Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

In pediatric patients with insulin-dependent diabetes mellitus, US reveals an inverse relationship between pancreatic size and the duration of diabetes (145); the pancreas was larger in children aged 3 to 7 years who had diabetes for 2 years or less than in older children who had had diabetes for >5 years. In children, diabetes thus appears to affect pancreatic growth.

Vascular Lesions

Hemosuccus pancreaticus, also called wirsungorrhagia in some of the French literature, signifies hemorrhage into the pancreatic duct. It is rarely detected.

Bleeding from the ampulla is either from the pancreatic or bile ducts. At times angiography is useful in locating and embolizing a specific site of bleeding. Most pancreatic bleeding is secondary to chronic pancreatitis.

Aneurysms or pseudoaneurysms of a pancreatic artery are rare in the absence of pancreatitis. Bleeding pseudoaneurysms in a setting of pancreatitis have already been discussed. These aneurysms are a cause of massive blood loss, even exsanguination. Aside from bleeding, a rare pseudoaneurysm results in bile duct obstruction due to compression.

Arteriovenous malformations are rare in the pancreas but are a cause of bleeding. Endoscopic Doppler US revealed a pulsatile waveform in one pancreatic malformation adjacent to the duodenum (146); arteriography confirmed an extensive vascular network. Resection of pancreatic head arteriovenous malformations is difficult; some of these patients require a Whipple procedure to control bleeding. Radiation therapy is an option for larger arteriovenous malformation (147).

Immunosuppression

Acquired immunodeficiency syndrome patients have developed non-Hodgkin’s lymphomas located primarily in the pancreas; imaging reveals diffuse infiltration. A percutaneous CTguided needle biopsy should be diagnosis.

Unusual infections abound. In one patient, AIDS initially manifested as a tuberculous pancreatic abscess (148).

ADVANCED IMAGING OF THE ABDOMEN

Bile duct abnormalities are well described in AIDS patients, but pancreatic duct changes are less well known. Over half have pancreatographic abnormalities consisting of dilation, an irregular contour to the main and secondary ducts, and main duct stenosis. Most patients with pancreatic duct abnormalities also have AIDS-related cholangitis.

Drug-induced pancreatitis develops in AIDS patients. Dideoxyinosine is an antiretroviral agent used to treat HIV infection; both pseudocyst formation and relapse of pancreatitis are associated with this drug.

Total amylase is of limited value in identifying pancreatic disease in HIV-infected patients; 28% of ambulant HIV-positive males had elevated total amylase values; however, almost half of these were due to an increased salivary fraction (149).

Postoperative Changes

Transplantation

Indications

A pancreatic transplantation is indicated in a patient with insulin-dependent diabetes mellitus who also requires a renal transplant, with both organs being transplanted at the same time. An isolated pancreatic transplant is performed in patients who already have a functioning renal transplant or in those with diabetic complications who do not have gross nephropathy. If transplantation is successful, the patient no longer depends on external insulin but has hyperinsulinemia and remains on chronic immunosuppressive therapy.

Pretransplant cardiac function is generally obtained; stress thallium scintigraphy is typical, with more invasive cardiac imaging reserved for specific situations.

Initially the transplanted pancreas and attached duodenum were placed in the right iliac fossa, and a side-to-side duodenovesical anastomosis was performed. Arterial supply to the donor pancreas is from the recipient common iliac artery, and venous drainage is to the common or external iliac vein. A simultaneously transplanted kidney is placed in the left iliac fossa. An alternate approach is primary enteric drainage. The native pancreas is not

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resected. Such a transplant results in two nonphysiologic events: (1) Pancreatic endocrine secretions are into the systemic circulation, bypassing the liver, and they lead to hyperinsulinemia. (2) Exocrine secretions into the bladder result in inflammation, infection with possible pancreatitis, and a metabolic acidosis. To overcome these problems a revised procedure was subsequently adopted by some centers: The donor duodenum is anastomosed to a jejunal loop and pancreatic venous drainage is to the recipient portal venous system.

Following total or subtotal pancreatectomy for chronic pancreatitis, pancreatic islet autotransplantation by intraportal infusion of pancreatic tissue leads to approximately half of these patients becoming insulin independent. Clinical pancreatic islet transplantation, however, is still in its infancy.

Results

Overall results among 500 simultaneous pan- creas-kidney transplants at the University of Wisconsin through October 1997, are as follows (150): patient survival at 1, 5, and 10 years was 96%, 89%, and 76%; kidney function, 89%, 80%, and 67%; and pancreas function, 87%, 78%, and 67%. Since June 1995, 1-year survival rates are as follows: patient, 98%; kidney, 94%; and pancreas, 93%. Conversion from bladder drainage to enteric drainage was required in 24% of patients, with primary indications for enteric conversion being leak in 14%, urethritis and extravasation in 7%, and chronic hematuria in 3% of patients.

Pancreatic allograft excretion and perfusion can be studied with secretin-augmented MR pancreatography and dynamic contrast enhanced MRI. Patients with a normally functioning graft produced 236ml (s. d. +/- 104) of pancreatic juice and those with dysfunctional grafts produced only 42ml (+/- 25) (151).

Thallium-201 scintigraphy provides static images of the transplanted pancreas. Tech- netium-99m–sestamibi scintigraphy provides both dynamic and static images, in some patients being superior to thallium-201 studies.

Complications

Rejection ranges up to 9% at 1 year. Other complications are only slightly less common, with

vascular thrombosis leading the list. Unlike renal transplant failure, pancreas transplant failure is more covert. Rejection, however, often involves both kidney and pancreas. The degree of parenchymal enhancement at dynamic contrast-enhanced MRI appears useful in detecting acute transplant rejection. Dynamic contrast-enhanced gradient-recalled echo (GRE) MR studies showed a mean parenchymal enhancement of 106% in patients with no biopsy evidence of rejection, 66% in those with mild, 62% in those with moderate, and 57% in those with severe acute rejection (152); infarcted transplants revealed parenchymal enhancement of 3% and could thus be readily identified. Overlap existed, however, between some patients in the normal and rejection groups, and a biopsy is still necessary in some.

Biopsy is performed either percutaneously or cystoscopically through the duodenal anastomosis. With the latter, an US-guided and cystoscopically directed approach appears advantageous. CTor US-guided core biopsies of pancreas grafts are satisfactory in most patients. The most common biopsy complication is self-limited bleeding from a biopsy site; less often encountered are major bleeding and asymptomatic hyperamylasemia.

Posttransplant CT is indicated in patients with fever, elevated serum amylase levels, and suspected fluid collections.

Ultrasonography of a transplanted pancreas is more difficult than of a kidney. Especially if enteric rather than bladder drainage is created, superimposed bowel and an indistinct margin make organ identification difficult. Fluid collections can be detected but their clinical significance is difficult to assess in the postoperative period. Neither absolute resistive indexes, obtained from US data, nor changes in resistive indexes correlate with acute rejection. Doppler US is useful, however, in detecting vascular thromboses and strictures.

Urologic complications are not uncommon with duodenum anastomosed to the urinary bladder and exocrine pancreatic secretions thus draining into the bladder. Complications of kidney-pancreas transplantations include urinary tract infections, hematuria, leaking duodenovesical anastomosis, and ureteral and urethral abnormalities. The most common postsurgical complication is a leak at the duodenovesical anastomosis, resulting in accumulation

of peritoneal fluid. A radionuclide cystogram confirms a communication between the bladder and peritoneal cavity. The duodenovesical anastomosis can also be evaluated by conventional cystography or CT cystography. Computed tomography cystography should be performed both with the bladder distended and after voiding; at times full distention will not detect a leak that is evident after voiding.

Urethral stricture and urethral disruption are complications of pancreatic transplantation in male patients; extravasation occurs at either the bulbar urethra or the bulbomembranous junction.

Magnetic resonance angiography is useful in detecting vascular complications. The absence of flow, vascular thrombosis, rejection, and infarction are detected. An MRA should detect almost all acute vascular compromise. Postcontrast 3D MR after kidney-pancreas transplantation readily detects larger transplanted arteries, seen as hyperintense structures, but transplanted venous vessels are less often identified. Gadolinium-enhanced 3D MRI in five patients with venous thrombosis and occlusion revealed serpentine voids within graft parenchyma or at the venous anastomosis during venous-phase imaging (153); the lack of graft enhancement or heterogeneous enhancement corresponded to gland necrosis, confirmed at pancreatectomy.

Allograft pancreatitis is relatively common. Other complications include fistulas and infection. Some patients develop hypoglycemia, at times years later.

Follow-up of patients with uremia and type 1 diabetes after a kidney-pancreas transplant and similar patients after kidney-only transplantation revealed that the double transplant patients were at lower risk for atherosclerosis than kidney-only transplant patients (154). Nevertheless, underlying atherosclerosis unrelated to transplantation is common in these patients.

Postpancreatic transplantation lymphoproliferative disorders manifest by diffuse allograft enlargement, an imaging appearance similar to that seen with transplant rejection or acute pancreatitis (155).

Resection

Traditionally, a Whipple procedure was performed when resecting the pancreatic head, which involves a pancreaticoduodenectomy and

ADVANCED IMAGING OF THE ABDOMEN

includes segments of extrahepatic bile ducts and pancreatic duct. The high mortality associated with a Whipple procedure a generation ago has dropped markedly and this procedure is now also being performed for some patients with chronic pancreatitis. Currently some surgeons are resecting only the pancreatic head and preserving the adjacent duodenum, the common bile duct, and the papilla of Vater. An end-to-end anastomosis is performed to connect the pancreatic duct to the intestinal tract.

After a pylorus-preserving pancreaticoduodenectomy, most patients have abnormal pyloric function (156); alkaline reflux is common and some patients develop gastritis.

Total pancreatectomy is rarely performed; in a setting of pancreatic cancer; it has not led to increased survival but does introduce major endocrine management problems.

One complication of a pancreaticoduodenectomy with portal and superior mesenteric vein resection is localized liver infarction. Computed tomography should detect most of these complications. A distinctly unusual occurrence after a Whipple procedure was rectal evacuation of a portal vein graft (157); presumably the underlying cause was graft infection.

Examination Complications

Laparoscopy

Laparoscopy has a limited role in pancreatic disease. Port site seeding after laparoscopic staging of a pancreatic carcinoma is a complication.

Endoscopic Retrograde

Cholangiopancreatography

The major complication of ERP and endoscopic sphincterotomy is pancreatitis, at times lifethreatening and resulting in massive peripancreatic necrosis. The risk of such pancreatitis is difficult to gauge. In a 1-year prospective study of 430 consecutive ERCP examinations in a single center, pancreatitis developed in 3% of patients (158); hyperamylasemia occurred in 8%. Both pancreatitis and hyperamylasemia are more common after difficult procedures. Over-

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injection of contrast is probably one of the causes. The high osmolality and ionic nature of iodinated contrast agents may play a role, although some studies suggest that the use of nonionic contrast agents does not decrease the risk of postprocedure pancreatitis. Some of the risk factors associated with postprocedure pancreatitis include the presence of prior or current pancreatitis, pancreatic duct stricture, and sphincter of Oddi abnormalities. Curiously, endoscopic sphincterotomy performed in a setting of nondilated bile ducts is also associated with an increased risk of pancreatitis. A direct correlation exists between postprocedure pancreatitis and number of pancreatic duct injections; also, those with difficult common bile duct cannulations are more to develop pancreatitis.

Computed tomography performed after ERCP papillotomy occasionally detects a pancreatic “tumor” (159).

Technetium-99m–HMPAO leucocyte scintigraphy has been disappointing in detecting mild acute pancreatitis after ERCP.

Biopsy

Pancreatitis, with fatal outcome, has followed percutaneous pancreatic biopsy. Needle tract tumor seeding is a complication. Whether there is an increased risk of seeding with the use of larger needles is conjecture.

In addition to percutaneous pancreatic biopsy, US-guided laparoscopic biopsy of the pancreas can be performed. Port site seeding after laparoscopic carcinoma biopsy has occurred.

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