MRI is different from all other diagnostic modalities in that the hardware configuration plays little role in the image contrasts that can be collected, these instead being dictated by software through the scanning sequences. Several new methods show potential. Perfusion measurements have become central to many neurological protocols (particularly in acute stroke), based around kinetic tracking of a bolus contrast agent injection. For the past 10 years alternative non-contrast agent approaches have been developing, known collectively as arterial spin labelling (ASL), and at 3T these sequences show real promise of providing clinically useful images of cerebral blood flow CBF [Figure 2, bottom right].
Image contrast generated by any MRI sequence relies (partly) upon the rate at which the transient MR signal is decaying away (the relaxation time T2) the time at which the signal is recorded (the echo time, TE). Recently clinical research findings have been published using ultra-short echo time (UTE) sequences where TE has been reduced from its typical range of 1-10 milliseconds down to 10-50 microseconds . This has created a whole new set of image contrasts allowing direct imaging of tissues which have traditionally not been seen by MRI
(e.g. ligaments and tendons, ).
While conventional T1 and T2 weighted scans will always remain the bed-rock of the clinical examination, it is clear that truly quantitative scan types will be needed as we enter the era of personalised medicine, where non-invasive monitoring of treatment response will be increasingly important. Integrated examination protocols collecting fully quantitative measures are already possible in the brain [Figure 2], providing data on metabolic, physiological, micro- and macro-structural tissue integrity, and these may well become the norm. These fundamental biological parameters are inter-related and the relationship between them is likely to provide additional detail on tissue injury or disease. Advanced image processing techniques are currently being developed that will provide the radiologist with tools to assimilate the many types of data.
New contrast agents
As clinical treatment of many conditions become increasingly personalised, improved diagnosis, and hence specific selection of therapeutic treatments based on knowledge of disease phenotype, will become important. For many years PET has offered the ability to map molecular pathways, receptor density, etc, via appropriate radioligands, but PET cannot provide the level of spatial resolution offered by MRI, and is limited in availability. MRI is responding to this challenge through the development of targeted MR contrast media that bind to specific molecular or cellular targets. This work is currently in its infancy, but recent data in animal models have shown the power to detect the earliest phase of brain injury . Although the transfer of such technology into useful clinical compounds requires extensive development and product licensing, these data show clear potential for this technique.
MRI has continued to rapidly develop since its introduction as a clinical tool in the early 1980s. Widespread use of 3T scanners is already becoming a reality and future developments in coil technology and new image contrasts will continue to provide new tools for clinical diagnosis. Combined modalities and targeted contrast media are further on the clinical horizon, but will become a reality for specialist referral centres in the coming years.
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For a more in-depth discussion about future MRI developments, the reader is directed to the recent review by the same author .
Andrew M. Blamire, B.Sc., Ph.D.
Professor of MR Physics,
Newcastle upon Tyne,