Browsing by Subject "Magnetic resonance imaging."

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Open Access
Biofunctionalization of superparamagnetic iron oxide nanoparticles
(2011) Sülek, Selim
Magnetic resonance imaging (MRI) has attracted intensive interest due to its non-invasive monitoring capacity. Gadolinium based contrast agents, most widely used CA, suffer from high level of toxicity and high threshold of detection. Superparamagnetic iron oxide nanoparticles (SPION) based contrast agents (CA) are good alternatives for gadolinium based CAs, since they have extraordinary magnetic properties within nanometer size and relatively low toxicity. Surface active group of SPIONs are mostly responsible for these advantages. In this thesis, we studied biofunctionalization of iron oxide magnetic nanoparticles with variety of peptide molecules for the solubilization and biofunctionalization of SPIONs. Particle synthesis was carried out via two methods: co-precipitation and thermal decomposition and they were compared by means of size and stability. Several characterization methods, such as Fourier Transform Infrared Spectroscopy (FT-IR), Circular Dichroism (CD), Rheology, X-ray diffraction (XRD) X-ray photon spectroscopy (XPS), vibrating sample magnetometer (VSM), Magnetic resonance imaging (MRI), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) were used in order to fully characterized the SPIONs prepared.methods were used in order to fully characterize the SPIONs. Thermal decomposition is the best method to control the particle size and avoid aggregation problems. Peptide amphiphile molecules are used to non-covalently functionalize SPIONs synthesized by thermal decomposition method to provide water solubility and biocompatibility. Particles are found to be around 35 nm with r2 values of 100.4 and 93.7 s-1mM-1 which are comparable with commercially available SPIONs. In vitro cell culture experiments revealed that peptide-SPION complexes are biocompatible and are localized around the cells due to their peptide coating. Finally, SPIONs were evaluated in terms of their potential use as MRI contrast agent.
Open Access
Design and implementation of internal MRI coils using ultimate intrinsic
(2006) Eryaman, Yiğitcan
In this thesis, methods for optimization of internal magnetic resonance imaging coils have been developed. As a sample optimization endorectal coils were optimized. As the first step ultimate intrinsic signal-to-noise ratio for internal coils was formulated. This value was used as a measure of performance of the internal MRI coils. The related optimization problem was divided into two sub problems: length optimization and cross-sectional optimization. For the length optimization length of a single loop rectangular coil was optimized in order to obtain maximum intrinsic signal to noise ratio at the prostate region. For the cross-sectional optimization designs with different cross-sectional geometries were implemented and tested in saline solution phantom. The designs that gave a significant improvement over the conventional loop coil were favored and the related design ideas were used for the design of an optimized coil. Finally optimized coil was tested in a saline solution phantom model and its performance was calculated.
Open Access
Endoluminal coils for interventional MRI procedures
(2010) Viskuşenko, V. Nikolay
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.
Open Access
Imitation of radiofrequency ablation with fiber delivered laser system for magnetic resonance guided treatment of atrial fibrillation
(2010) Kerse, M. Can
Atrial Fibrillation (AF) is among the most common cardiac arrhythmias with a high risk of mortality and morbidity. As a cure several minimally invasive catheter approaches are performed under imaging guidance. These treatments imitate linear and transmural cuts and sutures along the atrial walls similar to the widely accepted surgical Cox Maze procedure to block undesired currents. Catheter delivery of RF energy to the cardiac chamber is widely used and approved as safe and successful. The operation is commonly performed under X-Ray which is deprived of soft tissue contrast. Besides, combination of the image with ECG (electrocardiogram) data makes the operation technically difficult and time consuming. Due to the long exposure times, X-Ray burns may be seen on the patient. MR images can be taken during RF ablation with proper matching and tuning circuits, however, during the operation RF and ECG catheters may cause artifacts in the image for some orientations. On the other hand, fiber delivery of laser energy has no significant MR compatibility issues and can be used under MR guidance. Nevertheless, MR guided laser ablation is not in clinical practice as a minimally invasive technique for curing AF possibly because of the risk of perforating the myocardial wall. Excess light intensity at the end of the fiber tip causes rapid changes in the temperature gradients which may cause charring. This is an undesired effect and especially in cardiac ablations, light intensity should be diffused. There are several diffusing tip designs to emit light in cylindrical symmetry, but, due to their orientation with respect to the cardiac chamber, common RF delivery methods cannot be applied directly. In this thesis, we propose a novel multiple fiber laser energy delivery with catheter approach and a system that imitates the scars created with RF probes under MR guidance. The system closely imitates the ablation pattern of RF delivery and therefore is expected to have quick adaptation by physicians. As a proof of principle, we used 3 fibers oriented in different directions and obtained real time MR thermometry maps of the ex-vivo and in-vitro ablation zones during laser delivery. In addition, various light diffusion methods are considered for single fiber power delivery. We believe the combination of these methods will be the solution for the MR compatible RF laser ablation system.
Open Access
Magnetic resonance technologies based on reverse polarization for image-guided interventions
(2010) Çelik, Haydar
In this PhD dissertation, we presented four magnetic resonance (MR) technologies established upon reverse polarization for image guided interventions. The first three studies are based on tracking interventional devices, such as catheters, biopsy needles, and guidewires. The interventional devices cannot be seen using MRI without markers, coils, or extra devices. Our studies utilize different imaging modalities in order to obtain positional information of the interventional devices. The last study is a novel inductively coupled radio frequency birdcage coil design, which is a miniaturized version of a widely used volume coil. The new design can be 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. Using a linearly polarized inductively coupled radio frequency (ICRF) coil enables the reverse polarization mode, because a linearly polarized signal consists of both forward and reverse polarization signals. As a result, building the ICRF coil to interventional devices paves the way of using this method in interventional MRI. Performances of developed technologies were tested in phantom, animal, and volunteer studies. We believe that the studies explained in this dissertation contribute to obtaining better imaging systems.
Open Access
Miniaturized fiber optic transmission system for magnetic resonance imaging signals
(2005) Memiş, Ömer Gökalp
In conventional Magnetic Resonance Imaging (MRI) instruments, after echo signals are received by an MRI coil, they are transmitted through an ultra low noise transmission system consisting of electrical cables. Although this design proved to be effective over the years, there are recent developments in the MRI technology which require a better, more sophisticated design. One of these emerging technologies is “parallel imaging”, where total size of interconnections is the primary problem, and the other is “interventional MRI”, where safety needs to be improved. The Miniature Fiber Optic Transmission System was developed to serve these needs. The system consists of a receiver MRI coil with passive detuning, a two-stage low-noise preamplifier and a low-noise laser diode connected to a photodetector with fiber optic cable in between. The overall noise figure of the system is measured to be lower than 1 dB which guarantees that total signal-to-noise ratio (SNR) reduction in the images due to optical MRI signal transmission system is less than 15%. Total power consumption is 50mW and the device is switchable by another fiber optic line, which can also control active detuning circuit if it is present. A prototype device was tested in a GE 1.5 Tesla MRI instrument and several images were acquired with a slight SNR drop, due to problems with passive detuning. We believe that this design will significantly reduce the size of parallel imaging arrays and enable placement of internal coils into body cavities without providing any safety hazard to the patient, such as electrical shock or burns.
Open Access
Modes of shear wave in magnetic resonance elastography
(2014) Arıyürek, Cemre
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.
Open Access
Near-IR absorbing Bodipy functionalized Spions : a potential magnetic nanoplatform fo diagnosis and therapy
(2012) Ertem, Elif
Photodynamic therapy (PDT), especially with the recent advances in photosensitizer design has already been established as a noninvasive technique for cancer treatment. In PDT, photosensitizers (PSs) are targeted to tumor sites either actively or passively, and are irradiated with a laser of appropriate wavelength. The stimulated PSs transfer excitation energy to endogenous oxygen converting it to reactive oxygen species (ROS) that can kill tumor cells. Up to now, numerous nanomaterials tailored to suitable size, have been studied for effective delivery of PSs. Recently, Near IR-based absorbing nanomaterials which have a rising potency to implement light-triggered tumor ablation have attracted much attention since near-IR light in the 650–850 nm range penetrates more deeply in tissues. In addition, imaging of these nanomaterials carrying PSs is very important in order to prevent damage to the healthy tissues upon irradiation. Magnetic resonance imaging (MRI) is a powerful technique due to its excellent spatial resolution and depth for in vivo imaging. In this study, a multidisciplinary approach was utilized to create MRI active, near IR-based functional nanomaterials. This approach involves (i) nanochemistry to prepare silica coated super paramagnetic iron oxide (core-shell) nanoparticles, (ii) organic chemistry to synthesize four different type of near- IR absorbing Bodipy derivatives as PSs, and (iii) spectroscopy to verify singlet oxygen production. Four different type of Bodipy based PSs were covalently attached to MRI active, biocompatible, and nontoxic nanocarriers and generation of singlet oxygen capabilities were evaluated. It was demonstrated that these core-shell nanoparticles are promising delivery vehicles of PSs for the use in diagnosis and therapy.
Open Access
Novel implantable distributively loaded flexible resonators for MRI
(2011) Gökyar, Sayım
Magnetic resonance imaging (MRI) is an enabling technology platform for imaging applications. In MRI, the imaging frequency falls within the radio frequency (RF) range where the tissue absorption of electromagnetic power is conveniently very low (e.g., compared to X-ray imaging), making MRI medically safe. As a result, MRI has evolved into a major imaging tool in medicine. However, in MRI, it is typically difficult to receive a magnetic resonance signal from tissue near a metallic implant, which hinders imaging of the implant device neighborhood to observe, monitor, and make assessment of the recovery and tissue compatibility. This can be accomplished by using locally resonating implants, but such implantable local resonators compatible with MRI that simultaneously feature reasonable chip size are currently not available (although there are some MRI-guided catheter applications). In this thesis, we proposed and developed a new class of implantable chip-scale local resonators that operate at radio frequencies of MRI, despite their small size, for the purposes of enhancing the signal-to-noise ratio (SNR) and thus the resolution in their vicinity. Here we addressed the scientific challenge of achieving low resonance frequency while maintaining chip-scale size suitable for potential MR-compatible implants. Using only biocompatible materials (gold, nitrides, and silicon or polyimide) within a substantially reduced footprint (miniaturized by 2 orders of magnitude), we demonstrated novel chip-scale designs based on the basic concept of split ring resonators (SRRs). Different than classical SRRs or those loaded with lumped elements (e.g., thin-film lumped loading), however, in our designs we loaded the SRR geometry in a distributive manner with a micro-fabricated dielectric thin-film layer to increase effective capacitance. For a proof-of-concept demonstration, we fabricated 20 mm ´ 20 mm resonators that operate at the resonance frequency of 130 MHz (compatible with 3 T MRI system) when distributively loaded with the capacitive film, which would otherwise operate around 1.2 GHz as a classical SRR of the same size if not loaded. It is worth noting that this resonance frequency of 130 MHz would normally require a classical SRR of 20 cm ´ 20 cm, a chip size 100-fold larger than ours. Designing and fabricating flexible thin-film resonators, we also showed that this architecture can be tuned by bending and is appropriate for non-planar surfaces, which is often the case for in vivo imaging. The phantom images indicated that, depending on the resonator configuration, these novel self-resonating structures increase SNR of the received signal by a maximum factor of 4 to 150 and over an enhancement penetration up to 10 mm into the phantom. This corresponds to a resolution enhancement in the 2D image by a factor of 2 to 12, respectively, under the same RF power. These in vitro experiments prove that it is possible to operate our local resonators at reduced frequencies via the help of distributive loading on the same chip. These findings suggest that proposed implantable resonator chips make promising candidates for self-resonating MR-compatible implants.
Open Access
Novel methods and analysis of B0 and B1 gradients in magnetic resonance imaging
(2013) Türk, Esra Abacı
In this thesis, analysis of B0 gradients and B1 fields are performed and novel methods using B1 gradients instead of B0 gradients are proposed. The first contribution of this dissertation is expressing the nature of the interaction between the B0 gradient fields and the active implantable medical devices (AIMD). By utilizing the fact that gradient coils produce linear magnetic field in a volume of interest, the simplified closed form electric field expressions are defined inside a homogeneous cylindrical volume. Using these simplified expressions, the induced potential on an implant electrode has been computed approximately for various lead positions on a cylindrical phantom and verified by comparing with the measured potentials for these sample conditions. In addition, the validity of the method has been tested with isolated frog leg stimulation experiments. The results of both phantom and ex vivo experiments show that if the path of the implant lead is known, the induced voltage on the lead can be estimated analytically. The second topic in this dissertation is the Bloch-Siegert (BS) shift based B1 mapping method. The method is analyzed in terms of the effects of the off-resonance frequency, the RF pulse shape, and the duration of the RF pulse. Based on these analyses, a new theoretical model that relates the Fourier transform of the off-resonant BS RF pulse envelope to the phase shift is proposed. Utilizing Bloch simulations and phantom experiments the proposed frequency domain expression is verified. The results indicates that the proposed expression works well even for short pulse durations (< 2ms) and low offset frequencies (fRF < 500Hz) when the ratio of the RF field and the frequency offset of the RF pulse is smaller than 0.5. The last topic of this dissertation is on flow and shear wave imaging with B1 gradients instead of B0 gradients. In flow imaging, a novel sequence using a Bloch-Siegert pulse generated by a spatially dependent B1 field is proposed. The proposed method is experimentally verified by comparing the resultant velocity measurements with those obtained by using bipolar flow encoding B0 gradients. This comparison demonstrates the feasibility of using BS shift with B1 gradients in detecting the flow. The usage of B1 gradients is also proposed to detect shear waves at frequencies in kilohertz range and this method is experimentaly verified for 2kHz, 3kHz and 4kHz shear frequencies. The studies in this thesis indicate that extensive analysis of B0 gradients in Magnetic Resonance Imaging (MRI) is important for safety issues, and for scenarios where B0 gradients prove insufficient in encoding due to hardware limitations, utilizing B1 gradients can be considered as an alternative.
Open Access
Piezoelectric power generation using heart motion
(2006) Afacan, Onur
The presence of pacemakers and implantable cardioverter-defibrillators (ICD) is considered historically a contraindication to magnetic resonance (MR) imaging. Main reason behind this contraindication is the current induced on the pacing leads during the MRI examination which may damage the cardiac tissues by heating or the pulse generator of the pacemaker with a reverse current. In this thesis an approach towards the solution of this problem is stated. It has been shown in previous work that replacing the pacing leads with fiber-optic cables minimizes the current induced on the leads. Drawback of this system is the increase in the power consumption of the pacemaker because of the fiber-optic cables and also necessity of an additional pulse generator circuitry near the heart. In this thesis, feasibility of using a piezoelectric power generator for compensating the increased power consumption is investigated. A novel piezoelectric geometry increasing the effective length in a given volume is designed. With this device the resonance frequency of the generator was decreased and the power output for a given volume is increased compared to standard rectangular piezoelectric bimorphs. When connected to a simple heart phantom the novel design produced 2.83 microwatts power whereas the standard rectangular bimorphs produced less than 1 microwatts. Although the output power is increased with the novel design, it was not sufficient to power a pulse generator circuitry that will be used to pace the heart.
Open Access
Ultimate intrinsic SNR in magnetic resonance imaging by optimizing the EM field generated by internal coils
(2000) Abdel-Hafez, Imad Amin
A method to find the ultimate intrinsic signal-to-noise ratio (ISNR) in a magnetic resonance imaging experiment is applied to a human body model. The method uses cylindrical wave expansion to represent an arbitrary electromagnetic field inside the body. This field is optimized to give the maximum possible ISNR for some point of interest from which the signal is received, and repeated for all points inside the body. Optimization is conducted by finding the set of coefficients associated with expansion modes that give the maximum ISNR. Application of this method enables the determination of the ultimate ISNR and the associated optimum electromagnetic field without the necessity of finding the receiving coil configuration needed to obtain the ultimate value of ISNR. Results of this work can be used to examine the efficiency of already available commercial coils and how far they can be improved. Moreover, the solution can be used to determine the performance difference between internal and external Magnetic Resonance Imaging (MRI) coils. Finally, knowledge of the optimum electromagnetic field inside the human body can be used to find the coil configuration that can radiate this field by solving an inverse problem.