Steven Herwick 1 Frank H. Miller Ana L. Keppke

Transcription

1 Herwick et al. MRI of Islet ell Tumors of the Pancreas bdominal Imaging Pictorial Essay Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved M E D E N T U R I L I M G I N G JR 2006; 187:W472 W X/06/1875 W472 merican Roentgen Ray Society Y O Steven Herwick 1 Frank H. Miller na L. Keppke Herwick S, Miller FH, Keppke L F Keywords: abdominal imaging, MRI, pancreas, pancreatic neoplasms, pancreaticobiliary imaging DOI: /JR Received May 12, 2005; accepted after revision ugust 2, ll authors: Department of Radiology, Northwestern Memorial Hospital, Northwestern University, The Feinberg School of Medicine, 676 N St. lair St., Ste. 800, hicago, IL ddress correspondence to F. H. Miller (fmiller@northwestern.edu). WE This is a Web exclusive article. MRI of Islet ell Tumors of the Pancreas OJETIVE. T 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. ONLUSION. lthough 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. slet cell tumors are neoplasms arising from neuroendocrine cells I 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. ecause 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. T 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 ppearance 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 HSTE sequences, axial in-phase and opposed-phase images, and breath-hold W472 JR:187, November 2006

2 MRI of Islet ell Tumors of the Pancreas Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved Fig year-old man with known malignant islet cell tumor of pancreas. xial T2- weighted HSTE MR image shows low-signalintensity lobulated mass (arrow) in pancreatic tail. T1-weighted fat-suppressed spoiled gradient-echo shared prepulse (SHRP) 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 HSTE sequence were TR/TE, 1,000/63; slice thickness, 3 5 mm; matrix, ; and flip angle, 160. Our T1-weighted fatsuppressed SHRP sequences used 210/1.94; slice thickness, 6 mm; flip angle, 70 ; matrix, ; with 24 slices in a 16-second breath-hold with an integrated parallel acquisition technique (ipt) with a factor of 2. 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 outof-phase sequences were obtained as a combined sequence with the following parameters: 190/4.76 (in-phase); TE, 2.38 (opposedphase); flip angle, 70 ; and matrix, 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 HSTE 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 and 2). 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 (Fig. 2). This may be the best sequence to detect subtle tumors [1]. 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. lthough islet cell tumors are classically considered hypervascular in the arterial phase (Fig. 3), the degree, uniformity, and timing of enhancement can be highly variable. s a result, the ability to distinguish primary or secondary lesions from the sur- Fig year-old man with hypoglycemia and biochemical evidence of insulinoma., xial 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., xial T2-weighted HSTE MR image at same level shows nearly imperceptible lesion (arrow). JR:187, November 2006 W473

3 Herwick et al. Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved Fig 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., xial T1-weighted fat-suppressed spoiled gradient-echo MR image shows lowsignal-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., xial arterial phase T1-weighted fat-suppressed spoiled gradient-echo MR image shows lesion has marked enhancement (arrow) unlike adenocarcinoma, which tends to be hypovascular., xial 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. Fig year-old woman with history of hypoglycemia and clinical diagnosis of insulinoma, which could not be detected on multiple MDT scans over preceding 3 years., xial T1-weighted fat-suppressed spoiled gradient-echo MR image shows 1.4-cm lesion (arrow), which is hypointense relative to surrounding normal pancreas. (Fig. 4 continues on next page) W474 JR:187, November 2006

4 MRI of Islet ell Tumors of the Pancreas Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved Fig. 4 (continued) 52-year-old woman with history of hypoglycemia and clinical diagnosis of insulinoma, which could not be detected on multiple MDT scans over preceding 3 years., xial 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., xial venous phase gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image also shows lesion (arrow) enhancing to same extent as surrounding pancreas. Fig year-old man with history of gallstone pancreatitis and cystic lesion of pancreas thought to represent pseudocyst or cystic neoplasm at MDT., xial T1-weighted fat-suppressed spoiled gradient-echo MR image shows hypointense lesion (arrow) in tail of pancreas., xial gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image shows ring-enhancing lesion (arrow)., oronal gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image shows ring-enhancing lesion (arrow) due to pancreatic islet cell tumor. JR:187, November 2006 W475

5 Herwick et al. Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved Fig year-old man with von Hippel-Lindau disease and pancreatic mass diagnosed as serous cystadenoma on basis of findings at previous fine-needle aspiration. xial 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). Fig year-old woman (same patient as in Fig. 1) with malignant islet cell tumor of pancreas. oronal gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image shows irregular enhancement of mass (arrows) in pancreatic tail. Fig year-old woman with multiple endocrine neoplasia type I and multiple surgically proven islet cell tumors of pancreas., oronal venous phase gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image shows small (< 1 cm) hyperenhancing lesion (arrow) in pancreatic head., oronal 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. rounding 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 (Fig. 4). W476 JR:187, November 2006

6 MRI of Islet ell Tumors of the Pancreas Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved Fig year-old man with large neoplasm incidentally found at MDT., xial T2-weighted HSTE MR image shows lesion of intermediate signal intensity similar to that of muscle or liver. enter 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., xial 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. enter of lesion lacked enhancement due to necrosis (arrow). Fig year-old woman with cystic lesion of pancreatic tail seen at MDT, which proved to be islet cell tumor at fine-needle aspiration biopsy., xial T2-weighted HSTE MR image shows 1-cm lesion (arrow) in pancreatic tail with high signal intensity mimicking pseudocyst or cystic neoplasm of pancreas., xial arterial phase gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image shows rim enhancement of lesion (arrow). 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 (Fig. 5). lso, the uniformity of enhancement tends to be variable, with larger, more malignant lesions exhibiting more heterogeneous enhancement [4] (Figs. 6 and 7). onversely, small lesions are often benign and homogeneous in enhancement [4, 5] (Fig. 8). JR:187, November 2006 W477

7 Herwick et al. Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved Fig year-old man with epigastric pain. MDT showed only dilatation of most distal pancreatic duct in tail., oronal MR cholangiopancreatography image shows dilatation of pancreatic duct (arrow) in tail with normal-caliber pancreatic duct in remainder of gland (chevrons)., xial thin-section T2-weighted HSTE 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. Fig 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., xial T2-weighted HSTE MR image near portal vein origin shows heterogeneous mass expanding distal splenic vein and proximal portal vein., xial gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image at slightly more cephalad level than shows enhancing tumor expanding main portal vein. 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] (Fig. 9). ecause 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, W478 JR:187, November 2006

8 MRI of Islet ell Tumors of the Pancreas Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved and mucinous cystic neoplasms, particularly if multiple contrast-enhanced images are not acquired. rim of enhancement with signal intensity different from the adjacent gland may help characterize the cystic lesion as an islet cell tumor (Fig. 10). Fig 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., xial 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)., xial T1-weighted fat-suppressed spoiled gradient-echo MR image at slightly more cephalad level than shows multiple ring-enhancing hepatic metastases and smaller, homogeneously enhancing lesions (arrows). Fig year-old woman (same patient as in Figs. 1 and 7) with MDT reportedly showing focal nodular hyperplasia in liver., xial arterial phase gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image shows 1.3-cm lesion (arrow) in liver., xial venous phase gadolinium-enhanced T1-weighted fat-suppressed spoiled gradient-echo MR image does not show lesion. (Fig. 14 continues on next page) Local Effects Large and aggressive lesions are prone to secondarily affect adjacent structures such as biliary and pancreatic ducts (Fig. 9). 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 (Fig. 11). In addition, aggressive lesions may also cause complications secondary to compression or invasion of local organs or vascular structures (Fig. 12). JR:187, November 2006 W479

9 Herwick et al. Downloaded from by on 02/19/18 from IP address opyright RRS. For personal use only; all rights reserved 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. entral necrosis often occurs as metastases grow (Fig. 13). In a study by Semelka et al. [6], T2- weighted fat-suppressed images depicted more liver metastases than dynamic contrastenhanced T. 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 gadoliniumenhanced T1-weighted fat-suppressed sequences and T2-weighted sequences is essential (Fig. 14). onclusion lthough classically considered welldefined, 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 T in these patients, who are often young and may require long-term imaging follow-up. References 1. Thoeni RF, Mueller-Lisse UG, han R, Do NK, Shyn P. Detection of small, functional islet cell tumors of the pancreas: selection of MR imaging sequences for optimal sensitivity. Radiology 2000; 214: Fig. 14 (continued) 49-year-old woman (same patient as in Figs. 1 and 7) with MDT reportedly showing focal nodular hyperplasia in liver., oronal 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. onsequently, liver lesions were thought to represent metastases. 2. Sheth S, Fishman EK. Imaging of uncommon tumors of the pancreas. Radiol lin North m 2002; 40: Ichikawa T, Peterson MS, Federle MP, et al. Islet cell tumor of the pancreas: biphasic T versus MR imaging in tumor detection. Radiology 2000; 216: uetow P, Parrino TV, uck JL, et al. Islet cell tumors of the pancreas: pathologic imaging correlation among size, necrosis and cysts, calcification, malignant behavior, and functional status. JR 1995; 165: Fidler JL, Johnson D. Imaging of neuroendocrine tumors of the pancreas. Int J Gastrointest ancer 2001; 30: Semelka R, umming MJ, Shoenut JP, et al. Islet cell tumors: comparison of dynamic contrast-enhanced T and MR imaging with dynamic gadolinium enhancement and fat suppression. Radiology 1993; 186: Semelka R, ustodio M, em alci N, Woosley JT. Neuroendocrine tumors of the pancreas: spectrum of appearances on MRI. J Magn Reson Imaging 2000; 11: W480 JR:187, November 2006