fMRI repetition time (TR) and stimulus duration I realized that in many neuroscientific experiments, the duration of the stimulus (say the duration of a grating stimuli displayed to subjects) is considerably shorter than the "TR" (roughly speaking the time between consecutive fMRI data acquisitions of the same cortical location) - an example would be a stimulus duration of 500 ms and a TR of 1.5 seconds.

My question: Does this not mean that some measurements have a very low signal-to-noise ratio, or can this approach be justified by the BOLD responses relatively long duration?
(f)MRI does not have good temporal resolution. If that is needed something like EEG or MEG is used, but the latter have low spatial resolution (hard inverse problem). 
Also note that the low temporal resolution of fMRI is due to BOLD response time (i.e. it's not just the case  that the current fMRI technology sucks)


  fMRI’s temporal resolution is limited by hemodynamic response time; typically the BOLD response has a width of ~3s and a peak occurring ~5–6s after the onset of a brief neural stimulus. This is much slower than the underlying neural processes, and temporal information is thereby heavily blurred.


Without getting into the specifics of the experiment (what's that you're actually trying to measure) it's impossible to say whether fMRI (=BOLD) would be inappropriate.

Also noise and blur are not the same issue; loot at some illustrations and it should be obvious without getting into math.