I guess you try to remove the contrast agent from the vessels to make them transparent, so that you get an endovascular view.
The bad news that you cannot easily achieve this with masking, because you will have very high intensity voxels (remnants of the contrast agent or the walls/vessels) right next to very low intensity (blanked out voxels), which will be rendered with staircase artifacts. With some effort you could soften it (create a black-and-white volume, then soften the edges with Gaussian smoothing, then subtract it from the original volume), but you may end up with blurred walls.
The good news that you don’t need to rely on segmentation to render cardiac CT images in endocardial view. You just need to edit the volume rendering transfer functions to make contrast agent transparent.
Default CT Cardiac volume rendering preset:
With contrast agent (approximately 300-700HU) made transparent: