Home About us Contact
|
Home About us Contact | ||
|
|
||
Gradient Coil (gradient + coil)
Selected AbstractsSimple anatomical measurements do not correlate significantly to individual peripheral nerve stimulation thresholds as measured in MRI gradient coilsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2003Blaine A. Chronik PhD Abstract Purpose To examine peripheral nerve stimulation (PNS) thresholds for normal human subjects in magnetic resonance imaging (MRI) gradient coils, and determine if observed thresholds could be predicted based on gross physiologic measurements. Materials and Methods PNS thresholds for 21 healthy normal subjects were measured using a whole-body gradient coil. Subjects were exposed to a trapezoidal echo-planar imaging (EPI) gradient waveform and the total change in gradient strength (,G) required to cause PNS as a function of the duration of the gradient switching time (,) were measured. Correlation coefficients and corresponding P values were calculated for the PNS threshold measurements against simple physiologic measurements taken of the subjects, including weight, height, girth, and average body fat percentage, in order to determine if there were any easily observable dependencies. Results No convincing correlations between threshold parameters and gross physiologic measurements were observed. Conclusion These results suggest it is unlikely that a simple physiologic measurement of subject anatomy can be used to guide the operation of MRI scanners in a subject-specific manner in order to increase gradient system performance while avoiding PNS. J. Magn. Reson. Imaging 2003;17:716,721. © 2003 Wiley-Liss, Inc. [source] Consideration of magnetically-induced and conservative electric fields within a loaded gradient coilMAGNETIC RESONANCE IN MEDICINE, Issue 6 2006Weihua Mao Abstract We present a method to calculate the electric (E)-fields within and surrounding a human body in a gradient coil, including E-fields induced by the changing magnetic fields and "conservative" E-fields originating with the scalar electrical potential in the coil windings. In agreement with previous numerical calculations, it is shown that magnetically-induced E-fields within the human body show no real concentration near the surface of the body, where nerve stimulation most often occurs. Both the magnetically-induced and conservative E-fields are shown to be considerably stronger just outside the human body than inside it, and under some circumstances the conservative E-fields just outside the body can be much larger than the magnetically-induced E-fields there. The order of gradient winding and the presence of conductive RF shield can greatly affect the conservative E-field distribution in these cases. Though the E-fields against the outer surface of the body are not commonly considered, understanding gradient E-fields may be important for reasons other than peripheral nerve stimulation (PNS), such as potential interaction with electrical equipment. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Imaging single mammalian cells with a 1.5 T clinical MRI scannerMAGNETIC RESONANCE IN MEDICINE, Issue 5 2003Paula Foster-Gareau Abstract In the present work, we demonstrate that the steady-state free precession (SSFP) imaging pulse sequence FIESTA (fast imaging employing steady state acquisition) used in conjunction with a custom-built insertable gradient coil and customized RF coils can be used to detect individual SPIO-labeled cells using a commonly available 1.5 T clinical MRI scanner. This work provides the first evidence that single-cell tracking will be possible using clinical MRI scanners, opening up new possibilities for cell tracking and monitoring of cellular therapeutics in vivo in humans. Magn Reson Med 49:968,971, 2003. © 2003 Wiley-Liss, Inc. [source] A comparison between human magnetostimulation thresholds in whole-body and head/neck gradient coilsMAGNETIC RESONANCE IN MEDICINE, Issue 2 2001Blaine A. Chronik Gradient coil magnetostimulation thresholds were measured in a group of 20 volunteers in both a whole-body gradient coil and a head/neck gradient coil. Both coils were operated using both x and y axes simultaneously (xy oblique mode). The waveform applied was a 64-lobe trapezoidal train with 1-ms flat-tops and varying rise times. Thresholds were based on the subjects' perception of stimulation, and painful sensations were not elicited. Thresholds were expressed in terms of the total gradient excursion required to cause stimulation as a function of the duration of the excursion. Thresholds for each subject were fit to a linear model, and values for the threshold curve slope (SRmin) and vertical axis intercept (,Gmin) were extracted. For the body coil, the mean values were: SRmin = 62.2 mT/m/ms, ,Gmin = 44.4 mT/m. For the head/neck coil, the mean values were: SRmin = 87.3 mT/m/ms, ,Gmin = 78.9 mT/m. These curve parameters were combined with calculated values for the induced electric field as a function of position within the coil to yield the tissue specific parameters Er (electric field rheobase) and ,c (chronaxie). For tissue stimulated within the body coil, the mean values were: Er = 1.8 V/m, ,c = 770 ,s. For tissue stimulated within the head/neck coil, the mean values were: Er = 1.3 V/m, ,c = 1100 ,s. Scalar potential contributions were not included in the calculation of induced electric fields. The mean threshold curves were combined with the gradient system performance curves to produce operational limit curves. The operational limit curves for the head/neck coil system were verified to be higher than those of the whole-body coil; however, the head/neck system was also found to be physiologically limited over a greater range of its operation than was the body coil. Subject thresholds between the two coils were not well correlated. Magn Reson Med 46:386-394, 2001. © 2001 Wiley-Liss, Inc. [source] Simple linear formulation for magnetostimulation specific to MRI gradient coilsMAGNETIC RESONANCE IN MEDICINE, Issue 5 2001Blaine A. Chronik Abstract 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 , is introduced as being fundamental in the evaluation of gradient coil stimulation. , 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 , 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: SRmin and ,Gmin. SRmin is the slope of the threshold curve and represents the minimum slew rate required to cause stimulation in the limit of infinite gradient strength. ,Gmin 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 SRmin and ,Gmin are functions of both , and the standard tissue parameters Er (rheobase) and ,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. Magn Reson Med 45:916,919, 2001. © 2001 Wiley-Liss, Inc. [source] Simple anatomical measurements do not correlate significantly to individual peripheral nerve stimulation thresholds as measured in MRI gradient coilsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2003Blaine A. Chronik PhD Abstract Purpose To examine peripheral nerve stimulation (PNS) thresholds for normal human subjects in magnetic resonance imaging (MRI) gradient coils, and determine if observed thresholds could be predicted based on gross physiologic measurements. Materials and Methods PNS thresholds for 21 healthy normal subjects were measured using a whole-body gradient coil. Subjects were exposed to a trapezoidal echo-planar imaging (EPI) gradient waveform and the total change in gradient strength (,G) required to cause PNS as a function of the duration of the gradient switching time (,) were measured. Correlation coefficients and corresponding P values were calculated for the PNS threshold measurements against simple physiologic measurements taken of the subjects, including weight, height, girth, and average body fat percentage, in order to determine if there were any easily observable dependencies. Results No convincing correlations between threshold parameters and gross physiologic measurements were observed. Conclusion These results suggest it is unlikely that a simple physiologic measurement of subject anatomy can be used to guide the operation of MRI scanners in a subject-specific manner in order to increase gradient system performance while avoiding PNS. J. Magn. Reson. Imaging 2003;17:716,721. © 2003 Wiley-Liss, Inc. [source] Experimental determination of human peripheral nerve stimulation thresholds in a 3-axis planar gradient systemMAGNETIC RESONANCE IN MEDICINE, Issue 3 2009Rebecca E. Feldman Abstract In MRI, strong, rapidly switched gradient fields are desirable because they can be used to reduce imaging time, obtain images with better resolution, or improve image signal-to-noise ratios. Improvements in gradient strength can be made by either increasing the gradient amplifier strength or by enhancing gradient efficiency. Unfortunately, many MRI pulse sequences, in combination with high-performance amplifiers and existing gradient hardware, can cause peripheral nerve stimulation (PNS). This makes improvements in gradient amplifiers ineffective at increasing safely usable gradient strength. Customized gradient coils are one way to achieve significant improvements in gradient performance. One specific gradient configuration, a planar gradient system, promises improved gradient strength and switching time for cardiac imaging. The PNS thresholds for planar gradients were characterized through human stimulation experiments on all three gradient axes. The specialized gradient was shown to have significantly higher stimulation thresholds than traditional cylindrical designs (y-axis SRmin = 210 ± 18 mT/m/ms and ,Gmin = 133 ± 13 mT/m; x-axis SRmin = 222 ± 24 mT/m/ms and ,Gmin = 147 ± 17 mT/m; z-axis SRmin = 252 ± 26 mT/m/ms and ,Gmin = 218 ± 26 mT/m). This system could be operated at gradient strengths 2 to 3 times higher than cylindrical configurations without causing stimulation. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source] A comparison between human magnetostimulation thresholds in whole-body and head/neck gradient coilsMAGNETIC RESONANCE IN MEDICINE, Issue 2 2001Blaine A. Chronik Gradient coil magnetostimulation thresholds were measured in a group of 20 volunteers in both a whole-body gradient coil and a head/neck gradient coil. Both coils were operated using both x and y axes simultaneously (xy oblique mode). The waveform applied was a 64-lobe trapezoidal train with 1-ms flat-tops and varying rise times. Thresholds were based on the subjects' perception of stimulation, and painful sensations were not elicited. Thresholds were expressed in terms of the total gradient excursion required to cause stimulation as a function of the duration of the excursion. Thresholds for each subject were fit to a linear model, and values for the threshold curve slope (SRmin) and vertical axis intercept (,Gmin) were extracted. For the body coil, the mean values were: SRmin = 62.2 mT/m/ms, ,Gmin = 44.4 mT/m. For the head/neck coil, the mean values were: SRmin = 87.3 mT/m/ms, ,Gmin = 78.9 mT/m. These curve parameters were combined with calculated values for the induced electric field as a function of position within the coil to yield the tissue specific parameters Er (electric field rheobase) and ,c (chronaxie). For tissue stimulated within the body coil, the mean values were: Er = 1.8 V/m, ,c = 770 ,s. For tissue stimulated within the head/neck coil, the mean values were: Er = 1.3 V/m, ,c = 1100 ,s. Scalar potential contributions were not included in the calculation of induced electric fields. The mean threshold curves were combined with the gradient system performance curves to produce operational limit curves. The operational limit curves for the head/neck coil system were verified to be higher than those of the whole-body coil; however, the head/neck system was also found to be physiologically limited over a greater range of its operation than was the body coil. Subject thresholds between the two coils were not well correlated. Magn Reson Med 46:386-394, 2001. © 2001 Wiley-Liss, Inc. [source] Simple linear formulation for magnetostimulation specific to MRI gradient coilsMAGNETIC RESONANCE IN MEDICINE, Issue 5 2001Blaine A. Chronik Abstract 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 , is introduced as being fundamental in the evaluation of gradient coil stimulation. , 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 , 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: SRmin and ,Gmin. SRmin is the slope of the threshold curve and represents the minimum slew rate required to cause stimulation in the limit of infinite gradient strength. ,Gmin 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 SRmin and ,Gmin are functions of both , and the standard tissue parameters Er (rheobase) and ,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. Magn Reson Med 45:916,919, 2001. © 2001 Wiley-Liss, Inc. [source] | ||||||||||