Then the VEP would be what you're looking for, tests that usually are referred to as imaging, such as CT or MRI wouldn't be able to show you the signal being transmitted. As CT and MRI would only visualize the structure of the optic nerve and not the signal traveling along the optic nerve. It might be best to think of a CT and MRI like taking an x-ray of a TV, you'll be able to see all the bits and pieces that make up the tv but the X-ray won't be able to show you the image the tv was displaying at the time.
A PET-MRI, which is basically an MRI with color added to show activity, might be able to show the activity along the neuron (like showing which bits of the tv are ative), however a PET-MRI likely wouldn't be fast enough to capture a single signal and might not have the resolution needed to image a single nerve, even one as relatively big as the optic nerve. Especially given the location of the optic nerve, which is lying for the most part directly below the frontal lobe.
The pulses moving along neuron's are generally best captured by EEG. Which is what a VEP does (the bottom right figure is the read out of the EEG and the dips and spikes in it would be the signal traveling through the optic nerve to the brain.