Does a magnetic resonance venogram have superior image quality to a ct venogram?

No, CT spacial image: detail quality (partially older technology with more improvements over time) remains much superior to MRI. Yet CT is radiation, MRI (actually NMRI) no radiation, but slower, more noise & better at distinguishing soft tissue differences. Yet both only show location of dye in vein, not the vein itself, thus can easily miss issues present but not reflected in dye column.