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NEWER MODALITIES IN IMAGING THE PANCREAS

Meher Ursekar
Consulting Radiologist, CT and MRI, Bombay Hospital.

By virtue of its retroperitoneal location the pancreas has invariably proved to be a diagnostic challenge. Cross-sectional imaging however, has provided a means for visualizing the normal and abnormal pancreas, and has become essential for the initial evaluation of the patient suspected to have pancreatic disease. The techniques themselves have undergone profound developments in the last 20 years. Included amongst the newer modalities for the investigation of the pancreas are : helical computed tomography (CT), magnetic resonance imaging (MRI) and MR cholangiopancreatography (MRCP), endoscopic and intraductal pancreatic sonography, positron-emission tomography (PET) and radionuclide scintigraphy with indium 111 (111In) octreotide. The clinical applications techniques of the newer modalities are discussed in this article.

HELICAL CT

CT scanning with non helical scanners has long been used to evaluate the pancreas. Helical CT however, offers clear advantages over conventional scanning. It allows rapid scanning of a "volume" of tissue during a single breath hold. This reduces misregistration artifacts which occur during non helical scanning1 and also permits highly overlapped slices to be obtained retrospectively with no additional exposure of the patient to radiation for the overlapped slices. Small pancreatic masses and short segments of vascular encasement can be evaluated more readily with this high degree of overlapping reconstruction. The pancreas is typically scanned using a dual-helical, contrast enhanced technique. The first helical acquisition is obtained during the arterial phase of a rapidly injected intravenous bolus of non ionic contrast medium which is injected through a power injector at the rate of 3 to 5 ml/sec. The pancreas enhances upto 85 to 90 HU more than its unenhanced density during the arterial phase. The coeliac and superior mesenteric arteries are also optimally opacified at this time and demonstration of tumour infiltration around these vessels can be easily made out. The high degree of contrast enhancement of the pancreatic parenchyma during the arterial phase allows detection of small, hypoattenuating pancreatic carcinomas and hyperdense islet cell tumours.[2] The second helical acquisition is made during the portal phase (approximately 60 seconds after the onset of the injection). In this phase, the peripancreatic veins are readily detected and tumour infiltration of the veins can be seen. This phase is optimal for detecting hepatic metastases as the liver parenchyma is maximally enhanced.

Apart from the advantages of optimal contrast enhancement obtainable during the arterial and portal venous phases, the volumetric acquisition of helical CT also permits computer reconstruction of arterial phase axial images into 3-D images of the arterial anatomy (helical CT angiography). The depiction of arterial anatomy by helical CTA has been found to be equivalent to conventional angiography, thus obviating the need for conventional angiography for detecting anatomic variations, before pancreatic surgery.[3] The arterial phase axial images and the 3D images of the pancreas can be projected together for visualizing the tumour-vessel relationships in patients with pancreatic surgery. [3] The arterial phase axial images and the 3D images of the pancreas can be projected together for visualizing the tumour-vessel relationships in patients with pancreatic tumours.

Volume reconstructions of the CBD with minimum intensity (for visualization of bile) pixel reconstruction, are also possible.

The duodenal papilla and duodenal-pancreatic interface are well visualized on helical CT when water is used instead of opaque contrast medium to distend the duodenum. An ampullary carcinoma can be distinguished from pancreaic carcinoma more readily with water distension of the duodenum.

Accuracy of CT

The overall accuracy of CT for staging pancreatic head carcinomas is about 80%, and for diagnosing pancreatic carcinomas, 97%. Limitations of CT are its inability to detect small surface metastases over the liver and on the peritoneal surfaces, and inability to detect microscopic infiltration within lymph nodes.

Magnetic Resonance Imaging

There have been a number of advances in MRI technology which include the development of phased-array surface coils that wrap around the torso, newer and faster pulse sequences and development of newer contrast media like manganese-DPDP. These have permitted higher resolution imaging of the pancreas in shorter times. The pancreas is typically evaluated with conventional spin-echo and breath-held gradient echo sequences. Fat suppression techniques are very useful and allow the normal hyperintense pancreas to be differentiated from tumour, which is relatively hypointense on these sequences. Breath-held gradient echo sequences are performed with contrast enhancement and are useful for depicting small pancreatic tumours and hepatic metastases.

Accuracy of MRI

When compared with helical CT, both modalities are found to have a high degree of accuracy for detecting and staging pancreatic tumours. MRI is less freely available however, and in most centres, CT remains the primary modality for evaluating pancreatic diseases. In addition, MRI is more time-consuming, and state-of-the-art technology is limited to only a few major hospitals and centres. MRI is often reserved for those patients intolerant to iodinated CT contrast media, and in whom, contrast media are contraindicated (e.g. : renal failure).

Accuracy

There are no large series assessing the overall accuracy, sensitivity and specificity of MRI in detecting and staging pancreatic tumours. Reported accuracies in smaller series range from 90 to 100% for detection of tumours. [6,7]

MR Cholangiopancreatography

MRCP is a new technique that is entirely non invasive and allows direct visualization of the biliary system and pancreatic duct without the use of contrast agents. [8,9] MRCP is based on heavily T2-weighted pulse sequences which take advantage of the fact that stationary fluids with a long T2 relaxation time have high signal intensity while solid organs and flowing blood generate little signal. Optimal contrast between hyperintense bile and hypointense surrounding soft tissues is provided by a number of different techniques. These include gradient-echo, fast spin-echo, HASTE, and RARE pulse sequences.

Accuracy of MRCP

MRCP is able to visualize the normal extrahepatic bile ducts in nearly 100% of patients and the pancreatic duct in over 80%.[10] It is accurate in the diagnosis of anatomic variants of the biliary tree. In bile duct obstructions, the presence and level of obstruction can be diagnosed in over 90%.[11,12] In case of occlusion or high grade obstruction MRCP depicts the intrahepatic biliary tree more reliably than ERCP. MRCP is highlyaccurate in the detection of bile duct calculi - the sensitivity and specificity reported in the CBD being about 90% in several large series. In cases where the reason for the bile duct obstruction remains unclear, or is due to malignant or inflammatory disease, the MRCP is followed by conventional MR imaging. The high degree of accuracy of MRCP has lead to it being the primary test, replacing USG, CT and ERCP for evaluating suspected biliary tract disease, especially stones. Case selection for ERCP becomes more specific when prior MRCP is done - ERCP being reserved for those cases in which the MRCP has failed to answer the clinical question, or when biliary intervention is necessary.

SONOGRAPHY

Endoscopic ultrasonography (EUS) has been found to be useful for the detection of small pancreatic tumours that may not be demonstrated by CT,13,14 and for the localization of islet cell tumours. The superiority of EUS over other modalities is greatest for pancreatic tumours less than 20 to 30 mm in diameter. EUS provides high-resolution images of structures within close proximity to the gastrointestinal tract. Despite the high accuracy of EUS for the detection of pancreatic tumours, the technique is not foolproof. Detectability of tumours diminishes when there is associated chronic pancreatitis or if the tumour has similar echogenicity as the rest of the pancreatic parenchyma. Focal pancreatitis and neoplasm may be indistinguishable, without biopsy. More recently, the ability to obtain simultaneous, image-guided biopsy specimens of focal pancreatic masses has extended the usefulness of EUS. The EUS examination may be limited when a stent has been placed, or after sphincterotomy, which create air artifacts.

Intraductal pancreatic sonography involves the use of a rotating radial 20MHz ultrasound transducer, which can be passed through the biopsy channel of an ERCP endoscope and into the pancreatic duct. [15] It has been used for the evaluation of pancreatic duct strictures, distinguishing tumours from chronic focal pancreatitis as the cause for strictures, and for detection of intraductal papillary tumours.

RADIONUCLIDE SCINTIGRAPHY

The development of somatostatin analogues (octreotide) for imaging neuroendocrine tumours has led to renewed interest in pancreatic scintigraphy, which hitherto, was not used widely in clinical practice. Octreotide is avidly taken up by neuroendocrine tumours and their metastases.

PET scanning using fluodeoxyglucose F18 has been found to be useful in distinguishing carcinoma and inflammatory processes. False-positive PET images can be produced by inflammatory masses. False-negative results can occur when there are elevated plasma glucose levels.

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