Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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This chapter discusses the peritoneum, the intraand extra-peritoneal soft tissues, pelvic structures, diaphragm, and the anterior abdominal wall. The extraperitoneal abdominal structures are in continuity with the diaphragm superiorly, pelvic structures inferiorly, and the abdominal wall along both flanks. Some authors prefer the term extraperitoneal rather than retroperitoneal because it more accurately reflects the location, especially for pelvic structures, but both terms are in use synonymously.

The intraand extraperitoneal portions of the gastrointestinal and genitourinary tracts and other major organs are discussed in their respective chapters.

Technique

Computed Tomography

Multislice computed tomography (CT), especially coronal and three-dimensional (3D) reconstructions, are very useful in defining extraperitoneal tumor spread and tissue planes.

Computed tomography fluoroscopy is useful for guiding abdominal interventional procedures which cannot be managed with ultrasonography (US). Intermittent CT fluoroscopy using real-time fluoroscopic reconstruction at six frames per second confirms the needle position. Patient and operator radiation doses can be decreased with CT fluoroscopy compared to conventional CT guidance by limiting CT flu-

oroscopy to needle tip scanning rather than the entire needle.

Computed tomography peritoneography is occasionally useful in otherwise inapparent hernias or suspected peritoneal leaks in patients on peritoneal dialysis.

Ultrasonography

The primary limitations of abdominal US are the inherent physical limitations of this technique and, less well appreciated among clinicians, its considerable dependence on the operator. Proficiency in US, especially use of newer techniques and contrast agents, requires considerable training. Unskilled use of US is an invitation to disaster.

New US techniques and addition of contrast agents are changing this field. In the abdomen, tissue harmonic US using pulse-inversion techniques improves image quality and organ structure resolution compared to conventional US. A further improvement in overall image quality consists of combining tissue harmonic US with real-time spatial compound US (1). Yet the type of technique used may need to be tailored to study indication; thus what is optimal for detecting stone disease differs from is ideal in tumor detection.

Similar to intraperitoneal structures, US is useful for extraperitoneal lymph node biopsy. It is efficacious for deep nodes as small as 1cm in diameter.

Endoscopic US detects enlarged lymph node and tumors. Aspiration biopsy can also be performed using endoscopic US as a guide.

Probes for performing laparoscopic US allow the surgeon to increase visualization of surrounding structures. Such laparoscopic US has a high sensitivity in detecting liver and other metastases. Overall, it improves tumor staging, but additional trocar sites are necessary for the US probe and laparoscopy is prolonged.At some sites the insufflated carbon dioxide makes probe contact with an organ in question more difficult; at times instilling saline into the peritoneal cavity is helpful.

Magnetic Resonance Imaging

Echo planar imaging is a fast imaging technique with images obtained in 50 to 150msec. Breathing and peristaltic artifacts cause little image degradation with such fast scanning and relatively high-resolution images are obtained with a single breath hold.

Magnetic resonance imaging (MRI) is useful in differentiating extraperitoneal from intraperitoneal tumors. The superior magnetic resonance (MR) soft tissue contrast resolution (compared to CT) makes it more suitable for identifying tumor margins and tumor infiltration of adjacent structures. Similar to CT, a fast MRI technique, together with intraperitoneally instilled saline, visualizes the peritoneal surfaces, omentum, and adjacent mesentery; saline also aids intraperitoneal tumor visualization. Various intraperitoneal recesses are identified. In distinction to CT, retained barium in the bowel does not create artifacts on MRI.

Any fibrosis is accentuated against background fat on T1-weighted spoiled gradient echo (SGE) images. Tumors, abscesses, and other inflammation are best evaluated on intravenous (IV) contrast-enhanced images.

No uniform definition of MR lymphography exists. Whether a conventional MR study focusing on lymph nodes should be called MR lymphography is a matter of definition. Often a more appropriate study consists of heavily T2weighted images processed using maximum intensity projection accentuating stationary or very slowly flowing fluid, similar to magnetic resonance cholangiopancreatography (MRCP); a limitation of such a study of retroperitoneal structures is incomplete suppression of signals from veins.

ADVANCED IMAGING OF THE ABDOMEN

Ultrasmall superparamagnetic iron oxide (SPIO) particles are of potential use in MR lymphography (these agents are discussed in more detail in Chapter 7). Using SPIO as an endolymphatic contrast agent results in contrast uptake by normal lymph nodes but not by those replaced by tumor. Potentially, nodal uptake occurs even after IV injection if the iron oxide particles are sufficiently small. Magnetic resonance imaging is typically performed 24 to 36 hours after IV infusion of these contrast agents. The use of IV SPIO in patients with suspected lymph node metastases results in additional diseased nodes being detected than with precontrast scans. Normal lymph nodes decrease in signal intensity on enhanced T2-weighted and T2*-weighted gradient-echo images, indicating uptake of this contrast agent, but nodes infiltrated by tumor have no appreciable contrast uptake and thus reveal no signal change on preand postcontrast images (2). Also, postcontrast, T1-weighted signal intensity increases in metastatic nodes, probably due to altered node capillary permeability. These findings are not absolute, and nodes involved by benign disease also tend not to take up contrast.

Scintigraphy

Radionuclide agents used to image inflammation and abscesses are gallium 67, indium- 111–labeled leukocytes and technetium-99m (Tc-99m)–labeled leukocytes. Imaging with Ga 67 is performed 48 to 72 hours after injecting the radionuclide; because one of its excretion pathways is via the colon, residual colonic activity interferes with abscess detection, although to a large degree single photon emission computed tomography (SPECT) imaging overcomes this limitation. Indium 111 is cyclotron produced and is not as readily available as the other agents. It is distributed primarily in the liver, spleen, and bone marrow, and bowel activity is less of a concern than with the other two agents. Labeled leukocyte scintigraphy requires in vitro labeling of a sample of the patient’s blood with either In 111 or Tc-99m–hexamethylpropyle- neamine oxime (HMPAO), a somewhat involved procedure. Most of the nucleotide is labeled to neutrophils, and thus this procedure is most useful for neutrophil-involved inflammatory conditions.

Indium-111–satumomab pendetide (OncoScint) is the first murine origin monoclonal

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

antibody approved by the Food and Drug Administration for tumor imaging, a technique termed immunoscintigraphy. This specific antibody attaches to a colorectal or ovarian cancer–associated antigen and the attached radionuclide allows cancer detection. Either a diagnostic or therapeutic procedure is possible depending on the radionuclide selected. Its primary use is with suspected intraabdominal metastases, including carcinomatosis. Falsepositive tests do occur with satumomab pendetide. In an occasional patient increased upper abdominal uptake persists for days in regions of chronic inflammation. Isotope uptake in a nonfunctional adrenal adenoma has resulted in false-positive results.

Lymphoscintigraphy assesses the lymphatic system and, for the most part, has replaced direct lymphography. It is a useful study to investigate chyluria, chyloperitoneum, and chylothorax. Multiple views are obtained for several hours after intradermal injection of Tc- 99m–sulfur colloid. Both lymphatic channels and nodes can be evaluated, and the study is used to detect lymphatic obstruction and fistulas. More lymphatic channels and more lymph nodes are visualized using smaller filtration, thus increasing diagnostic certainty in detecting diseased lymph nodes.

The current primary clinical application of positron emission tomography (PET) is to detect cancer using the positron-emitting compound 2-[18F]-fluoro-deoxy-D-glucose (FDG). Transported across cell membranes of metabolically active tumors, the deoxy component of FDG prevents further metabolization, leading to intracellular accumulation and an increase in tumor-to-background of fluorine 18. The amount of FDG accumulating within a tumor is proportional, within broad limits, to the degree of malignancy. Thus in a setting of a tumor of unknown significance detected by another imaging modality, PET suggests whether the tumor is benign or malignant, depending on its metabolic activity. In the abdomen, PET is most useful for lymphoma, colon cancer, and metastases from melanomas and lung and breast carcinoma. It has a role in tumor staging and appears to be of prognostic significance. In general, increased tumor metabolic activity signifies a worse prognosis. Positron emission tomography is also useful after cancer therapy. Its ability to detect recurrent or residual metabolically active tumor is independent of under-

lying distortion due to fibrosis, and such tumor detection is superior to what is currently available with CT or MRI. Combining PET and CT images improves tumor localization significantly compared with each modality alone.

Test results are degraded by urinary and colon retention of FDG. These artifacts can be minimized by a colon cleansing regimen, patient hydration after FDG injection, administration of furosemide and bladder drainage and instillation of normal saline into the bladder prior to pelvic scanning. Use of oral Gastrografin to outline the intestines during a combined PET-CT study does not degrade FDG images. A limitation of FDG-PET in detecting small tumors is its minimal spatial resolution of about 5mm. Also, inflammation does lead to false-positive PET results, influencing detection specificity. Recurrent or residual tumor detection is thus best studied after postoperative inflammation has subsided.

Positron emission tomography using oxygen- 15–labeled water provides information on tissue perfusion; it is mostly a research tool with limited current clinical use.

Lymphography

Performed more often prior to the introduction of CT, MR, and lymphoscintigraphy, the current indications for lymphography have decreased considerably. It is a relatively prolonged, detailed procedure liked by few radiologists and even fewer radiology residents.

Although CT will detect an enlarged node, differentiation between most benign and malignant is not possible. Lymphography, on the other hand, can identify internal derangement even in a normal-sized node infiltrated by tumor.

The older literature reported rare but varied complications with lymphography, mostly related to contrast overinjection (3), including embolization to brain; a right-to-left cardiac shunt is responsible for some of these complications.

Peritoneography

Direct peritoneography using either a contrast agent or its scintigraphic counterpart has limited indications and is rarely performed. One

use is in a setting of peritoneal carcinomatosis prior to chemotherapy through a catheter implanted in the abdominal wall to evaluate drug distribution.

Biopsy

Either a conventional large core needle or a fineneedle aspiration technique is used for percutaneous biopsy of suspicious tumors; fine-needle aspiration is often preferred, but especially with benign lesions it achieves a low success rate. On average, core needle biopsies obtain a specific diagnosis in over 90% of biopsies, while fineneedle aspirations average only about 50%; the complication rate, chiefly bleeding, is grossly similar with both techniques.

Either CT or US (and occasionally fluoroscopy) is used for biopsy localization and other percutaneous interventional procedures. In general, in most hospitals US equipment is more readily accessible, the study is quicker, and it results in faster patient throughput.

Special interventional equipment designed to be used with MR guidance is available. Little used at present, MR interventional radiology is undergoing rapid expansion with tissue biopsies, fluid drainages, and guidance of other percutaneous therapeutic procedures. An open magnet provides ready patient access.

Endoscopic US-guided fine-needle aspiration biopsy is useful for diagnosis and staging of malignancies. Diagnostic sensitivities and specificities of about 90% are achieved with both extraluminal tumors and lymph nodes.

Nerve Block

Celiac plexus and splanchnic nerve block provides pain palliation for patients with unresectable upper abdominal malignancies. This technique was initially designed to be performed using fluoroscopic guidance, although CT provides more accurate guidance. An alcohol-contrast agent combination provides localization.

Congenital Abnormalities

Situs

Situs refers to orientation of certain organs to the midline. In general, the bilobed lung, left

Figure 14.1. Situs inversus in an 85–year-old woman. Polysplenia is present on the right (arrows). The stomach is also on the right.

atrium, descending aorta, spleen, and stomach are all located on one side of the body. When these structures are normally placed on the left side the condition is called situs solitus. Situs inversus is a mirror image and results when these structures are on the right side and the trilobed lung, right atrium, inferior vena cava, gallbladder, and liver are on the left (Fig. 14.1). The prevalence of situs inversus is about 1 in 10,000. These patients are at an increased risk for nasal polyposis, chronic sinusitis, and bronchiectasis (Kartagener’s syndrome) and a higher than normal risk for congenital heart disease.

Situs inversus can be associated with multiple spleens and other defects. Often a conventional radiograph is a good starting point in evaluating these anomalies.

Heterotaxy Syndrome

The term heterotaxy, or situs ambiguous, describes complex congenital abnormalities differing from situs solitus or situs inversus. Some authors use the terms asplenia and polysplenia to subdivide heterotaxy into specific categories, but these classifications are incomplete and at times confusing. Others use the terms left and right isomerism.

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

Complex familial inheritance patterns exist for heterotaxy. An X-linked recessive inheritance appears to account for a male preponderance.

These patients usually have complex congenital cardiac abnormalities that tend to overshadow abdominal findings. Complex bowel rotation anomalies are encountered with both right and left isomerism. Aortic and gastric positions are variable. Right-sided isomerism results in asplenia. The liver extends across both upper quadrants of the abdomen. Some of these patients have a left-sided inferior vena cava. Gallbladder duplications occur in right-sided isomerism.

Heterotaxy with left abdominal isomerism results in polysplenia. Cardiac anomalies tend to be less severe than in asplenia. Most patients have azygous continuation with interruption of the inferior vena cava. The spleen(s) is (are) on the same side as the stomach. An absent gallbladder is more common in left-sided isomerism. An association exists between polysplenia and biliary atresia. Midgut malrotation is a common associated finding, and midgut volvulus develops in some of these infants.

Traditionally, heterotaxy has been evaluated with angiocardiography. While both CT and pulse and color Doppler US are helpful in outlining some anomalies, MRI is evolving as the preferred modality.

Liver/spleen scintigraphy detects splenic tissue in suspected heterotaxy; in some infants with negative planar imaging, the presence of splenic tissue is shown by SPECT.

Abdominal Wall Defects

Bladder exstrophy is discussed in Chapter 11. Cloacal malformations are discussed in Chapter 12.

Omphalocele

Failure of the abdominal wall to close normally results in several defects.A defect cephalic to the umbilicus results in a supraumbilical ventral hernia, anterior diaphragmatic defects, and related conditions. Additional failure of lateral wall fusion results in an omphalocele. The degree of visceral herniation depends on the size of the defect. Peritoneum covers the viscera,

and the defect is obvious. Associated bowel malrotation is common.

Prognosis in these neonates is often limited by other associated abnormalities, at times major. The abdominal organs tend to be malpositioned, leading to unusual imaging findings.

Gastroschisis

Defects in gastroschisis involve the lateral abdominal wall. The gross appearance is similar to that of an omphalocele, but with gastroschisis the umbilicus is in its normal position. Among infants with gastroschisis, about two thirds have a simple defect, and in one third complex defects are identified, ranging from bowel atresia, stenosis, and perforation, to volvulus (4); survival of those with a simple defect was 100%, but those with a complex defect had a mortality rate of 28%.

Neonates with gastroschisis are often premature. Small bowel dysmotility and a prolonged transit time are common. Conventional radiography often reveals bowel wall thickening and lumen dilation. Delayed barium transit suggests obstruction, although actual small bowel obstruction is uncommon. Bowel atresia, if present, does result in a high mortality.

After surgical repair of gastroschisis these infants are at increased risk for necrotizing enterocolitis, although even then overall survival rate is quite high.

Prune Belly Syndrome (Eagle-Barrett

Syndrome)

The prune belly syndrome, named after the lax, wrinkled abdominal wall seen in this condition, consists of abdominal wall hypoplasia, genitourinary anomalies—including bilateral cryptorchidism in males—and other, at times major, systemic abnormalities. Occurring mostly in males, the more severely affected neonates die shortly after birth. An incomplete expression of this syndrome exists, and some neonates have only mild manifestations. Unilateral abdominal wall hypoplasia also occurs. An association between congenital cytomegaloviral infection and prune belly syndrome has been raised, but the precise etiology is unknown.

A minority of these neonates have a urethral obstruction such as atresia or valves; most, however, have functional bladder outlet