Acoustic and Electromagnetic Waves in MRI

Novel Strategies for Acoustic Noise Reduction, Transmit Field Characterization and Correction

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Abstract

Magnetic Resonance Imaging (MRI) is increasingly applied at high magnetic field strengths of 3T and above. This trend aids in achieving superior image quality, accelerated scan times, and enhanced diagnostic capabilities. However, the increased magnetic field strengths also amplify the acoustic and electromagnetic field interactions during MRI. This presents considerable challenges for comfortable and reliable imaging.
Firstly, the acoustic noise at high magnetic field strength MRI is greatly exacerbated. The sound may reach up to 130 dB, greatly exceeding the safe prolonged noise exposure limit at 90 dB. This causes anxiety in patients, elevate claustrophobic reactions, and are even associated with increased scan refusal rates. The sound pressure levels at modern scanners generally exceed the public safety threshold. Thus, if not adequately accounted for, it bears the risk of temporary or permanent hearing damage.
Secondly, achieving a homogeneous distribution of the transmitted radiofrequency (RF) field, which is used to create the magnetization signal, becomes increasingly challenging at high magnetic fields. The degree of constructive and destructive interference of the RF field components becomes exacerbated as the RF field wavelength shortens and starts to approximate the body dimensions in tissue. This results in spatial signal variation that cannot be universally predetermined, which can hinder diagnostic assessment in clinical imaging.
In this thesis, multiple challenges of high-field MRI were tackled by developing advanced devices and MR scanning methodologies. Firstly, a solution to the acoustic noise problem was proposed based on predicting the scanner sound and superimposing its anti-noise to reduce the sound pressure. The results indicate robust noise reduction of up to 13 dB with a proof-or-principle system across various scanning methods.
To tackle the transmitted field B1+ inhomogeneity, an adaptive shimming device was proposed, based on an array of electrically-coupled pads with high permittivity. Based on electromagnetic field simulations, the device prototype was constructed. Up to 11% modulation was measured in phantoms at 15 mm depth, with an ability to tune the location of shimming hotspot.
A novel scanning methodology to quantify B1+ magnitude in the heart was developed, using Bloch-Siegert shift-based preparations. The sequence has shown highly-robust 2D and 3D mapping in healthy subjects, and achieved superior noise-resilience, when compared to more established mapping methods.
The proposed novel B1+ mapping sequence was then adapted to enable conductivity mapping in the myocardium. The method provided the first complex B1+ field-based conductivity reconstruction in healthy subjects, with robust results in agreement with previous literature.
Finally, another MRI scanning methodology was proposed for quantitative cardiac imaging robust to field inhomogeneities.T2* - a common biomarker used in MRI - was reconstructed using phase-cycled preparations. Compared to conventional sequence, higher noise resilience was achieved in healthy subjects, while enabling a wider range of sequence tailoring options.