The objective is to enhance the contrast in MR image when the acquisition time is constrained to be limited in comparison to those of available MR sequences. Harmonic retrieval methods will be used for synthesizing the k-space from sparsely sampled, and incompletely acquired spin/gradient echo signal. A significant outcome is the development of a partial-echo sequence, in which the acquisition time per RF pulse application can be limited to 1.2-1.5TE for both spin and gradient echo sequences. A linear prediction-based algorithm embodies the principle underlying the possibility of recovering anatomical details of the proposed partial-echo sequence. Prior to clinical trials, the prototypical sequence and the associated image reconstruction method will be validated using an MR simulator, and anatomically realistic simulated data sets. The remainder of outcomes is related to clinical applications involving brain lesion identification and fat-water separation using the proposed partial-echo sequence..
Diffusion-weighted MRI (DWI) and Fluid Attenuated Inversion Recovery (FLAIR) form a combinational approach in the investigation of stroke imaging. DWI derived measures are used to detect the infarcted brain tissue and FLAIR images are used to detect regions with edema. Of these, the DWI employs Echo-Planar Imaging (EPI) type of acquisition in which the entire k-space is scanned with a single RF pulse, followed by echoes whose amplitudes are modulated in accordance with the distance from the k-space center. EPI data have inherently low Signal-to-Noise Ratio (SNR). In situations where only partial echoes are acquired, the low SNR of EPI data can further result in loss of fine structural details. In the case of rapid scanning, echo truncation in EPI will need extra processing to restore the lost information. Our method to investigate the incomplete (partial) acquisition of EPI and bring out corrective filtering steps to improve quantitative stroke imaging..
The key research components include development of spatial and k-space filters for elimination of truncation artefacts, enhancement and restoration of features in the magnitude image reconstructed from partial k-space of EPI data. As compared to other multi-pulse sequences, the filters required for restoration of partially acquired EPI are to be tailored for operation under low SNR conditions. A novel aspect of the current proposal is the elimination of the need for acquiring a major fraction of the echoes in the dephasing period..
MRI simulators using the Bloch equation have been extensively used to design pulse sequences, validate image reconstruction algorithms, develop methods for image artifact suppression, and aid in MRI education. Although many fundamental physical principles and software designs for creating an MRI simulator have been described, and the image artifacts due to static magnetic field in-homogeneities and other off-resonance effects have been simulated and validated for cases with homogeneous RF transmission, many previous works were based on assumptions of homogenous radiofrequency (RF) fields that did not take into consideration the multiple unique distributions of the RF receive (B1-) and transmit (B1+) circularly polarized magnetic field components that have allowed for parallel reception and transmission. Although at least one prior MRI simulator allows for consideration of specific B1 field patterns, including transmit and receive arrays, none have considered the associated electric field distributions (E1) throughout the sample for calculating protocol-specific specific absorption rate (SAR) in multichannel transmission and realistic, correlated noise received in each channel during multichannel reception.
In recent years, MRI-related field simulations have made knowledge of realistic distributions of all the above electromagnetic fields available to many in the MR community. Recently developed MRI simulation tools consider realistic representation of all these pertinent electromagnetic fields to accurately calculate not only signal with the Bloch equation but also noise and SAR useful in the design and evaluation of new pulses, sequences, reconstruction methods, and hardware before costly implementation on a real scanner. This enables MRI developers to perform preliminary evaluation of new ideas.
In parallel MRI, phased arrays are used as receivers, and a separate homogenous volume coil is used for transmission. Parallel MRI uses spatial information about the origin of the detected MR signal from sensitivity maps of the receiver coils. Hence the received signal is strongly dependent on the coil geometry. The MRI simulators discussed, do not take into account the coil sensitivity and geometry effects for calculation of the induced voltage in the receiver coil. Further, mutual inductances between coils can result in additive noise and loss of spatial information. Computation of flux lines from the simulator can be used for optimal design of phased array coils with minimum coupling and increased SNR. Although other simulators are available, we do not have access to the time varying spatial magnetization data that is required for computation of the flux lines. Hence the need for building an in-house simulator. .
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