All the brain imaging techniques I know fall into two categories:
The problem with the first approach is that it is fundamentally secondary in nature: it tracks the response of the vascular system to brain activity. Although the BOLD signal correlates well with neural activity, it is still an indirect indicator of firing rate, has time lags, and limited spatial (limited by distribution of capillaries) and temporal (limited by flow rate) resolution.
The second approach is better, but still not perfect from a pharmacology perspective. Although it gives us good information about the post-synaptic potential (EEG) or intraneuron potential (MEG), it still provides no information on specific neurotransmitters released. The approach also suffers from limitations on spatial resolution and the fact that EM-fields follow the superposition principle and thus it is mathematically impossible to reconstruct signals that interfere destructively outside the neuron but before the sensors.
The perfect approach for a pharmacologist would be to track individual types on neurotransmitters. Are there imagining techniques that use the resonance, absorption, or other properties of neurotransmitters (instead of hemoglobin)? Or is there a fundamental reason why this won't work?
I know that the same exact techniques as for hemoglobin won't work (since as far as I know most neurotransmitters don't contain nice things like iron to play with), but is there a reason other resonance or scattering techniques wouldn't work?
The closest work I know in this direction is the event-related optical signal (Gratton & Fabiani, 2001) because it tracks the light-scattering properties of something other than hemoglobin: neural tissue. Unfortunately, this is still tracking activity of the neuron and not neurotransmitters.