Homogeneous B1 Field Research Articles

An essentially radiation-transparent body coil integrated with a patient rotation system for MR-guided particle therapy.

The pursuit of adaptive radiotherapy using MR imaging for better precision in patient positioning puts stringent demands on the hardware components of the MR scanner. Particularly in particle therapy, the dose distribution and thus the efficacy of the treatment is susceptible to beam attenuation from interfering materials in the irradiation path. This severely limits the usefulness of conventional imaging coils, which contain highly attenuating parts such as capacitors and preamplifiers in an unknown position, and requires development of a dedicated radiofrequency (RF) coil with close consideration of the materials and components used. In MR-guided radiation therapy in the human torso, imaging coils with a large FOV and homogeneous B1 field distribution are required for reliable tissue classification. In this work, an imaging coil for MR-guided particle therapy was developed with minimal ion attenuation while maintaining flexibility in treatment. A birdcage coil consisting of nearly radiation-transparent materials was designed and constructed for a closed-bore 1.5 T MR system. Additionally, the coil was mounted on a rotatable patient capsule for flexible positioning of the patient relative to the beam. The ion attenuation of the RF coil was investigated in theory and via measurements of the Bragg peak position. To characterize the imaging quality of the RF coil, transmit and receive field distributions were simulated and measured inside a homogeneous tissue-simulating phantom for various rotation angles of the patient capsule ranging from 0° to 345° in steps of 15°. Furthermore, simulations with a heterogeneous human voxel model were performed to better estimate the effect of real patient loading, and the RF coil was compared to the internal body coil in terms of SNR for a full rotation of the patient capsule. The RF coil (total water equivalent thickness (WET) ≈ 420µm, WET of conductor ≈ 210µm) can be considered to be radiation-transparent, and a measured transmit power efficiency (B1 +/ ) between 0.17 µT/ and 0.26 µT/ could be achieved in a volume (Δz=216mm, complete x and y range) for the 24 investigated rotation angles of the patient capsule. Furthermore, homogeneous transmit and receive field distributions were measured and simulated in the transverse, coronal and sagittal planes in a homogeneous phantom and a human voxel model. In addition, the SNR of the radiation-transparent RF coil varied between 103 and 150, in the volume (Δz=216mm) of a homogeneous phantom and surpasses the SNR of the internal body coil for all rotation angles of the patient capsule. A radiation-transparent RF coil was developed and built that enables flexible patient to beam positioning via full rotation capability of the RF coil and patient relative to the beam, with results providing promising potential for adaptive MR-guided particle therapy.

Unilateral RF sensors based on parallel-plate architecture for improved surface-scan MRI analysis of commercial pouch cells

Rapid expansion of the Li-ion pouch cell market is driven by the looming problem of permanent depletion of natural gas reservoirs and by the growing demand for high-performance portable devices and electric vehicles. Safety and performance of Li-ion cells have been two main focal points of the extensive battery research. Surface-scan Magnetic Resonance Imaging (MRI) is an operando method designed for the accurate detection of substandard battery cells and for monitoring electrochemical processes with high spatial and temporal resolutions. Intercalation-dependent magnetism and charge transfer processes in the cell's electrodes give rise to characteristic magnetic field patterns outside the cell. For accurate mapping of such patterns, we proposed the concept of a unilateral radio-frequency (RF) sensor, a flat thin resonator encapsulating a proton-rich solid-state detection medium. When the pouch cell is placed in direct contact with the sensor, the magnetic field patterns propagate inside the detection medium, and the corresponding spatial distribution of Larmor precession frequencies can be detected with MRI. In this work, we developed and evaluated a series of RF sensor configurations based on parallel-plate architecture enhanced by arrays of distributed capacitors. The parallel-plate approach does not suffer from RF interference with pouch cells and provides excellent sensitivity and B1-field homogeneity. The optimal configuration of the parallel-plate sensor depends on the dimensions of the pouch cell and the distribution of parallel capacitors. This article includes the results of experimental tests, RF-field simulations, and strategies to further improve the surface-scan MRI method.

Towards high performance dielectric microwave resonators for X-band EPR spectroscopy

Microwave (MW) resonators in Electron Paramagnetic Resonance (EPR) spectroscopy concentrate the MW magnetic field (B1) at the sample and separate the MW electric field from the sample. There are numerous experimental methods in EPR spectroscopy which all impose different requirements on MW resonators (e.g. high or low quality factor, MW conversion, and B1-field homogeneity). Although commercial spectrometers offer standardized MW resonators for a broad application range, newly emerging and highly-specialized research fields push these spectrometers to or beyond their sensitivity limits. Optimizing the MW resonator offers one direct approach to improve the sensitivity. Here we present three low-cost optimization approaches for a commercially available X-band (9–10 GHz) MW resonator for three experimental purposes (continuous-wave (CW), transient and pulse EPR). We obtain enhanced MW conversion factors for all three optimized resonators and higher quality factors for two optimized resonators. The latter is important for CW and transient EPR. Furthermore, we fabricated a resonator which features an extended area of homogeneous B1-field and, hence, improved pulse EPR performance. Our results demonstrate that small changes to a commercial MW resonator can enhance its performance in general or for specific applications.

Design of a three-loop asymmetric coil producing a homogeneous radiofrequency B1 field gradient along the axis of a vertical sample tube

A coil system generating a vertical radio-frequency (rf) field gradient (B1 gradient) has been built for surrounding, in a horizontal magnet, a vertical sample (object) of axial symmetry. The system comprises three coaxial loops with an overall shape either spherical or ellipsoidal. The geometry has been theoretically and experimentally devised for producing a very uniform gradient (cancellation of B1 derivatives from second order up to sixth order) in the central region where a vertical receiver/transmitter coil is installed. The latter is of the saddle-shaped type and is geometrically and electrically decoupled from the gradient coil system. This receiver/transmitter coil not only ensures an optimal signal reception but, in addition, is able to deliver perfectly homogeneous rf hard pulses which are mandatory in most NMR experiments. In its present design, the system delivers a uniform gradient in a limited region but could be extended at will. Its main advantages over static field gradients (B0 gradients) appear clearly in the case of very short transverse relaxation times. This property has been emphasized in the case of experiments leading to the measurement of diffusion coefficients. Also, this system would be suitable for chemical shift imaging (CSI) experiments as confirmed by a preliminary test experiment.

Tailored birdcage resonator for magnetic resonance imaging at 7 T using 3D printing

This study outlines the design, construction and characterization of a tailored low-cost linear birdcage (BC) resonator for magnetic resonance imaging at 7T. Typically, different BC designs found in literature are well described in theory but lack crucial information for practical realization. This is challenging, as theoretical and practical aspects often differ greatly from each other, especially in the field of high frequency technology. We propose a simple and open-source 3D printable design which results in a working BC if the instructions in this publication are followed. The aim is to open up the possibility of building a functioning BC with simple means and a budget below 750€, even for users without a great deal of expertise in MRI coil building. We demonstrate that the BC can achieve a good B1 field homogeneity using the double angle method. The proposed design is qualitatively compared to a commercially available resonator. Both perform equally well in terms of SNR and image quality.

Optimized radio frequency coil for noninvasive magnetic resonance relaxation detection of human finger

Noninvasive NMR measurement of human tissues, such as fingers, to achieve early detection for metabolic diseases is of important significance. The NMR relaxation measurements have a wide application prospect due to simplicity, portability, and low cost, as the static magnetic field is not required to be highly homogeneous. However, the inhomogeneous radiofrequency (RF) magnetic field (B1) results in errors in the magnetic resonance relaxation times. This is inevitable in in-vivo localized human tissue measurements with a portable MR scanner, as signals from tissues close to the edge of RF coil are excited with a different B1 field amplitude. A novel RF coil termed T coil with high B1 field homogeneity is presented. Numerical simulation and phantom measurements were implemented. The novel RF coil was compared with a regular solenoid coil and a variable width coil. In-vivo experiments were performed. The T coil has a better B1 field homogeneity than the regular solenoid coil and the variable width coil, producing more accurate magnetic resonance relaxation times. Improved detection accuracy has been achieved with the T coil. This work may promote the development of noninvasive human tissue diagnosis based on NMR relaxation methods.

Implementation and prospective clinical validation of AI‐based planning and shimming techniques in cardiac MRI

PurposeCardiovascular magnetic resonance (CMR) is a vital diagnostic tool in the management of cardiovascular diseases. The advent of advanced CMR technologies combined with artificial intelligence (AI) has the potential to simplify imaging, reduce image acquisition time without compromising image quality (IQ), and improve magnetic field uniformity. Here, we aim to implement two AI‐based deep learning techniques for automatic slice alignment and cardiac shimming and evaluate their performance in clinical cardiac magnetic resonance imaging (MRI).MethodsTwo deep neural networks were developed, trained, and validated on pre‐acquired cardiac MRI datasets (>500 subjects) to achieve automatic slice planning and shimming (implemented in the scanner) for CMR. To examine the performance of our automated cardiac planning (EasyScan) and AI‐based shim (AI shim), two prospective studies were performed subsequently. For the EasyScan validation, 10 healthy subjects underwent two identical CMR protocols: with manual cardiac planning and with AI‐based EasyScan to assess protocol scan time difference and accuracy of cardiac plane prescriptions on a 1.5 T clinical MRI scanner. For the AI shim validation, a total of 20 subjects were recruited: 10 healthy and 10 cardio‐oncology patients with referrals for a CMR examination. Cine images were obtained with standard cardiac volume shim and with AI shim to assess signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), overall IQ (sharpness and MR image degradation), ejection fraction (EF), and absolute wall thickening. A hybrid statistical method using of nonparametric (Wilcoxon) and parametric (t‐test) assessments was employed for statistical analyses.ResultsCMR protocol with AI‐based plane prescriptions, EasyScan, minimized operator dependence and reduced overall scanning time by over 2 min (∼13 % faster, p < 0.001) compared to the protocol with manual cardiac planning. EasyScan plane prescriptions also demonstrated more accurate (less plane angulation errors from planes manually prescribed by a certified cardiac MRI technologist) cardiac planes than previously reported strategies. Additionally, AI shim resulted in improved B0 field homogeneity. Cine images obtained with AI shim revealed a significantly higher SNR (12.49%; p = 0.002) than those obtained with volume shim (volume shim: 32.90 ± 7.42 vs. AI shim: 37.01 ± 8.87) for the left ventricle (LV) myocardium. LV myocardium CNR was 12.48% higher for cine imaging with AI shim (149.02 ± 39.15) than volume shim (132.49 ± 33.94). Images obtained with AI shim resulted in sharper images than those obtained with volume shim (p = 0.012). The LVEF and absolute wall thickening also showed that differences exist between the two shimming methods. The LVEF by AI shim was shown to be slightly larger than LVEF by volume shim in two groups: 2.87% higher with AI shim for the healthy group and 1.70% higher with AI shim for the patient group. The LV absolute wall thickening (in mm) also showed that differences exist between shimming methods for each group with larger changes observed in the patient group (healthy: 3.31%, p = 0.234 and patient group: 7.29%, p = 0.059).ConclusionsCMR exams using EasyScan for cardiac planning demonstrated accelerated cardiac exam compared to the CMR protocol with manual cardiac planning. Improved and more uniform B0 magnetic field homogeneity also achieved using AI shim technique compared to volume shimming.

Performance evaluation of a PET insert for preclinical MRI in stand-alone PET and simultaneous PET\u2013MRI modes

BackgroundThis study aimed to evaluate the performance of a preclinical PET insert in three configurations: as a stand-alone unit outside the MRI bore, inside the bore of a cryogen-free 3T MRI and, finally, while performing simultaneous PET/MRI studies.MethodsThe PET insert consists of two rings of six detectors, each detector comprising 8 × 12 SiPMs reading out dual offset layers of pixelated LYSO crystals with a 1.4-mm pitch. The inner diameter is 60 mm, transaxial field of view (FoV) 40 mm and axial FoV 98 mm. Evaluation was based on NEMA NU 4-2008 guidelines with appropriate modifications. Spatial resolution and sensitivity were measured inside and outside the MR bore. Image quality, count rate and quantitative performance were measured in all three configurations. The effect of temperature stability on PET sensitivity during fast spin echo sequences was also evaluated. B0 field homogeneity and T1 and T2 relaxation times were measured using a water-filled phantom, with and without simultaneous PET operation. Finally, PET and MRI scans of a mouse injected with 10 MBq [18F]NaF and a mouse injected with 16 MBq [18F]FDG were performed in sequential and simultaneous modes.ResultsPeak absolute sensitivity was 10.15% with an energy window of 250–750 keV. Absolute sensitivity values outside and inside the MR bore with MR idle agreed to within 0.1%. Outside the MR bore, spatial resolution was 1.21/1.59 mm FWHM (radial/tangential) 5 mm from the centre of the FoV which compared well with 1.19/1.26 mm FWHM inside the MR bore. There were no substantial differences between all three scan configurations in terms of peak NEC rate (175 kcps at 17 MBq), scatter or random fractions. Uniformity and recovery coefficients were also consistent between scanning modes. B0 field homogeneity and T1 and T2 relaxation times were unaltered by the presence of the PET insert. No significant differences were observed between sequential and simultaneous scans of the animals.ConclusionsWe conclude that the performance of the PET insert and MRI system is not significantly affected by the scanning mode.

B0-Shimming Methodology for Affordable and Compact Low-Field Magnetic Resonance Imaging Magnets

Low-field (B0&amp;lt; 0.2 T) magnetic resonance imaging (MRI) is emerging as a low cost, point-of-care alternative to provide access to diagnostic imaging technology even in resource scarce environments. MRI magnets can be constructed based on permanent neodymium-iron-boron (NdFeB) magnets in discretized arrangements, leading to substantially lower mass and costs. A challenge with these designs is, however, a goodB0field homogeneity, which is needed to produce high quality images free of distortions. In this work, we describe an iterative approach to build a low-field MR magnet based on aB0-shimming methodology using genetic algorithms. The methodology is tested by constructing a small bore (inner bore diameter = 130 mm) desktop MR magnet (&amp;lt;15 kg) at a field strength ofB0= 0.1 T and a target volume of 4 cm in diameter. The configuration consists of a base magnet and shim inserts, which can be placed iteratively without modifying the base magnet assembly and without changing the inner dimensions of the bore or the outer dimensions of the MR magnet. Applying the shims,B0field inhomogeneity could be reduced by a factor 8 from 5,448 to 682 ppm in the target central slice of the magnet. Further improvements of these results can be achieved in a second or third iteration, using more sensitive magnetic field probes (e.g., nuclear magnetic resonance based magnetic field measurements). The presented methodology is scalable to bigger magnet designs. The MR magnet can be reproduced with off-the-shelf components and a 3D printer and no special tools are needed for construction. All design files and code to reproduce the results will be made available as open source hardware.

Development of an L-band resonator optimized for fast scan EPR imaging of the mouse head.

To develop a novel resonator for high-quality fast scan electron paramagnetic resonance (EPR) and EPR/NMR co-imaging of the head and brain of mice at 1.25 GHz. Resonator dimensions were scaled to fit the mouse head with maximum filling factor. A single-loop 6-gap resonator of 20 mm diameter and 20 mm length was constructed. High resonator stability was achieved utilizing a fixed position double coupling loop. Symmetrical mutually inverted connections rendered it insensitive to field modulation and fast scan. Coupling adjustment was provided by a parallel-connected variable capacitor located at the feeding line at λ/4 distance. To minimize radiation loss, the shield around the resonator was supplemented with a planar conductive disc that focuses return magnetic flux. Coupling of the resonator loaded with the mouse head was efficient and easy. This resonator enabled high-quality in vivo 3D EPR imaging of the mouse head following intravenous infusion of nitroxide probes. With this resonator and rapid scan EPR system, 4 ms scans were acquired in forward and reverse directions so that images with 2-scan 3,136 projections were acquired in 25 s. Head images were achieved with resolutions of 0.4 mm, enabling visualization of probe localization and uptake across the blood-brain barrier. This resonator design provides good sensitivity, high stability, and B1 field homogeneity for in vivo fast scan EPR of the mouse head and brain, enabling faster measurements and higher resolution imaging of probe uptake, localization, and metabolism than previously possible.

B0 field homogeneity recommendations, specifications, and measurement units for MRI in radiation therapy.

The purpose is: (a) Relate magnetic resonance imaging (MRI) quality recommendations for radiation therapy (RT) to B0 field homogeneity; (b) Evaluate manufacturer specifications of B0 homogeneity for 34 commercial whole-body MRI systems based on the MRI quality recommendations and RT application; (c) Measure field homogeneity in five commercial MRI systems and one commercial MRI-Linac used in RT and compare the results with their B0 homogeneity specifications. Magnetic resonance imaging quality recommendations for spatial integrity, image blurring, fat saturation, and null banding in RT were developed based on the literature. Guaranteed (maximum) and typical B0 field homogeneity specifications for various diameter spherical volumes (DSVs) were provided by GE, Philips, Siemens, and Canon. For each system, the DSV that conforms to each MRI quality recommendation and anatomical RT application was estimated based on the manufacturer specifications. B0 field homogeneity was measured on six MRI systems including Philips (1.5T), Siemens (1.5 and 3T), and ViewRay MRI (0.35T) systems using 24 and 35cm DSV spherical phantoms. Two measurement techniques were used: (a) MRI using phase contrast field mapping to measure peak-to-peak (pk-pk), volume root mean square (VRMS), and standard deviation (SD); and (b) Magnetic resonance (MR) spectroscopy by acquiring a volumetric free induction decay (FID) to measure full width at half maximum (FWHM). The measurements were used to assess: (a) conformance with the manufacturer specifications; and (b) the relationship between the various field homogeneity measurement units. Measurements were made with and without gradient shimming (gradshim) or second-order active shimming. Multiple comparisons, analysis of variance (ANOVA), and Pearson correlations were performed to assess the dependence of pk-pk, VRMS, SD, and FWHM measurements of field homogeneity on shim volume, level of shim, and MRI system. For a 40cm DSV, the B0 homogeneity specifications ranged from 0.35 to 5ppm (median=0.75ppm) VRMS for 1.5T systems and 0.2 to 1.4ppm (median=0.5ppm) VRMS for 3T systems. The usable DSVs ranged from 16 to 49cm (median=35cm) based on the image quality recommendations and the manufacturer specifications. There was general compliance between the six measured field homogeneities and manufacturer specifications although signal dephasing was observed in two systems at<35cm DSV. The relationships between pk-pk, VRMS, SD, and FWHM varied based on MRI system, shim volume, and quality of shim. However, VRMS and SD measurements were highly correlated. The delineation of the diseased lesion from organs at risk is the main priority for RT. Therefore, field homogeneity performance for RT must minimize image blurring and image artifacts (null bands and signal dephasing) while optimizing spatial integrity and fat saturation. Based on the specifications and recommendations for field homogeneity, some MRI systems are not well suited to meet the strict demands of RT particularly for the large imaging volumes used in body MRI. VRMS and SD measurements of B0 field homogeneity tend to be more stable and sensitive to field inhomogeneities in RT applications than pk-pk and FWHM.

MR spectroscopy using static higher order shimming with dynamic linear terms (HOS-DLT) for improved water suppression, interleaved MRS-fMRI, and navigator-based motion correction at 7T.

PurposeTo interleave global and local higher order shimming for single voxel MRS. Single voxel MR spectroscopy requires optimization of the B0 field homogeneity in the region of the voxel to obtain a narrow linewidth and provide high data quality. However, the optimization of local higher order fields on a localized MRS voxel typically leads to large field offsets outside that volume. This compromises interleaved MR sequence elements that benefit from global field homogeneity such as water suppression, interleaved MRS‐fMRI, and MR motion correction.MethodsA shimming algorithm was developed to optimize the MRS voxel homogeneity and the whole brain homogeneity for interleaved sequence elements, using static higher order shims and dynamic linear terms (HOS‐DLT). Shimming performance was evaluated using 6 brain regions and 10 subjects. Furthermore, the benefits of HOS‐DLT was demonstrated for water suppression, MRS‐fMRI, and motion corrected MRS using fat‐navigators.ResultsThe HOS‐DLT algorithm was shown to improve the whole brain homogeneity compared to an MRS voxel‐based shim, without compromising the MRS voxel homogeneity. Improved water suppression over the brain, reduced image distortions in MRS‐fMRI, and improved quality of motion navigators were demonstrated using the HOS‐DLT method.ConclusionHOS‐DLT shimming allowed for both local and global field homogeneity, providing excellent MR spectroscopy data quality, as well as good field homogeneity for interleaved sequence elements, even without the need for dynamic higher order shimming capabilities.

Improving B1 field homogeneity in dielectric tube resonators for EPR spectroscopy via controlled shaping of the dielectric insert

We propose an approach for improving the homogeneity of microwave magnetic field amplitude in a dielectric tube resonator for electron paramagnetic resonance spectroscopy at X-band. The improvement is achieved by “shaping” (controllable variation of the outer diameter of a dielectric insert along its axial direction). Various shaping scenarios based on the principle of discrete solenoids and electromagnetic calculations have been considered. The dielectric insert having the most promising shape was manufactured from a bismuth germanate single crystal. The shaped insert increases the area at B1 > 0.9 B1max from 5.06 to 7.36 mm. Higher sensitivity and lower likelihood of quantitative errors have been achieved in pulse EPR experiments for “long” samples (whose length was comparable to that of the dielectric insert) in a shaped dielectric insert.

Comparison of FASTMAP and &amp;lt;i&amp;gt;B&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;0&amp;lt;/sub&amp;gt; Field Map Shimming at 4T: Magnetic Field Mapping Using a Gradient-Echo Pulse Sequence

Local susceptibility variations result in B0 field inhomogeneities, causing distortions and signal losses in MR imaging. Susceptibility variations become stronger with increasing B0 magnetic field strength. Active shimming is used to generate corrective magnetic fields, which can be used to improve B0 field homogeneity. FASTMAP is an effective shimming technique for computing optimal coil currents, which uses data from six projection directions (or columns): this technique is routinely used for shimming cubic volumes of interest (VOIs). In this paper, we propose several improvements to FASTMAP at 4T. For each shim coil, using a modified 3D gradient-echo pulse sequence, we compute B0 inhomogeneity maps and project them onto eight 1st and 2nd order spherical harmonic functions. This process is repeated for shim currents between -15,000 to 15,000 with increments of 5000 Digital to Analog Converter (DAC) units, and is used to compute the gradient between spherical harmonic coefficients and DAC values for all 8 shim coils—along with the R2 values of linear fits. A method is proposed (based on R2 values) to further refine optimal shim currents in respective coils. We present an analysis that is numerically robust and completely flexible in the selection of the VOIs for shimming. Performance analyses, phantom results, and in vivo results of a human brain are presented, comparing our methods with the FASTMAP method.

Improving resolution of solid state NMR in dense molecular hydrogen

Recent advancements in radio frequency resonator designs have led to the implementation of nuclear magnetic resonance in diamond anvil cells (DACs) at pressures well above 100 GPa. However, a relatively low resolution and the absence of decoupling sequences complicate the analysis of the results of solid state NMR in DACs. Here, we present the first application of homonuclear Lee-Goldburg (LG) decoupling on high density molecular hydrogen up to 64 GPa. Lenz lens based two-dimensional resonator structures were found to generate a homogeneous B1 field across sample cavities as small as 12 pl, a prerequisite for optimal decoupling. At ideal LG conditions, the broad 1H resonance of molecular ortho-hydrogen was narrowed 1600-fold, resulting in linewidths of 3.1 ppm.

Effect of linewidth on estimation of metabolic concentration when using water lineshape spectral model fitting for single voxel proton spectroscopy at 7 T.

Good B0 field homogeneity is considered an essential requirement to obtain high-quality MRS data. Many commonly used spectral fitting methods assume that all metabolite signals have Lorentzian or Gaussian shapes. However, B0 inhomogeneity can both broaden the linewidth and modify the lineshape. In this study, it is hypothesized that a realistic metabolite fitting model, which accounts for B0 homogeneity on the basis of the water lineshape, will improve the accuracy of estimation of metabolite concentrations. In-vivo water suppressed/unsuppressed single voxel spectroscopy signals were acquired under three different B0 field homogeneity regimes. Individual realistic basis sets were created for each acquisition. Frequency-domain spectral fitting with LCModel was used to quantify the metabolite concentrations with fitting uncertainties given in terms of the Cramer-Rao lower bound. The quantification results obtained using the water lineshape basis set yielded similar concentrations independent of linewidth and showed a larger fitting error as the linewidth increased. The conventional approach, however quantifies metabolite concentrations with greater variations despite showing a supposedly improved fitting quality. The water lineshape basis set achieved single voxel spectroscopy accuracy that is less sensitive to the linewidth compared to the conventional spectral fitting method for the range of linewidths used in this study, but the precision deteriorated with worsening B0 field inhomogeneity. The beneficial effect was ascribed to a reduction in the number of degrees of freedom when using the water lineshape to generate the basis set.

Development of custom RF coils for use in a small animal platform for magnetic resonance-guided focused ultrasound hyperthermia compatible with a clinical MRI scanner

Three different magnetic resonance imaging (MRI) coils were developed and assessed for use with an experimental platform designed to generate hyperthermia in mice using magnetic resonance-guided focused ultrasound (MRgFUS). An ergonomic animal treatment bed was integrated with MRI coils. Three different coil designs optimized for small targets were tested, and performance in targeting and conducting accurate temperature imaging was evaluated. Two transmit/receive surface coils of different diameters (4 and 7 cm) and a transmit-only/receive-only (TORO) coil were used. A software platform was developed to provide real-time targeting and temperature maps and to deliver controlled ultrasound exposure. MR thermometry was conducted on different targets, including fresh chicken breasts and mouse cadavers. Multiple experiments were performed in which tissues were targeted with high reproducibility. The TORO coil was the most resilient to temperature drift, resulting in an increase in the calculated temperature of 0.29 ± 0.12 °C, compared to 1.27 ± 0.13 °C and 0.47 ± 0.04 °C for the medium and small coils, respectively. Controlled closed-loop hyperthermia exposure was successfully performed with all three coils. Considering all assessments, the TORO coil exhibited the best overall performance for thermometry acquisition when accounting for stability, precision, temperature spread and resilience to temperature drift. B1 maps of the three coils confirmed that the TORO coil exhibited the most homogeneous B1 field, which explained the improved thermometry performance. The use of coils specifically designed for small targets within the proposed experimental platform allowed accurate thermometry during hyperthermia.

Development and Testing of a Magnetic Resonance (MR) Conditional Afterloader for Source Tracking in Magnetic Resonance Imaging-Guided High-Dose-Rate (HDR) Brachytherapy

For the purpose of magnetic resonance imaging (MRI)-guided high-dose-rate (HDR) brachytherapy, a prototype magnetic resonance (MR) conditional afterloader was developed. This study demonstrates the development and testing of the prototype, while operating simultaneously with MRI. In combination with an MR-based method for HDR source localization, this development enables treatment verification of HDR brachytherapy. Additionally, this allows a direct reconstruction of the source dwell positions after catheter insertion (when using a dummy source) and introduction of a clinical workflow where the patient remains in the same position during dwell position reconstruction, treatment planning and irradiation. A prototype MR conditional afterloader was developed by providing radiofrequency (RF) shielding and a plastic source cable containing a dummy source. Simultaneous functioning of the afterloader and MRI acquisition was tested in an experimental setting where the afterloader was placed next to the scanner and programmed to send the source to predefined positions within a phantom, while acquiring MR images. The HDR source positions were determined using MR artifact simulation and matching of the MR images to the simulated artifact. Additionally, the impact of the presence and use of the afterloader on the MRI performance was investigated by assessment of RF interference, signal-to-noise ratio (SNR), and B0 field homogeneity. The experiments demonstrated that the prototype MR conditional afterloader and the MRI scanner fully functioned while operating simultaneously, without influencing the other system. The step sizes between the source positions obtained from the MR images corresponded with the afterloader settings. Besides, the MRI performance tests demonstrated no deterioration due to the presence or functioning of the afterloader next to the scanner. This research has demonstrated the feasibility of simultaneous MR acquisition and employment of an MR conditional afterloader. This development enables real-time HDR source localization for treatment verification of MRI-guided HDR brachytherapy using an MR conditional afterloader.

Multiple Parallel Round Leg Design for Quadrature Birdcage Coil in Ultrahigh-Field MRI

Copper foil has been widely employed in conventional radio frequency (RF) birdcage coils for magnetic resonance imaging (MRI). However, for ultrahigh-field (UHF) MRI, current density distribution on the copper foil is concentrated on the surface and the edge due to proximity effect. This increases the effective resistance and distorts the circumferential sinusoidal current distribution on the birdcage coils, resulting in low signal-to-noise ratio (SNR) and inhomogeneous distribution of RF magnetic (B1) field. In this context, multiple parallel round wires were proposed as legs of a birdcage coil to optimize current density distribution and to improve the SNR and the B1 field homogeneity. The design was compared with three conventional birdcage coils with different width flat strip surface legs for a 9.4 T (T) MRI system, e.g., narrow-leg birdcage coil (NL), medium-leg birdcage coil (ML), broad-leg birdcage coil (BL) and the multiple parallel round wire-leg birdcage coil (WL). Studies were carried out in in vitro saline phantom as well as in vivo mouse brain. WL showed higher coil quality factor Q and more homogeneous B1 field distribution compared to the other three conventional birdcage coils. Furthermore, WL showed 12, 10 and 13% SNR increase, respectively, compared to NL, ML and BL. It was proposed that conductor’s shape optimization could be an effective approach to improve RF coil performance for UHF MRI.

High-Quality 3-Dimensional 1H Magnetic Resonance Spectroscopic Imaging of the Prostate Without Endorectal Receive Coil Using A Semi-LASER Sequence.

Inclusion of 3-dimensional H magnetic resonance spectroscopic imaging (3D-H-MRSI) in routine multiparametric MRI of the prostate requires good quality spectra and easy interpretable metabolite maps of the whole organ obtained without endorectal coil in clinically feasible acquisition times. We evaluated if a semi-LASER pulse sequence with gradient offset independent adiabaticity refocusing pulses (GOIA-sLASER) for volume selection can meet these requirements. Thirteen patients with suspicion of prostate cancer and 1 patient known to have prostate cancer were examined at 3 T with a multichannel body-receive coil. A 3D-H-MRSI sequence with GOIA-sLASER volume selection (echo time, 88 milliseconds) was added to a routine clinical multiparametric MRI examination of these patients. Repetition times from 630 to 1000 milliseconds and effective voxel sizes of approximately 0.9 and 0.6 cm were tested. Spectral components were quantified by LCModel software for quality assessment and to construct choline and citrate maps. Three-dimensional MRSI of the prostate was successfully performed in all patients in measurement times of 5 to 10 minutes. Analysis of the multiparametric MRI examination or of biopsies did not reveal malignant tissue in the prostate of the 13 patients. In 1404 evaluated voxels acquired from 13 patients, the citrate resonance could be fitted with a high reliability (Cramér-Rao lower bound <30%), 100% for 7 × 7 × 7-mm voxels and 96 ± 7 in 6 × 6 × 6-mm voxels. The percentage of 7 × 7 × 7-mm voxels in which the choline signal was fitted with Cramér-Rao lower bound of less than 30% was approximately 50% at a TR of 630 milliseconds and increased to more than 80% for TRs of 800 milliseconds and above. In the patient with prostate cancer, choline was detectable throughout the prostate in spectra recorded at a TR of 700 milliseconds. The homogeneous B1 field over the prostate of the receive coil enabled the generation of whole organ metabolite maps, revealing choline and citrate variations between areas with normal prostate tissue, seminal vesicles, proliferative benign prostatic hyperplasia, and tumor. The good signal-to-noise ratio and low chemical shift artifacts of GOIA-sLASER at an echo time of 88 milliseconds enable acquisition of high-quality 3D-H-MRSI of the prostate without endorectal coil in less than 10 minutes. This facilitates reconstruction of easy interpretable, quantitative metabolite maps for routine clinical applications of prostate MRSI.