Certain Investigations On Design And Analysis Of Microstrip Rf Coil Structures For 1.5 Tesla Magnetic Resonance Imaging System

Abstract

In recent years, magnetic resonance imaging (MRI) is widely used to obtain the clear images of the human body, especially those of high water content tissues. The introduction of MRI technique into clinical practice gives a new dimension to the radiological study. Before the invention of MRI; X-ray, ultrasound and computed tomography (CT) were the widely accepted techniques for acquisition of images inside human anatomy. In the case of X-ray or CT medical imaging techniques, the ionizing radiation is used. But in MRI there is no ionization of molecules present in the human body. In addition to the advantage of safety, MRI is better for visualizing the images of soft tissues than CT.In the MRI system RF coil plays an important role to improve the quality of imaging. The RF coil transmits RF pulses at the Larmor frequency to excite the nuclei in the object to be imaged. When the RF excitation pulse is removed, the nuclei will relax; during relaxation the nuclei will emit the RF energy at the Larmor frequency. This NMR emitted RF signals are again collected by receive RF coil. The signals received by the receive RF coil are suitably processed inside a digital computer for getting the MR image of the anatomy. The RF coil performs the role of an antenna which radiates a pulse of the electromagnetic (EM) wave into the body to generate a nuclear magnetic resonance (NMR) signal and the same antenna receives the reradiated signal from the body. The quality of the image captured by MR scanners is highly dependent of the performance of RF coil. High field (greater than 3 Tesla)MRI scanners will give better quality images but the cost of imaging devices is very high. Instead of shifting towards high field MRI 2systems, it is better to improve the performance of RF coils in a low field environment for producing better quality images. So there is a need for more research participation to design and analysis the performance of RF coils for low field (1.5 Tesla) MRI applications. The objective of the research is to propose various microstrip based RF coil structures for 1.5 Tesla MRIsystem. The resonance frequency of the 1.5 Tesla MRI RF coil is 63.87 MHz. Microstrip based structures will radiate effectively for high frequency microwave applications. In other words preferring microstrip based structures for low frequency (63.87 MHz) operation will lead to physically a large sized RF coil. Therefore the challenging part of the RF coil design is, for the resonance frequency of 63.87 MHz the dimension of the microstrip based structure should not be much larger.In this research work a variety of RF coil structures are proposed for the needs of 1.5 Tesla Magnetic Resonance Imaging systems. The surface type RF coils are formed in a FR4 (Flame-Retardant Type 4) substrate with height 1.6 mm and the thickness of the top conducting transmission line is 35m. The designed RF coil structures are analysed with respect to their return loss and quality factor performance by using Advanced Design system (ADS) scattering parameter simulation. The aim of preferring surface type RF coils is to extend themfor phased array coil design.There are four different types of surface coil geometries are proposed for low field (1.5 Tesla) MRI requirements. The first surface coil design focuses the square shaped RF coil structure with L Type Matching network. The matching network consists of shunt and series configuration of capacitor and inductor. The impedance matching procedure indicated is so simple for matching source impedance with real as well as complex type of load impedances.In practice, inductors are made up of conducting wires of small diameter which may dissipate energy as heat loss. In order to avoid such limitations a new L type matching network with only capacitive elements is proposed. Such matching network is modified Type1 matching network.