Browsing by Author "Truong, Trong-Kha"

Alzheimer's disease (AD) accounts for 60%-80% of dementia. AD patients start by having mild memory, language, and thinking difficulties, then gradually lose more critical abilities, such as dressing, bathing, or walking. AD not only degrades patients’ life quality but also burdens caregivers and the health system. Specifically, there are 6.5 million AD cases in the U.S. today, and the annual health costs for 2022 are estimated to be $321 billion. AD diagnosis has been evolving in the past 30 years. The criteria established in 1984 recommended that AD cannot be identified until a post-mortem neuropathological test is performed. Recently, more biomarkers have gradually been discovered, such as brain atrophy, Positron Emission Tomography (PET) measures of glucose hypometabolism, and cerebrospinal fluid (CSF) and PET measures of pathological amyloid-beta and tau. However, these biomarkers lack the specificity to probe the damage in the neuronal microstructure that directly causes the disease, and they only provide late diagnoses when the AD progression is no longer reversible. Since the neuronal damages are believed to begin 20 years or more before symptoms start, biomarkers that can detect abnormalities in the neuronal microstructure would enable the diagnosis of AD at the very early stage of neurodegeneration, years before the onset of symptoms, and they could thus potentially enable better treatment outcomes since neuronal damage at the early stage could be reversible.Diffusion tensor imaging (DTI) is a magnetic resonance imaging technique that can noninvasively probe the microstructure of the human brain in vivo. Some regions in the cortical gray matter and hippocampus are known to experience early neurodegeneration in AD, and changes in DTI metrics in these regions could reflect the early stage of AD. However, the cortex is folded and is made of different cortical layers and cortical regions and the hippocampus is made of different subfields that have distinct neuronal populations with a specific microstructure. Additionally, neurodegeneration does not necessarily occur at the same time across different cortical depths or regions in the cortex or across different subfields in the hippocampus. As such, the development of early diagnostic biomarkers would require the ability to probe the neuronal microstructure at specific cortical depths and in specific cortical regions and hippocampal subfields in vivo. However, doing so with DTI has been challenging because the average cortical thickness is only 2.5 mm and the average hippocampal volume is only 2.84 mL. Therefore, a high-resolution DTI acquisition within a reasonable scan time is needed. In this dissertation, we first aim to develop DTI acquisition and reconstruction methodologies to acquire high-resolution (0.9-mm to 1.0-mm isotropic) whole-brain DTI images. Specifically, we used an efficient multi-band multi-shot echo-planar imaging sequence and a multi-band multiplexed sensitivity-encoding reconstruction. Furthermore, we aim to develop a data analysis pipeline that can quantitatively probe the microstructure and capture characteristic features: 1) in the cortex by performing a column-based cortical depth analysis of the diffusion anisotropy and radiality; and 2) in the hippocampus by investigating intra-hippocampal fiber tracts and connectomes, with the long-term goal of enabling the early diagnosis of AD. In the cortex, a column-based cortical depth analysis that samples the fractional anisotropy (FA) and radiality index (RI) along radially oriented cortical columns was performed to quantitatively analyze the FA and RI dependence on the cortical depth, cortical region, cortical curvature, and cortical thickness across the whole brain. We first studied young healthy subjects to optimize the data acquisition and analysis pipeline and to investigate the consistency of the results. The results showed characteristic FA and RI vs. cortical depth profiles, with an FA local maximum and minimum (or two inflection points) and a single RI maximum at intermediate cortical depths in most cortical regions, except for the postcentral gyrus where no FA peaks and a lower RI were observed. These results were consistent between repeated scans from the same subjects and across different subjects. They were also dependent on the cortical curvature and cortical thickness in that the characteristic FA and RI peaks were more pronounced i) at the banks than at the crown of gyri or at the fundus of sulci and ii) as the cortical thickness increases. We then performed a preliminary clinical study in a small cohort of AD patients and age-matched healthy controls (HC) to further examine if this methodology could be applied to detect differences in the FA and RI vs. cortical depth profiles between the AD and HC groups. The FA and RI at each cortical depth and in different regions of interest (ROIs) were sampled and compared between these two groups to look for any significant differences. Additionally, based on the results from the young healthy subjects, we minimized the dependence of these DTI metrics (FA and RI) on structural metrics such as cortical thickness and cortical curvature. The results showed significant differences (p < 0.05) in the FA and RI profiles between the AD and HC groups for specific cortical depths, curvature subsets, and ROIs. To generate intra-hippocampal fiber tracts and connectomes, the hippocampus of all subjects was registered to a common template and deterministic fiber tracking was performed. The fiber orientations across hippocampal subfields were investigated, and the connectivity among subfields was quantified. The results showed characteristic fiber orientations in different hippocampal subfields that were generally consistent between repeated scans and across all subjects: right/left in the middle of the CA4/dentate gyrus subfield and the inferior part of the subiculum; anterior/posterior in CA2/CA3; superior/inferior in the medial and inferior parts of the molecular layer and subiculum. These in vivo fiber orientations aligned with those obtained from an ex vivo specimen scanned over 21 hours at a 0.2-mm isotropic resolution. However, the ex vivo scan delineated the C-shaped molecular layer, which was not shown in the in vivo scans. The in vivo connectomes were generally consistent between repeated scans and across all subjects. The in vivo and ex vivo connectomes both showed more connectivity within the head than within the body of the hippocampus; however, the in vivo and ex vivo connectivity ranking across pairs of subfields was not exactly the same, which could be explained by altered diffusion properties in the ex vivo sample due to fixation or by the higher resolution in the ex vivo scan. In conclusion, the proposed high-resolution whole-brain DTI acquisition, column-based cortical depth analysis of the diffusion anisotropy and radiality, and intra-hippocampal fiber tracking captured characteristic features of FA and RI vs. cortical depth profiles in the cortex and characteristic fiber orientations and connectivity strengths across different subfields of the hippocampus, which were consistent between repeated scans from the same subjects and across different subjects. In addition, the cortical analysis applied in a preliminary clinical study of AD patients vs. HC showed significant differences in the FA and RI profiles between these two groups, showing the potential of this methodology to generate biomarkers for the early diagnosis of AD.

Integrated parallel reception, excitation, and shimming (iPRES) is a novel MRI coil design, in which radiofrequency (RF) currents and direct currents (DC) flow in the same coil elements to perform MR image acquisition and localized magnetic field shimming, respectively, with a single coil array. The purpose of this study was to implement dynamic shimming with a 32-channel iPRES head coil array, by dynamically updating the DC currents to shim individual slices within a single scan. Dynamic shimming is more effective than global static shimming, in which the same currents are used to shim the whole brain, and more efficient than slice-optimized static shimming, in which different currents are used to shim different slices, but in separate scans.

To implement dynamic shimming, a Python script was written to send new DC current amplitudes and polarities to a DC power supply and a switch box connected to the iPRES head coil array, respectively, for each slice acquisition. This current-update process was optimized by performing timing measurements with an oscilloscope and by modifying the Python script to ensure that the DC currents to shim the ith slice were updated as efficiently as possible after the data acquisition of the (i-1)th slice and before the excitation of the ith slice.

Magnetic field maps were acquired in a phantom and in a human brain with either dynamic shimming or slice-optimized static shimming, and the root-mean-square-error was calculated to evaluate the shimming performance of the dynamic shimming relative to slice-optimized static shimming. The results show that dynamic shimming with the iPRES head coil array was successfully implemented and that it was as effective as, but much more efficient than, slice-optimized static shimming.

In MRI, magnetic susceptibility differences at air/tissue interfaces result in main magnetic field (B0) inhomogeneities, which in turn cause artifacts such as distortions and signal loss, especially in gradient-echo echo-planar imaging (EPI). Because of the air cavities in the sinuses and ear canals, these artifacts are most prominent in the inferior frontal and temporal brain regions, and hinder our ability to accurately assess brain structure and function. Existing shimming techniques can correct for these artifacts by applying additional magnetic fields that compensate for the B0 inhomogeneities. However, passive shimming has limited flexibility and patient comfort, whereas spherical harmonic shimming cannot shim localized B0 inhomogeneities.

Integrated parallel reception, excitation, and shimming (iPRES) is a novel technology that allows radio-frequency and direct currents (DC) to flow in the same coil simultaneously, enabling image acquisition and localized B0 shimming, respectively. By using a single coil array placed close to the subject, iPRES saves valuable space in the magnet bore and maximizes both the signal-to-noise ratio and shimming performance. Two advancements to this technology are proposed in this thesis: (1) a method to effectively recover signal loss in functional MRI (fMRI) and (2) a plug-and-play battery- powered iPRES head coil array.

Previously, iPRES has been used to correct for distortions in EPI images. In this work, we propose to use it to recover signal loss in fMRI. Three types of approaches were simulated: modifications of the coil geometry, adjustments to the volume shimmed, and redefinition of the cost function used in the shim optimization. The simulation results from the modified cost function were the most promising and showed that there was a tradeoff between reduction in B0 inhomogeneity and signal loss, which could be adjusted by using a weight.

The shimming performance was further assessed in in vivo fMRI experiments by acquiring B0 maps and EPI images during a breathholding task, before and after shimming with the optimal weight. The results confirmed that the proposed method could effectively recover signal loss in fMRI, while also reducing the B0 inhomogeneity.

Existing iPRES coil arrays use a multi-channel DC power supply, which allows the DC currents to be optimized to shim individual subjects and/or slices, giving the best shimming performance. However, this implementation requires many cables and filters between the coil array and the machine room. Additionally, subject-specific shimming involves the acquisition of a B0 map and a DC current optimization for each subject, which requires additional time and technical expertise, and may not be practical for clinical applications.

In this work, an MR-compatible battery pack was developed to power an iPRES head coil array, eliminating the external DC power supply, cables, and filters, and reducing the cost and complexity of the system. The battery pack was designed to deliver fixed DC currents to a subset of iPRES coil elements, which were optimized in advance to shim an average subject’s brain, thus eliminating the subject-specific shim optimization.

The system was validated through bench-top measurements and MRI experiments to verify that the shim currents were accurate and remained stable within the operating time. B0 maps and EPI images acquired in vivo before and after shimming with the battery- powered iPRES head coil array showed that it could reduce the B0 inhomogeneity and geometric distortion. While it does not offer the flexibility to shim individual subjects or slices, this stand-alone, plug-and-play system is expected to enable a wider adoption of iPRES in clinical applications.

Magnetic resonance imaging (MRI) systems rely on wired radio-frequency (RF) coil arrays placed near the anatomical region of interest to acquire images. However, these wired arrays have a long setup time and RF currents induced on their cables can potentially burn nearby tissue. A novel coil design, termed an integrated RF/wireless (iRFW) coil, eliminates these issues by removing the cabled connection between the coil array and the scanner. In lieu of transmitting the acquired signal through wired connections, the iRFW coil transmits data wirelessly over the air at a WiFi frequency (2.442 GHz).Previous work has shown that the iRFW coil can be used for low data rate applications (e.g., ~10 Megabytes/sec), but further investigation at higher data rates (~300 Megabytes/sec) is needed in order to adequately transfer acquired MR image data. Electromagnetic simulations are performed in this study to computationally determine the optimal iRFW coil size and position in the MRI scanner bore. The simulations determine: 1. The similarity in MR signal-to-noise ratio between a traditional RF coil and an iRFW coil, 2. The amount of localized heating in a human subject’s body due to the transmitted wireless signal, 3. The strength of the wireless data transmission between the iRFW coil inside the scanner bore and connecting antennas outside the bore for different coil sizes and positions, and 4. The stability of the wireless transmission link for the optimal coil size and position with different subject body lengths. The optimal coil size and position are determined by considering the trade-off between reducing localized heating in human subject tissue and maximizing the transmission link between the iRFW coil and adjacent antennas. Completed simulations indicate that specific coil sizes and positions do result in an improved connection. In future work, these simulations will be validated by a physical model.

A novel integrated RF/wireless coil design, termed an integrated RF/wireless coil, which enables simultaneous MR image acquisition and wireless data transfer, has recently been proposed. The integrated RF/wireless coil design allows radio-frequency (RF) currents to flow on the coil simultaneously at the Larmor frequency, for MR image acquisition, and at the 2.4 GHz wireless communication frequency, for wireless data transfer from within the MRI scanner bore to a wireless Access Point (AP) on the scanner room wall. The integrated RF/wireless coil design provides a low-cost solution for wireless data transmission between the scanner bore and the console room that requires no mechanical modifications to the existing MRI system, which can 1) reduce the need for cumbersome cabling networks in the scanner, 2) increase patient comfort, and 3) decrease patient set up time.

In previous work, the radiated energy emitted from the integrated RF/wireless coil was not optimized for wireless data transfer between the coil in the scanner bore and the AP on the scanner room wall. The wireless data transfer from an integrated RF/wireless coil is optimal when the maximum amount of radiated power in the wireless communication band is transferred from the integrated RF/wireless coil to the AP, and the power deposited into the subject in the scanner is minimized. However, measurements of the radiated power from the integrated RF/wireless coil in the MRI environment (i.e., in the scanner bore and on a human) are not practical because they would require an excessively large anechoic chamber. Thus, electromagnetic simulations are performed to determine the optimal integrated RF/wireless coil design (e.g., size, position on the subject) that maximizes the radiated power delivered from the coil to an AP while 1) ensuring no degradation in SNR compared to a traditional RF coil and 2) minimizing the radiated power deposited into the subject in the scanner bore.

In this work, 3-D finite element electromagnetic co-simulations with an RF circuit designer are performed to optimize the gain of an integrated RF/wireless coil on a human phantom in the scanner bore and the corresponding S21 power transmission (i.e., link budget) between the coil and an AP on the scanner room wall. These simulations are verified by constructing an integrated RF/wireless coil and by using it to perform free-space radiated gain pattern measurements in an anechoic chamber and to acquire SNR maps of a uniform water phantom in an MRI scanner.

In the reconstruction of magnetic resonance (MR) images, two important assumptions that are made are that the main magnetic field B_0 is homogeneous and that there is no bulk movement of the subject. This work proposes various strategies using innovations in radio-frequency (RF) coil design to address the problems that arise when these assumptions are broken.

Typically, B_0 inhomogeneities are caused by susceptibility differences at air/tissue interfaces and result in image artifacts such as geometric distortions and signal loss. B_0 inhomogeneities can be corrected through a process called shimming, which generates magnetic field patterns that have a similar spatial distribution but are opposite in polarity. 2\nd order spherical harmonic shimming is used on most clinical scanners, however, it is unable to correct highly localized \B_0 inhomogeneities found in the inferior frontal and temporal brain regions. By integrating a direct current (DC) path onto an RF surface coil, thereby allowing both DC current and an RF current at the Larmor frequency to flow on the same coil, localized B_0 shimming and MR imaging can be performed with the same coil array. This technology, referred to as an integrated parallel reception, excitation, and shimming (iPRES) coil array, was previously used to correct for distortions in spin-echo echo-planar imaging (EPI) and is further developed here to also recover signal loss in gradient-echo EPI, which is used for blood-oxygenation level-dependent (BOLD) functional MRI (fMRI). This was done through modification of the cost function used in the shim optimization, which typically uses a single term representing the B_0 inhomogeneity, to include a second term representing the signal loss, with an adjustable weight to optimize the trade-off between distortion correction and signal recovery. Simulations and experiments were performed to investigate the shimming performance. Slice-optimized shimming with iPRES and the proposed cost function substantially reduced the signal loss in the inferior frontal and temporal brain regions compared to shimming with iPRES and the original cost function or 2nd-order spherical harmonic shimming with either cost function. In breath-holding BOLD fMRI experiments, the ∆B_0 and signal loss root-mean-square errors decreased by -34.3% and -56.2%, whereas the EPI signal intensity and number of activated voxels increased by 60.3% and 174.0% in the inferior frontal brain region.

In addition to the integration of DC currents, currents at a Wi-Fi frequency can be integrated onto RF coils as well to perform simultaneous MR imaging and wireless communication. This technology, called an integrated RF/wireless (iRFW) coil, has previously been used for wireless respiratory monitoring with a respiratory belt or to wirelessly control shim currents, and is further developed here to wirelessly transmit ultrasound data acquired with an organ-configuration motion (OCM) sensor. OCM sensors are small, ultrasound based sensors that attach to the skin, move with the subject, and provide information about internal physiological motion. They can be used to create synthesized MR images through machine learning techniques and to monitor patient motion, and ultimately can be used to improve various treatments and therapies. However, they currently require electronics that must remain outside of the scanner as well as various wired connections to those electronics, which limits their portability. By making the OCM sensors wireless and their associated electronics MR-compatible, setup time is decreased and the OCM sensor can accompany the patient throughout the hospital while monitoring motion. This was done by developing MR-compatible, battery-powered electronics to trigger the ultrasound sensor, digitize the received ultrasound signal, modulate it to a Wi-Fi frequency, and wirelessly transmit it via the iRFW coil to a nearby access point (AP). Phantom experiments were performed to ensure that 1) the MR data quality was unaffected with and without wirelessly transmitting ultrasound data and that 2) the ultrasound data was unaffected with and without acquiring MR images. In vivo experiments were performed to demonstrate the portability of the wireless ultrasound device and its ability to monitor motion.

AbstractPurpose: Integration of wireless data transfer in magnetic resonance imaging (MRI) would allow for the reduction of wired connections between the scanner subsystems and the control computers located outside the scanner room, which add to the cost and complexity of the scanner, reduce patient comfort, and impede the application of next-generation MRI technologies.

Methods:In this dissertation, a novel RF coil design, termed the wireless integrated parallel reception, excitation, and shimming (iPRES-W) coil design, is further developed to remove some of these wired connections by offering a compact and easy-to-integrate MR-compatible wireless data transfer solution, which requires no scanner modifications or additional antenna systems in the bore. The iPRES-W coil design allows both a direct current (DC) and radio-frequency (RF) currents at the Larmor frequency and in a wireless communication band to flow on the same coil element to enable simultaneous MR image acquisition and wireless data transfer, which enables: wireless localized B0 shimming; wireless peripheral system data transfer to augment imaging (e.g., respiratory tracking using a respiratory belt); wireless transfer of the scanner control signal to control on-coil electronics (e.g., Q-spoiling); and wireless power harvesting to power components of the iPRES-W system. To demonstrate the performance and utility of the iPRES-W coil design in clinically relevant applications, this dissertation has four aims: 1. To develop an iPRES-W spine coil array to perform simultaneous MR imaging and wireless localized B0 shimming of the spinal cord; 2. To develop a dual-stream iPRES-W head coil array for simultaneous MR imaging of the brain and multiple wireless data streams for control of separate peripheral systems, specifically, wireless localized B0 shimming and respiratory tracking using a respiratory belt; 3. To further develop an integrated RF/wireless coil design for wireless transfer of the scanner trigger signal to perform the Q-spoiling required for MR image acquisition; 4. To design a wireless power harvesting system to convert the high-energy RF energy emitted by the scanner during the transmit cycle into a DC voltage to charge the batteries to power in-bore electronics and B0 shim currents.

Results:The results from this dissertation demonstrate that the iPRES-W coil modifications did not degrade the signal-to-noise ratio (SNR) when implemented onto different radio-frequency coil structures (e.g., a conventional RF coil element and novel adaptive imaging receive (AIRTM) coil element). Wireless localized B0 shimming with the iPRES-W spine array and dual-stream iPRES-W head coil array substantially reduced the B0 root-mean-square-error (RMSE) by 70.1% and -41.2% in the spinal cord and frontal brain region, which corresponded to reduced DTI and EPI distortions, respectively. The wireless performance of the iPRES-W and iRFW coil elements measured in an anechoic chamber were minimally impacted by the introduction of a saline phantom representing tissue or during wireless Q-spoiling, respectively. The power harvesting experiments performed showed that the 4-channel power harvesting coil array could convert RF energy from the scanner into a DC voltage for recharging MR-compatible batteries for various scan parameters and imaging sequences.

Conclusions:The iPRES-W and iRFW coil designs can be integrated into different coil structures and arrays to perform simultaneous imaging, wireless localized B0 shimming, and wireless transfer of peripheral device data (e.g., respiratory tracking with a respiratory belt), with no SNR degradation, minimal change in wireless performance, and without scanner modifications or additional antenna systems within the scanner bore. Additionally, the power harvesting array can supply wireless power to charge MR-compatible batteries for various scan types and parameters.

Magnetic resonance imaging (MRI) generates anatomical images by utilizing a homogeneous static magnetic field (B0) generated by a magnet and radiofrequency (RF) signals transmitted to and received from the subject by RF coils. To enhance the acquired signal strength and improve the image signal-to-noise ratio, receive RF coils are placed close to the surface of the subject and multiple RF coil elements are typically combined to form an RF coil array. The number of RF coil elements in an array has continually increased over the years, requiring large cables, connectors, and added electronic components to be connected to the MRI scanner for imaging, which increases the integration complexity, cost, and weight of the RF coil arrays. Additionally, RF coil arrays are typically heavy and rigid, which makes them difficult and time consuming to setup and uncomfortable for the subjects. Finally, additional shim coils are required to correct for B0 inhomogeneities induced by the subject and to improve the image quality, but they currently provide suboptimal results. This work presents a highly innovative RF coil design to address all of these concerns.

First, a novel integrated RF/wireless coil design was proposed to enable simultaneous MR image acquisition and wireless communication with a single coil, thereby reducing or eliminating the wired connections for data transfer between the coil and the MRI scanner. Second, the RF/wireless coil design was combined with the integrated parallel reception, excitation, and shimming (iPRES) coil design to enable simultaneous MR image acquisition, wireless communication, and localized B0 shimming with a single coil, thereby further improving the B0 homogeneity and image quality (iPRES-W coil). Finally, the iPRES-W coil design was integrated with: 1) the revolutionary AIR coil technology to perform the same three functions, but with a flexible and ultra-lightweight coil, thereby increasing patient comfort and offering more flexible coil design opportunities and 2) a wireless bidirectional DC power supply for B0 shimming to further eliminate any cables between the MRI scanner and RF coil (iPRES-W AIR coil).

Experiments were conducted to demonstrate that the modifications made to the RF coil, to enable wireless communication and B0 shimming, did not degrade its imaging performance. Additionally, experiments were conducted to test the wireless data connection, transmission rate, and quality of the wireless link for the RF/wireless and iPRES-W coil designs. Finally, experiments were conducted to demonstrate the ability of the iPRES-W coil to simultaneously perform localized B0 shimming during wireless data transmission and image acquisition. The results presented show no degradation in image quality with the modifications made, excellent B0 shimming performance, and the ability to wirelessly transmit data within the MRI scanner bore. The iPRES-W coil design requires no modifications to the current MRI scanner and leads to a highly scalable, cost effective, wireless solution for a more efficient, comfortable, and beneficial MRI experience.