British Journal of Radiology 74 (2001),968-973 © 2001 The British Institute of Radiology
MRI of pancreatic neuroendocrine tumours
N J Owen, FRCS, FRCR
1
S A A Sohaib, MRCP, FRCR
1
P D Peppercorn, MRCP, FRCR
1
J P Monson, MD, FRCP
2
A B Grossman, MD, FRCP
2
G M Besser, MD, FRCP
2 and
R H Reznek, FRCP, FRCR
1
Departments of 1Diagnostic Imaging and 2Endocrinology, St Bartholomew's Hospital, West Smithfield, London EC1A 7BE, UK
Correspondence: Prof. R H Reznek, Academic Department of Radiology, Dominion House, 59 Bartholomew's Close, St Bartholomew's Hospital, West Smithfield, London EC1A 7BE, UK
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Abstract
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Neuroendocrine tumours of the pancreas are rare and are frequently difficult to demonstrate. Several imaging modalities have been used to demonstrate these tumours, but recent reports have suggested that MRI may have an important role in their localization. We review the spectrum of MRI appearances of pancreatic neuroendocrine tumours.
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Introduction
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Pancreatic neuroendocrine tumours (NETs) are rare tumours, which may be associated with clinical syndromes including Zollinger–Ellison syndrome, carcinoid syndrome and multiple endocrine neoplasia type 1. These tumours are frequently difficult to detect on imaging owing to their small size and the low inherent contrast between the tumour and the pancreatic parenchyma. Several imaging modalities have been used to demonstrate these tumours [1] and recent reports suggest that MRI may have an important role in their localization [2]. This pictorial review shows the spectrum of MRI appearances of pancreatic NETs.
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Materials and methods
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The cases demonstrated were drawn from a series of 19 patients (12 males, 7 females), median age 54 years (age range 25–85 years) with 31 pancreatic NETs investigated at our institution from 1994–1998. In 17 of the 19 patients the diagnosis of pancreatic NET was confirmed on histopathological examination of surgically resected pancreatic tissue. Amongst these 17 patients there were 29 lesions: 9 (31%) insulinomas, 5 (17%) gastrinomas, 4 (14%) carcinoids and 11 (38%) non-functioning tumours. Two insulinomas in the remaining two patients, treated medically, were diagnosed on the basis of characteristic insulin and glucose profiles and localization performed with selective arterial stimulation and hepatic venous sampling. The tumours were classified on the basis of hormone assays and immunohistocytochemistry. 5 of the 19 patients had multiple tumours. 6 (32%) of the 19 patients had the multiple endocrine neoplasia type 1 (MEN 1).
MRI was performed on a 1.5 T unit (Signa GE Medical Systems, Milwaukee, WI). The following sequences were performed: T1 weighted spin echo (TR 400–600 ms, TE 14–16 ms); T1 weighted spin echo with spectral fat saturation (TR 400–600 ms, TE 14–16 ms); fast spin echo T2 weighted (TR 5000–6500 ms, effective TE 95–120 ms, echo train length 8–16); and fast multiplanar spoiled gradient echo (TR 100–150 ms, TE 4.2 ms, flip angle 80–90°). All scans were performed in the axial plane with a slice thickness of 5–8 mm and an interslice gap of 1–2 mm. Intravenous contrast medium was administered as gadodiamide–DTPA 0.1 ml kg-1 (Omniscan; Nycomed, Amersham, UK).
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Results
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MRI identified 29 (94%) of the 31 lesions in the 19 patients. The two lesions missed on MRI were 5 mm and 7 mm non-functioning tumours in the tail of the pancreas. The positions of the tumours seen on MRI were as follows: 5 in the head, 8 in the body and 16 in the tail of the pancreas. The MRI signal characteristics of the 29 tumours identified are summarized in Table 1
. The tumours were predominantly of low signal on T1 weighted fat suppressed images (Figures 1 and 2), except for one insulinoma, which was of high signal on this sequence (Figure 3). 19 (66%) of the 29 tumours were demonstrated to best effect with the fat suppressed T1 weighted sequence. The signal characteristic on the T2 weighted scans was more variable; most were of high signal intensity (Figures 1 and 2), but some were of low signal intensity (Figure 4). Four lesions were cystic in nature (Figure 5).
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Figure 1. A 51-year-old man with an insulinoma in the tail of the pancreas, showing the commonest signal intensity changes for islet sell tumours on the various MR sequences. (a) The T1 weighted spin echo image (TR/TE 500/14 ms) shows the typical low signal intensity pattern (curved black arrow). (b) On the T2 weighted fast spin echo image (TR/TE 6315/105 ms) the insulinoma (curved black arrow) shows typical high signal intensity for this sequence. (c) The fat saturated axial T1 weighted spin echo image (TR/TE 500/14 ms) shows the characteristic high signal ofthe pancreatic parenchyma using this sequence, resulting in increased conspicuity of the insulinoma (arrow). (d) Following iv gadolinium, the insulinoma (curved white arrow) shows enhancement greater than the pancreatic parenchyma on the fat saturated axial T1 weighted spin echo image (TR/TE 500/14 ms). Note adjacent splenic enhancement. (e) CT shows a vascular lesion (open arrow) with rim calcification, a feature not appreciated on MRI.
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Figure 2. A small non-functioning islet cell tumour in a patient with multiple endocrine neoplasia type 1. (a) Axial T1 weighted spin echo image, (b) axial T2 weighted fast spin echo image, and fat suppressed T1 weighted spin echo axial images before (c) and after (d) iv gadolinium, showing the typical signal intensity pattern for an islet cell tumour (arrow) on the various sequences, in addition to the marked enhancement following gadolinium.
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Figure 3. A 72-year-old man with an insulinoma in the tail of the pancreas. This lesion shows the unusual signal intensity pattern, with high signal on the fat saturated T1 weighted spin echo image. (a) Fat saturated T1 weighted spin echo image (TR/TE 500/16 ms) shows high signal intensity in this insulinoma (curved white arrow). (b) T2 weighted fast spin echo image (TR/TE 6000/102 ms) shows the insulinoma has a low intensity rim with an intermediate intensity centre (curved black arrow). (c) Contrast enhanced CT shows a markedly vascular tumour (arrow) in the tail of the pancreas.
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Figure 4. A 41-year-old man with an insulinoma in the body of the pancreas. This lesion shows the uncommon low signal intensity pattern on the T2 weighted fast spin echo image. (a) T1 weighted spin echo image (TR/TE 400/14 ms). The tumour (arrow) appears as a low signal bulge in the contour of the pancreas. (b) T2 weighted fast spin echo image (TR/TE 4000/105 ms) shows the tumour (curved black arrow) is of low signal intensity, isointense to the pancreatic parenchyma. (c) Fat saturated T1 weighted image (TR/TE 400/14 ms) shows the tumour (curved white arrow) as low signal in the surrounding high signal pancreatic parenchyma. (d) Contrast enhanced fat saturated T1 weighted image (TR/TE 400/14 ms) shows that the tumour (white arrow) has become isointense to the pancreas. (e) Contrast enhanced CT shows the tumour only as a bulge (white arrow) in the pancreatic contour.
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Figure 5. A 42-year-old woman with multiple endocrine neoplasia type 1 (MEN 1) and multiple cystic islet cell tumours. (a) Axial fat saturated T1 weighted spin echo image (TR/TE 600/14 ms) and (b) axial fat saturated T2 weighted fast spin echo image (TR/TE 4000/105 ms) show the lesion in the neck of the pancreas (between the arrowheads) to have solid and cystic components, while the two lesions (arrows) in the tail of the pancreas are predominantly cystic.
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All the lesions showed enhancement after intravenous gadolinium. In seven tumours the hypointense lesion on the pre-contrast image became isointense to the enhancing pancreas (Figure 4
). In the remaining 22 tumours the lesion enhanced to a greater degree than the pancreas (Figures 1 and 2). The tumours enhanced homogeneously, except for the cystic lesions, which showed rim enhancement. Gadolinium enhancement did not demonstrate any lesions that were not seen on the non-contrast enhanced image.
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Discussion
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Recent reports have suggested an important role for MRI in the detection of pancreatic NET, in particular with the use of fast spin echo and fat saturation techniques [3]. Fat suppressed T1 weighted sequences have been reported to be particularly useful in imaging pancreatic lesions, especially islet cell tumours [3–6]. Our experience confirms the value of this sequence in visualizing these tumours. The normal pancreas is of relatively high signal intensity on fat saturated T1weighted images. Islet cell tumours are of lower signal intensity than normal pancreatic tissue. This increased contrast between tumours and pancreas explains the greater detection rate with fat suppressed T1 weighted images.
Most of the tumours imaged in this study exhibited the well documented signal characteristics of low signal on T1 weighted or fat suppressed T1 weighted images and high signal on T2 weighted images [2]. However, some islet tumours have low signal on the T2 weighted sequences, which may be due to abundant fibrous tissue [6]. The reason for the high signal of an insulinoma on fat suppressed T1 weighted imaging in this study is not clear, as the patient was managed medically and pathological correlation is not available. The high signal may have been due to a small amount of haemorrhage.
Cystic change within islet cell tumours may be more common than previously thought. A recent large series showed cystic or necrotic change pathologically or on imaging in 42% of cases [7]. Cystic tumours are more often larger and non-functioning compared with the typically solid, homogeneous and smaller functioning tumours. However, predominantly cystic islet cell tumours may be rarer. One of the tumours in our series was a purely cystic lesion, which filled in with solid tissue over 10 years [8].
Calcification may also occur in islet cell tumour, but this feature is not apparent on MRI (Figure 1) [1, 7].
Islet cell tumours, particularly insulinomas, are often vascular. Dynamic MRI with fast gradient echo sequences following bolus injection of contrast medium may aid the detection of these tumours [3]. All the lesions in this series showed enhancement, but in our experience contrast enhancement does not improve tumour visualization compared with non-contrast images. A recent study also found that fat suppressed T1 weighted images with delay after contrast medium injection were the best for detecting islet cell tumours [6].
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Summary
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In summary, the fat saturated T1 weighted spin echo sequence appears to be the most useful pulse sequence in the detection of these tumours. Most tumours are of low signal intensity on these images and of high signal intensity on the T2 weighted image.
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Footnotes
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S A Sohaib was supported by the Joint Research Board, St Bartholomew's Hospital, London. 
Received for publication December 10, 1999.
Revision received October 13, 2000.
Accepted for publication January 24, 2001.
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Abstract
Introduction
