CHICAGO – The results of a multicenter study have revealed that helical CT (HCT) is as good as electron beam CT for detecting and measuring coronary calcium, particularly when retrospectively gated single-slice HCT is used.
Dr. Kishore Acharya from GE Medical Systems in Waukesha, WI reported at the RSNA meeting on the results obtained from 106 patients imaged at three institutions: Highland Park Hospital in Highland Park, IL, Wake Forest University in Wake Forest, NC, and the National Institutes of Health (NIH) in Bethesda, MD. Other investigators included Dr. Jeffrey Carr from Wake Forest University and Dr. Alex Ling from the NIH.
All three sites used the same equipment and protocol: Single-slice HCT systems of capable of sub-second scanning with retrospective ECG gating. Scanning was done in helical mode at 3 mm-slice thickness with pitch ranging from 0.8:1 to 1.2:1 depending on the patient’s heart rate. Pitch was selected so that the table advanced by 3 mm or less in one heart cycle. Images were generated at 0.3 mm intervals using a half scan reconstruction algorithm, and the retrospective ECG signal was used to select an image per cardiac cycle during diastole.
Patients also underwent electron beam CT (EBCT) scans with 3 mm-slice thickness, no overlap, and 70 to 80% into the cardiac cycle. Conventional Agatston Janowitz (AJ) scoring was used for the EBCT images; a minimum lesion size of 0.25 mm2 was assigned to the images obtained with HCT.
“At NIH and Wake Forest, patients were scanned back to back,” Acharya said. “At Highland Park, scans were done within one week. The mA went from 100 to 250 depending on patient size and doctor preference.”
According to the results, calcium threshold in all 106 patients ranged from 0 to 4,000 (AJ threshold, 130 HU). The data was divided into four score groups: 0-10, 11-100, 101-500, and greater than 500.
In the 0-10 group, EBCT and HCT scores were the same in 38 patients, while HCT underscored four patients. In the 11-100 group, the results of 14 scans matched, two were underscored by HCT and five were overscored. In the 101-500 group, 13 matched, 1 was overscored, and six were underscored by HCT. Finally, for scores about 500, 21 matched and two were underscored.
“The EBCT score is slightly higher than HCT score,” Acharya said, adding that differences in temporal resolution, scanning protocol, and system characteristics contributed to the difference in score. But the close match in AJ scoring between EBCT and HCT was significant and highly correlative, he said.
Session moderator Dr. William Stanford from Iowa City asked if the difference in calcium scoring categories changed treatment protocol. Acharya responded that because the differences were so subtle, treatment protocol was never significantly altered for any of the patients.
A session attendee asked how the group avoided double-scoring. Acharya said retrospective gating was used so that “we looked at the heart rate and, based on that, advanced the table so that there was no overlap.”
In a previously published studies, the researchers from Wake Forest University pointed out that HCT was a viable “an alternative method for measuring coronary calcium because of its greater availability and lower cost. It could make population-based screening for coronary artery calcium more feasible, they said (American Journal of Roentgenology, April 2000, Vol.174:4, pp. 915-921).
By Shalmali PalAuntMinnie.com staff writer
November 26, 2000
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