Magnetic Resonance Imaging System Research Articles

Imaging modalities for the equine distal limb

There has been huge investment in computed tomography machines and standing equine magnetic resonance imaging systems across the UK in recent years, with several practices now offering both imaging modalities. It can be difficult to determine which of these imaging modalities is best for imaging the equine distal limb without a good understanding of how each of the imaging systems work practically, and which specific tissues are better evaluated using computed tomography compared to magnetic resonance imaging. It is often not known which tissue is injured when selecting an imaging modality to use. All standing systems, be it magnetic resonance imaging or computed tomography, are affected by patient motion, which can be markedly reduced by the use of general anaesthesia. This article describes the practical features of some of the various systems available for standing distal limb magnetic resonance imaging and computed tomography, and advises on case selection for each modality.

Gray–white matter contrast reversal on T1-weighted spin-echo in postmortem brain

PurposeThe image contrast of postmortem magnetic resonance imaging (MRI) may differ from that of antemortem MRI because of circulator arrest, changes in postmortem tissue, and low-body-temperature scanning conditions. In fact, we have found that the signal intensity of white matter (WM) on T1-weighted spin-echo (T1WSE) images of the postmortem brain was lower than that of gray matter (GM), which resulted in image contrast reversal between GM and WM relative to the living brain. However, the reason for this phenomenon is unclear. Therefore, the aim of this study is to clarify the reason why image contrast reversal occurs between GM and WM of the postmortem brain. Materials and methodsTwenty-three corpses were included in the study (mean age, 60.6 years; range: 19–60 years; mean rectal temperature at scan, 6.9℃; range: 4–11℃). On a 1.5 T MRI system, postmortem T1W-SE MRI of the brain was conducted in the 23 corpses prior to medico-legal autopsy. Next, T1 and T2 of the GM and WM at the level of the basal ganglia were determined in the same participants using inversion recovery and multiple SE sequences, respectively. The proton density (PD) was also calculated from the T1 and T2 images (in the same slice). ResultsT1W-SE image contrast between the GM and WM of all postmortem brains was inverted relative to the living brain. T1 (579 ms in GM and 307 ms in WM) and PD (64 in GM and 44 in WM) of the postmortem brain decreased compared with the living brain. While T1 of WM/GM remained below 1 even postmortem, the PD of WM/GM decreased. T2 (110 ms in GM and 98 ms in WM) of the postmortem brain did not differ from the living brain. ConclusionThe decrease in PD of WM/GM in the postmortem brain may be the major driver of contrast reversal between the GM and WM relative to the living brain.

Wearable Coaxially-Shielded Metamaterial for Magnetic Resonance Imaging.

Recent advancements in metamaterials have yielded the possibility of a wireless solution to improve signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI). Unlike traditional closely packed local coil arrays with rigid designs and numerous components, these lightweight, cost-effective metamaterials eliminate the need for radio frequency cabling, baluns, adapters, and interfaces. However, their clinical adoption is limited by their low sensitivity, bulky physical footprint, and limited, specific use cases. Herein, a wearable metamaterial developed using commercially available coaxial cable, designed for a 3.0T MRI system is introduced. This metamaterial inherits the coaxially-shielded structure of its constituent cable, confining the electric field within and mitigating coupling to its surroundings. This ensures safer clinical adoption, lower signal loss, and resistance to frequency shifts. Weighing only 50g, the metamaterial maximizes its sensitivity by conforming to the anatomical region of interest. MRI images acquired using this metamaterial with various pulse sequences achieve an SNR comparable or even surpass that of a state-of-the-art 16-channel knee coil. This work introduces a novel paradigm for constructing metamaterials in the MRI environment, paving the way for the development of next-generation wireless MRI technology.

Turbo spin-echo-based enhanced acceleration-selective arterial spin labeling without electrocardiography or peripheral pulse unit triggering and contrast enhancement for lower extremity MRA

PurposeLower extremity magnetic resonance angiography (MRA) without electrocardiography (ECG) or peripheral pulse unit (PPU) triggering and contrast enhancement is beneficial for diagnosing peripheral arterial disease (PAD) while avoiding synchronization failure and nephrogenic systemic fibrosis. This study aimed to compare the diagnostic performance of turbo spin-echo-based enhanced acceleration-selective arterial spin labeling (eAccASL) (TSE-Acc) of the lower extremities with that of turbo field-echo-based eAccASL (TFE-Acc) and triggered angiography non-contrast enhanced (TRANCE). MethodsNine healthy volunteers and a patient with PAD were examined on a 3.0 Tesla magnetic resonance imaging (MRI) system. The artery-to-muscle signal intensity ratio (SIR) and contrast-to-noise ratio (CNR) were calculated. The arterial visibility (1: poor, 4: excellent) and artifact contamination (1: severe, 4: no) were independently assessed by two radiologists. Phase-contrast MRI and digital subtraction angiography were referenced in a patient with PAD. Friedman's test and a post-hoc test according to the Bonferroni-adjusted Wilcoxon signed-rank test were used for the SIR, CNR, and visual assessment. p < 0.05 was considered statistically significant. ResultsNo significant differences in nearly all the SIRs were observed among the three MRA methods. Higher CNRs were observed with TSE-Acc than those with TFE-Acc (anterior tibial artery, p = 0.014; peroneal artery, p = 0.029; and posterior tibial artery, p = 0.014) in distal arterial segments; however, no significant differences were observed upon comparison with TRANCE (all p > 0.05). The arterial visibility scores exhibited similar trends as the CNRs. The artifact contamination scores with TSE-Acc were significantly lower (but within an acceptable level) compared to those with TFE-Acc. In the patient with PAD, the sluggish peripheral arteries were better visualized using TSE-Acc than those using TFE-Acc, and the collateral and stenosis arteries were better visualized using TSE-Acc than those using TRANCE. ConclusionPeripheral arterial visualization was better with TSE-Acc than that with TFE-Acc in lower extremity MRA without ECG or PPU triggering and contrast enhancement, which was comparable with TRANCE as the reference standard. Furthermore, TSE-Acc may propose satisfactory diagnostic performance for diagnosing PAD in patients with arrhythmia and chronic kidney disease.

Towards high-field applications: high-performance, low-cost iron-based superconductors

Abstract High magnetic fields play a crucial role in advancing basic sciences, fusion energy, and magnetic resonance imaging systems. However, the widespread use of high-field magnets requires affordable high-temperature superconducting wires that can carry large supercurrents. Iron-based superconductors offer an economically attractive solution to push forward important yet costly scientific programs, such as nuclear fusion reactors and next-generation particle accelerators. In this review, we start with the fabrication of iron-based superconducting wires and tapes and continue to discuss several key factors governing the current transport properties. State-of-the-art wires and tapes are introduced with emphasis on grain boundary characteristics, flux pinning, and anisotropy. The flexible conductor architecture enables low cost, large mechanical strength, and high thermal stability. Recent progress in practical applications, including superconducting joints and insert coils, is also reviewed. Finally, we propose several key questions faced by iron-based superconductors in future practical applications.

Noninvasive intervention by transcranial ultrasound stimulation: Modulation of neural circuits and its clinical perspectives.

Low-intensity focused transcranial ultrasound stimulation (TUS) is an emerging noninvasive technique capable of stimulating both the cerebral cortex and deep brain structures with high spatial precision. This method is recognized for its potential to comprehensively perturb various brain regions, enabling the modulation of neural circuits, in a manner not achievable through conventional magnetic or electrical brain stimulation techniques. The underlying mechanisms of neuromodulation are based on a phenomenon where mechanical waves of ultrasound kinetically interact with neurons, specifically affecting neuronal membranes and mechanosensitive channels. This interaction induces alterations in the excitability of neurons within the stimulated region. In this review, we briefly present the fundamental principles of ultrasound physics and the physiological mechanisms of TUS neuromodulation. We explain the experimental apparatus and procedures for TUS in humans. Due to the focality, the integration of various methods, including magnetic resonance imaging and magnetic resonance-guided neuronavigation systems, is important to perform TUS experiments for precise targeting. We then review the current state of the literature on TUS neuromodulation, with a particular focus on human subjects, targeting both the cerebral cortex and deep subcortical structures. Finally, we outline future perspectives of TUS in clinical applications in psychiatric and neurological fields.

Subwavelength dielectric waveguide for efficient travelling-wave magnetic resonance imaging

Magnetic resonance imaging (MRI) has diverse applications in physics, biology, and medicine. Uniform excitation of nuclei spins through circular-polarized transverse magnetic component of electromagnetic field is vital for obtaining unbiased tissue contrasts. However, achieving this in the electrically large human body poses a significant challenge, especially at ultra-high fields (UHF) with increased working frequencies (≥297 MHz). Canonical volume resonators struggle to meet this challenge, while radiative excitation methods like travelling-wave (TW) show promise but often suffer from inadequate excitation efficiency. Here, we introduce a new technique using a subwavelength dielectric waveguide insert that enhances both efficiency and homogeneity at 7 T. Through TE11-to-TM11 mode conversion, power focusing, wave impedance matching, and phase velocity matching, we achieved a 114% improvement in TW efficiency and mitigated the center-brightening effect. This fundamental advancement in TW MRI through effective wave manipulation could promote the electromagnetic design of UHF MRI systems.

Estimating the synaptic density deficit in Alzheimer's disease using multi-contrast CEST imaging.

In vivo noninvasive imaging of neurometabolites is crucial to improve our understanding of the underlying pathophysiological mechanism in neurodegenerative diseases. Abnormal changes in synaptic organization leading to synaptic degradation and neuronal loss is considered as one of the primary factors driving Alzheimer's disease pathology. Magnetic resonance based molecular imaging techniques such as chemical exchange saturation transfer (CEST) and magnetic resonance spectroscopy (MRS) can provide neurometabolite specific information which may relate to underlying pathological and compensatory mechanisms. In this study, CEST and short echo time single voxel MRS was performed to evaluate the sensitivity of cerebral metabolites to beta-amyloid (Aβ) induced synaptic deficit in the hippocampus of a mouse model of Alzheimer's disease. The CEST based spectra (Z-spectra) were acquired on a 9.4 Tesla small animal MR imaging system with two radiofrequency (RF) saturation amplitudes (1.47 μT and 5.9 μT) to obtain creatine-weighted and glutamate-weighted CEST contrasts, respectively. Multi-pool Lorentzian fitting and quantitative T1 longitudinal relaxation maps were used to obtain metabolic specific apparent exchange-dependent relaxation (AREX) maps. Short echo time (TE = 12 ms) single voxel MRS was acquired to quantify multiple neurometabolites from the right hippocampus region. AREX contrasts and MRS based metabolite concentration levels were examined in the ARTE10 animal model for Alzheimer's disease and their wild type (WT) littermate counterparts (age = 10 months). Using MRS voxel as a region of interest, group-wise analysis showed significant reduction in Glu-AREX and Cr-AREX in ARTE10, compared to WT animals. The MRS based results in the ARTE10 mice showed significant decrease in glutamate (Glu) and glutamate-total creatine (Glu/tCr) ratio, compared to WT animals. The MRS results also showed significant increase in total creatine (tCr), phosphocreatine (PCr) and glutathione (GSH) concentration levels in ARTE10, compared to WT animals. In the same ROI, Glu-AREX and Cr-AREX demonstrated positive associations with Glu/tCr ratio. These results indicate the involvement of neurotransmitter metabolites and energy metabolism in Aβ-mediated synaptic degradation in the hippocampus region. The study also highlights the feasibility of CEST and MRS to identify and track multiple competing and compensatory mechanisms involved in heterogeneous pathophysiology of Alzheimer's disease in vivo.

Magnetic Resonance Imaging in Breast Cancer Tissue In Vitro after PDT Therapy.

Photodynamic therapy (PDT) is increasingly used in modern medicine. It has found application in the treatment of breast cancer. The most common cancer among women is breast cancer. We collected cancer cells from the breast from the material received after surgery. We focused on tumors that were larger than 10 mm in size. Breast cancer tissues for this quantitative non-contrast magnetic resonance imaging (MRI) study could be seen macroscopically. The current study aimed to present findings on quantitative non-contrast MRI of breast cancer cells post-PDT through the evaluation of relaxation times. The aim of this work was to use and optimize a 1.5 T MRI system. MRI tests were performed using a clinical scanner, namely the OPTIMA MR360 manufactured by General Electric HealthCare. The work included analysis of T1 and T2 relaxation times. This analysis was performed using the MATLAB package (produced by MathWorks). The created application is based on medical MRI images saved in the DICOM3.0 standard. T1 and T2 measurements were subjected to the Shapiro-Wilk test, which showed that both samples belonged to a normal distribution, so a parametric t-test for dependent samples was used to test for between-sample variability. The study included 30 sections tested in 2 stages, with consistent technical parameters. For T1 measurements, 12 scans were performed with varying repetition times (TR) and a constant echo time (TE) of 3 ms. For T2 measurements, 12 scans were performed with a fixed repetition time of 10,000 ms and varying echo times. After treating samples with PpIX disodium salt and bubbling with pure oxygen, PDT irradiation was applied. The cell relaxation time after therapy was significantly shorter than the cell relaxation time before PDT. The cells were exposed to PpIX disodium salt as the administered pharmacological substance. The study showed that the therapy significantly affected tumor cells, which was confirmed by a significant reduction in tumor cell relaxation time on the MRI results.

Quantifying human gray matter microstructure using neurite exchange imaging (NEXI) and 300 mT/m gradients

Abstract Biophysical models of diffusion tailored to quantify gray matter microstructure are gathering increasing interest. The two-compartment Neurite EXchange Imaging (NEXI) model has been proposed recently to account for neurites, extra-cellular space, and exchange across the cell membrane. NEXI parameter estimation requires multi-shell multi-diffusion time data and has so far only been implemented experimentally on animal data collected on a preclinical magnetic resonance imaging (MRI) set-up. In this work, the translation of NEXI to the human cortex in vivo was achieved using a 3 T Connectom MRI system with 300 mT/m gradients, that enables the acquisition of a broad range of b-values (0 – 7.5 ms/µm²) with a window covering short to intermediate diffusion times (20 – 49 ms) suitable for the characteristic exchange times (10 – 50 ms). Microstructure estimates of four model variants: NEXI, NEXIdot (its extension with the addition of a dot compartment), and their respective versions that correct for the Rician noise floor (NEXIRM and NEXIdot,RM) that particularly impacts high b-value signal, were compared. The reliability of estimates in each model variant was evaluated in synthetic and human in vivo data. In the latter, the intra-subject (scan-rescan) versus between-subjects variability of microstructure estimates was compared in the cortex. The better performance of NEXIRM highlights the importance of correcting for Rician bias in the NEXI model to obtain accurate estimates of microstructure parameters in the human cortex, and the sensitivity of the NEXI framework to individual differences in cortical microstructure. This application of NEXI in humans represents a significant step, unlocking new avenues for studying neurodevelopment, aging, and various neurodegenerative disorders.

Utility of 1.5 Tesla MRI Scanner in the Management of Small Sample Sizes Driven from 3D Breast Cell Culture.

The aim of this work was to use and optimize a 1.5 Tesla magnetic resonance imaging (MRI) system for three-dimensional (3D) images of small samples obtained from breast cell cultures in vitro. The basis of this study was to design MRI equipment to enable imaging of MCF-7 breast cancer cell cultures (about 1 million cells) in 1.5 and 2 mL glass tubes and/or bioreactors with an external diameter of less than 20 mm. Additionally, the development of software to calculate longitudinal and transverse relaxation times is described. Imaging tests were performed using a clinical MRI scanner OPTIMA 360 manufactured by GEMS. Due to the size of the tested objects, it was necessary to design additional receiving circuits allowing for the study of MCF-7 cell cultures placed in glass bioreactors. The examined sample's volume did not exceed 2.0 mL nor did the number of cells exceed 1 million. This work also included a modification of the sequence to allow for the analysis of T1 and T2 relaxation times. The analysis was performed using the MATLAB package (produced by MathWorks). The created application is based on medical MR images saved in the DICOM3.0 standard which ensures that the data analyzed are reliable and unchangeable in an unintentional manner that could affect the measurement results. The possibility of using 1.5 T MRI systems for cell culture research providing quantitative information from in vitro studies was realized. The scanning resolution for FOV = 5 cm and the matrix was achieved at a level of resolution of less than 0.1 mm/pixel. Receiving elements were built allowing for the acquisition of data for MRI image reconstruction confirmed by images of a phantom with a known structure and geometry. Magnetic resonance sequences were modified for the saturation recovery (SR) method, the purpose of which was to determine relaxation times. An application in MATLAB was developed that allows for the analysis of T1 and T2 relaxation times. The relaxation times of cell cultures were determined over a 6-week period. In the first week, the T1 time value was 1100 ± 40 ms, which decreased to 673 ± 59 ms by the sixth week. For T2, the results were 171 ± 10 ms and 128 ± 12 ms, respectively.

Near-infrared-II fluorescence/magnetic resonance double modal imaging of transplanted stem cells using lanthanide co-doped gadolinium oxide nanoparticles.

To ensure maximum therapeutic safety and efficacy of stem cell transplantation, it is essential to observe the kinetics of behavior, accumulation, and engraftment of transplanted stem cells in vivo. However, it is difficult to detect transplanted stem cells with high sensitivity by conventional in vivo imaging technologies. To diagnose the kinetics of transplanted stem cells, we prepared multifunctional nanoparticles, Gd2O3 co-doped with Er3+ and Yb3+ (Gd2O3: Er, Yb-NPs), and developed an in vivo double modal imaging technique with near-infrared-II (NIR-II) fluorescence imaging and magnetic resonance imaging (MRI) of stem cells using Gd2O3: Er, Yb-NPs. Gd2O3: Er, Yb-NPs were transduced into adipose tissue-derived stem cells (ASCs) through a simple incubation process without cytotoxicity under certain concentrations of Gd2O3: Er, Yb-NPs and were found not to affect the morphology of ASCs. ASCs labeled with Gd2O3: Er, Yb-NPs were transplanted subcutaneously onto the backs of mice, and successfully imaged with good contrast using an in vivo NIR-II fluorescence imaging and MRI system. These data suggest that Gd2O3: Er, Yb-NPs may be useful for in vivo double modal imaging with NIR-II fluorescence imaging and MRI of transplanted stem cells.

Advancing MRI capacitors: Design, fabrication, and characterization

AbstractIn today's healthcare landscape, MRI (magnetic resonance imaging) plays a pivotal role in early diagnosis and treatment decisions. However, the quest for higher frequencies to achieve micron‐level resolutions poses technological challenges. This paper focuses on coils and capacitors, with special attention to air gap considerations for movable electrodes. The paper delves into the development of adjustable capacitors using high‐fired NP0 ceramic, tailored for 128‐MHz MRI systems. These capacitors boast impressive specs: a 3‐kV breakdown voltage (VBreakdown), a 3‐pF minimum capacitance (Cmin), a 30‐pF maximum capacitance (CMax), and a 1.5‐pF stepwise capacitance variation (Cvariation step). Notably, they enhance breakdown voltage while preserving frequency stability with MgTiCa ceramic between the fixed and mobile electrode, critical for adaptive blocks. Leveraging geometrical parameters analyzed through the ANSYS simulator, the authors meticulously designed an optimized 25–40 pF capacitor with a 15‐mm height and 1600 mm2 surface area. Post‐fabrication and sintering, extensive testing using a vector network analyzer across 10 MHz to 100 MHz validated performance against simulated expectations.

High-Q metasurface signal isolator for 1.5T surface coil magnetic resonance imaging on the go.

The combination of surface coils and metamaterials remarkably enhance magnetic resonance imaging (MRI) performance for significant local staging flexibility. However, due to the coupling in between, impeded signal-to-noise ratio (SNR) and low-contrast resolution, further hamper the future growth in clinical MRI. In this paper, we propose a high-Q metasurface decoupling isolator fueled by topological LC loops for 1.5T surface coil MRI system, increasing the magnetic field up to fivefold at 63.8 MHz. We have employed a polarization conversion mechanism to effectively eliminate the coupling between the MRI metamaterial and the radio frequency (RF) surface transmitter-receiver coils. Furthermore, a high-Q metasurface isolator was achieved by taking advantage of bound states in the continuum (BIC) for extremely high-resolution MRI and spectroscopy. An equivalent physical model of the miniaturized metasurface design was put forward through LC circuit analysis. This study opens up a promising route for the easy-to-use and portable surface coil MRI scanners.

Bridging the gap: improving correspondence between low-field and high-field magnetic resonance images in young people.

Portable low-field-strength magnetic resonance imaging (MRI) systems represent a promising alternative to traditional high-field-strength systems with the potential to make MR technology available at scale in low-resource settings. However, lower image quality and resolution may limit the research and clinical potential of these devices. We tested two super-resolution methods to enhance image quality in a low-field MR system and compared their correspondence with images acquired from a high-field system in a sample of young people. T1- and T2-weighted structural MR images were obtained from a low-field (64mT) Hyperfine and high-field (3T) Siemens system in N = 70 individuals (mean age = 20.39 years, range 9-26 years). We tested two super-resolution approaches to improve image correspondence between images acquired at high- and low-field: (1) processing via a convolutional neural network ('SynthSR'), and (2) multi-orientation image averaging. We extracted brain region volumes, cortical thickness, and cortical surface area estimates. We used Pearson correlations to test the correspondence between these measures, and Steiger Z tests to compare the difference in correspondence between standard imaging and super-resolution approaches. Single pairs of T1- and T2-weighted images acquired at low field showed high correspondence to high-field-strength images for estimates of total intracranial volume, surface area cortical volume, subcortical volume, and total brain volume (r range = 0.60-0.88). Correspondence was lower for cerebral white matter volume (r = 0.32, p = 0.007, q = 0.009) and non-significant for mean cortical thickness (r = -0.05, p = 0.664, q = 0.664). Processing images with SynthSR yielded significant improvements in correspondence for total brain volume, white matter volume, total surface area, subcortical volume, cortical volume, and total intracranial volume (r range = 0.85-0.97), with the exception of global mean cortical thickness (r = 0.14). An alternative multi-orientation image averaging approach improved correspondence for cerebral white matter and total brain volume. Processing with SynthSR also significantly improved correspondence across widespread regions for estimates of cortical volume, surface area and subcortical volume, as well as within isolated prefrontal and temporal regions for estimates of cortical thickness. Applying super-resolution approaches to low-field imaging improves regional brain volume and surface area accuracy in young people. Finer-scale brain measurements, such as cortical thickness, remain challenging with the limited resolution of low-field systems.

Modern low-field MRI.

This narrative review explores recent advancements and applications of modern low-field (≤ 1 Tesla) magnetic resonance imaging (MRI) in musculoskeletal radiology. Historically, high-field MRI systems (1.5T and 3T) have been the standard in clinical practice due to superior image resolution and signal-to-noise ratio. However, recent technological advancements in low-field MRI offer promising avenues for musculoskeletal imaging. General principles of low-field MRI systems are being introduced, highlighting their strengths and limitations compared to high-field counterparts. Emphasis is placed on advancements in hardware design, including novel magnet configurations, gradient systems, and radiofrequency coils, which have improved image quality and reduced susceptibility artifacts particularly in musculoskeletal imaging. Different clinical applications of modern low-field MRI in musculoskeletal radiology are being discussed. The diagnostic performance of low-field MRI in diagnosing various musculoskeletal pathologies, such as ligament and tendon injuries, osteoarthritis, and cartilage lesions, is being presented. Moreover, the discussion encompasses the cost-effectiveness and accessibility of low-field MRI systems, making them viable options for imaging centers with limited resources or specific patient populations. From a scientific standpoint, the amount of available data regarding musculoskeletal imaging at low-field strengths is limited and often several decades old. This review will give an insight to the existing literature and summarize our own experiences with a modern low-field MRI system over the last 3years. In conclusion, the narrative review highlights the potential clinical utility, challenges, and future directions of modern low-field MRI, offering valuable insights for radiologists and healthcare professionals seeking to leverage these advancements in their practice.

Preclinical magnetic resonance imaging and spectroscopy in the fields of radiological technology, medical physics, and radiology.

Magnetic resonance imaging (MRI) is an indispensable diagnostic imaging technique used in the clinical setting. MRI is advantageous over X-ray and computed tomography (CT), because the contrast provided depends on differences in the density of various organ tissues. In addition to MRI systems in hospitals, more than 100 systems are used for research purposes in Japan in various fields, including basic scientific research, molecular and clinical investigations, and life science research, such as drug discovery, veterinary medicine, and food testing. For many years, additional preclinical imaging studies have been conducted in basic research in the fields of radiation technology, medical physics, and radiology. The preclinical MRI research includes studies using small-bore and whole-body MRI systems. In this review, we focus on the animal study using small-bore MRI systems as "preclinical MRI". The preclinical MRI can be used to elucidate the pathophysiology of diseases and for translational research. This review will provide an overview of previous preclinical MRI studies such as brain, heart, and liver disease assessments. Also, we provide an overview of the utility of preclinical MRI studies in radiological physics and technology.

Encoding scheme design for gradient-free, nonlinear projection imaging using Bloch-Siegert RF spatial encoding in a low-field, open MRI system

Eliminating conventional pulsed B0-gradient coils for magnetic resonance imaging (MRI) can significantly reduce the cost of and increase access to these devices. Phase shifts induced by the Bloch-Siegert shift effect have been proposed as a means for gradient-free, RF spatial encoding for low-field MR imaging. However, nonlinear phasor patterns like those generated from loop coils have not been systematically studied in the context of 2D spatial encoding. This work presents an optimization algorithm to select an efficient encoding trajectory among the nonlinear patterns achievable with a given hardware setup. Performance of encoding trajectories or projections was evaluated through simulated and experimental image reconstructions. Results show that the encodings schemes designed by this algorithm provide more efficient spatial encoding than comparison encoding sets, and the method produces images with the predicted spatial resolution and minimal artifacts. Overall, the work demonstrates the feasibility of performing 2D gradient-free, low-field imaging using the Bloch-Siegert shift which is an important step towards creating low-cost, point-of-care MR systems.

Restricted diffusion characteristics in oscillating gradient spin echo with mesoscopic phantom

In diffusion magnetic resonance imaging, oscillating gradient spin echo (OGSE) has an extremely short diffusion time if motion probing gradient (MPG) is applied to the waveform. Further, it can detect microstructural specificity. OGSE changes sensitivity to spin displacement velocity based on the MPG phase. The current study aimed to investigate the restricted diffusion characteristics of each OGSE waveform using the capillary phantom with various b-values, frequencies, and MPG phases. We performed OGSE (b-value = 300, 500, 800, 1200, 1600, and 2000 s/mm2) for the sine and cosine waveforms using the capillary phantom (6, 12, 25, 50, and 100 μm and free water) with a 9.4-T experimental magnetic resonance imaging system and a solenoid coil. We evaluated the axial and radial diffusivity (AD, RD) of each structure size. The output current of the MPG was assessed with an oscilloscope and analyzed with the gradient modulation power spectra by fast Fourier transform.In sine, the sidelobe spectrum was enhanced with increasing frequency, and the central spectrum slightly increased. The difference in RD was detected at 6 and 12 μm; however, it did not depend on the structure scale at 50 or 100 μm and free water. In cosine, the diffusion spectrum was enhanced, whereas the central spectrum decreased with increasing frequency. Both AD and RD in cosine had a frequency dependence, and AD and RD increased with a higher frequency regardless of structure size. AD and RD in either sine or cosine had no evident b-value dependence. We evaluated the OGSE-restricted diffusion characteristics. The measurements obtained diffusion information similar to the pulsed gradient spin echo. Hence, the cosine measurements indicated that a higher frequency could capture faster diffusion within the diffusion phenomena.

Dynamic Imaging of Blood Coagulation Within the Hematoma of Patients With Acute Hemorrhagic Stroke.

The dynamics of blood clot (combination of Hb [hemoglobin], fibrin, and a higher concentration of aggregated red blood cells) formation within the hematoma of an intracerebral hemorrhage is not well understood. A quantitative neuroimaging method of localized coagulated blood volume/distribution within the hematoma might improve clinical decision-making. The deoxyhemoglobin of aggregated red blood cells within extravasated blood exhibits a higher magnetic susceptibility due to unpaired heme iron electrons. We propose that coagulated blood, with higher aggregated red blood cell content, will exhibit (1) a higher positive susceptibility than noncoagulated blood and (2) increase in fibrin polymerization-restricted localized diffusion, which can be measured noninvasively using quantitative susceptibility mapping and diffusion tensor imaging. In this serial magnetic resonance imaging study, we enrolled 24 patients with acute intracerebral hemorrhage between October 2021 to May 2022 at a stroke center. Patients were 30 to 70 years of age and had a hematoma volume >15 cm3 and National Institutes of Health Stroke Scale score >1. The patients underwent imaging 3×: within 12 to 24 (T1), 36 to 48 (T2), and 60 to 72 (T3) hours of last seen well on a 3T magnetic resonance imaging system. Three-dimensional anatomic, multigradient echo and 2-dimensional diffusion tensor images were obtained. Hematoma and edema volumes were calculated, and the distribution of coagulation was measured by dynamic changes in the susceptibilities and fractional anisotropy within the hematoma. Using a coagulated blood phantom, we demonstrated a linear relationship between the percentage coagulation and susceptibility (R2=0.91) with a positive red blood cell stain of the clot. The quantitative susceptibility maps showed a significant increase in hematoma susceptibility (T1, 0.29±0.04 parts per millions; T2, 0.36±0.04 parts per millions; T3, 0.45±0.04 parts per millions; P<0.0001). A concomitant increase in fractional anisotropy was also observed with time (T1, 0.40±0.02; T2, 0.45±0.02; T3, 0.47±0.02; P<0.05). This quantitative neuroimaging study of coagulation within the hematoma has the potential to improve patient management, such as safe resumption of anticoagulants, the need for reversal agents, the administration of alteplase to resolve the clot, and the need for surgery.