MRI Gradient Research Articles

Relation of regional articular cartilage morphometry and meniscal position by MRI to joint space width in knee radiographs

To ascertain the contribution of articular cartilage morphometry and meniscal position on MRI to joint space width (JSW) measured in the Lyon schuss radiograph of the knee. 62 obese women with knee OA and 99 non-obese female controls (mean age 56.6 years) were imaged using 3T MRI and coronal water excitation spoiled gradient echo sequences. Segmentation of femorotibial cartilage morphology and regional morphometric analysis was performed using custom software. Meniscal position was measured quantitatively in sagittal and coronal planes. Minimum space width (mJSW) was measured in the Lyon Schuss knee radiograph; Kellgren and Lawrence grades (KLG) were assigned on standing anteroposterior knee films. The relative contribution of regional cartilage thickness and meniscal position to mJSW was assessed initially in univariate models and subsequently with multivariable modelling. 65% of the variation in mJSW was explained by regional cartilage thickness measures, different KLG and meniscal coverage. Of these measures the medial tibia cartilage thickness measures and central region of the central medial femur (ccMF) play a consistent role in variations in mJSW observed across all KLG. Further ccMF and the addition of percent meniscal coverage to this model explains the remaining differences in mean mJSW found between those subjects with definite joint space narrowing (KLG3) and those without OA. The variation in radiographic mJSW is best described by five regional cartilage thickness measures and percent meniscal coverage. The magnitude of each measures contribution differs according to radiographic severity with more variability explained by cartilage thickness of ccMF cartilage thickness and percent meniscal coverage with more severe disease.

Gradient waveform synthesis for magnetic propulsion using MRI gradient coils

Navigating an untethered micro device in a living subject is of great interest for both diagnostic and therapeutic applications. Magnetic propulsion of an untethered device carrying a magnetic core in it is one of the promising methods to navigate the device. MRI gradients coils are thought to be suitable for navigating the device since they are capable of magnetic propulsion in any direction while providing magnetic resonance images. For precise navigation of the device, especially in the peripheral region of the gradient coils, the concomitant gradient fields, as well as the linear gradient fields in the main magnetic field direction, should be considered in driving the gradient coils. For simple gradient coil configurations, the Maxwell coil in the z-direction and the Golay coil in the x- and y-directions, we have calculated the magnetic force fields, which are not necessarily the same as the conventional linear gradient fields of MRI. Using the calculated magnetic force fields, we have synthesized gradient waveforms to navigate the device along a desired path.

Measurement of visual evoked potential during and after periods of pulsed magnetic field exposure

To study the effect of switched magnetic fields used in MR scanners on the visual evoked potential (VEP) in human subjects. We have used an MRI gradient coil, remote from an MRI magnet to produce a time-varying magnetic field (0.5 kHz, peak field approximately 8.7 T/second) in the human brain without the confounding effects of static field exposure or accompanying acoustic noise. The VEP response to a 2-Hz reversal, 8 x 8 checkerboard, occupying 20 degrees of the visual field was recorded from occipital locations O1 and O2. VEP recordings were made every five minutes before, during, and after a 10-minute magnetic field exposure period for seven subjects. In contradiction to studies previously reported in the literature for fields of 50 Hz and 60 mT, no significant effects on the peak amplitude or latency of the VEP P100 O1 and O2 responses were found. Switched magnetic fields of a level and frequency comparable to those used in MRI do not have a significant effect on primary retinal or visual processing.

Calculating the movement of MRI coils, and minimizing their noise

The design of gradient coils within Magnetic Resonance Imaging equipment is considered. These coils produce linear magnetic fields in each of the three orthogonal directions in physical space. In addition, they are turned on and off repeatedly to enhance the clarity of the image, but this produces a great deal of noise within the coil, as its shape distorts under the influence of Lorentz forces. We present a method for calculating the movement of the coil in the background magnetic field, and estimating the consequent noise levels. This involves solving for the current density in the coil coupled with equations for its elastic deformation, along with acoustic equations for the pressure in the surrounding air. Winding patterns are designed to minimize the noise produced by the Magnetic Resonance Imaging coil. References L. M. Delves and J. L. Mohamed. Computational Methods for Integral Equations. Cambridge University Press, Cambridge, 1985. L. K. Forbes and S. Crozier. A novel target-field method for finite-length magnetic resonance shim coils: Part 1: Zonal shims. J. Phys. D: Appl. Phys., 34:3447--3455, 2001. doi:10.1088/0022-3727/34/24/305 L. K. Forbes and S. Crozier. A novel target-field method for finite-length magnetic resonance shim coils: Part 2: Tesseral shims. J. Phys. D: Appl. Phys., 35:839--849, 2002. doi:10.1088/0022-3727/35/9/303 L. K. Forbes and S. Crozier. A novel target-field method for magnetic resonance shim coils: Part 3: Shielded zonal and tesseral coils. J. Phys. D: Appl. Phys., 36:68--80, 2003. doi:10.1088/0022-3727/36/2/302 E. M. Haacke, R. W. Brown, M. R. Thompson and R. Venkatesan. Magnetic Resonance Imaging: Physical principles and sequence design. Wiley, New York, 1999. J. Jin. Electromagnetic Analysis and Design in Magnetic Resonance Imaging. Biomedical Engineering Series, CRC Press, Boca Raton, 1999. P. Mansfield, B. Haywood and R. Coxon. Active acoustic control in gradient coils for MRI. Magn. Reson. Med., 46:807--818, 2001. C. K. Mechefske, G. Yao, W. Li, C. Gazdzinski and B. K. Rutt. Modal analysis and acoustic noise characterization of a 4T MRI gradient coil insert. Concepts Magn. Reson. B, 22:37--49, 2004. doi:10.1002/cmr.b.20013 W. Shao and C. K. Mechefske. Acoustic analysis of a gradient coil winding in an MRI scanner. Concepts Magn. Reson. B, 24:15--27, 2005. doi:10.1002/cmr.b.20023 W. Shao and C. K. Mechefske. Analysis of the sound field in finite length infinite baffled cylindrical ducts with vibrating walls of finite impedance. J. Acoust. Soc. Am., 117:1728--1736, 2005. A. P. Boresi and K. P. Chong. Elasticity in engineering mechanics. Elsevier, New York 1987. R. Turner. A target field approach to optimal coil design. J. Phys D: Appl. Phys., 19:147--151, 1986. M. T. Vlaadingerbroeck and J. A. den Boer. Magnetic Resonance Imaging: Theory and practice, 3rd edition. Springer Verlag, Berlin, 2003. M. A. Brideson, L. K. Forbes and S. Crozier. Determining complicated winding patterns for shim coils using stream functions and the target-field method. Concepts Magn. Reson., 14:9--18, 2002. doi:10.1002/cmr.10000 B. L. W. Chapman and P. Mansfield. A quiet gradient-coil set employing optimized, force-shielded, distributed coil designs. J. Magn. Reson. B, 107:152--157, 1995. S. Crozier and D. M. Doddrell. Gradient-coil design by simulated annealing. J. Magn. Reson. A, 103:354--357, 1993.

Gradient Coil Design in Permanent Magnet MRI

Based on target field method, the article designs gradient coils for a permanent MRI magnet model. By simulating, winding and testing, the result verifies the validity and feasibility of target field method used in gradient coils design of permanent magnet MRI; moreover, shielded coils can restrain eddy current and can be designed based on target field method easily. The simulated results show that the effect of restraining eddy current is notable. So, the permanent magnetic MRI gradient coil can be designed by target field method. Gradient coils generate the linearly gradient field; and shielded coils restrain the eddy current.

Coarctation of the aorta: pre and postoperative evaluation with MRI and MR angiography; correlation with echocardiography and surgery

To compare MRI and MRA with Doppler-echocardiography (DE) in native and postoperative aortic coarctation, define the best MR protocol for its evaluation, compare MR with surgical findings in native coarctation. 136 MR studies were performed in 121 patients divided in two groups: Group I, 55 preoperative; group II, 81 postoperative. In group I, all had DE and surgery was performed in 35 cases. In group II, DE was available for comparison in 71 cases. MR study comprised: spin-echo, cine, velocity-encoded cine (VEC) sequences and 3D contrast-enhanced MRA. In group I, diagnosis of coarctation was made by DE in 33 cases and suspicion of coarctation and/or aortic arch hypoplasia in 18 cases. Aortic arch was not well demonstrated in 3 cases and DE missed one case. There was a close correlation between VEC MRI and Doppler gradient estimates across the coarctation, between MRI aortic arch diameters and surgery but a poor correlation in isthmic measurements. In group II, DE detected a normal isthmic region in 31 out of 35 cases. Postoperative anomalies (recoarctation, aortic arch hypoplasia, kinking, pseudoaneurysm) were not demonstrated with DE in 50% of cases. MRI is superior to DE for pre and post-treatment evaluation of aortic coarctation. An optimal MR protocol is proposed. Internal measurement of the narrowing does not correspond to the external aspect of the surgical narrowing.

Intercentre Reproducibility of Magnetic Resonance T2* Measurements of Myocardial Iron in Thalassaemia

In transfusion-dependent thalassemia major, iron-induced cardiomyopathy is the predominant cause of morbidity and mortality. Assessment of myocardial iron loading using MRI gradient echo T2* measurements have been described, but has only been performed at one centre in London. We assessed the transferability of this method by comparing the results from three different MR scanners in three different countries. Ten patients with thalassemia major underwent myocardial T2* assessment using a Siemens Sonata Scanner in London. Patients were also scanned with either a similar T2* sequence on a GE Systems CVI scanner in Athens, or a GE Systems signa echospeed scanner in Cagliari. Two scans were performed at the respective site in all patients to assess interstudy reproducibility at each site. The mean difference and coefficient of variability for the heart between scanners was 0.08 ms and 9.7% between London and Athens; and 0.30 ms and 1.6% between London and Cagliari. The interstudy mean difference and coefficient of variability for the heart in Athens was 0.6 ms and 3.5%, and 0.2 ms and 2.4% in Cagliari. In conclusion, the myocardial iron estimations were consistent between the three centres with scanners of differing manufacture, suggesting that this technique may have widespread application in the assessment of patients with iron overload conditions such as thalassaemia.

Acoustic gating monitor for magnetic resonance imaging system

An MRI imaging system provides an audible feedback signal in the gantry room triggered by a physiological sensor on the patient. The feedback signal is a sound generated by one of the MRI gradient coils. The sensor output signal may be indicative of the patient's heartbeat, or other physiological event. The application of the sequence causes the coil to emit a sound that is associated with the sensor output signal.

Momentum-weighted conjugate gradient descent algorithm for gradient coil optimization.

MRI gradient coil design is a type of nonlinear constrained optimization. A practical problem in transverse gradient coil design using the conjugate gradient descent (CGD) method is that wire elements move at different rates along orthogonal directions (r, phi, z), and tend to cross, breaking the constraints. A momentum-weighted conjugate gradient descent (MW-CGD) method is presented to overcome this problem. This method takes advantage of the efficiency of the CGD method combined with momentum weighting, which is also an intrinsic property of the Levenberg-Marquardt algorithm, to adjust step sizes along the three orthogonal directions. A water-cooled, 12.8 cm inner diameter, three axis torque-balanced gradient coil for rat imaging was developed based on this method, with an efficiency of 2.13, 2.08, and 4.12 mT.m(-1).A(-1) along X, Y, and Z, respectively. Experimental data demonstrate that this method can improve efficiency by 40% and field uniformity by 27%. This method has also been applied to the design of a gradient coil for the human brain, employing remote current return paths. The benefits of this design include improved gradient field uniformity and efficiency, with a shorter length than gradient coil designs using coaxial return paths.

Echo planar imaging at 4 Tesla with minimum acoustic noise.

To minimize the acoustic sound pressure levels of single-shot echo planar imaging (EPI) acquisitions on high magnetic field MRI scanners. The resonance frequencies of gradient coil vibrations, which depend on the coil length and the elastic properties of the materials in the coil assembly, were measured using piezoelectric transducers. The frequency of the EPI-readout train was adjusted to avoid the frequency ranges of mechanical resonances. Our MRI system exhibited two sharp mechanical resonances (at 720 and 1220 Hz) that can increase vibrational amplitudes up to six-fold. A small adjustment of the EPI-readout frequency made it possible to reduce the sound pressure level of EPI-based perfusion and functional MRI scans by 12 dB. Normal vibrational modes of MRI gradient coils can dramatically increase the sound pressure levels during echo planar imaging (EPI) scans. To minimize acoustic noise, the frequency of EPI-readout trains and the resonance frequencies of gradient coil vibrations need to be different.

703 Poster IMRT fluency compensation of heterogeneous beam transmission through an MRI cryostat and gradient coils

703 Poster IMRT fluency compensation of heterogeneous beam transmission through an MRI cryostat and gradient coils

Toward shielding improvements in MRI gradients and other systems

Toward shielding improvements in MRI gradients and other systems

Toward shielding improvements in MRI gradients and other systems.

A new method is described for reducing the shielding-error function in the 'supershielding' approach to designing MRI systems. The method is thus shown to lead to significantly better shielding and better control of eddy current effects associated with gradient coils. To illustrate this technique, a set of results for a z-gradient coil is presented. A generalization to non-standard geometries can be made in a straightforward manner with the new method. The usefulness of the relationship of all fringe-field quantities to the shielding-error function is emphasized. The formal limit of perfect shielding in a 'least-squares' sense is shown for a simple strip-shield model along with a numerical eigenvalue study for comparison with the theoretical limit.

Simple linear formulation for magnetostimulation specific to MRI gradient coils.

A simple linear formulation for magnetostimulation thresholds specific to MRI gradient coils is derived based on established hyperbolic electrostimulation strength vs. duration relations. Thresholds are derived in terms of the gradient excursion required to cause stimulation, and it is demonstrated that the threshold curve is a linear function of the gradient switching time. A parameter beta is introduced as being fundamental in the evaluation of gradient coil stimulation. beta is a map of the induced electric field per unit gradient slew rate, and can be calculated directly from the gradient coil wire pattern. Consideration of beta alone is sufficient to compare stimulation thresholds between different gradient coil designs, as well as to evaluate the expected dependency of stimulation threshold on position within the gradient coil. The linear gradient threshold curve is characterized by two parameters: SR(min) and DeltaG(min). SR(min) is the slope of the threshold curve and represents the minimum slew rate required to cause stimulation in the limit of infinite gradient strength. DeltaG(min) is the vertical axis intercept of the curve and represents the minimum gradient excursion required to cause stimulation in the limit of infinite slew rate. Both SR(min) and DeltaG(min) are functions of both beta and the standard tissue parameters E(r) (rheobase) and tau(c) (chronaxie time). The ease with which both the gradient system performance and the stimulation thresholds can be plotted on the same axes is noted and is used to introduce the concept of a piece-wise linear operational limit curve for a gradient system.

Simple but Reliable Solutions for Spiral MRI Gradient Design

The efficiency of gradient design in MRI is limited by the simple fact that the gradient coil current and slew rate cannot exceed hardware threshold values. In spiral MRI, which requires gradients to be very rapidly switched between positive and negative values, minimization of the acquisition time is achieved by maintaining the current and slew rate as high as possible during the entire measurement. Since the current and slew rate compete against each other, an efficient gradient design consists of two parts in which current and slew rate are pushed alternatively to their limits. Values for these types of gradients can be obtained by solving numerically the equation of motion for the spiral trajectory. This paper shows that simple but reasonable mathematical approximations deliver reliable analytical solutions. Images obtained using these analytical solutions do not show evident distortions when compared with images obtained with numerical solutions.

Reduced MTR in the corticospinal tract and normal T 2 in amyotrophic lateral sclerosis

The objective of this study was to test the hypothesis that magnetization transfer ratios (MTR) are decreased in the corticospinal tract of patients with amyotrophic lateral sclerosis (ALS); to determine if T2 is increased in corticospinal tract or reduced in motor cortex in ALS; to determine if corticospinal tract MTR correlates with a clinical measure of motor neuron function in ALS. Ten ALS patients and 17 age-matched controls were studied. Double spin echo MRI and 3D gradient echo MRI with and without off-resonance saturation were acquired on each subject. 3D data sets were coregistered and resliced to match the spin echo data set. MTR was calculated for corticospinal and non-corticospinal tract white matter. T2 was calculated for corticospinal and non-corticospinal tract white matter, motor cortex and non-motor cortex. MTR was reduced by 2.6% ( p < .02) in corticospinal, but not in non-corticospinal, tract white matter in ALS. There was no difference in T2 in any brain region. The correlation between a clinical measure of motor neuron function and corticospinal tract MTR was statistically significant. These findings are consistent with the known pathology in ALS and suggest that MTR is more sensitive than T2 for detecting involvement of the corticospinal tract. Quantitative MTR of the corticospinal tract may be a useful, objective marker of upper motor neuron pathology in ALS.

5406204 Integrated MRI gradient coil and RF screen

5406204 Integrated MRI gradient coil and RF screen

5217010 ECG amplifier and cardiac pacemaker for use during magnetic resonance imaging

A device for monitoring a patient or pacing a patient is disclosed which can safely operate in a MRI system. The device uses unique RF filtering and shielding to attenuate voltages on the leads resulting from the high frequency RF signals produced in the MRI. The device is uniquely shielded to prevent induced currents from disrupting the amplifying and processing electronics. The device uses an optional secondary low pass or band reject filter to eliminate interference from the MRI's gradient magnetic field. The device uses optional inductors placed close to electrodes to limit RF currents through the electrodes. Several embodiments of the RF filter are taught which depend on the number of sensing leads, whether the leads are shielded, whether the RF filter is contained in the electronic shielded housing or whether single or multistage filtering is employed. The device may operate as an extended ECG monitor or may be an implantable MRI safe pacemaker.

MRI gradient waveform design by numerical optimization

This manuscript describes a method of gradient waveform design by nonlinear constrained optimization. Methods of formulation and solution of the waveform optimization problem are briefly described for minimization of root mean squared current and minimization of waveform moments. Waveforms generated using these objectives are presented and compared with those obtained with other objectives. The method uses waveforms which are defined as a set of discrete amplitudes in order to remove artificial constraints on waveform shape imposed by "multilobe" designs. These point-to-point amplitudes are the parameters determined in the optimization procedure which includes knowledge of the specific imaging conditions and the specific gradient hardware system. Some beneficial results of this design approach are: a) physically realizable waveforms which optimally achieve specific imaging and motion artifact reduction goals, b) waveforms which are guaranteed to be optimal with respect to one of several possible objective, c) less reliance on the experience of the designer, and d) a potential reduction in waveform design time.

Review of MRI gradient waveform design methods with application in the study of motion

AbstractMany design techniques for magnetic resonance gradient waveforms have aimed at controlling the phase that results from spin motion during the application of gradients. This paper discusses the evolution of gradient waveform design into the gradient‐moment‐nulling and motion‐encoding efforts common today. It also describes current design methods and technology.