Development of NMR: Magnetic Resonance Imaging During the Past Two Decades

The inception of spatially localized nuclear magnetic resonance (NMR) that led to medical imaging within a decade, and the ongoing refinements of the methodology during the past almost four decades, arguably represent one of the most significant advances in medical diagnostics during the second half of the twentieth century, which continues with a host of innovations. It is therefore not surprising that the two key contributors, Professors Paul Lauterbur and Sir Peter Mansfield, who conceived the basic ideas and first demonstrated the feasibility of NMR to generate spatial maps of hydrogen density, were awarded the 2003 Nobel Prize in Physiology or Medicine. The history of magnetic resonance imaging (MRI) has been reviewed previously (Wehrli, Progr NMR Spectr 1995) and notably, as part of a general review of the history of NMR, by Becker et al., in the original edition of this encyclopedia in 1996. The early phase comprising the period following the method's inception from 1972 to 1990 is therefore not covered in detail except where reference to critical precedents was deemed necessary. MRI is far richer than any other imaging technology since there are literally infinitely many ways to manipulate nuclear spins and tailor the experiment in such a manner that specific information can be extracted. While initially a structural imaging technique, it was soon recognized that the method was equally able to afford functional and physiological information, such as measurement of blood flow—both macroscopically and at the capillary level—entirely noninvasively—adding another dimension to the method. The discovery of the BOLD effect in 1990 and the subsequent demonstration that the response to neural stimulation manifests in a subtle change in local signal intensity, revolutionized neuroscience by providing unprecedented insight into the functioning of the human brain. Although the NMR signal is intrinsically weak, numerous technological advances have led to a clinical imaging modality that can detect disease with unprecedented sensitivity and acquire images at a speed approaching real time. The evolution of most technologies behaves as a sigmoid curve, with a slow beginning, followed by a phase of rapid advances, eventually decelerating during the final maturation phase. For a modality that approaches the beginning of the fifth decade since its inception, one would expect MRI to be a mature modality. However, MRI seems to defy this pattern as it continues to evolve at a vigorous pace as manifested by numerous technological advances that include ultrahigh-field imaging, parallel reception and excitation, the emergence of new detection schemes, the resurgence of radial imaging, in particular in conjunction with sparse sampling, hyperpolarization of rare spins, molecular and cellular imaging, and more, with no end in sight. These capabilities, in turn, drive the emergence of new applications, some of which are entirely unpredictable. Keywords: MRI history; MRI technology; magnetic field strength; radial imaging; spatial encoding; k-space; Fourier reconstruction; parallel imaging; compressed sensing; superconductivity