Superconducting MRI System Research Articles

Magnetic modeling of actively shielded rotating MRI magnets in the presence of environmental steel

Rotating MRI systems could enable novel integrated medical devices such as MRI-Linacs, MRI-xray-angiography systems, and MRI-proton therapy systems. This work aimed to investigate the feasibility of rotating actively shielded superconducting MRI magnets in the presence of environmental steel—in particular, construction steel in the floor of the installation site. Two magnets were investigated: a 1.0 T split bore magnet, and a 1.5 T closed bore magnet. Each magnet was scaled to emulate field strengths of 0.5, 1.0, and 1.5 T. Finite Element Modeling was used to simulate these magnets in the presence of a 3 × 4 m steel plate located 1250 mm or 1400 mm below the isocenter. There are two possible rotation directions: around the longitudinal (z) axis or around the transverse (x) axis. Each model was solved for rotation angles between 0 and 360° in 30° intervals around each of these axes. For each simulation, a 300 mm DSV was extracted and decomposed into spherical harmonics. For the closed-bore magnet, total induced perturbation for the zero degree rotation angle was 223, 432, and 562 μT peak-to-peak (pk–pk) for the 0.5, 1.0, and 1.5 T models respectively (steel at 1250 mm). For the split-bore magnet, the same numbers were 1477, 16747, and 1766 μT. The substantially higher perturbation for the split-bore magnet can be traced to its larger fringe field. For rotation around the z-axis, total perturbation does not change as a function of angle but is exchanged between different harmonics. For rotation around the x-axis, total perturbation is different at each rotation angle. For the closed bore magnet, maximum perturbations occurred for a 90° rotation around the transverse axis. For the split-bore magnet, the opposite was observed, with the same 90° rotation yielding total perturbation lower than the conventional position. In all cases, at least 95% of the total perturbation was composed of 1st and 2nd order harmonics. The presence of environmental steel poses a major challenge to the realization of an actively shielded rotating superconducting MRI system, requiring some novel form of shimming. Possible shimming strategies are discussed at length.

A target field approach to optimize cylindrical shim coils for superconducting MRI system

Target field method is used to design a specific form of the magnetic field which has been verified in many works of literature. And to restrict the current distribution on a rectangular or cylindrical surface with finite size, the current density is usually expanded by Fourier series with some unknown coefficients, which can be determined by setting the target-field points over the imaging region of interest, and then comparing the calculated field values from the Biot–Savart law and the ideal ones. The stream-function method is adopted to discrete the current density to generate winding patterns. But some design results can’t match the predetermined ones. Based on it, this paper proposed a new strategy for cylindrical shim coil design. Finally it can make the final design values meet the ideal fields with limited errors. This method can also be used to design gradient coils for high field superconducting MRI systems.

Design and Magnetic Field Simulation of 3.0T MRI Superconducting Magnet

The 3.0T superconducting MRI system has the advantages of high field strength and high magnetic field uniformity and stability. It has become the mainstream research direction of the modern clinical MRI system. This paper analyzes the research status of the MRI system and designs the superconducting magnet of the 3.0T MRI system according to the six-order coil principle. The Ansoft Maxwell magnetic field simulation software is used to simulate the superconducting magnetic field of the first 3.0T/850 superconducting MRI system magnet, which includes the passive shimming technology and the magnetic field shielding technology. According to the result of simulation analysis, the prototype of 3.0T MRI is build and optimized. Finally, the magnetic field strength, magnetic field uniformity, and magnetic field stability of the prototype are guided to achieve the expected target.

Determination of patellar ligament and anterior cruciate ligament geometry using MRI.

Ligament geometry is crucial to surgical treatment success in anterior cruciate ligament (ACL) injury. This study aimed to optimize the MRI technique to elucidate the geometry of the patellar ligament (PL) and ACL in vivo. A 1.5-T superconducting MRI system with a special surface coil and fast spin echo was used to acquire high-resolution T1-weighted images (H-T1WI) of the ACL. The sagittal plane angle was 10° to 15° towards the inner side of the vertical line of the tangent line axis of the femoral intercondylar fossa. The H-T1WI images of the PL were centered at the lower margin of the patella and the center of the tibial tuberosity. The lengths of the PL and ACL were measured using a Radworks 5.1 workstation. ACL and PL lengths were compared between left and right knees and between genders, and left PL length measurements obtained separately by three doctors underwent correlation analysis. The quality of the images satisfied the clinical measurement requirements. The duration of sagittal image acquisition was 2 min and 25 s. The average PL length was 42.20 ± 4.21 and 40.15 ± 4.00 mm, and the average ACL length was 36.98 ± 4.12 and 35.80 ± 4.67 mm, in male and female subjects, respectively. The intraclass correlation coefficients of the PL lengths obtained by the three specialists were greater than 0.997. This MRI technique provides highly stable and repeatable in vivo data of PL and ACL geometry relevant to ACL reconstruction surgery with PL grafts.

Abstract 94: Comparison of Ultrasound and Compact MRI for the Assessment of Cardiac Function in Mice

Introduction: Ultrasound (US) is a standard for the assessment of cardiovascular function. Pre-clinically, this is due the modality’s translational potential, its high temporal resolution and the absence of infrastructure. Recent advancements in novel high-performance compact MRI has made cardiac MRI a more accessible technique for assessing a variety of pathologies in murine models of CVD while reducing the complexity and costs traditionally associated with superconducting MRI systems. These developments may allow compact MRI to overcome some of the limitations of US meanwhile producing viable solutions for quantification along with new capabilities for cardiovascular biomarker assessment. Hypothesis: This study explores the relative capabilities of novel compact high-performance MRI compared to HFU for phenotypic analysis of mouse models of CVD. Methods: Four normal and one induced myocardial infarct C57BL/6 mice were imaged using both US and MRI. MI mice were prepared by ligation of the left anterior descending artery. In order to assess the ability of each modality to qualitatively and quantitatively characterize cardiac function, CINE loops of long axis and short axis slices were acquired with both systems and analyzed offline. In addition to traditional measures of cardiac function, strain analysis was performed using the HARP method for compact MRI and using speckle tracking for US. Finally, a cardiac MRI method for mapping and measuring infarct extent using gadolinium (Gd) contrast agents is described. Results and Conclusions: Although compact MRI has lower temporal resolution, when compared with US, it still provides many advantages in pre-clinical CVD imaging. In particular, because MRI signal is not attenuated by dense tissue or air, compact MRI can successfully generate artefact-free imaging of pathologies difficult or impossible to image with US, such as clearly visualizing and quantifying endo and epicardial borders. It also provides a whole body image and a “whole heart” image, making pre-clinical CV imaging easier for biologists and helping to reduce inter-operator variability. Finally, high sensitivity to Gd-based contrast agent with compact MRI enable new applications such as infarct quantification.

Frictional Behavior of Material Couples in Superconducting MRI Magnet Systems at 4.2 K

Windings in superconducting magnets operating at 4.2 K are highly susceptible to quenching caused by small frictional heat inputs. Small movements in the magnet system are inevitable during ramping due to the increasing electromagnetic forces. Friction pairs of polymer based materials have been investigated at 4.2 K to gain an understanding of their sliding behavior in conditions representative of superconducting MRI systems. The results indicate that polymer-polymer pairs experience unstable sliding behavior with repeated stick-slip whereas polymer-aluminum couples have stable sliding behavior up to high contact pressures of 20 MPa.

Research on Stability of ${\rm MgB}_{2}$ Superconducting Magnet for MRI

Magnesium diboride (MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) shows a wide application prospect in the field of low field DC application for its relatively simple structure and rather higher critical temperature than LTS. With the development of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite superconductor manufacturing technology and cryogenic technology, developing conduction cooled superconducting MRI system with MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite superconductor is becoming increasingly feasible. An MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting magnet for MRI was developed. The MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting magnet is used to investigate the fundamental scientific problem of the MRI main magnet, including field uniformity and stability, influencing factors of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite superconductor on field uniformity and stability. The superconducting magnet consists of 2 solenoids wound with MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wire with the length of 60 m. The magnet has an outer diameter of 169 mm, an iron core diameter of 135 mm, and air gap of 32 mm. The superconducting magnet is fabricated and tested. The central magnetic field reaches 1.0 T when the current of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> magnet is 290 A. The central magnetic field is 0.39 T when the current of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> magnet is 100 A. In this paper, the minimum quench energy, the quench propagation velocity and the minimum propagation zone of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite conductor are measured. Main magnet for MRI with MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite superconductor is analysed and proposed, including magnet design and test results. By theoretical modeling and simulation, the detailed quench propagation of the magnet is further analysed. Finally, the stability criteria of conduction-cooled MRI magnet are established.

Research on ${\rm MgB}_{2}$ Superconducting Magnet With Iron Core for MRI

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Magnesium diboride <formula formulatype="inline"><tex Notation="TeX">$({\rm MgB}_{2})$</tex></formula> shows a wide application prospect in the field of low field DC application for its relatively simple structure and rather higher critical temperature than LTS. With the development of <formula formulatype="inline"> <tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> composite superconductor manufacturing technology and cryogenic technology, developing conduction cooled superconducting MRI system with <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> composite superconductor is becoming increasingly feasible. An <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex> </formula> superconducting magnet for MRI was developed. The <formula formulatype="inline"> <tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> superconducting magnet is used to investigate the fundamental scientific problem of the MRI main magnet, including field uniformity and stability, influencing factors of <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> composite superconductor on field uniformity and stability. The superconducting magnet consists of 2 solenoids wound with <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> wire with the length of 60 m. The magnet has an outer diameter of 169 mm, an iron core diameter of 135 mm, and air gap of 32 mm. The superconducting magnet is fabricated and tested. The central magnetic field reaches 1.0 T when the current of the <formula formulatype="inline"> <tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> magnet is 290 A. The central magnetic field is 0.39 T when the current of the <formula formulatype="inline"> <tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> magnet is 100 A. In this paper, main magnet for MRI with <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> composite superconductor is analysed and proposed, including magnet design, test results, and field uniformity. </para>

Design and construction of a prototype high‐power B0 insert coil for field‐cycled imaging in superconducting MRI systems

AbstractField‐cycled MRI provides a noninvasive quantification of the concentration of bound (activated) targeted contrast agents. For field‐cycled imaging to be performed within a clinical full‐body superconducting MRI magnet, an auxiliary insertable magnet is required that can dynamically change the main field strength during the MR pulse sequence. Herein, the design and construction of a solenoidal insert coil is discussed. Design aspects including active shielding, thermal performance, forces, and ramping times are examined. Proper materials, construction methods, and interfacing for the insert coil with a clinical MRI are also covered. Finally, a field‐cycled experiment in a 1.5 T clinical MRI system is described, demonstrating the feasibility of this method and then future novel contrasts available with field‐cycled MRI. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 35B: 1–10, 2009

Development of a Compact Mouse MRI Using a Yokeless Permanent Magnet

A compact mouse MRI has been developed using a 1.0T yokeless permanent magnet and portable MRI console. The entire system was installed in a space measuring 2 m x 1 m. The imaging region was the cylindrical volume (35 mm diameter, 50 mm length) at the center of the magnet and was used for whole-brain or body imaging of mice. Whole-brain imaging took less than 90 min for T1- and T2-weighted 3D images with 2-mm slice thickness and 200-microm in-plane resolution. Body imaging took less than 30 min for T1-weighted spin-echo and FLASH 3D images with 0.5- to 1.0-mm slice thickness and 250- to 300-microm in-plane resolution. In addition to the compactness of the system, the mouse MRI has several advantages over high-field superconducting animal MRI systems in its accessibility to the specimen, similarity to clinical MRI in image contrast, capacity for biological isolation, and maintenance. The results obtained demonstrate the potential of this new system for routine imaging in biomedical laboratories.

Geometric distortion in clinical MRI systems

Recently, a 3D phantom that can provide a comprehensive and accurate measurement of the geometric distortion in MRI has been developed. Using this phantom, a full assessment of the geometric distortion in a number of clinical MRI systems (GE and Siemens) has been carried out and detailed results are presented in this paper. As expected, the main source of geometric distortion in modern superconducting MRI systems arises from the gradient field nonlinearity. Significantly large distortions with maximum absolute geometric errors ranged between 10 and 25 mm within a volume of 240 x 240 x 240 mm(3) were observed when imaging with the new generation of gradient systems that employs shorter coils. By comparison, the geometric distortion was much less in the older-generation gradient systems. With the vendor's correction method, the geometric distortion measured was significantly reduced but only within the plane in which these 2D correction methods were applied. Distortion along the axis normal to the plane was, as expected, virtually unchanged. Two-dimensional correction methods are a convenient approach and in principle they are the only methods that can be applied to correct geometric distortion in a single slice or in multiple noncontiguous slices. However, these methods only provide an incomplete solution to the problem and their value can be significantly reduced if the distortion along the normal of the correction plane is not small.

Design and evaluation of shielded gradient coils.

Techniques are described for the design of shielded gradient coils for superconducting MRI systems. These design methods are suited for constructing the most efficient gradient coil that meets a specified homogeneity requirement. Tradeoffs in coil design of efficiency with coil size and gap size are discussed. Residual eddy currents from coils constructed with a finite number of wires are calculated and give guidelines for the construction of efficient, whole-body gradient coils.

Book reviews

This book is based on the author's personal experience of more than 5000 documented cases studied with a 0.6 T superconducting MRI system in Amsterdam. A very brief summary of the basic physical principles of MRI is followed by a useful description of various technical considerations such as pulse sequences; some anatomical structures displayed by them; tissue differentiation; artefacts; flow phenomena and surface coils. A short section on MRI studies of special regions such as the pituitary follows. The 14 subsequent chapters cover all the major disease processes and include affections of the brain, orbit, larynx, neck and temporomandibular joints. Spinal disorders and the use of contrast agents are also covered.

5.超電導型MRIの臨床応用 : II.NMR-CT 特に画像と設置に対する問題点

Clinical applications of the superconducting MRI system were started in April of 1985 in our hospital. We had performed examinations of 575 cases for seven months at a field strength of 0.35T. In April of 1985 clinical studies using the 1.5T MRI system were started. Spatial resolution as much as X-ray CT was possible by the superconducting MRI system. It is thus very effective for the treatment of central nervous system, pelvis and extremity, disorders since it is hardly in fluenced by motion artifacts. In thoracic and abdominal regions spatial resolution by MRI was inferior to that by X-ray CT, but its high contrast resolution and ability to provide optional plane images show it highly useful for clinical diagnosis and therapeutic plan therapy. By 1.5T MRI system the spectroscopy of P-31 is possible as well as high resolutional MR imaging for high signal to noise ratio. Many problems, remain to be solved, such as those of motion artifacts and chemical shift artifacts.