Книги по МРТ КТ на английском языке / Advanced Imaging of the Abdomen - Jovitas Skucas

Figure 14.34. Small bowel obstruction in a paraduodenal hernia in a man who had previous testicular carcinoma resection and retroperitoneal lymph node dissection. Contrastenhanced CT reveals dilated small bowel loops posterior to the duodenum. Involved bowel wall is thickened and enhances considerably. Surgery found a strangulated paraduodenal hernia. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

serves as an anatomic landmark for CT. Occasionally small bowel loops are even located posterior to the pancreas.

A barium upper gastrointestinal study identifies a left paraduodenal hernia as loops of

A

Figure 14.35. Right paraduodenal hernia. A: Postcontrast CT reveals focal small bowel loops in the right paraduodenal region (arrows) (Waldeyer fossa). (Source: Okino Y, Kiyosue H, Mori H, et al. Root of the small bowel mesentery: correlative anatomy and CT features of pathologic conditions. Radiographics 2001;21:1475– 1490, with permission from the Radiological Society of North America.) B: Right paraduodenal hernia and midgut malrotation in another patient.

bowel located posterior to the stomach and left of the duodenum. A foramen of Winslow hernia and herniation through a mesenteric defect have a similar gross appearance but can generally be excluded by different bowel position.

A right paraduodenal hernia extends into the mesentericoparietal fossa of Waldeyer, posterior to the small bowel mesentery (Fig. 14.35). Computed tomography reveals the superior mesenteric artery and vein anterior to the hernia. These hernias develop in a setting of midgut malrotation.

Other Internal Hernias

Rare causes of small bowel obstruction and strangulation include herniation through a defect in the falciform ligament (76) and transmesosigmoid herniation; CT should define these. Most retropsoas small bowel herniations are incidental findings.

An uncommon hernia is through a defect in the levator ani muscles. These hernias contain bowel, fat, or simply ascitic fluid in communication with the peritoneal cavity. T2-weighted MR images are useful in detecting these hernias.

B

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

Coronal images allow comparison of one side with the other.

Paracecal hernias are rare. In children, Treves’ field hernias predominate. An occasional paracecal hernia in adults results in intermittent small bowel obstruction.A barium study should be diagnostic.

Fibrosis/Adhesions

In spite of an extensive surgical literature on the etiology and pathogenesis of intraperitoneal adhesions, why some patients develop adhesions while others are relatively adhesion-free is not clear. Undoubtedly surgically introduced foreign bodies play a role in postoperative intraperitoneal adhesion development.

Insertion of laparoscopic trocars in patients with previous abdominal surgery and scars can result in bowel injury due to adhesions. In these patients preoperative US of the anterior abdominal wall appears useful to establish regions free of adhesion because many of these patients have adhesions under their scar. Overall, however, US has a low sensitivity in detecting adhesions.

Mesenteric and extraperitoneal fibrosis was discussed earlier (see Diffuse Infiltration).

Fistula

Fistulas and sinus tracts extending to the skin are best studied by direct fistulography. Internal ones are evaluated mostly indirectly. Some fistulas are adequately studied by outlining an adjacent gastrointestinal or genitourinary tract structure with contrast and obtaining conventional radiographs, usually under fluoroscopic guidance. In some patients CT is superior because it not only identifies the full extent of a fistula but also reveals any associated complications such as a tumor, abscess, or even osteomyelitis. Contrast injected into a percutaneous fistula prior to CT often aids in defining internal communications.

Computed tomographic peritoneography can detect a transvaginal leak of peritoneal dialysate.

Magnetic resonance imaging is well suited to evaluate fistulas. Bladder fistulas are best evaluated with gadolinium-enhanced T1-weighted

images, while nonbladder fistulas are shown equally well both by nonenhanced and gadolin- ium-enhanced scans. Perianal fistulas are best studied by MR.

Pneumoperitoneum

A spontaneous pneumoperitoneum without associated peritonitis is not common. Rarely, intrathoracic trauma or an intrathoracic tumor results in a pneumoperitoneum. An uncommon cause of a spontaneous pneumoperitoneum is rupture of an intraabdominal abscess. In patients on mechanically assisted ventilation, air in the anterior mediastinum, or endothoracic fascia, dissects inferiorly through the diaphragm into the extraperitoneal anterior abdominal wall; on a CT study such anterior abdominal wall air can mimic an intraperitoneal location. Complicating the issue, at times this air extends into the peritoneal cavity; such dissection should not be confused with bowel perforation and thus lead to an unnecessary laparotomy.

A postoperative pneumoperitoneum disappears within several days. Generally of importance is not the amount of free air but rather whether it is decreasing from one day to the next; an increase in size should suggest bowel communication.

Over the years the upright posteroanterior chest radiograph was believed to be the most sensitive study to detect a subtle pneumoperitoneum. A lateral radiograph tends to detect a smaller pneumoperitoneum than a frontal radiograph.A major limitation of both of these radiographs is that many of these patients are old, debilitated, in pain, or obtunded, and cannot be upright. Still, as little as 1cc of intraperitoneal gas can be detected whenever this approach is feasible. Often a radiograph with a horizontal x-ray beam and the patient supine will reveal a pneumoperitoneum adjacent to the liver, in Morison’s pouch,or similar locations. Diaphragmatic muscle slips identified on supine abdominal or recumbent frontal chest radiographs or visualization of the ligamentum teres or even gallbladder on conventional radiographs is evidence of a pneumoperitoneum.

Computed tomography is accurate in detecting small amounts of intraperitoneal gas. Some authors have found that CT detects smaller

amounts of gas than conventional radiographs. Computed tomography reveals a pneumoperitoneum as gas between the liver and anterior abdominal wall, subhepatically, in the periumbilical region, in the pelvis, or even trapped between mesenteric folds. With small amounts of gas in unusual locations, differentiation from a small abscess is difficult.

Renal excretion of an oral water-soluble contrast agent should suggest bowel perforation. It should be kept in mind, however, that occasionally orally ingested contrast is absorbed from bowel even in the absence of disease, but these are exceptions.

Ultrasonography can also detect a pneumoperitoneum.

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Abnormal elevation of a hemidiaphragm is defined as eventration. It can be complete or partial, congenital or acquired. Congenital eventration is associated with a muscular defect. A rare congenital eventration is bilateral. Most acquired eventrations are secondary to an innervation defect; with time, muscle atrophy develops. Most partial eventrations are asymptomatic. Complete eventration leads to respiratory distress.

Diaphragmatic neoplasms, although quite rare, tend to manifest late. These tumors include rhabdomyosarcomas, malignant fibrous histiocytomas and others. They grow either superiorly and manifest as an intrathoracic tumor or grow inferiorly and often their imaging appearance mimics a liver, splenic or other upper abdomen organ tumor (77).

Pneumoretroperitoneum

Focal pneumoretroperitoneum is a hallmark of an abscess. Less often pneumoretroperitoneum alone or in association with pneumoperitoneum is a late sign of bowel ischemia.

Extraperitoneal gas and abdominal wall emphysema have developed after transanal excision of a rectal carcinoma.

Soft tissue gas within the back muscles can mimic a pneumoretroperitoneum on conventional radiographs.

Diaphragm

Diaphragmatic abnormalities include hemidiaphragmatic paralysis, inversion, and eventration.

The classic fluoroscopic “sniff test” detects hemidiaphragmatic paralysis, normally a simple procedure except in the presence of pleural effusion when additional maneuvers are necessary. Bedside US is an alternate technique to detect diaphragmatic paralysis. In some pediatric patients a barium study is useful to define the underlying anatomy and exclude bowel herniation.

Instead of a normal diaphragmatic concavity toward the abdomen, in diaphragmatic inversion the concavity is toward the chest. Inversion is most often secondary to a pleural effusion or neoplasm. The hemidiaphragm is either paralyzed or moves paradoxically.

Omentum

Omental torsion can be primary or secondary if adhesions involve the free omental edge. Primary omental torsion occurs in both children and adults; the condition mimics acute appendicitis, with acute onset of right lower quadrant pain. A rare omental cystic teratoma results in torsion around its pedicle. Uncorrected omental torsion leads to ischemia and infarction. Omental ischemia also develops in the absence of torsion. Edema, fat necrosis, and eventual fibrosis ensue.

Computed tomography and MRI of omental torsion reveal a characteristic whirling or twisted fatty tumor in the middle and lower abdomen anteriorly (78); MRI also identifies a heterogeneous omentum, suggesting edema.

Ultrasonography of omental torsion reveals a homogeneous noncompressible hyperechoic tumor that can be separated from a normalappearing gallbladder. Simple torsion is not associated with bowel wall thickening, and, in fact, bowel wall thickening should suggest another diagnosis, such as inflammation of an epiploic appendage. Exception occur, however, and the adjacent bowel wall is thickened if inflammation spreads via the omental tenia.

Ultrasonography and CT reveal omental infarction as hyperechoic, hyperdense regions, often adherent to peritoneum. These findings then evolve and change rapidly. Adjacent organ inflammation is in the differential diagnosis.

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

Endometriosis

Endometriosis can develop anywhere in the abdomen, including the abdominal wall, inguinal canal, umbilicus, or surgical scar. Endometriosis of the mesentery, omentum, or bowel wall is less common than peritoneal involvement. Endometriosis is associated with a cystic mesothelioma and disseminated leiomyomatosis. It is one of the causes of bloody ascites; in an occasional patient it results in peritonitis and eventually evolves into scarring. A rare endometrioid carcinoma develops at a scar endometriosis site.

Adnexal endometriosis is discussed in Chapter 12.

Most often intraperitoneal endometriosis consists of solid or partly cystic tumors,at times mimicking metastasis. Smaller peritoneal implants are not detected by US.The solid component in larger tumors often appears nodular, and Doppler US reveals blood flow within the tumor.

Magnetic resonance findings vary considerably. Larger endometriomas are readily detected with MRI; fat suppression aids in visualizing smaller foci. With a mostly solid, fibrotic endometrioma, T1-weighted images are isoto hyperintense and T2-weighted images hypointense. Endometrioma fluid, similar to a hematoma, is hyperintense on both T1and T2weighted images. Solid endometrioma nodules enhance postgadolinium.

Diffuse peritoneal endometriosis has an imaging appearance similar to that of carcinomatosis or a chronic infection. Imaging is useful in guiding a diagnostic biopsy.

Peritoneal Foreign Bodies

One of the complications of laparoscopic cholecystectomy is intraperitoneal spill of gallstones and clips. Stones tend to settle in the pelvis and right iliac fossa.Abdominal pain in such a patient often suggests a ureteric calculus or appendicitis and imaging findings of a right lower quadrant calcification may concur with this diagnosis. Surgery in these patients reveals calcifications surrounded by granulomas. Gallstones “lost” in the peritoneal cavity during laparoscopic cholecystectomy are discussed in Chapter 8.

One sequela of colonic epiploic appendiceal inflammation is loose foreign bodies in the

peritoneal cavity; some of these are quite large—they measured 6cm in diameter in one patient (79). Few other conditions lead to loose calcified peritoneal structures. The task of imaging is to prove that such a calcification is indeed loose. Some of these foreign bodies become encased by fibrosis, and then a calcified leiomyoma or similar tumor is in the differential.

Intrauterine contraceptive devices do migrate into the peritoneal cavity. Imaging aids in their localization for laparoscopic removal, although some become encased by adhesions.

Peritoneal spill of ovarian dermoid content or similar structure during resection results in a postoperative granulomatous peritonitis.

Retained barium in the peritoneal cavity or extraperitoneal tissues is discussed in chapter 5. Retained surgical sponges are discussed later (see Examination and Surgical Complications).

Immunosuppression

Most immunosuppression is encountered in HIV-positive patients and in those after organ transplantation. Also, occasionally detected is a primary immunodeficiency state, found mostly in children.

Acquired Immune Deficiency

Syndrome (AIDS)

Infection

An immunocompromised 15-year-old boy developed necrotizing myofasciitis of the anterior abdominal wall and clinically presented with an acute abdomen (80).

A common cause of death in African AIDS patients is pulmonary tuberculosis; a majority of these patients also had abdominal tuberculosis, with abdominal lymph nodes, liver, spleen and kidneys involved. These patients tend not to form tuberculous granulomas but do develop tuberculous ascites. Tuberculous HIV patients tend to have larger extraperitoneal and mesenteric adenopathy than nontuberculous HIV patients. Also, adenopathy with a hypodense appearance to lymph nodes suggests Mycobacterium tuberculosis source.

Extraperitoneal tuberculous abscesses in AIDS patients are amenable to percutaneous drainage under US guidance (81).

Salmonella bacteremia and septicemia are not uncommon in African seropositive AIDS patients.

Acquired immune deficiency syndrome patients are especially prone to develop systemic Trypanosoma cruzi infection, and an extensive literature exists on this topic.

Intestinal perforation secondary to cytomegalovirus (CMV) enteritis is the most common cause of peritonitis in AIDS patients; CMV peritonitis can also occur without a perforation.

Acquired immune deficiency syndrome patients develop bacillary angiomatosis and vascular lesions mimicking Kaposi’s sarcoma that are secondary to infection with Bartonella henselae. These lesions occur throughout the body, including the abdominal lymph nodes, liver, and spleen. Ascites develops with peritoneal involvement. These lesions contain numerous capillaries, and postcontrast CT reveals marked contrast enhancement.

ADVANCED IMAGING OF THE ABDOMEN

tion instilled into the peritoneal cavity. Images obtained with the peritoneal cavity filled and after drainage are evaluated for leaks, hernias, adhesions, and for any loculated fluid (82). A transvaginal leak of peritoneal dialysate can also be confirmed by CT peritoneography.

The most common causes of peritoneal dialysis catheter occlusion are infection and adjacent soft tissue abutting the end of the catheter. Guide wire-assisted catheter manipulation is often helpful with a suspected catheter occlusion; long-term catheter patency can often be reestablished.

Peritonitis in patients undergoing peritoneal dialysis is often caused by an unusual organism. These include both bacteria and fungi.

Patients undergoing hemodialysis are at risk for mesenteric ischemia involving either the large or small bowel. These ischemias are typically nonocclusive in nature. A mesenteric infarct is a not uncommon cause for an acute abdomen in this patient population.

Tumors

Kaposi’s sarcomas develop in AIDS patients not only in the skin but also in the intestinal wall, omentum, mesentery, and other sites. Chylous ascites is not common, except if Kaposi’s sarcoma involves the cisterna chyli.

A propensity for AIDS patients to develop lymphoma is well documented.

Posttransplantation

Chronic immunosuppression in patients after transplantation results in a lymphoproliferative condition ranging from lymphoid hyperplasia to lymphoma. Manifestations vary depending on organ involvement. The primary role of imaging is to detect any initial abnormality and in follow-up, with biopsies used to establish a specific diagnosis.

Examination and

Surgical Complications

Dialysis

Ventriculoperitoneal Shunts

Complications of ventriculoperitoneal shunts include shunt migration, shunt fracture, and superimposed infection; an occasional one results in loculated cyst formation.

Laparoscopy

A previous laparotomy is a relative contraindication to laparoscopy because the presence of abdominal wall adhesions increases the risk of bowel injury during pneumoperitoneum induction and trocar insertion.

Laparoscopy in both adults and children has a steep learning curve, but once established it is associated with few complications. Complications encountered include abscesses, hematomas (in the abdominal wall, intraperitoneal, and retroperitoneal), bowel perforation and obstruction, pancreatitis, and splenic infarction. Complications discovered intraoperatively generally result in open surgical intervention.

In peritoneal dialysis patients, MR peritoneography is performed by adding an MR contrast agent (about 20mL) to a 2000mL dialysate solu-

Hernia Repair

Repair of an inguinal hernia using an extraperitoneal approach should not be associated

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

with peritoneal complications, although such complication as small bowel obstruction do occur. Other complications encountered include hematoma, seroma, protrusion of the prosthetic mesh, infection, and hernia recurrence. Generally, however, no imaging study is necessary in most patients after laparoscopic hernia repair.

Laparoscopic ventral hernia repair using a mesh does not obliterate completely a preoperative hernial sac, and thus postoperative fluid collections here are common (83); CT differentiation of such fluid from an abscess or recurrent hernia is difficult and, in the absence of symptoms, serial follow-up appears reasonable.

thread was no longer present and the sponge was presumably spontaneously expelled.

Biopsy

A pneumothorax is relatively common after a lung biopsy. Occasionally not only a pneumothorax but also a pneumoperitoneum develops after a lung biopsy.

Abdominal biopsies and fluid aspirations are performed under sterile conditions, yet the introduction of bacteria and the resultant peritonitis is a recognized complication of these procedures.

An endoscopic suction biopsy can result in small bowel perforation.

Other Laparoscopic Complications

Incisional hernias through laparoscopic trocar sites are uncommon and develop mostly through trocar sites >10mm in diameter. Even acute appendiceal strangulation has developed within a laparoscopic port hernia (84).

One complication of laparoscopy is injury to extraperitoneal vessels, often due to blind insertion of needles and trocars. These patients manifest with acute hypotension intraoperatively and require conversion to an open laparotomy.

Small bowel necrosis or mesenteric infarction is rare.

Retained Sponges

A gossypiboma, or textiloma, is a tumor within the body composed of a cotton matrix, such as a laparotomy sponge. Conventional radiography identifies most retained sponges by their radiopaque markers. For a number of reasons CT does not identify radiopaque markers in all patients with retained sponges. Gas bubbles can develop within a gossypiboma without an abscess being present. In time, a sponge becomes encased in fibrosis, a phlegmon, or occasionally an abscess.A rare one migrates into the bowel lumen through a fistula and obstructs (85).

Colonoscopy identified the thread of a surgical sponge at the tip of a granuloma in the sigmoid colon of a patient with a prior hysterectomy (86); conventional radiography identified a radiopaque thread, and CT revealed a gossypiboma. Later study showed that the

Tumor Seeding

Tumor seeding is associated with laparoscopic, biopsy, and catheter drainage procedures and is discussed in the appropriate organ-related chapters. Subcutaneous tumor seeding has even developed along a needle track used for alcohol sclerotherapy of a hepatocellular carcinoma. The type of tumor and probably its pathologic grade are relevant factors in tumor spread. These implants tend to be homogeneous on precontrast CT and heterogeneous and enhancing on postcontrast CT.

The risk of abdominal wall tumor seeding during needle biopsies is probably underestimated in the literature. An unfortunate consequence of tumor seeding after a fine-needle biopsy is that it renders any subsequent attempted curative resection into a palliative one.

Numerous reports describe metastases at laparoscopic trocar sites. These have occurred with both extraperitoneal and intraperitoneal laparoscopy. The relative prevalence of these metastases compared to incisional recurrence after a laparotomy is difficult to place in perspective.

Although rare, tumor seeding also occurs along percutaneous drainage tracts in patients undergoing abscess drainage in a setting of an underlying cancer.

Heterotopic Calcifications

Heterotopic calcification in abdominal incisions is a known sequela of abdominal surgery. These

calcifications often are palpable, and some are painful. They are often visible with not only CT but also conventional radiography. Why they form is not known. Some patients appear to be predisposed to heterotopic bone formation because calcifications recur after the primary one is excised.

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Technique

Magnetic Resonance

Magnetic resonance (MR) relaxation characteristics of focal tumors tend to be similar to normal splenic tissue and thus difficult to detect. With the spleen located in the left upper abdomen close to the left hemidiaphragm, magnetic resonance imaging (MRI) of the spleen requires control of motion artifacts. A number of breath-hold techniques have been developed that allow dynamic contrast-enhanced images. Faster MRI techniques allow postcontrast images to be obtained during perfusion, equilibrium, and more delayed phases. Some focal lesions equilibrate with normal splenic parenchyma within several minutes after contrast injection and thus early postcontrast images accentuate tumors compared to normal tissue.

Superparamagnetic iron oxide contrast agents are taken up by the reticuloendothelial cells and shorten T2, making high signal intensity tumors more conspicuous. While these agents are theoretically advantageous in the spleen, research activity in this field peaked about a decade ago, and little current clinical application exists in the spleen.

Scintigraphy

Some of the more common radiopharmaceuticals useful in splenic imaging are technetium-

99m (Tc-99m)–sulfur colloid, indium-111– white blood cells, and Tc-99m–red blood cells.

Biopsy/Drainage

Splenic biopsies are performed in both adults and children with few complications. A variety of needles are used, with 20and 22-gauge needles being the most common (1). Biopsies establish a specific diagnosis in most patients with a focal splenic abnormality.

Congenital Anomalies

Malposition

The spleen is normally located in the left upper quadrant of the abdomen. Malposition is due to either a congenital maldevelopment or an acquired condition, such as a prior surgical procedure. Excessive mobility to the spleen is secondary to laxity of the splenic suspensory ligaments, including the lienorenal ligament. At times the splenic hilum is located along the superior aspect of the spleen, a condition that is probably a normal variant. The spleen can herniate into a prior lumbar incision.

Many patients with a hypermobile spleen are asymptomatic, and these “wandering spleens” are discovered incidentally when an imaging study is performed for other purposes. An asymptomatic wandering spleen can even be located in the right side of the pelvis, mimick-