Field Magnetic Resonance Imaging Systems Research Articles

Research on a portable shielding-free ultra-low field magnetic resonance imaging system

The portable light-weight magnetic resonance imaging system can be deployed in special occasions such as Intensive Care Unit (ICU) and ambulances, making it possible to implement bedside monitoring imaging systems, mobile stroke units and magnetic resonance platforms in remote areas. Compared with medium and high field imaging systems, ultra-low-field magnetic resonance imaging equipment utilizes light-weight permanent magnets, which are compact and easy to move. However, the image quality is highly susceptible to external electromagnetic interference without a shielded room and there are still many key technical problems in hardware design to be solved. In this paper, the system hardware design and environmental electromagnetic interference elimination algorithm were studied. Consequently, some research results were obtained and a prototype of portable shielding-free 50 mT magnetic resonance imaging system was built. The light-weight magnet and its uniformity, coil system and noise elimination algorithm and human brain imaging were verified. Finally, high-quality images of the healthy human brain were obtained. The results of this study would provide reference for the development and application of ultra-low-field magnetic resonance imaging technology.

Development of Ultra-Low Field Magnetic Resonance Imaging System at 1 mT

Ultra-low field (ULF) magnetic resonance imaging (MRI) is considered an innovative MRI technique that can be combined with magnetoencephalography (MEG). In this study, a 1-mT ULF-MRI system without a pre-polarization technique aiming at combining an MEG system is developed. The intensity of the static magnetic field B0 is decided considered to be 1 mT because of the limitation of the magnetic field exposed to our MEG sensor made of superconducting quantum interference devices (SQUIDs). A square-shaped (750 mm × 750 mm) B0-coil based on a Merritt-type configuration is fabricated. The inhomogeneity of B0 is less than 0.07 % in the ±125 mm area. Further, an induction coil is employed to detect the magnetic resonance signal instead of the SQUID sensors of the MEG to confirm the feasibility of realizing MRI measurement at 1 mT without the pre-polarization technique. Relaxation time measurements and MR imaging of a phantom made of a CuSO4 aqueous solution are performed. Reasonable longitudinal and transverse relaxation times are obtained, and clear shapes of the phantom are obtained using the gradient- and spin-echo pulse sequences with this ULF-MRI system. These results indicate the potential of the proposed 1-mT ULF-MRI to effectively integrate ULF-MRI and MEG systems without pre-polarization.

US regulatory considerations for low field magnetic resonance imaging systems.

Although there has been a resurgence of interest in low field magnetic resonance imaging (MRI) systems in recent years, low field MRI is not a new concept. FDA has a long history of evaluating the safety and effectiveness of MRI systems encompassing a wide range of field strengths. Many systems seeking marketing authorization today include new technological features (such as artificial intelligence), but this does not fundamentally change the regulatory paradigm for MR systems. In this review, we discuss some of the US regulatory considerations for low field magnetic resonance imaging (MRI) systems, including applicability of existing laws and regulations and how the U.S. Food and Drug Administration (FDA) evaluates low field MRI systems for market authorization. We also discuss regulatory considerations in the review of low field MRI systems incorporating novel AI technology. We foresee that MRI systems of all field strengths intended for general diagnostic use will continue to be evaluated for marketing clearance by the metric of substantial equivalence set forth in the premarket notification pathway.

Manipulating Bandwidth of Cryogenically Cooled Receive-Only MRI Detectors

Cryogenically cooled MRI coils demonstrate extremely high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factors leading to a very narrow bandwidth. This bandwidth can be increased in a controlled manner using a preamplifier decoupling. In this work, an MRI coil design procedure is explained and an example showing over 2 times bandwidth increase in a cryogenic coil for 13C isotope imaging in 3-tesla MRI systems is demonstrated. The achieved wider bandwidth preserves the spectral components of magnetic resonance signals and makes coils less susceptive to detuning due to parasitic coupling to surrounding structures. The presented method is also useful in other applications of high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> coils, for instance, in low field magnetic resonance imaging systems.

Cost-efficient analysis of a portable low field magnetic resonance imaging system for prostate cancer diagnosis

PurposeMagnetic resonance imaging (MRI)-guided procedures are typically performed under high magnetic field strengths (1.5-3 Tesla) and are expensive. Recently, technological progress has led to MRIs that are low-cost but similarly high performing despite lower field strengths (58-74mT). Our purpose was to evaluate the cost-efficiency of implementing a low-field MRI for targeted prostate cancer biopsy.Materials and methodsWe utilized time-driven activity-based costing (TDABC) to establish the overall costs based on cost rates and total times for every step. Thorough literature searches and consultations with two board certified radiologists, the process maps were created, and the duration of each step was determined by experience and previously published data. A sensitivity cost analysis was also performed to create several cost scenarios for the low-field MRI system.ResultsTotal procedure times, performed under moderate sedation, were 61 ± 14.5 mins and 57 ± 23 mins for transperineal targeted biopsy (TPB) with low-field MRI and in-bore MRI-guided transrectal biopsy (IB-TBx) with high-field MRI. In the low-field MRI pathway, 61% of intra-operative time was spent on patient preparation and performing the prebiopsy MRI, however, this step did not require a urologist or radiologist expertise, subsequently keeping costs lower. A sensitivity analysis of the low-field MRI cost at 5% to 50% (in 5% increments) of a high-field MRI produced a difference from $888.13 to $879.18 favoring the lower-field system, respectively.ConclusionsA portable, low field, office-based MRI system offers a cost-efficient alternative to the high-field MRI. Further studies assessing cost effectiveness while taking into consideration sampling accuracy are needed.

A Comparison of the Distortion in the Same Field MRI and MR-Linac System With a 3D Printed Phantom.

PurposeA 3D printed geometric phantom was developed that can be scanned with computed tomography (CT) and magnetic resonance imaging (MRI) to measure the geometric distortion and determine the relevant dose changes.Materials and MethodsA self-designed 3D printed photosensitive resin phantom was used, which adopts grid-like structures and has 822 1 cm2 squares. The scanning plan was delivered by three MRI scanners: the Elekta Unity MR-Linac 1.5T, GE Signa HDe 1.5T, and GE Discovery-sim 750 3.0T. The geometric distortion comparison was concentrated on two 1.5T MRI systems, whereas the 3.0T MRI was used as a supplemental experiment. The most central transverse images in each dataset were selected to demonstrate the plane distortion. Some mark points were selected to analyze the distortion in the 3D direction based on the plane geometric distortion. A treatment plan was created with the off-line Monaco system.ResultsThe distortion increases gradually from the center to the outside. The distortion range is 0.79 ± 0.40 mm for the Unity, 1.31 ± 0.56 mm for the GE Signa HDe, and 2.82 ± 1.48 mm for the GE Discovery-sim 750. Additionally, the geometric distortion slightly affects the actual planning dose of the radiotherapy.ConclusionGeometric distortion increases gradually from the center to the outside. The distortion values of the Unity were smaller than those of the GE Signa HDe, and the Unity has the smallest geometric distortion. Finally, the Unity’s dose variation best matched with the standard treatment plan.

Recent Progress in Birdcage RF Coil Technology for MRI System.

The radio frequency (RF) coil is one of the key components of the magnetic resonance imaging (MRI) system. It has a significant impact on the performance of the nuclear magnetic resonance (NMR) detection. Among numerous practical designs of RF coils for NMR imaging, the birdcage RF coil is the most popular choice from low field to ultra-high field MRI systems. In the transmission mode, it can establish a strong and homogeneous transverse magnetic field B1 for any element at its Larmor frequency. Similarly, in the reception mode, it exhibits extremely high sensitivity for the detection of even faint NMR signals from the volume of interest. Despite the sophisticated 3D structure of the birdcage coil, the developments in the design, analysis, and implementation technologies during the past decade have rendered the development of the birdcage coils quite reasonable. This article provides a detailed review of the recent progress in the birdcage RF coil technology for the MRI system.

RF shield parallel-plate waveguide for travelling-wave MRI experiments at 3 T

Ultra high field magnetic resonance imaging systems suffer from the standing wave problems produced by conventional radiofrequency coils. This causes inhomogeneous fields degrading the image quality. To overcome this problems, propagation of RF waves by antennas inside a waveguide is suggested. We assessed the feasibility of using the magnet RF shield with aluminium strips to form a parallel-plate waveguide for travelling-wave magnetic resonance imaging at 3 T. Imaging results showed that a number of constraints must be solved before good image quality can be obtained. A brief discussion is presented on the possible sources or error and interference. Despite these limitations, results are really encouraging to continue investigating the implementation of this approach.

Feasibility of low field MRI and proteomics for the analysis of Tissue Engineered bone

There is a need for the replacement of bone tissue that has been lost due to trauma. On average, there are an estimated 10 million cases of fracture-related injuries in the US every year [Amini et al 2012 Crit. Rev. Biomed. Eng. 40, 363–408]. A potential solution is offered by tissue engineering which aims to create biological substitutes for natural tissues. Magnetic resonance imaging (MRI) has been proposed to assess engineering outcomes, however one challenge is the use of high field, which is costly and with regulatory effect on the cells not well understood. This study aims to illustrate the capability of a table top, low field MRI system in monitoring the development of Tissue Engineered bone, supported through proteome analysis. MR images, spin-spin (T2) relaxation, and proteomics were acquired for samples assessed at three in vitro developmental time points: week 0, week 2, and week 4. A ∼45% decrease in the T2 relaxation time was observed from weeks 0 to 4. During cellular differentiation, proteomics revealed the appearance of proteins [alkaline phosphatase (ALP), collagen I and III, fibronectin, transforming growth factor beta (TGF-β), and latent transforming growth factor beta binding protein-1 (LTBP1)] consistent with osteogenesis. The data demonstrated that low field MRI can offer a cost-effective alternative to study the development of Tissue Engineered bone, in addition to proteomics analysis of the developing tissue.

“Molecular” MR imaging at high fields

Magnetic resonance imaging (MRI) and spectroscopy (MRS) have contributed considerably to clinical radiology, and a variety of MR techniques have been developed to evaluate pathological processes as well as normal tissue biology at the cellular and molecular level. However, in comparison to nuclear imaging, MRI has relatively poor sensitivity for detecting true molecular changes or for detecting the presence of targeted contrast agents, though these remain under active development. In recent years very high field (7T and above) MRI systems have been developed for human studies and these provide new opportunities and technical challenges for molecular imaging. We identify 5 types of intrinsic contrast mechanisms that do not require the use of exogenous agents but which can provide molecular and cellular information. We can derive information on tissue composition by (i) imaging different nuclei, especially sodium (ii) exploiting chemical shift differences as in MRS (iii) exploiting specific relaxation mechanisms (iv) exploiting tissue differences in the exchange rates of molecular species such as amides or hydroxyls and (v) differences in susceptibility. The increased signal strength at higher fields enables higher resolution images to be acquired, along with increased sensitivity to detecting subtle effects caused by molecular changes in tissues.

The first T2-weighted imaging of stage IB1–IIB cervical cancer by ultra–high field 7.0T MRI using an endorectal antenna: A prospective feasibility study

Objectives: Magnetic resonance imaging (MRI), particularly T2-weighted sequences, is increasingly used in the workup of cervical cancer. To improve image quality, we studied the feasibility of T2-weighted cervical cancer imaging on an ultra–high field MRI system. Central to our approach is the use of an endorectal antenna which should boost the signal-to-noise ratio (SNR), thus allowing higher resolutions. Here, we will assess patient discomfort and quantify its SNR increase. Methods: We conducted a prospective feasibility study on 18 stage IB1 to IIB histology-proven cervical cancer patients who had a routine 1.5T MRI in their workup. All underwent an additional 7.0T MRI, which included transversal, sagittal, and oblique T2-weighted turbo spin echo sequences. Seven external transmit and receive dipole antennas were positioned around the pelvis, and combined with a single in-house built endorectal monopole antenna. Its signal receive optimum was placed 6 to 10 cm beyond the anodermal transition. Patients scored the level of discomfort related to the antenna from 0 (none whatsoever) to 10 (worst imaginable). Results: Patients had a median age of 39.3 years (range, 25.3–66.5 years) and stage IB1 (n = 9), IB2 (n = 3), IIA (n = 1) or IIB (n = 5) cervical cancer. Discomfort of antenna placement and removal was low with a median score of 1 (range 0–5) and 0 (range 0–2), respectively. The endorectal antenna increased the SNR at the cervix by a mean factor of 1.8 and by 2.9 in a 30-mm radius from the antenna. No adverse events occurred related to the antenna. Although some artefacts remained, 7.0T MRI was found qualitatively superior to the clinical 1.5T MRI. In Fig. 1, the corresponding oblique (perpendicular to the cervical canal) T2-weighted slices of an exemplary stage IIB2 patient are shown. Conclusions: We demonstrated the feasibility and qualitative increase of cervical cancer imaging with 7.0T MRI. The use of an endorectal antenna is well tolerated and enables optimal signal capture with a substantial increase in SNR.

Incidence of elbow injuries in adolescent baseball players: screening by a low field magnetic resonance imaging system specialized for small joints.

The aim of this preliminary study was to examine the capability of screening for elbow injuries induced by baseball using a low field small joint MRI system. Sixty-two players in the 4th-6th elementary school grades, with ages ranging from 9 to 12 years, participated in this study. Screening for elbow injuries was performed using a low-magnetic-field (0.2-T) magnetic resonance imaging (MRI) system designed for examinations of small joints of the extremities. Gradient-echo coronal, sagittal, and short-tau inversion recovery (STIR) coronal images of the dominant arm used for pitching were obtained to identify medial collateral ligament (MCL) injuries with or without avulsion fracture and osteochondritis dissecans. All 62 examinations were performed successfully, with 26 players (41.9 %) showing positive findings, all being confined to the MCL. No child showed bone damage. All criteria in the MRI evaluation of injuries showed high agreement rates and kappa values between two radiologists. Screening for early detection of elbow injuries in junior Japanese baseball players can be successfully performed using a low-field MRI system specialized for small joints. The percentage of MCL injury without avulsion fracture was unexpectedly high (41.9 %).

Opportunities and Challenges of 7 Tesla Magnetic Resonance Imaging: A Review.

The desire to achieve clinical ultra-high magnetic resonance imaging (MRI) systems stems from the fact that higher field strength leads to higher signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and spatial resolution. During last few years 7T MRI systems have become a quasi standard for ultra-high field MRI (UhFMRI) systems. This review presents a detailed account of opportunities and challenges associated with a clinical 7T MRI system for cranial and extracranial imaging. As with all of the previous transitions to higher field strengths, the switch from high to UhFMRI is not easy. The engineering and scientific community have to overcome challenges like magnetic field inhomogeneity, patient safety and comfort issues, and cost and related problems in order to achieve a clinically viable UhFMRI system. In addition, a large number of clinical studies are still required to show the improvements in quality of diagnostics that would come with 7T MRI, in order to bring such a research tool to the clinic.

Design of Crisscrossed Double-Layer Birdcage Coil for Improving B1+Field Homogeneity for Small-Animal Magnetic Resonance Imaging at 300 MHz

We design a crisscrossed double-layer birdcage (DLBC) coil by modifying the coil geometry of a standard single-layer BC (SLBC) coil to enhance the homogeneity of transmitting magnetic flux density (B₁+) along the main magnetic field (B 0 )-direction for small-animal magnetic resonance imaging (MRI) at 300 MHz. The performance assessment of the crisscrossed DLBC coil is conducted by computational analysis with the finitedifference time domain method (FDTD) and compared with SLBC coil in terms of the B₁ and the B₁+ distribution. As per the computational calculation studies, the mean value in the two-dimensional B₁+ map obtained at the mid-axial slice with the proposed DLBC coil is slightly lower than that obtained with the SLBC coil, but the B₁ + value of the DLBC coil in the outermost plane (40 mm away from the central plane) shows improvements of 19.3% and 24.8% over the SLBC coil B₁+ value when simulating a spherical phantom and realistic mouse body modeling. These simulation results indicate that, the B₁ + homogeneity along the z-direction was improved by using DLBC configuration. Our approach enables B₁ + homogeneity improvement along the zdirection, and it can also be applied to ultra-high field (UHF) MRI systems.

Two-dimensional compressed sensing using the cross-sampling approach for low-field MRI systems.

A compressed sensing method using a cross sampling and self-calibrated off-resonance correction is proposed. Estimation of the magnetic field inhomogeneity based on image registration enables the off-resonance correction with no additional radio-frequency pulses or acquisitions. In addition to this advantage, a fast and straightforward calculation was achieved by using the first-order components of the magnetic field inhomogeneity. Imaging experiments using a phantom and a chemically fixed mouse demonstrated practical benefits in improving blurring and artifacts in magnetic resonance images in low field magnetic resonance imaging systems.

Magnetic Resonance Imaging of Surgical Implants Made from Weak Magnetic Materials

Materials with high magnetic susceptibility cause local inhomogeneities in the main field of the magnetic resonance (MR) tomograph. These inhomogeneities lead to loss of phase coherence, and thus to a rapid loss of signal in the image. In our research we investigated inhomogeneous field of magnetic implants such as magnetic fibers, designed for inner suture during surgery. The magnetic field inhomogeneities were studied at low magnetic planar phantom, which was made from four thin strips of magnetic tape, arranged grid-wise. We optimized the properties of imaging sequences with the aim to find the best setup for magnetic fiber visualization. These fibers can be potentially exploited in surgery for internal stitches. Stitches can be visualized by the magnetic resonance imaging (MRI) method after surgery. This study shows that the imaging of magnetic implants is possible by using the low field MRI systems, without the use of complicated post processing techniques (e.g., IDEAL).

Optimization of Flux Transformer for Optically Pumped Atomic Magnetometer in Ultra-Low Field MRI Systems

An ultra-low field (ULF) magnetic resonance imaging (MRI) system with an optically pumped atomic magnetometer (AM) has recently been proposed. Because AM does not require cryogenic cooling, it can easily measure extremely small magnetic fields. However, to measure magnetic resonance (MR) signals with high sensitivity in ULF-MRI systems with AMs, the resonant frequencies of the sample and alkali metal in the AM must be same. To satisfy this requirement, a flux transformer (FT) has been proposed to detect MR signals. In this study, the simulations in the output coil of the FT and pseudo-MR signal measurements were performed to improve the signal-to-noise ratio (SNR) in the remote detection of MR signals by using AM with FT. The simulations and measurement results indicate that, to improve the SNR of the detector, the output coil of the FT should be placed in the vicinity of a glass cell, and the number of turns and radius of the output coil need to be optimized.

Radiofrequency-induced Heating near Fixed Orthodontic Appliances in High Field MRI Systems at 3.0 Tesla

To assess radiofrequency (RF)-induced heating of fixed orthodontic appliances during acquisition of three different sequences in magnetic resonance imaging (MRI) at 3 Tesla. Ten commonly used fixed orthodontic appliances were investigated utilizing a phantom head and simulating the in vivo intraoral situation. A 3 Tesla MRI system (Intera, Philips Medical Systems, Best, The Netherlands) was used to acquire T1w spin-echo (T1 SE), T1w turbo spin-echo (T1 TSE) and T1w gradient-echo (T1 GRE) sequences in axial orientation. Continuous temperature measurement was performed with a dedicated four channel fluoroptic thermometry system. For each orthodontic appliance temperature probes were placed at three predefined sites in order to perform temperature measurements during MR imaging. The fourth temperature probe was fixed to the neck of the phantom head and served as the reference. Mean temperature alterations were determined for all appliances. Temperature elevations ranged from -0.3 degrees C to 0.2 degrees C and were negligible for all orthodontic appliances investigated. There was no difference in mean temperature alteration for any of the three imaging sequences. Based on the data of our experimental study the radiofrequency-induced heating of orthodontic brackets during high field MRI at 3 Tesla can be categorized as negligibly low. Even the clinical routine examination of the head at 3 Tesla using high-energetic pulse sequences can be applied without hesitation in patients with fixed orthodontic appliances.

Optic Nerve Imaging in Multiple Sclerosis

In this article I review the last 10 years of progress in the imaging of the optic nerve with a particular focus on applications to multiple sclerosis (MS). Development of magnetic resonance imaging (MRI) of the optic nerve has lagged behind imaging of other parts of the CNS. These limitations are due to technical challenges related to the small size and mobility of the optic nerves and artefacts caused by surrounding cerebrospinal fluid, orbital fat, and air-bone interfaces. Nonetheless the last 10 years has seen significant progress with regard to detecting optic nerve atrophy following optic neuritis, the use of fat- and CSF-suppressed high resolution imaging, the ability to measure magnetization transfer ratio and diffusivity in the optic nerve, and the emergence of SPIR-FLAIR for increasing sensitivity to inflammatory demyelination. Remaining challenges include further reduction of movement artifacts, testing ultra-high field MRI systems and dedicated surface coils, and developing automated segmentation techniques to improve the reproducibility of quantitative measurements. Finally the role of optic coherence tomography as a marker of retinal damage needs to be clarified further through correlations with MRI, clinical, and electrophysiologic data.

Low magnetic moment PIN diodes for high field MRI surface coils

Positive-intrinsic-negative (PIN) silicon diodes are commonly used in magnetic resonance imaging (MRI) coils to perform active or passive blocking and detuning, or to disable circuit functions. However, diode packages with large magnetic moments are known to cause image artifacts in high field MRI systems. In this study, diode packages with low magnetic moment were designed by compensating components of ferromagnetic nickel and paramagnetic tungsten with diamagnetic silver. The new diodes have an initial positive susceptibility up to fields of 1 T and a negative susceptibility from 1 to 7 T. Their magnetic moments are one to two orders of magnitude smaller than those of standard diodes; moments as small as 20 nJ/T at 7 T were achieved.