20 research outputs found
Optimized Quantification of Spin Relaxation Times in the Hybrid State
Purpose: The analysis of optimized spin ensemble trajectories for relaxometry
in the hybrid state.
Methods: First, we constructed visual representations to elucidate the
differential equation that governs spin dynamics in hybrid state. Subsequently,
numerical optimizations were performed to find spin ensemble trajectories that
minimize the Cram\'er-Rao bound for -encoding, -encoding, and their
weighted sum, respectively, followed by a comparison of the Cram\'er-Rao bounds
obtained with our optimized spin-trajectories, as well as Look-Locker and
multi-spin-echo methods. Finally, we experimentally tested our optimized spin
trajectories with in vivo scans of the human brain.
Results: After a nonrecurring inversion segment on the southern hemisphere of
the Bloch sphere, all optimized spin trajectories pursue repetitive loops on
the northern half of the sphere in which the beginning of the first and the end
of the last loop deviate from the others. The numerical results obtained in
this work align well with intuitive insights gleaned directly from the
governing equation. Our results suggest that hybrid-state sequences outperform
traditional methods. Moreover, hybrid-state sequences that balance - and
-encoding still result in near optimal signal-to-noise efficiency. Thus,
the second parameter can be encoded at virtually no extra cost.
Conclusion: We provide insights regarding the optimal encoding processes of
spin relaxation times in order to guide the design of robust and efficient
pulse sequences. We find that joint acquisitions of and in the
hybrid state are substantially more efficient than sequential encoding
techniques.Comment: 10 pages, 5 figure
Cram\'er-Rao Bound Optimized Subspace Reconstruction in Quantitative MRI
We extend the traditional framework for estimating subspace bases that
maximize the preserved signal energy to additionally preserve the Cram\'er-Rao
bound (CRB) of the biophysical parameters and, ultimately, improve accuracy and
precision in the quantitative maps. To this end, we introduce an
\textit{approximate compressed CRB} based on orthogonalized versions of the
signal's derivatives with respect to the model parameters. This approximation
permits singular value decomposition (SVD)-based minimization of both the CRB
and signal losses during compression. Compared to the traditional SVD approach,
the proposed method better preserves the CRB across all biophysical parameters
with negligible cost to the preserved signal energy, leading to reduced bias
and variance of the parameter estimates in simulation. In vivo, improved
accuracy and precision are observed in two quantitative neuroimaging
applications, permitting the use of smaller basis sizes in subspace
reconstruction and offering significant computational savings
Hybrid-State Free Precession in Nuclear Magnetic Resonance
The dynamics of large spin-1/2 ensembles in the presence of a varying
magnetic field are commonly described by the Bloch equation. Most magnetic
field variations result in unintuitive spin dynamics, which are sensitive to
small deviations in the driving field. Although simplistic field variations can
produce robust dynamics, the captured information content is impoverished.
Here, we identify adiabaticity conditions that span a rich experiment design
space with tractable dynamics. These adiabaticity conditions trap the spin
dynamics in a one-dimensional subspace. Namely, the dynamics is captured by the
absolute value of the magnetization, which is in a transient state, while its
direction adiabatically follows the steady state. We define the hybrid state as
the co-existence of these two states and identify the polar angle as the
effective driving force of the spin dynamics. As an example, we optimize this
drive for robust and efficient quantification of spin relaxation times and
utilize it for magnetic resonance imaging of the human brain
Generalized Bloch model: a theory for pulsed magnetization transfer
Purpose: The paper introduces a classical model to describe the dynamics of
large spin-1/2 ensembles associated with nuclei bound in large molecule
structures, commonly referred to as the semi-solid spin pool, and their
magnetization transfer (MT) to spins of nuclei in
Theory and Methods: Like quantum-mechanical descriptions of spin dynamics and
like the original Bloch equations, but unlike existing MT models, the proposed
model is based on the algebra of angular momentum in the sense that it
explicitly models the rotations induced by radio-frequency (RF) pulses. It
generalizes the original Bloch model to non-exponential decays, which are,
e.g., observed for semi-solid spin pools. The combination of rotations with
non-exponential decays is facilitated by describing the latter as Green's
functions, comprised in an integro-differential equation.
Results: Our model describes the data of an inversion-recovery
magnetization-transfer experiment with varying durations of the inversion pulse
substantially better than established models. We made this observation for all
measured data, but in particular for pulse durations small than 300s.
Furthermore, we provide a linear approximation of the generalized Bloch model
that reduces the simulation time by approximately a factor 15,000, enabling
simulation of the spin dynamics caused by a rectangular RF-pulse in roughly
2s.
Conclusion: The proposed theory unifies the original Bloch model, Henkelman's
steady-state theory for magnetization transfer, and the commonly assumed
rotation induced by hard pulses (i.e., strong and infinitesimally short
applications of RF fields) and describes experimental data better than previous
models
Cram\'er-Rao bound-informed training of neural networks for quantitative MRI
Neural networks are increasingly used to estimate parameters in quantitative
MRI, in particular in magnetic resonance fingerprinting. Their advantages over
the gold standard non-linear least square fitting are their superior speed and
their immunity to the non-convexity of many fitting problems. We find, however,
that in heterogeneous parameter spaces, i.e. in spaces in which the variance of
the estimated parameters varies considerably, good performance is hard to
achieve and requires arduous tweaking of the loss function, hyper parameters,
and the distribution of the training data in parameter space. Here, we address
these issues with a theoretically well-founded loss function: the Cram\'er-Rao
bound (CRB) provides a theoretical lower bound for the variance of an unbiased
estimator and we propose to normalize the squared error with respective CRB.
With this normalization, we balance the contributions of hard-to-estimate and
not-so-hard-to-estimate parameters and areas in parameter space, and avoid a
dominance of the former in the overall training loss. Further, the CRB-based
loss function equals one for a maximally-efficient unbiased estimator, which we
consider the ideal estimator. Hence, the proposed CRB-based loss function
provides an absolute evaluation metric. We compare a network trained with the
CRB-based loss with a network trained with the commonly used means squared
error loss and demonstrate the advantages of the former in numerical, phantom,
and in vivo experiments.Comment: Xiaoxia Zhang, Quentin Duchemin, and Kangning Liu contributed equally
to this wor
On multi-path longitudinal spin relaxation in brain tissue
The purpose of this paper is to confirm previous reports that identified
magnetization transfer (MT) as an inherent driver of longitudinal relaxation in
brain tissue by asserting a substantial difference between the relaxation
times of the free and the semi-solid spin pools. Further, we aim to identify an
avenue towards the quantification of these relaxation processes on a
voxel-by-voxel basis in a clinical imaging setting, i.e. with a nominal
resolution of 1mm isotropic and full brain coverage in 12min. To this end, we
optimized a hybrid-state pulse sequence for mapping the parameters of an
unconstrained MT model. We scanned 4 people with relapsing-remitting multiple
sclerosis (MS) and 4 healthy controls with this pulse sequence and estimated
s and s for the free and semi-solid
spin pool of healthy WM, respectively, confirming previous reports and
questioning the commonly used assumptions or s.
Further, we estimated a fractional size of the semi-solid spin pool of , which is larger than previously assumed. An analysis of
in normal appearing white matter revealed statistically significant differences
between individuals with MS and controls. In conclusion, we confirm that
longitudinal spin relaxation in brain tissue is dominated by MT and that the
hybrid state facilitates a voxel-wise fit of the unconstrained MT model, which
enables the analysis of subtle neurodegeneration
Rapid quantitative magnetization transfer imaging: utilizing the hybrid state and the generalized Bloch model
Purpose: To improve spatial resolution and scan time of quantitative
magnetization transfer (qMT) imaging without constraints on model parameters.
Theory and Methods: We combine two recently-proposed models in a
Bloch-McConnell equation: the dynamics of the free spin pool is confined to the
hybrid state and the dynamics of the semi-solid spin pool is described by the
generalized Bloch model. We numerically optimize the flip angles and durations
of a train of radio frequency pulses to enhance the encoding of three marked
qMT parameters while accounting for an 8-parameter model. We sparsely sample
each time frame along this spin dynamics with a 3D radial koosh-ball
trajectory, reconstruct the data with sub-space modeling, and fit the qMT model
with a neural network for computational efficiency.
Results: We were able to extract qMT parameter maps of the whole brain with a
nominal resolution of 1mm isotropic and high SNR from a 12.6 minute scan. In
lesions of multiple sclerosis subjects, we observe a decreased size of the
semi-solid spin pool and slower relaxation, consistent with previous reports.
Conclusion: The encoding power of the hybrid state, combined with regularized
image reconstruction, and the accuracy of the generalized Bloch model provide
an excellent basis for highly efficient quantitative magnetization transfer
imaging
JakobAsslaender/MRIgeneralizedBloch.jl: v0.8.5
<h2>MRIgeneralizedBloch v0.8.5</h2>
<p><a href="https://github.com/JakobAsslaender/MRIgeneralizedBloch.jl/compare/v0.8.4...v0.8.5">Diff since v0.8.4</a></p>
<p><strong>Merged pull requests:</strong></p>
<ul>
<li>New version: v0.8.4 (#31) (@JakobAsslaender)</li>
<li>CompatHelper: bump compat for LsqFit to 0.14, (keep existing compat) (#32) (@github-actions[bot])</li>
<li>CompatHelper: bump compat for LsqFit to 0.15, (keep existing compat) (#33) (@github-actions[bot])</li>
</ul>
Minimization of Eddy Current Artifacts in Sequences with Periodic Dynamics
Purpose: To minimize eddy-current artifacts in periodic pulse sequences with
balanced gradient moments as, e.g., used for quantitative MRI.
Theory and Methods: Eddy current artifacts in balanced sequences result from
large jumps in k-space. In quantitative MRI, one often samples some spin
dynamics repeatedly while acquiring different parts of k-space. We swap
individual k-space lines between different repetitions in order to minimize
jumps within each repetition. This reordering can be formulated as a travelling
salesman problem and we tackle the discrete optimization with a simulated
annealing algorithm.
Results: The proposed approach is highly effective at minimizing the change
of the readout direction of a 3D koosh-ball trajectory. This decreases the
eddy-current induced perturbations of the magnetization and reduces artifacts
in the derived parameter maps, as demonstrated in the example of a hybrid-state
free precession sequence.
Conclusion: Beyond this example application, we believe that the approach can
be used for most k-space trajectories, other quantitative MRI methods, as well
as for cine cardiac imaging.Comment: 7 pages, 5 figure