An active TX/RX NMR probe for real-time monitoring of MRI field imperfections

In this paper, we present a PCB-based active miniaturized MR field probe for real-time monitoring of the magnetization's phase evolution during magnetic resonance (MR) experiments. The data obtained with the presented sensor can be used to correct gradient field imperfections which uncorrected result in significant distortions in the reconstructed MR images. The presented active field probe consists of a susceptibility matched solenoidal MR coil and a complete homodyne transceiver. Thanks to the local generation of the radio frequency signal required for the excitation of the spin ensemble and the downconversion of the recorded MR signal to low frequencies, the proposed architecture significantly reduces the crosstalk between the probe head and the MR imaging object compared to existing designs. MR measurements performed in an ultra high field 9.4 T full-body scanner prove the compatibility of the presented sensor with commercial MR imaging systems and demonstrate its excellent MR phase tracking performance

Real-time Feedback of B0 Shimming at Ultra High Field MRI

Magnetic resonance imaging(MRI) is moving towards higher and higher field strengths. After 1.5T MRI scanners became commonplace, 3T scanners were introduced and once 3T scanners became commonplace, ultra high field (UHF) scanners were introduced. UHF scanners typically refer to scanners with a field strength of 7T or higher. The number of sites that utilise UHF scanners is slowly growing and the first 7T MRI scanners were recently CE certified for clinical use. Although UHF scanners have the benefit of higher signal-to-noise ratio (SNR), they come with their own challenges. One of the many challenges is the problem of inhomogeneity of the main static magnetic field(B0 field). This thesis addresses multiple aspects associated with the problem of B0 inhomogeneity. The process of homogenising the field is called "shimming". The focus of this thesis is on active shimming where extra shim coils drive DC currents to generate extra magnetic fields superimposed on the main magnetic field to correct for inhomogeneities. In particular, we looked at the following issues: algorithms for calculating optimal shim currents; global static shimming using very high order/degree spherical harmonic-based (VHOS) coils; dynamic slice-wise shimming using VHOS coils compared to a localised multi-coil array shim system; B0 field monitoring using an NMR field camera; characterisation of the shim system using a field camera; and designing a controller based on the shim system model for real-time feedback