Resolution and Imaging Times

The typically attainable spatial resolution for brain MRI is approximately 1 x 1 x 3 mm3. Somewhat higher resolution can be attained in special circumstances (e.g., by using magnetic field strengths greater than 1.5 T or limited field-of-view RF coils). In general, the resolution is principally dependent on the signal-to-noise ratio (SNR) of the MRI signal produced by a single voxel. MRI ''noise'' results from the random movement of electric currents in the detection circuitry and in the subject. The noise level can generally be assumed to be a constant and only weakly dependent on the signal detection technique. Unfortunately, the MRI signal is relatively weak compared to that detected in many other forms of spectroscopy. It is often not appreciably larger than the noise. Reducing the size of the volume element to increase spatial resolution results in a proportional decrease in the SNR and this leads to an increased level of image ''graininess," complicating visualization of subtle features. SNR may be increased through signal averaging (i.e., by spending more time at image acquisition), although this provides meager returns. In general, the SNR is directly proportional to the square root of the time spent at image acquisition. Doubling the image acquisition time leads to only a V2 (about 40%) improvement in SNR. If one wishes to improve the resolution by twofold without loss of image clarity (i.e., maintain the SNR), one must increase the imaging time by fourfold.

The imaging time is also limited by the phase encoding process. A twofold increase in the resolution along the phase encoding axis requires that one spend twofold more time at phase encoding. Certain rapid imaging techniques partially circumvent this limitation. EPI and fast spin echo imaging procedures are examples. However, additional imperfections in image quality must be accepted as a consequence of more rapid imaging.

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