Arterial-phase CT finds hepatic fungal infection

Immunocompromised patients are at high risk of invasive fungal infections, especially of the liver and spleen. Typical hosts include AIDS and marrow transplant patients, or patients with prolonged neutropenia. Candidiasis is by far the dominant filamentous fungal infection of the liver, but other forms, such as aspergillosis and zygomycosis in liver transplant patients, are appearing with increasing frequency.

Diagnosis has always been hit-or-miss. In fact, only about 9% of invasive fungal infections of the liver are diagnosed ante-mortem, according to Dr. Ur Metser, a radiology fellow at the University of Toronto. At the 2003 RSNA meeting, he presented a new study that relies on contrast-enhanced CT to diagnose infection and pave the way to antifungal therapy when appropriate.

"Fungal infection of the liver is one of the complications of prolonged neutropenia in patients treated for hematologic malignancies," Metser said. "Diagnosis is difficult because it's based on biopsy specimens, which are often negative.... Therefore, imaging plays an important role in both the diagnosis and follow-up of these patients."

The purpose of the study was to assess the clinical utility of bi- or triphasic CT (non-contrast, arterial phase and portal-venous phase) in patients with suspected fungal infections of the liver, based on the guidelines of the European Organization for the Research and Treatment of Cancer and Mycoses Study Group (EORTC/MSG).

The organization's guidelines came from a study in which "patients were proven positive for fungal infection, where 'proven' means aspiration of fungal cells from the liver, and 'probable infection' means unexplained fever during antibiotic treatment, abnormal liver function test, and proof of decrease in size or number of liver lesions on follow-up after antifungal therapy," he explained. The difficulty of obtaining positive biopsy results meant that only a few of the infections in the study were, in fact, "proven."

Metser and colleagues retrospectively examined 60 bi- or triphasic CT exams in the 39 immunocompromised patients with suspected fungal hepatic infections. CT images of the liver were acquired at 120 kVp, 230-330 mAs, table speed of 0.5 mm per rotation, high-quality mode, and pitch of 3:1. CT data were reconstructed using 5-mm-thick slices with 50% overlap. Nonionic intravenous contrast material was administered with the use of a power injector at a dose of 2 mL/kg up to a maximum of 200 mL, at a rate of 3 mL/sec with a 25-second delay for arterial phase, and 60-second delay for portal venous phase.

Patient with fever after prolonged neutropenia. Above, arterial-phase CT of liver shows a type 2b lesion (arrow). Unenhanced CT (below) shows a corresponding hypoattenuating lesion, while portal-venous-phase CT (bottom) shows a tiny nonspecific hypodensity (type 1 lesion). This case demonstrates how arterial-phase CT adds specificity. Although portal-venous phase depicted the lesion, it would be extremely difficult to "call" it an abscess. The enhancement pattern on arterial phase CT makes for a more confident diagnosis. All images courtesy of Dr. Ur Metser.

"All phases of all studies were randomized and presented to a panel of two radiologists who were blinded to the clinical and laboratory data," he said. "Decisions were made by consensus. The panel assessed the number of lesions in the liver, conspicuity on a four-point scale, (the presence of) focal splenic lesions, and the enhancement patterns of the lesion."

A phase was considered positive if at least three type 1 lesions, and one type 2 to 4 lesions, were identified. A study was considered positive if at least one of its phases was positive. And based on the available literature on fungal infections of the liver, type 2 and 4 lesions were classified as "lesions suggestive of infection," Metser said.

The four lesion types included:

• Type I: hypoatteunating (Ia= sub-centimeter in diameter; Ib= at least 1 cm in diameter);
• Type II: hypoatteunating center with a hyperatteunating rim (IIa= thickness of hyperatteunating rim equal to or less than the radius of hypoatteunating center; IIb = thickness of hyperatteunating rim is greater than radius of hypoatteunating center);
• Type III: any lesion with an associated transient hepatic enhancement (defined as an adjacent wedge-shaped region of increased enhancement);
• Type IV lesion: hyperatteunating lesion (brighter than surrounding liver).