Adjusting Opacity of Volume Rendering to Visualize Segmented Lesions

I performed volume rendering on a volume from a CT scan.
I segmented the lesions within the said volume. Now, I have displayed both parts of the content in a 3D window.
However, the content of the volume rendering overlays the segmented content, preventing me from seeing the relative position of the segmented lesion with the head (as seen in Figure 1).

I am wondering if there is a way to adjust the opacity of the entire volume rendering so that I can visualize the segmented content.
I have been able to achieve this effect with other software, such as MRIcroGL (as shown in Figure 2).

Yes, you can via the Scalar Opacity maps. Please read the Volume Rendering section of the user documentation.

Another approach is not to waste computational resources and limit your rendering options by using volume rendering for displaying the skin surface. Instead, you can segment the skin surface using simple thresholding Warp solidify effect. You can then adjust the opacity of the skin surface segment to make the lesion in the brain visible.