FORUMS
IN G A S T R O I N T E S T I N A L
ROENTGENOLOGY
Radiology Editors: WYLIE J. DODDS, MD, AND HENRY L. GOLDBERG,MD Consultants: WALTERJ. HOGAN, MD, ROBERT K. OCKNER, MD, AND RICHARD L. WECHSLER, MD
Ultrasonic Diagnosis of Abdominal Ab scess BRUCE D. DOUST, MB, and v I V i E N N E L. DOUST, MB
Ultrasound is high-frequency mechanical vibration. It is completely unrelated to x rays, radio waves, or gamma rays, The frequency of ultrasound most commonly used for abdominal diagnosis is 2.25 million cycles per second. H i g h e r frequencies give higher resolution but fail to penetrate to the depths of the abdomen so that deeply situated lesions are not adequately demonstrated. The vast majority of currently available scanners produce images that are two-dimensional displays of structures that reflect sound. The scanner builds up a two-dimensional laminographic picture as the transducer, which both generates and receives the sound, is swept across the area under investigation. The most striking advantage of ultrasonic images is that they clearly distinguish between solid soft tissue and fluid-filled structures. A solid mass contains echoing elements, whereas a fluid-filled structure, whether it be a renal cyst or an abscess, or the abdominal aorta, is echo-free. Modern scanners are also capable of demonstrating fine differences in the acoustical characteristics of solid tissues. Bowel gas and air-containing lung, barium sulfate, and trabecular bone all reflect the sonic energy striking them so that sound does not reach deeper structures, which therefore are not imaged. Thus, the esophagus and thoracic aorta, surrounded by the lungs and the vertebral column, cannot be examined; the aorta cannot be examined through the vertebral column, and abdominal ultrasonic examinations cannot be satisfactorily performed on patients who have large amounts of intestinal barium sulfate or gas. Recently, diagnostic ultrasonic scanners which display information in eight shades of grey have beFrom the Department of Radiology, Medical College of Wisconsin, Milwaukee, Wisconsin and Veterans Administration Hospital, Wood, Wisconsin. Address for reprint requests: Dr. Bruce D. Doust, Department of Radiology, Medical College of Wisconsin, 8700 West Wisconsin Avenue, Milwaukee, Wisconsin 53226. Digestive Diseases, Vol. 21, No. 7 (July 1976)
come commercially available. Older instruments displayed the images in black and white only, with no grey shades, and the images contained considerably less diagnostic information. Grey-scale scanners represent a major advance in diagnostic ultrasound technology. Ultrasound beams of the intensities used in diagnostic work do not produce ionization of the tissue through which they pass, and there are no known hazards to ultrasonic investigation. It is conceivable that at some future time some undesirable effect of low-energy ultrasound may be demonstrated, so that frivolous use of diagnostic ultrasound investigation is best avoided. However, provided there is a reasonable chance that the patient will benefit from the study, repeated examinations can be performed without fear. Pregnancy is not a contraindication to ultrasonic examination. TECHNIQUE The skin of the patient's abdomen is covered with aqueous jelly or mineral oil to ensure airless contact between the transducer and the skin. Without airless contact, sound cannot be transmitted into the body nor echoes received, thus no image is formed. A minimal amount of cover is left over open surgical wounds. It is not necessary to bring the transducer into contact with an open wound as the area deep to the wound can be examined by an oblique approach through the adjacent intact skin. In all cases both transverse and longitudinal scans should be performed. Whenever a subphrenic or perinephric abscess is suspected, the patient should also be examined through the kidneys in order to detect fluid collections in the posterior subphrenic and subhepatic spaces--regions that are poorly demonstrated by the anterior approach. If it is not possible for the patient to lie prone, this part of the examination may have to be done with the patient in the lateral
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Fig 1. Intrahepatic abscess. This transverse section through the liver shows the abscess (A) as an area containing relatively few echoes, with an adjacent strongly echoing zone which probably represents compressed liver parenchyma. Intrahepatic abscesses are in general spherical so that they appear circular in both transverse and longitudinal scans.
decubitus position. When excessive bowel gas or barium prevents an adequate examination, a repeat examination on the following day usually provides images of areas previously obscured by bowel gas. Simethicone has been recommended as a means of reducing the amount of bowel gas, but to be effective, this has to be given for several days. Where a pelvic abscess is suspected, it is best if the patient's bladder is distended. A distended bladder displaces bowel loops out of the pelvis, acts as an anatomical reference point and also provides an ultrasonic standard of fluidity against which a lesion can be compared. In general, it is inadvisable to interpret an ultrasonic study on the basis of the pictures alone, since some of the information obtained during the study cannot be adequately recorded in the pictures. In this respect, ultrasonic examinations are similar to fluoroscopy. Ultrasonic e x a m i n a t i o n s require very little patient cooperation. Suspension of respiration is an advantage, especially in examinations of the liver and kidneys. However, adequate studies can be obtained even when there is pronounced tachypnea. An ultrasonic image takes from 5 to 30 sec to pro570
duce. Most ultrasound abdominal studies can be completed in 20 min or less. However, difficult studies may take as long as 45 min. Examination for intraabdominal abscess in patients with no localizing signs are particularly laborious. U L T R A S O N I C PROPERTIES OF ABSCESSES
IN GENERAL Fluid collections are echo-free, well-defined structures which conduct sound very well. Thus, echoes from structures deep to an abscess (or any fluid collection) are much stronger than echoes from structures at the same depth that are overlaid by solid tissue. There are exceptions to these general rules. For instance, abscesses may contain debris, particularly in the dependent part of the abscess, which produces echoes in parts of the abscess, but which does not affect the overall echo-free quality of the fluid collection. Although the walls of an abscess are usually well defined, they are not, in general, as sharply defined as the walls of fluid-filled cavities that have a preformed membrane for their walls. That is, the walls of abscesses are less sharp Digestive Diseases, Vol. 21, No. 7 (July 1976)
ULTRASONIC DIAGNOSIS OF ABDOMINAL ABSCESS
Fig 2. Subhepatic fluid collection (not pus). This is a longitudinal scan (cranial to the left, caudal to the right) about 8 cm to the right of the midline in a patient who had recently undergone bilateral nephrectomy for polycystic kidneys. The fluid collection (C) is behind and caudal to the liver (L) and is separate from the gallbladder (arrow). Its outline is sharply defined, but irregular.
than those of renal cysts or pockets of ascites, or other noninflammatory fluids. Compare Figures 1 and 4 with Figure 2. Abscesses are usually ellipsoidal, that is, football-shaped. While there are obviously exceptions, the approximation is useful because it allows a simple determination of the volume of the abscess (1). INTRAHEPATIC ABSCESSES
Most of the liver can be demonstrated in conventional longitudinal and transverse scans. However, the uppermost parts of the right lobe must be examined by a subcostal, angled approach. The transducer is angled steeply up under the right costal margin from both the front and the back, because the overlying lung blocks sound transmission, preventing a direct antero-posterior approach (2). Intrahepatic abscesses may be multiple so that it is important to examine the entire liver with some care, even when a l a r g e abscess is found early in the Digestive Diseases, Vol. 21, No. 7 (July 1976)
course of the examination. The walls of amebic abscesses (Figure 1) may be less well defined than those of pyogenic abscesses (3). Failure to demonstrate an abscess is good evidence that the liver does not contain an abscess larger than 2 or 3 cm in diameter. Some investigators have reported demonstration of intrahepatic structures as small as 3 mm in diameter using a specially built ultrasonic scanner (4). The newer grey-scale scanners may make lesions smaller than 2 cm in diameter regularly demonstrable, but at present the limit of reliable resolution has not been determined. RIGHT-SIDED SUBPHRENIC ABSCESSES
Right-sided subphrenic abscesses may be purely subphrenic or may have an associated intrahepatic component. The key to differentiating between a purely intrahepatic abscess high in the right lobe of the liver and a subphrenic abscess is the demonstration of fluid outside the liver capsule (1). Where there is a subphrenic, ie, extrahepatic component, 57 1
DOUST
AND DOUST
Fig 3. Left subphrenic abscess. This is a longitudinal scan about 3 cm medial to the left posterior axillary line. The patient is sitting and facing the left of the picture. At the top of the picture the chest wall shows a scalloped appearance due to artifacts from ribs. At the bottom, a longitudinal section through the left kidney (k) can be seen. The renal cortex (black) surrounds the pelvi-calyceal system (white). Immediately above the upper pole of the left kidney is a subphrenic abscess (a). Its appearance is indistinguishable from that of the spleen. However, this patient was known to have undergone splenectomy. The left dome of the diaphragm (--*) is incompletely demonstrated.
the liver is displaced medially from the lateral abdominal wall and downward from the diaphragm by an echo-free zone. Confusion can arise if there is only a small amount of fluid, which may be difficult to recognize because of reverberations produced by overlying ribs. SUBHEPATIC ABSCESS
Subhepatic abscesses are not unlike those situated in the general peritoneal cavity, but are easily confused with the gallbladder. Usually the characteristic shape of the gallbladder, even when distended, will allow differentiation. Demonstration of two fluid collections in the subhepatic region is proof positive that at least one of them is not the gallbladder (Figure 2). Occasionally gallstones can be demonstrated within a distended gallbladder (57). Occasionally, it may be useful to give the patient
572
a fatty meal to study for evidence of contraction in a subhepatic fluid collection. Usually, however, the patient's clinical condition makes this procedure impractical. LEFT-SIDED SUBPHRENIC ABSCESSES
The most difficult area of the abdomen in which to diagnose abscess is the left subphrenic area. Access to this region is difficult because of the aerated lung above, the gas-filled stomach and splenic flexure anteriorly, and the overlying ribs laterally. The best approach is through the left lower ribs posteriorly and obliquely upward through the left kidney and the spleen. Diagnostic difficulties arise because the spleen is of variable shape and size and may contain few echoes. On the other hand, reverberations from overlying ribs or debris within the fluid may interfere with the identification of a left-sided subDigestive Diseases, Vol. 21, No. 7 (July 1976)
ULTRASONIC
DIAGNOSIS
OF ABDOMINAL
ABSCESS
Fig 4. Pelvic abscess. (A) This 28-yr-old male became febrile about 10 days after surgical repair of a perforation of the descending colon. The longitudinal midline scan (cranial to the left, caudal to the right) shows the rounded, echo-free abscess (A) immediately behind the urinary bladder (B). (B) This is a transverse scan performed 4 cm above the pubic symphysis, with the transducer angled obliquely downward into the pelvis. The abscess (A) appears oval in this section (B = bladder).
Digestive Diseases, Vol. 21, No. 7 (July 1976)
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Fig 5. Right-sided pleural effusion. This longitudinal scan was taken in the right posterior axillary line. The top of the picture is at the level of the patient's shoulder. The solid vertical curved line to the right of the picture represents the surface of the postero-lateral aspect of the right chest wall. Deeper structures are to the left of this. The diaphragm (--->) separates a subpulmonic effusion (e) above, from the liver (L) below. Differentiation of pleural fluid from subphrenic collections is simple provided the diaphragm can be demonstrated. Identification of the diaphragm may be ditficult when there is much fibrosis at the lung base.
phrenic fluid collection. Therefore, it is important to know whether the patient has undergone a splenectomy prior to the examination. An appearance similar to that of a normal spleen in a patient who has undergone splenectomy is highly suggestive of left subphrenic abscess (Figure 3). When the spleen is still in place, a clearly defined junction between the spleen and the fluid collection must be demonstrated before it is safe to diagnose a left subphrenic fluid collection. Otherwise, the echo-free zone under the left dome of the diaphragm is probably the spleen alone. Diagnostic difficulty can sometimes be overcome by comparing the size of the spleen as seen on radionuclide scan to that of the echo-free left subphrenic region seen on the ultrasonic study.
pelvic contents can be examined by an oblique suprapubic approach provided the bladder is distended (Figures 4a and b). The urinary bladder may occasionally be mistaken for an abnormal pelvic fluid collection. A postvoiding film showing a reduction in the volume of a pelvic fluid collection proves that the collection is the bladder and not an abscess. OTHER ABSCESSES
Ultrasonic delineation of an abscess in the anterior abdominal wall has been described (8). The planes of the anterior abdominal wall are disrupted by the echo-free abscess. Renal carbuncles can also be demonstrated ultras o n i c a l l y . T h e y m a y be u l t r a s o n i c a l l y indistinguishable from a solid intrarenal lesion, such as a hypernephroma (9).
PELVIC ABSCESSES Ultrasonic access to the pelvic cavity is limited by the bony pelvis, which prevents examination from the side and from the direct AP approach. The
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DIFFERENTIAL DIAGNOSIS
Ultrasonic investigation is helpful in deciding whether or not a patient with signs of sepsis reDigestive Diseases, Vol. 21, No. 7 (July 1976)
ULTRASONIC DIAGNOSIS OF ABDOMINAL ABSCESS quires surgical drainage of an abscess. However, not all fluid collections are abscesses and many fluid collections do not require surgical drainage so that some caution in the diagnosis of abscess is advisable. An abscess must be distinguished from a fluidfilled normal hollow viscus particularly the gallbladder (Figure 2). Fluid in a distended bowel loop may rarely cause confusion. At least three reports (1012) describe distended hollow viscera having appearances similar to those of an abscess. Bowel loops almost always contain a little gas or are filled with echo-producing material and scarcely ever remain unchanged in appearance throughout the duration of the ultrasonic investigation. Other types of intraabdominal fluid cannot usually be distinguished purely on the basis of the ultrasonic appearances (13). Distinction between an abscess and a urinoma or hematoma is not particularly important since these conditions usually require surgical drainage. Distinction from ascites, however is important. Massive ascites appears as an echo-free crescent on either side of the mesentery and small bowel (14). Confusion between massive ascites and abscess is not likely. However, difficulty may arise when the ascites is loculated. The presence of fluid lateral to the liver and the widespread nature of the process generally allows distinction of ascites from multiple abscesses. When there is a right-sided, extrahepatic fluid collection, it is important to decide whether it represents a subphrenic abscess with lateral extension, or purely ascites. Examination of the subphrenic area and the remainder of the abdomen usually clarifies the problem. Change in the depth of the fluid collection with change in posture may provide a clue and provides an estimate of the volume of ascites (15). Differentiation from pleural effusion depends on identification of the diaphragm (Figure 5), best achieved by longitudinal scans in the decubitus or upright positions. Time-motion (M-mode) scans with the transducer idented below the costal margin and pointing crainially can also be used to demonstrate diaphragmatic position (16, 17). Differentiation from lymphoceles is based on the clinical picture and the history of kidney transplant (18). Differentiation of abscesses from pancreatic pseudocysts is not usually a problem because of the differences in the mode of clinical presentation. The nature of the fluid collection can be further clarified by needle aspiration. Ultrasonic guidance Digestive Diseases, Vol. 21, No. 7 (July 1976)
of needle puncture is easy and effective (1%21). Although published reports suggest that needle aspiration of a b s c e s s e s and h e m a t o m a s is safe, insufficient published data exist at present to estimate its morbidity with certainty.
RELATIONSHIP OF ULTRASOUND TO
OTHER DIAGNOSTIC MODALITIES Ultrasonic investigation is particularly helpful in distinguishing between fluid-filled and solid lesions. In a small minority of cases a plain x ray of the abodomen will demonstrate an abscess by virtue of the gas within it. Sometimes decubitus films of the upper abdomen taken after oral administration of barium sulphate will convincingly outline a left subphrenic abscess (22). Barium studies and IVP examinations may demonstrate displacement of normal structures, but the displacement must be so large that it can be confidently recognized as abnormal. Even then, the study does not identify the nature of the lesion. Visceral angiography can demonstrate abscesses, sometimes with great clarity. However, it is the most traumatic and expensive method available short of exploratory surgery. ~7Ga gallium scanning does not distinguish between inflammatory tissue alone and an abscess containing drainable fluid. It also does not provide anatomical landmarks, and there is an appreciable time delay (1-2 days) between the administration of the radionuclide and the time of the Gallium scan. ~TGa gallium may be useful in distinguishing inflammatory from noninflammatory fluid collections that have been previously demonstrated ultrasonically. 99mTc technetium sulfur colloid scans can be used to demonstrate filling defects within the liver and, in combination with technetium-labeled macroaggregated albumen, can be used to demonstrate separation of the liver from the right lung in patients with subphrenic abscess. The fluid nature of an intrahepatic defect is not identified by radionuclide scans, and the position of the diaphragm relative to fluid collections cannot be ascertained from the liver-lung scan. The value of computerized axial tomography in the diagnosis of abscess is not yet clear. Overall, ultrasonic examination provides the best currently available means of identifying, localizing, and following the progress of intraabdominal fluid collections. Reported accuracy varies (1, 23) but an accuracy in excess of 90% is probable.
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REFERENCES 1. Maklad NF, Doust BD, Baum JK: Ultrasonic diagnosis of postoperative intraabdominal abscess. Radiology 113:417422, 1974 2. Leyton B, Halpern S, Leopold G, Hagen S: Correlation of ultrasound and colloid scintiscan studies of the normal and diseased liver. J Nucl Med 14:27-33, 1973 3. Matthews AW, Gough KR, Davies ER, Ross FGM, Hinchliffe A: The use of combined ultrasonic and isotope scanning in the diagnosis of amoebic liver disease. Gut 14:50-53, 1973 4. Taylor K J, Carpenter DA, McCready VR: Ultrasound and scintigraphy in the differential diagnosis of obstructive jaundice. J Clin Ultrasound 2:105-I 16, 1974 5. Doust BD, Maklad NF: Ultrasonic B-mode examination of the gallbladder. Radiology 110:643-647, 1974 6. Goldberg BB, Harris K, Broocker W: Ultrasonic and radiographic cholecystography radiology 111:405-409, 1974 7. Tabrisky J, Lindstrom RR, Herman MW, Castagna J, Sarti D: Value of gallbladder B-scan ultrasonography. Gastroenterology 68:1246-1252, 1975 8. Weiner CI, Diaconis JN: Primary abdominal wall abscess diagnosed by ultrasound. Arch Surg 110:341-342, 1975 9. Pedersen JF, Hancke S, Kristensen K: Renal carbuncle: Antiobiotic therapy governed by ultrasonically guided aspiration. J Urol 109:77%778, 1973 10. Hauser JB, Stanley RJ, Geisse G: The ultrasound findings in an obstructed afferent loop. J Clin Ultrasound 2:287-289, 1974 11. Holm HH, Rasmussen SN, Kristensen JK: Errors and pitfalls in ultrasonic scanning of the abdomen. Br J Radiol 45:835-840, 1972 12. Jensen F, Pedersen JF: The value of ultrasonic scanning in
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13.
14. 15.
16. 17.
18.
19.
20.
21. 22.
23.
the diagnosis of intraabdominal abscesses and hematomas. Surg Gynecol Obstet 139:326-328, 1974 Kaplan GN, Sanders RC: B-Scan ultrasound in the management of patients with occult abdominal hematomas. J Clin Ultrasound 1:5-13, 1973 HOnig R, Kinser J: The diagnosis of ascites by ultrasonic tomography (B-scan) Br J Radio146:325-328, 1973 Goldberg BB, C!earfield HR, Goodman GA, Morales JO: Ultrasonic determination of ascites. Arch Int Med 131:217220, 1973 Doust BD, Baum .IK, Maklad NF, Doust VL: Ultrasonic evaluation of pleural opacities radiology 114:135-140, 1975 Haber K, Asher WM, Freimanis AK: Echograpbic evaluation of diaphragmatic motion in intraabdominal disease. Radiology 114:141-144, 1975 Morley P, Barnett E, Bell PR, Briggs JK, Caiman KC. Hamilton DN, Paton AM: Ultrasound in the diagnosis of fluid collections following renal transplantation. Clin Radiol 26:199-207, 1975 Smith EH, Bartrum RJ: Ultrasonically guided percutaneous aspiration of abscesses. Am J Roentgenol Radium Ther Nucl Med 122:308-312, 1974 Holm HH, Rasmussen SN, Kristensen JK: Ultrasonically guided percutaneous puncture technique. J Clin Ultrasound 1:2%31, 1973 Goldberg BB, Pollack HM: Ultrasonic aspiration-biopsy transducer. Radiology 108:667-671, 1973 Sanders RC: Radiological and radioisotopic diagnosis of perihepatic abscess. CRC Crit Rev Clin Radiol Nucl Med 5: !65-211, 1974 Hill BA, Yamaguchi K, Flynn J J, Miller DR: Diagnostic sonography in general surgery. Arch Surg 110:1089-1094, 1975
Digestive Diseases, Vol. 21, No. 7 (July 1976)
IN G A S T R O I N T E S T I N A L
ROENTGENOLOGY
Radiology Editors: WYLIE J. DODDS, MD, AND HENRY L. GOLDBERG,MD Consultants: WALTERJ. HOGAN, MD, ROBERT K. OCKNER, MD, AND RICHARD L. WECHSLER, MD
Ultrasonic Diagnosis of Abdominal Ab scess BRUCE D. DOUST, MB, and v I V i E N N E L. DOUST, MB
Ultrasound is high-frequency mechanical vibration. It is completely unrelated to x rays, radio waves, or gamma rays, The frequency of ultrasound most commonly used for abdominal diagnosis is 2.25 million cycles per second. H i g h e r frequencies give higher resolution but fail to penetrate to the depths of the abdomen so that deeply situated lesions are not adequately demonstrated. The vast majority of currently available scanners produce images that are two-dimensional displays of structures that reflect sound. The scanner builds up a two-dimensional laminographic picture as the transducer, which both generates and receives the sound, is swept across the area under investigation. The most striking advantage of ultrasonic images is that they clearly distinguish between solid soft tissue and fluid-filled structures. A solid mass contains echoing elements, whereas a fluid-filled structure, whether it be a renal cyst or an abscess, or the abdominal aorta, is echo-free. Modern scanners are also capable of demonstrating fine differences in the acoustical characteristics of solid tissues. Bowel gas and air-containing lung, barium sulfate, and trabecular bone all reflect the sonic energy striking them so that sound does not reach deeper structures, which therefore are not imaged. Thus, the esophagus and thoracic aorta, surrounded by the lungs and the vertebral column, cannot be examined; the aorta cannot be examined through the vertebral column, and abdominal ultrasonic examinations cannot be satisfactorily performed on patients who have large amounts of intestinal barium sulfate or gas. Recently, diagnostic ultrasonic scanners which display information in eight shades of grey have beFrom the Department of Radiology, Medical College of Wisconsin, Milwaukee, Wisconsin and Veterans Administration Hospital, Wood, Wisconsin. Address for reprint requests: Dr. Bruce D. Doust, Department of Radiology, Medical College of Wisconsin, 8700 West Wisconsin Avenue, Milwaukee, Wisconsin 53226. Digestive Diseases, Vol. 21, No. 7 (July 1976)
come commercially available. Older instruments displayed the images in black and white only, with no grey shades, and the images contained considerably less diagnostic information. Grey-scale scanners represent a major advance in diagnostic ultrasound technology. Ultrasound beams of the intensities used in diagnostic work do not produce ionization of the tissue through which they pass, and there are no known hazards to ultrasonic investigation. It is conceivable that at some future time some undesirable effect of low-energy ultrasound may be demonstrated, so that frivolous use of diagnostic ultrasound investigation is best avoided. However, provided there is a reasonable chance that the patient will benefit from the study, repeated examinations can be performed without fear. Pregnancy is not a contraindication to ultrasonic examination. TECHNIQUE The skin of the patient's abdomen is covered with aqueous jelly or mineral oil to ensure airless contact between the transducer and the skin. Without airless contact, sound cannot be transmitted into the body nor echoes received, thus no image is formed. A minimal amount of cover is left over open surgical wounds. It is not necessary to bring the transducer into contact with an open wound as the area deep to the wound can be examined by an oblique approach through the adjacent intact skin. In all cases both transverse and longitudinal scans should be performed. Whenever a subphrenic or perinephric abscess is suspected, the patient should also be examined through the kidneys in order to detect fluid collections in the posterior subphrenic and subhepatic spaces--regions that are poorly demonstrated by the anterior approach. If it is not possible for the patient to lie prone, this part of the examination may have to be done with the patient in the lateral
569
DOUST AND DOUST
Fig 1. Intrahepatic abscess. This transverse section through the liver shows the abscess (A) as an area containing relatively few echoes, with an adjacent strongly echoing zone which probably represents compressed liver parenchyma. Intrahepatic abscesses are in general spherical so that they appear circular in both transverse and longitudinal scans.
decubitus position. When excessive bowel gas or barium prevents an adequate examination, a repeat examination on the following day usually provides images of areas previously obscured by bowel gas. Simethicone has been recommended as a means of reducing the amount of bowel gas, but to be effective, this has to be given for several days. Where a pelvic abscess is suspected, it is best if the patient's bladder is distended. A distended bladder displaces bowel loops out of the pelvis, acts as an anatomical reference point and also provides an ultrasonic standard of fluidity against which a lesion can be compared. In general, it is inadvisable to interpret an ultrasonic study on the basis of the pictures alone, since some of the information obtained during the study cannot be adequately recorded in the pictures. In this respect, ultrasonic examinations are similar to fluoroscopy. Ultrasonic e x a m i n a t i o n s require very little patient cooperation. Suspension of respiration is an advantage, especially in examinations of the liver and kidneys. However, adequate studies can be obtained even when there is pronounced tachypnea. An ultrasonic image takes from 5 to 30 sec to pro570
duce. Most ultrasound abdominal studies can be completed in 20 min or less. However, difficult studies may take as long as 45 min. Examination for intraabdominal abscess in patients with no localizing signs are particularly laborious. U L T R A S O N I C PROPERTIES OF ABSCESSES
IN GENERAL Fluid collections are echo-free, well-defined structures which conduct sound very well. Thus, echoes from structures deep to an abscess (or any fluid collection) are much stronger than echoes from structures at the same depth that are overlaid by solid tissue. There are exceptions to these general rules. For instance, abscesses may contain debris, particularly in the dependent part of the abscess, which produces echoes in parts of the abscess, but which does not affect the overall echo-free quality of the fluid collection. Although the walls of an abscess are usually well defined, they are not, in general, as sharply defined as the walls of fluid-filled cavities that have a preformed membrane for their walls. That is, the walls of abscesses are less sharp Digestive Diseases, Vol. 21, No. 7 (July 1976)
ULTRASONIC DIAGNOSIS OF ABDOMINAL ABSCESS
Fig 2. Subhepatic fluid collection (not pus). This is a longitudinal scan (cranial to the left, caudal to the right) about 8 cm to the right of the midline in a patient who had recently undergone bilateral nephrectomy for polycystic kidneys. The fluid collection (C) is behind and caudal to the liver (L) and is separate from the gallbladder (arrow). Its outline is sharply defined, but irregular.
than those of renal cysts or pockets of ascites, or other noninflammatory fluids. Compare Figures 1 and 4 with Figure 2. Abscesses are usually ellipsoidal, that is, football-shaped. While there are obviously exceptions, the approximation is useful because it allows a simple determination of the volume of the abscess (1). INTRAHEPATIC ABSCESSES
Most of the liver can be demonstrated in conventional longitudinal and transverse scans. However, the uppermost parts of the right lobe must be examined by a subcostal, angled approach. The transducer is angled steeply up under the right costal margin from both the front and the back, because the overlying lung blocks sound transmission, preventing a direct antero-posterior approach (2). Intrahepatic abscesses may be multiple so that it is important to examine the entire liver with some care, even when a l a r g e abscess is found early in the Digestive Diseases, Vol. 21, No. 7 (July 1976)
course of the examination. The walls of amebic abscesses (Figure 1) may be less well defined than those of pyogenic abscesses (3). Failure to demonstrate an abscess is good evidence that the liver does not contain an abscess larger than 2 or 3 cm in diameter. Some investigators have reported demonstration of intrahepatic structures as small as 3 mm in diameter using a specially built ultrasonic scanner (4). The newer grey-scale scanners may make lesions smaller than 2 cm in diameter regularly demonstrable, but at present the limit of reliable resolution has not been determined. RIGHT-SIDED SUBPHRENIC ABSCESSES
Right-sided subphrenic abscesses may be purely subphrenic or may have an associated intrahepatic component. The key to differentiating between a purely intrahepatic abscess high in the right lobe of the liver and a subphrenic abscess is the demonstration of fluid outside the liver capsule (1). Where there is a subphrenic, ie, extrahepatic component, 57 1
DOUST
AND DOUST
Fig 3. Left subphrenic abscess. This is a longitudinal scan about 3 cm medial to the left posterior axillary line. The patient is sitting and facing the left of the picture. At the top of the picture the chest wall shows a scalloped appearance due to artifacts from ribs. At the bottom, a longitudinal section through the left kidney (k) can be seen. The renal cortex (black) surrounds the pelvi-calyceal system (white). Immediately above the upper pole of the left kidney is a subphrenic abscess (a). Its appearance is indistinguishable from that of the spleen. However, this patient was known to have undergone splenectomy. The left dome of the diaphragm (--*) is incompletely demonstrated.
the liver is displaced medially from the lateral abdominal wall and downward from the diaphragm by an echo-free zone. Confusion can arise if there is only a small amount of fluid, which may be difficult to recognize because of reverberations produced by overlying ribs. SUBHEPATIC ABSCESS
Subhepatic abscesses are not unlike those situated in the general peritoneal cavity, but are easily confused with the gallbladder. Usually the characteristic shape of the gallbladder, even when distended, will allow differentiation. Demonstration of two fluid collections in the subhepatic region is proof positive that at least one of them is not the gallbladder (Figure 2). Occasionally gallstones can be demonstrated within a distended gallbladder (57). Occasionally, it may be useful to give the patient
572
a fatty meal to study for evidence of contraction in a subhepatic fluid collection. Usually, however, the patient's clinical condition makes this procedure impractical. LEFT-SIDED SUBPHRENIC ABSCESSES
The most difficult area of the abdomen in which to diagnose abscess is the left subphrenic area. Access to this region is difficult because of the aerated lung above, the gas-filled stomach and splenic flexure anteriorly, and the overlying ribs laterally. The best approach is through the left lower ribs posteriorly and obliquely upward through the left kidney and the spleen. Diagnostic difficulties arise because the spleen is of variable shape and size and may contain few echoes. On the other hand, reverberations from overlying ribs or debris within the fluid may interfere with the identification of a left-sided subDigestive Diseases, Vol. 21, No. 7 (July 1976)
ULTRASONIC
DIAGNOSIS
OF ABDOMINAL
ABSCESS
Fig 4. Pelvic abscess. (A) This 28-yr-old male became febrile about 10 days after surgical repair of a perforation of the descending colon. The longitudinal midline scan (cranial to the left, caudal to the right) shows the rounded, echo-free abscess (A) immediately behind the urinary bladder (B). (B) This is a transverse scan performed 4 cm above the pubic symphysis, with the transducer angled obliquely downward into the pelvis. The abscess (A) appears oval in this section (B = bladder).
Digestive Diseases, Vol. 21, No. 7 (July 1976)
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DOUST AND DOUST
Fig 5. Right-sided pleural effusion. This longitudinal scan was taken in the right posterior axillary line. The top of the picture is at the level of the patient's shoulder. The solid vertical curved line to the right of the picture represents the surface of the postero-lateral aspect of the right chest wall. Deeper structures are to the left of this. The diaphragm (--->) separates a subpulmonic effusion (e) above, from the liver (L) below. Differentiation of pleural fluid from subphrenic collections is simple provided the diaphragm can be demonstrated. Identification of the diaphragm may be ditficult when there is much fibrosis at the lung base.
phrenic fluid collection. Therefore, it is important to know whether the patient has undergone a splenectomy prior to the examination. An appearance similar to that of a normal spleen in a patient who has undergone splenectomy is highly suggestive of left subphrenic abscess (Figure 3). When the spleen is still in place, a clearly defined junction between the spleen and the fluid collection must be demonstrated before it is safe to diagnose a left subphrenic fluid collection. Otherwise, the echo-free zone under the left dome of the diaphragm is probably the spleen alone. Diagnostic difficulty can sometimes be overcome by comparing the size of the spleen as seen on radionuclide scan to that of the echo-free left subphrenic region seen on the ultrasonic study.
pelvic contents can be examined by an oblique suprapubic approach provided the bladder is distended (Figures 4a and b). The urinary bladder may occasionally be mistaken for an abnormal pelvic fluid collection. A postvoiding film showing a reduction in the volume of a pelvic fluid collection proves that the collection is the bladder and not an abscess. OTHER ABSCESSES
Ultrasonic delineation of an abscess in the anterior abdominal wall has been described (8). The planes of the anterior abdominal wall are disrupted by the echo-free abscess. Renal carbuncles can also be demonstrated ultras o n i c a l l y . T h e y m a y be u l t r a s o n i c a l l y indistinguishable from a solid intrarenal lesion, such as a hypernephroma (9).
PELVIC ABSCESSES Ultrasonic access to the pelvic cavity is limited by the bony pelvis, which prevents examination from the side and from the direct AP approach. The
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DIFFERENTIAL DIAGNOSIS
Ultrasonic investigation is helpful in deciding whether or not a patient with signs of sepsis reDigestive Diseases, Vol. 21, No. 7 (July 1976)
ULTRASONIC DIAGNOSIS OF ABDOMINAL ABSCESS quires surgical drainage of an abscess. However, not all fluid collections are abscesses and many fluid collections do not require surgical drainage so that some caution in the diagnosis of abscess is advisable. An abscess must be distinguished from a fluidfilled normal hollow viscus particularly the gallbladder (Figure 2). Fluid in a distended bowel loop may rarely cause confusion. At least three reports (1012) describe distended hollow viscera having appearances similar to those of an abscess. Bowel loops almost always contain a little gas or are filled with echo-producing material and scarcely ever remain unchanged in appearance throughout the duration of the ultrasonic investigation. Other types of intraabdominal fluid cannot usually be distinguished purely on the basis of the ultrasonic appearances (13). Distinction between an abscess and a urinoma or hematoma is not particularly important since these conditions usually require surgical drainage. Distinction from ascites, however is important. Massive ascites appears as an echo-free crescent on either side of the mesentery and small bowel (14). Confusion between massive ascites and abscess is not likely. However, difficulty may arise when the ascites is loculated. The presence of fluid lateral to the liver and the widespread nature of the process generally allows distinction of ascites from multiple abscesses. When there is a right-sided, extrahepatic fluid collection, it is important to decide whether it represents a subphrenic abscess with lateral extension, or purely ascites. Examination of the subphrenic area and the remainder of the abdomen usually clarifies the problem. Change in the depth of the fluid collection with change in posture may provide a clue and provides an estimate of the volume of ascites (15). Differentiation from pleural effusion depends on identification of the diaphragm (Figure 5), best achieved by longitudinal scans in the decubitus or upright positions. Time-motion (M-mode) scans with the transducer idented below the costal margin and pointing crainially can also be used to demonstrate diaphragmatic position (16, 17). Differentiation from lymphoceles is based on the clinical picture and the history of kidney transplant (18). Differentiation of abscesses from pancreatic pseudocysts is not usually a problem because of the differences in the mode of clinical presentation. The nature of the fluid collection can be further clarified by needle aspiration. Ultrasonic guidance Digestive Diseases, Vol. 21, No. 7 (July 1976)
of needle puncture is easy and effective (1%21). Although published reports suggest that needle aspiration of a b s c e s s e s and h e m a t o m a s is safe, insufficient published data exist at present to estimate its morbidity with certainty.
RELATIONSHIP OF ULTRASOUND TO
OTHER DIAGNOSTIC MODALITIES Ultrasonic investigation is particularly helpful in distinguishing between fluid-filled and solid lesions. In a small minority of cases a plain x ray of the abodomen will demonstrate an abscess by virtue of the gas within it. Sometimes decubitus films of the upper abdomen taken after oral administration of barium sulphate will convincingly outline a left subphrenic abscess (22). Barium studies and IVP examinations may demonstrate displacement of normal structures, but the displacement must be so large that it can be confidently recognized as abnormal. Even then, the study does not identify the nature of the lesion. Visceral angiography can demonstrate abscesses, sometimes with great clarity. However, it is the most traumatic and expensive method available short of exploratory surgery. ~7Ga gallium scanning does not distinguish between inflammatory tissue alone and an abscess containing drainable fluid. It also does not provide anatomical landmarks, and there is an appreciable time delay (1-2 days) between the administration of the radionuclide and the time of the Gallium scan. ~TGa gallium may be useful in distinguishing inflammatory from noninflammatory fluid collections that have been previously demonstrated ultrasonically. 99mTc technetium sulfur colloid scans can be used to demonstrate filling defects within the liver and, in combination with technetium-labeled macroaggregated albumen, can be used to demonstrate separation of the liver from the right lung in patients with subphrenic abscess. The fluid nature of an intrahepatic defect is not identified by radionuclide scans, and the position of the diaphragm relative to fluid collections cannot be ascertained from the liver-lung scan. The value of computerized axial tomography in the diagnosis of abscess is not yet clear. Overall, ultrasonic examination provides the best currently available means of identifying, localizing, and following the progress of intraabdominal fluid collections. Reported accuracy varies (1, 23) but an accuracy in excess of 90% is probable.
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REFERENCES 1. Maklad NF, Doust BD, Baum JK: Ultrasonic diagnosis of postoperative intraabdominal abscess. Radiology 113:417422, 1974 2. Leyton B, Halpern S, Leopold G, Hagen S: Correlation of ultrasound and colloid scintiscan studies of the normal and diseased liver. J Nucl Med 14:27-33, 1973 3. Matthews AW, Gough KR, Davies ER, Ross FGM, Hinchliffe A: The use of combined ultrasonic and isotope scanning in the diagnosis of amoebic liver disease. Gut 14:50-53, 1973 4. Taylor K J, Carpenter DA, McCready VR: Ultrasound and scintigraphy in the differential diagnosis of obstructive jaundice. J Clin Ultrasound 2:105-I 16, 1974 5. Doust BD, Maklad NF: Ultrasonic B-mode examination of the gallbladder. Radiology 110:643-647, 1974 6. Goldberg BB, Harris K, Broocker W: Ultrasonic and radiographic cholecystography radiology 111:405-409, 1974 7. Tabrisky J, Lindstrom RR, Herman MW, Castagna J, Sarti D: Value of gallbladder B-scan ultrasonography. Gastroenterology 68:1246-1252, 1975 8. Weiner CI, Diaconis JN: Primary abdominal wall abscess diagnosed by ultrasound. Arch Surg 110:341-342, 1975 9. Pedersen JF, Hancke S, Kristensen K: Renal carbuncle: Antiobiotic therapy governed by ultrasonically guided aspiration. J Urol 109:77%778, 1973 10. Hauser JB, Stanley RJ, Geisse G: The ultrasound findings in an obstructed afferent loop. J Clin Ultrasound 2:287-289, 1974 11. Holm HH, Rasmussen SN, Kristensen JK: Errors and pitfalls in ultrasonic scanning of the abdomen. Br J Radiol 45:835-840, 1972 12. Jensen F, Pedersen JF: The value of ultrasonic scanning in
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the diagnosis of intraabdominal abscesses and hematomas. Surg Gynecol Obstet 139:326-328, 1974 Kaplan GN, Sanders RC: B-Scan ultrasound in the management of patients with occult abdominal hematomas. J Clin Ultrasound 1:5-13, 1973 HOnig R, Kinser J: The diagnosis of ascites by ultrasonic tomography (B-scan) Br J Radio146:325-328, 1973 Goldberg BB, C!earfield HR, Goodman GA, Morales JO: Ultrasonic determination of ascites. Arch Int Med 131:217220, 1973 Doust BD, Baum .IK, Maklad NF, Doust VL: Ultrasonic evaluation of pleural opacities radiology 114:135-140, 1975 Haber K, Asher WM, Freimanis AK: Echograpbic evaluation of diaphragmatic motion in intraabdominal disease. Radiology 114:141-144, 1975 Morley P, Barnett E, Bell PR, Briggs JK, Caiman KC. Hamilton DN, Paton AM: Ultrasound in the diagnosis of fluid collections following renal transplantation. Clin Radiol 26:199-207, 1975 Smith EH, Bartrum RJ: Ultrasonically guided percutaneous aspiration of abscesses. Am J Roentgenol Radium Ther Nucl Med 122:308-312, 1974 Holm HH, Rasmussen SN, Kristensen JK: Ultrasonically guided percutaneous puncture technique. J Clin Ultrasound 1:2%31, 1973 Goldberg BB, Pollack HM: Ultrasonic aspiration-biopsy transducer. Radiology 108:667-671, 1973 Sanders RC: Radiological and radioisotopic diagnosis of perihepatic abscess. CRC Crit Rev Clin Radiol Nucl Med 5: !65-211, 1974 Hill BA, Yamaguchi K, Flynn J J, Miller DR: Diagnostic sonography in general surgery. Arch Surg 110:1089-1094, 1975
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