Browsing by Subject "Magnetic resonance imaging"

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A novel hybrid magnetic particle imaging and low-field magnetic resonance imaging scanner
(2024-08) Karaca, Sefa
Magnetic particle imaging (MPI) is an emerging medical imaging modality, in which the spatial distribution of the magnetic nanoparticles (MNPs) are imaged using their non-linear magnetization curve. Since biological tissues do not exhibit such magnetic behavior, MNPs serve as the sole source of the MPI signal, making it a promising in vivo imaging modality with high contrast and sensitivity. However, anatomical information is also essential for many applications. To address this issue, MPI can be combined with other imaging modalities, such as computed tomography (CT) or magnetic resonance imaging (MRI), using standalone or hybrid systems. It is particularly advantageous to combine MPI with MRI, given that they are both non-ionizing imaging modalities. Moreover, since both MPI and MRI utilize magnetic fields, a hybrid system that integrates these modalities can potentially reduce costs via shared utilization of hardware. This thesis introduces a novel preclinical-size hybrid MPI and low-field (LF) MRI scanner. The proposed system features an MPI mode with a field free line (FFL) topology and a selection field gradient of 0.25 T/m, alongside a LF-MRI mode with a B0 field strength of 50 mT. The primary advantage of this hybrid system lies in the interchangeable use of coil groups between MPI and LF-MRI modes, facilitating the generation of a multimodal image that features high sensitivity and contrast imaging of MNP distribution by MPI, alongside anatomical information from LF-MRI. Additionally, the coil configuration of this hybrid system features an open-sided design, with the exception of the Tx/Rx coil of MRI, which utilizes a solenoid design for experimental studies. This coil can be substituted with a surface coil, facilitating the development of an open-sided hybrid system. First, the feasibility of multimodal imaging with the proposed hybrid scanner is evaluated by characterizing the magnetic fields in the system. Next, the effects of system-induced deviations on image quality are investigated via an in-house MRI simulator. The experimental imaging results demonstrate that the proposed preclinical-size hybrid MPI and LF-MRI scanner can successfully perform multimodal imaging.
Open Access
Accelerated phase-cycled SSFP imaging with compressed sensing
(Institute of Electrical and Electronics Engineers Inc., 2015) Çukur, T.
Balanced steady-state free precession (SSFP) imaging suffers from irrecoverable signal losses, known as banding artifacts, in regions of large B0 field inhomogeneity. A common solution is to acquire multiple phase-cycled images each with a different frequency sensitivity, such that the location of banding artifacts are shifted in space. These images are then combined to alleviate signal loss across the entire field-of-view. Although high levels of artifact suppression are viable using a large number of images, this is a time costly process that limits clinical utility. Here, we propose to accelerate individual acquisitions such that the overall scan time is equal to that of a single SSFP acquisition. Aliasing artifacts and noise are minimized by using a variable-density random sampling pattern in k-space, and by generating disjoint sampling patterns for separate acquisitions. A sparsity-enforcing method is then used for image reconstruction. Demonstrations on realistic brain phantom images, and in vivo brain and knee images are provided. In all cases, the proposed technique enables robust SSFP imaging in the presence of field inhomogeneities without prolonging scan times. © 2014 IEEE.
Open Access
Amphiphilic peptide coated superparamagnetic iron oxide nanoparticles for in vivo MR tumor imaging
(Royal Society of Chemistry, 2016) Ozdemir, A.; Ekiz, M. S.; Dilli, A.; Güler, Mustafa O.; Tekinay, A. B.
Magnetic resonance imaging (MRI) is a noninvasive imaging technique that provides high spatial resolution and depth with pronounced soft-tissue contrast for in vivo imaging. A broad variety of strategies have been employed to enhance the diagnostic value of MRI and detect tissue abnormalities at an earlier stage. Superparamagnetic iron oxide nanoparticles (SPIONs) are considered to be suitable candidates for effective imaging due to their small size, versatile functionality and better biocompatibility. Here, we demonstrate that coating SPIONs with proline-rich amphiphilic peptide molecules through noncovalent interactions leads to a water-dispersed hybrid system suitable as an MRI contrast agent. Cellular viability and uptake of amphiphilic peptide coated SPIONs (SPION/K-PA) were evaluated with human vascular endothelial cells (HUVEC) and estrogen receptor (ER) positive human breast adenocarcinoma (MCF-7) cells. The efficiency of SPION/K-PA as MRI contrast agents was analyzed in Sprague-Dawley rats with mammary gland tumors. MR imaging showed that SPION/K-PA effectively accumulated in tumor tissues, enhancing their imaging potential. Although nanoparticles were observed in reticuloendothelial system organs (RES) and especially in the liver and kidney immediately after administration, the MR signal intensity in these organs diminished after 1 h and nanoparticles were subsequently cleared from these organs within two weeks. Histological observations also validated the accumulation of nanoparticles in tumor tissue at 4 h and their bioelimination from the organs of both healthy and tumor-bearing rats after two weeks.
Open Access
Approximate fourier domain expression for bloch-siegert shift
(John Wiley and Sons Inc., 2015) Turk, E. A.; Ider, Y. Z.; Ergun, A. S.; Atalar, Ergin
Purpose: In this study, a newsimple Fourier domain-based analytical expression for the Bloch-Siegert (BS) shift-based B1 mapping method is proposed to obtain |B1+| more accurately while using short BS pulse durations and small off-resonance frequencies.Theory and Methods: A new simple analytical expression for the BS shift is derived by simplifying the Bloch equations. In this expression, the phase is calculated in terms of the Fourier transform of the radiofrequency pulse envelope, and thus making the off- and on-resonance effects more easily understandable. To verify the accuracy of the proposed expression, Bloch simulations and MR experiments are performed for the hard, Fermi, and Shinner-Le Roux pulse shapes.Results: Analyses of the BS phase shift-based B1 mapping method in terms of radiofrequency pulse shape, pulse duration, and off-resonance frequency show that |B1+| can be obtained more accurately with the aid of this new expression.Conclusions: In this study, a new simple frequency domain analytical expression is proposed for the BS shift. Using this expression, |B1+| values can be predicted from the phase data using the frequency spectrum of the radiofrequency pulse. This method works well even for short pulse durations and small offset frequencies.
Open Access
Assessment of third ventriculostomy patency with the 3D-SPACE technique: a preliminary multicenter research study
(American Association of Neurological Surgeons, 2015) Algın, Oktay; Uçar, M.; Ozmen, E.; Borcek, A.; Ozisik, P.; Ocakoglu, G.; Tali, E.
Object The goal of this study was to determine the value of the 3D sampling perfection with application-optimized contrasts using different flip-angle evolutions (3D-SPACE) technique in the evaluation of endoscopic third ventriculostomy (ETV) patency. Methods Twenty-six patients with ETV were examined using 3-T MRI units. Sagittal-plane 3D-SPACE with variant flip-angle mode, 3D T1-weighted (T1W), and 3D heavily T2-weighted (T2W) images were obtained with isotropic voxel sizes. Also, sagittal-axial plane phase-contrast cine (PC)-MR images were obtained. The following findings were evaluated: diameters of stoma and third ventricle, flow-void sign on 3D-SPACE and PC-MR images, integrity of the third ventricle on heavily T2W images, and quantitative PC-MRI parameters of the stoma. Obtained sequences were evaluated singly, in combination with one another, and all together. Results The mean area, flow, and velocity values measured at the level of stoma in patients with patent stoma were significantly higher than those measured in patients with closed stoma (p < 0.05). There was significant correlation among PC-MRI, 3D-SPACE, and 3D heavily T2W techniques regarding assessment of ETV patency (p < 0.001). The 3D-SPACE technique provided the lowest rate of ambiguous results. Conclusions The 3D-SPACE technique seems to be the most efficient one for determination of ETV patency. The authors suggest the use of 3D-SPACE as a stand-alone first-line sequence in addition to routine brain MRI protocols in assessing patients with ETV, thereby decreasing scan time and reserving the use of a combination of additional sequences such as PC-MRI and 3D heavily T2W images in suspicious or complex cases.
Open Access
Biomacromolecules, molecules and functional nanoparticles for therapeutic and diagnostic applications
(2016-03) Özdemir, Ayşe
Cancer is one of the most important global health problem. In the last decade, researchers have focused on the development of novel sensitive diagnostic agents and potential therapeutic molecules to further contribute to the success of cancer treatment and increase survival rates of cancer patients. Magnetic resonance imaging (MRI) is a powerful diagnostic tool and used in clinics for cancer imaging. Superparamagnetic iron oxide nanoparticles (SPIONs) are used as a negative contrast agent to increase sensitivity of MRI. SPIONs can be coated with biocompatible natural or synthetic materials to maintain stability and improve their blood distribution profile. SPIONs can also be non-covalently functionalized with peptide amphiphile (PA) molecules through hydrophobic interactions to render them water soluble and biocompatible. In addition, several efforts have been made to improve specificity and sensitivity of SPIONs by attaching cancer targeting agents such as peptides. For cancer therapy, metal based drugs have attracted attention because of their biological and pharmaceutical properties over the past decades. The understanding of interactions between potential agents and biomolecules is important for designing novel anticancer drugs against tumors to overcome the toxicity of currently used chemotherapeutic drugs and achieve more precision. Herein, I investigated the potential of proline-rich PA coated SPIONs as a negative contrast agent for cancer diagnosis by MRI. To achieve water solubility and cancer targeting, positively charged K and LPPR peptide sequences were presented on the PA micelles. PA functionalization provided a water-dispersible hybrid system. Biocompatibility and efficient uptake of the SPIONs were found to be improved with PA coating. This hybrid system provided enhancement in the MR imaging of tumor tissue in chemically induced breast cancer model. In addition, in vivo experiments and histological examinations revealed the biodistribution and bioelimination profile of the nanoparticles. These SPION/PA system can potentially be used as a contrast agent in cancer diagnosis by MRI. In addition, I analyzed the interactions between metal based molecules that can be used as cancer therapeutics and calf thymus DNA or human serum albumin (HSA) by spectroscopic and calorimetric methods which showed the binding modes, affinities and the effects on the structure of these biomacromolecules. Although similar structures demonstrated similar binding characteristics, each molecule has different association with DNA or HSA. The obtained results are promising for the development of metal or half metal based anticancer agents targeting DNA and carried by HSA.
Open Access
Bismuth nano-Hall probes fabricated by focused ion beam milling for direct magnetic imaging by room temperature scanning Hall probe microscopy
(The Institution of Engineering and Technology (IET), 2001) Sandhu, A.; Masuda, H.; Kurosawa, K.; Oral, A.; Bending, S. J.
Bismuth nano-Hall probes fabricated by using focused ion beam (FIB) milling were studied. The nano-Hall probes were used for direct magnetic imaging of domain structures in low coercivity garnets and demagnetized strontium ferrite permanent magnets. The analysis was performed using room temperature scanning Hall probe microscopy and it was found that the Bi nano-probes could overcome limitations due to surface depletion and large series resistances.
Open Access
BRAPH: A graph theory software for the analysis of brain connectivity
(Public Library of Science, 2017) Mijalkov, M.; Kakaei, E.; Pereira, J. B.; Westman, E.; Volpe, G.
The brain is a large-scale complex network whose workings rely on the interaction between its various regions. In the past few years, the organization of the human brain network has been studied extensively using concepts from graph theory, where the brain is represented as a set of nodes connected by edges. This representation of the brain as a connectome can be used to assess important measures that reflect its topological architecture. We have developed a freeware MatLab-based software (BRAPH–BRain Analysis using graPH theory) for connectivity analysis of brain networks derived from structural magnetic resonance imaging (MRI), functional MRI (fMRI), positron emission tomography (PET) and electroencephalogram (EEG) data. BRAPH allows building connectivity matrices, calculating global and local network measures, performing non-parametric permutations for group comparisons, assessing the modules in the network, and comparing the results to random networks. By contrast to other toolboxes, it allows performing longitudinal comparisons of the same patients across different points in time. Furthermore, even though a user-friendly interface is provided, the architecture of the program is modular (object-oriented) so that it can be easily expanded and customized. To demonstrate the abilities of BRAPH, we performed structural and functional graph theory analyses in two separate studies. In the first study, using MRI data, we assessed the differences in global and nodal network topology in healthy controls, patients with amnestic mild cognitive impairment, and patients with Alzheimer’s disease. In the second study, using resting-state fMRI data, we compared healthy controls and Parkinson’s patients with mild cognitive impairment. © 2017 Mijalkov et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Open Access
Cerebrospinal fluid velocity changes of idiopathic scoliosis: a preliminary study on 3-T PC-MRI and 3D-SPACE-VFAM data
(Springer, 2021-10-06) Algın, Oktay; Koç, Ural; Yalçın, N.
Objectives To the best of our knowledge, there is no study on 3-Tesla (3-T) phase-contrast MRI (PC-MRI) and three-dimensional sampling perfection with application-optimized contrasts using diferent fip-angle evolutions (3D-SPACE-VFAM) in the evaluation of idiopathic scoliosis. This study aimed to investigate CSF abnormalities in the scoliotic spine using 3-T PC MRI and 3D-SPACE-VFAM techniques. Methods Thirty-four patients and 14 controls were examined with spinal PC-MRI and T2-weighted 3D-SPACE-VFAM techniques. Inter- and intra-reader agreements of fow-void phenomenon on 3D-SPACE-VFAM images, and velocity values on PC-MRI data were also evaluated. Results There are statistically signifcant diferences between scoliosis and control groups based on the highest and mean peak velocity values on PC-MRI images (p=0.005 and p=0.023, respectively). The main thoracic (MT) group’s highest peak CSF velocity values were higher than the control group (p=0.022). There is a signifcant diference between the patient and control groups regarding fow-void phenomenon scores on 3D-SPACE-VFAM images (p=0.036). Inter- and intra-reader agreement values related to PC-MRI velocity measurements were perfect for all PC-MRI readings. Inter- and intra-reader agreement values of the fow-void phenomenon scores were moderate. Conclusions Our study has led us to conclude that idiopathic scoliosis is associated with CSF fow disturbances in parallel with the literature. MRI can demonstrate these abnormalities in a non-invasive and radiation-free way.
Open Access
Cisternography of arachnoid cysts
(Springer Link, 2023-01-01) Güneş, Y. C.; Algın, Oktay; Turgut, Mehmet; Akhaddar, Ali; Turgut, Ahmet T.; Hall, Walter A.
Conventional magnetic resonance (MR) sequences are useful for the diagnosis of arachnoid cysts (ACs), but they are not sufficient to show the communication between ACs and adjacent cerebrospinal fluid (CSF)-containing areas. Contrast material-enhanced computed tomography (CT) and MR cisternography play a crucial role in demonstrating this relationship. These tests create a contrast difference in the CSF in the ventricular system and in the cisternal spaces. Compared with radionuclide cisternography and CT cisternography (CTC), the main advantages of contrast material-enhanced MR cisternography (CE-MRC) are its high contrast-noise ratio, multiplanar analysis capacity, images with thin section thickness, and not causing radiation exposure. ACs that were filled with intrathecal contrast on early-phase postcontrast (after intrathecal administration) images were included in the full communicating group that did not fill at the 24th hour later or showed minimal filling was included in the noncommunication group. In addition, 3D-SPACE with variant flip-angle mode (VFAM) and PC-MRI techniques have special capabilities for evaluating CSF flow. It was successful in showing the communication between ACs and the adjacent CSF-containing areas. In cases with suspicious findings on phase-contrast MRI images, the final decision can be made with CE-MRC. CE-MRC is considered the gold standard in demonstrating the relationship between ACs and CSF-containing spaces. Isotope cisternography has obvious disadvantages, and it is no longer used today. In conclusion, demonstrating the relationship of ACs with adjacent CSF-filled structures is important in making surgical decisions.
Open Access
Compressed sensing techniques for accelerated magnetic resonance imaging
(2017-07) Ilıcak, Efe
Magnetic resonance imaging has seen a growing interest in the recent years due to its non-invasive and non-ionizing nature. However, imaging speed remains a major concern. Recently, compressed sensing theory has opened new doors for accelerated imaging applications. This dissertation studies compressed sensing based reconstruction strategies for accelerated magnetic resonance imaging, speci cally for angiography and multiple-acquisition methods. For magnetic resonance angiography, we propose a novel approach that improves scan time e ciency while suppressing background signals. In this study, we attain high-contrast angiograms from undersampled data by utilizing a two-stage reconstruction strategy. Simulations and in vivo experiments demonstrate that the developed strategy is able to relax trade-o s between image contrast and scan e ciency without compromising vessel depiction. For multiple-acquisition balanced steady state free precession imaging, we develop a framework that jointly reconstructs undersampled phasecycled images. This approach is able to improve banding artifact suppression while maintaining scan e ciency. Results show that the proposed method is able to attain high-quality reconstructions even at high acceleration factors. Overall, the ndings presented in this thesis indicate that compressed sensing reconstructions represent a promising future for rapid magnetic resonance imaging. Consequently, compressed sensing reconstruction techniques hold a great potential to change the time-consuming clinical imaging practices.
Open Access
Convection-reaction equation based magnetic resonance electrical properties tomography (cr-MREPT)
(Institute of Electrical and Electronics Engineers Inc., 2014) Hafalir, F. S.; Oran, O. F.; Gurler, N.; Ider, Y. Z.
Images of electrical conductivity and permittivity of tissues may be used for diagnostic purposes as well as for estimating local specific absorption rate distributions. Magnetic resonance electrical properties tomography (MREPT) aims at noninvasively obtaining conductivity and permittivity images at radio-frequency frequencies of magnetic resonance imaging systems. MREPT algorithms are based on measuring the B1 field which is perturbed by the electrical properties of the imaged object. In this study, the relation between the electrical properties and the measured B1 field is formulated for the first time as a well-known convection-reaction equation. The suggested novel algorithm, called 'cr-MREPT,' is based on the solution of this equation on a triangular mesh, and in contrast to previously proposed algorithms, it is applicable in practice not only for regions where electrical properties are relatively constant but also for regions where they vary. The convective field of the convection-reaction equation depends on the spatial derivatives of the B1 field, and in the regions where its magnitude is low, a spot-like artifact is observed in the reconstructed electrical properties images. For eliminating this artifact, two different methods are developed, namely 'constrained cr-MREPT' and 'double-excitation cr-MREPT.' Successful reconstructions are obtained using noisy and noise-free simulated data, and experimental data from phantoms.
Open Access
Current constrained voltage scaled reconstruction (CCVSR) algorithm for MR-EIT and its performance with different probing current patterns
(Institute of Physics Publishing, 2003) Birgül, Ö.; Eyüboğlu, B. M.; İder, Y. Z.
Conventional injected-current electrical impedance tomography (EIT) and magnetic resonance imaging (MRI) techniques can be combined to reconstruct high resolution true conductivity images. The magnetic flux density distribution generated by the internal current density distribution is extracted from MR phase images. This information is used to form a fine detailed conductivity image using an Ohm's law based update equation. The reconstructed conductivity image is assumed to differ from the true image by a scale factor. EIT surface potential measurements are then used to scale the reconstructed image in order to find the true conductivity values. This process is iterated until a stopping criterion is met. Several simulations are carried out for opposite and cosine current injection patterns to select the best current injection pattern for a 2D thorax model. The contrast resolution and accuracy of the proposed algorithm are also studied. In all simulation studies, realistic noise models for voltage and magnetic flux density measurements are used. It is shown that, in contrast to the conventional EIT techniques, the proposed method has the capability of reconstructing conductivity images with uniform and high spatial resolution. The spatial resolution is limited by the larger element size of the finite element mesh and twice the magnetic resonance image pixel size.
Open Access
Deep learning for accelerated MR imaging
(2021-02) Dar, Salman Ul Hassan
Magnetic resonance imaging is a non-invasive imaging modality that enables multi-contrast acquisition of an underlying anatomy, thereby supplementing mul-titude of information for diagnosis. However, prolonged scan duration may pro-hibit its practical use. Two mainstream frameworks for accelerating MR image acquisitions are reconstruction and synthesis. In reconstruction, acquisitions are accelerated by undersampling in k-space, followed by reconstruction algorithms. Lately deep neural networks have oﬀered signiﬁcant improvements over tradi-tional methods in MR image reconstruction. However, deep neural networks rely heavily on availability of large datasets which might not be readily available for some applications. Furthermore, a caveat of the reconstruction framework in general is that the performance naturally starts degrading towards higher accel-eration factors where fewer data samples are acquired. In the alternative syn-thesis framework, acquisitions are accelerated by acquiring a subset of desired contrasts, and recovering the missing ones from the acquired ones. Current syn-thesis methods are primarily based on deep neural networks, which are trained to minimize mean square or absolute loss functions. This can bring about loss of intermediate-to-high spatial frequency content in the recovered images. Fur-thermore, the synthesis performance in general relies on similarity in relaxation parameters between source and target contrasts, and large dissimilarities can lead to artifactual synthesis or loss of features. Here, we tackle issues associated with reconstruction and synthesis approaches. In reconstruction, the data scarcity is-sue is addressed by pre-training a network on large readily available datasets, and ﬁne-tuning on just a few samples from target datasets. In synthesis, the loss of intermediate-to-high spatial frequency is catered for by adding adversarial and high-level perceptual losses on top of traditional mean absolute error. Fi-nally, a joint reconstruction and synthesis approach is proposed to mitigate the issues associated with both reconstruction and synthesis approaches in general. Demonstrations on MRI brain datasets of healthy subjects and patients indicate superior performance of the proposed techniques over the current state-of-the art ones.
Open Access
Design of a novel MRI compatible manipulator for image guided prostate interventions
(IEEE, 2005-02) Krieger, A.; Susil, R. C.; Ménard, C.; Coleman, J. A.; Fichtinger, G.; Atalar, Ergin; Whitcomb, L. L.
This paper reports a novel remotely actuated manipulator for access to prostate tissue under magnetic resonance imaging guidance (APT-MRI) device, designed for use in a standard high-field MRI scanner. The device provides three-dimensional MRI guided needle placement with millimeter accuracy under physician control. Procedures enabled by this device include MRI guided needle biopsy, fiducial marker placements, and therapy delivery. Its compact size allows for use in both standard cylindrical and open configuration MRI scanners. Preliminary in vivo canine experiments and first clinical trials are reported.
Open Access
Disrupted network topology in patients with stable and progressive mild cognitive impairment and alzheimer's disease
(Oxford University Press, 2016) Pereira, J. B.; Mijalkov, M.; Kakaei, E.; Mecocci, P.; Vellas, B.; Tsolaki, M.; Kłoszewska, I.; Soininen, H.; Spenger, C.; Lovestone, S.; Simmons, A.; Wahlund, L.-O.; Volpe, G.; Westman, E.
Recent findings suggest that Alzheimer's disease (AD) is a disconnection syndrome characterized by abnormalities in large-scale networks. However, the alterations that occur in network topology during the prodromal stages of AD, particularly in patients with stable mild cognitive impairment (MCI) and those that show a slow or faster progression to dementia, are still poorly understood. In this study, we used graph theory to assess the organization of structural MRI networks in stable MCI (sMCI) subjects, late MCI converters (lMCIc), early MCI converters (eMCIc), and AD patients from 2 large multicenter cohorts: ADNI and AddNeuroMed. Our findings showed an abnormal global network organization in all patient groups, as reflected by an increased path length, reduced transitivity, and increased modularity compared with controls. In addition, lMCIc, eMCIc, and AD patients showed a decreased path length and mean clustering compared with the sMCI group. At the local level, there were nodal clustering decreases mostly in AD patients, while the nodal closeness centrality detected abnormalities across all patient groups, showing overlapping changes in the hippocampi and amygdala and nonoverlapping changes in parietal, entorhinal, and orbitofrontal regions. These findings suggest that the prodromal and clinical stages of AD are associated with an abnormal network topology.
Open Access
Effects of pulse duration on magnetostimulation thresholds
(Wiley-Blackwell Publishing, Inc., 2015-06) Saritas, E. U.; Goodwill, P. W.; Conolly, S. M.
Purpose: Medical imaging techniques such as magnetic resonance imaging and magnetic particle imaging (MPI) utilize time-varying magnetic fields that are subject to magnetostimulation limits, which often limit the speed of the imaging process. Various human-subject experiments have studied the amplitude and frequency dependence of these thresholds for gradient or homogeneous magnetic fields. Another contributing factor was shown to be number of cycles in a magnetic pulse, where the thresholds decreased with longer pulses. The latter result was demonstrated on two subjects only, at a single frequency of 1.27 kHz. Hence, whether the observed effect was due to the number of cycles or due to the pulse duration was not specified. In addition, a gradient-type field was utilized; hence, whether the same phenomenon applies to homogeneous magnetic fields remained unknown. Here, the authors investigate the pulse duration dependence of magneto stimulation limits for a 20-fold range of frequencies using homogeneous magnetic fields, such as the ones used for the drive field in MPI. Methods: Magnetostimulation thresholds were measured in the arms of six healthy subjects (age: 27±5 yr). Each experiment comprised testing the thresholds at eight different pulse durations between 2 and 125 ms at a single frequency, which took approximately 3040 min/subject. A total of 34 experiments were performed at three different frequencies: 1.2, 5.7, and 25.5 kHz. A solenoid coil providing homogeneous magnetic field was used to induce stimulation, and the field amplitude was measured in real time. A pre-emphasis based pulse shaping method was employed to accurately control the pulse durations. Subjects reported stimulation via a mouse click whenever they felt a twitching/tingling sensation. A sigmoid function was fitted to the subject responses to find the threshold at a specific frequency and duration, and the whole procedure was repeated at all relevant frequencies and pulse durations. Results: The magnetostimulation limits decreased with increasing pulse duration (Tpulse). For Tpulse < 18 ms, the thresholds were significantly higher than at the longest pulse durations (p < 0.01, paired Wilcoxon signed-rank test). The normalized magnetostimulation threshold (BNorm) vs duration curve at all three frequencies agreed almost identically, indicating that the observed effect is independent of the operating frequency. At the shortest pulse duration (Tpulse ≈ 2 ms), the thresholds were approximately 24% higher than at the asymptotes. The thresholds decreased to within 4% of their asymptotic values for Tpulse > 20 ms. These trends were well characterized (R2 = 0.78) by a stretched exponential function given by BNorm = 1+αe?(Tpulse/β)γ, where the fitted parameters were α = 0.44, β = 4.32, and γ = 0.60. Conclusions: This work shows for the first time that the magnetostimulation thresholds decrease with increasing pulse duration, and that this effect is independent of the operating frequency. Normalized threshold vs duration trends are almost identical for a 20-fold range of frequencies: the thresholds are significantly higher at short pulse durations and settle to within 4% of their asymptotic values for durations longer than 20 ms. These results emphasize the importance of matching the human-subject experiments to the imaging conditions of a particular setup. Knowing the dependence of the safety limits to all contributing factors is critical for increasing the time-efficiency of imaging systems that utilize time-varying magnetic fields. © 2015 American Association of Physicists in Medicine.
Open Access
Evaluation of cortical thickness and brain volume on 3 Tesla magnetic resonance imaging in children with frontal lobe epilepsy
(Springer, 2020) Kural-Rahatlı, F.; Sezer, T.; Has, Arzu Ceylan; Ağıldere, A. M.
Background Frontal lobe epilepsy (FLE) is the most common epilepsy syndrome in the pediatric population; however, brain magnetic resonance imaging (MRI) of the children with FLE is frequently normal. We use both cortical thickness and brain volume measurements to report on cortical changes in children with FLE. Our aim was to determine cortical thickness and brain volume changes on 3 Tesla MRI of children with FLE and normal brain magnetic resonance imaging. Methods Twenty-seven children with FLE and 27 healthy controls received brain magnetic resonance imaging. Cortical thickness and regional brain volumes were assessed using three-dimensional volumetric T1-weighted imaging and patients were compared with controls. Results In children with FLE, statistically significant (p < 0.05) cortical thinning were found in the bilateral middle frontal gyrus, bilateral occipitotemporal and medial lingual gyrus, left subcallosal gyrus, left short insular gyrus, and right long insular gyrus. Statistically significant volume reductions in right and left hemisphere cortical white matter, total cortical white matter, bilateral thalamus, bilateral putamen, bilateral globus pallidus, right caudate nucleus, brain stem, and right cerebellar cortex were found. Conclusion Cortical thinning in frontal and extra-frontal lobes and volume loss in a variety of brain regions were found in children with FLE.
Open Access
Evaluation of internal MRI coils using ultimate intrinsic SNR
(John Wiley & Sons, 2004) Çelik, H.; Eryaman, Y.; Altıntaş, A.; Abdel-Hafez, I. A.; Atalar, Ergin
The upper bounds of the signal-to-noise ratio (also known as the "ultimate intrinsic signal-to-noise ratio" (UISNR)) for internal and external coils were calculated. In the calculation, the body was modeled as a dielectric cylinder with a small coaxial cylindrical cavity in which internal coils could be placed. The calculated UISNR values can be used as reference solutions to evaluate the performance of internal MRI coils. As examples, we evaluated the performance of a loopless antenna and an endourethral coil design by comparing their ISNR with the UISNR.
Open Access
Evaluation of spontaneous third ventriculostomy by three-dimensional sampling perfection with application-optimized contrasts using different flip-angle evolutions (3D-SPACE) sequence by 3 T MR imaging: preliminary results with variant flip-angle mode
(Elsevier, 2013) Algın, Oktay; Turkbey, B.; Ozmen, E.; Ocakoglu, G.; Karaoglanoglu, M.; Arslan, H.
Purpose This prospective study aimed to evaluate the use of three-dimensional sampling perfection with application-optimized contrasts using different flip-angle evolutions (3D-SPACE) in the diagnosis of spontaneous third ventriculostomy (STV) and to compare it with phase-contrast cine magnetic resonance imaging (PC-MRI). Methods PC-MRI and 3D-SPACE images were obtained in 15 patients with hydrocephalus for evaluation of STV as well as in 10 control cases. The presence of STV was evaluated visually by two experienced radiologists on both PC-MRI and 3D-SPACE images, and the results were statistically compared. Results There was a strong correlation between PC-MRI and SPACE scores for both readers (correlation coefficient [r] = 0.784; P = 0.001). There was also a good correlation between PC-MRI scores and consensus-based results. Interobserver reliabilities were strong for all STV scores. In addition, there was excellent correlation between 3D-SPACE scores and consensus-based results (first reader's kappa value: 0.87; second reader's kappa value: 1). Conclusion 3D-SPACE can provide morphological–physiological information for the evaluation of STV with no need for additional PC-MRI analysis or other sequences. As a non-invasive test, it can also be included among the first line of choices of MRI sequences for patients with obstructive hydrocephalus.