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Date of Award
Master of Science in Electrical Engineering (MS)
College, School or Department Name
Department of Electrical and Computer Engineering
RF coils serving for both transmitting and receiving functions are necessary parts of the MRI systems. Coils generate a linearly polarized oscillating magnetic field (denoted usually as B1), which is normal to the static magnetic field (B0). If oscillations of the B1 field match the natural precession frequency (Larmor frequency) of the nuclear spins around the direction of B0, the energy of the B1 field can be transmitted to the spin system causing a change in its net alignment ("on resonant condition") that implies excitation to higher energy states. Turning the B1 on takes usually only a brief period of time (a few msec). After turning the B1off, the spin system gradually releases the excessive energy and returns to the initial state. During such relaxation the RF-signal portion of the net magnetization can be detected by controlling the induced electric current in the same RF coil as that used for excitation. To obtain a high quality image, a homogeneous B0 field, as well as a fairly homogeneous B1 field distribution within the volume of interest, is needed. Areas with higher or lower magnitudes of B1 field can result in hypo intense areas in the image. In addition to homogeneity, the coils should provide for the maximal magnitudes of B1 fields within the volumes of interest that presents an additional challenge for coil designing. Although various rather complicated coil designs have been proposed, small objects have often being studied with employment of simple solenoid coils. The homogeneity and field strength provided by solenoid coils can be acceptable for some applications  , . However, these coils are known to have higher fields at the center of the coil and a reduced field strength toward the coil ends. In  it was proposed to divide the RF coil into several decoupled segments (down to separate loops) and to integrate them by the input matching network providing for the control of current magnitudes in each of the segments. However, developing specific networks complicates measurements and makes them time consuming. In addition, it can’t guarantee the high field strength. In  it was mentioned that the maximum field strength inside the solenoid coils can be achieved when the coil length is a factor of 1.41 larger than the coil diameter, however, at frequencies higher than 100 MHz this approach is not applicable and other methods should be utilized . In this work, we investigate an opportunity to provide for better field homogeneity inside the solenoid coils by using non-uniform coil wrapping. First, a uniformly wrapped coil probe for MR imaging at the static magnetic field of 14 T has been designed and studied. Then the homogeneity of the RF magnetic field has been improved by sectioning the coil in parts with different density of wrapping. The field distributions along the RF coil axes were simulated by using the CST Microwave studio software package. Secondly, we considered different challenges in proposing matching circuit and we proposed several methods to solve this issue.
Seifi, Bahram, "MODIFIED DESIGN OF THE COIL PROBE FOR HIGH FIELD MRI", Master's report, Michigan Technological University, 2015.