for infection by opportunistic agents. Secondary psoas abscess is more common and may result
from spread of infection from adjacent structures, including colon, kidney, and bone (6).
The CT findings of psoas abscess include enlargement of the muscle by a low-attenuation
lesion that displays rim enhancement after IV contrast administration. Other findings include
obliteration of normal fat planes as well as bone destruction and gas formation. Gas within a
psoas abscess may also be related to an underlying bowel fistula, such as in Crohn’s disease or
diverticulitis. MRI has generally shown no increased diagnostic benefit and has no role in the
diagnosis of psoas abscess, unless concurrent examination of the spine and thecal sac is
indicated. Abnormal uptake on a Ga-67 scan may also be used for diagnosis, although other
entities, such as lymphoma, also show increased uptake; this finding is therefore not specific.
An indium-111 white blood cell scan alternatively can be used to confirm infection if needed
and should be more specific, although percutaneous aspiration (and drainage) can be
performed for more definitive diagnosis and therapy (6–8).
Mimic of Psoas Abscess
Differentiation from tumor, such as lymphoma, can be difficult with imaging alone, as both
can present as low-attenuation lesions, although the presence of gas makes the diagnosis of
abscess far more likely. Adjacent structures should be examined to determine if there is a
source of secondary infection. In the case of lymphoma originating from para-aortic lymph
nodes, a potential helpful differentiating feature is that there may be medial or lateral
displacement of the muscle by tumor, rather than extension into the muscle, as would be seen
in an abscess (9,10).
Clinical and Radiologic Diagnosis of Prostate Abscess
Prostatic abscess occurs as a complication of acute bacterial prostatitis. The most common
organism isE. coli.Diabetic and immunocompromised patients are especially prone to this
complication. The symptoms are similar to acute bacterial prostatitis, including fever, chills,
and urinary frequency, with focal prostatic tenderness on physical exam (11).
Both CT and ultrasound are used for diagnosis, with ultrasound also having therapeutic
utility in transrectal drainage. Abscesses can occur anywhere in the prostate, although they are
usually centered away from the midline. Findings on ultrasound include focal hypoechoic or
anechoic masses, with thickened or irregular walls, septations, and internal echoes. On CT,
findings include an enlarged gland containing multiple well-demarcated, non-enhancing fluid
collections within the gland and/or periprostatic tissues. These collections may be multi-
septated or demonstrate enhancing rims (3,12).
Mimic of Prostate Abscess
A potential mimicker of prostate abscess is prostate carcinoma. Prostate cancer is the most
common noncutaneous cancer in American men and the second most common cause of male
cancer deaths after lung cancer. Unlike prostate abscess, which can occur anywhere in the
gland, prostate cancer occurs mainly in the peripheral zones. Diagnosis is made via a
combination of digital rectal exam findings, elevated PSA level, transrectal ultrasound, and
MR, with a definitive diagnosis established by biopsy (3,13).
Ultrasound findings are somewhat similar to abscess in that carcinoma appears as an
anechoic to hypoechoic mass. The contour is classically asymmetric or triangular with the base
close to the capsule and extending centrally into the gland based on the pattern of tumor
growth. While CT is an excellent means for diagnosing and following treatment of prostatic
abscess, it has limited use in the diagnosis of carcinoma due to relatively poor sensitivity and
specificity for detection of cancer within the gland compared with MRI. CT findings may
include an enlarged gland with evidence of extracapsular extension in more advanced tumors
(obliterated periprostatic fat plane, invasion of adjacent bladder or rectum) and pelvic
lymphadenopathy (Fig. 5). T2-weighted MRI demonstrates prostate cancer as a low-intensity
area within the gland, whereas abscess should demonstrate areas of central high signal
intensity related to the fluid content (13,14).
Radiology of Infectious Diseases and Their Mimics in Critical Care 79