Spatial and Temporal Resolution in High Speed MRI

The physiology of the circulatory system and the physics of the MRI devices constrain the spatial and temporal resolution of fMRI. Today, it is routine to obtain 1 x 1 x 1-mm structural MR images and 5 x 5 x 5-mm functional MR images. The temporal resolution of fMRI is on the order of 1-3 sec. Neither the spatial nor the temporal resolution numbers are indicative of absolute limits in terms of the physiology or the imaging hardware. Rather, they represent a snapshot in the development of ever-improving resolutions. Moreover, at any give stage of technical development in MRI, the various imaging parameters can be manipulated to emphasize one aspect of resolution in exchange for another.

When investigators approach experimental design in fMRI, they must recognize that the key physical variables—spatial resolution, temporal resolution, brain coverage, and signal-to-noise ratio—are quantities whose values can be manipulated by trading one off against the others. For example, extremely high spatial resolutions are possible, but the techniques needed to achieve them involve reduced temporal resolution, limited brain coverage, and/or decreased signal-to-noise ratio. Alternately, extremely rapid imaging can be performed, but at the cost of spatial resolution and/or brain coverage. Trade-offs will continue to exist even as the overall power of scanning technology improves.

As an indication of the numbers associated with these issues, and the manner in which they are changing, consider the issue of the rate at which individual images can be collected. The first whole-body high-speed (EPI) fMRI system could collect 20 images per second for about 1 min, and then it would overheat. At a slower operating rate of 10 EPI images per second (which was still very fast in 1992), there was no overheating, but the subject (in an fMRI experiment) had to wait a long time between scans for reconstruction of the images from the raw data. At an even slower rate of 5 EPI images per second, the scanner could operate continuously and reconstruct the images in real time, but the memory for buffering those images would fill after about 2000 images. In contrast, modern machines can operate at 20 images per second continuously.

Analogous improvement is ongoing in all of the mentioned domains. Higher field MRI (from 3 to 8 T) will improve spatial resolution and will yield the added signal that may improve the practicality of single-event fMRI designs and the possible use of the "initial dip'' in oxygenation to improve temporal resolution. However, these high-field machines are not as widely available as 1.5 T machines, and they are considerably more expensive, difficult, and dangerous to use.

Currently, the message for experimental design is simply that these resolution limits must be taken into account. There is little point to designing a conventional experiment to detect changes in a structure that is much smaller than one's spatial resolution permits, nor in designing a study that requires the detection of temporal changes that are too rapid for one's current technology. At the same time, because these imaging parameters can be traded off, one should not dismiss difficult-sounding experiments too quickly.

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