|Gray Versus White Matter: Native Image Contrast Mechanism|
The Molecular Basis for Gray and White Matter
Contrast in Phase Imaging
Kai Zhong1, Jochen Leupold2, Dominik von Elverfeldt2, Oliver Speck1
1Otto-von-Guericke University, Magdeburg, Germany; 2University Hospital Freiburg, Freiburg, Germany
Direct magnetic resonance phase images acquired at high field have been shown to yield superior gray and white matter contrast up to 10-fold higher compared to conventional magnitude images. However, the underlying contrast mechanism is not yet understood. This study demonstrates that the water macromolecule exchange is the dominant effect that determines the in vivo phase image contrast. Therefore, magnetic resonance phase imaging could be applied for in vivo studies of pathologies on a macromolecular level
Using Field Simulations to Understand Susceptibility
Related Phase Contrast in High Field Gradient Echo Images
Andreas Schäfer1, Penny A. Gowland1, Richard Bowtell1
1University of Nottingham, Nottingham, UK
Phase images acquired at 7T, have been shown to provide enhanced grey to white matter contrast compared with modulus data and to allow the delineation of cortical sub-structure. The nature of this contrast has not been explored in detail and important questions remain about its quantitative nature and level of locality. Here, a Fourier-based method for simulating the field perturbations produced by a general magnetic susceptibility distribution have been used to explore the relationship between the form of the susceptibility distribution and phase contrast. The results have been compared with experimental data acquired at 7T from human subjects.
Optimization of Phase Contrast in
Susceptibility Weighted Imaging at 7T
Yulin Ge1, Samuel Barnes, Yingbiao Xu, Jaladhar Neelavalli, Robert I. Grossman1, E. Mark Haacke
1New York University Medical Center, New York, New York, USA
Imaging small transcereberal veins and venules is now possible using susceptibility weighted imaging at 7T. In this work, small venules within cerebral cortex are clearly identified and structures such as the arcuate fibers in the gray matter are also demonstrated using SWI filtered phase images.
|11:06||879.||Visualization of the Subthalamic Nuclei at High
Spatial Resolution and High Contrast with Susceptibility Weighted Phase
Alexander Rauscher1, Stephan Witoszynskyj2, Shannon Kolind1, Volker Coenen1, David Li1
1UBC, Vancouver, Canada; 2Friedrich Schiller Universitaet Jena, Jena, Germany
The stereotactic procedure of placing electrodes into the subthalamicnuclei (STN) to alleviate the symptoms of Parkinson disease requires areliable identification of the STNs' shape and location. This caneither be achieved by defining their position relative to landmarks orby direct targeting using imaging data that show the STN. Currently,heavily T2-weighted spin echo MR imaging is the method of choice fordirect visualization, using the iron content as a source ofcontrast. We present a fast method based on susceptibility-weighted phase imaging that allows the visualization of the STN at high spatial resolutuion with high contrast.
Delineation of the Subthalamic Nucleus (STN)
on High-Resolution Maps of R2*
Gunther Helms1, Tabea Gringel1, Michael Knauth1, Peter Dechent1, Erck Elolf1
1Göttingen University, Göttingen, Germany
The subthalamic nucleus (STN) is the target structure for deep brain stimulation for neurosurgical treatment of Parkinson’s disease. It is notoriously difficult to detect on T1-weighted structural MRI, but exhibits a short T2* due to high iron content. Maps of R2*=1/T2* with 0.95 mm resolution were obtained using 3D multi-echo FLASH (TR = 30 ms,  = 20 , 8 bipolar echoes at TE = 2.23 to 23.2 ms, 370 Hz/pixel). The center and extension of the STN was determined in 20 healthy adults by a thresholded overlay. Marked differences to the generally accepted STN position were found.
|High Resolution R2* Maps Reveal Laminar Structure of
Human Visual Cortex in Vivo.
Masaki Fukunaga1, Marta Bianciardi1, Peter van Gelderen1, Jacco A. de Zwart1, Jeff H. Duyn1
1National Institutes of Health, Bethesda, Maryland, USA
High resolution T2* weighted MRI at high field strength shows substantial contrast variation across laminae in cortical gray matter. This contrast variation is most prominent in phase images and indicates a substantial bulk susceptibility shift. In this work, we investigated whether this susceptibility shift leads to observable changes in R2*. In-vivo R2* maps obtained from human visual cortex at 7T indeed show a prominent, laminar specific R2* increase in the line of Gennari. This finding suggests that, similar to phase data, R2* maps might aid in revealing cortical laminar structure.
Post-Mortem MRI of Human Brain
Hemispheres: Effects of Formaldehyde Fixation on T2 Relaxation
Robert John Dawe1, David A. Bennett2, Julie A. Schneider2, Sunil K. Vasireddi1, Konstantinos Arfanakis1
1Illinois Institute of Technology, Chicago, Illinois, USA; 2Rush University Medical Center, Chicago, Illinois, USA
Postmortem MRI of the human brain allows histological examination of the tissue specimen following the MR scan. Correlating postmortem MR and histological results may allow the development of novel diagnostic techniques. However, the effects of fixation on the MRI properties of postmortem tissue are not well understood. In this study, five cadaveric human brain hemispheres, immersed in formaldehyde, underwent weekly MR scans for three months postmortem. In deep tissue, the T2 values initially decreased and subsequently increased to reach a plateau at approximately 3 months postmortem. In contrast, the plateau was reached within days postmortem for tissue near the surface.
Magnetisation Transfer Effects in an IR-TSE Study of
Cortical Layers in the Area V1
Ana-Maria Oros-Peusquens1, Sandro Romanzetti1, N. Jon Shah1, Robert Turner2
1Institute of Medicine, Research Centre Juelich, Juelich, Germany; 2Max-Planck-Institute for Human Cognitive and Brain Sciences, Leipzig, Germany
Magnetization transfer effects are known to be important in the contrast of 2D-turbo-spin-echo brain images. While using 3D turbo-spin echo imaging at 3 T to identify human cortical layer structure in vivo with isotropic voxels, an apparent shortening of T1 was observed. This can only be explained by invoking a magnetization transfer effect, despite the use of thick slabs for volume imaging. Optimal 3D-TSE sequence parameters for best grey-white contrast thus require empirical determination.
Direct Saturation MRI: Theory and Application to
Imaging Brain Iron
Seth A. Smith1, 2, Jeff W.M Bulte2, Peter C.M. van Zijl1, 2
1Kennedy Krieger Institute, Baltimore, Maryland, USA; 2Johns Hopkins University, Baltimore, Maryland, USA
We show that direct water saturation can differentiate gray matter (GM) structures with different iron contents. Saturation depends on T1, T2, and the strength, length and frequency offset of RF irradiation. T2 decrease, e.g. due to presence of iron, broadens the saturation spectrum and can be selectively detected when minimizing magnetization transfer effects. A direct saturation ratio (DSR) was determined analogous to magnetization transfer ratios (MTR). DSR in GM correlated with iron concentration. We also show that DSR imaging can be used to determine T1 without influence from compartmental averaging and that T1 relaxation is largely unaffected by iron content
Iron as a Source of Laminar Contrast in MRI of Human
Tie-Qiang Li1, Masaki Fukunaga1, Peter van Gelderen1, Jacco de Zwart1, Steve Dodd1, Afonso Silva1, Kant Matsuda2, Bing Yao1, Karin Shmueli1, Hellmut Merkle1, Jeff Duyn1
1NINDS, Bethesda, Maryland, USA; 2NIH, Bethesda, Maryland, USA
We recently showed that resonance frequency changes induced by magnetic susceptibility variations in grey matter allow in-vivo visualization of laminar architecture with unprecedented contrast, and suggested that the contrast might be due to laminar variation in iron and / or myelin content. Here, we compare MRI scans of post-mortem brain tissues before and after chemical extraction of iron, and find that most of the intra-cortical frequency shifts are caused by tissue iron.