Matching Items (5)
Filtering by

Clear all filters

147605-Thumbnail Image.png
Description

This thesis details the design process of a variable gain amplifier (VGA) based circuit which maintains a consistent output power over a wide range of input power signals. This effect is achieved by using power detection circuitry to adjust the gain of the VGA based on the current input power

This thesis details the design process of a variable gain amplifier (VGA) based circuit which maintains a consistent output power over a wide range of input power signals. This effect is achieved by using power detection circuitry to adjust the gain of the VGA based on the current input power so that it is amplifier to a set power level. The paper details the theory behind this solutions as well as the design process which includes both simulations and physical testing of the actual circuit. It also analyses results of these tests and gives suggestions as to what could be done to further improve the design. The VGA based constant output power solution was designed as a section of a larger circuit which was developed as part of a senior capstone project, which is also briefly described in the paper.

ContributorsMeyer, Sheldon (Author) / Aberle, James (Thesis director) / Chakraborty, Partha (Committee member) / Electrical Engineering Program (Contributor) / School of Mathematical and Statistical Sciences (Contributor) / Barrett, The Honors College (Contributor)
Created2021-05
147550-Thumbnail Image.png
Description

The honors thesis presented in this document describes an extension to an electrical engineering capstone project whose scope is to develop the receiver electronics for an RF interrogator. The RF interrogator functions by detecting the change in resonant frequency of (i.e, frequency of maximum backscatter from) a target resulting

The honors thesis presented in this document describes an extension to an electrical engineering capstone project whose scope is to develop the receiver electronics for an RF interrogator. The RF interrogator functions by detecting the change in resonant frequency of (i.e, frequency of maximum backscatter from) a target resulting from an environmental input. The general idea of this honors project was to design three frequency selective surfaces that would act as surrogate backscattering or reflecting targets that each contains a distinct frequency response. Using 3-D electromagnetic simulation software, three surrogate targets exhibiting bandpass frequency responses at distinct frequencies were designed and presented in this thesis.

ContributorsSisk, Ryan Derek (Author) / Aberle, James (Thesis director) / Chakraborty, Partha (Committee member) / Electrical Engineering Program (Contributor, Contributor) / Barrett, The Honors College (Contributor)
Created2021-05
165040-Thumbnail Image.png
Description
The ability of magnetic resonance imaging (MRI) to image any part of the human body without the effects of harmful radiation such as in CAT and PET scans established MRI as a clinical mainstay for a variety of different ailments and maladies. Short wavelengths accompany the high frequencies present in

The ability of magnetic resonance imaging (MRI) to image any part of the human body without the effects of harmful radiation such as in CAT and PET scans established MRI as a clinical mainstay for a variety of different ailments and maladies. Short wavelengths accompany the high frequencies present in high-field MRI, and are on the same scale as the human body at a static magnetic field strength of 3 T (128 MHz). As a result of these shorter wavelengths, standing wave effects are produced in the MR bore where the patient is located. These standing waves generate bright and dark spots in the resulting MR image, which correspond to irregular regions of high and low clarity. Coil loading is also an inevitable byproduct of subject positioning inside the bore, which decreases the signal that the region of interest (ROI) receives for the same input power. Several remedies have been proposed in the literature to remedy the standing wave effect, including the placement of high permittivity dielectric pads (HPDPs) near the ROI. Despite the success of HPDPs at smoothing out image brightness, these pads are traditionally bulky and take up a large spatial volume inside the already small MR bore. In recent years, artificial periodic structures known as metamaterials have been designed to exhibit specific electromagnetic effects when placed inside the bore. Although typically thinner than HPDPs, many metamaterials in the literature are rigid and cannot conform to the shape of the patient, and some are still too bulky for practical use in clinical settings. The well-known antenna engineering concept of fractalization, or the introduction of self-similar patterns, may be introduced to the metamaterial to display a specific resonance curve as well as increase the metamaterial’s intrinsic capacitance. Proposed in this paper is a flexible fractal-inspired metamaterial for application in 3 T MR head imaging. To demonstrate the advantages of this flexibility, two different metamaterial configurations are compared to determine which produces a higher localized signal-to-noise ratio (SNR) and average signal measured in the image: in the first configuration, the metamaterial is kept rigid underneath a human head phantom to represent metamaterials in the literature (single-sided placement); and in the second, the metamaterial is wrapped around the phantom to utilize its flexibility (double-sided placement). The double-sided metamaterial setup was found to produce an increase in normalized SNR of over 5% increase in five of six chosen ROIs when compared to no metamaterial use and showed a 10.14% increase in the total average signal compared to the single-sided configuration.
ContributorsSokol, Samantha (Author) / Sohn, Sung-Min (Thesis director) / Allee, David (Committee member) / Jones, Anne (Committee member) / Barrett, The Honors College (Contributor) / Electrical Engineering Program (Contributor)
Created2022-05
190806-Thumbnail Image.png
Description
In 1946 Felix Bloch first demonstrated the phenomenon of nuclear magnetic resonance using continuous-wave signal generation and acquisition. Shortly after in 1966, Richard R. Ernst demonstrated the breakthrough that nuclear magnetic resonance needed to develop into magnetic resonance imaging: the application of Fourier transforms for sensitive pulsed imaging. Upon this

In 1946 Felix Bloch first demonstrated the phenomenon of nuclear magnetic resonance using continuous-wave signal generation and acquisition. Shortly after in 1966, Richard R. Ernst demonstrated the breakthrough that nuclear magnetic resonance needed to develop into magnetic resonance imaging: the application of Fourier transforms for sensitive pulsed imaging. Upon this discovery, the world of research began to develop high power radio amplifiers and fast radio switches for pulsed experimentation. Consequently, continuous-wave imaging placed on the backburner.Although high power pulses are dominant in clinical imaging, there are unique advantages to low power, continuous-wave pulse sequences that transmit and receive signals simultaneously. Primarily, tissues or materials with short T2 time constants can be imaged and the peak radio power required is drastically reduced. The fundamental problem with this lies in its nature; the transmitter leaks a strong leakage signal into the receiver, thus saturating the receiver and the intended nuclear magnetic resonance signal is lost noise. Demonstrated in this dissertation is a multichannel standalone simultaneous transmit and receive (STAR) system with remote user-control that enables continuous- wave full-duplex imaging. STAR calibrates cancellation signals through vector modulators that match the leakage signal of each receiver in amplitude but opposite in phase, therefore destructively interfering the leakage signals. STAR does not require specific imaging coils or console inputs for calibration. It was designed to be general- purpose, therefore integrating into any imaging system. To begin, the user uses an Android tablet to tune STAR to match the Larmor frequency in the bore. Then, the user tells STAR to begin calibration. After self-calibrating, the user may fine-tune the calibration state of the system before enabling a low-power mode for system electronics and imaging may commence. STAR was demonstrated to isolate two receiver coils upwards of 70 dB from the transmit coil and is readily upgradable to enable the use of four receive coils. Some primary concerns of STAR are the removal of transceivers for multichannel operation, digital circuit noise, external noise, calibration speed, upgradability, and the isolation introduced; all of which are addressed in the proceeding thesis.
ContributorsColwell, Zachary Allen (Author) / Sohn, Sung-Min (Thesis advisor) / Trichopoulos, Georgios (Thesis advisor) / Aberle, James (Committee member) / Sadleir, Rosalind (Committee member) / Arizona State University (Publisher)
Created2023
190874-Thumbnail Image.png
Description
Magnetic Resonance Imaging has become an increasingly reliable source of medical imaging to obtain high quality detailed images of the human anatomy. Application specific coil or an array of coils when placed closely to the anatomy produces high quality image due to the improved spatial signal to noise ratio. Elastic

Magnetic Resonance Imaging has become an increasingly reliable source of medical imaging to obtain high quality detailed images of the human anatomy. Application specific coil or an array of coils when placed closely to the anatomy produces high quality image due to the improved spatial signal to noise ratio. Elastic RF coils have been shown to conform to the shape of the patient’s body and drastically reduce the gap between coil and anatomy. First, a major challenge faced by these elastic RF coils is the changing impedance condition as the coil takes a different shape for every individual. Next, an area that could benefit from the improved image quality and patient comfort that comes from flexible RF coil design is endorectal prostate imaging. Demonstrated in the first part of this dissertation is a modular solution to compensate the impedance mismatch. Standalone Wireless Impedance Matching (SWIM) system is an automatic impedance mismatch compensation system that can function independently of the MR scanner. The matching network consists of a capacitor array with RF switches to electronically cycle through different input impedance conditions. The SWIM system can automatically calibrate an RF coil in 3s with a reflection coefficient of less than -15dB resulting in improved Signal-to-noise ratio (SNR) of the sample image by 12% - 24%, based on sample size, when compared to a loaded coil without retuning. For the second part, we propose a novel elastic and inflatable RF coil integrated with the SWIM system for endorectal prostate imaging at 9.4T. A silicone polymer substrate filled with liquid metal alloy is designed and fabricated with a cavity to create ii inflation. This inflatable RF coil is combined with the SWIM system to automatically tune and match after inflating the RF coil for individual levels of inflation. The imaging results have shown a ~10%, ~19%, and ~25 % increase in SNR due to inflation of RF coil at different ROIs in the acquired image. Overall, the methods proposed and discussed in this thesis are a step towards a new generation of RF coil systems for both existing applications and upcoming ones.
ContributorsKandala, Sri Kirthi (Author) / Sohn, Sung-Min (Thesis advisor) / Kdibagkar, Vikram (Committee member) / Sadleir, Rosalind J (Committee member) / Beeman, Scott (Committee member) / Trichopoulos, Georgios (Committee member) / Arizona State University (Publisher)
Created2023