Multislice CT and 3D propel pancreatic imaging forward

Buoyed by effective postprocessing techniques, modern multislice CT has swept away some of the modality's limitations in visualizing the complex pancreas and created new challenges for radiologists in assessing incidentally detected lesions.

At the Stanford International Symposium on Multidetector-Row CT held in San Francisco this past summer, radiologists demonstrated how the latest scanners are propelling CT forward in the pancreas, pushed by the steam of sophisticated multiplanar reformations and minimum intensity projections, which are now considered crucial.

Traditionally, evaluating the pancreatic duct with CT has been challenging. The relatively small caliber of the duct in normal patients, the complex anatomy of the duct within the pancreas and of the pancreas in the abdomen, and the inherent low contrast between the duct and glandular parenchyma, even after contrast enhancement, all made CT imaging difficult, said Dr. Jorge Soto, an associate professor of radiology at Boston University, who spoke at the Stanford symposium.

Sixty-four-slice units address some of these difficulties. Scanning with much thinner slices yields much higher spatial resolution. Faster speed ensures peak enhancement in the glandular parenchyma and eliminates motion artifact.

"We are now seeing the normal duct in all patients, whereas we could not do that in the past. And we now see lesions affecting the duct and congenital anomalies that were hidden to us before," said Soto in a postconference interview with Diagnostic Imaging.

Major applications of ductal imaging with MSCT include assessment of ductal anomalies, strictures, cystic lesions, and stones. In the past, if pancreatic lesions affecting the duct were seen on abdominal CT, the patient was typically referred for MR cholangiopancreatography (MRCP) or endoscopic retrograde pancreatography. As the advantages of 64-slice CT are becoming apparent, such referrals increasingly become unnecessary, even if the initial scan is not done using protocols optimized for the pancreas.

"If the initial CT is performed well, it is really very unlikely that another CT or MR will be necessary," Soto said.

POSTPROCESSING CRITICAL

Postprocessing options, which may be performed on dedicated workstations, in a 3D lab, or with PACS-integrated 3D software, are particularly important for assessing pancreatic findings and communicating them with other physicians. Radiologists at Soto's institution use the multiplanar function in all abdominal CT studies and then generate reformations in direct coronal and sagittal planes.

Minimum intensity projections (MinIPs) and curved planar (linear) reformations are also often generated, and they provide vital information. These options are advantageous for displaying anatomy and disease of the complete pancreatic duct in a single image. The combination of MinIPs and curved planar reformations rivals the diagnostic information provided by MRCP, Soto said. He stressed, however, that the CT reformations should be created by a highly trained operator who knows exactly what anatomy and pathology need to be displayed.

"There is always a risk with curved linear reformations of false representations of anatomy. That is important to keep in mind," he said.

Sixty-four-slice CT is also proving useful at providing staging information in patients with pancreatic cancer. CT is comparable to MR in providing information about the pancreatic duct and bile duct for planning therapy. MR used to be the study of choice for congenital anomalies such as pancreas divisum, but now 64-slice CT, using MinIPs and curved linear reformations, is stepping into that role.

Another speaker at the Stanford symposium, Dr. R. Brooke Jeffrey, agreed that postprocessing techniques, notably MinIP and curved planar displays, are crucial in pancreatic imaging. Jeffrey's lecture covered the use of MSCT to distinguish islet cell tumors from ductal adenocarcinoma.

"Islet cell tumors represent only a small percentage of pancreatic neoplasms, but they are important because they have a much better prognosis than the more common ductal adenocarcinoma," said Jeffrey, a professor of radiology at Stanford University.

Unlike ductal adenocarcinoma, islet cell tumor cells are usually hypervascular and rarely cystic. Features typical of ductal adenocarcinoma, such as local invasion, vascular encasement, and pancreatic duct obstruction, are far less common in islet cell tumors. Calcifications are present in up to 15% of islet cell tumors, whereas they are extremely rare in ductal adenocarcinoma cases.

For smaller tumors and multiple tumors, intraoperative ultrasound should be combined with CT for maximum sensitivity. As lesions get larger, they tend to be much more heterogeneous and are able to obstruct veins but not encase the arteries. It's also important to bear in mind that not all hypervascular lesions are necessarily islet cell tumors-they may actually be renal cell carcinoma metastases.

"For reasons that are poorly understood, the pancreas is the sanctuary organ for renal cell metastases," Jeffrey said.

INCIDENTAL FINDINGS

Greater use of MSCT in abdominal imaging has resulted in greater detection of incidental cystic lesions in the pancreas. One large study showed that cystic lesions in the pancreas are very common incidental findings on abdominal MR scans, present in 20% of cases (Radiology 2002;223:547-553). Most of these lesions had a simple appearance, said Dr. Michael Macari, another speaker at the Stanford symposium. Most of these lesions are benign.

"This is an epidemic. We don't see them as frequently on CT, due to lower contrast resolution, but we do see them often in daily practice. The radiologist and clinician need to decide what to do with these findings," said Macari, section chief of abdominal imaging at the New York University School of Medicine.

Cystic lesions can be benign or malignant, and there is often overlap in the imaging appearance of the various types, making confident diagnosis difficult. Most cystic lesions (85%) are pseudocysts, lesions that are filled with fluid and debris, and these are typically associated with a prior history of pancreatitis. Occasionally, pseudocysts have septations. The other main types of cystic lesions are mucinous cystic tumor, serous cystadenoma, solid psuedopapillary tumor, and intraductal papillary mucinous tumor.

Aside from prior history of pancreatitis, examiners must consider patient age, sex, and-most important-the size of the lesion. Tiny lesions are often seen on MSCT scans, but most are clinically irrelevant.

In general, lesions smaller than 2.5 cm with no malignant characteristics such as nodularity or thick irregular enhancing septations can be safely followed. Sometimes incidental findings of cystic lesions result in surgeries that harm the patient. The Whipple operation, for example, is associated with complications such as bowel obstruction. The procedure was once associated with 25% mortality rates, raising concern about lesions that could possibly never have become clinically significant.

"Mortality is exceedingly rare, but morbidity is still quite high. We try to put the brakes on surgeons operating if there are benign characteristics," Macari said.

Since most cystic lesions are incidentally detected, it may be necessary to perform another CT scan with protocols optimized for imaging the pancreas, such as dual-phase acquisition and thin-section MSCT. Multiplanar reformats enable a more confident assessment of cystic lesions. MinIPs are useful in the assessment of ductal communication.

In cases of difficult diagnosis, it may prove helpful to perform cyst aspiration or endoscopic ultrasound, Macari said. Occasionally, for example, the serous cystadenoma will be unilobular and difficult to differentiate from peripheral mucinous cystic tumors. Cyst aspiration could clarify the true nature of this finding. Occasionally, surgery is still necessary to make a confident diagnosis.

Ms. Hayes is feature editor for Diagnostic Imaging.