Hybrid MR-PET Imaging: Systems, Methods and Applications
Published:29 Nov 2018
Special Collection: 2018 ebook collectionSeries: New Developments in NMR
Hybrid MR-PET Imaging: Systems, Methods and Applications, ed. N. J. Shah, The Royal Society of Chemistry, 2018, pp. P007-P009.
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In the latter half of the twentieth century, the advent of positron emission tomography (PET) and magnetic resonance imaging (MRI) revolutionised in vivo tomographical imaging. PET rapidly became an invaluable research tool and later migrated into clinical diagnostics. MRI followed the same path some years later. Notwithstanding their similar trajectories, the reasons for their general acceptance and uptake in their respective communities differed enormously, even from the outset. Whereas PET excelled as an in vivo metabolic technique, MRI quickly established itself as the method of choice for structural imaging, providing excellent soft-tissue contrast with exceedingly good spatial resolution, which today approaches <100 microns in ultra-high field human scanners.
Even since its initial conception, MRI has shown remarkable versatility. This largely stems from the fact that as MRI is based on NMR, it can be seen as NMR spectroscopy, with the use of gradient magnetic fields to encode space. Given this fact, it is not difficult to appreciate that many NMR techniques—which have a long and rich history—can be turned into MRI methods, with each one potentially delivering a new contrast mechanism. Moreover, the often-quoted maxim that “one man's artefact is another man's effect”, has also been put to good use by the MRI community. Functional MRI is based largely on the susceptibility ‘artefact’ created by paramagnetic haemoglobin, for example. Furthermore, MR methods, such as diffusion weighted/tensor imaging, and chemical shift imaging, now provide connectivity and metabolic information beyond anatomy. Unlike other imaging modalities such as computerised tomography (CT), MRI does not involve ionising radiation, making it suitable for repeated scanning or, in particular, for use in the paediatric population.
Not to be left by the wayside, developments in PET have concentrated on the use of more and more tracers, each one specifically targeting a particular disease. This, coupled with the fact that PET has an exquisite sensitivity many orders of magnitude better than that of MRI, has led to new areas of research as well as diagnostic questions.
Naturally, the underlying physics of the two methods ensured that their development remained separate for many years, as the strong magnetic fields needed for MRI remain the antithesis of the environmental requirements for the operation of photomultiplier tubes in PET. Nonetheless, a number of groups recognised the complementarity of these two imaging modalities and began to lay the foundations for what has now become a fully-fledged field of research and development.
When one considers the complementary advantages of MRI and PET, and the success of the hybridisation of PET and CT into one combined PET/CT instrument, the benefit of combining MRI with PET in one device is obvious. Previously, PET and MRI had already been used in combination, whereby images from the two modalities had been registered and fused by using appropriate software tools, and such a combination had been particularly successful in brain imaging, where only a rigid-body registration is necessary. However, the combination of MRI and PET in one instrument with one patient bed is most advantageous in whole-body imaging. Utilising the specific advantages of MRI and PET, an integrated PET/MR-scanner or MR-PET scanner, as it is commonly called, allows a multi-parametric investigation in a “one-stop-shop” manner.
There is a lack of textbooks addressing the needs of readers wishing to gain an oversight of hybrid MR-PET. Reaching for two separate texts is not particularly helpful in this case, since the specific issues associated with bringing the two modalities together are usually not discussed in sufficient detail. This book is concerned with the underlying principles of MRI and PET and, in particular, hybrid MR-PET. In practical terms, at the level of scanner construction and operation, hybrid MR-PET is a relatively new area of research and is opening the door to novel diagnostic approaches. Whilst initial approaches were based on the work of academic research groups, the field has now moved into the commercial sector with academia pursuing further advances such as hybrid MR-PET at 7 T for human applications.
The book is divided into five parts—from A to E—and each part is further sub-divided into sections. Part A, Section I addresses the basics of MRI complete with selective applications and the up-to-date topic of ultra-high field MRI. Part A, Section II addresses the basics of PET and includes PET quantification and kinetic modelling. Part B gives a technical overview of hybrid MR-PET and addresses the issues involved once these two modalities have been brought together. Part C deals with applications in human of MR-PET dividing them into brain and whole-body applications. Part D outlines preclinical applications including a brief discussion of the hardware of preclinical hybrid scanners. Finally, PET would be nothing without tracers and this topic is therefore presented in Part E, outlining tracer production.
The book is aimed at several audiences simultaneously, as befits a book on simultaneous hybrid imaging. A novice graduate student entering the field will benefit from the whole book, whereas experts in MRI will find Part A, Section II on PET to be instructive before going on to read Parts C, D, and E on hybrid MR-PET, applications and tracer production, respectively. Conversely, the PET expert will benefit from Part A, Section I on the basics of MRI and the other parts as described above.
I wish to thank the many authors who contributed to the various chapters and sections whose names are included with each chapter.
Finally, I would like to extend my gratitude to Claire Rick for invaluable assistance throughout the editorial process and to Martina Bunn for administrative help.
N. J. Shah