Clinical Radiology (1990) 42, 154-156
Editorial Ultrasound in the Diagnosis of Abdominal Abscesses Imaging has long played an important role in the detection and diagnosis of abdominal abscesses and fluid collections. For many years plain films and contrast studies were the only techniques available. Chest and abdominal radiographs still play a role in the investigation of suspected abscesses, particularly in the postoperative patient but have been superseded largely by the new techniques of ultrasound, isotope scanning and computed tomography (CT). However, no consensus exists in the radiological or surgical literature on which of these techniques should be used as the initial investigation. Given certain reservations, we believe that ultrasound may be used as the first, and often the only investigation in detection of abdominal collections. It should be stressed that no imaging modality can reliably distinguish infected from non-infected serous collections or haematomas, and aspiration is an integral part of the diagnostic approach to suspected abscesses. Abdominal abscesses were described by Connell et al. (1980) as 'a continuing and deadly problem'. The mortality of untreated abdominal abscess approaches 100% (Altemeier et al., 1973). It is over 50 years since Ochsner and DeBakey (1938) reported a reduction of their operative mortality rate for subphrenic abscess when accurate pre-operative localization allowed a targeted approach to drainage. Despite the introduction of antibiotics, the reported mortality rate for intra-abdominal abscess remained around 30% over the ensuing four decades (Ariel and Kazarian, 1971; Fry et al., 1980; Doberneck and Mittelman, 1982). Prolonged sepsis due to delay in diagnosis results in multiple organ failure, and failure to localize the abscess necessitates a transabdominal midline approach, with its higher risk of surgical complications (Fry et al., 1980). Saini et al. (1983) noted that their overall mortality rate for intra-abdominal abscesses fell from 28% between 1970 and 1975, to 13% between 1975 and 1981, the improvement being attributed to more accurate pre-operative localization. The commonest sites of intra-abdominal abscesses have been described in a large series of 540 abscesses by Altemeier et al. (1973). Thirty six per cent were found to be intraperitoneal, of which half were in the right lower quadrant. The left lower quadrant and subphrenic spaces each accounted for 5% of the overall total. Retroperitoneal abscesses accounted for 38%, and visceral (e.g. hepatic, pancreatic and renal) abscesses 26% of the total. Unfortunately the proportion of patients who developed sepsis subsequent to surgery is not disclosed. The site of abscesses is determined by aetiology. In the U K abdominal abscesses are frequently secondary to surgery, particularly operations on the colon, appendix, biliary tree and stomach (Doberneck and Mittelman, 1982; Saini et al., 1983). Although blunt and penetrating trauma has been recorded as the commonest cause of abscess (Fry et al., 1980) this is not our experience nor has it been confirmed by other reports. In a series of abscesses unrelated to surgery, 81% were in the pelvis or lower quadrants, compared with 8% in the subphrenic spaces (Field and Pickleman, i985). In this series 28% of abscesses were thought to be aetiologically related to appendiceal pathology, 28% related to tubo ovarian disease, and 18% to diverticulitis; the average age of
patients with diverticular disease was more than 20 years older than the other two groups. Other authors have stressed the importance of bowel pathology in relation to abscess formation (Connell et al., 1980; Saini et al., 1983) and in our own experience diverticulitis appears to have replaced appendicitis as the commonest cause of portal pyaemia leading to the formation of liver abscesses. Few would deny that early detection, accurate localiza. tion and prompt drainage are the mainstays of successful management of abdominal abscesses and radiologists need to establish a system which will achieve these goals. A number of studies have assessed sensitivity and specificity of ultrasound, CT and nuclear medicine in the detection of intra-abdominal abscess and they are summarized in Table 1 (adapted from Baker et al., 1986). These comparative figures show some interesting features. Paradoxically, the sensitivity of ultrasound is highest in the two earliest studies (Taylor et al., 1978; Korobkin et al., 1979) with subsequent studies reporting sensitivities between 60% and 82%. It is possible that the ultrasound pioneers were better motivated and more expert than those who have followed them. Certainly some authors admit to relative inexperience in the use of ultrasound (Halber et al., 1979). It is generally accepted that ultrasound is particularly 'operator dependent' and its accuracy critically dependent on the experience and perseverence of the operator. Another perhaps more important factor in the apparent falling sensitivity is the change in clinicians' referral practices (Andriole, 1983). With modern real time equipment, radiologists are prepared to examine 'difficult' patients who would previously have been considered unsuitable for ultrasound. Lundstedt et al. (1986), in the only prospective study quoted, give two figures for ultrasound sensitivity and specificity. When all scans are included a low sensitivity resulted; recalculation after exclusion of studies deemed suboptimal as a result of intra-abdominal gas, open wounds etc., gave a figure more than 20% higher. Furthermore, these authors identified the sites of abscess where difficulty arose, from which it may be inferred that a determined attempt to detect abscesses in the less readily accessible parts of the abdomen will improve diagnostic yield. Figures for CT scanning appear impressive. However, Korobkin and colleagues (1978) acknowledge that CT, which was found to be 100% accurate, was, in all their cases, the final imaging technique used, "and (occasional) knowledge of the results of the previous tests may have introduced a source of bias. Moir and Robins (1982) als0 reported 100% sensitivity and specificity for CT, but in every instance CT was preceded by 67-Gallium (67Ga) scans or ultrasound which were highly suspicious of an abscess. Knochel et al. (1980)]:eport a higher overall accuracy for CT than for ultrasound or isotope scanning. However, the fact that 136 ultrasound scans and 136 isotope studies were performed, compared with 80 CT scans, implies that CT may have been used for confirrna" tion or clarification. Carroll et al. (1981) and Knochel et al. (1980) consider that multiple tests improve diagnostic accuracy. Investi" gative algorithms have been suggested utilizing combina~ tions of all three modalities (Mintz et al., 1983; Gagliardl
155
CLINICAL RADIOLOGY
Table I Author
Taylor et al. (1978) gorobkin et aL (1978) l-/alber et al. (1979) Knochel et al. (1980) Carroll et al. (1981) Idoir and R o b i n s (1982) saverymuttu et al. (1984) Lundstedt et al. (1986)
, No. o f patients
220 29 40 170 163 75 90 40
US (sens/spec ) (%) 93/98.6 100/93 80/100 82/94.5 81/95 82/91 60/83 ~ 61/55" ( 82/78
CT ( sens/spec ) (%)
Isotopes rain
67Ga
-i00/100 96/86 97.5/95 -100/100 -78/85
---86/95 84/95 -95/99 65/55
100/43 75/86 ---96/65 ---
*, see text
et al., 1988). This approach which may be useful, and is
frequently employed, sometimes subconsciously, by radiologists involved in investigation of intra-abdominal sepsis, must take into account the advantages and disadvantages of each technique. The principal advantages of ultrasound are that it is usually the most readily available of the techniques, is portable, and is the most reliable for identification of fluid collections. These factors make ultrasound the first choice investigation for the critically ill or post-operative patient. The diagnostic yield is highest in the right upper quadrant, perirenal areas and pelvis. In patients whose clinical findings suggest sepsis in the areas acknowledged as more difficult to examine (left upper quadrant, midabdomen and retroperitoneum), problems such as overlying gas can usually be overcome with persistence, skill and the use of multiple scan planes. Computed tomography shares some of these advantages; results are immediately available and as with ultrasound a suspected abscess may be confirmed by needle aspiration and drained if wished at the same sitting. Excellent anatomical detail is available, and the retroperitoneum and body wall are easy to evaluate. Confusion may arise if contrast opacification of the bowel is impossible or suboptimal. Unopacified bowel loops may mimic or obscure collections of pus, and meticulous bowel preparation is mandatory. Surgical clips and other metallic devices cause artefacts which may also limit interpretation. CT is the most expensive of these techniques, the least available in the UK, and it involves the highest radiation exposure. Unlike ultrasound the patient must always be transported to the radiology department. Isotope scanning with 67Ga is a sensitive method of detecting infection, but activity in liver, spleen and bowel may limit interpretation. Uptake in tumours and noninfective inflammatory processes may occur and results in low specificity. Results are not available for 24-48 hours and scintigraphy cannot be used as a direct guide to needle aspiration. Labelled white cell scanning using Indium o r 99Tcm hexamethylpropyleneamineoxime (Tct-IMPAo) is more specific than gallium for infective processes and gives positive results within 3 hours. HOWever, delayed studies may be required to distinguish abscesses from activity within the bowel. Tc-HMPAO white cell scanning is less well established and interpretation is more difficult due to renal and biliary excretion of activity. Normal uptake of labelled white cells with both Indium and Tc-HMPAO make the diagnosis of small abscesses in or around the liver and spleen difficult if not Impossible. Active migration of leucocytes into the abscess is a
prerequisite for the success of labelled leucocyte scans; this may not always be present in more chronic abscesses and particularly in tuberculous abscesses. False positive scans may be obtained with turnouts (Saverymuttu et al., 1986), non-infected haematomas and recent surgical wounds. Labelling leucocytes is time consuming and success depends on the skills of an experienced radiopharmacist. False negative results may be due to damage to cells in labelling. The principal current role of isotope scanning is as a back up technique if ultrasound and CT have failed to identify an abscess when clinical suspicion remains high; in these cases it may reveal increased uptake allowing reexamination of a particular area, which is occasionally distant to that under clinical suspicion. In our unit during the past 5 years, only two infective collections have been identified by isotope scanning when ultrasound was normal, although Seabold et al. (1984) reported unsuspected extra abdominal sites of infection detected by rain WBC scintigraphy in 16% of patients. In our experience, clinically useful serendipitous findings are rare. Magnetic resonance imaging (MRI) can detect abscesses (Cohen et al., 1985) but its relative unavailability in the UK, and the difficulty of performing needle aspiration preclude its consideration as a frontline diagnostic method for abdominal abscess. Our own algorithm starts with ultrasound in all patients. When abscesses are suspected in the right upper quadrant, renal areas and pelvis, a negative examination is usually considered to represent an end point. If the left upper quadrant, mid-abdomen and retroperitoneum, are under suspicion CT should follow an inconclusive ultrasound examination. Isotope scaning is employed only if there are no localizing clinical features and suspicion of an abscess is high. When a request is made for ultrasound in a case of suspected abdominal abscess, certain rules should be followed. Delays must be avoided and the investigation should not be delegated to an inexperienced member of staff. If a collection is found, percutaneous aspiration and drainage should be attempted if feasible (Halasz and Van Sonnenberg, 1983). Although many authors quote speed as one of the main advantages of ultrasound in abscess detection, it is important not to be hasty. If necessary, the stomach and bladder must be filled with fluid, and dressings should be removed. With sufficient care, a sensitivity in excess of 90% should be achievable. A sound knowledge of the anatomical sites where abscesses occur is invaluable. Meyers (1982) has elegantly described the anatomy of the peritoneal and extra peritoneal spaces and fluid dynamics in the abdomen.
156
U L T R A S O U N D IN D I A G N O S I S O F A B D O M I N A L ABSCESSES
Clinical history a n d signs m a y guide the u l t r a s o n o g r a p h e r to a p a r t i c u l a r site. H o w e v e r , the ramifications o f the p e r i t o n e a l a n d e x t r a p e r i t o n e a l spaces a n d passage o f infection between t h e m m e a n t h a t an abscess m a y be f o u n d at a site d i s t a n t to t h a t u n d e r suspicion, a n d multiple abscesses m a y be present. Evidence o f n o n specific i n f l a m m a t o r y c h a n g e m a y be a s s o c i a t e d with a d i s t a n t abscess; for e x a m p l e , p a r a r e n a l i n f l a m m a t i o n m a y be a s s o c i a t e d with a n a p p e n d i x abscess (Belli a n d Joseph, 1988). T h e r e f o r e a systematic scanning technique is necessary to e v a l u a t e all o f the spaces described (Meyers, 1982), a n d a comlzlete s o n o g r a p h i c e x a m i n a t i o n o f the entire a b d o m e n is m a n d a t o r y in all patients with suspected i n t r a - a b d o m i n a l abscess even if an o b v i o u s abscess is identified within m o m e n t s o f starting the scan. Special a t t e n t i o n s h o u l d be p a i d to the left s u b p h r e n i c space where abscesses m a y be o b s c u r e d b y air in the lung a n d extension o f the left l o b e o f the liver m a y m i m i c a n abscess; likewise in the i n t e r l o o p a r e a a n d lower r e t r o p e r i t o n e u m abscesses are easily missed. A n excellent m e t h o d i cal technique has been t h o r o u g h l y described b y B a k e r et al. (1986). The textbook description of an intra-abdominal abscess is o f a n o v o i d o r r o u n d anechoic lesion p r o d u c i n g a mass effect on a d j a c e n t structures. The wall is i r r e g u l a r a n d there is p o s t e r i o r echo e n h a n c e m e n t . H o w e v e r , this a p p e a r a n c e is seen in only h a l f o f all a b d o m i n a l abscesses (Schwerk a n d D u r r , 1981). Abscesses m a y d e m o n s t r a t e internal echoes to such a n extent t h a t they m i m i c solid lesions. C o n v e r s e l y solid lesions in the liver m a y s h o w r e d u c e d a t t e n u a t i o n c o m p a r e d to the s u r r o u n d i n g tissue that p o s t e r i o r echo e n h a n c e m e n t m a y result causing a ' p s e u d o c y s t i c ' a p p e a r a n c e ( I g n o t u s et al., 1990). G a s within an abscess causes b r i g h t echoes; the a p p e a r a n c e m a y m i m i c a l o o p o f b o w e l which is n o r m a l l y distinguished by the presence o f peristalsis. Collections o f sterile fluid such as acute a n d chronic s e r o m a s a n d loculated ascites are often indistinguishable s o n o g r a p h i cally f r o m abscesses, u n d e r l i n i n g the i m p o r t a n c e o f needle a s p i r a t i o n as a n intrinsic p a r t o f the u l t r a s o u n d evaluation. Conversely, absence o f the 'classic' features o f abscess s h o u l d n o t be r e g a r d e d as a c o n t r a d i c t i o n to aspiration. In s u m m a r y , we r e c o m m e n d u l t r a s o u n d as the p r i m a r y r a d i o l o g i c a l investigation w h e n an a b d o m i n a l abscess is suspected. The experience a n d perseverenc e o f the o p e r a tor a n d a m e t h o d i c a l a n a t o m i c a l a p p r o a c h will i m p r o v e the sensitivity o f the p r o c e d u r e , a n d j u d i c i o u s use o f a needle will increase its specificity. W h e n u l t r a s o u n d is inconclusive, C T scanning will o v e r c o m e m o s t o f the p r o b l e m s which r e n d e r e d the u l t r a s o u n d unsatisfactory. I s o t o p e scanning is best reserved for puzzling cases which are unresolved by these two techniques. REFERENCES
Altemeier, WA, Culbertson, WR, Fullen, WD & Shook, CD (1973). Intra abdominal abscesses. American JournalofSurgery, 125, 71-79. Andriole, VT (1983). The Clinicians viewpoint. In: Clinics in Diagnostic Ultrasound: Ultrasound in Inflammatory Disease, eds Joseph A.E.A. & Cosgrove D.C., pp. 1-13. Churchill Livingstone, Edinburgh. Ariel, IM & Kazarian, KK (1971). Diagnosis and Treatment of Abdominal Abscesses, pp. 203. Williams and Wilkins, Baltimore. Baker, ME, Blinder, RA & Rice, RP (1986). Diagnostic imaging of abdominal fluid collections and abscesses. Critical Reviews in Diagnostic Imaging, 25, 233-278. Belli, A-M & Joseph AEA (1988). The renal rind sign: a new ultrasound indication of inflammatory disease in the abdomen. British Journal of Radiology, 61, 806-810.
Carroll, B, Silverman, DM, Goodwin, DA & McDougall, IR (1981) Ultrasonography and III Indium white blood celt scanning for the detection of intra-abdominal abscesses. Radiology, 140, 155-160. Cohen, JM, Weinreb, JC & Maravilla, KR (1985). Fluid collectionsin the intraperitoneal and extraperitoneal spaces: comparison on MR and CT. Radiology, 155, 705 708. Connell, TR, Stephens, DH & Carlson, HC (1980). Upper abdominal abscesses: A continuing and deadly problem. American Journalof Roentgenology, 134, 759-765. Doberneck, RC & Mittelman, J (1982). Reappraisal of the problemsof intra abdominal abscess. Surgery, Gynaecology and Obstetrics, 154, 875 879. Field, TC & Pickleman, J (1985). Intra abdominal abscesses unass0, ciated with prior operation. Archives of Surgery, 120, 821-824. Fry, DE, Garrison, RN, Heitsch, RC, Calhoon, K & Polk, HC (1980). Determinants of death in patients with intra abdominal abscess. Surgery, 88, 517 523. Gagliardi, PD, Hoffer, PB & Rosenfield, AT (1988). Correlative imaging in abdominal infection: An algorithmic approach using nuclear medicine, ultrasound and computed tomoraphy. Seminarsin Nuclear Medicine, 18, 320-334. Hatasz, NA & Van Sonnenberg, E (1983). Drainage ofintra-abdominal abscesses: tactics and choices. American Journal of Surgery, 146, 113-115. Halber, MD, Daffner, RH, Morgan, CL, Trought, WS, Thompson, WM, Rice, RP & Korobkin, M (1979). Intra abdominal abscess: Current concepts in radiologic evaluation. American Journal of Roentgenology, 133, 9-13. Ignotus, PI, Sandrasagra, FA, Adam, EJ & Joseph, AEA (1990). Posterior echo enhancement behind solid intrahepatic metastases:a potential pitfall in diagnosis. Clinical Radiology (in press). Knochel, JQ, Koehler, PR, Lee, TG & Welch, DM (1980). Diagnosisof abdominal abscesses with computed tomography, ultrasound and III Indium leucocyte scans. Radiology, 137, 425 432. Korobkin, M, Callen, PW, Filly, RA, Hoffer, PB, Shimshak, RR & Kressel, HY (1978). Comparison of Computed Tomography, ultrasonography and gallium-67 scanning in the evaluation of suspected abdominal abscess. Radiology, 129, 89-93. Lundstedt, C, Hederstrom, E, Brisma, J, Holmin, T & Strand, S-l~ (1986). Prospective investigation of radiologic methods in the diagnosis of intra abdominal abscesses. Acta Radiologica Diagnosis, 27, 49-54. Meyers, MA (1982). Dynamic Radiology of the Abdomen, 2nd Edition. Springer Verlag, New York. Mintz, MC, Arger, PH & Kressel, HY (1983). An algorithmic approach to the radiologic evaluation of a suspected abdominal abscess. Seminars in Ultrasound, 4, 80-90. Moir, C & Robins, RE (1982). Role of ultrasonography, gallium scanning, and computed tomography in the diagnosis of intra abdominal abscess. American Journal of Surgery, 143, 582-585. Ochsner, A & DeBakey, M (1938). Subphrenic abscess: collective review and analysis of 3,608 collected and personal cases. Surgery, Gynaecology and Obstetrics, 66, 426-433. Saini, S, Kellum, JM, O'Leary, MP, O'Donnell, TF, Tally, FP, Carter, C, Deterlihg, RA & Curtis, LE (1983). Improved localisation and survival in patients with intra abdominal abscesses. American Journal of Surgery, 145, 136-142. Saverymuttu, S, Croxton, ME, Peters, AM & Lavender, JP (1984)~ Indium III tropolonate leucocyte scanning in the detection of intra abdominal abscesses. Clinical Radiology, 34, '593 596. Saverymuttu, SH, Maltby, P, Batman, P, Joseph, AEA & Maxwell,I) (1986). False positive localisation of Indium III granulocyte ill colonic carcinoma. British Journal of Radiology, 59, 773 777. Schwerk, WB & Durr, HK (1981). Ultrasound grey scale pattern and guided aspiration puncture of abdominal abscesses. Journal of Clinical Ultrasound, 9, 389. Seabold, JF, Wilson, DG, Lieberman, LM & Boyd, CM (1984). Unsuspected extra abdominal sites of infection: Scintigraphic detection with Indium III labelled leucocytes. Radiology, 151, 213217. Taylor, KJW, Sullivan, DC, Wasson, JFM & Rosenfield, AT (1978!, Ultrasound and gallium for the diagnosis of abdominal and pelvic abscesses. Gastrointestinal Radiology, 31, 28 I-5. A. E. A. J O S E P H A N D D. M A C V I C A g D e p a r t m e n t o f RadiologY S t George's Hospital Blackshaw Road London S W 1 7 OO1
Editorial Ultrasound in the Diagnosis of Abdominal Abscesses Imaging has long played an important role in the detection and diagnosis of abdominal abscesses and fluid collections. For many years plain films and contrast studies were the only techniques available. Chest and abdominal radiographs still play a role in the investigation of suspected abscesses, particularly in the postoperative patient but have been superseded largely by the new techniques of ultrasound, isotope scanning and computed tomography (CT). However, no consensus exists in the radiological or surgical literature on which of these techniques should be used as the initial investigation. Given certain reservations, we believe that ultrasound may be used as the first, and often the only investigation in detection of abdominal collections. It should be stressed that no imaging modality can reliably distinguish infected from non-infected serous collections or haematomas, and aspiration is an integral part of the diagnostic approach to suspected abscesses. Abdominal abscesses were described by Connell et al. (1980) as 'a continuing and deadly problem'. The mortality of untreated abdominal abscess approaches 100% (Altemeier et al., 1973). It is over 50 years since Ochsner and DeBakey (1938) reported a reduction of their operative mortality rate for subphrenic abscess when accurate pre-operative localization allowed a targeted approach to drainage. Despite the introduction of antibiotics, the reported mortality rate for intra-abdominal abscess remained around 30% over the ensuing four decades (Ariel and Kazarian, 1971; Fry et al., 1980; Doberneck and Mittelman, 1982). Prolonged sepsis due to delay in diagnosis results in multiple organ failure, and failure to localize the abscess necessitates a transabdominal midline approach, with its higher risk of surgical complications (Fry et al., 1980). Saini et al. (1983) noted that their overall mortality rate for intra-abdominal abscesses fell from 28% between 1970 and 1975, to 13% between 1975 and 1981, the improvement being attributed to more accurate pre-operative localization. The commonest sites of intra-abdominal abscesses have been described in a large series of 540 abscesses by Altemeier et al. (1973). Thirty six per cent were found to be intraperitoneal, of which half were in the right lower quadrant. The left lower quadrant and subphrenic spaces each accounted for 5% of the overall total. Retroperitoneal abscesses accounted for 38%, and visceral (e.g. hepatic, pancreatic and renal) abscesses 26% of the total. Unfortunately the proportion of patients who developed sepsis subsequent to surgery is not disclosed. The site of abscesses is determined by aetiology. In the U K abdominal abscesses are frequently secondary to surgery, particularly operations on the colon, appendix, biliary tree and stomach (Doberneck and Mittelman, 1982; Saini et al., 1983). Although blunt and penetrating trauma has been recorded as the commonest cause of abscess (Fry et al., 1980) this is not our experience nor has it been confirmed by other reports. In a series of abscesses unrelated to surgery, 81% were in the pelvis or lower quadrants, compared with 8% in the subphrenic spaces (Field and Pickleman, i985). In this series 28% of abscesses were thought to be aetiologically related to appendiceal pathology, 28% related to tubo ovarian disease, and 18% to diverticulitis; the average age of
patients with diverticular disease was more than 20 years older than the other two groups. Other authors have stressed the importance of bowel pathology in relation to abscess formation (Connell et al., 1980; Saini et al., 1983) and in our own experience diverticulitis appears to have replaced appendicitis as the commonest cause of portal pyaemia leading to the formation of liver abscesses. Few would deny that early detection, accurate localiza. tion and prompt drainage are the mainstays of successful management of abdominal abscesses and radiologists need to establish a system which will achieve these goals. A number of studies have assessed sensitivity and specificity of ultrasound, CT and nuclear medicine in the detection of intra-abdominal abscess and they are summarized in Table 1 (adapted from Baker et al., 1986). These comparative figures show some interesting features. Paradoxically, the sensitivity of ultrasound is highest in the two earliest studies (Taylor et al., 1978; Korobkin et al., 1979) with subsequent studies reporting sensitivities between 60% and 82%. It is possible that the ultrasound pioneers were better motivated and more expert than those who have followed them. Certainly some authors admit to relative inexperience in the use of ultrasound (Halber et al., 1979). It is generally accepted that ultrasound is particularly 'operator dependent' and its accuracy critically dependent on the experience and perseverence of the operator. Another perhaps more important factor in the apparent falling sensitivity is the change in clinicians' referral practices (Andriole, 1983). With modern real time equipment, radiologists are prepared to examine 'difficult' patients who would previously have been considered unsuitable for ultrasound. Lundstedt et al. (1986), in the only prospective study quoted, give two figures for ultrasound sensitivity and specificity. When all scans are included a low sensitivity resulted; recalculation after exclusion of studies deemed suboptimal as a result of intra-abdominal gas, open wounds etc., gave a figure more than 20% higher. Furthermore, these authors identified the sites of abscess where difficulty arose, from which it may be inferred that a determined attempt to detect abscesses in the less readily accessible parts of the abdomen will improve diagnostic yield. Figures for CT scanning appear impressive. However, Korobkin and colleagues (1978) acknowledge that CT, which was found to be 100% accurate, was, in all their cases, the final imaging technique used, "and (occasional) knowledge of the results of the previous tests may have introduced a source of bias. Moir and Robins (1982) als0 reported 100% sensitivity and specificity for CT, but in every instance CT was preceded by 67-Gallium (67Ga) scans or ultrasound which were highly suspicious of an abscess. Knochel et al. (1980)]:eport a higher overall accuracy for CT than for ultrasound or isotope scanning. However, the fact that 136 ultrasound scans and 136 isotope studies were performed, compared with 80 CT scans, implies that CT may have been used for confirrna" tion or clarification. Carroll et al. (1981) and Knochel et al. (1980) consider that multiple tests improve diagnostic accuracy. Investi" gative algorithms have been suggested utilizing combina~ tions of all three modalities (Mintz et al., 1983; Gagliardl
155
CLINICAL RADIOLOGY
Table I Author
Taylor et al. (1978) gorobkin et aL (1978) l-/alber et al. (1979) Knochel et al. (1980) Carroll et al. (1981) Idoir and R o b i n s (1982) saverymuttu et al. (1984) Lundstedt et al. (1986)
, No. o f patients
220 29 40 170 163 75 90 40
US (sens/spec ) (%) 93/98.6 100/93 80/100 82/94.5 81/95 82/91 60/83 ~ 61/55" ( 82/78
CT ( sens/spec ) (%)
Isotopes rain
67Ga
-i00/100 96/86 97.5/95 -100/100 -78/85
---86/95 84/95 -95/99 65/55
100/43 75/86 ---96/65 ---
*, see text
et al., 1988). This approach which may be useful, and is
frequently employed, sometimes subconsciously, by radiologists involved in investigation of intra-abdominal sepsis, must take into account the advantages and disadvantages of each technique. The principal advantages of ultrasound are that it is usually the most readily available of the techniques, is portable, and is the most reliable for identification of fluid collections. These factors make ultrasound the first choice investigation for the critically ill or post-operative patient. The diagnostic yield is highest in the right upper quadrant, perirenal areas and pelvis. In patients whose clinical findings suggest sepsis in the areas acknowledged as more difficult to examine (left upper quadrant, midabdomen and retroperitoneum), problems such as overlying gas can usually be overcome with persistence, skill and the use of multiple scan planes. Computed tomography shares some of these advantages; results are immediately available and as with ultrasound a suspected abscess may be confirmed by needle aspiration and drained if wished at the same sitting. Excellent anatomical detail is available, and the retroperitoneum and body wall are easy to evaluate. Confusion may arise if contrast opacification of the bowel is impossible or suboptimal. Unopacified bowel loops may mimic or obscure collections of pus, and meticulous bowel preparation is mandatory. Surgical clips and other metallic devices cause artefacts which may also limit interpretation. CT is the most expensive of these techniques, the least available in the UK, and it involves the highest radiation exposure. Unlike ultrasound the patient must always be transported to the radiology department. Isotope scanning with 67Ga is a sensitive method of detecting infection, but activity in liver, spleen and bowel may limit interpretation. Uptake in tumours and noninfective inflammatory processes may occur and results in low specificity. Results are not available for 24-48 hours and scintigraphy cannot be used as a direct guide to needle aspiration. Labelled white cell scanning using Indium o r 99Tcm hexamethylpropyleneamineoxime (Tct-IMPAo) is more specific than gallium for infective processes and gives positive results within 3 hours. HOWever, delayed studies may be required to distinguish abscesses from activity within the bowel. Tc-HMPAO white cell scanning is less well established and interpretation is more difficult due to renal and biliary excretion of activity. Normal uptake of labelled white cells with both Indium and Tc-HMPAO make the diagnosis of small abscesses in or around the liver and spleen difficult if not Impossible. Active migration of leucocytes into the abscess is a
prerequisite for the success of labelled leucocyte scans; this may not always be present in more chronic abscesses and particularly in tuberculous abscesses. False positive scans may be obtained with turnouts (Saverymuttu et al., 1986), non-infected haematomas and recent surgical wounds. Labelling leucocytes is time consuming and success depends on the skills of an experienced radiopharmacist. False negative results may be due to damage to cells in labelling. The principal current role of isotope scanning is as a back up technique if ultrasound and CT have failed to identify an abscess when clinical suspicion remains high; in these cases it may reveal increased uptake allowing reexamination of a particular area, which is occasionally distant to that under clinical suspicion. In our unit during the past 5 years, only two infective collections have been identified by isotope scanning when ultrasound was normal, although Seabold et al. (1984) reported unsuspected extra abdominal sites of infection detected by rain WBC scintigraphy in 16% of patients. In our experience, clinically useful serendipitous findings are rare. Magnetic resonance imaging (MRI) can detect abscesses (Cohen et al., 1985) but its relative unavailability in the UK, and the difficulty of performing needle aspiration preclude its consideration as a frontline diagnostic method for abdominal abscess. Our own algorithm starts with ultrasound in all patients. When abscesses are suspected in the right upper quadrant, renal areas and pelvis, a negative examination is usually considered to represent an end point. If the left upper quadrant, mid-abdomen and retroperitoneum, are under suspicion CT should follow an inconclusive ultrasound examination. Isotope scaning is employed only if there are no localizing clinical features and suspicion of an abscess is high. When a request is made for ultrasound in a case of suspected abdominal abscess, certain rules should be followed. Delays must be avoided and the investigation should not be delegated to an inexperienced member of staff. If a collection is found, percutaneous aspiration and drainage should be attempted if feasible (Halasz and Van Sonnenberg, 1983). Although many authors quote speed as one of the main advantages of ultrasound in abscess detection, it is important not to be hasty. If necessary, the stomach and bladder must be filled with fluid, and dressings should be removed. With sufficient care, a sensitivity in excess of 90% should be achievable. A sound knowledge of the anatomical sites where abscesses occur is invaluable. Meyers (1982) has elegantly described the anatomy of the peritoneal and extra peritoneal spaces and fluid dynamics in the abdomen.
156
U L T R A S O U N D IN D I A G N O S I S O F A B D O M I N A L ABSCESSES
Clinical history a n d signs m a y guide the u l t r a s o n o g r a p h e r to a p a r t i c u l a r site. H o w e v e r , the ramifications o f the p e r i t o n e a l a n d e x t r a p e r i t o n e a l spaces a n d passage o f infection between t h e m m e a n t h a t an abscess m a y be f o u n d at a site d i s t a n t to t h a t u n d e r suspicion, a n d multiple abscesses m a y be present. Evidence o f n o n specific i n f l a m m a t o r y c h a n g e m a y be a s s o c i a t e d with a d i s t a n t abscess; for e x a m p l e , p a r a r e n a l i n f l a m m a t i o n m a y be a s s o c i a t e d with a n a p p e n d i x abscess (Belli a n d Joseph, 1988). T h e r e f o r e a systematic scanning technique is necessary to e v a l u a t e all o f the spaces described (Meyers, 1982), a n d a comlzlete s o n o g r a p h i c e x a m i n a t i o n o f the entire a b d o m e n is m a n d a t o r y in all patients with suspected i n t r a - a b d o m i n a l abscess even if an o b v i o u s abscess is identified within m o m e n t s o f starting the scan. Special a t t e n t i o n s h o u l d be p a i d to the left s u b p h r e n i c space where abscesses m a y be o b s c u r e d b y air in the lung a n d extension o f the left l o b e o f the liver m a y m i m i c a n abscess; likewise in the i n t e r l o o p a r e a a n d lower r e t r o p e r i t o n e u m abscesses are easily missed. A n excellent m e t h o d i cal technique has been t h o r o u g h l y described b y B a k e r et al. (1986). The textbook description of an intra-abdominal abscess is o f a n o v o i d o r r o u n d anechoic lesion p r o d u c i n g a mass effect on a d j a c e n t structures. The wall is i r r e g u l a r a n d there is p o s t e r i o r echo e n h a n c e m e n t . H o w e v e r , this a p p e a r a n c e is seen in only h a l f o f all a b d o m i n a l abscesses (Schwerk a n d D u r r , 1981). Abscesses m a y d e m o n s t r a t e internal echoes to such a n extent t h a t they m i m i c solid lesions. C o n v e r s e l y solid lesions in the liver m a y s h o w r e d u c e d a t t e n u a t i o n c o m p a r e d to the s u r r o u n d i n g tissue that p o s t e r i o r echo e n h a n c e m e n t m a y result causing a ' p s e u d o c y s t i c ' a p p e a r a n c e ( I g n o t u s et al., 1990). G a s within an abscess causes b r i g h t echoes; the a p p e a r a n c e m a y m i m i c a l o o p o f b o w e l which is n o r m a l l y distinguished by the presence o f peristalsis. Collections o f sterile fluid such as acute a n d chronic s e r o m a s a n d loculated ascites are often indistinguishable s o n o g r a p h i cally f r o m abscesses, u n d e r l i n i n g the i m p o r t a n c e o f needle a s p i r a t i o n as a n intrinsic p a r t o f the u l t r a s o u n d evaluation. Conversely, absence o f the 'classic' features o f abscess s h o u l d n o t be r e g a r d e d as a c o n t r a d i c t i o n to aspiration. In s u m m a r y , we r e c o m m e n d u l t r a s o u n d as the p r i m a r y r a d i o l o g i c a l investigation w h e n an a b d o m i n a l abscess is suspected. The experience a n d perseverenc e o f the o p e r a tor a n d a m e t h o d i c a l a n a t o m i c a l a p p r o a c h will i m p r o v e the sensitivity o f the p r o c e d u r e , a n d j u d i c i o u s use o f a needle will increase its specificity. W h e n u l t r a s o u n d is inconclusive, C T scanning will o v e r c o m e m o s t o f the p r o b l e m s which r e n d e r e d the u l t r a s o u n d unsatisfactory. I s o t o p e scanning is best reserved for puzzling cases which are unresolved by these two techniques. REFERENCES
Altemeier, WA, Culbertson, WR, Fullen, WD & Shook, CD (1973). Intra abdominal abscesses. American JournalofSurgery, 125, 71-79. Andriole, VT (1983). The Clinicians viewpoint. In: Clinics in Diagnostic Ultrasound: Ultrasound in Inflammatory Disease, eds Joseph A.E.A. & Cosgrove D.C., pp. 1-13. Churchill Livingstone, Edinburgh. Ariel, IM & Kazarian, KK (1971). Diagnosis and Treatment of Abdominal Abscesses, pp. 203. Williams and Wilkins, Baltimore. Baker, ME, Blinder, RA & Rice, RP (1986). Diagnostic imaging of abdominal fluid collections and abscesses. Critical Reviews in Diagnostic Imaging, 25, 233-278. Belli, A-M & Joseph AEA (1988). The renal rind sign: a new ultrasound indication of inflammatory disease in the abdomen. British Journal of Radiology, 61, 806-810.
Carroll, B, Silverman, DM, Goodwin, DA & McDougall, IR (1981) Ultrasonography and III Indium white blood celt scanning for the detection of intra-abdominal abscesses. Radiology, 140, 155-160. Cohen, JM, Weinreb, JC & Maravilla, KR (1985). Fluid collectionsin the intraperitoneal and extraperitoneal spaces: comparison on MR and CT. Radiology, 155, 705 708. Connell, TR, Stephens, DH & Carlson, HC (1980). Upper abdominal abscesses: A continuing and deadly problem. American Journalof Roentgenology, 134, 759-765. Doberneck, RC & Mittelman, J (1982). Reappraisal of the problemsof intra abdominal abscess. Surgery, Gynaecology and Obstetrics, 154, 875 879. Field, TC & Pickleman, J (1985). Intra abdominal abscesses unass0, ciated with prior operation. Archives of Surgery, 120, 821-824. Fry, DE, Garrison, RN, Heitsch, RC, Calhoon, K & Polk, HC (1980). Determinants of death in patients with intra abdominal abscess. Surgery, 88, 517 523. Gagliardi, PD, Hoffer, PB & Rosenfield, AT (1988). Correlative imaging in abdominal infection: An algorithmic approach using nuclear medicine, ultrasound and computed tomoraphy. Seminarsin Nuclear Medicine, 18, 320-334. Hatasz, NA & Van Sonnenberg, E (1983). Drainage ofintra-abdominal abscesses: tactics and choices. American Journal of Surgery, 146, 113-115. Halber, MD, Daffner, RH, Morgan, CL, Trought, WS, Thompson, WM, Rice, RP & Korobkin, M (1979). Intra abdominal abscess: Current concepts in radiologic evaluation. American Journal of Roentgenology, 133, 9-13. Ignotus, PI, Sandrasagra, FA, Adam, EJ & Joseph, AEA (1990). Posterior echo enhancement behind solid intrahepatic metastases:a potential pitfall in diagnosis. Clinical Radiology (in press). Knochel, JQ, Koehler, PR, Lee, TG & Welch, DM (1980). Diagnosisof abdominal abscesses with computed tomography, ultrasound and III Indium leucocyte scans. Radiology, 137, 425 432. Korobkin, M, Callen, PW, Filly, RA, Hoffer, PB, Shimshak, RR & Kressel, HY (1978). Comparison of Computed Tomography, ultrasonography and gallium-67 scanning in the evaluation of suspected abdominal abscess. Radiology, 129, 89-93. Lundstedt, C, Hederstrom, E, Brisma, J, Holmin, T & Strand, S-l~ (1986). Prospective investigation of radiologic methods in the diagnosis of intra abdominal abscesses. Acta Radiologica Diagnosis, 27, 49-54. Meyers, MA (1982). Dynamic Radiology of the Abdomen, 2nd Edition. Springer Verlag, New York. Mintz, MC, Arger, PH & Kressel, HY (1983). An algorithmic approach to the radiologic evaluation of a suspected abdominal abscess. Seminars in Ultrasound, 4, 80-90. Moir, C & Robins, RE (1982). Role of ultrasonography, gallium scanning, and computed tomography in the diagnosis of intra abdominal abscess. American Journal of Surgery, 143, 582-585. Ochsner, A & DeBakey, M (1938). Subphrenic abscess: collective review and analysis of 3,608 collected and personal cases. Surgery, Gynaecology and Obstetrics, 66, 426-433. Saini, S, Kellum, JM, O'Leary, MP, O'Donnell, TF, Tally, FP, Carter, C, Deterlihg, RA & Curtis, LE (1983). Improved localisation and survival in patients with intra abdominal abscesses. American Journal of Surgery, 145, 136-142. Saverymuttu, S, Croxton, ME, Peters, AM & Lavender, JP (1984)~ Indium III tropolonate leucocyte scanning in the detection of intra abdominal abscesses. Clinical Radiology, 34, '593 596. Saverymuttu, SH, Maltby, P, Batman, P, Joseph, AEA & Maxwell,I) (1986). False positive localisation of Indium III granulocyte ill colonic carcinoma. British Journal of Radiology, 59, 773 777. Schwerk, WB & Durr, HK (1981). Ultrasound grey scale pattern and guided aspiration puncture of abdominal abscesses. Journal of Clinical Ultrasound, 9, 389. Seabold, JF, Wilson, DG, Lieberman, LM & Boyd, CM (1984). Unsuspected extra abdominal sites of infection: Scintigraphic detection with Indium III labelled leucocytes. Radiology, 151, 213217. Taylor, KJW, Sullivan, DC, Wasson, JFM & Rosenfield, AT (1978!, Ultrasound and gallium for the diagnosis of abdominal and pelvic abscesses. Gastrointestinal Radiology, 31, 28 I-5. A. E. A. J O S E P H A N D D. M A C V I C A g D e p a r t m e n t o f RadiologY S t George's Hospital Blackshaw Road London S W 1 7 OO1
