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

(Tc-99m)–hexamethylpropyleneamine oxime (HMPAO) white blood cell scintigraphy in women with an acute abdomen, suspected of appendicitis but having atypical clinical findings, achieved a sensitivity and specificity of >90% in diagnosing appendicitis (30).

Unusual Appearance

In the majority of patients appendicitis eventually localizes in the right lower quadrant. Occasionally inflammation evolves in another location, such as in the pelvis (Fig. 6.5). Subhepatic appendicitis, at times with a fecalith, mimics acute cholecystitis. Several experienced clinicians relate encountering a gangrenous appendix in an incarcerated right inguinal hernia.

A commonly accepted dogma both in the literature and among practicing radiologists is that the presence of a contrast-filled appendix on CT excludes appendicitis. Occasionally, however, appendicitis involves only the appendiceal tip. A previous generation of radiologists practicing barium studies were taught to look for the bulbous dilatation near the tip of the appendix as a sign that the entire appendiceal lumen has

ADVANCED IMAGING OF THE ABDOMEN

been visualized; this finding is more difficult to detect with CT. Computed tomography in patients with distal appendicitis (the bulbous blind end is defined as distal) identifies a normal proximal appendix but does defect periappendiceal fat stranding; as expected with distal appendicitis, no abnormality is detected at the cecal base. Ultrasound can also detect distal appendicitis. Regardless of the imaging modality used, the entire appendiceal length should be studied to exclude distal appendicitis.

A right lower quadrant phlegmon or abscess is a late finding.An appendiceal abscess tends to involve adjacent bowel. Thus distal small bowel obstruction is common with neglected appendicitis. Strictures, presumably from chronic inflammation, have developed both in the ileum and sigmoid.

Appendicitis with a perforation allows appendicolith migration to unusual locations, including the pouch of Douglas; some of these calcifications are identified by postoperative CT.

In a rare presentation, scrotal US detected a hypoechoic lesion but a normal testis and epididymis in a 10-year-old boy with an inguinal hernia in the left scrotum (31); torsion was suspected,but surgery found pus in the tunica vaginalis together with surrounding inflammation. Subsequent abdominal US and MRI revealed an abscess posterior to the bladder, and laparotomy yielded a gangrenous appendix.

A rare complication of appendicitis is a vesicoappendicular fistula.

Figure 6.5. Neglected appendicitis that has evolved into a pelvic abscess. The appearance mimics a tumor in the pouch of Douglas (arrows).

Inflammatory Bowel Disease

Appendicitis appears to occur less often than expected in individuals with ulcerative colitis but roughly at the same rate as in the general population in Crohn’s patients.

The clinical presentation of Crohn’s disease of the appendix and typical acute appendicitis is similar. Indeed, it is conjecture whether a histologic diagnosis of idiopathic granulomatous appendicitis is the same entity as Crohn’s disease limited to the appendix. Some patients with histologic acute and chronic appendicitis have granulomas but no systemic or enteric granulomatous disease. In general, the complication rate in a setting of granulomatous appendicitis is low. Some investigators suggest that if an appendectomy reveals granulomatous appen-

APPENDIX

dicitis, further investigation is warranted to ensure that no underlying Crohn’s disease exists in either the small bowel or colon.Also,presence of noncaseating granulomas is found in appendiceal sarcoidosis.

Pregnancy

Appendicitis does occur in pregnancy, during any trimester and also in the postpartum period. Maternal morbidity is essentially the same throughout the gestational period but perinatal outcome appears worse if appendicitis occurs in the second trimester. An extrauterine infection, such as appendicitis, appears to be associated with increased neonatal abnormalities.

Graded compression US is of similar value in pregnant women as in the general population.

MRI is often indicated in these patients.

Older patients developing acute appendicitis are considerably more likely to have a gangrenous or perforated appendix than younger patients. Among patients aged 70 to 89 years, the site of abdominal pain was atypical in half, 45% presented with bowel obstruction, a preoperative diagnosis of appendicitis was made only in half, surgery was performed within 24 hours of admission in only 40%, and appendiceal perforation occurred in 70% (32); complications developed in 70%, mostly related to perforation.

AIDS Related

AIDS patients are prone to unusual infections. Even cytomegalovirus appendicitis can be expected in a patient with HIV infection.

Old Patients

A clinical diagnosis of appendicitis is difficult in the very young and very old (Fig. 6.6).

Figure 6.6. Barium enema reveals a tumor at the cecal base (arrows) in an elderly woman. A cancer was suspected but surgery revealed appendicitis.

Therapy

The primary purpose of early therapy of acute appendicitis is to arrest the inflammatory process prior to perforation. Mortality and morbidity rise with perforation. On the other hand, attempts to perform early appendectomy result in an increased normal appendectomy rate. An oft-repeated statement holds that an inverse relationship exists between a normal appendectomy rate and perforation rate, although the validity of such a relationship has been modified considerably by better diagnosis.

Laparoscopy

Laparoscopic appendectomy was initially greeted rather skeptically by the surgical community, yet numerous studies have concluded that, compared to open appendectomy, a laparoscopic approach leads to a shorter hospital stay. Nevertheless, a review of randomized, controlled trials of laparoscopic appendectomy in adults failed to establish superiority for the laparoscopic approach, and the authors concluded that “nothing is definitively well established” (33). The reported complication rates range from being similar for both open and laparoscopic approaches to the laparoscopic group having fewer infections and an earlier return to normal activity.

Diagnostic laparoscopy appears useful in a clinical equivocal diagnosis of appendicitis. A laparoscopic diagnosis of acute appendicitis can be immediately followed by a laparoscopic appendectomy.

Some surgeons consider acute complicated appendicitis,such as a gangrenous or perforated appendix, a relative contraindication to laparoscopic appendectomy.

Colonoscopy

Anecdotal reports describe colonoscopy both for diagnosis and therapy of unsuspected acute appendicitis. Colonoscopy reveals an inflamed appendiceal orifice. Intubation and pus aspiration are reputedly therapeutic but these patients invariably require an eventual elective appendectomy.

Other Therapy

Occasionally a patient with suspected appendicitis, especially if clinical symptoms are mild, is treated conservatively. Some of these patients then undergo an elective appendectomy.

Most periappendiceal abscesses are amenable to percutaneous drainage. Some of these can be drained using an endorectal route. Most surgeons recommend an interval appendectomy.

An incidental vaginal appendectomy can be performed in women during laparoscopicassisted vaginal hysterectomy.

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Torsion of the right ovary mimics appendicitis; imaging should distinguish between these two conditions.

A cecal or appendiceal carcinoma not uncommonly manifests as appendicitis, with the clinical presentation in some patients suggesting chronic rather than acute appendicitis. Most published CT and US studies of appendicitis do not include cecal carcinoma in their alternate diagnosis groups, yet the not rare reports of cecal carcinoma mimicking CT and US findings of appendicitis are disturbing. Even in a younger adult a carcinoma often needs to be in the differential. A simple appendectomy in a setting of cecal cancer has grave consequences.

Appendiceal myeloid sarcoma in a patient with myeloid leukemia mimicked appendicitis both clinically and with CT (34). Also known as granulocytic sarcoma, myeloid sarcoma occurs in various myeloproliferative disorders, leukemia and polycythemia vera.

Thrombosis of an ovarian vein in a postpartum patient can mimic acute appendicitis both clinically and by US. Other conditions that mimic some of the imaging findings of appendicitis include Crohn’s disease, tuboovarian abscess, typhlitis, cecal diverticulitis, and an adjacent neoplasm. Most often endometriosis has a different clinical presentation, but occasionally is a cause of acute appendicitis.

Differential Conditions

An otherwise normal appendix can be thickened by reactive inflammation due to an adjacent lesion, with the most relevant being mesenteric lymphadenitis. Complicating the issue is that lymph nodes enlarge both in appendicitis and mesenteric lymphadenitis.

Various gynecologic abnormalities, right ureter obstruction, cecal diverticulitis or cancer, and ischemia are some other conditions occasionally mimicking clinical acute appendicitis. Clinically, a rectus sheath hematoma presents similarly to appendicitis, although the correct diagnosis can be suspected either by CT or US.

Chronic Appendicitis

Several recent authors have “rediscovered” the presence of recurrent episodes of abdominal pain due to appendicitis, a condition already described by Fitz in his seminal article in 1886. Many of these patients are treated conservatively, although recurrent bouts of appendicitis are common (35) (Fig. 6.7); patients with an appendiceal diameter >8mm are more prone to develop recurrence than those with a smaller diameter. In spite of the initial clinical and imaging diagnosis of appendicitis, a basic question is often left unanswered: Did the patients indeed have appendicitis or was some other disorder responsible for the patients’ symptoms?

Tumors

Many appendiceal neoplasms are directly related to appendicitis, presumably by obstructing the lumen, or they are discovered incidentally by a pathologist in a resected appendix. An occasional appendiceal malignancy invades the adjacent cecum and eventually is labeled as a cecal carcinoma.

In general, the prevalence of both synchronous and metachronous colorectal neoplasms is increased in the presence of an appendiceal tumor; a study of nearly 8000 appendectomies found synchronous or metachronous colorectal cancers in 10% of patients with appendiceal carcinoids, in 33% of those with benign tumors, in 55% with secondary malignancies, and in 89% of those with primary malignancies (36).

Mucocele

A mucocele describes a grossly dilated appendiceal lumen filled with accumulated mucus. The term is descriptive in nature and does not imply a specific etiology; it is found with mucosal hyperplasia with lumen obstruction, a cystadenoma, cystadenocarcinoma, and even a mucin-producing tumor metastatic to the

appendix. Some authors include the dilated appendices found normally in patients with cystic fibrosis under mucoceles, but this is not a universal practice.

Appendiceal mucoceles are associated with an increased prevalence of synchronous colon neoplasms. Thus colon tumor surveillance appears worthwhile in these patients.

An occasional mucocele is huge. Rarely, one develops postoperatively, at times even years later. Rupture of a mucocele leads to pseudomyxoma peritonei (discussed in Chapter 14). Most spontaneously rupturing mucoceles are malignant. Cecocolic intussusception by an appendiceal mucocele is very rare (37).

The barium enema appearance of a mucocele is that of a smoothly outlined mass extrinsic to the cecum in the region of the appendix. A similar finding is seen with CT (Fig. 6.8).

Ultrasonography shows an inhomogeneous tumor, often containing hyperechoic components and having posterior enhancement. Endosonography reveals a hypoechoic tumor containing hyperechoic foci. Endosonography should be able to distinguish between an appendiceal and an ovarian origin for mucoceles.

Magnetic resonance imaging identifies a cystic hypointense mass on T1and a hyperintense mass on T2-weighted images. Their signal

Figure 6.8. Appendiceal mucocele. CT reveals a cystic structure extending inferiorly from the cecum and an abscess was suspected. Needle puncture under US guidance yielded gelatinous material. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

intensity varies depending on protein content. The mucocele wall tends to be thick and enhances postcontrast.

The surgeon faces a dilemma once a mucocele is identified. If the condition is due to a simple retention cyst, an appendectomy suffices. At the other extreme, an underlying cystadenocarcinoma of the appendix requires a more extensive cancer resection.

Endometriosis

The most common site for endometriosis is the rectosigmoid, followed by the appendix and ileocecal region. Appendiceal endometriosis is usually associated with right ovarian endometriosis and is often clinically silent, being detected by the pathologist after an incidental appendectomy.

An occasional patient with appendiceal endometriosis presents with massive lower gastrointestinal bleeding or even an acute abdomen suggesting acute appendicitis. Intratumoral bleeding and necrosis presumably account for the acute presentation.

Carcinoid

A carcinoid is the most common appendiceal neoplasm. The vast majority of carcinoids are benign and, if detected incidentally in a resected appendix, are of little consequence. A rare one

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infiltrates the adjacent cecum and obstructs the bowel. Carcinoid induced fistulas are rare.

Somatostatin scintigraphy, performed after an incidental appendiceal carcinoid was discovered, detected focal tracer activity at the previous appendectomy site (38). No residual carcinoid was found at subsequent right hemicolectomy.

A rare and distinct entity is an adenocarcinoid, also called goblet cell carcinoid (39). These have histologic features of both a carcinoid and an adenocarcinoma and are aggressive tumors metastasizing readily.At surgery an appendiceal adenocarcinoid can mimic metastatic ovarian carcinoma.

Cystic Neoplasms

Appendiceal cystadenomas and cystadenocarcinomas typically present as soft tissue tumors extrinsic to the cecum but indenting it. The cystic nature of the tumor can be suspected from barium enema findings and confirmed either by US or CT. Occasionally mucin leaks into the surrounding soft tissues and incites inflammation and granuloma formation, at times to the point that even at surgery a carcinoma is suspected. Some appendiceal mucinous cystadenomas are associated with an elevated serum carcinoembryonic antigen (CEA) level.

Occasionally found are synchronous solid or cystic neoplasms of the ovaries and appendix. Although some authors consider them as representing two independent neoplasms, mutation analysis suggests that they are not independent but one originates from the other.

Computed tomography of primary mucinous appendiceal cystadenocarcinomas identifies not only a cystic component but also enhancing wall nodules. A gynecologic tumor is often in the differential diagnosis.

Adenoma/Adenocarcinoma

Appendiceal adenocarcinoma are uncommon. Most are not suspected preoperatively and many cancers have already metastasized at the initial presentation. Appendiceal adenocarcinomas have developed in a setting of chronic ulcerative colitis and in familial polyposis syndrome. A rare appendiceal villous adenoma caused acute appendicitis.

APPENDIX

A necrotic appendiceal adenocarcinoma can form a fistula to adjacent structures, including bladder.

Other Tumors

A fibrous histiocytic tumor at the appendiceal tip led to appendiceal torsion (40). A hyperplastic polyp is occasionally detected in a resected appendix. Rare appendiceal ganglioneuromas and pheochromocytoma develop mostly in patients with neurofibromatosis type 1.

Metastases to the appendix are probably more common than the literature suggests, especially in a setting of carcinomatosis. A rare metastasis results in acute appendicitis. The role of appendectomy in staging and cytoreductive surgery in a setting of ovarian cancer and suspected carcinomatosis is debatable.

In spite of the rich surrounding adenopathy, lymphoma involving the appendix is rare.

the patients; the seroma gradually resolves. After laparoscopic appendectomy fluid collections are also relatively common even in asymptomatic patients. In the symptomatic patient these innocuous fluid collections can be difficult to distinguish from a postoperative abscess.

Even a scrotal abscess has developed after laparoscopic appendectomy.

Occasionally an incomplete appendectomy is performed. Some of these patients continue to have symptoms; in others appendicitis develops later.A postoperative appendicocutaneous fistula suggests an incomplete appendectomy or possibly retained stone.

Dropped appendicoliths are associated with necrotic appendicitis and occur more often during a laparoscopic appendectomy. Not uncommonly they result in a pelvic abscess (42).

A cecocolic intussusception develops rarely after an appendectomy.

Intussusception

Appendicocecal intussusception is rare. These intussusceptions range from complete to partial. Some are idiopathic, others have an underlying malignancy, endometriosis, cecal lymphangioma (41), or some other lead point.

Clinically, these intussusceptions range from acute to chronic.

The intussuscepted appendix appears as a finger-like projection in the cecal lumen. Some intussusceptions are reduced during a barium enema. Once an intussusception has progressed beyond the point of appendiceal invagination, the imaging appearance begins to mimic a cecocolic or even an ileocolic intussusception; these latter intussusceptions are discussed in Chapter 5.

Examination and Surgical

Complications

Postoperative complications develop in about 10% of patients after simple acute appendicitis versus about 20% in those with gangrenous or perforated appendicitis.

Following conventional appendectomy, US identifies a seroma under the scar in about half

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16.Kamel IR, Goldberg SN, Keogan MT, Rosen MP, Raptopoulos V. Right lower quadrant pain and suspected appendicitis: nonfocused appendiceal CT— review of 100 cases. Radiology 2000;217:159–163.

17.Weltman DI, Yu J, Krumenacker J Jr, Huang S, Moh P. Diagnosis of acute appendicitis: comparison of 5– and 10–mm CT sections in the same patient. Radiology 2000;216:172–177.

18.Sivit CJ, Dudgeon DL, Applegate KE et al. Evaluation of suspected appendicitis in children and young adults: helical CT. Radiology 2000;216:430–433.

19.Mullins ME, Kircher MF, Ryan DP, et al. Evaluation of suspected appendicitis in children using limited helical CT and colonic contrast material. AJR 2001;176:37–41.

20.Jacobs JE, Birnbaum BA, Macari M, et al. Acute appendicitis: comparison of helical CT diagnosis focused technique with oral contrast material versus nonfocused technique with oral and intravenous contrast material. Radiology 2001;220:683–690.

21.Kaiser S, Finnbogason T, Jorulf HK, Soderman E, Frenckner B. Suspected appendicitis in children: diagnosis with contrast-enhanced versus nonenhanced Helical CT. Radiology 2004;231:427–433.

22.Fefferman NR, Roche KJ, Pinkney LP, Ambrosino MM, Genieser NB. Suspected appendicitis in children: focused CT technique for evaluation. Radiology 2001;220: 691–595.

23.Rao PM, Feltmate CM, Rhea JT, Schulick AH, Novelline RA. Helical computed tomography in differentiating appendicitis and acute gynecologic conditions. Obstet Gynecol 1999;93:417–421.

24.Pohl D, Golub R, Schwartz GE, Stein HD. Appendiceal ultrasonography performed by nonradiologists: does it help in the diagnostic process? J Ultrasound Med 1998;17:217–221.

25.van Breda Vriesman AC, Kole BJ, Puylaert JB. Effect of ultrasonography and optional computed tomography on the outcome of appendectomy. Eur Radiol 2003; 13:2278–2282.

26.Jahn H, Mathiesen FK, Neckelmann K, Hovendal CP, Bellstrom T,Gottrup F. Comparison of clinical judgment and diagnostic ultrasonography in the diagnosis of acute appendicitis: experience with a score-aided diagnosis. Eur J Surg 1997;163:433–443.

27.Kessler N, Cyteval C, Gallix B, et al. Appendicitis: evaluation of sensitivity, specificity, and predictive values of US, Doppler US, and laboratory findings. Radiology 2004;230:472–478.

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37.Coulier B, Pestieau S, Hamels J, Lefebvre Y. US and CT diagnosis of complete cecocolic intussusception caused by an appendiceal mucocele. Eur Radiol 2002;12: 324–328.

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Technique

Computed Tomography

In a solid organ such as the liver, computed tomography (CT) reveals characteristic attenuation alterations and morphologic changes of diffuse disorders such as cirrhosis and fatty infiltration. Similar changes are also detected with magnetic resonance imaging (MRI). Currently the primary limitation of both CT and MRI is that a number of liver disorders have overlapping imaging findings, thus limiting specificity.

The terms helical CT and spiral CT are used interchangeably. In helical CT the patient table moves at a constant speed while the x-ray tube and detectors rotate continuously, and thus scanning is in a helix rather than a circle as with conventional CT. Images obtained with helical CT do not define a specific circular body slice and are not identical to those obtained with conventional CT. There is little argument that in evaluating abdominal disease in general, and liver disorders in particular, helical CT is preferred over conventional CT.

Initial helical CT scanners could cover either large body parts or thin sections of a limited volume, but not both, a limitation largely overcome by the introduction of multidetector CT (also known as multirow CT and multislice CT) in the late 1990s. As a basic concept, multidetector CT generates more than one slice per x-ray tube rotation. Multidetector CT, with 16

detectors being readily available, 32 detectors being tested, and 64 or more detectors on the drawing boards, offers several advantages: a larger volume scanned during a given time, reduced time required to scan a given volume, narrower collimation and thus increased resolution, and shorter enhancement intervals after contrast. Multidetector helical CT allows simultaneous acquisition of multiple slices, and complex, single breath-hold techniques are thus feasible (a breath-hold is typically defined as 20sec or less). Three-dimensional (3D) CT arteriograms without venous overlay are readily obtained by using first-pass data from a multirow detector CT scanner. One by-product of multidetector CT is a considerable increase in the number of images available for review, thus adding to study complexity. Simply decreasing the number of images evaluated is not a viable option because overlapping images at various phases of contrast flow improve disease detection.

In general, precontrast CT scanning identifies fewer liver lesions than postcontrast images. In many institutions precontrast CT is limited to specific indications such as in detecting calcifications or hemorrhage.

Correct arterial phase timing is obtained by using an initial test dose. Automatic bolus tracking initiates scanning after injection of contrast by monitoring a region-of-interest cursor placed in the abdominal aorta; a typical scenario is to set a threshold level at 100 Hounsfield units (HU) over the aortic baseline

CT level and initiate scanning about 10 seconds later. A similar approach is to start arterial phase imaging when splenic enhancement reaches a certain HU value above baseline.

On average, CT arterial phase begins to enhance about 15 to 20 seconds after the start of intravenous (IV) contrast injection, followed by portal venous enhancement about 30 seconds later and parenchyma enhancement shortly after that. Images during the arterial phase map the major hepatic artery branches, and portal phase images outline portal and hepatic venous systems. Such a biphasic or dual-phase CT technique refers to the two discrete imaging sequences obtained and not to a biphasic contrast injection. For some indications, a liver parenchyma enhancement phase, also called an equilibrium phase, obtained several minutes after the start of contrast injection, is useful.

The literature is inconsistent about defining biphasic and triphasic CT imaging. Some authors include a precontrast phase as part of these terms, but others do not. In this book a precontrast phase is not included as part of either biphasic or triphasic imaging, and the use of these terms refers to postcontrast phases only. Even here confusion exists; does“biphasic” refer to the arterial, portal venous or equilibrium phases (or any other phase for that matter)? The term double arterial phase imaging signifies that early and late arterial phase images are obtained during a single breath-hold study. Some use quadruple phase to mean that images are obtained precontrast and at three times after the start of contrast injection. No sharp boundary exists between various phases. Ideally, authors should include the specific times after the start of injection when scanning is initiated.

Computed tomography CT data are viewed either as traditional transverse images or displayed in coronal, sagittal, or 3D projections. The latter allows a direct estimate of tumor size, information at times useful to the oncologist or surgeon.

Computed Tomography Angiography

Computed tomography angiography (CTA) is a general term used in a sense similar to conventional angiography but is often applied to a technique of injecting IV contrast and obtaining images during the arterial phase. Use of

ADVANCED IMAGING OF THE ABDOMEN

multidetector CT is especially well suited for such vascular studies.

Computed tomography angiography can also be performed with contrast injected through a catheter advanced into a major abdominal artery; whether one obtains CT arterial portography or CT hepatic arteriography (or any other specific vessel angiography) depends on the artery used and image timing.

Computed Tomography

Arterial Portography

Computed tomography arterial portography consists of angiographic placement of a catheter in the superior mesenteric artery or splenic artery, transfer of the patient to a CT suite, injection of a contrast bolus through the intraarterial catheter, followed by liver imaging during the portal venous phase. This technique maximizes attenuation differences between a neoplasm having a primarily arterial blood supply and normal liver parenchyma primarily supplied by the portal vein. It is superior to the usual intravenous contrast-enhanced CT imaging; over 80% of tumors <1cm in diameter are imaged. It is used in some centers preoperatively for anatomic localization of lesions and in evaluating whether a patient is indeed a surgical candidate. A refinement of this technique consists of a dual-phase study: during the first phase images are obtained about 30 seconds after the start of the contrast injection, and second-phase images are obtained at 70 seconds.

In general, there is no difference in hepatic enhancement whether CT arterial portography is performed via the superior mesenteric artery or the splenic artery. In the presence of an anomalous right hepatic artery originating from the superior mesenteric artery, the catheter needs to be positioned distal to this site. Computed tomography arterial portography is of limited use in patients with portal hypertension and collateral portal blood flow away from the liver.

Altered blood flow anomalies result in a number of artifacts during arterial portography. Inhomogeneous perfusion is most common near the porta hepatis, falciform ligament, and gallbladder. Perfusion defects can either resemble a neoplasm or even mask the presence of one. A liver zebra pattern, consisting of alternating regions of hyperand hypoperfusion,

LIVER

is identified in some patients, regardless of whether the splenic artery or superior mesenteric artery is injected.

Computed tomography arterial portography is an invasive procedure and was more in vogue during the 1990s. It has a high sensitivity for detecting lesions but a low specificity for diagnosis due to numerous false-positive findings. Currently, it is gradually being supplanted by other advances in CT and magnetic resonance (MR) technology.

Computed Tomography

Hepatic Arteriography

Computed tomography hepatic arteriography consists of contrast injected through a catheter placed in the hepatic artery and arterial phase images obtained. Tumors having primarily an arterial blood supply are thus enhanced compared to liver parenchyma, which obtains a large part of its blood supply from the portal vein. This technique is also prone to artifacts because liver parenchyma tends not to enhance homogeneously, mostly due to blood supply anomalies. These artifacts (often called pseudolesions), especially around the gallbladder fossa, can be minimized by first injecting prostaglandin into the superior mesenteric artery (1).

An additional refinement is combined CT arterial portography and CT hepatic angiography in an attempt to improve lesion detection.

Contrast Agents

A biphasic CT study typically consists of continuous IV injection of 60 to 200mL of a contrast agent at a rate of up to 5mL per second. In general, a uniphasic injection is superior to biphasic injection. The degree of liver enhancement depends on the amount of iodine delivered. Ionic and nonionic contrast agents provide the same degree of contrast enhancement. The nonionic agents, however, are associated with less patient discomfort, fewer motion artifacts, and fewer side effects, and are preferred over ionic agents, especially for more complex examinations such as 3D reconstruction where minimizing patient motion is paramount.

Portal venous blood flow increases after a meal. Thus an indirect method of increasing liver CT portal contrast enhancement would be

to perform a study shortly after a meal, but this adds further complexity.

How common is hepatobiliary excretion of vascular contrast media? Functioning hepatocytes excrete small amounts of contrast material into the biliary system. Several-hour delayed images obtained after arterial portography result in enhancement of normal liver parenchyma; non–iodine-excreting neoplasms tend to appear hypodense. Gallbladder opacification is occasionally seen after CT angiography in patients with cirrhosis or other liver disease.

The use of iodized oil during CT is essentially limited to the study of hepatocellular carcinomas, and this topic is covered later (see Hepatocellular Carcinoma).

Ultrasonography

Conventional (Gray Scale)

Most ultrasonography (US) imaging consists of real-time, gray-scale, B-mode display, with the reflected signal amplitude displayed as a variation in brightness. This technique is referred to as gray-scale US or simply as US to distinguish it from Doppler US. The transducer is most often in contact with a patient’s skin (conventional US), although it can be placed in body cavities (endoluminal US) either directly or endoscopically (endoscopic US or endosonography).

As with CT and MR, it is hoped that continued advances in US equipment design will lead to more accurate diagnoses. For instance, the introduction of tissue and contrast harmonic imaging with stimulated acoustic emission has resulted in clearer images and improved tumor detection compared to more conventional US.

One novel approach is the use of two frequencies. Preliminary data suggest that metastases have an increase in contrast-to-noise ratios at higher frequencies, in distinction to hemangiomas, which have a decreased ratio.

Intraoperative

Special transducers are available for laparoscopic US. Introduced through a 10-mm laparoscopic port, a flexible transducer tip allows contact with the curved liver surface. The oftenused 7-MHz transducer provides only limited penetration of the liver; a 5-MHz transducer