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

COLON AND RECTUM

bowel disease from superficial mucosal biopsies has already been mentioned (see Ulcerative Colitis). Some of these patients undergo steroids therapy until a correct diagnosis of lymphoma is made. In fact, lymphoma should be considered in the differential diagnosis of a dense lymphocytic infiltrate obtained from a segment of bowel simulating either ulcerative colitis or Crohn’s disease.

Although not common, patients with leukemia have developed colon lymphoma.

Overall, colonic lymphomas appear similar to those seen in the small bowel. A common appearance is that of an intramural infiltrating, sharply marginated tumor. Involved colon tends to be thickened and distorted, an aid in differentiating lymphomas from adenocarcinomas. A lymphomatous large, ulcerated mass is less common. A rare appearance is aneurysmal dilation of the affected colonic segment.

Unlike cecal adenocarcinomas, extension across the ileocecal valve is common with lymphomas, and the site of origin is often difficult to determine; some authors use the term ileocecal lymphoma to describe these tumors. Their differential diagnosis includes a mesenchymal tumor and localized Crohn’s disease.

Double-contrast barium enema findings in patients with peripheral T-cell lymphoma range from diffuse colonic involvement to focal, and from aphthae, gross ulcers, polyps, and circumferential narrowing to simply ileocecal deformity (180). An occasional lymphoma manifests as multiple polyps in the proximal gastrointestinal tract and numerous aphthae in the colon.

An occasional colonic lymphoma presents as diffuse polyposis. These polyps tend to vary in size, and the barium enema appearance mimics familial polyposis, although the two entities can usually be differentiated on clinical grounds. Most often such lymphomatous polyposis represents B-cell lymphoma.

Sarcoma

Leiomyosarcomas are the most common primary colorectal sarcomas. These sarcomas tend to be larger than carcinomas at first presentation. After resection the 5-year survival depends on the tumor grade.

Endoscopic US identifies rectal leiomyosarcomas as hypoechoic tumors.

Liposarcoma

Magnetic resonance imaging of liposarcomas reveal several patterns. Well-differentiated liposarcomas have MRI characteristics similar to those of a lipoma, consisting of a wellmarginated tumor hyperintense on T1weighted images, hypointense on T2-weighted images,and showing little if any contrast enhancement; less well-differentiated liposarcomas tend toward a heterogeneous appearance, with many containing varying amounts of necrosis. In general, tumor necrosis varies inversely with the degree of tumor differentiation.

Angiosarcoma

Colonic angiosarcomas are rare. A cecal angiosarcoma occasionally intussuscepts.

Histiocytoma

Primary colonic malignant fibrous histiocytomas are rare. Initially these sarcomas are confined to the colon wall, but with growth ulcerate through the mucosa and bleed, or they invade the adjacent soft tissues. Peritoneal implants and lymph node metastases are evident with some. Also, an adjacent extraperitoneal malignant fibrous histiocytoma readily invades the colon and appears as an infiltrating serosal tumor.

Imaging reveals a solid, generally large tumor suggesting a sarcoma or lymphoma.

Pathologic identification of a histiocytoma is not always straightforward. At times an inflammatory fibrosarcoma and a leiomyosarcoma are in the differential.

Carcinosarcoma

Only a few colonic carcinosarcomas have been reported. A question is occasionally raised about whether these represent two separate collision neoplasms, although most carcinosarcomas appear to represent differentiation of a single progeny into two cell types. The presence of retroviral particles in the sarcomatous cells of some of these tumors supports the theory of tumor differentiation from a carcinomatous into a sarcomatous component.

An elevated, fungated, ulcerated tumor is a typical appearance. Most carcinosarcomas carry a poor prognosis.

Melanoma

Most of the rare primary melanomas are found in the rectum, and these patients present with rectal bleeding. They tend not to obstruct.

Primary rectal malignant melanomas tend to be polypoid or fungating, but often already extending to the pelvic side wall at initial presentation (181). A cancer is often suspected. Adenopathy is common. At times a biopsy contains few melanocytes but considerable inflammation, and only after tumor excision is a correct diagnosis made.

A dual rectal melanoma and adenocarcinoma are occasionally reported; presumably such dual collision neoplasms develop by chance.

Both primary and metastatic melanomas have a high tumor-to-background PET activity and FDG-PET scanning is useful to detect unsuspected metastases; PET is also commonly employed for follow-up after therapy.

Metastasis or Direct Invasion to Colon

Metastasis to the colon is not common; more common is direct invasion from an adjacent structure. Thus gynecologic malignancies invade the rectum, or a hepatocellular carcinoma invading the adjacent splenic flexure results in massive bleeding, even to the point of exsanguination.

In a comparison of CT and MRI in predicting bladder or rectal invasion in women with uterine carcinoma, MRI was slightly, but not statistically, superior to CT (182); both provided similar results as rectoscopy.

A long segment of circumferential rectal wall thickening, having a rectal linitis plastica appearance, is most often due to metastatic gastric cancer, but it can be found with other causes of peritoneal carcinomatosis and rectal metastasis (183). Obstructions and fistulas are rare manifestations of lung and breast metastases. A renal cell carcinoma is one cause of a hypervascular metastasis.

A recurrent bladder or prostatic carcinoma invading the rectum can simulate a rectal leiomyoma or result in an annular constricting lesion.

A malignant colon obstruction, either primary colonic or extrinsic, most often due to spread of a gynecologic tumor or peritoneal seeding, is a difficult management problem.

ADVANCED IMAGING OF THE ABDOMEN

Most of these tumors are unresectable, but palliation of bowel obstruction is desirable. Palliation consists, at best, of a proximal colostomy. An occasional option with a single major obstruction is metallic stent placement for decompression if access under fluoroscopic guidance is feasible. Most such treatable obstructions are in the rectum and rectosigmoid, but occasionally a stent can be inserted through a more proximal obstruction (184). Many of these patients, however, have widespread metastases, including to the small bowel, and no viable bypass is feasible.

Rhabdoid Tumor

The rare colonic malignant rhabdoid tumor is diagnosed by a pathologist detecting rhabdoid cells. This tumor is more common in the kidneys. No specific imaging features have been described.

Neuroendocrine Tumors

Colonic neuroendocrine tumors are uncommon, and some are difficult to place in proper perspective.An occasional colorectal poorly differentiated neuroendocrine carcinoma presents with widespread liver metastasis.

These tumors range from benign to malignant. Most are solid, intramural tumors, with an occasional mesenteric one appearing as an extraserosal tumor.

Carcinoid

Rectal carcinoids are more common than colonic ones; a cecal location is most common in the colon. Synchronous carcinoids occur occasionally. Similar-appearing rectal carcinoids have developed in siblings. The malignant potential of rectal carcinoids varies considerably.

Small rectal carcinoids are palpable on digital examination as firm nodules. Endoscopy visualizes small polyps covered by normal-appearing mucosa, with either erythema or a central depression occasionally being found.

Carcinoid syndrome develops mostly in a setting of metastases, with only an occasional rectal carcinoid directly responsible for this syndrome.

Table 5.5 summarizes the clinical findings from two studies. Between 1964 and 1988, the Alberta Cancer Registry compiled 36 malignant colon carcinoids (excluding ileocecal region and rectum) (186); average age at diagnosis was 68 years, and at presentation 22% of patients were already Dukes C and 86% had invaded pericolic fat. Many malignant carcinoids have already metastasized at initial presentation, with the most common metastatic sites being the liver and lung. In fact, the presence of metastases is often the unequivocal finding establishing malignancy of these tumors. Survival with colonic carcinoids is lower than with rectal or appendiceal carcinoids (or even with colon adenocarcinomas).

Carcinoids range in appearance from a simple polyp to an apple-core infiltrating tumor mimicking an adenocarcinoma. An unusual barium enema finding in the presence of a carcinoid is colon jejunization. Such jejunization presumably is secondary to colonic wall foreshortening induced by the desmoplastic reaction commonly associated with these tumors. A carcinoid located in the posterior rectal wall or adjacent tissues widens the presacral space.

Transrectal US in patients with rectal carcinoids reveals increased echogenicity and a heterogeneous internal echo pattern in some. Small rectal carcinoids tend to be hypoechoic. Pathologically, these findings are associated with increased fibrotic interstitium around

nodular tumor nests. Transrectal US can also often reveal depth of invasion and suggest lymph node metastasis.

I-123-metaiodobenzylguanidine (MIBG) scintigraphy evaluates metastatic carcinoids.

Some small carcinoids have been resected endoscopically, although most require surgical excision, similar to adenocarcinomas.

Other Tumors

Schwannomas, or primary nerve sheath tumors, originate more often from peripheral nerves and are rare in the colon. Most are benign. Their imaging appearance is similar to other stromal tumors. A cystic component is occasionally detected.

A rare colonic ganglioneuroma presents as filiform polyposis.

Neurofibromatosis type 1 (von Recklinghausen’s disease) is discussed in Chapter 14. Gastrointestinal neurofibromatosis is uncommon and is a late manifestation of von Recklinghausen’s disease. Only rarely is colonic neurofibromatosis an initial presentation.

Gastrointestinal neurofibromas range from solitary, to multiple, to plexiform in appearance. An occasional colonic plexiform neurofibroma and neuronal hyperplasia result in disordered mobility, a megacolon, and proximal bowel dilation, similar to other causes of adynamic ileus. Some manifest through gastrointestinal bleeding.

Granular cell myoblastomas are more common in the esophagus. They are rare in the colon. Most are single, but occasional reports describe multiple tumors scattered throughout the large bowel.

Dilatation

Mechanical Obstruction

The two most common causes of colonic obstruction in adults, namely colon cancer and diverticulitis, have already been discussed. Hernias are covered in Chapter 14. Colon obstruction in a setting of cystic fibrosis was covered in a previous section (see Cystic Fibrosis).

Primary causes of intestinal obstruction in elderly patients requiring surgery are an incarcerated hernia and colonic neoplasms.

Figure 5.34. Cecal volvulus. CT identifies a massively dilated cecum displaced to the left of midline (arrows). Dilated loops of small bowel on the right are secondary to small bowel obstruction. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

Volvulus

Cecal

Traditionally, cecal volvulus was suggested with conventional radiography and confirmed either with a barium enema or colonoscopy. Most often cecal volvulus is idiopathic (Fig. 5.33), but occasionally it is induced by a more distal

partial obstruction. Computed tomography appears to be more accurate than conventional radiography in suggesting the diagnosis (187), and although CT is often performed for suspected cecal volvulus,few studies have evaluated whether it is superior or even equal to a barium enema (Fig. 5.34).

COLON AND RECTUM

In distinction to sigmoid volvulus, a successful therapeutic barium enema or colonoscopy is achieved only in a minority of these patients, and most undergo surgery.

Sigmoid

Sigmoid volvulus ranges from an acute condition, often associated with strangulation, to a chronic setting, with the patient presenting with a gradual onset or intermittent obstruction. An immediate concern is to ascertain that this is indeed idiopathic sigmoid volvulus rather than a sigmoid or rectal cancer-induced colonic obstruction. Sigmoid volvulus occasionally develops during pregnancy and after gynecologic and other abdominal surgery.

Imaging findings of sigmoid volvulus are familiar to most radiologists (Fig. 5.35). A CT whirl pattern consists of a twisted, dilated sigmoid loop and its associated vessels around the mesocolon. If the transverse colon can be identified on radiographs (with the patient supine), a dilated sigmoid colon located cephalad to the transverse colon is an accurate finding of sigmoid volvulus (188).

The preferred therapy for acute sigmoid volvulus is decompression either by endoscopy or barium enema, followed, if indicated, by elective sigmoid resection. Simple sigmoid decompression does relieve obstruction but volvulus

Figure 5.35. Sigmoid volvulus. A lateral view from a barium enema identifies a typical twist (arrow), shows barium in a dilated sigmoid and excludes a carcinoma as etiology for the obstruction.

tends to recur if no resection or fixation is performed. A surgical nonresective procedure consists of extraperitonealization of the sigmoid colon by placing it in the infraumbilical abdominal wall.

Transverse Colon

Transverse colon volvulus is rare. It is more common in women. Patients with Chilaiditi’s syndrome appear more prone to developing a transverse colon volvulus; lax colonic ligaments predispose to such torsion.

In adults, conventional radiographs are rarely diagnostic of transverse colon volvulus. Barium enema findings vary; even a coil-spring appearance mimicking an intussusception has been reported.

Other

Only rare reports describe splenic flexure volvulus. It occurs in association with systemic sclerosis and has developed in patients with a wandering spleen, generally around the splenic pedicle. It can be associated with small bowel obstruction.

Descending colon volvulus can develop in a setting of an anomalous mesocolon and a redundant bowel.

Intussusception

In an intussusception, a segment of bowel, the intussusceptum, invaginates into the lumen of an adjacent intussuscipiens. Any part of bowel can intussuscept, although a mobile intraperitoneal bowel loop and its associated mesentery are most often involved. The intussusceptum usually invaginates distally, although occasional proximal invagination does occur (for example, a jejunogastric intussusception after a Billroth II operation). Intussusceptions range from transient to fixed. As discussed below, some are reduced with pressure.

By its bulk, an intussusception should obstruct the bowel lumen, although in distinction to pediatric patients, bowel obstruction is not a prominent feature of adult intussusceptions. A more serious consequence, especially in the younger patient, is vascular occlusion of the intussusceptum, generally venous, and resultant ischemia.

Adults

Intussusceptions in most adults have an identifiable lead point and range from enteroenteric, to ileocolic, to colocolic, to rectal prolapse. The most common lead point is a cecal adenocarcinoma (Fig. 5.36); less common is cecal lymphoma or a benign polyp. Rarer lead points in adults consist of pseudomembranous colitis, Meckel’s diverticulum, a rare duplication, endometrioma (Fig. 5.37), or even calcified cecal fecaliths. An appendiceal polyp in a patient with Peutz-Jeghers syndrome acted as a lead point for intussusception (189). Although most colonic lipomas are intramural and sessile, they are prone to becoming pedunculated and act as lead points for an intussusception. Not all of these are at the ileocecal region; a number of sigmoid lipoma-associated sigmoidorectal intussusceptions have been reported.

A rectal intussusception is usually a transient phenomenon occurring during straining, is idiopathic, and is associated with constipation. Proctography shows circular infolding of the rectal wall during straining. The criteria defining when such infolding is abnormal are not well established, and minor changes probably are best considered normal variants, but solitary rectal ulcer syndrome (discussed later) is in the differential diagnosis.

Figure 5.37. Colocolic intussusception (arrows). The lead point was an endometrioma, a highly unusual source for an intussusception.

Computed tomography and MR detect most adult ileocolic intussusceptions but, aside from a lipoma, identification of a lead point is difficult. At times even endoscopic biopsy fails to provide an etiology, and the diagnosis is established only after a right hemicolectomy. Computed tomography findings of an intussus-

COLON AND RECTUM

ception consist of a target or sausage-shaped inhomogeneous soft tissue tumor. The appearance varies depending on the relative orientation of the x-ray beam and intussusception. Colocolic intussusceptions caused by a colonic lipoma can be suggested by US; CT is diagnostic if fat is detected in the lead point, although the lack of fat in the lead point due to infarction and necrosis of an intussuscepted tumor does not exclude a lipoma.

Unenhanced CT has a role if ischemia is suspected in adults with an intussusception; CT findings of a hypodense layer in the intussusceptum or surrounding fluid or gas should suggest vascular compromise (190); lumen obstruction is not always present in an ischemic or necrotic intussusception.

Overlying pneumatosis cystoides intestinalis and enteritis cystica profunda are uncommon associated finding of a colocolic intussusception.

Magnetic resonance imaging also readily identifies intussusceptions, with findings similar to those found with CT. Magnetic resonance imaging reveals concentric bowel rings.

Pediatrics

Clinical

An acute ileocolic intussusception in a young child is a common emergency. Most intussusceptions occur before the age of 2 years and are idiopathic in origin. The rare identifiable lead points, more common in older children, consist of a Meckel’s diverticulum, polyp, or even a duplication. Why the reported prevalence of intussusception is greater in some parts of the world is puzzling.

The typical clinical presentation and conventional radiographic findings are well known. Occasionally encountered, however, is an atypical presentation, for instance, bilious vomiting due to an ileocolic mass resulting in extrinsic duodenal obstruction.

One variant is an ileoileocolic intussusception. Prereduction findings are similar to those of an ileocolic intussusception. Once the intussusception was reduced to the cecum, air enemas in nine children with ileoileocolic intussusceptions identified the intussusceptum as two or more separate polypoid components, in contrast to ileocolic intussusceptums, which

tend to be either smoothly marginated or somewhat lobular in appearance (191).

Sigmoidorectal intussusceptions also occur in infants and children. In some, the typical clinical presentation of a palpable abdominal mass and colicky pain is absent. These intussusceptions can be misdiagnosed as simple rectal prolapse.

Presumably a surgical consultation has been obtained and a surgeon has examined the child prior to attempted intussusception reduction. The child should be in stable condition, and both the surgeon and the radiologist should be confident that no contraindication exists to a therapeutic enema. Contraindications for reduction include bowel perforation, peritonitis, and hypovolemic shock.

A long-term outcome study in children found an overall recurrence rate of 9%, with about two thirds of children having a single recurrence (192); reducibility was 95% for recurrent intussusceptions, with no perforations. Also, recurrence did not predict an abnormal lead point.

Imaging

Although conventional abdominal radiographs are often obtained first, their value has been questioned. Even experienced observers often differ whether in children with clinically suspected intussusception it is indeed present or absent; the best predictor of intussusception is a soft tissue mass and decreased large bowel gas (Fig. 5.38).

In some centers, US is the initial imaging modality of choice when suspecting an intussusception (Fig. 5.39). In experienced hands US has a high sensitivity and specificity in detecting an intussusception and a contrast enema is then limited to therapy. Viewed in a transverse section, prereduction US shows an intussusception as a doughnut or target lesion; it has a reniform shape (pseudokidney is the term often used) when viewed in longitudinal section. Scans close to the lead point of an intussusception reveal the intussusceptum as a hypoechoic central structure; scans away from the lead point have a hyperechoic crescent appearance due to mesentery and related vessels being drawn in by the intussusceptum. Although such a US appearance should suggest an intussusception, neither a target nor reni-

form appearance is pathognomonic. Necrotizing enterocolitis, volvulus, or even stool may mimic this appearance.

At times, because of obscure symptomatology, these patients are studied with CT. Findings of intussusception are straightforward in most. Computed tomography reveals an intraluminal tumor and a target sign–like appearance of alternating layers of high and low attenuation. With obstruction, more proximal bowel loops distend with fluid. Necrosis manifests as inflammation, loss of tissue planes, and presence of intraperitoneal fluid.

An extensive ileocolic intussusception distorts normal superior mesenteric vessel anatomy. Thus with the lead point of an intussusceptum at the sigmoid colon or distally, the superior mesenteric vein is located to the left of the superior mesenteric artery.

Published successful intussusception reduction rates range between 70% and 85%, with an occasional report of 90%, regardless of whether a liquid or air is used. A comparison of different contrast agents used is difficult unless the procedure used is standardized. A major factor influencing success rates is the intraluminal pressure achieved rather than any other techni-

cal factor. A barium enema bag at 1-m elevation produces greater intraluminal pressure than a typical water-soluble contrast agent or water at the same height. Pressure during pneumatic reduction varies considerably.

At times US identifies fluid within an intussusception, representing trapped peritoneal fluid, seen on axial images as an anechoic crescent between the intussusceptum and intussuscipiens.

Ultrasonography during reduction of an ileoileocolic intussusception reveals a complex frond-like appearance. The intussuscepted small bowel is also surrounded by cecal fluid. These intussusceptions are likewise difficult to reduce.

Contrast Agents

Historically, a barium enema was performed in the pediatric patient suspected of an intussusception. The study not only established a diagnosis but also was therapeutic.

Some radiologists still use barium for reducing intussusceptions, although teaching and pediatric hospitals have changed to a pneumatic technique.

A pneumatic reduction of intussusception is safe and successful in most children. Although fluoroscopy is useful for this procedure and is employed by many radiologists, a lead point is difficult to identify. Thus even with successful reduction, the presence of a tumor lead point is not excluded. Still, many radiologists believe that pneumatic reduction is quicker, safer, and more effective than hydrostatic reduction.

Reduction success rate varies with the duration of signs and symptoms. Thus success rate for air reduction was 89% for those symptomatic for <12 hours, 83% for those with symptoms for 12 to 24 hours, and 74% for those symptomatic for >24 hours (193). Several trials of air reduction increase the success rate. A success rate of 70% with one trial of air reduc-

tion increased to 91% after a policy of up to three trials was instituted (193).

Some radiologists perform pneumatic reduction under US control. In patients who underwent 52 US-guided pneumatic intussusception reductions, the overall success rate was 92% (194); a pressure of 60mmHg was maintained for 30 seconds, and if an intussusception failed to reduce, the procedure was repeated at a pressure of 120mm Hg. Perforation occurred in two others. The published data of pneumatic reduction under US control are difficult to place in the proper perspective because of the subjective nature of many of these studies.

In some parts of the world pneumatic reduction is performed with no imaging, and the success of reduction is evaluated purely on clinical grounds.

Color Doppler US evaluates whether blood flow is present in an intussusception. The success rate for air reduction is significantly greater in those children with blood flow in the intussusception than in those with absent flow. Lack of Doppler evidence for blood flow, however, should not be a contraindication to attempted reduction, and practical application of such Doppler study remains to be established.

A more recent technique is US-guided intussusception reduction using a saline enema or Hartmann’s solution. The sonographic criteria of intussusception reduction include an initial target sign that is later no longer identified, visualization of the ileocecal valve, and fluid refluxing into small bowel; this technique has a success rate of over 90% in reducing an intussusception.

ADVANCED IMAGING OF THE ABDOMEN

the rectosigmoid without invasion. A barium enema is diagnostic.

A pregnant patient with an ileal pouch–anal anastomosis presented at 36 weeks’ gestation with bowel obstruction (196); the obstruction cleared after delivery.

Obstruction by Gallstones

Gallstone ileus most often obstructs in the ileum (discussed in Chapter 4). With a cholecystoduodenal fistula, if a stone manages to pass through the ileocecal valve, colonic gallstone ileus occurs almost always only proximal to a stricture.

Another scenario of colonic gallstone ileus occurs if a cholecystocolic fistula develops; obstruction by the gallstone most often is in the sigmoid colon. If the obstruction is incomplete, a barium enema identifies a cholecystocolic fistula.

Complications

In some infants only partial intussusception reduction is achieved. In these infants, instead of performing immediate laparotomy, in consultation with the surgeon and if the infant is clinically stable, another attempt at intussusception reduction may be appropriate.

The risk of bacteremia during intussusception reduction is low.

What are the sequelae of a perforation during an intussusception reduction? In 14 perforations (seven using barium and seven air) all children with barium reduction required bowel resection, but only four of the seven with air required resection (195); in addition, the anesthesia time was longer and hospital stay longer in the barium group. Of interest is that perforations are through necrotic bowel only in a minority of these children; presumably increased pressure plays a major role in these perforation. In general, all other factors being equal, the perforation rate is probably similar regardless whether barium or air is used.

Extrinsic Obstruction

Occasionally a distended bladder compresses the rectosigmoid against the sacrum and obstructs on either an acute or chronic basis. Similarly, some gynecologic tumors compress

Obstruction Due to

Motility Abnormalities

Pseudo-Obstruction (Ogilvie’s Syndrome)

Etiology

A number of etiologies have been proposed for acute colonic pseudo-obstruction (Ogilvie’s syndrome), including an imbalance between sympathetic inhibitory and parasympathetic excitatory colonic innervation. It occurs most often after surgery or trauma (Table 5.6).

Table 5.6. Conditions associated with Ogilvie’s syndrome

Common

Recent surgery

Recent trauma

Severe medical condition

Uncommon Postcesarean section Leukemia

von Recklinghausen’s disease

Multiple endocrine neoplasia (MEN) syndrome type 2 Botulism in infants

Herpes zoster infection Hypothyroidism Myotonic dystrophy Drug therapy

Imipramine Tocolytic therapy