Ultrasound studies were acquired according to a standard protocol using a GE-Logiq-700 (Issaquoah, Illinois). A high-resolution M12L transducer operating at a frequency of 13MHz was used to image the common carotid artery and a 9 MHz frequency for the carotid artery bulb and proximal internal carotid arteries. Sonographers were trained centrally and then underwent a certification process.
Image acquisition was made at end-diastole by a certified sonographer selecting the image with the lowest arterial diameter and then saving the selected images on a super VHS-videotape. The image series used to select the images were also recorded so that the selection could subsequently be confirmed during the reading process. On each side, one image was obtained at the level of the common carotid, before the bifurcation. Two images were then acquired at the carotid artery bulb, and two images were obtained in the proximal 2 cm of the internal carotid artery proper after the flow divider. The first image was taken at approximately 45 degree to the horizontal while the second was set more vertical near to 20–25 degrees.
A certified reader reviewed the videotape and digitized images with the aid of an image analysis workstation. The software integrated validated image analysis algorithms interfaced with an Access database to store the IMT measurements. The high-resolution images of the different carotid artery segments were used to calculate the intima-media thickness of the far or near wall on each image after the operator traced the respective lumen-intima and media-adventitia interfaces over a 1 cm distance with the aid of a Wacom imaging tablet. Any atherosclerotic plaque was included as part of the intima-media and a note was made about the extent of stenosis that existed anywhere in the right or left carotid artery. The mean of the maximum wall thickness of the respective carotid artery segment was defined as the mean of the mean near and far wall thickness for each of the images taken on the left and right side, four for the common carotid arteries and eight segments for the bulb and internal carotid arteries respectively. Carotid ultrasound studies were acquired in 3258 of the 3549 (91.8%) participants seen in the clinics. Pearson correlation coefficients based on 58 replicate studies were 0.86 for the common carotid artery, 0.72 for the bulb and 0.88 for the internal carotid artery.
Image acquisition was made at end-diastole by a certified sonographer selecting the image with the lowest arterial diameter and then saving the selected images on a super VHS-videotape. The image series used to select the images were also recorded so that the selection could subsequently be confirmed during the reading process. On each side, one image was obtained at the level of the common carotid, before the bifurcation. Two images were then acquired at the carotid artery bulb, and two images were obtained in the proximal 2 cm of the internal carotid artery proper after the flow divider. The first image was taken at approximately 45 degree to the horizontal while the second was set more vertical near to 20–25 degrees.
A certified reader reviewed the videotape and digitized images with the aid of an image analysis workstation. The software integrated validated image analysis algorithms interfaced with an Access database to store the IMT measurements. The high-resolution images of the different carotid artery segments were used to calculate the intima-media thickness of the far or near wall on each image after the operator traced the respective lumen-intima and media-adventitia interfaces over a 1 cm distance with the aid of a Wacom imaging tablet. Any atherosclerotic plaque was included as part of the intima-media and a note was made about the extent of stenosis that existed anywhere in the right or left carotid artery. The mean of the maximum wall thickness of the respective carotid artery segment was defined as the mean of the mean near and far wall thickness for each of the images taken on the left and right side, four for the common carotid arteries and eight segments for the bulb and internal carotid arteries respectively. Carotid ultrasound studies were acquired in 3258 of the 3549 (91.8%) participants seen in the clinics. Pearson correlation coefficients based on 58 replicate studies were 0.86 for the common carotid artery, 0.72 for the bulb and 0.88 for the internal carotid artery.
