Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Mesenteric stranding, often in association with adjacent blood, suggests mesenteric injury, although laceration of an adjacent loop of bowel results in similar findings.

With rare exceptions, the presence of extraluminal gas (either pneumoperitoneum or extraperitoneal gas) is diagnostic of bowel perforation. Extraluminal gas is readily detected with both conventional radiography and CT. The inability to reliably and consistently detect a pneumoperitoneum is a limitation of US.

The small bowel normally contains little gas, and a number of bowel perforations manifest later as an intraabdominal abscess rather than as an immediate pneumoperitoneum. Colonic perforation, on the other hand, commonly results in a pneumoperitoneum, which is readily detected. Other indirect signs for perforation include intraperitoneal fluid, bowel wall thickening, bowel wall contrast enhancement, and bowel lumen dilation. None of the latter signs is specific for a perforation.

Bleeding

Bleeding leads to mesenteric or bowel hematomas, identified by CT as hazy streaking in mesenteric fat, or results in peritoneal or extraperitoneal fluid. Computed tomography scans of direct extravasation of IV contrast is evidence of active bleeding, and such bleeding in a setting of trauma is assumed to represent an injury to the involved viscera. Diffuse extravasation is implied by detecting extravasated contrast material, keeping in mind that extravasated contrast usually has a lower attenuation than the aorta. Not all intraor extraperitoneal bleeding is due to trauma; a ruptured aneurysm, a vessel weakened by tumor, anticoagulation therapy, or even venous obstruction and superimposed ischemia lead to bleeding and hematoma formation. Other rare causes of hemorrhage include severe pancreatitis or even an intraabdominal pregnancy.

Intraperitoneal blood pools in dependent spaces. Thus with upper abdominal bleeding a common site is Morison’s pouch and subphrenic spaces. More inferior locations include paracolic gutters and pouch of Douglas.

Computed tomography attenuation of intraperitoneal blood varies with age; initially it is isodense to intravascular blood. Hemoglobin concentration when blood clots, occurring

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within several hours, raises the attenuation to over 50 Hounsfield units (HU). Subsequent clot lysis, in a matter of days, gradually leads to an attenuation decrease, and in several weeks may approach the attenuation of water. Lower CT attenuation values are found in patients with preexisting anemia or if blood mixes with ascites or other fluid; thus fluid having a low attenuation does not exclude acute bleeding.

A hematoma often is not homogeneous in appearance. The rate of clot lysis in a hematoma varies, and a lower attenuation may be present at the periphery. Likewise, intermittent bleeding leads to simultaneous clotting and lysis and results in regions containing different attenuation values. If contrast-enhanced CT is performed during active arterial bleeding, the extravasating blood is isodense to adjacent arterial blood. Invariably an associated hematoma is present. A recent bleed can be denser than the rest of a hematoma, and such a sentinel clot tends to be located close to the site of bleeding.

At times nonhemorrhagic ascites also enhances with CT IV contrast.

The MR appearance of a hematoma (and intraperitoneal blood) also varies depending on clot age. Within a day or so of bleeding a hematoma is hypointense on both T1and T2weighted images. Then within several days it gradually becomes isointense to hyperintense on T1but remains hypointense on T2-weighted images. This prominent hypointensity on T2weighted images allows differentiation of blood from ascites, which is very hyperintense on T2weighted images. A pneumoperitoneum is also hypointense on T2-weighted images, but other MR sequences and the relative location of gas versus fluid in the peritoneal cavity allow differentiation. Within a week or so a hematoma becomes hyperintense on both T1and T2weighted images,but while evolving to this stage some hematomas reveal a hyperintense rim surrounding a hypointense central portion on T1weighted images. Eventually, if fibrosis develops around a prior hematoma, a hypointense rim on both T1and T2-weighted images encloses this region.

Diaphragmatic Injury

Diaphragmatic injury is one cause of visceral herniation into the chest. A majority of hemidiaphragmatic ruptures occur on the left side.

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

Figure 14.2. Traumatic rupture of left hemidiaphragm. A scout view localizer prior to computed tomography (CT) reveals mediastinal shift to the right, partial left lung atelectasis and an elevated stomach. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

The most common site for rupture is at the diaphragmatic dome, and the least common is at the rib muscular insertions (15). A number of these posttraumatic diaphragmatic ruptures are not initially apparent; herniation increases in size with time, and thus delayed imaging is nec-

essary. Some of these hernias are detected only months later. Intubation appears to hinder the detection of diaphragmatic rupture. Thus an initial chest radiograph or CT detects only about half of diaphragmatic ruptures (Fig. 14.2). These hernias became clinically symptomatic from days to years after trauma, and either conventional chest radiographs or upper gastrointestinal studies are diagnostic (Fig. 14.3). Strangulation of intestinal content has developed, including delayed gastric perforation into the pleural cavity.

A rare cause of diaphragmatic rupture is cardiopulmonary resuscitation.

Computed tomography detection sensitivities for diaphragmatic rupture are disappointing, especially for right hemidiaphragmatic rupture, and are of limited use; keep in mind that detection rates vary with time after trauma. Diaphragmatic crura are not thickened in patients with an injured diaphragm (16); coronal and sagittal reconstructions are also of limited value in detecting subtle diaphragmatic injury. Computed tomography usually does not reveal diaphragmatic discontinuity even with thin sectioning (except in the rare diaphragmatic avulsion); rather, intestinal content not confined by the diaphragm but spilling into the thorax is diagnostic of a hernia, and in the appropriate clinical setting provides indirect

evidence for diaphragmatic rupture. A waistlike intestinal constriction at the site of herniation is occasionally detected if rupture is limited in scope. These traumatic hernias need to be distinguished from congenital diaphragmatic hernias and from hernias through the esophageal hiatus.

Ultrasonography findings in patients with diaphragmatic rupture due to blunt trauma range from diaphragmatic disruption to a nonvisualized diaphragm. Occasionally detected is a diaphragm surrounded by fluid or abdominal content herniating through a diaphragmatic defect.

Preliminary reports suggest that MRI is reliable in detecting diaphragmatic injury; coronal and sagittal MRI reveal the site of a diaphragmatic tear and detect abdominal visceral herniating into the thorax, but keep in mind the limitation on early detection, as discussed previously.

Scintigraphy using intraperitoneally instilled Tc-99m–macroaggregated albumin (MAA) detects a diaphragmatic rupture but is rarely necessary.

Arecdotal reports describe spontaneous diaphragmatic rupture.

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requiring surgical intervention. Less common etiologies for an acute abdomen include lymphoma infiltrating the bowel and resulting in perforation, a perforating primary small bowel neoplasm, and a perforated bowel duplication cyst with spill of the contents into the peritoneal cavity.

In pediatrics, perforation is more common in neonates than in older children. Among neonates with gastrointestinal perforation, most common etiologies are necrotizing enterocolitis, isolated ileal perforations, a combination and sequella of malrotation/volvulus. Etiologic factors in children are trauma, Meckel’s diverticula complications, intussusception, pseudomembranous colitis, and post-operative complications.

In children, screening US detects an abdominal abnormality in about half of those with acute or subacute abdominal pain.

Past teaching has been to study an acute abdomen with conventional radiographs, an approach supplanted by CT, generally without IV contrast. At times images with and without IV contrast are useful (Fig. 14.4). Computed tomography has had a major impact in the diag-

Barotrauma

A pneumoperitoneum is a rare complication of mechanical ventilation. Detection of free gas in these generally rather sick patients leads to a diagnostic dilemma—Is the pneumoperitoneum secondary to an unsuspected bowel perforation? A number of these patients undergo surgical exploration.

Acute Abdomen

The causes of an acute abdomen are legion, including infection, bowel perforation, inflammation, obstruction, ischemia, volvulus of various structures, gynecologic abnormalities, and tumor infiltration; these conditions are discussed in their respective chapters. At times the first evidence of a serious underlying disease is an acute abdomen, such as Crohn’s disease manifesting as bowel perforation. Colonic epiploic appendagitis, a condition diagnosable by imaging, is an example of an acute abdomen not

Figure 14.4. Acute abdomen secondary to jejunal perforation. Oral and intravenous (IV) contrast-enhanced CT reveals ascites and pneumoperitoneum. Higher density material is present within this fluid adjacent the liver (arrow) and also in the left upper quadroon (curved arrow). Although angiography revealed patent vessels, surgery suggested emboli and ischemia for the patient’s perforation. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

nosis and subsequent management of patients presenting with an acute abdomen. Some studies suggest that CT is superior to clinical evaluation in diagnosing a cause for an acute abdomen. Such an approach appears to hold up regardless of the duration of signs and symptoms and in patients with no prior disease. Nevertheless, rather than use CT in a shotgun approach for all patients presenting with an acute abdomen, a more selective choice of imaging studies often establishes a diagnosis more quickly. For instance, with suspected cholecystitis, US should be the initial imaging modality; suspected acute uncomplicated pancreatitis generally requires little or no imaging, except possibly endoscopic retrograde cholangiopancreatography (ERCP), while pancreatic necrosis calls for contrast-enhanced CT or MR. Ultrasonography is more commonly employed in pediatric patients. CT is especially useful in obese patients, nondiagnostic US, or with suspected bowel obstruction.

In some centers US is used liberally for the initial study of patients with an acute abdomen. It is readily performed and detects a number of acute conditions. One limitation is the presence of dilated bowel. Also, while in experienced hands such diagnoses as appendicitis are readily made, a normal US examination does not exclude appendicitis, pyelonephritis, and other disorders. Likewise, early pancreatitis and bowel ischemia do not have specific US findings.

Laparoscopy is still preferred by some as a diagnostic and therapeutic modality in patients presenting with an acute abdomen. Even if conversion to an open laparotomy is necessary, laparoscopic findings are useful as a guide for the subsequent incision.

Infection/Inflammation

Abscess

Intraperitoneal

Clinical

Some abscesses develop spontaneously, although most are secondary to postoperative complications or spread from a source in an adjacent structure, such as diverticular disease, appendicitis, cholecystitis, and so on. Fluid collections communicating with bowel can become

Figure 14.5. Postoperative abscess extending from the left hemidiaphragm inferiorly into left lower quadrant (arrows), communicating with the stomach. Barium sulfate was the contrast material used; it does not affect abscess healing.

huge, and patients have few symptoms due to the internal drainage (Fig. 14.5). At times an abscess and peritonitis coexist, and the initial inciting event is difficult to identify.

A gallstone falling into the peritoneal cavity during laparoscopic cholecystectomy may not be readily retrievable. Although many of these intraperitoneal gallstones are innocuous, they do serve as a potential nidus for abscess formation, with some of these abscesses manifesting years later. An occasional dropped appendicolith, occurring mostly during laparoscopic appendectomy, results in a similar finding. At times the specific etiology for such an abscess is suggested by CT or US.

Imaging

Computed tomography, US, MRI, or scintigraphy should detect and localize most intraabdominal abscesses, and most can then be drained percutaneously, generally under US guidance. Numerous comparison studies have shown CT and US accuracies of over 90% in detecting intraabdominal abscesses. Whether the greater resolution of CT or the greater portability of US determine the modality used, clinically the availability is the deciding factor.

A note about subphrenic abscesses. It is almost unheard of to have a subphrenic abscess without an associated pleural effusion. Even a chest radiograph should detect such an effusion, and the absence of effusion essentially excludes a subphrenic abscess. If imaging identifies a suspicious abscess beneath the right hemidiaphragm but no pleural effusion is detected, an intrahepatic rather than a subphrenic abscess is more likely.

Gas in a fluid collection generally implies an abscess,but gas bubbles are also seen in retained surgical sponges even without an abscess. Large amounts of gas suggest bowel communication, a finding seen with other benign and malignant conditions (Fig. 14.6).

Computed tomography of a typical abscess shows a fluid-filled structure surrounded by a contrast-enhancing rim. Such a finding is not limited to abscesses and is also seen with some necrotic tumors and other benign conditions such as a hematoma and various cystic structures. Also, not all abscesses have this appearance. Differentiation of an abscess and a benign fluid collection is difficult, especially if the wall is thick. Loculated fluid after abdominal surgery tends to develop primarily in the abdomen and

Figure 14.6. Left subphrenic abscess secondary to a perforated gastric fundal adenocarcinoma. The entire fundus is amputated by tumor and abscess (arrows). The study was performed primarily for unexplained weight loss.

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after pelvic surgery loculated fluid is mostly in the pelvis, but this is of limited use in differentiating benign fluid from an abscess.

Abscesses are hypointense on T1and hyperintense on T2-weighted MR images; about half are homogeneous in appearance. Gadoli- nium-enhanced T1-weighted fat-suppressed images identify abscesses as fluid collections surrounded by a contrast-enhancing rim. Gas within an abscess appears as a signal void on both T1and T2-weighted images. Coronal and sagittal reconstruction aids in differentiating an abscess from bowel. Fluid layering occurs in some abscesses, with hypointense material, presumably representing protein, being dependent on T2-weighted images, and such a finding in the peritoneal cavity is strong presumptive evidence of an abscess. Overall, MR sensitivity in detecting abscesses is close to 100%.

Scintigraphy detects most abdominal abscesses. Useful radiopharmaceuticals include gallium-67 citrate, indium-111 leukocytes, and Tc-99m leukocytes. A major limitation of Ga-67 citrate scintigraphy is the prolonged time required to perform the study.

Therapy

Percutaneous abdominal abscess drainage is an established technique, and almost all welldefined unilocular abscesses can be successfully drained. A majority of abscesses are cured with initial drainage. Recurrent abscesses can be drained percutaneously in most patients and surgery avoided in about half (17). Complex abscesses consisting of loculated, poorly confined, or multiple abscesses or those associated with a fistula have a lower success rate and often require several drains. A single abscess is often drained using US guidance, but multiple abscesses are easier to drain with CT guidance. Distinguishing an abscess from necrotic tissue can be difficult. At times aspirate cytology is helpful. Similar to surgical drainage, attempts to drain infected necrotic tumors percutaneously are rarely successful. Conversion to surgical drainage (and often associated resection) is required with the presence of unhealing abscesses or fistulas and bowel or pancreatic necrosis. Catheter-induced bleeding is an occasional complication requiring surgical correction.

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Crohn’s disease abscesses can be drained percutaneously using image guidance, and the patient is thus stabilized. These abscesses tend not to resolve completely, especially if they involve an enteric fistula.

Some left subphrenic abscesses cannot be readily drained using a transabdominal approach, and a transpleural approach is necessary. At times a drainage catheter is inserted through the pleura. Regardless of catheter position, most abscesses are successfully drained, although a transpleural approach risks a pneumothorax, requiring its own therapy.

Abscess drainage using a transrectal or transvaginal approach with a combination of endoluminal US and fluoroscopy for needle advancement, tract dilation, and catheter insertion, combined with appropriate antibiotics, is effective therapy for most pelvic abscesses. Patients undergoing transrectal aspiration or drainage have less procedure-related pain and catheter pain than those with a transvaginal approach (18). A viable option for some pelvic abscesses is US-guided transperineal catheter drainage.

Pelvic abscesses are readily drained in children and adolescents. The average hospital stay for children after image-guided transrectal drainage of pelvic abscesses tends to be shorter than after open surgical drainage. Surgical drainage is associated with more complications than percutaneous drainage, but comparison studies often have a built-in bias against surgery—patients undergoing surgical drainage tend to be sicker.

Computed tomography–guided transgluteal percutaneous drainage of deep pelvic abscesses through the greater sciatic foramen is an option in both adults and children (19).

A majority of vancomycin-resistant enterococcal abscesses can be drained percutaneously, although the rate of successful therapy is lower than with more conventional abscesses (20); at times drainage provides a first clue to the presence of vancomycin-resistant enterococci.

Abdominal Wall Abscess

Occasionally diverticulitis or cholecystitis evolves into an abdominal wall abscess. Likewise, an occasional biliary or other neoplasm leads to an abdominal wall abscess. Imaging

readily differentiates those abscesses involving the rectus abdominis muscle from intraabdominal conditions.

Psoas Muscle Abscess

An abnormal fluid collection in the psoas muscle region most often is an abscess, and less often a hematoma. In a setting of pancreatitis, a pseudocyst is also in the differential. A primary iliopsoas abscess is not common; a number of these occur in IV drug users and those positive for human immunodeficiency virus. More often these abscesses develop from a gastrointestinal, genitourinary, or spinal source. Some retroperitoneal abscesses involve not only the psoas muscles but also spread along soft tissue planes into adjacent compartments. Psoas abscesses develop in Crohn’s patients with disease.

Gram stain and a culture of the abscess contents should establish the responsible organism. Blood cultures are less often helpful. Both grampositive and gram-negative organisms are involved. In some parts of the world a tuberculous psoas abscess is more common than a pyogenic abscess; a tuberculous abscess tends to involve the adjacent vertebrae. Tuberculous psoas abscesses can be successfully drained percutaneously, although abscess recurrence often requires repeat drainage.

The clinical triad of fever, flank or thigh pain, and limitation of hip movement is found only in about half or fewer patients with a psoas abscess. Sepsis is common.

Computed tomography readily detects psoas abscesses; however, differentiation from a tumor purely on CT criteria is problematic (Fig. 14.7). A hematoma is also often in the differential. Image-guided needle aspiration should be diagnostic and percutaneous catheter drainage therapeutic.

Magnetic resonance imaging is very useful in evaluating psoas muscles. Normal psoas muscle is hypointense on T2-weighted images, while abscesses and the occasional psoas muscle tumor are hyperintense. Contrast-enhanced MR of a psoas abscess reveals a signal void surrounded by intense enhancement.

Conventional therapy of these abscesses is surgical drainage, although percutaneous drainage using CT or US guidance is becoming