Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

in some patients several techniques are necessary to adequately visualize the entire colonic wall. A typical practice is to use primarily 2D multiplanar reformatted imaging for analysis and reserve 3D imaging for specific problems; endoluminal views appear necessary to differentiate polyps from folds. Several commercial 3D endoluminal volume rendering and navigational systems are available and further improvements are to be expected. Adding color (translucency rendering) to 3D aids in ruling out false polyps (13). Interpretation of 2D images is faster; on the other hand, navigation with 3D endoluminal imaging mimics the conventional colonoscopic appearance (Fig. 5.1). A panoramic view perpendicular to the centerline is also feasible, with sequential panoramic video views scanning the colon surface.

Results

Polyp detection sensitivity varies depending on scanning and image reconstruction parameters employed and on polyp size. No consensus exists on preferred viewing modes. For larger polyps a panoramic display results in greater sensitivity than a virtual endoluminal display and 3D displays are more sensitive than 2D dis-

plays. Experienced abdominal radiologists achieve similar polyp detection rates using 2D multiplanar reformation and 3D display techniques (14). Computed tomography colonography using axial 2D data and a cine mode and 3D “fly-through” with surface-rendered and multiplanar reformatted images identified the same number of polyps with both techniques (15). A metaanalysis of reported accuracy of CT colonography found a pooled per-patient sensitivity for polyps >10mm to be 88%, for polyps 6–9mm 84% and for polyps 5 mm or smaller 65% (16); per-polyp sensitivity for polyps >10mm was 81%.

CT colonoscopy perforation rates are low and should be similar to those with a barium enema. Rectal perforation is a potential complication with blind air insufflation in a setting of more proximal rectosigmoid obstruction.

Future Studies

Attempts to circumvent a colon cleansing regimen are theoretically feasible by tagging colonic content with ingested barium sulfate, with subsequent digital subtraction of this material. In patients with suspected or known colonic polyps, sensitivity for identifying

patients with polyps 1cm or larger was 80% to 100% if these patients ingested multiple dilute contrast doses over a prior 48 hours (17).

Using signal processing of CT colonography data to identify tumors protruding into bowel lumen, an automated polyp detection algorithm achieved a 64% sensitivity for detecting polyps 10mm or greater (18). Another approach is to use tumor contrast enhancement superimposed on a virtual double contrast and endoscopic display (19) (Fig. 5.2). Shape-based polyp detection and polyp edge enhancement are helpful in identifying polyps.

Teleradiology of CT colonography using wavelet compression to 1:1, 10:1, and 20:1 ratios detected all lesions >10mm for all compression ratios, but sensitivities for smaller lesions fell off with increasing compression ratios (20).

A claimed advantage of CT colonography is that extracolonic abnormalities are also detected. Among consecutive patients undergoing CT colonography, important extracolonic findings in 11% led to further imaging studies, and as a result several patients underwent surgery (21). Nevertheless, being designed for optimal colonic imaging, CT colonography is limited in evaluating solid organs (compared to a fine-tuned CT study of a specific organ or abnormality in question).

Magnetic Resonance Colonography

Similar to CT colonography, MR colonography is feasible, with relative advantages of CT versus MR colonography still evolving. Two broad approaches are possible:

1.Bright lumen MR colonography relies on colon filling with a paramagnetic contrastwater enema and T1-weighted gradientrecalled echo (GRE) single breath-hold acquisition,which results in a hyperintense luminal image, with other tissues being hypointense. Multiplanar reformatted 3D images and virtual colonoscopic images are then obtained.

2.Dark lumen MR colonography obtained by colon filling with a tap-water enema, which is hypointense on T1-weighted GRE imaging and an intravenous paramagnetic contrast agent to produce a hyperintense colonic wall. A variant technique is to use gas or air to distend the lumen; gas has no signal.

Another technique consists of colonic distention with fluid and use of T2-weighted spin echo (SE) imaging. The colon is studied both in cross section and using a virtual intraluminal outline. Often a coronal plane is useful.

The disadvantages of the bright lumen technique include retained air bubbles, which mimic polyps. Therefore, both prone and supine views are necessary. Surface-rendered MR virtual colonoscopic views, orthogonal sections and water-sensitive single-shot fast spin echo (FSE) MR images in patients (post–gadolinium-water enema) achieved a 93% sensitivity and 99% specificity in detecting tumors >10mm (22). To illustrate the complexity of these studies, they often require instillation of a gadolinium-water enema, prone and supine positioning, breathhold 3D spoiled gradient recalled echo (SGRE) sequences and also 2D images pre– and post–intravenous contrast, and, wherever necessary, virtual intraluminal images.

Comparing manganese chloride, iron glycerophosphate, and gadolinium-based enemas for use in T1 shortening 3D GRE MR colonography, the contrast-to-noise ratios for the iron enema were highest (23); the authors suggest replacing gadolinium with iron due to cost considerations.

Using a gadopentetate-water enema and breath-hold 3D SGE sequences, a virtual double-contrast display is achieved by calculating signal intensity differences between adjacent voxels and making adjacent voxels with similar intensities lucent while adjacent voxels with different intensities are made opaque (24); the resultant colonic display can be magnified

and rotated around its axis for detailed study from different planes (Fig. 5.3).

Some authors use the term hydro-MRI when a water, saline, or some other contrast enema is administered prior to MRI. Such an approach is helpful when the study is performed primarily for suspected colonic disease, and it is a step toward formal MR colonography. Confusing the issue is that hydro-MRI is also used by some authors if oral water or contrast is ingested prior to a MR small bowel study. Thus the type of contrast (including water and air) and route of administration need to be specified. The term double-contrast MR imaging is used by some when both an MR contrast enema and an MR intravenous (IV) contrast agent are employed. Due to its ambiguity, it is probably best avoided.

A typical technique for colon neoplasms consists of breath-hold T2-weighted halfFourier acquisition single-shot turbo spin echo (HASTE) and gadolinium-enhanced breathhold fat-suppressed T1-weighted SGE images (25); inflammatory changes appear best on the gadolinium-enhanced breath-hold fatsuppressed T1-weighted SGE images.

Air bubbles with the dark lumen technique are hypointense and blend into the surrounding hypointense water; thus only one patient position is necessary, resulting in a shorter scan time. Likewise, polyps enhance with IV contrast and stool does not. Potentially, the dark lumen

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technique is also useful to evaluate colitis because of bowel wall enhancement.

Using lumen distention with CO2,breath-hold single-shot FSE MRI performed during a CO2 enema in seven patients with known colon carcinoma detected cancers in all and correctly identified tumor extension through muscularis propria in four (26).

Colon cleansing is required with most of the above methods. Similar to CT colonography, use of an oral paramagnetic contrast agents to label stool (called fecal tagging) in an unprepared colon results in stool being hyperintense and thus blending in with a bright lumen. Or, concentrated oral barium sulfate, which is hypointense and thus useful with a dark lumen technique, is combined with IV gadolinium to enhance the colonic wall and any associated tumors (27). These MR stool-tagging techniques are still in their infancy.

Ultrasonography

Compared to other intraabdominal sites, colon US is rather limited. In some centers it has found a niche in following patients with colonic Crohn’s disease but appears less useful with ulcerative colitis and diverticulitis. Its role in pediatric intussusception reduction is well established, less so in adult tumor detection and staging. Ultrasonography findings are not disease-specific. Also, in general, a negative US examination does not exclude disease.

Colon US performed after a water enema is called hydrocolonic sonography (occasionally a methylcellulose-water mixture is used). Hydrocolonic US does detect larger polyps but it is very operator dependent and has been overshadowed by CT colonography.

Doppler US evaluates colonic blood flow. Thus viability of an obstructed bowel segment is suspected if Doppler US detects no blood flow. Similarly, in inflammatory bowel disease and in acute appendicitis Doppler flow through a thickened bowel segment can suggest an acute or ongoing inflammation.

The published terminology is somewhat inconsistent for US performed with rectal probes: Endoscopic and endoluminal US include either a rectal or vaginal probe, whereas the terms transrectal and endorectal US are used interchangeably. Anorectal echo-endoscopy and similar terms are also in use.

Endovaginal US evaluates the rectum and adjacent structures, including puborectalis muscle thickness, sphincter thickness, and sphincter defects. Distending the rectum with a water enema better delineates perirectal tissue planes.

Endorectal US defines surrounding structures. Urogenital structures and perirectal spaces are readily imaged. Both proctography and endorectal US evaluate internal and external rectal sphincters. Three-dimensional endorectal US appears to provide more accurate control of a biopsy needle toward a perirectal lesion than is available with other modalities, although the data for this are sparse. The current primary use of endorectal US is in a setting of rectal cancer and in the workup of evacuation disorders.

Available US miniprobes fit through the working channel of an endoscope. Similar to upper gastrointestinal endoscopic US, the role of flexible colonoscopic US in detecting and staging neoplasms is not yet clear.

Scintigraphy

A gamma camera scintigraphic technique estimates colonic transit, an infrequently used study in clinical practice.

The application of scintigraphy in inflammatory bowel disease and in the bleeding patient is covered in each respective section later in this chapter. Abdominal positron emission tomography (PET) scanning is discussed in more detail in Chapter 14.

Congenital Abnormalities

Malposition

Midgut malrotation is discussed in Chapter 4. The interposition of small or large bowel into

the right subphrenic space, first described by Chilaiditi in 1910, is rarely symptomatic and should be considered a normal variant. Bilateral bowel interposition is rare (28). Some authors use the term Chilaiditi’s syndrome to describe all such bowel interposition; others limit the term only to the symptomatic patient. Prevalence of such bowel hepatodiaphragmatic interposition appears to depend on patient position and is identified more often with the patient supine.

Detection of bowel interposition is generally straightforward with conventional radiography and barium studies. The appearance is confusing with US, where interposed bowel loops mimic an abnormal mass. Computed tomography detects ascending colon interposition between kidney and psoas muscle (pararenal space) in about 1% (29). Retropsoas interposition is slightly more common with the ascending colon (3%) and descending colon (2%) (30); little retroperitoneal fat favors interposition. A colon interposed posterior to the pancreas, between the spleen and left hemidiaphragm, is rare.

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neonate are lumped together as low intestinal obstructions.

A microcolon in a neonate is a descriptive term of a contrast enema finding rather than a specific disorder and usually suggests distal small bowel obstruction. Colonic disorders associated with a small caliber of the entire colon include total colonic aganglionosis of Hirschsprung’s disease and a microcolon seen with prematurity. Conventional radiography usually differentiates between a high and a low intestinal obstruction. A contrast enema is necessary to define the obstruction further.

Duplication

Gastrointestinal duplications are associated with both vertebral and genitourinary tract abnormalities. They occur roughly in one out of 4000 births. Least common are hindgut duplications. Most are detected in the young.

Colonic duplications have either a spherical or tubular appearance, with some long duplications mimicking a second colon lumen. Most of these duplications do not communicate with the lumen and occur along the mesenteric border. Occasionally a duplication contains noncolonic mucosa, such as heterotopic gastric mucosa, small bowel, pancreatic, and even respiratory epithelium. The mucosa of some duplications continues secreting and a noncommunicating duplication thus increases in size with time. Imaging identifies an abdominal cystic tumor.

A rare duplication intussuscepts; it acts as a source of cecal volvulus, or is an incidental palpable mass at initial presentation. An occasional one develops a fistula to an adjacent structure. Magnetic resonance imaging is useful with the rare rectal duplication in a neonate to show no posterior extension, thus excluding a meningocele.

Obstruction

Common distal ileal obstructions in neonates are due to meconium ileus and ileal atresia; in the colon obstructions include imperforate anus, meconium plug syndrome, and Hirschsprung’s disease. From a practical viewpoint, distal ileal and colonic obstructions in the

Atresia

In rectal atresia the anus is normal, with the atretic segment located more proximal. No bowel fistula is identified. Atresia proximal to the rectum is uncommon, although it can occur anywhere in the colon. Colonic stenosis is rare. A contrast enema reveals a small caliber colon distal to the obstructed, atretic segment.

Imperforate Anus

The most common neonatal colonic obstruction is an imperforate anus. Although the term imperforate anus implies a single and simple defect, in reality this is a complex deformity often also involving genitourinary tract structures and other anomalies. Cryptorchidism is common; in general, a more superior level of anorectal malformation increases the risk of cryptorchidism.

Rectal atresia differs from an imperforate anus. With an imperforate anus the hindgut does not descend and communicate with the anus, but either ends blindly or forms a fistula in an abnormal location (ectopic anus). An imperforate anus is classified as being high or low using the puborectalis sling as a dividing line. The differentiation between a high and low lesion is often made clinically, and imaging plays a limited direct role. The presence of a perineal dimple or passage of meconium from the genitourinary tract is a useful guide. In some boys conventional radiography reveals gas in the bladder.

The puborectalis muscle tends to be hypoplastic with a high obstruction.With a high lesion, the rectum can end blindly, although more often in boys it terminates in the posterior

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urethra, and less often in the bladder or anterior urethra. In girls the rectum tends to terminate in the vagina. Prior to definitive surgery, most high lesions are treated with a bypass colostomy. The underlying anatomy is then studied through the distal colostomy limb (mucous fistula) or, if needed, by cystography and urethrography.

A low lesion is usually associated with a perineal dimple, and no communication exists with the genitourinary tract. A variant of a low lesion is a congenital triad consisting of an anorectal malformation, sacral abnormality, and a presacral tumor, first described by Currarino et al. (31) in 1981 (Table 5.1). Magnetic resonance imaging is useful in this triad to detect a tethered cord.

Imaging aids in detecting any associated renal or sacral abnormalities. Magnetic resonance imaging outlines the hindgut, bony and muscular pelvic anomalies, including the puborectalis muscle and external sphincter, and other surrounding anatomy. T1-weighted images establish whether the puborectalis muscle is hypoplastic. Magnetic resonance imaging tends not to identify small fistulous tracts, however, and a contrast study is useful to define them.

Some boys have a mix of meconium and urine and the meconium calcifies; these calcifications are intraluminal in location, thus distinguishing them from meconium peritonitis. Such a mix of meconium and urine does not occur in girls with anal atresia; calcified intraluminal content in a girl should suggest a cloacal

Table 5.1. Currarino triad: anorectal malformation, sacral abnormality and presacral mass findings in 11 patients

malformation consisting of communication between the urethra and rectum, generally through a single perineal channel.

After surgical correction of an anorectal mal- formation—such as rectal pull-through(peri- neoplasty) or posterior sagittal reconstruction (anorectoplasty)—MRI is helpful in detecting complications and to evaluate muscle integrity. Residual internal and external sphincter disruptions are identified.

Megacystis-Microcolon-Intestinal

Hypoperistalsis Syndrome

In the megacystis-microcolon-intestinal hypoperistalsis syndrome the bladder is markedly distended and a contrast enema shows what initially looks like a small-caliber colon. Although the initial appearance suggests an obstruction, no mechanical obstruction is found. A shortened small bowel, at times malrotated, reveals poor or absent peristalsis. Hydronephrosis is common. Hydrometrocolpos and segmental colonic dilation also occur. Etiology of this rare autosomal-recessive disorder is unknown. Neuronal dysplasia is identified in some. This syndrome also occurs without megacystis; with such a presentation it blends into the general category of functional intestinal obstruction in neonates.

The diagnosis should be considered in a newborn with a markedly distended bladder and suspected intestinal obstruction.

Meconium Plug Syndrome

Meconium plug syndrome and small left colon syndrome are probably the same entity. Diabetes in the mother is common. These full-term neonates have colonic obstruction due to inspissated intestinal contents. The colonic lumen is narrowed distally and distended proximally. An abrupt transition between dilated and nondilated bowel is evident in some.

Hirschsprung’s disease is in the differential diagnosis.

Hirschsprung’s Disease

Pediatric

Source: Date from Lee et al. (32).

Hirschsprung’s disease is caused by incomplete caudal migration of neural cells, with bowel

distal to the point of migration arrest constituting an aganglionotic segment. By definition, the aganglionotic segment is continuous and extends to the anus; still, rare patients have segmental skip regions. Most often aganglionosis involves the rectum, but occasionally this segment extends more proximally, including the right colon, and the neonate presents with a microcolon. Also uncommon is for aganglionosis to be limited to the internal sphincter region only. To give an example of the varied involvement, in one Central European hospital among 142 children treated for Hirschsprung’s disease, 52% had typical rectal involvement, 30% a long colonic segment, in 13% only a very short rectal segment was involved, and 4% suffered from total colonic aganglionosis (33).

For unknown reasons Hirschsprung’s disease is uncommon in prematures. The prevalence in boys is several times greater than in girls.

An association exists between multiple endocrine neoplasia (MEN) type IIA and Hirschsprung’s disease. Mutations in the RET proto-oncogene are found in both entities (patients with MEN type IIB also have colonic abnormalities, including chronic constipation, but any relationship with aganglionosis is not clear). Hirschsprung’s disease is more common in patients with Down syndrome. Patients with Ondine’s curse (congenital hypoventilation

tend toward total colonic aganglionosis. Both Hirschsprung’s disease and ganglioneuroblastomas manifest aberrations of neural crest cell growth and development.

Radiologists generally perform a barium enema when suspecting Hirschsprung’s disease. A low-osmolality water-soluble contrast enema has also been used. Although a contrast enema tends to be diagnostic in most, in neonates a transition zone is not well defined during the first several weeks of life, and a normal examination does not exclude the diagnosis (Fig. 5.4). At times uncoordinated contractions are detected in the aganglionic segment.

In total colonic aganglionosis a contrast enema reveals a microcolon or a transition zone in the small bowel, or, rarely, it is even normal.

A definitive diagnosis is made by rectal biopsy. In some infants a full-thickness biopsy is necessary. At times biopsy reveals ganglion cells in the face of an abnormal barium enema, and

ADVANCED IMAGING OF THE ABDOMEN

Figure 5.4. Hirschsprung’s disease. Barium enema identifies a narrowed rectum (arrows) and a dilated colon more proximally. (Courtesy of Luann Teschmacher, M.D., University of Rochester.)

in such a setting an allergic colitis should be considered in the differential diagnosis.

The usual therapy for established Hirschsprung’s disease is an initial colostomy, followed by endorectal pull-through (Soave procedure).

Adult

Occasionally a mild form of what appears to be Hirschsprung’s disease is detected in adults. This acquired intestinal aganglionosis is often labeled adult Hirschsprung’s disease, but this term is tenuous at best. Biopsy often reveals a ganglionitis and loss of neurons. In some patients, with time, the involved segment becomes more extensive. Whether such acquired intestinal aganglionosis is indeed a variant of Hirschsprung’s disease, an allergic manifestation, or some other as yet undefined condition, is speculation.

Presentation in adults is generally similar but milder to that seen in children. It is diagnosed with a barium enema, anorectal manometry, and tissue biopsy. Occasionally adult Hirschsprung’s disease mimics rectal Crohn’s