Sub-Millimeter Spiral fMRI

Poster No:

2168 

Submission Type:

Abstract Submission 

Authors:

Lars Kasper1,2, Maria Engel2, Jakob Heinzle1, Matthias Müller-Schrader1, Jonas Reber2, Thomas Schmid2, Christoph Barmet2, Bertram Wilm2, Klaas Enno Stephan1, Klaas Pruessmann2

Institutions:

1Translational Neuromodeling Unit, University of Zurich & ETH Zurich, Zurich, Switzerland, 2Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland

First Author:

Lars Kasper  
Translational Neuromodeling Unit, University of Zurich & ETH Zurich|Institute for Biomedical Engineering, ETH Zurich and University of Zurich
Zurich, Switzerland|Zurich, Switzerland

Co-Author(s):

Maria Engel  
Institute for Biomedical Engineering, ETH Zurich and University of Zurich
Zurich, Switzerland
Jakob Heinzle  
Translational Neuromodeling Unit, University of Zurich & ETH Zurich
Zurich, Switzerland
Matthias Müller-Schrader  
Translational Neuromodeling Unit, University of Zurich & ETH Zurich
Zurich, Switzerland
Jonas Reber  
Institute for Biomedical Engineering, ETH Zurich and University of Zurich
Zurich, Switzerland
Thomas Schmid  
Institute for Biomedical Engineering, ETH Zurich and University of Zurich
Zurich, Switzerland
Christoph Barmet  
Institute for Biomedical Engineering, ETH Zurich and University of Zurich
Zurich, Switzerland
Bertram Wilm  
Institute for Biomedical Engineering, ETH Zurich and University of Zurich
Zurich, Switzerland
Klaas Enno Stephan  
Translational Neuromodeling Unit, University of Zurich & ETH Zurich
Zurich, Switzerland
Klaas Pruessmann  
Institute for Biomedical Engineering, ETH Zurich and University of Zurich
Zurich, Switzerland

Introduction:

fMRI with sub-millimeter spatial resolution enables the detailed study of functional brain organization at the level of cortical layers or columns [1–3].
Spiral readouts of gradient-echo (GRE) BOLD appear well suited for this task due to their superior average k-space speed compared to EPI [4], improving acquisition efficiency for increased resolution.
However, their spatial specificity is typically hampered by point spread function blurring (PSF) in the presence of B0 inhomogeneity, as well as macrovascular contributions to the gradient-echo BOLD signal [5].
Recently, anatomically veridical single-shot 2D spirals without conspicuous blurring have been deployed at 7T [6,7], owing to an expanded signal model incorporating static and dynamic B0 field changes [8]. This approach also yields raw phase data of high quality [9], intrinsically unwrapped by the B0 map demodulation in the model.
Here, we therefore explore the spatial specificity of BOLD data acquired with single-shot 2D spirals of 0.8mm resolution in a visual paradigm. We further investigate the utility of the corresponding high-resolution phase data to detect and remove macrovascular BOLD effects apparent as task-locked coherent phase change [10,11].

Methods:

Six young healthy volunteers were scanned on a 7T Philips Achieva system, using a 1-channel Tx, 32-channel Rx coil (Nova Medical). To monitor the spiral trajectories, 16 19F-NMR field probes [12] were positioned between the coils and connected to a separate MR acquisition system [13].
Sequence, Trajectories and Image Reconstruction
A 2D single shot spiral trajectory (4x undersampled FOV 220 mm) with 0.8mm in-plane resolution and time-optimal readout (57ms, Fig.1) for our gradient specifications was used [14]. In total, 36 transverse-oblique slices (0.9mm+0.1mm gap) were acquired covering the visual cortex with TR=3.3s and TE=20ms. Concurrent field recording with NMR probes was performed for every 3rd spiral trajectory at 1Mhz bandwidth.
The signal model for image reconstruction comprised these maps as well as the measured spiral field dynamics (k0, kx,y,z) to express the measured coil data, and was inverted by an iterative cg-SENSE reconstruction algorithm [8,16]. This yielded complex reconstructed images, of which modulus and argument were taken for subsequent separate time-series analysis. No spatial phase unwrapping or background phase removal was performed.
The employed paradigm stimulated visual quarter-field for 5.5 minutes (100 volumes, 15 s block design) with flickering checkerboards in upper left and lower right (ULLR), or upper right and lower left (URLL) visual field.

Results:

Single-subject activation maps for the magnitude data, overlaid on the mean functional images, contain typical stimulation patterns in visual cortex (Fig.1A), which follow gyrus anatomy when overlaid on the multi-echo reference image (Fig.1B). Spatial specificity is evident in contrast changes for adjacent voxels for both differential and individual contrasts with little overlap between conditions (Fig. 1C,D). These results are reproduced in all 6 subjects (Fig. 2).
For the phase data, a two-sided contrast of the individual conditions gave the strongest activation maps, pointing to a different mechanism of activation (Fig. 1E). Activation sites partially coincide with the magnitude contrasts, but are more confined to darker locations (short T2*), presumably larger vessels.
Supporting Image: Figure2.png
Supporting Image: Figure1.png
 

Conclusions:

Single-shot spiral fMRI with sub-mm spatial specificity has been shown. On a standard gradient system acquisition efficiency was enhanced to a 220x220x36 mm FOV brain image at 0.8 mm nominal resolution (i.e., a matrix size of 275x275x36) at a typical TR of 3.3s. The magnitude fMRI data shows discriminative activation down to the voxel level, while phase data highlights possible macrovascular contributions, suitable for masking. Future work will expand this phase analysis to include phase time courses into the GLM or analyze it in a functional QSM framework [17,18].

Modeling and Analysis Methods:

Exploratory Modeling and Artifact Removal

Novel Imaging Acquisition Methods:

Anatomical MRI 2
BOLD fMRI 1

Keywords:

fMRI CONTRAST MECHANISMS
FUNCTIONAL MRI
MRI
MRI PHYSICS
NORMAL HUMAN
Other - Spiral;Magnetic Field Monitoring

1|2Indicates the priority used for review

My abstract is being submitted as a Software Demonstration.

No

Please indicate below if your study was a "resting state" or "task-activation” study.

Task-activation

Healthy subjects only or patients (note that patient studies may also involve healthy subjects):

Healthy subjects

Was any human subjects research approved by the relevant Institutional Review Board or ethics panel? NOTE: Any human subjects studies without IRB approval will be automatically rejected.

Yes

Was any animal research approved by the relevant IACUC or other animal research panel? NOTE: Any animal studies without IACUC approval will be automatically rejected.

Not applicable

Please indicate which methods were used in your research:

Functional MRI
Structural MRI

For human MRI, what field strength scanner do you use?

7T

Which processing packages did you use for your study?

SPM

Provide references using author date format

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[2] J. Goense, H. Merkle, N.K. Logothetis, Neuron, 2012, 76, 629.
[3] S. Kashyap et al., Scientific Reports, 2018, 8, 1.
[4] G.H. Glover, NeuroImage, 2012, 62, 706.
[5] K. Uludağ, B. Müller-Bierl, K. Uğurbil, NeuroImage, 2009, 48, 150.
[6] M. Engel et al., Magnetic Resonance in Medicine, 2018, 80, 1836.
[7] L. Kasper et al., in Proc. Intl. Soc. Mag. Reson. Med. 25, 2017, 582.
[8] K.P. Pruessmann et al., Magnetic Resonance in Medicine, 2001, 46, 638―651.
[9] L. Kasper et al., NeuroImage, 2018, 168, 88.
[10] M. Bianciardi, P. van Gelderen, J.H. Duyn, Human Brain Mapping, 2013, n/a.
[11] R.S. Menon, Magnetic Resonance in Medicine, 2002, 47, 1.
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[13] B.E. Dietrich et al., Magnetic Resonance in Medicine, 2016, 75, 1831.
[14] M. Lustig, S.-J. Kim, J.M. Pauly, IEEE Transactions on Medical Imaging, 2008, 27, 866.
[15] B.J. Wilm et al., Magnetic Resonance in Medicine, 2011, 65, 1690.
[16] L.C. Man, J.M. Pauly, A. Macovski, Magnetic Resonance in Medicine, 1997, 37, 785.
[17] D.Z. Balla et al., NeuroImage, 2014, 100, 112.
[18] V.D. Calhoun et al., Magnetic Resonance in Medicine, 2002, 48, 180.