Doctoral Thesis: Mitigating Inhomogeneity in High-Field MRI Excitations: Multiphoton Parallel Transmission and Arbitrary Waveform Optimization 

By: John Drago

Details

• Date: Friday, March 14
• Time: 11:00 am - 1:00 pm
• Category: Thesis Defense
• Location: 36-428 (Haus Room)

Additional Location Details:

High-field magnetic resonance imaging (MRI) using a standard volume coil suffers a flip angle inhomogeneity problem due to the complex interactions of electromagnetic fields from current-carrying elements surrounding the imaging region. Parallel transmission (pTx) mitigates this issue by employing multiple high-power, independently controlled transmit elements for more precise excitation control. However, current pTx methods do not enable real-time, subject-specific pulse design during scanning and create complex electric field distributions that require careful monitoring to manage tissue power deposition, quantified as the specific absorption rate (SAR).

We introduce a method for optimizing patient-specific pulses using a global waveform (Ritz) approach, enabling rapid, in-scanner pulse optimization. While pTx effectively addresses flip angle inhomogeneity, it remains costly and introduces challenges in SAR management. To address these limitations, we develop and characterize the MP-pTx method, which leverages the multiphoton phenomenon to improve homogeneity using a standard volume coil supplemented with low-frequency (kilohertz) parallel channels. MP-pTx reduces costs and simplifies SAR management by employing off-resonant RF excitations from a birdcage coil, complemented by an oscillating z-directed field from a shim array and/or gradient channels at a frequency that satisfies the resonance condition for spin state transitions.

Host

• John Drago