Synopsis
Since the introduction of the first 7 Tesla
system in 1999, steady improvements in instrumentation and ever expanding
armamentarium of image acquisition and engineering solutions to challenges
posed by ultrahigh fields (UHF) has brought UHF imaging to exquisite anatomical
detail and biological information content in many organ systems of the human
body. However, like all technologies, magnetic resonance applications at UHF
have applications-specific advantages and limitations. This lecture will aim to
clarify the primary advantages that we should exploit (“what we want”), while
avoiding some of the pitfalls that detract from these advantages (“what you
don’t want”).
TARGET/Audience
Investigators interested in using ultrahigh
field (7 Tesla and higher) magnetic resonance imaging and spectroscopy for research and/or
clinical applicationsPURPOSE
Following early efforts in applying nuclear
magnetic resonance (NMR) spectroscopy to study biological processes in intact
systems, and particularly since the introduction of 4 Tesla human scanners
circa 1990, rapid progress has been made in imaging and spectroscopy studies of humans
at 4 Tesla and animal models at 9.4 Tesla, leading to the introduction of 7
Tesla and higher magnetic fields (i.e. Ultrahigh Fields (UHF)) for human
investigation at about the turn of the century (1). Work conducted on these platforms revealed the
existence of significant challenges (e.g. (1,2)) to working with objects the size of the human head and the torso at such high
magnetic fields. Emergence of numerous technological solutions to these challenges
(e.g. (1-4)), however, have ultimately led to the demonstration of significant UHF advantages in signal-to-noise
ratio (SNR), contrast-to-noise ratio (CNR), and biological information content (e.g.
(1,4-8)). Nevertheless,
it is still common to encounter not only skeptics who often posit that
everything can be accomplished at 3 or even 1.5T, but also ultra-optimists who expound
unrealistic capabilities. For example, it is virtually impossible today to
encounter a presentation on UHF imaging and/or spectroscopy application in
humans or animal model systems that would not enthusiastically list as a
foregone conclusion that SNR increases with magnetic field strength. Indeed it
does (e.g. (1,9)),
but this increase is spatially non-uniform, and depends a lot on transmit and receive RF coil geometries and the methods employed for spin-excitation and signal detection. In addition, the intrinsic gain in SNR can be
lost in many applications during contrast encoding or due to suboptimal
instrumentation and/or pulse sequences. This lecture, as
the title suggests, will aim to clarify the primary advantages that we should
exploit (“what we want”), while avoiding some of the pitfalls that detract from
these advantages (“what you don’t want”).
DISCUSSION
The primary difference of UHF from lower fields is the behavior of electromagnetic waves employed at the radiofrequencies (RF) corresponding
to resonance frequency of hydrogen nuclei (i.e. protons). Particularly at 7
Tesla and higher magnetic fields, the RF increasingly moves away from the near-field towards the
far-field electromagnetic regime as the wavelengths become smaller than the object dimensions. Consequently, RF behavior at UHF becomes characterized by attenuated
traveling waves in the human body; this leads to image non-uniformities for a
given sample-coil configuration because of spatially varying destructive and
constructive interferences. These non-uniformities were initially considered
detrimental to progress of imaging at high magnetic field strengths. However, they are
advantageous for parallel imaging in signal reception and transmission, two
critical technologies that account, to a large extend, for the success of
ultrahigh fields (1,2), including the mitigation of power deposition (i.e. SAR). The use of these new technologies, together
with improvements in instrumentation and imaging methods, have led to
unprecedented gains in spatial resolution and specificity at 7T for functional imaging
(e.g. (1,5,6,10)).
However, there are numerous significant considerations that go into choosing
the functional contrast encoding method with respect to the specificity (i.e.
fidelity or accuracy) relative to the actual sites of neuronal activity; a
single ideal solution that is applicable in all cases does not yet exist (1,11-13).
With respect to revealing brain anatomy, studies of
microstructure in the brain benefit tremendously from UHF due to new contrast
mechanisms that become robustly feasible (7).
However, techniques like diffusion imaging for tractography (dMRI), a popular
approach employed in probing anatomical connections in the brain, faces
challenges: While there are gains to be realized at UHF from the elevation in
intrinsic SNR, losses are encountered in dMRI during contrast encoding due to shorter T2
at UFH (14,15);
such losses can be minimized so that UHF provide a net advantage for dMRI (14,15)
but this requires exceptional gradient performance, which has its unique
challenges (16).
Spectroscopy measurements with 1H and spectroscopy
and/or imaging studies with X- nuclei (e.g., 31P, 17O,
and 23Na) obtained at 7T, and subsequently at 9.4T by several
laboratories have provided neurochemical and metabolic information in the human
brain with increasing biomedical relevance (e.g. (17,18)).
Unique kinetic studies of intracellular
enzymatic rates that were previously possible only in cells in suspension or
perfused organs, were performed two decades later for the first time in the
human brain at 7T. However, significant
gains in spatial resolution are still necessary for these techniques to achieve
widespread utility and biomedical impact.
Until recently, all the ultrahigh field studies were in
the brain. Imaging in the human torso at 7T is a significantly more challenging
goal due to the relative dimensions of the targeted object versus the RF wavelength employed. Nevertheless, 7T imaging in the
human torso was demonstrated in 2008 and 2009, thus starting a new burgeoning
activity in several laboratories (4). However, further significant technological developments, primarily with respect to RF coils,
parallel transmit methods, and pulse sequences are still needed
for optimal applications that can actually compete with the refined protocols
of 3Tesla.
CONCLUSIONS
Since the introduction of the first 7T system in
1999, steady improvements in high field instrumentation and ever expanding
armamentarium of image acquisition and engineering solutions to challenges
posed by ultrahigh fields has brought imaging to exquisite anatomical detail
and biological information content in many organ systems of the human body. However, obtaining such data requires cognizance of the increased complexity that characterizes UHF imaging, and judicious choice of hardware, imaging approaches, and acquisition parameters. Especially with the introduction of commercial instruments with increasing
sophistication, UHF advances are sure to continue unabated for years to come
as the incessant creativity of the MR community produces new capabilities and
overcome impediments. Further translation of these improvements to lower field
clinical systems to achieve new capabilities and to magnetic fields
significantly higher than 7 Tesla to enable human imaging with increasing
biological importance is inescapable.Acknowledgements
The work relevant to this lecture carried out by the author and colleagues in the Center for Magnetic Resonance Research (CMRR), University of Minnesota was supported by NIH grants P41 EB015894, U54 MH091657 (the Human Connectome Project), U01 EB025144, R24 MH106049, and P30 NS076408.References
All references provided in this abstract are to review
articles. Readers can access individual contributions relevant for this topic through the references given in these review
articles.
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