Rapid MR Imaging: Techniques and Performance Characteristics

The strength of the NMR signal reflects the rate of quantum state changes of protons - particles of infinitesimal size. Remarkably, however, these transitions occur very slowly, on a time scale of tenths of seconds to seconds for most biological materials. In this chapter, we shall see how the magnetic relaxation rate constants T1 and T2, which describe the rates of these quantum state changes, impose limitations on the practical speed of the MR imaging experiment. These limitations cause MRI as currently practised to be among the slowest of the diagnostic imaging modalities. Our primary focuses, however, will be on techniques developed in recent years to overcome some of these speed restrictions, on consequent changes in MR image contrast, and on applications the techniques engender. In particular, the reduced flip angle methods, known generally by the acronyms FLASH, GRASS, CE-FAST and MP-RAGE, will be discussed. Next, methods which improve the efficiency of the MR experiment by increasing the portion of the MR image that is collected after each excitation will be considered. Finally, the imaging results of the various methods will be compared. The period from 1985 to the present has seen an explosive growth in the range and power of high speed MR imaging methods. Consequently, it is possible to discuss only a few of the diverse techniques designed to increase MR acquisition rates in a single, short chapter. Threrefore, the goal here is to provide a basis for understanding developments that shall continue to emerge in pace with the importance of MRI in radiology.