000267497 001__ 267497
000267497 005__ 20190708160922.0
000267497 0247_ $$a10.5075/epfl-thesis-9555$$2doi
000267497 037__ $$aTHESIS
000267497 041__ $$aeng
000267497 088__ $$a9555
000267497 245__ $$aEngineering Parallel Transmit/Receive Radio-Frequency Coil Arrays for Human Brain MRI at 7 Tesla
000267497 260__ $$aLausanne$$bEPFL$$c2019
000267497 269__ $$a2019
000267497 300__ $$a180
000267497 336__ $$aTheses
000267497 502__ $$aProf. Frédéric Mila (président) ; Prof. Rolf Gruetter, Özlem Ipek (directeurs) ; Prof. Giovanni Boero, Prof. Andrew Webb, Prof. Alexander Raaijmakers (rapporteurs)
000267497 520__ $$aMagnetic resonance imaging is widely used in medical diagnosis to obtain anatomical details of the human body in a non-invasive way. Clinical MR scanners typically operate at a static magnetic field strength (B0) of 1.5T or 3T. However, going to higher field is of great interest since the signal-to-noise ratio is proportional to B0. Therefore, higher image resolution and better contrast between the human tissues could be achieved. Nevertheless, new challenges arise when increasing B0. The wavelength associated with the radio-frequency field B1+ has smaller dimensions - approx. 12 cm for human brain tissues - than the human brain itself (20 cm in length), the organ of interest in this thesis. The main consequence is that the transmit field distribution pattern (B1+) is altered and the final MR images present bright and dark signal spots. These effects prevent the ultra-high field MR scanners (>= 7T) to be used for routine clinical diagnosis. Parallel-transmit is one approach to address these new challenges. Instead of using an RF coil connected to a single power input as it is commonly done at lower magnetic fields, multiple RF coils are used with independent power inputs. The subsequent distinct RF signals can be manipulated separately, which provides an additional degree of freedom to generate homogeneous B1+-field distributions over large or specific regions in the human body. A transmit/receive RF coil array optimized for whole-brain MR imaging was developed and is described in this thesis. Dipoles antennas were used since they could provide a large longitudinal (vertical axis-head to neck) coverage and high transmit field efficiency. Results demonstrated a complete coverage of the human brain, and particularly high homogeneity over the cerebellum. However, since the receive sensitivity over large field-of-views is related to the number of channels available to detect the NMR signal, the next work was to add a 32-channel receive loop coil array to the transmit coil array. The complete coverage of the human brain was assessed with a substantial increase in signal-to-noise compared to the transmit/receive dipole coil array alone. Moreover, acquisition time was shortened since higher acceleration factors could be used. To optimize the individual RF fields and generate an homogeneous B1+-field, a method was developed making use of the particle-swarm algorithm. A user-friendly graphical interface was implemented. Good homogeneity could be achieved over the whole-brain after optimization with the coil array built in this study. Moreover, the optimization was shown to be robust across multiple subjects. The last project was focused on the single transmit system. Local volume coils (single transmit) present pronounced transmit field inhomogeneities in specific regions of the human brain such as the temporal lobes. A widely used approach to address locally these challenges is to add dielectric pads inside the volume coils to enhance the local transmit field efficiency. It was shown in this thesis that constructing dedicated surface coils is a valuable alternative to the dielectric pads in terms of transmit field efficiency and MR spectroscopy results. Two RF coil setups were developed for the temporal and frontal lobes of the human brain, respectively. This thesis provides extensive insight on MR engineering of RF coils at ultra-high field and the potential of parallel-transmit to address the future needs in clinical applications.
000267497 592__ $$b2019
000267497 6531_ $$aultra-high field
000267497 6531_ $$aphased array
000267497 6531_ $$acenter-shortened dipoles
000267497 6531_ $$aparallel imaging
000267497 6531_ $$aparallel transmission
000267497 6531_ $$aRF phase shimming
000267497 6531_ $$aparticle-swarm algorithm
000267497 6531_ $$asurface coils
000267497 6531_ $$a7T
000267497 6531_ $$awhole-brain imaging
000267497 700__ $$aClément, Jérémie Daniel$$g184201
000267497 720_2 $$aGruetter, Rolf$$edir.$$g161735
000267497 720_2 $$aIpek, Özlem$$edir.$$g231389
000267497 8564_ $$uhttps://infoscience.epfl.ch/record/267497/files/EPFL_TH9555.pdf$$s30383316
000267497 909C0 $$pLIFMET
000267497 909CO $$pthesis-public$$pDOI$$pSB$$ooai:infoscience.epfl.ch:267497$$pthesis
000267497 918__ $$aSB$$cIPHYS$$dEDPY
000267497 919__ $$aLIFMET
000267497 920__ $$a2019-06-24$$b2019
000267497 970__ $$a9555/THESES
000267497 973__ $$sPUBLISHED$$aEPFL
000267497 980__ $$aTHESIS