Synopsis
The recent emphasis on value in medical imaging, and the ever greater strain on many
nations’ healthcare budgets as their population ages, has focused attention on
what MRI and other similarly expensive imaging technologies can deliver. This
extends to scientific advances, where grant funding bodies need to be persuaded
that limited money spent on expensive technologies will lead to important scientific
insights. The talk overviews a number of areas where high-end MRI seeks to make concrete scientific contributions.
Introduction
The recent emphasis on value in medical imaging, and the ever greater strain on many
nations’ healthcare budgets as their population ages, has focused attention on
what MRI and other similarly expensive imaging technologies can deliver. This
extends to scientific advances, where grant funding bodies need to be persuaded
that limited money spent on expensive technologies will lead to important scientific
insights.Costs:
When specifically applied to magnetic resonance
imaging scanners, a significant current expense is in the magnet itself,
especially in the case of ultra-high-field systems, where the amount of superconducting
wire needed scales non-linearly with field strength. Also, the ‘standard’ alloy
that is usually used as the wire in super-conducting magnets, copper combined
with niobium-titanium, does not operate as effectively above about 10 Tesla,
where either extra effort must be made to cool the wires so that they can
operate at slightly higher fields, or different (and more complex/expensive) superconducting
wires such as copper combined with niobium-tin must be used.Ultra-high field:
Increasing the field strength leads to a
corresponding increase in signal-to-noise ratio, which can be used in a variety
of ways, including improving the spatial resolution of the image (Polimeni et
al, 2010; Yacoub et al. 2008), or the spectral separation of an MRS experiment
(Berrington et al. 2018), or the acquisition of time-limited data in less time.
Examples of novel scientific insights that have been obtained at ultra-high
field include exquisite cortical fMRI, enabling the directionality of input and
output flows of information via prior anatomical layer information; or the activity
of single (large) cells in a high-field pre-clinical system (Radecki et al.
2014). Even higher field MRI systems are proposed, albeit requiring funding at
a national level to be realized.Improved gradient technology:
An alternative tactic to gaining improved
scientific insights is in the use of specialist gradient coil technology. For
example, three human Connectom magnets (Setsompop et al. 2013), which
incorporate advanced gradient coils offering 300mT/m gradient strength, have
been deployed to sites in the USA and Europe. These gradient strengths allow far
higher b-values than usually achievable, to study white matter (and grey
matter) connectivity using diffusion MRI. These scanners have led to new insights
into sheet-like axonal connections close to the cortex. An alternative approach
to improving gradient technology is to focus on improving the gradient slew
rate (Foo et al. 2018), for which improved echo-train characteristics can be
achieved. Ultimately, slew rate improvements are limited by peripheral nerve
stimulation (and eventually by cardiac stimulation), but high slew rates of up
to 700 T/m/s can be achieved in head-dedicated MRI scanners.Hyperpolarized MRI:
The main motivation for increasing the field
strength of an MRI scanner (with the attendant cost implications outlined
above) is to gain additional equilibrium magnetization and hence
signal-to-noise ratio. For protons, the effective magnetization available
(spins aligned with the static field versus spins aligned against the static
field) for 1H spins at room temperature improves from 5.1x10-6
at 1.5 Tesla to 2.4x10-5 at 7 Tesla. Nevertheless, the effective
fraction of possible spins contributing signal remains very small. A strategy
to address this is to polarize the spin system externally and then inject or
otherwise introduce the hyperpolarized agent into the body. Both liquid-state
(13C using dissolution dynamic nuclear polarization) and gas-phase
hyperpolarized agents (3He and 129Xe using optical pumping) have been developed.
These offer substantial increases in the available polarization (>1000X),
albeit with a lifetime limited by the T1 relaxation of the agent once
injected/inhaled in vivo. As well as
important biomedical applications, a number of scientific insights have also
been made using hyperpolarized agents, such as being able to gain insights into
very specific flux pathways (Timm et al. 2017), cell necrosis (Gallagher et al.
2009) or inflammation (Lewis et al. 2018).High-N MRI:
A number of exciting projects have been funded
to study large cohorts of subjects. Prominent examples of this phenomenon
include the NIH-funded Human Connectome Project, the EU-funded Developing Human
Connectome Project, and the MRC/Wellcome-funded UK Biobank Imaging Extension. These
studies have provided substantial new insights into brain connectivity, and
associations between imaging phenotypes, lifestyle and genetics (Miller et al. 2016).Acknowledgements
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