Volume rendering uses a finite sample spacing, so in order to avoid those ripple artifacts, you must ensure that the highest spatial frequencies in the image are below the Nyquist frequency of the sampling.
I highly suggest some documentation on this fact in the beginner examples or on volumetric rendering, or something that will communicate this to beginner users of VTK. I’ve been wondering about these ripples for about a year now, and your statement makes a lot of sense now.