Coil combination using linear deconvolution in k-space for phase imaging

Background: The combination of multi-channel data is a critical step for the imaging of phase and susceptibility contrast in magnetic resonance imaging (MRI). Magnitude-weighted phase combination methods often produce noise and aliasing artifacts in the magnitude images at accelerated imaging sceneries. To address this issue, an optimal coil combination method through deconvolution in k-space is proposed in this paper. Methods: The proposed method firstly employs the sum-of-squares and phase aligning method to yield a complex reference coil image which is then used to calculate the coil sensitivity and its Fourier transform. Then, the coil k-space combining weights is computed, taking into account the truncated frequency data of coil sensitivity and the acquired k-space data. Finally, combining the coil k-space data with the acquired weights generates the k-space data of proton distribution, with which both phase and magnitude information can be obtained straightforwardly. Both phantom and in vivo imaging experiments were conducted to evaluate the performance of the proposed method. Results: Compared with magnitude-weighted method and MCPC-C, the proposed method can alleviate the phase cancellation in coil combination, resulting in a less wrapped phase. Conclusions: The proposed method provides an effective and efficient approach to combine multiple coil image in parallel MRI reconstruction, and has potential to benefit routine clinical practice in the future

Susceptibility induced gray–white matter MRI contrast in the human brain

AbstractMR phase images have shown significantly improved contrast between cortical gray and white matter regions compared to magnitude images obtained with gradient echo sequences. A variety of underlying biophysical mechanisms (including iron, blood, myelin content, macromolecular chemical exchange, and fiber orientation) have been suggested to account for this observation but assessing the individual contribution of these factors is limited in vivo.For a closer investigation of iron and myelin induced susceptibility changes, postmortem MRI of six human corpses (age range at death: 56–80years) was acquired in situ. Following autopsy, the iron concentrations in the frontal and occipital cortex as well as in white matter regions were chemically determined. The magnetization transfer ratio (MTR) was used as an indirect measure for myelin content. Susceptibility effects were assessed separately by determining R2* relaxation rates and quantitative phase shifts. Contributions of myelin and iron to local variations of the susceptibility were assessed by univariate and multivariate linear regression analysis.Mean iron concentration was lower in the frontal cortex than in frontal white matter (26±6 vs. 45±6mg/kg wet tissue) while an inverse relation was found in the occipital lobe (cortical gray matter: 41±10 vs. white matter: 34±10mg/kg wet tissue). Multiple regression analysis revealed iron and MTR as independent predictors of the effective transverse relaxation rate R2* but solely MTR was identified as source of MR phase contrast. R2* was correlated with iron concentrations in cortical gray matter only (r=0.42, p<0.05).In conclusion, MR phase contrast between cortical gray and white matter can be mainly attributed to variations in myelin content, but not to iron concentration. Both, myelin and iron impact the effective transverse relaxation rate R2* significantly. Magnitude contrast is limited because it only reflects the extent but not the direction of the susceptibility shift

Signal compartments in ultra-high field multi-echo gradient echo MRI reflect underlying tissue microstructure in the brain

Gradient recalled echo magnetic resonance imaging (GRE-MRI) at ultra-high field holds great promise for new contrast mechanisms and delineation of putative tissue compartments that contribute to the multi-echo GRE-MRI signal may aid structural characterization. Several studies have adopted the three water-pool compartment model to study white matter brain regions, associating individual compartments with myelin, axonal and extracellular water. However, the number and identifiability of GRE-MRI signal compartments has not been fully explored. We undertook this task for human brain imaging data. Multiple echo time GRE-MRI data were acquired in five healthy participants, specific anatomical structures were segmented in each dataset (substantia nigra, caudate, insula, putamen, thalamus, fornix, internal capsule, corpus callosum and cerebrospinal fluid), and the signal fitted with models comprising one to six signal compartments using a complex-valued plane wave formulation. Information criteria and cluster analysis methods were used to ascertain the number of distinct compartments within the signal from each structure and to determine their respective frequency shifts. We identified five principal signal compartments with different relative contributions to each structure's signal. Voxel-based maps of the volume fraction of each of these compartments were generated and demonstrated spatial correlation with brain anatomy

On Nature of the Gradient Echo MR Signal and Its Application to Monitoring Multiple Sclerosis

Multiple Sclerosis is a common disease, affecting 2.5 million people world-wide. The clinical course is heterogeneous, ranging from benign disease in which patients live an almost normal life to severe and devastating disease that may shorten life. Despite much research, a fully effective treatment for MS is still unavailable and diagnostic techniques for monitoring MS disease evolution are much needed. As a non-invasive tool, Magnetic resonance imaging: MRI) plays a key role in MS diagnosis. Numerous MRI techniques have been proposed over the years. Among most widely used are conventional T1-weighted: T1W), T2-weighted: T2W) and FLuid Attenuated Inversion Recovery: FLAIR) imaging techniques. However their results do not correlate well with neurological findings. Several advanced MRI techniques are also used as research tools to study MS. Among them are magnetization transfer contrast imaging: MT), MR spectroscopy: MRS), and Diffusion Tensor Imaging: DTI) but they have not penetrated to clinical arena yet. Gradient Echo Plural Contrast Imaging: GEPCI) developed in our laboratory is a post processing technique based on multi-echo gradient echo sequence. It offers basic contrasts such as T1W images and T2* maps obtained from magnitude of GEPCI signal, and frequency maps obtained from GEPCI signal phase. Phase information of Gradient Echo MR signal has recently attracted much attention of the MR community since it manifests superior gray matter/ white matter contrast and sub-cortical contrast, especially at high field: 7 T) MRI. However the nature of this contrast is under intense debates. Our group proposed a theoretical framework - Generalized Lorentzian Approach - which emphasizes that, contrary to a common-sense intuition, phase contrast in brain tissue is not directly proportional to the tissue bulk magnetic susceptibility but is rather determined by the geometrical arrangement of brain tissue components: lipids, proteins, iron, etc.) at the cellular and sub-cellular levels - brain tissue magnetic architecture . In this thesis we have provide first direct prove of this hypothesis by measurement of phase contrast in isolated optic nerve. We have also provided first quantitative measurements of the contribution to phase contrast from the water-macromolecule exchange effect. Based on our measurement in protein solutions, we demonstrated that the magnitude of exchange effect is 1/2 of susceptibility effect and to the opposite sign. GEPCI technique also offers a scoring method for monitoring Multiple Sclerosis based on the quantitative T2* maps generated from magnitude information of gradient echo signal. Herein we demonstrated a strong agreement between GEPCI quantitative scores and traditional lesion load assessment. We also established a correlation between GEPCI scores and clinical tests for MS patients. We showed that this correlation is stronger than that found between traditional lesion load and clinical tests. Such studies will be carried out for longer period and on MS subjects with broader range of disease severity in the future. We have also demonstrated that the magnitude and phase information available from GEPCI experiment can be combined in multiple ways to generate novel contrasts that can help with visualization of neurological brain abnormalities beyond Multiple Sclerosis. In summary, in this study, we 1) propose novel contrasts for GEPCI from its basic images; 2) investigate the biophysical mechanisms behind phase contrast; 3) evaluate the benefits of quantitative T2* map offered by GEPCI in monitoring disease of Multiple Sclerosis by comparing GEPCI results to clinical standard techniques; 4) apply our theoretical framework - Generalized Lorentzian Approach - to better understand phase contrast in MS lesions

Technical developments for clinical MR applications at 7 T

The aim of this thesis was to develop methods to enable the use of 7 Tesla MRI in clinical practice. A number of technical developments have been performed to facilitate clinical studies and to achieve the full potential of high field. Studies have been performed for imaging of different anatomies of the human body, including visualization of the right coronary artery and high resolution imaging of the brain in patients with neurodegenerative diseasesPhilips HealthcareUBL - phd migration 201