MDCT of the Pancreas II.3

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Introduction

With multidetector-row computed tomography (MDCT), images of the pancreas can be acquired with less than 1-mm collimation and viewed with reconstructed slice thickness ranging from 1 to 5 mm. Thus images are reconstructed from isotropic or nearly isotropic voxels, depending upon the specific acquisition parameters used. This technique enables very high-quality three-dimensional, multiplanar and curved planar reconstructions to be acquired. Consequently, large CT data sets can be viewed efficiently as volumes, rather than as individual transverse slices. The ability to view the image data as a volume also makes it eas- ier to communicate the results efficiently to refer- ring physicians, using a limited number of selected volume-rendered or multiplanar images. These images can be reconstructed to demonstrate the pancreatic duct, bile ducts, and peripancreatic vessels in a clinically relevant fashion.

MDCT Technique

Prior to imaging, patients drink approximately 1 l of fluid to opacify the stomach, duodenum, and jejunum. Water is preferred, rather than iodine or barium-based positive contrast materials, to avoid interference with volume-rendered reconstructions of the peripancreatic vessels. For the detection or staging of pancreatic adenocarcinoma, a volume of 150 ml of low-osmolar contrast material is administered at a rate of 5 ml/s [1, 2]. Images are acquired in two phases: a pancreatic parenchy- mal phase (beginning at 40–45 s from the start of contrast medium injection) and a venous or hepat- ic parenchymal phase (beginning 70 s from the start of contrast medium injection) [3, 4]. The pancreatic phase images are acquired with less than 1 mm collimation, whereas the venous phase im-

ages are acquired with 1.5-mm or 2.5-mm collimation, depending upon whether 4-detector or 16-detector CT is used (Tables 1 and 2). The images are reconstructed with 3-mm slice thickness at 2-mm intervals (1-mm slice thinkness at 1-mm intervals for 3D reconstructions). The data can be viewed on a workstation where three-dimensional volume-rendered, maximum intensity projection and multiplanar images can be viewed [5-9]. In addi- tion, curved planar images can be constructed through key anatomical structures, such as the peripancreatic arteries and veins, the common bile duct, and the pancreatic duct [10, 11] (Figs. 1–5).

Adenocarcinoma of the Pancreas

MDCT is established as the primary initial imaging method for both the detection and staging of suspected pancreatic carcinoma. Most studies have found that CT is highly reliable when it demonstrates features indicating that a tumor is unresectable [12, 13]. The positive predictive value of a diagnosis of unresectability with single-detector helical CT (SDCT) has ranged from 92% to 100%

[14, 18]. SDCT is less reliable, however, for predict- ing whether a tumor is resectable [negative predictive value (NPV) 76%–90%] [14–19]. In a recent study using eight-detector MDCT, the NPV and accuracy for vascular invasion were 100% and 99%, respectively [20]. The overall NPV in this study, however, was 87% due to undetected small hepatic and peritoneal metastases, which continue to be a limitation of CT, even MDCT.

Although criteria for unresectability vary among surgeons, imaging features that generally indicate unresectability include vascular invasion, lymph node metastases beyond those in the im- mediate vicinity of the pancreas, and distant metastases. Metastases most commonly involve the liver or peritoneum.

II.3

MDCT of the Pancreas

Jay P. Heiken

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Recent studies have demonstrated the useful- ness of CT for evaluating vascular invasion by pancreatic carcinomas [20-24]. When a pancreatic tumor is not contiguous with a critical peripancreatic vessel such as the hepatic artery, superior mesenteric artery, superior mesenteric vein, or portal vein (i.e., when an intervening fat plane is present), vas-

cular invasion is almost never present. When the tumor is contiguous with less than one-quarter of the vessel circumference, it is resectable in the majority of cases, but when the tumor is contiguous with one-quarter to one-half of the vessel circumference, it is unresectable in the majority of cases. It is in the group of patients in which the tumor con-

a b c

Fig. 1a-c.Unresectable pancreatic cancer. a The transaxial image demonstrates a large heterogeneous mass (m) involving the tail of the pancreas. The posterior superior pancreatoduodenal vein (yellow arrow) and the gastroepiploic vein (white arrow) are dilated, indicating invasion of the superior mesenteric vein and splenic vein, respectively. b A coronal maximum intensity projection image shows the dilated posterior superior pancreaticoduodenal vein (black arrow) and gastroepiploic vein (yellow arrows). The portal vein is occluded at its con- fluence with the superior mesenteric vein (white arrow). c A transaxial maximum intensity projection image also demonstrates the dilated posterior superior pancreaticoduodenal (white arrow) and gastroepiploic (yellow arrows) veins

Table 1. 4-row MDCT Pancreas: Dual Phase

Indications Evaluation of known or suspected pancreatic neoplasms Evaluation for complications of pancreatitis

Scan range Pre-contrast: Standard abdomen

Arterial: (craniocaudal) from third duodenum or inferior tip of liver, whichever is more caudal to the top of the pancreas.

For suspected islet cell tumors, scan up to the diaphragm Venous: (craniocaudal) from iliac crest to diaphragm mA selection 180-250 mA-adjust for patient size

Detector collimation Pre-contrast: 2.5 mm Arterial: 1 mm

Venous: 2.5 mm

Rotation time 0.5 s

Table speed (pitch) Pre-contrast: 10–18 mm/rot (1-1.75) Arterial: 4–7 mm/rot (1-1.75) Venous: 13–18 mm/rot (1-1.75)

Slice thickness Pre-contrast 5 mm

Arterial: 3 mm and 1.25 mm (3-D)

Venous: 3 mm

Reconstruction int. 3 mm for the 5-mm slices 1 mm for the 1.25-mm slices

IV contrast type 350–370 mgI/ml

Volume: 150 ml Injection rate: 5 ml/s

Scan delay Arterial: 40 s (for suspected islet cell tumors, this should be decreased to 30 s) Venous: 60 s (or ASAP)

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tacts up to one-half of the vessel circumference that endoscopic ultrasonography may be of value in assessing vascular invasion. Otherwise, surgical ex- ploration is needed to determine resectability. Tu- mors contacting more than one-half of the circumference of the vessel are nearly always unresectable.

Another sign of unresectability of adenocarcinoma

of the head of the pancreas is a teardrop shape of the superior mesenteric vein, which represents ei- ther direct tumor infiltration of the vein or peritu- moral fibrosis adherent to the vessel [25].

Assessment of the peripancreatic veins can also provide information regarding the likelihood of vascular invasion by pancreatic carcinoma. In pa- Table 2. 16-row MDCT

Pancreas: Dual Phase

Indications Evaluation of known or suspected pancreatic neoplasms Evaluation for complications of pancreatitis

Scan range Pre-contrast: standard abdomen

Arterial: (craniocaudal) from diaphragm to bottom of liver Venous: (craniocaudal) from diaphragm to iliac crest

Effective mA 180

Detector collimation Pre-contrast: 1.5 mm Arterial: 0.75 mm Venous: 1.5 mm

Rotation time 0.5 s

Table speed (pitch) Pre-contrast: 24–30 mm/rot (1–1.25) Arterial: 12–15 mm/rot (1–1.25) Venous: 24–30 mm/rot (1–1.25)

Slice thickness Pre-contrast 5 mm

Arterial: 3 mm and 1 mm (3-D)

Venous: 3 mm

Reconstruction int. 3 mm for the 5-mm slices 0.8 mm for the 1-mm slices

IV contrast type 350–370 mgI/ml

Volume: 150 ml Injection rate: 5 ml/s

Scan delay Arterial: 45 s (for suspected islet cell tumors, this should be decreased to 35 s) Venous: 65 s (or ASAP)

Fig. 2.Resectable pancreatic cancer. Curved planar reformation through the pancreatic duct demonstrates a mass (m) in the body of the pancreas. The tail of the pancreas is atrophic with a dilated pancreatic duct (yellow arrows). The pancreatic duct in the head and neck of the pancreas (white arrow) is normal

Fig. 3.Pancreatic adenocarcinoma. Curved planar reformatted image through the common bile duct (white arrow) and pancreatic duct (black arrow) shows obstruction of both ducts by an adenocarcinoma that diffusely infiltrates the pancreatic head

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tients with pancreatic carcinoma, dilatation of the posterior superior pancreaticoduodenal vein or the gastrocolic trunk is a sign of portal or superior mesenteric vein invasion [26-29] (Fig. 1). However, a dilated gastrocolic trunk should not be used as an independent sign of surgical unresectability [24].

Pancreatitis

MDCT is the imaging method of choice for evaluating complications of acute and chronic pancreatitis. Three-dimensional, multiplanar, and curved planar reconstructions of the data are useful for demonstrating the extent of pancreatic necrosis (Fig. 5) and peripancreatic fluid collections, and for demonstrating intraductal pancreatic calculi.

CT angiography is an excellent technique for de- lineating pseudoaneurysms of the peripancreatic vessels.

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