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Tissue Magnetic Susceptibility Matched Pyrolytic Graphite Foam for Improved MRI


In MRI, a powerful and uniform static B0 field is responsible for image signal and contrast. However, regions of different magnetic susceptibilities in the static field give rise to field inhomogeneities that cause image artifacts. For example, air and tissue interfaces for a patient in a MRI study induce up to ±5 ppm static B0 field perturbations within the patient. Unfortunately shim coils are too large to compensate for the very steep field variations found near the breast, cervical spine, shoulders, occipital lobe, and other regions of the body. Many MRI applications are vulnerable to B0 inhomogeneity, including robust fat suppression, which requires better than ±1 ppm homogeneity.

To address this challenge, we have developed a flexible, conforming composite foam with magnetic susceptibility matched to human tissue. We surround the region with enough matching material to move the field gradients outside of the body where they cannot cause MRI artifacts. Our matching material is a pyrolytic graphite (PG) composite foam. It has many advantages over existing matching agents: it is lightweight; it adds no noise; and it is safe to embed receiver coils within the PG foam.

In this thesis, we propose theory that describes the magnetic susceptibility matching properties of composite PG foam. We show experimental proof of concept susceptibility matching and safety in phantom experiments. We also experimentally demonstrate that PG foam cushions improve the B0 field uniformity to the critical threshold of ±1 ppm in both phantoms and in the neck of 6 normal volunteers at 3T. The tissue susceptibility matched PG foams consistently mitigate signal drop out, improve image SNR, and enable far more robust frequency selective fat suppression in T1-weighted FLASH images in volunteers. PG foam has many practical applications in MR applications that require a pristine B0 field.

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