Proceedings of the meeting - Department of Physics - University of ...

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I1Bill Moore Memorial Lecture: Human Brain MRI at 4.7 TeslaRoger Ordidge, David Thomas, David Carmichael, Enrico DeVita and Robert TurnerUniversity College London, United KingdomIntroductionAn aim of this research is to develop an approach for high resolution MRI of the humanbrain that could produce high quality images in a clinically feasible scan time using a 4.7Tesla magnet. Higher resolution images are desirable in order to visualise the smaller andmore intricate brain structures that are currently not observable in standard MR images. Amajor limitation to obtaining higher spatial resolution is the limited signal-to-noise ratio(SNR) of the MR data acquisition.MethodsIn order to achieve this goal, it was necessary to:(1) Increase the main magnetic field strength to 4.7 Tesla.(2) Perform pulse sequence optimisation. At higher field strengths, the RF powerrequired for a given excitation flip angle is proportionally larger. This means thatthe power deposition (SAR) limits are reached more quickly. Also, the transmitRF field is less uniform due to field focusing effects which can lead to nonuniformimages with variable contrast. This problem proliferates in sequenceswith multiple RF pulses, where the net effect becomes more pronouncedfollowing each RF pulse.(3) Use RF receiver coil arrays. Over recent years, array coils have been usedincreasingly because they enable higher sensitivity to be achieved over a largervolume.(4) Improve the point spread function (PSF). Novel signal acquisition strategiescombined with parallel imaging methodology can be used to mitigate the effectson the PSF of T2 relaxation during multiple-echo MR imaging. The deleteriouseffects of related imaging ghosting can also be minimised.Results and DiscussionIn this presentation, the configuration of our high field 4.7T MR scanner, customised forneuroimaging applications, will be described. High resolution T2-weighted human brainimages have been acquired using a modified fast spin echo (FSE) imaging sequence and aprototype 4-channel array coil. The array coil was designed to fit inside a standard designhead birdcage coil and to fit closely to the contours of the human head, enablingmaximum gradient performance, minimum SAR and optimum image SNR. Using thisapproach, we have obtained high quality images with an in-plane pixel resolution of352µm ×352µm (slice thickness = 2mm) in an acquisition time of 5mins 40s for a 17slice data set. For T1-weighted MRI, a modified MDEFT sequence was developed toovercome the effects of field-focussing.
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