Is there a difference between visual sensation and imagination in the brain?
Short answer
Visual perception and visual imagery draw on much of the same neural machinery.

Background
I have interpreted your question as: What are the common neural circuitries between visual sensation and the imagination of sensation? 

In neuroimaging, mental imagery of visual images is a big deal. For example, there is a large body of literature on cross-modal activation of visual areas in the brain in blind people. It is known that visual deprivation results in neural plasticity and the recruitment of visual areas in the brain for other sensory systems. For example, blind Braille readers show activation of the primary visual cortex when reading Braille (reviewed in Stronks et al., 2015). However, interpretation of these findings is difficult in late-blind individuals, because they have experienced visual input earlier in life. Hence, while Braille reading they can be mentally reproducing the visual representation of the Braille cells using visual neural circuitry.

Indeed, it has been shown with fMRI that visual imagery and visual perception draw on most of the same neural machinery (Ganis et al., 2004). However, the spatial overlap of the activated regions is neither complete nor uniform. The overlap in this study was more pronounced in frontal and parietal regions than in temporal and occipital regions, indicating that cognitive control processes function comparably in both imagery and perception, but not identically.

Various studies reveal different results, however. In another imaging study, 'just' two-thirds of brain regions overlapped in visual sensation and imagery (Kosslyn et al., 1997). Indeed, the experimental task used may have important effects on study outcomes.

Most notably in this regard is that approximately half of the studies done have found activation of the primary visual cortex during imagery (Kosslyn et al., 1999). This is interesting, because the primary visual cortex is generally thought to be an early, low-level area in the visual system that depends on thalamic input that relays information from the optic nerve to the brain. I.e., it is generally believed to depend on sensory stimulation. Generally, only the higher-level associative visual areas are associated with higher cognitive processes.  

References
- Ganis et al., Cognitive Brain Res (2004); 20: 226–41
- Kosslyn et al., Neuriomage (1997); 6; 320–34
- Kosslyn et al., Science (1999); 284; 167–70
- Stronks et al. Brain Res (2015); 1624: 140–52