NEW YORKThinner multislice CT scans are increasing the effectiveness of lung cancer screening, a German investigator said at the 3rd International Conference on Screening for Lung Cancer. U. Joseph Schoepf, MD, of the Institute of Clinical Radiology, University of Munich, showed images from 1 mm slices achieved with the Siemens Volume Zoom. "Obviously, we use the fastest rotation mode that we have available," he said. "That’s a 500 millisecond rotation. With a collimation of 1 mm, that allows us to cover the entire chest in a single breath-hold of about 20 to 25 seconds."
Because the population being screened is generally healthy, the radiation dose is of concern to Dr. Schoepf. He has, therefore, opted for doses of 10 to 40 mAs at 120 kV, depending on body type. With a dose of 10 mAs, he noted, "we have an effective dose exactly equivalent to the chest x-ray in two planes."
At Dr. Schoepf’s institution, one of the cutting-edge multislice scanners is dedicated to screening. "We’re doing anywhere from 50 to 60 cases a day on that machine," he said. In addition to screening for lung lesions, the scanner is used to check for cardiac and colon disease.
To "defray" the data load generated by multiple 1 mm slices, Dr. Schoepf reported, "we’re just doing one reconstruction that is saved." A number of visualization techniques are then applied to that single data set, enabling the radiologist to read the scan.
Because the data set is composed of 1 mm pixels, Dr. Schoepf said, "you can reconstruct such a data set in arbitrary imaging planes, and that helps with the visualization of the exact anatomic location of nodules. This is a good way of dealing with data."
Image noise, Dr. Schoepf noted, may interfere with visualization, particularly with low doses of radiation. The Siemens equipment, he said, provides techniques to remove image noise while retaining the view of the nodule.
Nodule characterization is another advantage of the 1 mm slice scans, Dr. Schoepf said. With a 10 mm slice, he pointed out, "we don’t really know what’s going on within such a nodule." When it is viewed in 1 mm slices, however, calcifications indicating a benign lesion can be seen within the nodule.
With this technology, he noted, the need to call people back for high-resolution studies is eliminated. "We have this data set available right away, and we can look at it any time," Dr. Schoepf said. "This is a great improvement in terms of work flow, how you organize your work."
One of the tools Siemens provides with the scanner allows the radiologist to rotate around the nodule. Being able to do so, Dr. Schoepf said, makes it possible to determine whether a nodule is attached to a vessel or even whether a suspicious spot is actually a nodule.
This technology, Dr. Schoepf said, can be used to help determine whether a nodule is malignant or benign. By segmenting away data such as attached vessels, "you get a much better volume assessment of the nodule," he said. "Documented nodule growth is the only thing that we have available to tell malignant from benign lesions. This allows us to avoid unnecessary thoracotomies and thus decrease the comorbidity caused by our screening tests ."
If a lung cancer is detected, the 1 mm slice "gives us a unique opportunity," he said, for staging and planning treatment. In some instances, he noted, it is possible to visualize tumor invasion of the mediastinum "in an unprecedented way." In others, it is possible to see distinctly small lymph nodes that in a larger slice would "just be fused together by volume averaging" into a large node that would be suspected of being malignant.
Nodule visualization is especially important in preoperative planning, Dr. Schoepf commented. "Our colleagues in thoracic surgery do a lot of video-assisted thoracoscopy," he said, "and this is a very good road map to tell them where to go and where to do their resection." The scans, he noted, enable the surgeons to see whether a nodule is attached to vessels and to localize the site of tiny lesions.
For automated reading of lung scans, Dr. Schoepf sees a definite advantage with 1 mm slices because the conspicuity "is so much greater than on a thicker slice." Thicker slices have "a lot of what we call volume averaging going on," he said. "But if you have a 1 mm slice, your lesion detection and characterization should definitely improve." He and his colleagues are testing a prototype of a machine to make computer-aided diagnoses.
"I believe that multislice CT is a very promising tool for the early detection of lung cancer," Dr. Schoepf said. With the technical ability to fully characterize nodules from the scans, he added, the effectiveness of lung cancer screening should be increased by use of multislice CT.