that is either hyperechoic or hypoechoic, with low-level internal echoes and disruption of the
corticomedullary junction. The “comet sign,” consisting of internal echogenic foci, indicates the
presence of gas within the lesion. Contrast-enhanced CT demonstrates a round, well-
marginated, low-attenuation mass with wall enhancement (Fig. 3). Gas may or may not be
present within the lesion, and there is no enhancement centrally within the lesion. Perinephric
inflammatory change is also often seen. A white blood cell scan may also be helpful for
diagnosis. Uptake of indium-111-labeled leukocytes within the abscess can be seen, although
false-negative results may occur if the patient has already been on antibiotic therapy, if the
abscess is walled off, or if there is a poor inflammatory response (3,4).
Mimic of Renal Abscess
Renal cell carcinoma may mimic renal abscess on imaging examinations. Both are mass-like
lesions within the kidney; however, unlike renal abscess, which does not enhance centrally,
renal cell carcinoma typically demonstrates heterogeneous enhancement. Internal calcifications
may or may not be present (Fig. 4). For this reason, the recommendation for imaging known or
suspected renal masses that could be cancers rather than abscesses includes non-enhanced CT
followed by multiphasic contrast-enhanced CT performed at the same sitting (3,5).
Clinical and Radiologic Diagnosis of Psoas Abscess
Primary psoas abscess is rare and usually idiopathic. The most common causative pathogens
areS. aureusand mixed Gram-negative organisms. Immunocompromised patients are at risk
Figure 3 Contrast-enhanced CT scan of the
abdomen in a 15-year-old female demonstrates
as abscess in the interpolar region of the left
kidney (arrow) with rim enhancement and adja-
cent diffuse low density and left renal swelling
representing pyelonephritis.Figure 4 Non-enhanced CT scan of the abdo-
men in a 47-year-old male, who presented with
left flank pain, demonstrates a large mixed solid
and cystic tumor in the upper pole of the right
kidney, containing internal foci of calcification,
which subsequently proved to be renal cell
carcinoma.78 Luongo et al.