¹H-MRS Detection of Dopamine
The concentration of dopamine is low
The concentration of other protons is very high
The magnetic field is not homogeneous.  
The brain cannot be rapidly rotated the way a laboratory solution state NMR sample is to average magnetic field inhomogenity
Compare to laboratory solution state NMR, the viscosity in the brain would be high causing further line broadening.  


If you look at the spectra in 1H NMR Probe for in Situ Monitoring of Dopamine Metabolism and Its Application to Inhibitor Screening J. Am. Chem. Soc., 2012, 134 (30), pp 12398–12401, you'll see that even in tissue samples, there are too many other stronger signals for conventional 1H NMR to be successful, so C-13 labeling is used in combination with 1H NMR.