William T Clarke1, Lukas Hingerl2, Wolfgang Bogner2, Ladislav Valkovic3,4, and Christopher T Rodgers3,5
1Wellcome Centre for Integrative Neuroimaging, NDCN, University of Oxford, Oxford, United Kingdom, 2High-field MR Centre, Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna, Austria, 3Oxford Centre for Clinical Magnetic Resonance Research, Radcliffe Department of Medicine, University of Oxford, Oxford, United Kingdom, 4Department of Imaging Methods, Institute of Measurement Science, Slovak Academy of Sciences, Bratislava, Slovakia, 5Wolfson Brain Imaging Centre, Department of Clinical Neurosciences, University of Cambridge, Cambridge, United Kingdom
Synopsis
A density-weighted concentric
ring trajectory MRSI sequence is implemented for cardiac 31P-MRS at
7T. The sequence is characterised in phantoms and in five healthy participants.
Quantitative comparisons are made against a previously implemented acquisition
weighted CSI sequence with matched acquisition time and voxel size. The
proposed sequence (CONCEPT) was found to robustly measure 3D localised PCr/ATP
ratios of the human myocardium 2.59 times faster or with 1.73 times smaller nominal
volume than standard acquisition weighted CSI.
Introduction
Phosphorus magnetic resonance
spectroscopy (31P‐MRS) allows measurement of the metabolism of the
human heart in vivo, specifically the ratio of phosphocreatine to adenosine triphosphate
concentrations (PCr/ATP), which is a biomarker of heart failure (1).
To date, 3D resolved 31P‐MRS
measurements of the human heart have used Cartesian-sampled chemical shift
imaging (CSI) or single voxel 3D-ISIS pulse sequences (2,3). While CSI offers optimal SNR per-unit-time, the point-by-point
sampling of k-space restricts the minimum scan time at higher spatial
resolutions (4).
Concentric ring (CONCEPT)
MRSI has recently been shown to be an effective pulse-sequence for
high-resolution 1H MRSI of the brain (5,6). In this work we demonstrate a 31P density-weighted CONCEPT
sequence for MRSI of the human heart at 7T (7). The proposed sequence is compared against previously
published sequences for reduced acquisition durations or increased resolution (8).Methods
A density-weighted 3D CONCEPT
sequence was created from a previously published equidistant ring 3D CONCEPT
sequence (9). Density was described as a Hamming Window: by placing
rings at particular radii in the kxy-plane, and at particular planes
in the kz-direction (Fig 1). The number of rings was attuned to
elliptically sample k-space in the kz-direction. The number of
points in each ring was constant. Sequence duration was adjusted by varying the
number of rings and z-direction partitions whilst keeping the maximum k-space
coverage identical. Excitation used a 2.4 ms shaped constant phase pulse
designed to uniformly excite metabolites between -3 and 8 ppm (2,3-DPG, PDE,
PCr and g-ATP) (3) when centred 270Hz above PCr. A single BISTRO saturation band to
suppress the chest wall signal was added to the sequence (10).
The sequence SNR and
point-spread function was characterised relative to an acquisition weighted CSI
sequence on a point-source phantom (11).
Five subjects (4M/1F, 74±11
kg, 30±3 years) were scanned in a supine position using a whole-body Siemens
Magnetom 7T scanner (Erlangen, Germany) equipped with a quadrature
transmit-receive surface coil positioned over the heart (12). Each subject was scanned using a previously
described CSI sequence (8), a matrix size of 8x16x8 (four averages at the
centre of k-space, 6:37 min), and with symmetrical matrix size of 10x10x10
(4&1 averages, 6:31&4:21 min). Then the CONCEPT sequence was repeated five
times with a matched symmetric matrix size of 10x10x10 (20, 16, 14, 12 and 10
rings/partitions; 6:32, 4:33, 3:19, 2:31 and 1:37 mins) and once with a matrix
size of 12x12x12 (19 rings/partitions, 6:55 mins). CSI was run with 240x240x200
mm3 FoV, 1 s TR, 8 kHz bandwidth, and 2048 time-samples.
CONCEPT had matched FoV and TR, but 2778 Hz bandwidth with 720 time-samples.
CSI data were reconstructed online and CONCEPT
data offline using the non‐uniform FFT (NUFFT) toolbox with min‐max
Kaiser‐Bessel kernel interpolation and twofold oversampling (13) in MATLAB
(MathWorks, Natick, USA). No density compensation is required. Individual coil
data was combined using the WSVD algorithm (14). CSI/MRSI
data fitting was performed using the OXSA toolbox (15).
Sequences were compared using
PCr/γ‐ATP ratios in
anterior-, mid-, and posterior-septal voxels from apical, mid and basal slices
of the heart, without correcting for partial saturation or blood contamination.Results
Phantom experiments showed
that the measured CONCEPT point-spread-function (PSF) matched the predicted low
ripple PSF (Fig 2). Reducing the number of rings to 10 from 19 did not cause
significant PSF deterioration. SNR performance of the time-matched sequence was
found to match that of the acquisition weighted CSI for acquisitions >4:12 min
total and exceed CSI for shorter scans (because post-acquisition reweighting of
1avg CSI causes SNR losses) (Fig 3).
In the in vivo experiments (Fig 4), the mean PCr/ATP
ratio showed no strong divergence from the time-matched CSI sequences even at
1.5 minutes duration (Fig 5). For every scan duration except the shortest (1:37
min) the mid-septal PCr/ATP was quantified with CRLB < 30% in all subjects.Discussion
Density-weighted CONCEPT MRSI
has been demonstrated for fast (2:31 min) 3D localised cardiac 31P-MRS
at 7T. Phantom measurements showed no loss of SNR compared to SNR optimal CSI
encoding. In vivo PCr/ATP maps are consistent with maps generated from
previously published sequences. This demonstrates a 2.59x and 1.73x reduction
in scan time compared to previously described (8) and the shortest feasible Cartesian-sampled CSI
sequence at matched TR.
Using CONCEPT trajectories has
some potential limitations compared to CSI. Incoherent aliasing artefacts could
influence measured metabolite ratios, especially in the case of cardiac 31P-MRS
with close proximity of potentially contaminating tissues (skeletal muscle and
liver) containing the same metabolites at different concentrations. Care must
also be taken with the increased susceptibility to off-isocentre distortions
caused by gradient non-linearity and strong spatial blurring associated with
aliased spectral peaks (16). Neither of these effects are observed in this data,
the latter due to the use of a spectrally selective excitation pulse.Conclusion
The proposed sequence is able
to measure the PCr/ATP ratio in the human septal myocardium in 2½ min, which is
2.59 times faster than a standard CSI sequence with the same nominal voxel
size of 11.5 mL. Or we can achieve comparable SNR with a nominal voxel size of 6.6
mL in only 6:55 min versus 8:35 min predicted for CSI.Acknowledgements
CTR and LV are funded by a Sir Henry Dale Fellowship from the
Wellcome Trust [098436/Z/12/B]. The support of the Slovak Grant Agencies VEGA
[2/0001/17] and APVV [#15–0029] is also gratefully acknowledged.
The Wellcome Centre for Integrative Neuroimaging
is supported by core funding from the Wellcome Trust (203139/Z/16/Z). CTR and LV contributed equally to this work.References
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