Imaging



Fig. 9.1
Trans-abdominal US and CEUS imaging findings of small PanNENs. Small iso-echoic tumor (arrows) located on the pancreatic head (a), showing early intense enhancement (arrows) on CEUS (b). Typical small hypoechoic insulinoma of the pancreatic head (arrows in c), seen as a hypervascular focal lesion (arrows); for comparison, normal parenchyma as seen on CEUS (d)



Trans-abdominal US usually permits the correct identification of large PanNENs. The majority of these tumors appear as inhomogeneous hypo-/isoechoic masses relative to the normal pancreatic and liver parenchyma (Fig. 9.2) [25]. Most NF-PanNENs are large at presentation, with a well-defined multilobulated border and a compressive rather than an infiltrative pattern of growth [20, 25].

NF-PanNENs cannot be reliably characterized using conventional US. (Figs. 9.1, 9.2); however, once they are detected on baseline scan, CEUS allows for their dynamic evaluation and characterization. The early and intense enhancement and the slow washout of hypervascularized tumors can be documented (Figs. 9.1, 9.2) and is a key feature in the differential diagnosis with adenocarcinoma, which usually appears as a hypovascular mass [20, 25].

Small lesions, measuring < 3 cm in diameter, usually enhance homogenously [17].

Intralesional necrosis and or hemorrhagic areas, frequently encountered in large lesions, result in inhomogeneous central hypoechoic areas better depicted at CEUS [15, 20, 25].

Large lesions may be associated with the encasement of arterial or venous vessels and with peri-tumoral lymphadenopathy.

Metastatic disease in the liver occurs in about 30% of cases [13, 25] and represents a clear sign of malignancy. The US appearance of the liver metastases is variable. Hyperechoic nodules or a target-like appearances at baseline US suggests an endocrine nature of the primary tumor, but this pattern is not specific. In other cases, the metastases show a non-specific hypoechoic pattern [15, 25]. A dedicated CEUS study of a suspected liver lesion may point out the typical hypervascularity shown by endocrine tumors during their early dynamic study (Fig. 9.3).

A978-88-470-2673-5_9_Fig2_HTML.jpg


Fig. 9.2
Trans-abdominal US and CEUS imaging findings of NF-PanNENs. A large hypoechoic tumor (arrows) located on the pancreatic body (a), showing early intense enhancement (arrows) on CEUS (b). Large well-defined hypoechoic NF-PanNEN of the pancreatic head (arrows in c), seen as a hypervascular focal lesion (arrows) at CEUS (d)

EUS visualizes islet-cell tumors as a relatively hypoechoic area compared with the adjacent pancreas, with smooth and at times slightly irregular margins that are well demarcated.

In the identification of small lesions located in the pancreatic head, EUS has high sensitivity but it may be limited in completely evaluating the tail of the pancreas, depending on its location [26]. A recently published study demonstrated the value of EUS in the preoperative assessment of patients with MEN1 [27].

In the identification of small F-PanNENs during surgery, a combination of intraoperative palpation and IOUS was found to achieve the best results due to the complementary nature of these two techniques.

A978-88-470-2673-5_9_Fig3_HTML.jpg


Fig. 9.3
Large liver metastasis from a NF-PanNEN/C studied using trans-abdominal US and CEUS and MDCT. A large, slightly hypoechoic, focal liver lesion (arrows) can be appreciated in the left lobe (a). The lesion shows early intense enhancement (arrows) on CEUS (b). At MDCT, during arterial pancreatic phase (c), the lesion has the typical hypervascular pattern of endocrine tumors, with subsequent washout during portal venous phase (d)




9.3 Computed Tomography



9.3.1 Imaging Technique


Multidetector computed tomography (MDCT) is a widely available imaging technique capable of providing, in a short examination time, images characterized by excellent spatial and contrast resolution. The MDCT protocol should include both unenhanced and enhanced scans, as the former, obtained at baseline, can be useful in the detection of intralesional calcifications (which may occur in F- and NF-PanNENs) (Fig. 9.4), and to accurately plan the dynamic contrast-enhanced study.

The intravenous administration of iodinated contrast agent is needed to optimally visualize the pancreatic parenchyma, increasing contrast resolution [28].

PanNENs are frequently hypervascular focal lesions, appearing as high-attenuating lesions during early contrast-enhancement phases [29, 30] (Figs. 9.5, 9.6). Accordingly, the protocol should include at least two contrast-enhanced phases [3032], acquired, respectively, with a 40- to 45-s delay (arterial pancreatic phase) and a 70- to 80-s delay (portal venous phase) after the administration of contrast material (calculated by using the bolus-tracking technique).

A978-88-470-2673-5_9_Fig4_HTML.jpg


Fig. 9.4
Calcification in PanNENs. Unenhanced CT scan points out a tiny F-PanNEN located in the pancreatic tail, with intralesional calcifications (a). These are not depicted on T1-weighted MRI (b). A large NF-PanNEN located in the pancreatic head, with intralesional calcifications depicted on unenhanced axial (c) and coronal (d) scans

However, about 30% of these tumors will have an atypical vascular pattern, resulting in iso- or even hypoattenuating lesions with respect to adjacent pancreatic parenchyma (Fig. 9.7) [13].

In our experience, in most cases the best enhancement is obtained during the arterial pancreatic phase [13]; nonetheless, additional contrast-enhanced scans should be taken in selected cases. An early arterial phase, acquired with a delay of 20–25 s (vascular arterial phase), may be useful in the detection of small tumors characterized by subtle brief enhancement. In addition, it may allow detailed arterial vascular mapping, which is useful for staging locally advanced tumors and for treatment planning (Fig. 9.8).

A late venous phase, acquired with a delay of about 120 s, may be useful for depicting a delayed hypervascular enhancing pattern. A multi-phase imaging protocol therefore offers the advantage of increasing the possibility of demonstrating the typical hypervascular pattern of PanNENs, to allow locoregional staging and the detection of liver metastases [13].

A978-88-470-2673-5_9_Fig5_HTML.jpg


Fig. 9.5
Typical tiny insulinoma studied by MDCT. The lesion (arrow) appears isodense on baseline unenhanced scan (a). During a contrast-enhanced study, the lesion (arrow) shows the typical hyperdensity on axial (b) and para-coronal reconstructed images obtained from the arterial pancreatic phase (c), showing washout during the venous phase (d)

Curvilinear reconstructions should be used to highlight the relationship between primary tumors and the pancreatic and biliary ductal systems (Fig. 9.9).

The patient can be administered a glass of water immediately before the examination, to assure optimal filling of the stomach and duodenum with a low-contrast medium and for better definition of the gastric and duodenal wall. We usually avoid administrating oral iodinated contrast material, to avoid the misinterpretation or masking of hypervascular lesions ectopically located within the duodenal or small-bowel wall.


9.3.2 Imaging Findings


Baseline CT usually depicts PanNENs as isodense masses with respect to normal parenchyma (Figs. 9.5a, 6a, 7a). Thus, the tumor might be missed on an unenhanced scan, unless it has a large diameter and/or is associated with a distorted morphology of the gland [13].

A978-88-470-2673-5_9_Fig6_HTML.jpg


Fig. 9.6
Typical enhancement pattern of NF-PanNEN/Cs studied by MDCT. The lesion (star) appears isodense on baseline unenhanced scan (a). During contrast-enhanced study, the tumor (star) shows the typical hyperdensity on axial images obtained during arterial pancreatic (b) and portal venous phases (c). Para-coronal reconstructed images obtained during the portal venous phase better depicts encasement of the superior mesenteric vein (arrows in d)

Tumor inhomogeneity may be caused by globular or lamellar calcifications, seen both in F-panNENs (mostly insulinomas) and in NF-PanNENs, in these cases often associated with large areas of necrosis (Fig. 9.4a, b, d) [33].

During dynamic contrast-enhanced study, both functioning and non-functioning PanNENs usually appear as well-defined round or oval-shaped hypervascular masses [33]. At MDCT, the most frequent peak-enhancement phase is the arterial pancreatic phase whereas rapid washout is frequently seen in the portal venous and late venous phases (Fig. 9.6).

Achieving an intense enhancement is also useful to better define the dimensions of the tumors and to evaluate the relationship with adjacent structures [34] (Fig. 9.10).

Enhancement is usually homogeneous in small lesions, measuring < 3 cm in diameter [13, 33]. Conversely, large tumors, measuring > 3 cm in diameter, are typically inhomogeneous because of the presence of intralesional necrotic areas or cystic degeneration [13, 35], appearing as hypodense areas compared with the viable hypervascularized neoplastic tissue (Fig. 9.9).

A978-88-470-2673-5_9_Fig7_HTML.jpg


Fig. 9.7
Atypical enhancement pattern of PanNEN/Cs studied by MDCT. The lesion (arrow) appears isodense on baseline unenhanced scan (a). During contrast-enhanced study, the tumor (arrow) is isodense on axial images obtained during arterial pancreatic (b) and portal venous phases (c, d). Multiple, tiny, slightly hypodense liver metastases are seen in segments V and VI (arrowhead)

MDCT, due to its high spatial, contrast, and temporal resolution, is advantageous in loco-regional staging, depicting the encasement of both the arterial (superior mesenteric artery or the celiac axis) and venous (superior mesenteric vein and portal vein) vessels [13, 33]. Three-dimensional reconstruction of the peri-pancreatic vessels may be of help in treatment planning (Fig 9.8).

Neoplastic thrombus within the peri-pancreatic veins has the same density as the mass from which it derives, thus displaying a slightly lower density than the vascular lumen.

Secondary phenomena, such as bile duct dilation or vascular encasement, may be well-demonstrated by means of MDCT, using axial native images and multi-planar dedicated reconstructions (Fig. 9.9b) [13]. Also, dilatation of the biliary tree may be depicted in case of tumors located in the pancreatic head.

Depending on its location, the primary tumor may dislocate or compress adjacent structures such as the stomach and duodenum, spleen and left kidney, and the adrenal gland (Fig. 9.11). Frank invasion into adjacent viscera is rare and is usually associated with the presence of an endocrine carcinoma.

A978-88-470-2673-5_9_Fig8_HTML.jpg


Fig. 9.8
Vascular encasement demonstrated using MDCT reconstructions. A large inhomogeneous NF-PanNEN/C of the pancreatic head is associated with complete encasement of multiple vessels (arrows), depicted on axial images. The volume-rendering reconstructed image obtained from the arterial pancreatic dataset (b) clearly shows the encasement of multiple vessels, including the superior mesenteric artery (arrows). Maximum-intensity projection (MIP) reconstructions (c, d) in another patient show involvement of the celiac axis

Even if less accurate than MRI in identifying liver metastases, MDCT represents a reliable tool for the identification of metastatic involvement of the liver in case of PanNENs [13, 33].

There is no difference between the metastases of F-PanNENs and NFPanNENs. These lesions usually share imaging features of primary tumors, i.e., slightly hypodense compared with the normal parenchyma on unenhanced CT scan, and hyperdense hypervascular lesions (sometimes with a target-like pattern) on arterial enhanced scan [30, 36]. Liver metastases typically show washout during portal venous and late venous phases, resulting in lesions hypodense to normal liver parenchyma (Fig. 9.3c, d). Calcifications may be present as well.

Hypovascular liver metastases may be associated with hypovascular primary tumors (Fig. 9.7).

A978-88-470-2673-5_9_Fig9_HTML.jpg


Fig. 9.9
Involvement of the main pancreatic duct, demonstrated using MDCT and MRI. A large inhomogeneous NF-PanNEN/C of the pancreatic head is associated with the complete upstream dilatation of the main pancreatic duct (arrows in a). The dilatation is better depicted using curvilinear reconstruction in the para-coronal plane (b). Dilatation of the main pancreatic duct (arrows) is well-depicted using contrast enhanced MRI, as seen on the axial plane (c) and on the MRCP image (d)

The typical features of NF-PanNENs, such as a well-demarcated hypervascular mass with a compressive pattern of growth, are present in about 70% of patients [13, 15, 28, 33]. In the other 30%, the pattern is non-specific and a reliable differential diagnosis with ductal adenocarcinoma is not possible [37], since the tumor is mainly hypodense compared with the pancreatic parenchyma (Fig. 9.7).

When the mass is large and well-circumscribed, a ductal adenocarcinoma can be excluded, but the problem of differential diagnosis from other rare, solid tumors remains, including solid variants of micro-cystic cystadenoma and pancreatic metastases.

PanNENs may appear as iso-attenuating to normal pancreas in pancreatic phase CT images, and sometimes are better delineated in the portal venous phase [13, 38]. Late enhancement of the tumor may be explained by extensive necrosis, resulting in a slower washout from the mass, due to the reduced vascularization [39].

A978-88-470-2673-5_9_Fig10_HTML.jpg


Fig. 9.10
Involvement of peri-tumoral vessels, assessed using MDCT and MRI. A relatively small but inhomogeneous NF-PanNEN/C of the pancreatic head is located close to the superior mesenteric vein (arrow) on axial portal-enhanced MRI (a) and MDCT (b). Coronal reconstructions obtained using MRI (c) do not rule out vessel, involvement (arrows). Conversely, MDCT (d), with its higher spatial resolution, demonstrates the presence of an adipose interface between tumor and vessel (arrows)


9.4 Magnetic Resonance Imaging



9.4.1 Imaging Technique


In patients with PanNENs, the information obtained with MRI is similar to that obtained with CT, with the additional advantage that the patient is spared radiation exposure. However, due to its relatively limited availability and the longer examination time, MRI is not as widely used as CT for imaging pancreatic tumors.

State-of-the-art MRI of pancreatic neoplasms is optimally performed with 1.5 Tesla gradient systems using phased-array coils to improve the signal-tonoise ratio, optimized with thin slices and a small field of view [13, 40]. Breath-hold acquisitions are obtained with fast spin echo (FSE) or gradient echo (GRE) sequences and echo planar imaging. A moderately T2-weighted FSE and single-shot FSE (SSFSE) should be obtained, followed by T1-weighted in-phase GRE and T1-weighted opposed-phase GRE.

A978-88-470-2673-5_9_Fig11_HTML.jpg


Fig. 9.11
Infiltration of adjacent structures as assessed using MDCT (a, b) and MRI (c, d). A large inhomogeneous NF-PanNEN/C of the pancreatic tail infiltrates the splenic hilum and peri-pancreatic fat (arrow), as well-depicted with both modalities

T1-weighted images with fat suppression have proven to be useful for imaging the pancreatic gland, allowing high contrast resolution between the normal bright parenchyma and the surrounding hypointense retroperitoneal fat [41]. Coronal and axial magnetic resonance cholangiopancreatography (MRCP) with SSFSE accurately depicts the pancreatic ducts.

For the evaluation of PanNENs, fat-suppressed three-dimensional spoiled GRE sequences after the administration of gadolinium-DTPA are acquired in arterial phase (30–40 s), portal phase (70–80 s), and equilibrium phase (180 s) [13, 40, 41]. The acquired images should cover the upper abdomen, including the entire liver, thus improving the detection and characterization of locoregional lymph-nodes and hepatic lesions [13].

Similar to contrast-enhanced helical CT, additional gadolinium enhanced scans can be obtained in the early arterial (scan delay 20 s) or late venous (120 s) phases, which increases the possibility of imaging the typical hypervascular enhancement pattern of these tumors [13, 31, 42].


9.4.2 Imaging Findings


On T1-weighted images, the normal pancreas exhibits medium to high signal intensity, similar to or slightly less than that of liver, but lower than that of retroperitoneal fat.

Fat suppression should be used, especially for T1 sequences, to increase pancreatic conspicuity, since with this technique the pancreas assumes a bright signal intensity that facilitates the detection of focal lesions [41, 42]. In patients with fatty involution of the pancreas, the signal intensity of this organ increases on T1-weighted sequences according to the amount of fat present within the parenchyma. Consequently, the pancreatic bed appears as an area of very low signal intensity on fat-sat sequences, which therefore are of little use in the visualization of small tumors (Fig. 9.12).

T2-weighted images with fat suppression may be useful for the evaluation of peri-pancreatic structures and inflammatory changes, but they are not strictly needed for imaging panNENs. On TSE T2-weighted images, the pancreas demonstrates intermediate signal intensity, similar to that of the liver. The signal may be intermediate to low on HASTE sequences.

A978-88-470-2673-5_9_Fig12_HTML.jpg


Fig. 9.12
Typical MRI finings of insulinoma. The on baseline unenhanced scan shows the lesion (arrow) as hypointense on the fat-saturated, T1-weighted, axial image (a) and hyperintense on the coronal T2-weighted image (b). During the contrast-enhanced study, the lesion (arrow) shows the typical hypervascularity on axial images obtained during the arterial pancreatic phase (c) and subsequent washout during late venous phase (d)

The surface of the normal pancreatic parenchyma may be either smooth or lobulated.

Pancreatic ducts appear as low-signal intensity tubular structures on T1- weighted sequences and as high-signal intensity structures on heavily T2- weighted scans. At MRCP, heavy T2-weighting and fat suppression provide a cholangiogram useful for the evaluation of pancreatic and biliary duct involvement.

The typical MRI features of panNENs include a pancreatic mass of low signal intensity on T1-weighted images and of intermediate to high intensity on T2-weighted images (Fig. 9.12). As previously stated, the better intrinsic contrast resolution of this imaging technique may be advantageous for the identification of very small primary tumors, which frequently appear hypointense relative to the normal parenchyma on T1-weighted sequences. Lesion conspicuity is usually enhanced by fat-suppression.

Small lesions, which account for the majority of F-panNENs and some incidentally detected NF-PanNENs, are often quite homogeneous. Conversely, larger tumors may appear markedly inhomogeneous due to intralesional necrosis or hemorrhage, which may be seen as hyperintensity on T1-weighted images [43] and inhomogeneous hyperintensity on T2-weighted images (Fig. 9.13).

Cystic tumors have been described [35] and are often associated with widespread intralesional necrosis. Cystic lesions are usually unilocular, with contents that are hypointense on T1-weighted and hyperintense on T2-weighted images [44]. The cystic wall may show variable thickness [44]. The appearance of these tumors may be similar or even identical to that of other cystic tumors of the pancreas. Sometimes, intense enhancement of a peripheral ring-shaped viable tumor will suggest the diagnosis of cystic NF-panNENs (Fig. 9.14); however, in the majority of cases, a definitive diagnosis can only be obtained by histological examination of the resected specimen.

Among their atypical features, some islet tumors may have a low signal intensity on T2-weighted images due to the presence of abundant fibrous tissue [45]; in such cases they may be indistinguishable from ductal adenocarcinoma.

As seen for CT, during dynamic contrast-enhanced study, the majority of PanNENs show a typical hypervascularity [13, 31, 33, 46], resulting in hyperintense lesions compared to normal pancreatic and liver parenchyma (Figs. 9.11c, d, 9.12, 9.13).

In general, small tumors are depicted as homogeneously enhancing lesions (Fig. 9.12), whereas large tumors may appear markedly inhomogeneous during dynamic studies, since central necrotic areas remain hypointense on T1- weighted images even after contrast medium administration (Fig. 9.13).

Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Jun 14, 2017 | Posted by in GENERAL SURGERY | Comments Off on Imaging

Full access? Get Clinical Tree

Get Clinical Tree app for offline access