Editorial

This special issue is devoted to a topic that has had an extraordinary history. After a few initial explorations, it lay dormant for more than a decade and then in the space of a few years it burst into fruition, rapidly making a transition from a basic research topic to widely used commercial products with significant impact on almost every facet of practice of Magnetic Resonance Imaging (MRI). Notwithstanding this rapid exploitation path, this topic remains an active area of research in its own right, maintaining a rapid pace of development in both concepts and applications. The field has become known as Parallel Imaging (PI); its defining feature is making use of spatial encoding inherent in the signals from multiple receiver coils arranged as arrays. Pioneering papers (1–3) all articulated in various ways the idea that reception of signals from a single object using multiple coils with distinct, if overlapping, regions of sensitivity provides inherent spatial information that can be regarded as a temporally parallel spatial encoding scheme. This powerful idea was not adopted by the mainstream of the MRI community, perhaps partly because of the clarity and comprehensive treatment of array coils provided by Roemer in his now classic 1990 paper ‘The NMR phased array’ (4). The Roemer paper did in fact contain ideas that directly related to PI; it was Roemer who observed that an optimal signal to noise ratio (SNR) image is formed from array coils only when the spatial variation in sensitivity of those coils is known. This requires what are now termed the coil ‘sensitivity maps’ to be determined and if this approach had been adopted, it would surely have lead to more rapid adoption of the early PI proposals. However, the opportunity was missed perhaps due largely to Roemer’s own elegantly practical solution of combining the signals from individual coils using a sums of squares at each image location, avoiding any need to calibrate coil arrays at time of use. As a result of this simple approach the array coil became synonymous with achieving large fields of view with improved SNR and remained in this paradigm for most of the next decade. The modern era of PI was initiated by Dan Sodickson in 1997, who started from what appeared at first to be a totally different formalism from the previous early proposals. The SMASHmethod (5) encapsulated its mathematics in k-space and was received with great enthusiasm at the ISMRM Vancouver meeting. It catalysed thought and action in this area by many individuals and groups. Prüssmann and co-workers followed up rapidly to provide a thorough image domain formalism, SENSE (6) that directly linked back to the early papers and introduced some key concepts that have become the bedrock of the language used in PI today. Many other important papers followed and the number of apparently distinct PI methods exploded over the next few years. As the subject matured, clarity gradually increased, and apparently disparate methods were revealed to be closely related or even equivalent, but with different combinations of processing in the image and Fourier domains. The importance of PI as a general tool has also emerged; it delivers not just reduced acquisition times but other benefits, such as reduced distortion in echo planar imaging (EPI). We are delighted that virtually all of the key researchers who contributed to the modern era of PI have contributed to this special issue. We also have taken great pleasure in soliciting contributions from some of the most talented newcomers to the field, proving we hope that there really is a vibrant PI community well into its second scientific generation. At the same time as the basics of PI were being vigorously debated and articulated in the research community the major manufacturers were taking advantage of their existing array coil receiver infrastructure to rapidly launch viable PI products that were immediately found to be useful and so incorporated into virtually every kind of MRI examination, from routine clinical study to advanced applications. This special issue is organised to take advantage of the state of knowledge and research findings at the time of writing. Accordingly some articles provide more mature in depth reviews, whereas others present rapidly emerging ideas that are yet to be fully explored. A particular case in point is the developing topic of parallel transmit in which the concepts that have proved so powerful in the reception and processing of signals are extended to the excitation process. We are grateful to all those who contributed to writing for this special issue and commend their papers to you the reader. We would also like to thank these same authors for the many stimulating debates and discussions they have contributed to during the rapid development of PI. As is inevitable in a field that is still evolving, we can expect new and exciting developments to come. However we hope this special issue will provide not just a snapshot of the present status, but also a valuable resource for the future.