Image Formation

Spatial Encoding with Gradients

The suggestion that the NMR signal could be used to form images was made first by Lauterbur[7] in 1973. Reasoning that the precessional frequency of the atomic nuclei depended upon the local magnetic field, he proposed that by forming a spatially varying magnetic field, it would be possible to separate the signal from different locations according to frequency. With a sample placed within a linear magnetic field gradient, for example, the Fourier transform of the signal would show its strength at each frequency, and thus at each position. Present day MR imaging instruments use three mutually orthogonal sets of electromagnetic "gradient coils" to encode the three spatial coordinates of the MR signal.

Imaging Speed

Detecting small differences in frequency (which in MRI, as discussed above, are equivalent to small differences in position) requires sampling the signal for a relatively long time; the smaller the frequency difference, the longer the time needed. In commercial MRI, one of the challenging tasks is to switch on and off the large magnetic field gradients needed for adequate spatial encoding in the limited time that the MR signal is available (which, as you will recall, is limited by T2). In conventional imaging instruments, this problem is handled by performing, in effect, only part of the spatial encoding at a time, and later re-exciting the MR signal to perform further encoding, repeating this process as many as several hundred times to form a complete image. For this reason, MR imaging times have traditionally been extremely long: from 3 to 15 minutes for an imaging series. By minimizing the perturbation of the magnetization from its equilibrium, a method known as FLASH [8,9] enables a reduction of the time between successive excitation pulses, and has recently permitted imaging times of less than a second, with some penalty in total contrast.

A method first proposed in 1977 by Sir Peter Mansfield, known as echo-planar imaging (EPI) [10], performs all required spatial encoding during the several tens of milliseconds that the MR signal is present, without resorting to repeated excitation-sampling cycles. This technically challenging method, reduced to practice in 1984 [11], and to high magnetic field whole body imaging in 1987 [12,13], makes it possible to form complete MR images in as little as 20 msec. Various modifications to EPI have been developed over the years that followed, to bring high resolution and controllable contrast to the technique, enabling a wide variety of novel medical and scientific applications [14,15]. Commercial devices with EPI capability are now available from several manufacturers. Both FLASH scanning (in about 6 seconds per image) and EPI (in about 0.1 second per image) achieve excellent functional MR imaging results. While the FLASH method allows good control of both T1 and T2* contrast, the EPI method is more flexible in controlling the relative contributions of T2m and T2D to the images.