to be
malignant islet cell tumor, and liver metastases (short arrows).
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Fig. 7 —49-year-old woman (same patient as in
Fig. 1) with malignant islet
cell tumor of pancreas. Coronal gadolinium-enhanced T1-weighted fat-suppressed
spoiled gradient-echo MR image shows irregular enhancement of mass
(arrows) in pancreatic tail.
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Fig. 8A —56-year-old woman with multiple endocrine neoplasia type I
and multiple surgically proven islet cell tumors of pancreas. Coronal venous
phase gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR
image shows small (< 1 cm) hyperenhancing lesion (arrow) in
pancreatic head.
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Fig. 8B —56-year-old woman with multiple endocrine neoplasia type I
and multiple surgically proven islet cell tumors of pancreas. Coronal
gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image
shows similar small lesion (arrow) in pancreatic tail. Multiple
lesions are more common in patients with multiple endocrine neoplasia type I
and von Hippel-Lindau disease than in sporadic cases.
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Fig. 9A —33-year-old man with large neoplasm incidentally found at
MDCT. Axial T2-weighted HASTE MR image shows lesion of intermediate signal
intensity similar to that of muscle or liver. Center of lesion is high signal
intensity due to necrosis (long arrow). Note dilatation of distal
pancreatic duct (short arrows) with atrophy of this portion of
gland.
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Fig. 9B —33-year-old man with large neoplasm incidentally found at
MDCT. Axial gadolinium-enhanced T1-weighted fat-suppressed spoiled
gradient-echo MR image shows lesion with thick enhancing wall, which is
typical of nonfunctioning islet cell tumor. Center of lesion lacked
enhancement due to necrosis (arrow).
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Fig. 10A —62-year-old woman with cystic lesion of pancreatic tail seen
at MDCT, which proved to be islet cell tumor at fine-needle aspiration biopsy.
Axial T2-weighted HASTE MR image shows 1-cm lesion (arrow) in
pancreatic tail with high signal intensity mimicking pseudocyst or cystic
neoplasm of pancreas.
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Fig. 10B —62-year-old woman with cystic lesion of pancreatic tail seen
at MDCT, which proved to be islet cell tumor at fine-needle aspiration biopsy.
Axial arterial phase gadolinium-enhanced T1-weighted fat-suppressed spoiled
gradient-echo MR image shows rim enhancement of lesion (arrow).
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Other enhancement characteristics of islet cell tumors may aid in their
detection and distinction from other pancreatic neoplasms. Often, a
characteristic ringlike enhancement is seen on early or delayed
contrast-enhanced images (Figs.
5A,
5B, and
5C). Also, the uniformity of
enhancement tends to be variable, with larger, more malignant lesions
exhibiting more heterogeneous enhancement
[4] (Figs.
6 and
7). Conversely, small lesions
are often benign and homogeneous in enhancement
[4,
5] (Figs.
8A and
8B).
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Fig. 11A —52-year-old man with epigastric pain. MDCT showed only
dilatation of most distal pancreatic duct in tail. Coronal MR
cholangiopancreatography image shows dilatation of pancreatic duct
(arrow) in tail with normal-caliber pancreatic duct in remainder of
gland (chevrons).
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Fig. 11B —52-year-old man with epigastric pain. MDCT showed only
dilatation of most distal pancreatic duct in tail. Axial thin-section
T2-weighted HASTE MR image shows subtle 1-cm minimally hypointense lesion
(short arrows) causing pancreatic duct dilatation (long
arrow). Subsequent fine-needle aspiration biopsy confirmed pancreatic
islet cell tumor.
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Fig. 12A —46-year-old woman who presented with bleeding gastric varices
and was found to have malignant islet cell tumor of pancreas with splenic vein
(SV) and portal vein (PV) invasion. Axial T2-weighted HASTE MR image near
portal vein origin shows heterogeneous mass expanding distal splenic vein and
proximal portal vein.
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Fig. 12B —46-year-old woman who presented with bleeding gastric varices
and was found to have malignant islet cell tumor of pancreas with splenic vein
(SV) and portal vein (PV) invasion. Axial gadolinium-enhanced T1-weighted
fat-suppressed spoiled gradient-echo MR image at slightly more cephalad level
than A shows enhancing tumor expanding main portal vein.
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Islet cell tumors, especially if large, may show cystic or necrotic areas
of nonenhancement, which sometimes occupy most of the lesion. Nonfunctioning
islet cell tumors are more frequently cystic or necrotic and present later,
often with metastases at the time of diagnosis
[4] (Figs.
9A and
9B). Because of their high
signal intensity on T2-weighted images, some lesions closely resemble
pseudocysts or cystic pancreatic neoplasms such as intraductal papillary
mucinous neoplasms, microcystic adenomas, and mucinous cystic neoplasms,
particularly if multiple contrast-enhanced images are not acquired. A rim of
enhancement with signal intensity different from the adjacent gland may help
characterize the "cystic" lesion as an islet cell tumor (Figs.
10A and
10B).
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Fig. 13A —52-year-old man with von Hippel-Lindau disease (same patient
as in Fig. 6) and pancreatic
islet cell neoplasm metastatic to regional lymph nodes and liver. Axial
arterial phase gadolinium-enhanced T1-weighted fat-suppressed spoiled
gradient-echo MR image shows intense enhancement of multiple porta hepatis
lymph nodes (chevrons) and multiple enhancing liver lesions
(arrows).
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Fig. 13B —52-year-old man with von Hippel-Lindau disease (same patient
as in Fig. 6) and pancreatic
islet cell neoplasm metastatic to regional lymph nodes and liver. Axial
T1-weighted fat-suppressed spoiled gradient-echo MR image at slightly more
cephalad level than A shows multiple ring-enhancing hepatic metastases
and smaller, homogeneously enhancing lesions (arrows).
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Fig. 14A —49-year-old woman (same patient as in Figs.
1 and
7) with MDCT reportedly
showing focal nodular hyperplasia in liver. Axial arterial phase
gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image
shows 1.3-cm lesion (arrow) in liver.
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Fig. 14B —49-year-old woman (same patient as in Figs.
1 and
7) with MDCT reportedly
showing focal nodular hyperplasia in liver. Axial venous phase
gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image
does not show lesion.
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Local Effects
Large and aggressive lesions are prone to secondarily affect adjacent
structures such as biliary and pancreatic ducts (Figs.
9A and
9B). Due to the small size of
functioning islet cell tumors at presentation, this is less often seen.
However, when located along the course of ducts, even relatively small lesions
may cause an obstruction (Figs.
11A and
11B). In addition, aggressive
lesions may also cause complications secondary to compression or invasion of
local organs or vascular structures (Figs.
12A and
12B).
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Fig. 14C —49-year-old woman (same patient as in Figs.
1 and
7) with MDCT reportedly
showing focal nodular hyperplasia in liver. Coronal T1-weighted fat-suppressed
spoiled gradient-echo MR image shows intensely enhancing lesion
(arrow) in pancreas. This hypervascular pancreatic lesion is highly
suggestive of islet cell tumor, which surgery confirmed. In addition,
hypervascular metastases, as seen in this example, are not typical of
adenocarcinoma of pancreas. Consequently, liver lesions were thought to
represent metastases.
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Metastases
Metastases from islet cell tumors most frequently involve the liver and
peripancreatic lymph nodes. The signal characteristics of metastases tend to
resemble those of the primary lesion with prominent enhancement typically
seen. Central necrosis often occurs as metastases grow (Figs.
13A and
13B).
In a study by Semelka et al.
[6], T2-weighted fat-suppressed
images depicted more liver metastases than dynamic contrast-enhanced CT. The
authors also observed a ring-enhancing pattern in liver metastases and
suggested that this may be characteristic of islet cell tumors
[6,
7]. Like the primary lesions,
metastases may be subtle; careful analysis of unenhanced and
gadolinium-enhanced T1-weighted fat-suppressed sequences and T2-weighted
sequences is essential (Figs.
14A,
14B, and
14C).
Conclusion
Although classically considered well-defined, arterially enhancing lesions
that are bright on T2-weighted sequences, pancreatic islet cell tumors have a
quite broad spectrum of appearances. MRI is well suited for detecting and
characterizing pancreatic islet cell tumors and their local effects and
metastases. MRI provides high contrast between the normal pancreas and the
lesion on T1-weighted fat-suppressed (and often T2-weighted) sequences and
provides the ability to acquire multiplanar dynamic contrast-enhanced images.
In addition, the use of MRI avoids the repeated radiation exposure of CT in
these patients, who are often young and may require long-term imaging
follow-up.
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Abstract
Introduction
