Researchers at Ohio State University have developed a technique that fuses the images produced by nuclear medicine scans with images produced by CT scans.
The composite image combines the ability of nuclear medicine to identify the presence of certain tumors with the ability of CT to produce detailed images of the tissue in which the tumor resides, thereby increasing the accuracy of both techniques.
The combination technique promises to improve the treatment of certain tumors by providing surgeons with a more precise idea of a tumor's location, size, and extent before they begin to operate.
"Both of these imaging methods are powerful tools for cancer diagnosis, but both have deficiencies," said William Bennett, assistant professor of radiology at the Arthur G. James Cancer Hospital and Research Institute. The deficiencies are reduced when the images are combined.
Combination Technique Most Useful in Problem Cases
"Nuclear medicine scans can suggest the presence of a tumor, and a CT scan can show where the tumor is located." For this reason, he said, "the fusion technique is most useful in problem cases where there's confusion about whether the nuclear medicine study is truly positive, or if there's a question about the location of the tumor."
Nuclear medicine scans can often detect malignant tumors as small as 1 cm (less than 1/2 in), about the size of an individual lymph node. Objects that small would be difficult to recognize as cancerous by using a CT scan alone.
In a nuclear medicine scan, a tumor shows up as a bright spot on an x-ray-like image. The surrounding organs, however, are indistinct or not visible, which is one drawback to a nuclear medicine scan when used alone.
Also, the radioactive compounds used to label tumors in nuclear medicine studies are sometimes taken up by tissues that are not cancerous. This leads to false-positive test results in 10% to 15% of cases.
A CT scan of the same area of the body will show the internal organs in sharp detail, but the area of the tumor, while often visible, may not be large enough to be considered cancerous. As a result, tumor tissue can sometimes be missed when viewing a CT scan alone, producing a false-negative test result. False-negative CT scans occur in about 20% to 30% of cases, depending on the type of tumor and region of the body.
"But when the computer is used to color the image of the tumor in a nuclear medicine scan, and that image is combined with a CT scan of the same area of the body, they reveal exactly where the tumor is located," said Bennett.
New Technique Tested in Patients With Neuroendocrine Tumors
Bennett presented the technique at the December 1996 meeting of the Radiological Society of America. He and a team of researchers tested the method using nuclear medicine and CT scans from 35 patients who had already completed treatment. All the patients had neuroendocrine tumors, a relatively rare type of tumor that can occur in areas of the body such as the intestines, adrenals, pancreas, and lung.
In one case involved in the study, the fused image revealed a cancerous lymph node that had been missed by the patient's original CT scan. In this way, the new method could reduce the number of falsely positive nuclear medicine scans and falsely negative CT scans.
"This technique might have changed the therapy in about 25% to 30% of cases used for this study," said Bennett.
The method does have limitations, however. It requires that a patient receive both a CT scan and a nuclear medicine scan. It also takes additional time and expertise to fuse the two images. Moreover, both scans must include an identical landmark that the computer can use to match the two images correctly. The agent used for the nuclear medicine scan in this study was taken up by the kidneys, liver, and spleen, which are also imaged by the CT scan.
"We told the computer to use the kidneys, liver, or spleen as landmarks in both sets of scans," said Bennett. The computer then figured out how everything in the two images relates to the landmarks."
This work was done by researchers at Arthur G. James Cancer Center in conjunction with software developers at Picker International, Cleveland, Ohio, who produce nuclear medicine imagers and CT scanners.
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