Carotid Stenosis on CT Angiography

OsiriX and Horos Course · Vascular Surgery

In this lesson I demonstrate how to evaluate the carotid arteries on CT angiography using OsiriX, from the overview in 3D Volume Rendering to measuring the stenosis on the proper slice, comparing the NASCET and ECST methods. The main case shows an occlusion of the right internal carotid artery and a stenosis of the left internal carotid artery, with well-developed compensatory vertebral arteries. At the end, I show a second case with a glomus tumor to illustrate how 3D helps in diagnosis and planning. 1) 3D overview (before measuring) I scroll through the axial images to identify the aortic arch and the supra-aortic trunks I open 3D Volume Rendering and apply a vascular preset (e.g., Soft Tissue / Standard) In 3D it is already evident: Occlusion of the right internal carotid artery Stenosis of the left internal carotid artery Compensatory vertebral artery (with a larger caliber to supply the circulation) I also demonstrate the use of Crop to isolate the intracranial circulation and better visualize the vessels. 2) Confirming the occlusion (MPR) On the right side, in MPR: I rotate the axes and show that no internal carotid artery appears at any angle at the expected point of the bifurcation Practical confirmation of occlusion. 3) Finding the ideal slice of the stenosis (contralateral side) To measure the stenosis correctly: I zoom in I scroll up/down to the bifurcation I rotate to get the best view of the plaque (calcium + contrast-enhanced lumen) I adjust brightness/contrast to differentiate: contrast calcium vessel wall (estimated) 4) Two ways to measure: NASCET and ECST (with a numerical example) I show the two classic methods: NASCET Uses the diameter distal to the lesion (“normal” internal carotid artery beyond the plaque) Compares it with the diameter at the point of stenosis ECST Uses the estimated diameter of the bulb/vessel at the site of the lesion Compares it with the diameter at the point of stenosis In the video example: Distal diameter (NASCET): 5 mm Estimated diameter at the site (ECST): 8 mm Diameter at the point of stenosis: ~1–1.2 mm (I show the measurements on screen) Applying the formulas: NASCET ≈ 76% ECST ≈ 85% Important: there is a relevant difference between the methods, because the “reference diameter” is different. 5) Warning: do not use MIP to measure carotid stenosis I reinforce a warning: MIP may make the image look nice, but it can underestimate the stenosis if there is a thin “thread” of contrast passing beside the plaque, or overlapping structures that misrepresent the true lumen. To measure stenosis, avoid MIP. 6) Practical uses for planning (open and endovascular) With 3D and the correct slice, you can: Estimate the height of the lesion relative to the mandible (access planning for open surgery) Assess whether the carotid is straight enough to accommodate a filter (endovascular) “Clean up” the field with the scissors to better see the supra-aortic arch and anticipate how easy catheterization will be 7) Additional case: glomus tumor I show a second case where: 3D reveals a large glomus tumor On the axial view, the tumor appears to involve the internal and external carotid arteries (characteristic of a difficult surgical case) I also briefly explain the choice between Resample and Clip when the software warns about dynamic range/gray levels: Resample: “compresses” values to fit the scale (heavier) Clip: cuts off the extremes (faster)