Ultrasound is the usual first line examination for abdominal imaging with computed tomography (CT), magnetic resonance imaging (MRI) and radio-isotope examinations used to provide further information. CT is the particularly useful for imaging intra-abdominal collections because it provides excellent anatomical detail and clearly demonstrates the relation of the fluid collection to the nearby structures . Typically on CT, an abscess appears as a soft tissue density mass ranging from 0-25 Hounsfield units (HU). The abscess wall is represented by a higher density irregular rim which typically enhances after intravenous contrast http://dx.doi.org/10.1016/S0720-048X(02)00156-0. Gas may be seen within the cavity and the adjacent fat planes may be obliterated or thickened . In our department, images are acquired using a 64 slice CT scanner (Lightspeed, General Electric) and images are reconstructed as 2mm slices. When performing an abdominal CT for the purpose of detecting intra-abdominal collections/abscesses, it is essential to opacify the bowel by using either dilute water soluble contrast or dilute barium. Otherwise, fluid filled loops of bowel may be mistaken for an abscess or conversely and abscess may be mistaken for a fluid filled loop of bowel. Our protocol includes oral preparation (20mls Omnipque 300 (Iohexol, GE Healthcare, Amersham Place, UK) in 1000mls of water) taken orally 1-2h prior to the CT examination. Images are acquired in full inspiration during the porto-venous phase (70s delay post injection of 100mls Omnipaque 300 i.v.) from the diaphragm to the pubic symphysis.
Intra-abdominal abscesses may be classified into visceral, intra-peritoneal and retroperitoneal. The next section illustrates common intra-abdominal abscesses with a brief description of specific imaging features. Percutaneous abscess drainage under either CT or ultrasound guidance is increasingly performed as an alternative to traditional surgery. It is safe, effective and widely used to treat patients with abdominal or pelvic sepsis http://dx.doi.org/10.1308/003588408X285928 http://dx.doi.org/10.1148/radiol.2283020924
Summary of typical CT findings of intra-abdominal visceral abscess:
(A) Liver abscess:
Pyogenic liver abscesses are almost uniformly fatal and early intervention is crucial to successful treatement http://dx.doi.org/10.1016/S0891-5520(05)70120-3. On CT, pyogenic liver abscesses typically appear as hyodense homogenous lesions with or without thick rims, fluid levels or microbubbles [Fig 1]. Liver abscesses may be single or multiple [Fig 2]. Chou et al found that single liver abscess are likely to be cryptogenic in origin whilst multiple liver abscesses had a biliary origin http://dx.doi.org/10.1007/PL00012258. Percutaneous drainage for a single abscess is usually by ultrasound but for multiple absesses are usually done under CT guidance.
Summary of typical CT findings of intra-abdominal intra-peritoneal abscesses:
Pancreatic abscesses may occur either within the pancreas as a result of secondary infection of necrotic pancreatic tissue or secondary infection of peri-pancreatic fluid collection (pseudocysts). Infected pancreatic necrosis is potentially the most serious complication of acute pancreatitis. It occurs in only 5% of patients with acute pancreatitis but has an associated mortality of 80%. Early aggressive treatment is vital . On CT, pancreatic abscesses appears as thick-rimmed low density unenhancing areas surrounded by normal enhancing pancreatic tissue [Fig 3] [Fig 4].
However, it is often difficult to distinguish a sterile fluid collection from an infected collection on imaging alone. Clinical correlation and sometimes a diagnostic aspiration under CT guidance to provide a sample for microbiological assessment may be required. Traditional open surgical debridement was the treatment of choice for infection pancreatic necrosis but percutaneous CT guided drains (10-24Fr) are increasingly used and can be an effective means of treatment. It is a lengthy undertaking which avoids high risk surgery but one which usually requires serial drainages and a team approach with close communication with the surgical team 
(B) Enteric abscess:
Intra-abdominal abscesses may occur secondary to enteric peroration as in the case with diverticular disease and appendicitis or direct extension of the disease through the bowel wall as in the case of Crohn’s disease http://dx.doi.org/10.1016/S0720-048X(02)00156-0. Other enteric abscesses may occur post surgically and are usually associated with a leak at the site of surgical anastamosis or secondary infection of a post-operative abdominal fluid collection. The collection does not communicate with and the adjacent bowel and there is usually stranding of the surrounding mesenteric fat and loss of the adjacent fat planes http://dx.doi.org/10.1308/003588408X285928 http://dx.doi.org/10.1007/s00247-008-0874-1 [Fig 5] [Fig 6].
Summary of typical CT findings of intra-abdominal retro-peritoneal abscesses:
(A) Psoas abscess
Psoas abscesses may be either due to haematogenous spread (primary) or from direct spread from contiguous structures such as the hip, vertebrae or bowel (secondary). They were previously thought to be relatively rare, but now with the increased use of CT to investigate patients with sepsis of unknown origin, they are being diagnosed more frequently . Percutaneous CT guided drainage either via a trans-peritoneal or retro-peritoneal approach is the established mode of treatment http://dx.doi.org/10.1016/S0009-9260(03)00274-5 [Fig 7] [Fig 8].
Patients with renal abscess usually have associated underlying renal disease such as nephrolithiasis, septicaemia or renal tract obstruction . Renal abscesses may rupture and present as a peri-nephric abscess or rupture into the renal collecting system and result in pyonephrosis. On CT, they typically appear as low attenuation, well defined and surrounded by normal enhancing renal parenchyma. The thick irregular enhancing wall may sometimes be difficult to appreciate against enhancing surrounding renal parenchyma.
Peri-nephric abscesses usually occur from rupture of an intra-nephric (renal) abscess (60-80%) but can also occur by direct seeding from adjacent infection e.g. diverticulitis or by haematogenous seeding (up to 30%) . From the peri-nephric area, infection may penetrate the flank and psoas muscles or spread caudally between the diverging layers of Gerota’s fascia to present as an abscess in the para-vesical or groin area . Percutaneous drainage is safe and effective means of treatment and may be performed under either CT or ultrasound guidance [Fig 9] [Fig 10].
Authors: Dr. Vivek Shrivastava, Dr. Daniel Chung, Dr. Raman Uberoi