Theses supervised by Prof. Dr. Ergin Atalar
25 theses · İhsan Doğramacı Bilkent University
7T'de serebellum manyetik rezonans görüntülemesi için optimize edilmiş RF güvenlik izleme
The primary objective of this thesis is to develop optimized RF safety assessment techniques for cerebellar imaging using 7T MRI systems, with a specific focus on Spinocerebellar Ataxias (SCAs). Due to the unavailability of detailed electromagnetic simulation data from the manufacturer, this study focused on predicting the electromagnetic behavior of the Nova 8Tx/32Rx coil'S 8 pTx channels to ensure accurate and safe imaging. Accurate prediction of the coil's electromagnetic performance is essential for both RF safety and imaging quality, particularly in managing RF exposure and minimizing tissue heating. The approach involved replicating the electromagnetic fields of the Nova coil through simulations, validating these predictions against experimental measurements, and implementing an algorithm for calculating the temperature-based Virtual Observation Points (tVOPs) in future works for rapid RF safety assessments. While the initial simulations captured key aspects of the coil's \(B_{1+}\) field distribution, discrepancies between predicted and experimental results revealed challenges, especially in random-phase shimming configurations. The limitations of using ideal current sources and the reduced dataset for optimization highlighted the need for more comprehensive data and realistic models. The findings underscore the importance of integrating empirical measurements with refined simulations to bridge the gap between theoretical models and real-world performance. Future work should focus on enhancing current source models, mitigating noise and experimental inaccuracies, and expanding the application of these techniques to broader clinical scenarios. By addressing these challenges, this research can contribute to improving both the safety and quality of high-field MRI, ultimately advancing its reliability for both clinical and research applications.
Gradyan dizi sistemlerine yönelik modüler spektrometre mimarileri
Gradient arrays are gaining popularity in the magnetic resonance imaging (MRI) community due to their superior performance and flexibility compared to conventional gradient coils. The technology can be used to excite multiple slices simultaneously, mitigate the B1+ inhomogeneity, reduce the power loss on the cryostat, and increase peripheral nerve stimulation (PNS) thresholds. However, despite their potential, fully standalone MRI systems based on gradient arrays are yet to be realized. With an increasing number of channels, scalable design methodologies become essential to resolve the hardware complexity. In this thesis, we present the development of modular spectrometer architectures for gradient array coils and parallel radio frequency (RF) transmit arrays to work towards a fully standalone gradient array magnetic resonance imaging system. Building upon previous work, the spectrometer submodules are implemented entirely on field programmable gate array (FPGA) hardware and consist of three main subsystems: a sequence parser, a gradient controller, and an RF controller. The sequence parser reads PulSeq-formatted conventional sequences and converts them into hardware-compatible memory structures. The gradient controller accesses these memory-mapped sequences and generates time-resolved gradient amplitudes, which are projected onto array channels and sent to an external driver chain. Lastly, the RF controller synthesizes a carrier clock and drives external RF power amplifiers using pulse-width-modulated (PWM) envelope signals. Unlike traditional RF synthesizers that rely on digital-to-analog converters (DACs) or mixers, the RF clock is generated entirely using FPGA primitives, simplifying the overall architecture and reducing scaling costs.
MRG'de bobin üzeri uygulama için kapı kiplemeli sayısal kontrollü değiştirilmiş sınıf-e güç yükselteci
The switch-mode RF power amplifiers, known for their high output power capability and good efficiency, have proved valuable for on-coil applications in MRI hardware. The class-D and class-E amplifier topologies have been demonstrated to be promising candidates to replace the conventional inefficient linear RF power amplifiers used in MR hardware which are placed away from the scanner room. Conventionally, the amplitude modulation of the output waveform in such switch-mode RF power amplifier applications is achieved either by implementing an amplitude modulation block at the drain of the amplifier, or by encoding the amplitude modulation information in the phase of the carrier signal at the gate of the amplifier. Both these approaches require additional hardware, thus increasing the cost and complexity of the system. Considering the aforementioned background, a novel technique of modulating both the amplitude and frequency of the output waveform, without the need for any additional hardware other than the driver circuitry for the amplifier itself, is presented and implemented for class-E amplifier topology. At 64 MHz (1.5 T), the analytical models of the amplifier for both the switch-on and switch-off cases are first derived and implemented in software. The period of the digital carrier signal at the gate of the amplifier is then divided into k bits, where k is greater than 2. It is then noted that both the amplitude and frequency of the output waveform can be controlled by altering this digital input in a certain manner. For a typical 2 ms 1.5 T MRI RF pulse, the digital carrier bitstream would consist of k×128000 bits. This would require testing 2^(k×128000) bitstream combinations to achieve the desired output waveform, requiring infeasible computational power. It is however shown that by intelligently programming the bitstream patterns for a selected number of periods, and by repeating those patterns for a chosen duration of time, the desired amplitude and frequency modulation of the output waveform can be achieved. The normalized root mean square error (NRMSE) for a 2 ms sinc pulse designed using such an approach is calculated to be 11%. The designed bitstreams are tested on hardware as well, both in bench-top and MRI experiments. Bench-top experiment results correlate well with the software predictions. The amplifier shows a peak drain efficiency of 89% at 50 W input power. The MR images obtained at 50 W input power using a 2 ms sinc pulse designed using the presented approach show no artifacts. The ultimate goal of the current research is to design a 32-channel transmit array coil for the MRI, capable of delivering a total of approximately 10 kW output power. Each amplifier element should therefore be able to deliver about 300 W output power. In this regard, further research needs to be conducted to achieve such output power level using the presented modulation approach. Nonetheless, the approach is general and can be implemented to other switch-mode RF power amplifier topologies as well. It promises to provide a performance equivalent to the other modulation approaches while reducing the overall cost and complexity of the system at the same time.
Z-gradyan dizisinin tasarımı ve manyetik rezonans görüntülemedeki uygulamaları
Array of gradient coils driven by independent power amplifiers can generate gradient fields with dynamically changing gradient field profiles. Nine channel z-gradient coil array together with the driver system are designed and manufactured. A first order circuit model including the mutual coupling is proposed and validated in bench-top and magnetic resonance imaging (MRI) experiments. Firstly, linear gradient in variable Volume of Interests (VOIs) with variable linearity errors are optimized for four different performance parameters such as maximization of gradient strength, maximization of slew rate, minimization of current norm and peak vector B-field. Smaller VOI and/or more linearity error improves performance parameters more than five times among the sweep ranges. This advantage is demonstrated in Diffusion Weighted Imaging (DWI). Maximization of gradient fields inside the slice rather than entire coil volume results in increased signal to noise ratio (SNR) and better estimate for the apparent diffusion coefficient (ADC). Secondly, two novel applications based on nonlinear gradients are proposed. Nonlinear gradients are used to encode multiple slice locations to the same frequency. Excitation of multiple slices is achieved with a single band radio frequency (RF) pulse without increase in specific absorption rate (SAR) and peak power. Moreover, a single channel nonlinear gradient field is simultaneously used with linear gradients during spoke excitation to mitigate the B1+ inhomogeneity. Simulations and MRI experiments validates that simultaneous use of linear and nonlinear gradients provides better B1+ homogeneity compared to using only linear gradients or only nonlinear gradients.
MR RF vurucu kanallarının interaktif gerçek zamanlı kontrolü
Cardiac catheterization is one of the heavily researched areas of the real-time interventional studies in Magnetic Resonance Imaging (MRI), where elongated conductive wires are prone to excessive radiofrequency (RF) heating and tracking of the devices might be challenging. Previous studies have proposed several techniques for heating reduction at the conductive wire tip and device visualization, using multiple transmit channels but the software platform for real-time RF control of multiple transmit channels has been missing for interventional procedures. In this study, we are presenting a software framework capable of interactive real-time RF control of MRI transmit channels and reception of dynamic images from MR image reconstruction computer. The software consists of three main programs running on three different operating systems (Linux, Windows and VxWorks) that communicate with each other over TCP/IP connection. Besides socket programming, multi-threading/multi-tasking is implemented for each platform along with the synchronization semaphores. The graphical user interface end is developed with Qt. Siemens' work-in-progress tip-tracking pulse sequence (BEAT_IRTTT) source code is modified to serve as the other end in our software system. The interactive real-time experiments are conducted on a copper sulfate phantom including a conductive wire. Dual-port body coil which is a product of Siemens is used as the transmit antenna and each port is driven independently. Results have shown the feasibility of the real-time RF control in the MRI, with an effective total update latency of two frames on the dynamic image-series. We believe this framework will contribute to real-time interventional procedures in terms of RF safety and catheter tracking.
Kuplajlı E-sınıfı RF güç yükselteçlerinin 1.5t MRG verici dizisi sistemi için optimizasyonu
Radio Frequency (RF) field is generated mostly by linear RF power amplifiers in Magnetic Resonance Imaging (MRI). These amplifiers have relatively low efficiency and are placed far from the transmit coil in MRI system room. Additional cooling systems and long transmission cables increase the cost of MRI hardware. Coupled Class-E on-coil RF amplifiers for a Transmit Array System is proposed instead of linear RF amplifiers. Class-E RF amplifier is a nonlinear switching amplifier which has 100% drain efficiency ideally. State-space models for single Class-E amplifier and coupled Class-E amplifiers are derived and steady-state operation of coupled Class-E amplifiers is investigated. The state-space models are verified with the simulation results of the Class-E amplifier. Effect of coupling between channels on the output characteristics of the transmit system is observed. Instead of implementing decoupling methods, coupling is maintained and optimization of circuit parameters for coupled Class-E amplifiers is proposed to achieve high drain efficiency and high output RF power. Output power of 600 W and overall drain efficiency higher than 90% are achieved by two coupled Class-E amplifiers even with high coupling levels in the state-space model. Power and efficiency measurements are done with coupled amplifiers and MRI experiments are performed in 1.5T Scimedix MRI Scanner. In conclusion, coupled Class-E RF power amplifiers can be operated as on-coil amplifiers in a Transmit Array system with high output characteristics by optimization of circuit parameters.
Eş zamanlı alıcı verici yapılı görüntüleme için dinamik ayrıştırma ve gürültü analizi
In simultaneous transmission and reception (STAR) MRI, along with the coupling of the excitation pulse to the received signal, noise, and undesired distortions (spurs) coming from the transmit chain also leak into the acquired signal and degrade image quality. The properties of this coupled noise and its relationship with the transmit amplifier gain, transmit chain noise density, isolation performance, and imaging bandwidth are analyzed. The importance of achieving high isolation and careful selection of the corresponding parameters are demonstrated. A cancellation algorithm, together with a vector modulator, is used for transmit-receive isolation. With higher isolation, coupled transmit noise can be reduced to the point that the dominant noise source becomes acquisition noise, as in the case for pulsed MRI. Amplifiers with different gain and noise properties are used in the experiments to verify the derived noise-transmit parameter relation. With the proposed technique, more than 80 dB isolation in the analog domain is achieved. The leakage noise and the spurs coupled from the transmit chain are reduced. It is shown that the transmit gain plays the most critical role in determining sufficient isolation, whereas the amplifier noise figure does not contribute as much. Additionally, the active cancellation technique mentioned above is adapted to Magnetic Particle Imaging (MPI) for active cancellation of the direct-feedthrough. A significant increase in detection sensitivity at the fundamental harmonic on an in-house Arbitrary Waveform Relaxometer (AWR) is demonstrated.
RF verici dizisi sargılarının yenilikçi tasarımları ve implante edilmiş DBS'li hastaların RF ısınma analizi
A safe and efficient magnetic resonance imaging (MRI) test would rely on informed specification, design, implementation, assessment, and application of appropriately selected radiofrequency (RF) coils. Towards these goals, this dissertation comprises three contributions to novel RF transmit array (TxArray) coil design techniques and two contributions to RF heating reduction of deep brain stimulation (DBS) implants in RF transmit coils. TxArray coils with multiple transmit elements provide the additional degrees of freedom that can be used to enhance field uniformity, accelerate acquisition time, enable RF shimming while intending to mitigate specific absorption rate (SAR) hotspots, and increase power efficiency. How a TxArray coil is designed can have a significant impact on its gain from parallel transmission technology. Thus, the first contribution of this dissertation is on the eigenmode analysis of the scattering matrix for the design of TxArray coils to obtain their efficient operation modes and achieve an efficient RF shimming in terms of power consumption. The algorithm is tested for the design of four 3T TxArray coils with 8 to 32 channels, and it is shown that it can enlarge the dimension of the excitation space by up to 50% compared with the commonly used design techniques. The next contribution is to establish a fast finetuning procedure to precisely design an imperfectly manufactured TxArray coil using its corresponding equivalent circuit model. By fitting the measured scattering parameters to a lumped circuit model, all inductances/resistances of an 8-channel 3T TxArray coil are estimated. The manufactured coil is then appropriately tuned only in a single iteration. As another contribution, a theoretical coil size optimization is introduced to minimize the magnetic coupling between non-adjacent transmit channels of a TxArray coil. By calculating all self/mutual-inductances of a 12-channel 3T TxArray coil and minimizing mutual-inductances, its sizes are determined. The finite element simulations are performed to demonstrate the feasibility of this approach. One of the safety considerations of RF transmit coils is the localized SAR amplification due to the interaction of metallic implants with the coil's electric fields. Therefore, the fourth contribution is on evaluating SAR mitigation performance at tips of patient-derived realistic DBS implants using a 3T patient-adjustable transmit coil, which uses a mechanically rotating linearly polarized birdcage resonator. The reconfigurable coil system decreases the SAR on average by 83% for unilateral leads and by 59% for bilateral leads in comparison to a conventional quadrature birdcage coil. In the final study, the local SAR amplification surrounding the tips of a large cohort of DBS implants with realistic lead trajectories in a commercially available vertical open-bore 1.2T coil and a standard horizontal closed-bore 1.5T birdcage coil is presented. On average, SAR is decreased by 31-fold in the 1.2T vertical coil compared to the 1.5T horizontal coil. Overall, this dissertation proposes innovative approaches for designing TxArray coils and heating assessment and SAR reduction of DBS patients, which mainly contribute to improving the performance of RF transmit coils in terms of power efficiency and patient safety. Keywords: Magnetic resonance imaging (MRI), Radiofrequency (RF) coil, Transmit array coil, Eigenmode analysis, Equivalent circuit model, Coupling, MRI safety, Deep brain stimulation (DBS), Specific absorption rate (SAR)
3T MRG için dinamik faz kontrolü özellikli bobin-üzeri 600W E-sınıfı güç yükselteci dizisi
Due to their size and cooling constraints, conventional Magnetic Resonance Imaging (MRI) places radio frequency (RF) amplifiers away from the scanner. These RF amplifiers have relatively low efficiency due to the matching of 50Ω output impedance for means of transmission with cables. Switching Class-E amplifiers on the other hand, by default need a bare RLC network as their load and thus can be directly integrated with the bare unmatched coils and reduce the cost and power losses significantly. This thesis aims to build up on the previous theses' line of work. Instead of mitigating the symptoms, chronic problems of artifacts have been fixed by focusing on their root causes in the FPGA side of the updated design. The driver has been updated, timing problems have been resolved. FPGA design is also extended to support multi-channel phase control. A dual channel imaging configuration of on-coil Class-E amplifiers with on-the-fly digital fine phase control is presented for 3T MRI. The system can control the phase with less than 2$^\circ$ granularity (this setting can be fine-tuned down to 0.15$^\circ$). Without any mechanical intervention with the coil setup, using merely phase control, illuminated slice depth is modulated to three times its base size during scan time. B1 field maps are also extracted for another setup. %Decoupling is achieved using overlap decoupling. Periodically linear switching (PLS) circuit model of the Class-E amplifier is derived and computed, yielding a simulator with fast and customizable optimization capability. The PLS model is also verified by SPICE and theoretical analysis.
Manyetik rezonans elastografi'deki makaslama dalgalarının modları
Manual palpation is used for diagnosing change in stiffness of tissues, due to a pathological state. Unfortunately, this diagnosis tool is limited with organs close to the surface of the body. Magnetic resonance elastography (MRE), also known as palpation by magnetic resonance imaging (MRI), can be used in detecting changes in material properties of the heart, liver, muscle, breast and brain. Alteration in stiffness of tissues can be detected by MRE, by simply measuring the wavelength of the induced shear wave by the actuator, from the phase difference images obtained by MR scanner. In addition to wavelength information, dependence of shear wave displacement amplitude to the frequency and excitation direction carry important information about material properties of the tissue. Modes of shear waves in MRE have not been studied previously. Change in material properties of the tissue, may affect modes of shear waves in MRE. Hence, a shift in natural frequencies may indicate a pathological state in the tissue. We propose a novel method to detect change in stiffness of tissues, by analyzing modes of shear waves and detecting frequency shift in peak displacement of shear waves in MRE. Eigenfrequency simulations are computed for a simple geometric object whose eigenfrequencies are known analytically. Validating simulation results with theoretical values, we are encouraged to continue with eigenfrequency analysis of the brain model. For different directions of motions of head, it is demonstrated by eigenfrequency analysis that brain has modes at certain frequencies. Results of frequency domain analysis indicates that modes of shear waves can be observed in brain by exciting head at its eigenfrequencies with correct excitation in that frequency. Results of frequency domain analysis repeated for neurodegenerative brain model are compared with the findings in healthy brain model. Comparing frequencies of peak displacements in neurodegenerative and healthy model, a constant frequency shift is observed in all frequencies of peak displacements. Preliminary results of modes of shear waves in brain MRE are presented, by sweeping mechanical excitation frequency. This method can be used in detecting change in stiffness of tissues for diagnosing diseases by observing shift in frequency of peak displacement and be beneficial for patient follow-up.
Mrg tarama sırasında vücudun içindeki ince iletkenler üzerinde oluşan akımların analizi
The aim of this thesis is to develop a method to analyze currents on thin conductor structures inside the body during Magnetic Resonance Imaging (MRI) scan based on Modified Transmission Line Method (MoTLiM). In this thesis, first, Active Implantable Medical Devices (AIMDs) are modeled and the tissue heating problem, which is a result of coupling between AIMD and incident Radio Frequency (RF) fields, is examined. Then, usage of MoTLiM to analyze the currents on the guidewires is shown by solving currents on guidewire when a toroidal transmit receive coil is used with guidewire. At first, a method to measure MoTLiM parameters of leads using a network analyzer is shown. Then, IPG case and electrode are modeled with a voltage source and impedance. Values of these parameters are found using the Modified Transmission Line Method (MoTLiM) and the Methods of Moments (MoM) simulations. Once the parameter values of an electrode/IPG case model are determined, they can be connected to any lead, and tip heating can be analyzed. To validate these models, both MoM simulations and MR experiments are used. The induced currents on the leads with the IPG case or electrode connections are solved using the proposed models and MoTLiM. These results are compared with the MoM simulations. In addition, an electrode is connected to a lead via an inductor. The dissipated power on the electrode is calculated using MoTLiM by changing the inductance and the results are compared with the specific absorption rate results that are obtained using MoM. Then, MRI experiments are conducted to test the IPG case and the electrode models. To test the IPG case, a bare lead is connected to the case and placed inside a uniform phantom. During a MRI scan the temperature rise at the lead is measured by changing the lead length. The power at the lead tip for the same scenario is also calculated using the IPG case model and MoTLiM. Then an electrode is connected to a lead via an inductor and placed inside a uniform phantom. During a MRI scan the temperature rise at the electrode is measured by changing the inductance and compared with the dissipated power on the electrode resistance. Second, based on the similarity between currents on guidewires and transmission lines, currents on the catheter are solved with MoTLiM. Current distributions on an insulated guidewire are solved and $B_1$ distribution along the catheter is calculated. Effect of stripping the tip on the tip visibility is analyzed. It is shown that there is an increase in the $B_1$ at the insulation and bare guidewire boundary. Then, a characteristic impedance is defined for the guidewires and impedance seen at the point where guidewire is inserted into the body is calculated. It is shown with EM simulations that if the impedance converges to the characteristic impedance of the guidewire, tip visibility of the guidewire is lost. At last, a new method to measure electrical properties of a phantom material is proposed. This method is used for validation of the coaxial transmission line measurement (CTLM) fixture, which is designed for measurement of electrical properties of viscous phantom materials at MRI frequencies, and which is previously presented by our group. The new method depends on the phenomena of the lead tip heating inside a phantom during MRI scan. Electrical properties of a phantom are influential on the relationship between tip temperature increase and the lead length. MoTLiM is used and the relationship between the lead length and the tip temperature increase is formulated as a function of conductivity and permittivity of the phantom. By changing the lead length, the tip temperature increase is measured and the MoTLiM formulation is fitted to these data to find the electrical properties of the phantom. Afterwards the electrical properties of the phantom are measured with the CTLM fixture and the results that are obtained with both methods are compared for an error analysis. To sum, electrical models for the IPG case and electrode are suggested, and the method is proposed to determine the parameter values. The effect of the IPG case and electrode on tip heating can be predicted using the proposed theory. An analytical analysis of guidewire with toroidal transceiver is shown. This analysis is helpful for better usage and improvements of toroidal transceiver. Also, MoTLiM analysis can be extended to other MRI guidewire antennas.
Manyetik rezonans görüntülemesi için Z-gradyan dizisi
In this thesis, a z-gradient array system for use in MRI is proposed. In MRI scanners, in order to encode the MR signal gradient coils are used. Conventionally, encoding in the z-direction (along the bore of the magnet) carried out using a single gradient coil. Proposed z-gradient coil array is a flexible and novel design to achieve high gradient strength while observing the peripheral nerve stimulation (PNS) limits. The design has a potential to decrease both the complexity and the cost of the gradient power amplifiers (GPA). In order to prove the concept, a three-channel system was designed and built. By using different combination of the generated fields of these channels, effective length of the coil can be modified according to the application. For the same PNS limitations, gradient strength can be increased in a smaller region, hence decreasing the dead-time in most sequences and acquiring images with smaller pixel sizes. In order to proof of concept, this system is designed and implemented in the MRI scanner. In our preliminary experiment, each channel of the system is composed of two coils with 16 turns each on a cylindrical former with the diameter of 7.5 cm, placed symmetrically about the center with equal current in opposite direction at each side. The overall length of the structure is 245 mm and the current ratio and the distance between the coils optimized such that three sizes of volume of interest (VOI) with more than 95% linearity is achievable. In order to drive each channel independently, three H-bridge GPAs with 30 A current capability at 50 V providing 0.5 A/µs slew-rate for the load with 46 µH inductance, 30µH maximum coupling and 390 mΩ resistance is built which are controlled digitally by an FPGA (Xilinx XUPV5) board. Although in this prototype the distance and the current ratio between the coils are the only parameters that were optimized, other parameters of the system can also be optimized according to the different criteria as well. In addition to the coil simulations, phantom experiments conducted to show the feasibility of this system. 2D coronal slices were acquired for three sizes of VOI; large, mid-size and small volume and different advantages of each mode verified experimentally. The gradient array is a promising design for the future of the gradient systems in magnetic resonance imaging. In addition to linear fields, gradient array system is also flexible in terms of producing non-linear fields by proper combination of the fields generated by each channel. This is noticeable when higher number of independent channels are used. Gradient array system provides flexible and effective field generation along with less complex amplifier design.
Sıfır eko-zamanı görüntüleme için tam çift yönlü MRG kullanımı
In this thesis a new method for decoupling of RF transmit and receive coils in MRI is presented. A modified version of isolation concept used in the full-duplex radios in communication systems is applied to acquire MRI signal using concurrent excitation and acquisition (CEA) method. Since in MRI transmit power is many orders of magnitude larger than receive signal, a weak coupling might dominate the MR signal during CEA in MRI. In our new method, a small copy of RF transmit signal is attenuated and delayed to generate the same coupling signal which is available in the receiver coil then it is subtracted from the receive signal in order to detect the MRI signal. The proposed decoupling method is developed and implemented in two designs. First a semi-automatic controllable decoupling design which uses a programmable attenuator and coaxial cables for the purpose of time delay. After estimating the length of coaxial cables an optimization algorithm finds the amount of attenuation factor. Using this method we could achieve more than 75 dB decoupling. Second design is a fully-automatic controllable decoupling design which contains four delay and attenuator lines. In this design four fixed phase shifters are used in order to generate the same phase delay between transmit and receive coils. A genetic optimization algorithm is used to find the amount of attenuation factors of each line. It is shown that this method provides more than 100 dB decoupling between transmit and receive coils which is good enough for detecting MRI signals during excitation from tissues with very short relaxation time. This study shows feasibility of applying full duplex electronics which is used in telecommunications, to decouple transmit and receive coils for MRI with CEA, using a clinical MRI system. This device can automatically tune the cancellation circuit and it is a potential tool for recovering signal from tissues with extremely short T2 in clinical MR systems.
Manyetik resonans görüntüleme için eşdeğer devre modeli kullanılarak kuşkafesi benzeri radyo frekans iletim dizisi sargısı tasarımı
One of the conditions to have a good magnetic resonance (MR) image is applying a homogeneous radio-frequency (RF) excitation (magnetic field) with efficiently high intensity to the region of interest. However, there are some limitations such as specific absorption rate (SAR) which is not allowed to exceed some standard levels. Since SAR level directly depends on the electric field and the electric field is coupled to the magnetic field, there is a trade-off between high-intensity RF-excitation and low SAR level. Moreover, in conventional RF coils (birdcage) for the MRI, the magnetic field profile is almost constant so that its intensity is pretty high at the center of the coil and decreases toward the coil. In such a coil, it is not possible to aim an off-center small region of interest and make the homogeneity concentrated at that region. Transmit array (Tx-array) coils provide high controllability on both electric and magnetic field, so, they would be good solutions for all of these issues, although, they come across the efficiency problem at the center when the same performance of a conventional RF coil is required. This problem has been already handled using a birdcage-like Tx-array coil, however, there are some difficulties to design and tune such a coil. In this thesis, we proposed a novel design method for birdcage-like Tx-array coil; an eight-channel birdcage-like Tx-array coil is designed using the equivalent lumped-element circuit model. This design profits controllability feature of an array and high transmit efficiency of a birdcage coil at the center, simultaneously. A capacitive decoupling method is utilized in order to get rid of reactive interactions between channels of the array. Then, an optimization (the steepest-descent method) with constraints based on minimizing the electric field and smoothing the magnetic field is applied to the voltage-excitations of the Tx-array coil. The proposed decoupling method provides 15dB matching for each channel and higher than 12dB decoupling between adjacent channels and at least 19dB for nonadjacent channels. This Tx-array coil provides only 3% less efficiency versus the birdcage coil at the center of the coil, while, at the regions close to the surface of the phantom we achieved more than 72% better efficiency in comparison to the birdcage coil. Furthermore, we demonstrated that the Tx-array is capable to produce a homogeneous magnetic field at an arbitrary (off-center) region of interest. This adjustment can be performed for the electric field as well such that the electric field and so the SAR can be minimized locally. Consequently, the proposed configuration of the Tx-array coil provides an efficient excitation while capability of local RF shimming and local electric-field-reduction can be achieved.
MRG cihazları için tasarlanmış sayısal kontrollü güç yükselteci
Radio-frequency (RF), or B1 field is used for slice selection purposes in Magnetic Resonance Imaging (MRI). In an MRI scanner this field is handled by the RF chain which consists on a pulse generator unit, a high power amplifier and a transmit coil. Relatively low efficiency linear power amplifiers are used. These amplifiers are placed in the system room, far from the transmit coil. In this work we propose to design the amplifier and the coil as a single unit, aiming to decrease cost and complexity while improving performance. Splitting the coil into multiple elements makes possible to drive these elements with individual amplifiers in Transmit Array (TxArray) mode. Also these elements are integrated as the load network of the amplifier, in this case a high efficiency Class-E amplifier. The Class-E amplifier was modified for the intended application and it was digitally controlled. For the pulse generation two different methods were applied. One was by controlling separately the phase and amplitude of the pulse. The other method generates simultaneously phase and amplitude by controlling the switching pattern of the amplifier. An amplifier of output power 100W with efficiency up to 88% was developed. As a step toward 36 channel complete system, 2 element prototype's operation was tested in a 3T Tim Trio Siemens scanner. In overall, Class-E amplifier is shown to be a promising candidate for on-coil RF excitation in TxArray in terms of size, cost, efficiency and complexity.
64 MHz verici dizisi sistemi için yüksek verimli 300 W değiştirilmiş class-E RF yükselteci
The conventional MRI system uses high power linear RF amplifiers which are placed away from scanner room, have low efficiency, high cost and need a cooling system. We aim to use parallel transmit system with on-coil amplifiers that has shown to be advantageous in improving B1 field homogeneity, slice selectivity and SAR reduction. In this work the on-coil class-E RF switching amplifier is suggested for MRI to decrease the cost and complexity of the current system while improving performance. The amplifier is digitally controlled and for pulse generation purpose supply modulation is used. The transmit coil acts as the load network of the amplifier so the need for matching circuit is eliminated. The amplifier has an output power of 300 W with maximum efficiency of 92%. The efficiency does not drop below 75% in 1 MHz bandwidth. The performance of amplifier at high temperature is also evaluated and it is established that at low duty cycles due to high efficiency no cooling system is required but for high duty cycle applications a cooling system might be needed for the uninterrupted operation. The ultimate goal of this research is to design a 32-channel transmit array using on-coil amplifiers, as a step forward towards this goal a prototype for two channels is designed. Our results depict that the behavior of the class-E amplifiers under coupled operations is acceptable. The dual-channel prototype was tested on Scimedix 1.5 T and no artifacts were observed in the images due to the presence of amplifiers near transmit coils inside the bore. To sum up, the class-E amplifier is proved to be a favorable candidate for on-coil applications in RF excitation due to its small size, reduced complexity and high efficiency.
Manyetik rezonans görüntülemede metalik implantların güvenliği
Magnetic Resonance Imaging (MRI) is safe only if we take safety precautions. Inthe presence of a metallic implant inside the body, three types of magnetic ï¬eldsencountered in MRI (Static magnetic ï¬eld, radiofrequency ï¬eld, gradient ï¬eld)may become the sources of safety problems. In this thesis, temperature increasecreated by a pacemaker under MRI is investigated. Electromagnetic simulationsare performed, in-vivo, phantom experiments are conducted and ï¬nally bioheatequation is solved to ï¬nd the corresponding temperature increase. Using thistemperature increase the input power can be limited to ensure safe scans. MRIcompatible lead design is the essential innovation of this thesis which is directlyapplicable to any kind of metallic, wire shaped interventional MRI device.Keywords: Magnetic Resonance Imaging, metallic implant, pacemaker, bioheattransfer, interventional MRI.iii
Manyetik sıvı kullanarak yapılan odaklanmış radyo dalga ablasyonu
In most developed countries, cancer is presently responsible for about 25% of alldeaths. Heat therapies like hyperthermia and thermoablation are very promisingapproaches in the treatment of the cancer. Since these are physical treatmentmethods they have fewer side affects compared to chemo- and radio-therapy.Currently, various types of heat treatment modalities are available like microwave,ultrasound, RF capacitance hyperthermia, RF probe hyperthermia, magnetic fluidhyperthermia, but non of these methods have the ability to accurately deliver highheat energy to deeply seated tumors without damaging the healthy surroundingtissues.In this thesis, a novel RF ablation system was developed capable of focusing theheat in to very small areas in the order of millimeters, which will allow heating ofthe tumors without destroying collateral normal tissues. Generally, in this systemthe tumor ablation is achieved via coupling RF energy on the magnetic fluids whichare previously dispersed in to the tumor tissue.By considering the human safety limits (nerve stimulation and tissue eddy currentheating safeties) optimum treatment parameters like particle size of the magneticfluids, frequency and strength of the applied RF field are obtained. The utilizationof the optimum parameters may lead to the very effective operation of the ablationsystem where treatments can be done with very small amounts of fluid injections, inshort durations.We believe that by the studies conducted in this thesis, magnetic fluid hyperthermia(tumor ablations using magnetic fluids) can be a much more effective method sothat it can be used as the one of the most important tumor treatment techniques infuture.Key words: Tumor Therapy, Hyperthermia, Thermo ablation, Magnetic FluidHyperthermia, Magnetic Fluids, Focused Ablation.
Vücuda takılabilen tıbbi elektronik üreteçlerin MR görüntülenmesinin güvenlik indeksi kullanılarak radyo frekans modellendirilmesi
Magnetic Resonance Imaging (MRI) is known as a safe imagingmodality that can be hazardous for patients with active implantable medicaldevices, such as a pacemakers or deep brain stimulators. The primary reason forthat is the radio frequency (RF) heating at the tips of the implant leads. In thepast, this problem has been analyzed with phantom, animal and humanexperiments. The amount of temperature rise at the lead tip of these implants,however, has not been theoretically analyzed. In this thesis, a simpleapproximate formula for the safety index of implants, which is the temperatureincrease at the implant lead tip per unit deposited power in the tissue without theimplant in place, was derived.For that purpose, an analytical quadrature birdcage coil model wasdeveloped and the longitudinal incident electric field distribution inside the bodywas formularized as follows:E z ( R) = â Ï Âµ H â Rin which Ï is the angular frequency, µ is the magnetic permeability of the tissue,H- is the left hand rotating component of the RF magnetic field and R is theradial distance from the center of the body. This formula was examined bysimulations and phantom experiments. The analytical, simulation andexperimental results of that model are in good agreement.iiiThen, depending on the quadrature birdcage coil model safety index (SI)formula for active implants with short leads was derived as shown below:âTmax 1 2Rl + Ae jθSI = = f ( Dv)SARpeak 2α ct Rb2where âTmax is the maximum temperature increase in the tissue, SARpeak is themaximum deposited power in the body when there is no implant in the body, αis the diffusivity of the tissue, ct is the heat capacity of the tissue, Rb is radius ofthe body, R is the radial distance from the center of the body, l is the length ofthe implant lead, A is the area of the curvature of the lead, θ is the angle thatcurvature of the implant makes with the radial axis, and f(Dv) is the perfusioncorrection factor, which is function of the diameter of the electrode andperfusion. The safety index formula was tested by simulations. Simulationresults showed that the theoretical safety index formula approximates andidentifies the RF heating problem of active implants with short leads accurately.The safety index formula derived in this thesis is valid for only shortwires. However, the formulation for long wires is currently under investigation.Despite the fact that the results obtained for short leads can not be generalizedfor the safety of patients with active implants, it is believed that this study is thefirst step towards safety of these patients. Using safety index as a measure ofsafety is very beneficial to ensure the safety of patients with active implants.Because, it uses the MR scanner-estimated deposited power that does not takethe existence of the implant in the patient body into account. This formulation isthe first study illustrating the advantage of the safety index metric for RFheating studies of active implants.Keywords: MRI, RF heating, Active Implants, RF Safety, Safety Index,Quadrature Birdcage Coiliv
MR görüntüleme sırasında gradyan sargılar içinde oluşan elektromanyetik alanın analizi ve hastada görülebilecek kalp ve sinir uyarımının incelenmesi
During magnetic resonance imaging (MRI), the static magnetic field, the radio frequency field and the gradient fields are utilized. In the literature, there are different studies in order to understand the bioeffects of these fields. Due to the time varying gradient fields, the electric field is induced in the body and that may cause the nerve and cardiac stimulation. In order to investigate this risk, first researchers have been worked on solving the induced electric field analytically and by using computational methods and the field distribution inside the body has been investigated. It is also vital in order to verify the risk of MRI to the patients with an implant. In order to improve the understandability of the field pattern, the field expression should be as simple as possible.In this thesis, the simplified expressions of induced electric field are derived inside and outside the cylindrical, homogenous volume taken as a human body model. For this derivation, low frequency based assumptions are used and gradient field is assumed to be perfectly uniform. The result satisfies the expected conditions for the electric and magnetic fields. The field patterns obtained with these simplified expressions are compared with the former studies and the maximum electric field values obtained for a different gradient field and slew rate values are used to investigate the stimulation risk. Moreover, by using these electric field values the worst position for an implant lead and the length of the lead is determined.We believe that with these simplified expressions, the understandability of the field distribution is increased and to comment on the risk of the MRI to the patient with implant becomes easier.
Manyetik resonans görüntülemede değişken hızlı seçici uyarım için iyileştirilmiş radyo frekans darbe tasarlama yöntemi
A novel radio frequency (RF) pulse design method for magnetic resonance imaging (MRI) and an improvement to an existing method that reduces specific absorption rate (SAR) in MRI are presented. The new RF pulse design method, variable rate selective excitation optimal RF pulse design method for parallel transmission (VERSEp), is developed for parallel transmission and aim of the method is to design RF pulses with lowest SAR after SAR reduction with variable rate selective excitation (VERSE) method. This is achieved by modifying the SAR optimal RF pulse deisgn method for parallel transmission. Performance of the VERSEp method is tested by comparing VERSE-SAR reduced SAR of the RF pulses designed with SAR optimal RF pulse design method and VERSE-SAR reduced SAR of theRF pulses designed using VERSEp. In the simulations, SAR reductions up to 47% are obtained. Different aspects of VERSEp are also shown with simulations. The second contribution of this work is an improvement made to an existing constrained VERSE-SAR reduction method. The existing VERSE-SAR reduction method uses a peak RF constaint for SAR reduction. In this work, peak square root power constraint is used instead of peak RF constraint in the VERSE-SAR reduction method. In the simulation results, the SAR of the RF pulses designed using the improved method were compared with SAR of the RF pulses designed using the method before improvement. SAR reductions up to 50% are obtained by using peak square root power constrained SAR reduction instead of peak RF constrained SAR reduction.
Modifiye iletim hattı metodu kullanılarak radyo frekans dalgaları kaynaklı akımların modellenmesi
Magnetic resonance imaging (MRI) is widely used diagnosis technique. DuringMRI radio frequency (RF) fields are utilized to excite the spins. If these RF fieldsincidence on metallic implants, currents will be induce on the metallic parts ofimplants. Inside the body these induced currents on metallic implants causeheating of tissue and sometimes cause severe burning of tissue. This phenomena makes MRI hazardous for patients with metallic implants. Much work has been done to understand this phenomena. However, most of these work based on purely experimental or numerical methods. So to understand and to obtain a good intuition on this problem a lot of cases must be solved computationally or tested experimentally.In this study lumped element model of the transmission line is modified in order to model the conductive wires of implants inside the body. This model is based on the similarity between the damped oscillatory behavior of transmission line currents and induced currents on wires inside the body. A voltage source is added to model the effect of the incident electric field. Voltages and currents on a infinitesimally small portion of wire are solved. Solving currents and voltages simultaneously on the modified lumped element model lead to a non- homogeneous differential equation for the current. The solution of this differential equation gives the analytical solution for the induced current on the implant lead. To test the validity of this solution, wire under the uniform incident electric field is solved with the Modified Transmission Line Method (MoTLiM) and compared to Methods of Moment (MoM) solution. The results are also verified using phantom experiments. For experimental verification, the distorted flip angle distribution due to induced currents are measured using flip angle imaging techniques. In addition to this, the flip angle distribution around the wire is calculated using results obtained from MoTLiM. Finally these results are compared and an error analysis is carried out
MRG prosedürleri için vücut boşluklarına girebilen sargılar
In this study we designed endoluminal magnetic resonance imaging (MRI) coils to be used for interventional procedures under the guidance of MRI. The first coil we developed is a two-channel endocervical coil for the treatment of cervical cancer. The coil was embedded into the brachytherapy applicator without interfering with its functions. It provides magnetic resonance (MR) images of the cervix with high signal-to-noise ratio (SNR) that is required for a more accurate radiation dose calculation in the treatment of cervical cancer with high dose rate brachytherapy (HDRB). The performance of this coil was tested with phantom experiments and the results proved that the design worked properly.Second, we developed an MRI guidewire and an MR EP catheter for the treatment of atrial fibrillation (AF). The MRI guidewire had similar mechanical properties with the common cardiovascular guidewires and it was proved successful in obtaining high SNR images of the heart. The MR EP catheter could also provide high SNR images as well as clean intracardiac electrocardiogram (IECG) signal during the MR scan. Due to the loopless antenna embedded inside both of these catheters, they could be navigated in the body under the MRI. They may be used to guide complex interventional procedures such as RF ablation. The performance of these catheters was tested and confirmed with in vitro experiments.To sum up, these two technologies can play a significant role in the treatment of cervical cancer and AF as well as contributing to the development of interventional MRI.
Görüntü rehberli girişimler için ters kutuplaşma temelli manyetik rezonans teknolojileri
In this PhD dissertation, we presented four magnetic resonance (MR) technologies establishedupon reverse polarization for image guided interventions. The first three studies are based ontracking interventional devices, such as catheters, biopsy needles, and guidewires. Theinterventional devices cannot be seen using MRI without markers, coils, or extra devices. Ourstudies utilize different imaging modalities in order to obtain positional information of theinterventional devices. The last study is a novel inductively coupled radio frequency birdcagecoil design, which is a miniaturized version of a widely used volume coil. The new design canbe used for prostate biopsy or imaging intestines.The reverse polarization is a mode of magnetic field that is not sensitive to anatomy signal.Therefore, it had been useless until the introduction of the reverse polarization concept. Usinga linearly polarized inductively coupled radio frequency (ICRF) coil enables the reversepolarization mode, because a linearly polarized signal consists of both forward and reversepolarization signals. As a result, building the ICRF coil to interventional devices paves the wayof using this method in interventional MRI.Performances of developed technologies were tested in phantom, animal, and volunteerstudies. We believe that the studies explained in this dissertation contribute to obtaining betterimaging systems.Key words: Magnetic resonance imaging, interventional MRI; catheter tracking; fiducialmarker, inductively coupled RF (ICRF) coil, transmit array system, birdcage coil