Imaging of infection provides a classic example of nuclear medicine's strengths and weaknesses. Radiopharmaceutical tracers can locate infection sites with great accuracy. As with oncology imaging, however, the absence of anatomic landmarks makes it difficult to determine the location to which the hot spot on a color map corresponds. A better road map, capable of locating the position of signal more precisely, would make it easier to diagnose the cause of infection and plan the most appropriate treatment.
A number of groups have begun investigating the value of combining SPECT and CT data for imaging infection. Early results suggest considerable promise for hybrid imaging in improving diagnostic specificity. More work is needed, however, to assess the true role of SPECT/CT in this application and to identify any added value that multislice CT might bring.
Dr. Marius Horger, a radiologist at the Eberhard-Karls University in Tubingen, Germany, began to explore the use of combined SPECT/CT in patients with chronic osteomyelitis when his department purchased a SPECT/CT scanner with single-slice CT in December 2000. The results of combined CT and immunoscintigraphy using technetium-99m-labeled antigranulocyte antibodies proved so successful that the protocol has been adopted in clinical practice.
"We were looking at patients who had undergone repeated surgery or trauma and had known bone distortion with remodeling. It was very difficult to correctly localize bone infection in these patients," Horger said.
At Tor Vergata University in Rome, SPECT/CT helps differentiate bone infection from soft-tissue involvement. Dr. Orazio Schillaci, a nuclear medicine physician, uses leukocytes labeled with either Tc-99m or indium-111 to assess the site of inflammation. The information from the SPECT scan is combined with CT data from an integrated single-slice scanner to gain an accurate map.
Further evidence that combining SPECT and CT data boosts diagnostic decision making in infection imaging comes from researchers at Boston University Medical Center. Two nuclear medicine radiologists reviewed SPECT images, side-by-side SPECT and CT images, and software-fused data maps for 26 patients with known abnormalities. Their sample included 11 In-111 white blood cell studies and six gallium-67 scans for a variety of infections.
The researchers scored the value of each image for abnormality identification, anatomic localization, and diagnostic confidence. They found that, overall, SPECT/CT fusion imaging outperformed side-by-side comparison for all three parameters. The group presented their results at the 2004 RSNA meeting.
"The study is limited in that we had selected cases that we already judged as abnormal, so it is difficult to judge sensitivity and specificity," said Dr. Elizabeth Oates, section head of nuclear radiology at Boston University. "But the strength of the fusion came in the localization and the confidence we felt that disease was truly present, and we knew where it was and felt better about what we were seeing."
The key question remains the likely clinical impact: Does added diagnostic confidence translate into improved patient care? The answer is a resounding yes, according to Prof. Ora Israel, director of nuclear medicine at the Rambam Medical Center in Haifa, Israel.
Israel's team is currently collating data from about 100 such cases. Each patient underwent SPECT/CT using either Ga-67- or In-111-labeled leukocytes. Results indicate that this approach could prove valuable in diagnosing infection and in localizing and tailoring treatment for patients with known infectious processes, Israel said.