Dinesh Deelchand1
1University of Minnesota, United States
Synopsis
This
lecture focuses on the pros and cons of utilizing single-voxel proton MR
spectroscopy at ultra-high fields (UHF) of 7T and beyond in both human and animal
brains. Advantages include higher signal-to-noise ratio, higher spectral
dispersion i.e. less overlap between metabolites and these benefits lead to
improved quantification of metabolites. However in human brain, going to UHF is
associated with B1 inhomogeneity and increased RF power requirements
and these can be mitigated by using dielectric pads or B1 shimming
techniques. In addition, relaxation times of metabolites and water tissue signals
change as B0 field increases.
Introduction
Proton
magnetic resonance spectroscopy (1H MRS) is a non-invasive technique
that enables measurement of the concentration of at least 18 metabolites in the
human and animal brains [1-4]. MRS data can be acquired using single-voxel
spectroscopy (SVS) or with magnetic resonance spectroscopic imaging (MRSI). SVS
is the most commonly used technique due to high spectral quality (e.g. good localization,
optimized B0 shim and water suppression). However, the method is
limited to only single volume-of-interest (VOI). Commonly utilized SVS
sequences are STEAM [5], PRESS [6], SPECIAL [4], LASER [7] and semi-LASER [8-9].
The benefits and issues associated with performing SVS acquisitions at ultra-high
fields (UHF) of 7T and higher are discussed below. Benefits of higher magnetic fields
- Increased signal-to-noise ratio (SNR) especially
in the time domain. However in the frequency domain, the SNR of NAA
levels off with modest increase above 3 T in the human brain [10]. In rodent
brain, the SNR of NAA levels off beyond 16 T [11].
- Increased in spectral dispersion [12]. Less overlap between metabolites. For instance,
glutamate and glutamine can be resolved beyond 7 T in both human and animal
brains [13].
- Simplification of J-coupled spectral patterns. Based on increased spectral linewidth (mostly
from microscopic susceptibility effects) and dispersion with higher B0
field [2].
·
- Improved quantification precision of
metabolite. This is reflected in the Cramer-Rao Lower
bounds (CRLB). Several studies have
reported decreased in CRLB from 3 or 4 T to 7 T [14-15]. For singlets, CRLB
improves as ~sqrt(B0) [2]. For J-coupled
metabolites (e.g. glutamate, glutamine, myo-inositol),
the gain in quantification precision is much more and ranges from sqrt(B0)
to B0 [2].
Issues associated with UHF
- Shorter transverse (T2) relaxation
times of water and metabolites. T2 decreases exponentially in the
human brain [2].
- Longer longitudinal (T1) relaxation
times of water and metabolites. Modest increase in T1 values in
human and animal brains [2].
- RF power requirement: Minimize chemical shift displacement error
(CSDE): need high-bandwidth RF pulses to cover the larger spectral dispersion. For refocusing pulses, gradient-modulated RF
pulses such as FOCI [16] or GOIA [17-18] pulses can be utilized to reduce
required B1 field.
- RF field uniformity goes down. B1 issues especially in humans brain
[19]. This can be mitigated by using B1 shimming routine when using
multi-channel coils for transmit or dielectric pad.
Localization MRS sequences for UHF
- Requirements: large
RF bandwidth, small CSDE, short or relatively short echo-time (TE) sequences
(due to shorter T2 relaxation times).
- STEAM
or semi-adiabatic SPECIAL: these can achieve relatively short TE
although SNR is halved compared with STEAM compared to spin-echo sequences.
- LASER
or semi-LASER: T2 relaxation times of water and
metabolites are lengthened due to pairs of adiabatic RF pulses in these
sequences [20]. In addition, J-modulation
is suppressed [20]. This in turn means that relatively long TE MR spectrum
resembles short-TE spectra acquired with STEAM.
Acknowledgements
No acknowledgement found.References
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