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Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student-submitted PDF version of thesis., Includes bibliographical references (page 65)., MRI is an attractive tool for fetal imaging due to its unique ability to provide detailed anatomical and physiological data in an inherently safe manner. In practice, the problem of unpredictable and nonrigid fetal motion limits fetal MRI to fast single shot T2 weighted sequences such as HASTE, which have poor tissue contrast, low SNR, and cannot provide detailed physiological information. In this work, we designed, built, and tested a semi-adjustable anatomically shaped 64 channel array coil for fetal imaging at 22 weeks of pregnancy. The coil's performance was compared to that of the vendor's standard configuration consisting of an 18 channel flexible body array and 16 channels from a 32 channel spine array. The fetal coil provides roughly 5% better SNR in the fetal brain region of an anthropomorphic phantom and allows increasing SENSE acceleration factor from R = 4 to R = 5 in the right-left direction and R = 3 to R = 4 in the head-foot direction., by Mark H. Spatz., M. Eng.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014., Cataloged from PDF version of thesis., Includes bibliographical references (pages 143-153)., Purpose: As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as intensive care units (ICUs), physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods: A truly portable (<100kg) proof-of-concept MRI scanner has been constructed and validated, which replaces conventional gradient encoding with a rotating lightweight, cryogen-free, low-field magnet. When rotated about the object, an inhomogeneous magnetic field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections and encode the iteratively reconstructed 2D images. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results: The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but this will be improved with modifications to the magnet design. Conclusion: This novel MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating gradient coils. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology., by Clarissa Zimmerman Cooley., Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014., Cataloged from PDF version of thesis., Includes bibliographical references (pages 143-153)., Purpose: As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as intensive care units (ICUs), physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods: A truly portable (<100kg) proof-of-concept MRI scanner has been constructed and validated, which replaces conventional gradient encoding with a rotating lightweight, cryogen-free, low-field magnet. When rotated about the object, an inhomogeneous magnetic field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections and encode the iteratively reconstructed 2D images. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results: The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but this will be improved with modifications to the magnet design. Conclusion: This novel MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating gradient coils. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology., by Clarissa Zimmerman Cooley., Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014., Cataloged from PDF version of thesis., Includes bibliographical references (pages 143-153)., Purpose: As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as intensive care units (ICUs), physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods: A truly portable (<100kg) proof-of-concept MRI scanner has been constructed and validated, which replaces conventional gradient encoding with a rotating lightweight, cryogen-free, low-field magnet. When rotated about the object, an inhomogeneous magnetic field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections and encode the iteratively reconstructed 2D images. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results: The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but this will be improved with modifications to the magnet design. Conclusion: This novel MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating gradient coils. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology., by Clarissa Zimmerman Cooley., Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014., Cataloged from PDF version of thesis., Includes bibliographical references (pages 143-153)., Purpose: As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as intensive care units (ICUs), physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods: A truly portable (<100kg) proof-of-concept MRI scanner has been constructed and validated, which replaces conventional gradient encoding with a rotating lightweight, cryogen-free, low-field magnet. When rotated about the object, an inhomogeneous magnetic field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections and encode the iteratively reconstructed 2D images. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results: The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but this will be improved with modifications to the magnet design. Conclusion: This novel MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating gradient coils. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology., by Clarissa Zimmerman Cooley., Ph. D.

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2011., Cataloged from PDF version of thesis., Includes bibliographical references., Magnetic resonance imaging (MRI) has demonstrated the potential for non-invasive mapping of structure and function (fMRI) in the human brain. In this thesis, we propose a series of methodological developments towards improved fMRI of auditory processes. First, the inefficiency of standard fMRI that acquires brain volumes one slice at a time is addressed. The proposed single-shot method is capable, for the first time, of imaging the entire brain in a single-acquisition while still maintaining adequate spatial resolution for fMRI. This method dramatically increases the temporal resolution of fMRI (20 fold) and improves sampling efficiency as well as the ability to discriminate against detrimental physiological noise. To accomplish this it exploits highly accelerated parallel imaging techniques and MRI signal detection with a large number of coil elements. We then address a major problem in the application of fMVIRI to auditory studies. In standard fMRI, loud acoustic noise is generated by the rapid switching of the gradient magnetic fields required for image encoding, which interferes with auditory stimuli and enforces inefficient and slow sampling strategies. We demonstrate a fMRI method that uses parallel imaging and redesigned gradient waveforms to both minimize and slow down the gradient switching to substantially reduce acoustic noise while still enabling rapid acquisitions for fMRI. Conventional fMRI is based on a hemodynamic response that is secondary to the underlying neuronal activation. In the final contribution of this thesis, a novel image contrast is introduced that is aimed at the direct observation of neuronal magnetic fields associated with functional activation. Early feasibility studies indicate that the imaging is sensitive to oscillating magnetic fields at amplitudes similar to those observed by magnetoencephalography., by Thomas Witzel., Ph.D.

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2011., Cataloged from PDF version of thesis., Includes bibliographical references., Magnetic resonance imaging (MRI) has demonstrated the potential for non-invasive mapping of structure and function (fMRI) in the human brain. In this thesis, we propose a series of methodological developments towards improved fMRI of auditory processes. First, the inefficiency of standard fMRI that acquires brain volumes one slice at a time is addressed. The proposed single-shot method is capable, for the first time, of imaging the entire brain in a single-acquisition while still maintaining adequate spatial resolution for fMRI. This method dramatically increases the temporal resolution of fMRI (20 fold) and improves sampling efficiency as well as the ability to discriminate against detrimental physiological noise. To accomplish this it exploits highly accelerated parallel imaging techniques and MRI signal detection with a large number of coil elements. We then address a major problem in the application of fMVIRI to auditory studies. In standard fMRI, loud acoustic noise is generated by the rapid switching of the gradient magnetic fields required for image encoding, which interferes with auditory stimuli and enforces inefficient and slow sampling strategies. We demonstrate a fMRI method that uses parallel imaging and redesigned gradient waveforms to both minimize and slow down the gradient switching to substantially reduce acoustic noise while still enabling rapid acquisitions for fMRI. Conventional fMRI is based on a hemodynamic response that is secondary to the underlying neuronal activation. In the final contribution of this thesis, a novel image contrast is introduced that is aimed at the direct observation of neuronal magnetic fields associated with functional activation. Early feasibility studies indicate that the imaging is sensitive to oscillating magnetic fields at amplitudes similar to those observed by magnetoencephalography., by Thomas Witzel., Ph.D.