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.@en

Purpose: To develop and evaluate a robust cardiac B+1 mapping sequence at 3 T, using Bloch–Siegert shift (BSS)-based preparations.

Methods: A longitudinal magnetization preparation module was designed to encode |B+1 |. After magnetization tip-down, off-resonant Fermi pulses, placed symmetrically around two refocusing pulses, induced BSS, followed by tipping back of the magnetization. Bloch simulations were used to optimize refocusing pulse parameters and to assess the mapping sensitivity. Relaxation-induced B+1 error was simulated for various T 1 /T 2 times. The effective mapping range was determined in phantom experiments, and |B+1 | maps were compared to the conventional BSS method and subadiabatic hyperbolic-secant 8 (HS8) pulse-sensitized method. Cardiac B+1 maps were acquired in healthy subjects, and evaluated for repeatability and imaging plane intersection consistency. The technique was modified for three-dimensional (3D) acquisition of the whole heart in a single breath-hold, and compared to two-dimensional (2D) acquisition.

Results: Simulations indicate that the proposed preparation can be tailored to achieve high mapping sensitivity across various B+1 ranges, with maximum sensitivity at the upper B+1 range. T 1 /T 2 -induced bias did not exceed 5.2%. Experimentally reproduced B+1 sensitization closely matched simulations for B+1 ≥ 0.3B+1, max (mean difference 0.031±0.022, compared to 0.018±0.025 in the HS8-sensitized method), and showed 20-fold reduction in the standard deviation of repeated scans, compared with conventional BSS B+1 mapping, and an equivalent 2-fold reduction compared with HS8-sensitization. Robust cardiac B+1 map quality was obtained, with an average test-retest variability of 0.027±0.043 relative to normalized B+1 magnitude, and plane intersection bias of 0.052±0.031. 3D acquisitions showed good agreement with2D scans (mean absolute deviation 0.055±0.061).

Conclusion: BSS-based preparations enable robust and tailorable 2D/3D cardiac B+1 mapping at 3 T in a single breath-hold.@en

With sound pressure levels reaching up to 130 dB, acoustic noise in Magnetic Resonance Imaging (MRI) is one of the main sources of patient discomfort in otherwise one of the safest medical imaging modalities. In this work, a noise prediction-based approach, termed predictive noise cancelling (PNC), is applied, for the first time, to suppress noise in MRI. In PN C the noise from the scanner gradient coils is predicted based on linear time-invariant models, which relate the individual gradient coil (X, Y and Z) input to the acoustic noise output. A model setup was constructed of a custom speaker box and MRI -compatible microphone to demonstrate live noise reduction. Additional tuning steps, including output channel equalization and clock mismatch correction, were performed to maximize noise reduction. A calibration sequence was designed to determine the model and tuning parameters. Analysis of actual scanner noise shows an upper limit of 21 dB noise reduction with the proposed linear model. For the components of a clinical example sequence, the setup demonstrated in-bore live noise reduction of up to 10 dB (7.01 ± 0.31 dB, 6.42 ± 2.04 dB and 9.28 ± 0.26 dB for X, Y and Z gradient coils respectively) in the presence of system imperfections. Clinical relevance - The results indicate promising noise attenuation without the need to modify scanner hardware or compromises in acquisition speed or quality. This has potential to substantially and cost effectively improve patient comfort in clinical MRI.

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Cardiovascular disease is the leading cause of death and a significant contributor of health care costs. Noninvasive imaging plays an essential role in the management of patients with cardiovascular disease. Cardiac magnetic resonance (MR) can noninvasively assess heart and vascular abnormalities, including biventricular structure/function, blood hemodynamics, myocardial tissue composition, microstructure, perfusion, metabolism, coronary microvascular function, and aortic distensibility/stiffness. Its ability to characterize myocardial tissue composition is unique among alternative imaging modalities in cardiovascular disease. Significant growth in cardiac MR utilization, particularly in Europe in the last decade, has laid the necessary clinical groundwork to position cardiac MR as an important imaging modality in the workup of patients with cardiovascular disease. Although lack of availability, limited training, physician hesitation, and reimbursement issues have hampered widespread clinical adoption of cardiac MR in the United States, growing clinical evidence will ultimately overcome these challenges. Advances in cardiac MR techniques, particularly faster image acquisition, quantitative myocardial tissue characterization, and image analysis have been critical to its growth. In this review article, we discuss recent advances in established and emerging cardiac MR techniques that are expected to strengthen its capability in managing patients with cardiovascular disease. Level of Evidence: 5. Technical Efficacy: Stage 1.

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