DOI: 10.1148/rg.272055098
Necrotizing Enterocolitis: Review of State-of-the-Art Imaging Findings with Pathologic Correlation1
Monica Epelman, MD2,
Alan Daneman, MD,
Oscar M. Navarro, MD,
Iris Morag, MD,
Aideen M. Moore, MD,
Jae Hong Kim, MD,
Ricardo Faingold, MD3,
Glenn Taylor, MD and
J. Ted Gerstle, MD
1 From the Department of Diagnostic Imaging (M.E., A.D., O.M.N., R.F.) and Divisions of Neonatology (I.M., A.M.M., J.H.K.), Pathology (G.T.), and General Surgery (J.T.G.), Hospital for Sick Children and University of Toronto, 555 University Ave, Toronto, Ontario, Canada M5G 1X8. Recipient of a Certificate of Merit award and an Excellence in Design award for an education exhibit at the 2004 RSNA Annual Meeting. Received April 19, 2005; revision requested June 21; final revision received August 10, 2006; accepted August 23. J.H.K. is a consultant for Abbott Nutrition, Abbott Park, Ill; all other authors have no financial relationships to disclose.
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Figure 1a. Pathologic findings in NEC. (a) Histologic section of small bowel (original magnification, x100; hematoxylin-eosin stain). Intramural gas is seen as rounded bubbles in the submucosa (arrows). There is hyperemia of the serosa. (b) Histologic section of small bowel (original magnification, x100; hematoxylin-eosin stain). The bowel is affected much more severely than in a. There is necrosis of the mucosa, submucosa, and muscularis with intraluminal necrotic debris on the mucosal side of the bowel wall (m). Only the serosa appears intact. s = serosal surface of bowel wall. (c) Postmortem photograph of bowel involved with severe NEC. The arrows indicate areas of the bowel wall where there has been so much necrosis and sloughing of the mucosa, submucosa, and muscularis that only the serosa is intact. One can see through these areas of the serosa into the lumen of the bowel.
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Figure 1b. Pathologic findings in NEC. (a) Histologic section of small bowel (original magnification, x100; hematoxylin-eosin stain). Intramural gas is seen as rounded bubbles in the submucosa (arrows). There is hyperemia of the serosa. (b) Histologic section of small bowel (original magnification, x100; hematoxylin-eosin stain). The bowel is affected much more severely than in a. There is necrosis of the mucosa, submucosa, and muscularis with intraluminal necrotic debris on the mucosal side of the bowel wall (m). Only the serosa appears intact. s = serosal surface of bowel wall. (c) Postmortem photograph of bowel involved with severe NEC. The arrows indicate areas of the bowel wall where there has been so much necrosis and sloughing of the mucosa, submucosa, and muscularis that only the serosa is intact. One can see through these areas of the serosa into the lumen of the bowel.
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Figure 1c. Pathologic findings in NEC. (a) Histologic section of small bowel (original magnification, x100; hematoxylin-eosin stain). Intramural gas is seen as rounded bubbles in the submucosa (arrows). There is hyperemia of the serosa. (b) Histologic section of small bowel (original magnification, x100; hematoxylin-eosin stain). The bowel is affected much more severely than in a. There is necrosis of the mucosa, submucosa, and muscularis with intraluminal necrotic debris on the mucosal side of the bowel wall (m). Only the serosa appears intact. s = serosal surface of bowel wall. (c) Postmortem photograph of bowel involved with severe NEC. The arrows indicate areas of the bowel wall where there has been so much necrosis and sloughing of the mucosa, submucosa, and muscularis that only the serosa is intact. One can see through these areas of the serosa into the lumen of the bowel.
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Figure 2a. Supine (a) and cross-table lateral (b) radiographs of the abdomen obtained in a neonate with NEC. The bowel is mildly dilated with gas, mainly on the left side. The bubbly pattern of gas seen mainly in the right lower quadrant represents intramural gas. Free intraperitoneal gas is present anteriorly (arrows in b).
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Figure 2b. Supine (a) and cross-table lateral (b) radiographs of the abdomen obtained in a neonate with NEC. The bowel is mildly dilated with gas, mainly on the left side. The bubbly pattern of gas seen mainly in the right lower quadrant represents intramural gas. Free intraperitoneal gas is present anteriorly (arrows in b).
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Figure 3a. Sonographic appearances of normal bowel in a neonate. (a) Gray-scale image shows the typical appearance of collapsed bowel. The echogenic linear markings represent the mucosal interface with the lumen, and the surrounding hypoechoic bands represent the muscularis of the bowel wall. (b) Color Doppler image of normal collapsed bowel shows color dots of flow in the arteries of the bowel wall.
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Figure 3b. Sonographic appearances of normal bowel in a neonate. (a) Gray-scale image shows the typical appearance of collapsed bowel. The echogenic linear markings represent the mucosal interface with the lumen, and the surrounding hypoechoic bands represent the muscularis of the bowel wall. (b) Color Doppler image of normal collapsed bowel shows color dots of flow in the arteries of the bowel wall.
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Figure 4. Supine radiograph of the abdomen of a normal neonate shows a normal bowel gas pattern. Gas is distributed throughout the small and large bowel, and it is difficult to differentiate the small bowel from the large bowel. Each loop causes impressions on adjacent loops, giving each loop a multifaceted appearance; the overall pattern resembles that of a mosaic. The loops are generally not rounded or elongated.
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Figure 5a. Supine abdominal radiographs obtained at initial presentation in two neonates with NEC show abnormal bowel gas patterns with loss of the normal mosaic pattern seen in Figure 4 and absence of convincing evidence of intramural gas. (a) The bowel loops are dilated with gas and have assumed a more rounded and elongated appearance. There appears to be a small amount of portal venous gas. (b) The bowel loops have assumed a more nonspecific appearance, which may reflect bowel wall thickening or an accumulation of fluid within the lumen or in the peritoneal cavity.
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Figure 5b. Supine abdominal radiographs obtained at initial presentation in two neonates with NEC show abnormal bowel gas patterns with loss of the normal mosaic pattern seen in Figure 4 and absence of convincing evidence of intramural gas. (a) The bowel loops are dilated with gas and have assumed a more rounded and elongated appearance. There appears to be a small amount of portal venous gas. (b) The bowel loops have assumed a more nonspecific appearance, which may reflect bowel wall thickening or an accumulation of fluid within the lumen or in the peritoneal cavity.
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Figure 6a. Serial radiographs of a neonate who developed severe NEC on the 14th day of life. (a) Radiograph obtained at presentation shows generalized bowel dilatation with gas, intramural gas mainly in the large bowel, and portal venous gas. The infant’s condition deteriorated clinically, and the parents refused surgery. (b, c) Follow-up images obtained at 8-hour intervals show gradual disappearance of intramural gas and portal venous gas. This finding demonstrates that disappearance of intramural gas and portal venous gas is not a good indicator of clinical progress. The images also show a decrease in the amount of intraluminal gas, which persists as fixed, dilated loops. This appearance is an ominous sign, suggesting the presence of necrotic loops. At no time was there evidence of free intraperitoneal gas.
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Figure 6b. Serial radiographs of a neonate who developed severe NEC on the 14th day of life. (a) Radiograph obtained at presentation shows generalized bowel dilatation with gas, intramural gas mainly in the large bowel, and portal venous gas. The infant’s condition deteriorated clinically, and the parents refused surgery. (b, c) Follow-up images obtained at 8-hour intervals show gradual disappearance of intramural gas and portal venous gas. This finding demonstrates that disappearance of intramural gas and portal venous gas is not a good indicator of clinical progress. The images also show a decrease in the amount of intraluminal gas, which persists as fixed, dilated loops. This appearance is an ominous sign, suggesting the presence of necrotic loops. At no time was there evidence of free intraperitoneal gas.
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Figure 6c. Serial radiographs of a neonate who developed severe NEC on the 14th day of life. (a) Radiograph obtained at presentation shows generalized bowel dilatation with gas, intramural gas mainly in the large bowel, and portal venous gas. The infant’s condition deteriorated clinically, and the parents refused surgery. (b, c) Follow-up images obtained at 8-hour intervals show gradual disappearance of intramural gas and portal venous gas. This finding demonstrates that disappearance of intramural gas and portal venous gas is not a good indicator of clinical progress. The images also show a decrease in the amount of intraluminal gas, which persists as fixed, dilated loops. This appearance is an ominous sign, suggesting the presence of necrotic loops. At no time was there evidence of free intraperitoneal gas.
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Figure 7a. Abdominal radiographs show bowel loops that exhibit the linear pattern of intramural gas. (a) There is a band of radiolucency or black band (black arrow) and a band of soft-tissue opacity or white band (white arrow) along the margin of the involved loop. The black band represents the intramural gas in the serosal layer of the bowel wall, and the white band represents the submucosa and mucosa, which is contrasted on one side by the intramural gas and on the other side by the gas in the lumen. (b) There is extensive intramural gas (arrows) involving almost all of the intestine as well as the stomach in the left upper quadrant. In this patient, the intramural gas is seen as multiple curvilinear black bands, but there are no white bands because the lumen is filled with fluid and there is no gas in the lumen to contrast with the inner aspect of the mucosa.
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Figure 7b. Abdominal radiographs show bowel loops that exhibit the linear pattern of intramural gas. (a) There is a band of radiolucency or black band (black arrow) and a band of soft-tissue opacity or white band (white arrow) along the margin of the involved loop. The black band represents the intramural gas in the serosal layer of the bowel wall, and the white band represents the submucosa and mucosa, which is contrasted on one side by the intramural gas and on the other side by the gas in the lumen. (b) There is extensive intramural gas (arrows) involving almost all of the intestine as well as the stomach in the left upper quadrant. In this patient, the intramural gas is seen as multiple curvilinear black bands, but there are no white bands because the lumen is filled with fluid and there is no gas in the lumen to contrast with the inner aspect of the mucosa.
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Figure 8. Supine abdominal radiograph of a neonate with clinically very mild or benign NEC. The intraluminal gas present is limited almost entirely to the large bowel, and there is no significant dilatation. There is extensive intramural gas involving the entire large bowel despite the benign clinical course. There are well-developed black and white bands (described in Fig 7) (arrows), which are seen very well in the descending colon and even in the rectum.
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Figure 9. Cross-table lateral abdominal radiograph of a neonate with clinically severe NEC. There is marked dilatation of bowel with gas and fluid and extensive intramural gas (arrows). The intramural gas is seen as multiple curvilinear black bands, and in some areas white bands are also visible. The bowel loops that are markedly distended with fluid are delineated by the black bands of the intramural gas outlining the dependent part of the wall of the loops (black arrows). The white arrow and arrowhead indicate a loop anteriorly that is filled with fluid and gas and thus has an air-fluid level. The intramural gas appears as a black band (arrowhead) in the dependent part of the loop, which contains fluid. In the anterior part of the loop, which contains the intraluminal gas, both a black band and a white band are visible (white arrow).
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Figure 10. Supine abdominal radiograph of a neonate with NEC shows dilatation of the bowel with gas and extensive intramural gas, which is seen as a bubbly pattern of radiolucency mainly in the right lower quadrant. This bubbly pattern is due to submucosal intramural gas.
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Figure 11a. Sonograms of the bowel wall in NEC show the appearances of varying amounts of intramural gas. (a) The bowel wall is thickened and has a layered appearance. One hyperechoic focus within the wall (close to the cursors) is thought to represent intramural gas. (b) Multiple bowel loops are seen surrounded by some free fluid. There is extensive intramural gas involving all of these loops. The intramural gas has a typical hyperechoic, granular pattern in the bowel wall with posterior reverberation artifacts. With this amount of intramural gas, it is difficult to assess the thickness of the bowel wall.
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Figure 11b. Sonograms of the bowel wall in NEC show the appearances of varying amounts of intramural gas. (a) The bowel wall is thickened and has a layered appearance. One hyperechoic focus within the wall (close to the cursors) is thought to represent intramural gas. (b) Multiple bowel loops are seen surrounded by some free fluid. There is extensive intramural gas involving all of these loops. The intramural gas has a typical hyperechoic, granular pattern in the bowel wall with posterior reverberation artifacts. With this amount of intramural gas, it is difficult to assess the thickness of the bowel wall.
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Figure 12. Sonogram of a bowel loop shows differentiation of intraluminal gas from intramural gas. The intraluminal gas (L) is surrounded by a thickened bowel wall. Within the bowel wall are multiple hyperechoic foci (arrows), which represent intramural gas. In this example, the posterior artifact caused by gas is seen only with the larger volume of the intraluminal gas. The bubbles of intramural gas in this patient are too small to cause the posterior artifact, in contrast to the appearance seen in Figure 11b.
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Figure 13. Sonogram shows a bowel loop with a large amount of intramural gas (arrows) in the more dependent and vertically oriented parts of the loop. This gives the bowel wall a typical granular appearance and causes a posterior artifact.
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Figure 14. Differentiation of intraluminal gas from intramural gas in the nondependent part of a bowel loop. Sonogram shows bowel loops with marked abnormality of the echogenicity of the wall and loss of the hypoechoic halo of the muscularis. In one loop, two hyperechoic foci are noted within the wall (arrows), which represent intramural gas. In the other loop, there is intraluminal gas (arrowhead) floating on the intraluminal fluid and causing a posterior artifact.
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Figure 15. Bowel wall calcification in a neonate with congenital bowel obstruction and no evidence of NEC. Sonogram of the left hemiabdomen shows two bowel loops (arrows) with thickened walls, which have granular increased echogenicity. Note the marked posterior acoustic shadow caused by the abnormal wall echogenicity. These findings correspond to bowel wall calcification, which was seen in the left hemiabdomen at plain abdominal radiography and confirmed at surgery. Bowel wall calcification can mimic intramural gas; however, such calcification may cause a clear acoustic shadow, whereas extensive intramural gas tends to cause a posterior reverberation artifact, as seen in Figures 11b and 13.
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Figure 16a. Radiographic and sonographic appearances of portal venous gas. (a) Supine abdominal radiograph shows a small bubble of portal venous gas projected over the liver (arrow). (b) Cross-table lateral radiograph obtained immediately after a shows that portal venous gas (arrow) may be depicted more extensively in the liver on this view. (c) Sonogram of another patient shows extensive portal venous gas, which is depicted as punctate and linear, branching areas of echogenicity in the portal branches within the liver.
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Figure 16b. Radiographic and sonographic appearances of portal venous gas. (a) Supine abdominal radiograph shows a small bubble of portal venous gas projected over the liver (arrow). (b) Cross-table lateral radiograph obtained immediately after a shows that portal venous gas (arrow) may be depicted more extensively in the liver on this view. (c) Sonogram of another patient shows extensive portal venous gas, which is depicted as punctate and linear, branching areas of echogenicity in the portal branches within the liver.
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Figure 16c. Radiographic and sonographic appearances of portal venous gas. (a) Supine abdominal radiograph shows a small bubble of portal venous gas projected over the liver (arrow). (b) Cross-table lateral radiograph obtained immediately after a shows that portal venous gas (arrow) may be depicted more extensively in the liver on this view. (c) Sonogram of another patient shows extensive portal venous gas, which is depicted as punctate and linear, branching areas of echogenicity in the portal branches within the liver.
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Figure 17a. Cross-table lateral abdominal radiographs show free intraperitoneal gas. (a) There is a small amount of free intraperitoneal gas anterior to the liver (arrows). (b) There are even smaller amounts of free intraperitoneal gas anteriorly, which are seen as subtle triangular areas of lucency between the anterior abdominal wall and loops of bowel (arrows) and as a linear area of lucency anterior to the stomach (arrowhead).
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Figure 17b. Cross-table lateral abdominal radiographs show free intraperitoneal gas. (a) There is a small amount of free intraperitoneal gas anterior to the liver (arrows). (b) There are even smaller amounts of free intraperitoneal gas anteriorly, which are seen as subtle triangular areas of lucency between the anterior abdominal wall and loops of bowel (arrows) and as a linear area of lucency anterior to the stomach (arrowhead).
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Figure 18a. Sonograms show the spectrum of appearances of free intraperitoneal gas. (a) A large amount of free intraperitoneal gas appears as a long, linear area of echogenicity (arrow) just deep to the abdominal wall and causes a posterior reverberation artifact. This finding may be difficult to differentiate from the appearance caused by several bowel loops filled with gas. (b) Smaller amounts of free intraperitoneal gas are depicted between the anterior abdominal wall and the left lobe of the liver as echogenic foci (arrow). These foci may vary in size, and the amount of gas present will determine whether there is a posterior artifact. The space between the liver and abdominal wall is ideal to evaluate for free gas, as bowel is seldom present in this location. (c) Bowel loops are seen with free fluid between them and the abdominal wall. During real-time imaging, echogenic foci (arrow) could be seen bubbling out of one of the loops; the echoes rose in the free fluid until they were just deep to the abdominal wall. The echogenic foci represent free gas bubbling out through an area of perforation.
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Figure 18b. Sonograms show the spectrum of appearances of free intraperitoneal gas. (a) A large amount of free intraperitoneal gas appears as a long, linear area of echogenicity (arrow) just deep to the abdominal wall and causes a posterior reverberation artifact. This finding may be difficult to differentiate from the appearance caused by several bowel loops filled with gas. (b) Smaller amounts of free intraperitoneal gas are depicted between the anterior abdominal wall and the left lobe of the liver as echogenic foci (arrow). These foci may vary in size, and the amount of gas present will determine whether there is a posterior artifact. The space between the liver and abdominal wall is ideal to evaluate for free gas, as bowel is seldom present in this location. (c) Bowel loops are seen with free fluid between them and the abdominal wall. During real-time imaging, echogenic foci (arrow) could be seen bubbling out of one of the loops; the echoes rose in the free fluid until they were just deep to the abdominal wall. The echogenic foci represent free gas bubbling out through an area of perforation.
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Figure 18c. Sonograms show the spectrum of appearances of free intraperitoneal gas. (a) A large amount of free intraperitoneal gas appears as a long, linear area of echogenicity (arrow) just deep to the abdominal wall and causes a posterior reverberation artifact. This finding may be difficult to differentiate from the appearance caused by several bowel loops filled with gas. (b) Smaller amounts of free intraperitoneal gas are depicted between the anterior abdominal wall and the left lobe of the liver as echogenic foci (arrow). These foci may vary in size, and the amount of gas present will determine whether there is a posterior artifact. The space between the liver and abdominal wall is ideal to evaluate for free gas, as bowel is seldom present in this location. (c) Bowel loops are seen with free fluid between them and the abdominal wall. During real-time imaging, echogenic foci (arrow) could be seen bubbling out of one of the loops; the echoes rose in the free fluid until they were just deep to the abdominal wall. The echogenic foci represent free gas bubbling out through an area of perforation.
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Figure 19. Free intraperitoneal fluid in a neonate with NEC and perforation. Sonogram shows echogenic fluid between the abdominal wall (A) and liver (L). The free fluid contains a large amount of echogenic debris, which is more suggestive of perforation than is free fluid with no echogenic debris. No free intraperitoneal gas was seen at plain abdominal radiography or sonography.
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Figure 20. Focal fluid collection and abscess formation in NEC after perforation. Sonogram shows a fluid collection with septa adjacent to a bowel loop (arrows). Note the high echogenicity of some of the valvulae in the bowel loop. Sonography is helpful to determine the site of such collections. However, samples of the fluid may be required to determine whether infection or pus is present.
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Figure 21a. Sonograms show part of the spectrum of appearances of the abnormalities of the bowel wall that may be seen in NEC. (a) The bowel wall is thickened and has relatively homogeneous echogenicity, as the hypoechoic halo of the muscularis and the echogenicity of the mucosa cannot be differentiated. (b) The bowel wall is thickened and has a somewhat layered appearance (arrow). The echogenicity of the bowel wall is similar to that of the bowel content. (c) The bowel loops are distended by fluid, and the wall appears somewhat more echogenic than normal (arrows).
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Figure 21b. Sonograms show part of the spectrum of appearances of the abnormalities of the bowel wall that may be seen in NEC. (a) The bowel wall is thickened and has relatively homogeneous echogenicity, as the hypoechoic halo of the muscularis and the echogenicity of the mucosa cannot be differentiated. (b) The bowel wall is thickened and has a somewhat layered appearance (arrow). The echogenicity of the bowel wall is similar to that of the bowel content. (c) The bowel loops are distended by fluid, and the wall appears somewhat more echogenic than normal (arrows).
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Figure 21c. Sonograms show part of the spectrum of appearances of the abnormalities of the bowel wall that may be seen in NEC. (a) The bowel wall is thickened and has relatively homogeneous echogenicity, as the hypoechoic halo of the muscularis and the echogenicity of the mucosa cannot be differentiated. (b) The bowel wall is thickened and has a somewhat layered appearance (arrow). The echogenicity of the bowel wall is similar to that of the bowel content. (c) The bowel loops are distended by fluid, and the wall appears somewhat more echogenic than normal (arrows).
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Figure 22a. Sonograms show part of the spectrum of appearances of the abnormalities of the bowel wall that may be seen in NEC. (a) Gray-scale image shows abnormal small bowel in a neonate with severe NEC. The valvulae are extremely well defined and are particularly hyperechoic. The appearance has a herringbone or zebra pattern. (b) Color Doppler image shows marked hyperemic flow in the valvulae, which is depicted as multiple parallel, linear vessels. This finding gives the bowel a zebra pattern in color.
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Figure 22b. Sonograms show part of the spectrum of appearances of the abnormalities of the bowel wall that may be seen in NEC. (a) Gray-scale image shows abnormal small bowel in a neonate with severe NEC. The valvulae are extremely well defined and are particularly hyperechoic. The appearance has a herringbone or zebra pattern. (b) Color Doppler image shows marked hyperemic flow in the valvulae, which is depicted as multiple parallel, linear vessels. This finding gives the bowel a zebra pattern in color.
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Figure 23. Thinning of the bowel wall in NEC. Sonogram shows a bowel loop filled with content of mixed echogenicity and surrounded by clear free fluid. The bowel wall is of various thicknesses but is particularly focally thinned at one site (arrow).
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Figure 24a. Color Doppler sonograms show hyperemic bowel loops in NEC. Each loop appears thickened and has lost the normal bowel wall echogenicity. (a) There is a "Y" pattern of flow outlining the mesenteric and subserosal vessels of the bowel loop (arrow). (b) There is a "ring" pattern of flow with flow seen around the entire circumference of the bowel loop (arrow). The mesenteric vessels are also prominent, and there is flow around most of the adjacent loop. These hyperemic patterns of flow are not seen in normal neonates, and in neonates with NEC these patterns indicate viable loops.
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Figure 24b. Color Doppler sonograms show hyperemic bowel loops in NEC. Each loop appears thickened and has lost the normal bowel wall echogenicity. (a) There is a "Y" pattern of flow outlining the mesenteric and subserosal vessels of the bowel loop (arrow). (b) There is a "ring" pattern of flow with flow seen around the entire circumference of the bowel loop (arrow). The mesenteric vessels are also prominent, and there is flow around most of the adjacent loop. These hyperemic patterns of flow are not seen in normal neonates, and in neonates with NEC these patterns indicate viable loops.
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Figure 25a. NEC and bowel necrosis in a neonate. (a) Color Doppler sonogram shows three bowel loops (arrows). The two outer loops (long arrows) are markedly hyperemic, with flow around the entire circumference of each loop and some prominent mesenteric vessels. The central loop has an irregular wall with focal thinning at one site (short arrow) and small hyperechoic foci suggestive of intramural gas. There is no flow in the wall, a finding highly suggestive of necrosis. (b) Intraoperative photograph of the bowel shows the necrotic bowel loop (arrow).
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Figure 25b. NEC and bowel necrosis in a neonate. (a) Color Doppler sonogram shows three bowel loops (arrows). The two outer loops (long arrows) are markedly hyperemic, with flow around the entire circumference of each loop and some prominent mesenteric vessels. The central loop has an irregular wall with focal thinning at one site (short arrow) and small hyperechoic foci suggestive of intramural gas. There is no flow in the wall, a finding highly suggestive of necrosis. (b) Intraoperative photograph of the bowel shows the necrotic bowel loop (arrow).
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Figure 26. Summary of the authors’ concept of the sequence of changes in bowel wall thickness and perfusion in NEC. The sequence is depicted with simplified diagrams of a transverse section of a bowel loop (top) and color Doppler sonograms (bottom). (a) There is normal flow to normal bowel. The diagram shows normal bowel wall thickness and perfusion. (b) The changes of NEC are shown with bowel wall thickening and hyperemia. (c) The bowel wall thickening persists, but the perfusion has diminished. (d) As the process progresses in more severely affected neonates, the mucosa starts to slough and the bowel wall becomes much thinner, although some perfusion persists. (e) Sloughing continues, the bowel wall becomes asymmetrically thinned, and blood flow ceases. The authors believe that progression from the phase of bowel wall thickening and hyperemia (b) to bowel wall thinning and absent perfusion (e) may take a variable time in different patients. However, it may be an extremely rapid process, and the latter findings may indeed be present on abdominal sonograms at presentation.
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Copyright © 2007 by the Radiological Society of North America.
