Given the strong technical components of fMRI, perhaps it is appropriate to close with a discussion of the ultimate spatial and temporal resolutions that might be obtainable in the near future. The temporal resolution of fMRI is not likely to be limited by the imaging tool but, rather, by the vagaries of the hemodynamic response. We are probably close to that limit already. Although there are aspects of temporal properties of hemodyamics that we can detect on the timescale of 100 msec, for most practical purposes the fMRI temporal resolution limit is likely to remain approximately 1 sec. Significant improvements in temporal resolution associated with fMRI are likely to result from integration with modalities such as EEG and MEG, whose intrinsic temporal resolution is the millisecond range.

Technological developments, especially in terms of higher field magnets and more sensitive and versatile imaging coils, will increase the effective spatial resolution of fMRI. Because higher field strength increases the signal-to-noise ratio, less averaging of signals across space is required, allowing for smaller voxels and thus greater spatial resolution. Although there are some safety issues that arise as field strength increases, there is no known reason why MRI cannot be done with humans at much higher field strength than the conventional 1.5 T. An increasing number of 3- and 4-T scanners are in routine use, and a handful of 7- and 8-T scanners for use on humans are either currently in use or are being built. Even without the higher field strengths, it would be possible to increase spatial resolution by using longer imaging times. However, the ultimate spatial resolution limits will not be determined by the MR scanner. Rather, they will be determined by hemodynamic limits, i.e., by the spatial resolution of the smallest vessels that show local changes with neural activity. Based on the available evidence, this limit is approximately the size of a cortical column—between 0.1 mm and 1 mm in linear dimension. If that spatial resolution could be achieved in routine fMRI-based experiments, it should represent a dramatic leap in the ability to develop and test far more interesting and explicit models of functioning neural systems in the human brain.

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