Novel Electromagnet for MRI Applications

Summary form only given. Magnetic Resonance Imaging is one of the most useful tools for medical diagnosis around the world. It is applied in diagnosis and treatment of many types of cancer, soft tissue injuries, in-vivo brain experiments and studies with drugs. Despite its development since the 70s [1], MRI still has to deal with problems that affect directly the sensitivity of the signal and the spatial resolution of the image. Some of these problems are related to the Nuclear Magnetic Resonance phenomenon in solid-state samples [2], and others come from the MRI technique itself [3]. Our work is focused on the development of a novel design of MRI magnet able to mitigate the spatial resolution problems in MRI experiments. RF-MAFS: Resolution problems in MRI are related to different inter and intramolecular phenomena [4] and experimental imperfections. In NMR, they are usually mitigated by applying the Magic Angle Spinning technique where the sample is tilted with respect to the main static magnetic field and spun around its principal axis, averaging to zero the effect of nuclear interactions on the NMR spectrum [5]. In MRI, the spatial resolution is also affected by changes in magnetic susceptibility along the sample, movements of the patient during the experiment and magnetic fi eld inhomogeneities [3]. Applying MAS techniques in MRI experiments is not straightforward. Although some experiments have been performed on rats spun at very low spinning rate [6], the mechanical spinning of a human patient during MILT experiments looks extremely risky. A solution was proposed early in the 60s [7], thought for low -field NMR applications [8], [9] and implemented with permanent magnets for MRI purposes [10], where the main magnetic fi eld is tilted with respect to the main axis of the sample and spun around it at very low spinning rates. The resultant field rotates around the sample at an angle of 54.74° with respect to the sample main axis. Our approach follows the work developed in [9], where RF signals will produce a rotating magnetic field in the transverse plane with respect to the main axis of the sample and an optimized solenoid will produce a static fi eld aligned with that axis. This is what we call RF-MAFS and it is able to achieve, in theory, ultra -high fi eld spinning rates. To do so, we propose a magnet consisting of two Double -Helix Dipole coils [11] and an optimized solenoid [12] (see Fig. 1). RF-MAFS Magnet Development: Fig. 1 plots the concept for the RF-MAFS magnet. The magnet includes two Double -Helix Dipole coils of 81 turns, 10 0 tilting angle and 1.5 mm effective conductor diameter with radii as specified. Theoretically (Biot-Savart integration) they produce two linearly polarized magnetic density fl uxes of 2.4 mT/A 1.7 mT/A respectively. If the coils are placed in spatial "quadrature", the RF signals feeding the coils are compensated in amplitude and they are also phase shifted 90 0 with respect to each other, the total magnetic fl ux density corresponds to a rotating magnetic field of B rat = 1.7 mT/A over a large volume inside the coils. In practice, we have built two DID coils with 5 mm spaced grooves (2x2 mm section) where we have placed 1.2 mm diameter copper wire. The grooves are carved on 4 G10 material tubes 3 mm thick resulting in two -900 mm assemblies. Their impedances have been measured with a VNA in the 1 kHz -1.5 MHz band (see Fig. 1) for better tune and match. The optimized solenoid will be placed concentric to this assembly producing the static field component, B z , needed to obtain the RF-MAFS fi eld. Fig. 2 shows the measurement system developed to obtain the magnetic fl ux density generated by the DID coils assemblies along their axis. The system consists of two pick-up coils of 5x5 mm section oriented in spatial "quadrature". The voltage induced by the time varying magnetic fl ux is amplified by two instrumentation amplifiers working up to 15 kHz with G v = 500 and filtered by a low-pass filter before going to an AGILENT-Infmiium oscilloscope. Fig. 2 also shows the normalized magnetic flux density, B rat , produced by the two DM coils when a 2 Vpp signal is amplified by an AR3501AH amplifier delivering -1 A (-10 W) when its gain is set to 50°4. The measurement was taken every 2 mm along the z-axis with an almost constant SNR of 25 dB. The frequencies were kept in a range where the value of 141 reach a minimum keeping a considerable Signal to Noise Ratio Mk, @9 kHz = 632 ['and IZELa @4 kHz = 10.35 0). B rat was normalized to the value at the geometric center of th e coil (z = 0 in Fig. 2). Fig. 2 demonstrates that the assembly produces a fl at region of' -40 mm along the axis. In order to better evaluate the volume of homogeneity, a measurement system with 5 pairs of 2x2 mm pick-up coils placed on the surface of a 15 mm radius cylinder is being implemented at the moment The coils will be moved 20 mm around the geometrical center of the assembly and a Cylindrical Harmonics Decomposition will be performed. The system will also include a hall detector per pair of coils in order to analyze in the same way the longitudinal magnetic fi eld generated by the optimized solenoid, B. At the end, we will be able to deploy the RF-MAFS magnet optimized to provide a magnetic fl ux density -2 mT/A with enough homogeneity to perform MRI experiments inside a cylindrical volume of 2.9x10 -5 m3.