Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 17

Paul Callaghan [1] and others [11–14] showed that the imaging resolution of MR microscopy is fundamentally limited by three factors: the diffusion coefficient of molecules within the sample, the line broadening due to magnetic susceptibility effects, and the specified SNR per voxel. While the limits placed by the first two factors can be pushed by stronger field gradients and dedicated pulse sequences [15], the rather poor SNR of the MR signal remains as the ultimate fundamental limit of resolution. This can be clearly seen from the following equation [16,17], which summarizes the factors that determine the achievable resolution for a specified SNR of the image:

(1.2)

where V_voxel is the voxel volume, d is the coil diameter, t_acq is the total acquisition time, and B₀ is the field strength. According to this equation, maintaining the image SNR while, for instance, halving the isotropic resolution (reducing the voxel volume by a factor of 8) necessitates either miniaturizing the coil by a factor of 8, increasing the acquisition time by a factor of 64, or increasing the B₀ field by a factor of 3.28. This explains why high-resolution MR images take excruciatingly long to acquire, and why most groups decrease coil diameter. However, at room temperature, coil diameter cannot be reduced indefinitely without disadvantageously increasing coil resistance, so that quality factor Q will ultimately limit this strategy.