CT and MR Imaging of Pancreatic Cancer Part 1

Abstract
Computed tomography (CT) and magnetic reso-nance imaging (MRI) are emerging noninvasive techniques for imaging of the pancreas. Based on multislice technology, CT enables multiplanar imaging of the pancreas (multislice-CT, MSCT) with a high contrast between vessels and paren-chyma. In addition, MRI of the pancreas includ-ing the imaging of the lumina of the biliary tree and the pancreatic duct (magnetic resonance cholangiopancreaticography, MRCP) and the abdominal vessels (MR angiography, MRA) has become available for daily clinical practice in most hospitals. The addition of multiplanar and curved reformations may increase the sensitivity of CT and improves its agreement with surgical findings. Beyond abdominal MR imaging, tech-niques such as magnetic resonance cholangio-pancreaticography (MRCP) and MR angiogra-phy should be integrated in the imaging protocol whenever possible.

2.1      Introduction
Pancreatic cancer is the third most common malignancy of the gastrointestinal tract, the in-cidence rate is estimated by 10 cases per 100,000 people per year. Surgery is the only therapy with curative intention and may result in long-term survival in those cases where the cancer is still confined to the organ itself. Surgery of the more advanced stages is being performed in some centers, but so far it has not become a routine procedure. Differentiating between pancreatic malignancy and focal chronic pancreatitis—in particular when taking into account that long-term pancreatitis is a risk factor in pancreatic cancer—is still a clinical challenge.
The main tasks of diagnostic imaging of the pancreas are the detection of pancreatic lesions and the differentiation between malignant and benign (e.g., inflammatory) changes in the pan-creas. In addition, computed tomography/mag-netic resonance (CT/MR) imaging should ideally be able to permit staging of pancreatic tumors in-cluding the detection of malignant infiltration of lymph nodes and distant metastases (Malka et al. 2002). When surgery is considered, resectability of the tumor is usually defined by the presence or absence of the infiltration of the portal vein, the venous confluens, or the superior mesenteric vein. Moreover, the detection of anatomic varia¬tions of the branches of the celiac trunk and the superior mesenteric artery is of crucial impor¬tance (Fuhrman et al. 1994).
CT and MRI are emerging noninvasive techniques for imaging of the pancreas. Based on multislice technology, CT enables multiplanar imaging (multislice-CT, MSCT) with a high contrast between vessels and parenchyma. In addition, MRI of the pancreas including the imaging of the lumina of the biliary tree and the pancreatic duct (magnetic resonance cholangio-pancreaticography, MRCP) and the abdominal vessels (MR angiography, MRA) has become available for daily clinical practice in most hospitals. Therefore, invasive digital subtraction angiography (DSA) has lost its importance in many institutions for preoperative diagnosis of the pancreas.

2.2     Technical Aspects

2.2.1    CT of the Pancreas
CT is a well-established modality in diagnostic imaging and staging of pancreatic cancer. The introduction of fast multidetector spiral CT has made it possible to study the pancreas during different perfusion phases of the organ (Rich-ter et al. 1996). Raw data of multislice images of the different perfusion phases may be used for the reconstructions of the parenchyma of the pancreas (usually 3-5 mm) as well as for recon-structions of the arterial and venous vessels (CT angiography, CTA) based on the thinner slices (0.5-1.0 mm; Fig. 2.1). CT scanning during the arterial phase of the perfusion is advantageous in the diagnostic workup of pancreatic cancer due to the presence of low perfused fibrotic and necrotic tissue in pancreatic cancer. For practical considerations, CT of the pancreas is mostly per-formed in multiple phases including at least the
arterial and the portal venous phase, enabling an accurate detection of distant metastases in the liver as well as the imaging of the abdominal veins (Graf et al. 1997). Very recently, Ichikawa et al. (2006) demonstrated that there is some use for an additional parenchymal scan between the arterial and portal venous phases in combination with multiplanar reconstructions for tumor de-tection based on a population of 35 patients.
Hydro-CT (oral contrast enhancement with 800-1,000 ml of water) has become standard clinical practice in many institutions due to a better delineation of the pancreas from the pos-terior gastric wall (Baum et al. 1999; Schima and Ba-Ssalamah 1999; Richter et al. 1996).
Curved reformation techniques as recently introduced for CT imaging of the pancreatic duct may be play an important role in daily clini¬cal practice because of a better availability due to faster reconstruction techniques (Sahani et al.
2006).


2.2.2    Magnetic Resonance
Imaging of the Pancreas
While 3 T and more seems to be the new stan¬dard for imaging of the central nervous system, 1.5 T is still state-of-the-art for MRI of the abdo¬men. MRI of pancreatic neoplasms is optimally performed by using breath-hold acquisitions im¬plicating that the patient should be able to stop breathing for more than 15 s. Conventional T1 weighted and T2 weighted images in transversal orientation are usually performed. In addition, dynamic, three-dimensional sequences after the administration of gadolinium-diethylenetri-aminepentaacetic acid (DTPA) or other nephro-tropic contrast media are obtained in arterial, portal, and delayed phases. On post-contrast im¬ages, fat suppression techniques that increase the contrast between uninvolved fat tissue and the enhancing tumor are recommended (Martin and
Semelka 2000).
MRCP are usually obtained to evaluate the pancreatic duct and to exclude pancreatic cysts. Moreover, MRCP is an additional tool to increase the sensitivity of MRI for the detection of small pancreatic tumors (Fig. 2.2). Based on heavily T2 weighted sequences, MRCP sequences enable the selective imaging of fluids inside the biliary system and the pancreatic duct.
Single breath-hold MRCP techniques provide selective views of the whole pancreatic duct in-cluding the extrahepatic biliary tract without artifact using thick (2- to 8-cm) sections. Some authors prefer oral dark lumen contrast media to avoid an overlay based on adjacent organs such as stomach or duodenum. However, additional thin slice MRCP may enable a precise localiza¬tion of the fluid structures. Some authors prefer secretin administration in improving pancreatic
ductal details in MRCP (Petersein et al. 2002;
Fulcher and Turner 1999). Based on an exogenous administration of secretin, the secretion of pancreatic juice is stimulated, which conse¬quently increases the volume of stationary fluid in the pancreatic ducts, which may improve di¬agnosis in small pancreatic tumors.
Mangafodipir trisodium (Mn-DPDP) was introduced as a tissue-specific contrast agent a couple of years ago. Some authors recommend the use of Mn-DPDP for the detection of subtle pancreatic neoplasms in equivocal cases. The mechanism of action of Mn-DPDP is that nor¬mal pancreatic parenchyma enhances following administration and becomes hyperintense on T1 weighted images whereas most of pancreatic neoplasms do not enhance (Kettritz et al. 1996; Gehl et al. 1993). It has been reported that Mn-DPDP-enhanced MRI provides better delinea-tion of the pancreatic tumor but does not sig-nificantly improve the detection rate and staging accuracy of focal pancreatic lesions over MRI without this contrast medium (Romijn et al. 2000). However, no clinical data exist to date that underline an impact on differential diagnosis of pancreatic masses when compared with dynamic MRI based on gadopentetate dimeglumine (Gd-
DTPA) or other Gd-chelates (Fig. 2.3).
MR angiography based on Gd-chelates may be used in conjunction with abdominal MRI including MRCP in one session using 3D gradi¬ent echo sequences. These so-called "all-in-one" MRI are becoming increasingly popular in clini¬cal practice because of their versatility in patients with pancreatic tumors for the evaluation for surgical resection (Fig. 2.4). There is a slight dif-ference between the MR sequences used for dy-namic images (better soft tissue contrast due to higher signal of the pancreas, thicker slices) than for MR angiography (higher contrast between vessels and parenchyma) in theory. Morakkabati-Spitz et al. (2002) did not observe any advantages of dynamic imaging in patients with a suspicion of pancreatic cancer and concluded that the in¬jection of contrast material should preferably be used for the performance of a contrast-enhanced MR angiography at the expense of a dynamic MR examination.