The choice and sequence of initial imaging techniques to investigate suspected abdominal sepsis must largely be tempered by a focused differential diagnosis surmised by a careful history, physical examination, and analysis of the initial laboratory data. Guided by the patient-specific differential diagnosis, an imaging modality is then selected. Four of the five classic “Ws” of post-surgical pyrexia may help guide decision-making:

In this day and age much emphasis has been placed on the early utilization of the various cross-sectional imaging modalities (US, CT, MRI). However, the plain film remains a useful initial diagnostic imaging modality. Particularly in intubated and ventilated ICU patients, sepsis of pulmonary origin is high on the list of potential differential diagnoses (see Chap. 24).

Ultrasound is easily performed at the bedside, provides a good overview of common pathologies and can guide the need for further imaging. It is also useful for picking up some of the less common sources of sepsis (Fig. 29.1). Computerized Tomography (CT) remains the preferred imaging modality for intra-abdominal infections in many ICU settings across the world. Various complementary applications can often be employed with the obtained data set, such as 3D volume-rendered reconstructions of various anatomic structures. As the data set is obtained quickly, often in a matter of seconds, it suffers less from motion artifacts commonly seen in MRI or Nuclear Radiology studies. CT can also be utilized for image-guided procedures in addition to diagnostic indications.

Scintigraphic studies are used in extremely rare circumstances in ICU patients. They have a role in cases where other modalities have failed to yield a diagnosis, mainly when looking for an infective focus (Fig. 29.2a, b). These studies are cumbersome and slow and they require the patient to be returned to the Nuclear Medicine Suite on more than one occasion (e.g., delayed Gallium Scan Images at 4 and 24 h to confirm absence of infectious source).

They are thought to be the most common overall source of ICU fever and are often catheter-related. It is possible that the incidence of catheter-related urinary tract infections has been overestimated because many studies do not distinguish between asymptomatic bacteriuria and a genuine urinary tract infection. If unrecognized they may progress to pyelonephritis but urosepsis is the ultimate endpoint that needs to be prevented. The diagnosis is typically made clinically in conjunction with laboratory. The role of imaging is typically to exclude more advanced infections of the urinary tract or their complications, such as advanced cystitis, pyonephrosis and both emphysematous and non-emphysematous pyelonephritis. Ultrasound provides the first line test, followed by CT (Fig. 29.3).