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

COLON AND RECTUM

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193.Gorenstein A, Raucher A, Serour F, Witzling M, Katz R. Intussusception in children: reduction with repeated, delayed air enema. Radiology 1998;206:721–724.

194.Yoon CH, Kim HJ, Goo HW. Intussusception in children: US-guided pneumatic reduction—initial experience. Radiology 2001;218:85–88.

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198.deSouza NM, Williams AD, Wilson HJ, Gilderdale DJ, Coutts GA, Black CM. Fecal incontinence in scleroderma: assessment of the anal sphincter with thinsection endoanal MR imaging. Radiology 1998;208: 529–535.

199.Kelvin FM, Hale DS, Maglinte DD, Patten BJ, Benson JT. Female pelvic organ prolapse: diagnostic contribution of dynamic cystoproctography and comparison with physical examination. AJR 1999;173:31–37.

200.Pucciani F, Rottoli ML, Bologna A, et al. Anterior rectocele and anorectal dysfunction. Int J Colorectal Dis 1998;11:1–9.

201.Kelvin FM, Hale DS, Maglinte DD, Patten BJ, Benson JT. Female pelvic organ prolapse: diagnostic contribution of dynamic cystoproctography and comparison with physical examination. AJR 1999;173:31–37.

202.Mellgren A, Lopez A, Schultz I, Anzen B. Rectocele is associated with paradoxical anal sphincter reaction. Int J Colorectal Dis 1998;13:13–16.

203.Salzano A, Grassi R, Habib I, et al. [The defecographic and clinical aspects of the solitary rectal ulcer syndrome.] [Italian] Radiol Med 1998;95:588–592.

204.Salzano A, Muto M, De Rosa A, et al. [Defecography in rectal wall prolapse conditions.] [Italian] Radiol Med 1999;97:486–490.

205.Halligan S, Malouf A, Bartram CI, Marshall M, Hollings N, Kamm MA. Predictive value of impaired evacuation at proctography in diagnosing anismus. AJR 2001;177:633–636.

206.Karoui S, Savoye-Collet C, Koning E, Leroi AM, Denis P. Prevalence of anal sphincter defects revealed by sonography in 335 incontinent patients and 115 continent patients. AJR 1999;173:389–392.

207.Stewart LK,Wilson SR. Transvaginal sonography of the anal sphincter: reliable, or not? AJR 1999;173:179– 185.

208.Rociu E, Stoker J, Eijkemans MJ, Schouten WR, Lameris JS. Fecal incontinence: endoanal US versus endoanal MR imaging. Radiology 1999;212:453–458.

209.Beets-Tan RG, Morren GL, Beets GL, et al. Measurement of anal sphincter muscles: endoanal US, endoanal MR imaging, or phased-array MR imaging? A study with healthy volunteers. Radiology 2001;220: 81–89.

210.Malouf AJ, Williams AB, Halligan S, Bartram CI, Dhillon S, Kamm MA. Prospective assessment of accuracy of endoanal MR imaging and endosonography in patients with fecal incontinence. AJR 2000;175: 741–745.

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212.Ellul JP, Mannion S, Khoury GA. Spontaneous rupture of the rectum with evisceration of the small intestine through the anus. Eur J Surg 1995;161:925–927.

213.Spencer JA, Chapple K, Wilson D, Ward J, Windsor AC, Ambrose NS. Outcome after surgery for perianal fistula: predictive value of MR imaging. AJR 1998;171:403–406.

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Congenital Abnormalities

Duplication of the vermiform appendix is rare. A double appendicitis developed in a patient with appendiceal duplication (1). A rare anomalous appendiceal insertion is not at the junction of colonic taenia but more distal.

Ultrasonography (US) identifies the appendix in only a minority of patients with cystic fibrosis; the appendix is normally dilated in these patients—the appendiceal mean diameter in cystic fibrosis patients was 9.8mm, versus a noncystic fibrosis normal control group diameter of 3.9mm (2). A normal appendiceal wall in cystic fibrosis patients is not thickened.

Appendicitis

Clinical

Appendicitis is a common cause of an acute abdomen in the Western world but is rare in parts of Africa and Asia. Apparently appendicitis was uncommon prior to the 19th century, at which time the term typhlitis was applied to right lower quadrant inflammation. Finally, in 1886, Reginald Fitz (3) put this condition in a correct perspective and coined the term appendicitis.

A number of unusual etiologies account for appendicitis, with the final common pathway being lumen obstruction. In children, obstruction due to lymphoid hyperplasia predomi-

nates. In adults, among others causes of obstruction are primary neoplasms and metastases to the appendix (Fig. 6.1). Ingested lead shot has been found in an inflamed appendix. Patients have developed acute appendicitis immediately after colonoscopy and after trauma, although any relationship is conjecture. Although schistosomiasis is common worldwide, schistosomal appendicitis is rare. Appendicitis does occur in renal transplant patients, and the clinical differential diagnosis needs to be expanded to include complications related to the transplanted kidney.

Occasionally periappendiceal inflammation involves the right ureter and causes an unsuspected obstruction. Even bilateral ureteric obstruction and acute tubular necrosis has developed (4).

A typical scenario in appendicitis is an acute appendiceal inflammation; next an intraluminal abscess and an increase in intraluminal pressure, followed by vascular occlusion, necrosis, and eventual perforation. A time-dependent sequence is apparent during the course of appendicitis and the prevalence of acute perforation increases when the diagnosis is delayed; an increased risk of perforation becomes apparent with a delay of more than 12 hours.

Occasionally a barium enema reveals an appendiceal diverticulum (Fig. 6.2). Most of these are curiosities of no significance, but similar to diverticula at other locations, obstruction, inflammation, and resultant diverticulitis are feasible. Indeed, appendiceal

diverticulitis does exist, but differentiating conventional appendicitis from appendiceal diverticulitis is problematic even for a pathologist. From a clinical viewpoint both entities are similar.

Figure 6.2. Appendiceal diverticulosis.

A clinical diagnosis of appendicitis has about 20% false-positive and false-negative rates. These rates are increased in the very young and very old and in women under age 40 who have underlying gynecologic abnormalities.

Recurrence of surgically treated appendicitis is possible, presumably representing appendicitis of a residual stump. Discussions with experienced surgeons suggest that this condition is more common than the literature suggests. Recurrent appendicitis also develops in an appendiceal stump after laparoscopic appendectomy.

In most younger patients with acute appendicitis, no imaging studies are necessary. With an atypical presentation, a clinical differential diagnosis includes a number of other gastrointestinal and genitourinary conditions and can be evaluated by specific imaging tests. Mesenteritis and ileitis, regardless of etiology, are often in the differential diagnosis and are difficult to exclude. Also, in adolescent girls and women of reproductive age gynecological abnormalities need be considered and in these US should be the first imaging modality considered.

Imaging complements but does not replace clinical evaluation. A number of patients with normal imaging studies have been found to have acute appendicitis.

APPENDIX

Diagnosis

An interesting observation in adults is that if the leukocyte count and C-reactive protein values are normal at admission and remain normal, acute appendicitis can be excluded and neither imaging nor resection is believed to be warranted (5); the authors emphasize that this finding applies only to adults, because in children with acute appendicitis both tests can be within normal limits.

Part of the problem in evaluating the efficacy of various imaging modalities is establishing a reproducible patient population. Most investigators agree that a majority of patients with suspected acute appendicitis do not need preoperative imaging, and it is this resultant patient selection, together with variations in the inherent delay in obtaining an imaging study, that introduces a bias. In addition, adults, with their more readily visualized periappendiceal fat, should not be compared to children. Even if a study does show that imaging reduces false-pos- itive and false-negative appendectomy rates, the results should be judiciously balanced by the inherent increased delay to surgery and resultant increased risk of perforation for truepositive patients.

What is “early” appendicitis? Inflammation initially involves only the appendix, then spreads to surrounding tissues. Signs of early appendicitis are centered around an enlarged, fluid-filled appendix, which then presumably evolves into a thickened, inflamed wall, then periappendiceal inflammation, and eventually a phlegmon and abscess. The specific imaging findings are different between these categories, yet most studies lump patients into a single category and the relative mix of early and not-early appendicitis patients affects the prevalence of specific signs and study results.

Does imaging aid in the overall diagnostic accuracy in suspected acute appendicitis? Although most publications answer this question in the affirmative, an occasional dissent appears in the literature. Thus, in a controversial study of 766 consecutive patients undergoing appendectomy for suspected appendicitis at a United States university tertiary care center, neither computed tomography (CT) nor US improved diagnostic accuracy or the negative appendectomy rate (6); in fact, preoperative US delayed surgical consultation and appendec-

tomy by 6 hours; the delay due to CT was 8 hours. A delay increases the risk of perforation with its increased morbidity. In the final analysis, whether to operate or not remains mostly a clinical decision.

Another relevant question is whether imaging has any effect on a negative appendectomy rate (normal pathologic findings). A negative appendix was found in 14% of children who underwent no imaging, 17% of those who had US (with or without CT), and only 2% of those who underwent CT only (7).

Presence of Appendicolith

Computed tomography (without enteric contrast) detects an appendicolith in about two thirds of children with appendicitis (8); for comparison, only 14% of 44 children initially suspected of appendicitis but with this diagnosis later excluded had an appendicolith. Most appendicoliths are solitary, but exceptions do occur. Some are several centimeters in diameter. Complicating the issue is that in an age-matched control population (CT obtained for trauma), 3% of children had an appendicolith. Thus in a clinical setting of suspected acute appendicitis, detection of an appendicolith is highly suggestive but not pathognomonic of appendicitis, a conclusion already established decades ago in the pre-CT era. Exceptions do occur and not all right lower quadrant calcifications are associated with appendicitis. Among other etiologies, a fecalith or ureterolith can mimic an appendicolith.

More attention needs to be placed on identifying an appendicolith than the literature suggests. A retained postappendectomy appendicolith predisposes to an abscess. Thus any preoperative imaging detection of an appendicolith should be communicated to the surgeon.

Other foreign bodies occasionally act as a nidus for appendicitis. In certain cultures lead shot appendicitis is a known entity (Fig. 6.3).

Appendiceal Gas

The earlier literature provides conflicting views about appendiceal gas; some authors suggested that gas is a normal finding; others thought gas to be a sign of appendicitis. In reality, intraluminal gas is found in both clinical settings. Computed tomography detects gas in slightly

Figure 6.3. Lead shot appendicitis. (Courtesy of Klas Mare, M.D., Göteborg, Sweden.)

over half of normal appendices; when present, it appeared as small bubbles or a tubular collection. An occasional individual has a gas–fluid level in a normal appendix. Similar findings, albeit less often, are found in patients with appendicitis (gas outside the lumen, on the other hand, is always abnormal). Ultrasound detects intraluminal gas in a majority of normal individuals and in about 15% of patients with acute appendicitis (9). Contrary to some statements in the literature, in any one patient a finding of intraluminal gas is of little diagnostic value. Gas in an appendicolith per se is of little significance. The presence of intramural or extraserosal gas generally implies a perforation.

Comparison Studies

A comparison of CT and US in suspected appendicitis is difficult to place in proper perspective. Ultrasound is rapid and requires no patient preparation, but is very operator dependent. Yet in most comparison studies CT achieves a slightly greater sensitivity and specificity than US, keeping in mind the added complexity of contrast administration (discussed below). The lack of periappendiceal fat in pediatric patients adds another variable to study comparisons, and results appear to be

ADVANCED IMAGING OF THE ABDOMEN

influenced by whether one is dealing with adults or children. Most comparison studies do not distinguish “early” or “perforated” appendicitis patients, yet disease status does influence study results. Thus US sensitivity and specificity in detecting appendicitis decrease after a perforation occurs, presumably because a decrease in appendiceal diameter makes it difficult to locate in the surrounding inflammation, and in this clinical setting CT appears to be preferred.

Unenhanced focused CT in children achieved a 97% sensitivity and 100% specificity, whereas a historical cohort of children with graded compression US reached a sensitivity of 100% and specificity of 88% (10); even with such a focused CT study, an alternate diagnosis was suggested by CT in 35% and by US in 28% of children without appendicitis.

Helical CT achieved a higher sensitivity (95% versus 78%) than graded compression US for detecting appendicitis in 386 children, adolescents, and young adults, with both techniques having similar specificity (11); of interest is that for discordant CT and US findings among these patients, CT was correct 85% of the time. In a prospective study of consecutive patients with clinical acute appendicitis, unenhanced focused single-detector helical CT and graded compression US performed in a Netherlands community teaching hospital by both body imaging radiologists and general radiology staff achieved similar accuracy for diagnosing acute appendicitis with both modalities (12): sensitivity of CT and US was 76% and 79%, respectively, and specificity was 83% and 78%.

In patients with clinically suspected acute appendicitis, unenhanced focused appendiceal CT, abdominopelvic CT, and focused appendiceal CT using colonic contrast achieved sensitivities of 78%, 72%, and 80% and specificities of 86%, 91%, and 87%, respectively (13); US without and with colonic contrast had low sensitivity (33% to 35%) but high specificity (85% to 89%), and all three CT techniques were superior to US. Another view from this study is that abdominopelvic CT provided the greatest diagnostic confidence for patients with negative findings, but focused appendiceal CT with colonic contrast provided the most confidence for patients with positive findings (13).

Finally, radiologists’ diagnostic confidence appears greater with CT. Thus in children and

APPENDIX

young adults with equivocal clinical findings for appendicitis, US was interpreted with a high or very high confidence level in only 58% of patients (14); CT was performed if US results were equivocal or appendix was not visualized (these are thus more atypical patients), yet CT was interpreted with a high or very high confidence in 92% of patients.

Conventional Radiographs

Although previously commonly obtained, conventional abdominal radiographs have been supplanted by other imaging tests. Likewise, a barium enema, although relatively accurate in detecting acute appendicitis, is rarely performed except in an older patient in whom a cecal carcinoma is suspected.

Elderly radiologists are familiar with barium retention in a normal appendix for prolonged time; if the patient develops appendicitis some time after a barium study, any residual appendiceal contrast should be placed in the proper perspective; it was situations such as this that gave rise to the surgical myth of bariuminduced appendicitis.

The presence of focal adynamic ileus and mechanical ileus generally represents sequelae of a phlegmon or focal peritonitis (Fig. 6.4).

Figure 6.4. Appendicitis presenting both with adynamic ileus and small bowel obstruction.

Computed Tomography

A considerable, but often contradictory, literature discusses CT in a setting of suspected appendicitis. One should keep in mind that a number of variables affect CT study results:

1.Studies are different in pediatric patients than in adults. Children have less fat, an important component when identifying the appendix, and periappendiceal inflammation is thus less readily detected. Published CT accuracies in evaluating appendicitis are best approached by distinguishing children from adults. In younger children sedation is also an issue.

2.Should oral, rectal, or no contrast be used? Contrast does aid diagnosis, but of necessity the use of oral contrast introduces a delay in diagnosis that in certain situations is not acceptable to surgeons. Rectal contrast, administered blindly, introduces additional complexity to these studies.

3.Does intravenous contrast aid in detecting inflamed tissue or is its use superfluous in a setting of suspected appendicitis?

4.Should an abdominopelvic screening study be performed or is a study focused on the appendix using thin collimation preferred? The answer to this question is often based on whether one is dealing with a child or an adult and on overall clinical suspicion.

Evidence suggests that with unenhanced multislice CT the radiation dose can be lowered considerably without loss of diagnostic information when evaluating for appendicitis (15). Most studies agree that helical multislice CT using close collimation improves abnormality detection. In treating children, the trend is toward CT focused on the appendix rather than a general abdominopelvic scan if clinical suspicion suggests appendicitis. In adults, in whom alternate diagnoses are more common, this trend is less evident; an abdominopelvic CT scan (nonfocused scan) is more often the initial study for appendicitis, and a focused appendiceal CT study, using colonic contrast, is reserved for unusual situations. In a small minority of patients clinically suspected of having appendicitis, abnormalities are found outside the pelvis (16). The differential diagnosis often includes gynecologic abnormalities, Crohn’s

disease, Yersinia enterocolitica infection, lymphoma, and, in adults, cecal diverticulitis and cancer.

Although intuitively obvious, one of the reasons for missing appendicitis with a focused study is if the inflamed appendix is not included in the imaged field of view. Scanning in a decubitus position is useful in some patients. In patients with clinically suspected acute appendicitis, 5-mm collimated sections through the lower abdomen and upper pelvis achieved a sensitivity of 99% and specificity of 98%, whereas 10-mm collimated sections from the diaphragm to the pubic symphysis reached a sensitivity of 82% and specificity of 95% (17); even narrower collimation is to be expected with the introduction of multislice units. In general, CT using 5-mm collimation but without oral, intravenous, or rectal contrast in adults with suspected acute appendicitis yields about 95% sensitivity and almost 100% specificity in diagnosing acute appendicitis. Even in children, focused CT achieves about 95% sensitivity and specificity (18,19).

Does intravenous, oral, and rectal contrast aid CT interpretation in suspected appendicitis? Even on such a simple topic published opinions vary. The sensitivity of detecting an inflamed appendix improves significantly with the use of intravenous (IV) contrast (20,21). The use of oral and IV contrast in children with suspected appendicitis achieved a 97% sensitivity and 93% specificity (22). Some protocols include both oral and rectal contrast. A disadvantage of oral contrast is the time involved before the contrast outlines the ileocecal region, a delay frowned upon by surgeons. A contrast enema sufficient to opacify the cecum aids in defining adjacent structures and cecal wall thickening. A contrast enema is more helpful in children, who have little abdominal fat, than in adults.

The typical criteria for a CT diagnosis of appendicitis include appendiceal wall thickening (>1mm) or an abscess, phlegmon, or pericecal inflammation associated with appendicolith(s).An isolated appendicolith or isolated periappendiceal inflammation are secondary findings and in the absence of a thickened appendix should not be used to diagnose appendicitis. On the other hand, if the entire length of the appendix is visualized and very little or no lumen fluid is evident, no appendicolith is present, and periappendiceal fat appears

ADVANCED IMAGING OF THE ABDOMEN

normal, then appendicitis is excluded from the differential. Also, appendiceal nonvisualization on a technically correct CT scan should exclude appendicitis, keeping in mind that in some patients a phlegmon or abscess,presumably secondary to progression of inflammation, will render an appendix not visible even on narrow CT windows.

The cecal bar sign consists of a curved soft tissue bar separating an appendicolith from adjacent cecal contrast. Occasionally cecal contrast assumes a funnel-shaped outline, surrounded by thickened cecal wall and pointing toward the appendix, termed the arrowhead sign.

In any one patient the diagnosis is not always straightforward. Individually, many described signs are not pathognomonic of appendicitis and are also seen with other disorders.A normal appendix, identified by helical CT in its entire length, excludes appendicitis. An unopacified appendix >6mm in diameter is strong presumptive evidence of appendicitis. Fat-stranding is a very sensitive but nonspecific finding in appendicitis.

Ileocolic lymph nodes tend to enlarge in appendicitis, but published detection sensitivities vary in different studies. In either case, adenopathy is found in a number of other diseases and cannot be relied on to suggest appendicitis.

Any small bowel adjacent to the appendix eventually develops a thickened wall due to inflammation or edema. Some phlegmons or abscesses are small and lie adjacent to the appendix; an occasional one extends into the pelvis and fills the pouch of Douglas. Adjacent vascular involvement ranges from mild engorgement to thrombosis.

Computed tomography in nonpregnant women clinically suspected of having either appendicitis or an acute gynecologic condition achieved 100% sensitivity and 97% specificity for diagnosing appendicitis and 87% sensitivity and 100% specificity for acute gynecologic conditions (23); CT is able to differentiate appendicitis and acute gynecologic conditions in the vast majority of women and influences both surgical and medical management in a number of them.

In summary, the use of CT, narrow collimation, and rectal contrast to opacify the cecum increases diagnostic confidence in a setting

APPENDIX

of suspected appendicitis, especially in children. Intravenous contrast appears to be less important.

Ultrasonography

Ultrasonography in a clinical setting of suspected appendicitis has achieved mixed results, partly due to operator dependent factors. For example, appendiceal US studies performed by unsupervised technicians at night achieved a sensitivity of 26% compared with a sensitivity of 61% when studies were performed during the day by supervised technicians (24).

Some European hospitals have a policy to perform imaging, generally US, in all patients with a clinical suspicion of appendicitis (25). A further refinement is to add CT if US is equivocal. In general, US sensitivity and specificity are superior to those obtained with a clinical diagnosis only. Ultrasound allows a diagnosis of acute appendicitis in more patients more often and more quickly than is possible with clinical evaluation alone. Patients with true negative studies generally have other intestinal, mesenteric, gallbladder, or ovarian abnormalities. A number of studies show an overall sensitivity of about 85% for an US diagnosis of appendicitis. Nevertheless, to dampen some of the generally enthusiastic findings, a Danish prospective, double-blind diagnostic US study achieved poor results, reaching a sensitivity of only 49% and specificity of 88% (26).A high false negative rate US should not be used to exclude appendicitis in patients with a classic clinical presentation. Likewise, with a low clinical probability of appendicitis US is not recommended for screening because of the high false positive rate.

Visualization of a normal appendix in its entire length during US-graded compression is infrequent but when seen excludes appendicitis. A retrocecal appendix can be difficult to visualize. At times appendicitis is excluded by showing an alternative condition to account for the patient’s symptomatology. It should be kept in mind, however, that appendicitis does occur synchronously with another disease having similar symptoms.

The most accurate US finding of acute appendicitis is presence of a noncompressible blindending tubular multilayered appendix having a maximal diameter >6mm, a finding having a sensitivity and, in some studies, specificity in

the high 90% (27). Others find a much lower specificity, primarily due to an appendiceal diameter of 6mm or more in controls (28). One of the causes of an appendiceal diameter >6mm in non-appendicitis patients is lymphoid hyperplasia, regardless of etiology. Other causes include excessive intraluminal content.

An appendiceal wall thickness >3mm is generally found in acute appendicitis. The loss of appendiceal wall definition suggests ischemia or gangrene. An appendicolith appears as a bright hyperechoic structure with distal shadowing. Enlarged lymph nodes are common. Visualization of the inflamed appendix is generally accepted proof that appendicitis is present. Yet it should be realized that the appendix is also inflamed by adjacent disorders such as Crohn’s disease, a tubo-ovarian abscess, colon carcinoma, and certain infections, and at times these mimic the sonographic appearance of appendicitis.

At times endovaginal US detects an appendix located in the pelvis when suprapubic US cannot locate it.

Doppler US reveals increased blood flow in the wall of an inflamed appendix. Appendiceal hyperemia is common in children with a nonperforating appendicitis but often resolves with a perforating or gangrenous appendix.

Magnetic Resonance Imaging

Magnetic resonance imaging is not commonly employed for suspected appendicitis, yet it has considerable advantages. It appears highly accurate in detecting acute appendicitis. In children with acute appendicitis, T2-weighted ultra–turbo spin echo (TSE) images revealed a markedly hyperintense region and a slightly hyperintense thickened wall, surrounded by a markedly hyperintense periappendiceal zone (29); unenhanced axial T2-weighted spin-echo imaging was the most sensitive sequence.

Peri-appendiceal inflammation appears on T1-weighted images as hypointense stranding within the hyperintense fat. Inflammatory tissue, including abscess wall, enhances postcontrast.

Scintigraphy

Scintigraphy is not commonly obtained in suspected appendicitis. Technetium-99m