I. Definition.
Acute pancreatitis is an acute chemical inflammation of the pancreas and its surrounding tissues caused by self-digestion of pancreatic enzymes activated in the pancreas, and is one of the common acute abdominal conditions. Clinically characterized by acute abdominal pain, fever with nausea, vomiting, increased blood and urine amylase, according to the pathological histology and clinical manifestations can be divided into acute interstitial (edematous) pancreatitis (light) and acute hemorrhagic necrotizing pancreatitis (heavy) two.
II. Etiology.
Biliary tract disease is the most common cause in China, alcoholism is the main cause in Europe and the United States, there are other causes, such as overeating, ischemia, metabolic abnormalities, sphincter of Oddi dysfunction, drugs, surgery and trauma, parasites, tumors, etc.
Three, acute pancreatitis imaging methods.
1, X-ray examination: nowadays the role of ordinary X-ray examination has been quite limited, it can not directly observe the pancreatitis itself, let alone reflect its severity, but in some occasions still have a certain role.
2, ultrasonography: simple, economic, is still one of the main means of examination, but the pancreas is located in the retroperitoneum, ultrasonography is susceptible to intestinal gas and abdominal wall fat, the resolution is poor.
3.CT examination: it is the best and easiest examination method with fast scanning speed and high spatial resolution, and has high sensitivity for pancreatic morphological changes, distribution of exudate and necrotic areas, and pancreatic complications.
MRI examination: It has high soft tissue resolution and is more sensitive than CT in terms of pancreatic volume changes, peripancreatic inflammatory exudation, bleeding, and intrapancreatic fluid accumulation.
The CT performance of normal pancreas.
The neck is located in front of the superior mesenteric artery, the body is caudal in front of the splenic vein, and the hooked part of the head of the pancreas is triangular in shape, located in the medial duodenum, in front of the inferior vena cava and behind the superior mesenteric vein. The pancreatic parenchyma is homogeneous and slightly less dense than that of the spleen, with clear lobular structures at the margins. On enhancement scan, the pancreatic parenchyma shows uniform and consistent enhancement. The size of the pancreas is compared with the transverse diameter of the 2nd lumbar vertebra. The normal transverse diameter of the head of the pancreas/vertebra is 1/2-1, the body of the pancreas is 1/3-2/3, and the tail of the pancreas is 1/5-2/5; the normal extension of the leptomeninges to the left does not exceed half of the transverse diameter of the superior mesenteric artery; the normal diameter of the pancreatic duct does not exceed 3 mm.
MRI performance of the normal pancreas.
The morphological and structural characteristics of the normal pancreas on MRI are the same as those on CT. The pancreatic parenchyma has a uniform medium signal in T1-weighted and uniform uniform reinforcement after enhancement; it has a uniform slightly low signal in T2-weighted and the edge of the pancreas is not very smooth, and after lipid suppression, it has a uniform high signal; the normal pancreatic duct has a low signal in T1-weighted and a high signal in T2-weighted.
VI. CT manifestations of acute interstitial pancreatitis.
The pancreas is locally or diffusely enlarged, the edges are locally poorly defined, the density is uniform or heterogeneous when scanning, the fat layer around the pancreas is blurred, there is a small amount of peripancreatic fluid, the prerenal fascia is thickened (the site of thickening is related to the site of the lesion), the pancreatic parenchyma is uniformly enhanced after enhancement, there is no liquefied necrotic area, there are usually no complications, 10%-20% of cases may not have positive CT performance.
MRI manifestations of acute interstitial pancreatitis.
The pancreatic volume increases, low signal on T1WI, high signal on T2WI, and signal is obviously inhomogeneous, the edge of the pancreas is obviously blurred, and the peripancreatic strips or sheets of abnormal high signal are seen on T2WI lipid suppression. Dynamic enhancement scan shows heterogeneous enhancement of the pancreatic parenchyma.
CT manifestations of acute hemorrhagic necrotizing pancreatitis.
①Pancreatic volume changes:The volume of the pancreas is significantly increased and the margins of the pancreas are blurred.
The CT value of pancreatic edema is lower than that of normal pancreas (40-50Hu), and the CT value of necrotic area is even lower, while the CT value of hemorrhagic area is higher than that of normal pancreas, reaching (50-70Hu), and the necrotic area is non-enhanced hypodense area after enhancement.
2. Peripancreatic changes:
① Peripancreatic fluid: pancreatic fluid often extravasates into the peripancreatic fatty space, accumulates in the small omental sac (the most common), the anterior pararenal space (the left side is most commonly involved), and can also infiltrate the right pararenal space when the pancreatic head is inflamed. There may be communication between the left and right pararenal space via the midline, so inflammation in one side of the pararenal space can spread to the opposite side.
Gastrointestinal changes: Inflammation can affect the adjacent gastric wall and produce focal gastric wall thickening changes; if it affects the duodenum, small intestine and transverse colon, it can reflexively cause intestinal pneumatization and fluid accumulation; the descending duodenum can be compressed by the enlarged pancreatic head and peripancreatic edema; peripancreatic fluid and pseudocyst can compress the adjacent intestine and cause obstruction; the lesion can also spread from the transverse colon and the root of the small intestine mesentery to the mesentery, which can cause edema, cellulitis, fluid accumulation, and obstruction. The lesions can also spread from the root of the transverse colon and small intestine to the mesentery, which can cause edema, cellulitis, effusion, bleeding, necrosis, etc., of which the first two are the most common.
3, complications:
① Cellulitis: often occurs in the body and tail of the pancreas, so it often involves the small omental sac and the anterior space of the left pararenal area, the lesion is mild when the CT only shows the thickening of the anterior fascias of the kidney, in severe cases, it shows a large hypodense soft tissue shadow, irregular shape, no envelope, no enhancement. It can spread along the transverse colon and small intestine mesentery, down to the pelvis and upper thighs, and can be followed by necrosis and infection to form abscesses.
Abscess: usually occurs more than 3 weeks into the disease, intra- and extra-pancreatic fluid, necrotic tissue, and cellulitis can all be secondary to infection to form an abscess, with a lower density than cellulitis, but higher than the general fluid and fluid in pseudocysts.
(3) Pseudocyst: usually formed in the 4th-6th weeks of the course of the disease, when the fluid is not emptied and absorbed in time, and is surrounded by fibrous sacs or granulation tissue. Cysts can occur in any part of the pancreas and are distributed in the same way as the fluid spreads. The lesion morphology is round, varying in size, mostly unicompartmental, the cyst wall is more uniform, varying in thickness, the cyst wall has varying degrees of enhancement, the liquid within the general low density, with more hemorrhage or necrotic tissue, the density within the uneven, mixed density.
MRI manifestations of acute hemorrhagic necrotizing pancreatitis.
The volume of the pancreas increases significantly, with low signal on T1WI and significant inhomogeneous high signal on T2WI. The extent of pancreatic exudate can be clarified on T2WI lipid suppression, while hemorrhage and bloody fluid show high signal on T1WI and T2WI. Dynamic enhancement scan shows inhomogeneous enhancement of the pancreas and no enhancement of the necrotic area, which shows a significant low signal area. MRI is also valuable in determining the complications of pancreatitis such as pseudocysts, hemorrhage, and abscesses.