Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Utility Patent
1999-04-27
2001-01-02
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000
Utility Patent
active
06169403
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for simulating the electrical stimulations which are generated in an examination subject by gradient coils of an MR device.
2. Description of the Prior Art
In the known MR devices, rapidly switched gradient fields with a high amplitude are superimposed on a basic magnetic field.
In MR exams, the patients can be stimulated by switching the gradient impulses (known as MR stimulation). The stimulations are caused by the effect of an electrical field on the patient. The electrical field is induced by the alteration, according to Maxwell's equations, of the magnetic flux B which is generated by each of the three gradient coils. For a given MR device, the magnitude of the electrical field that is generated by switching a gradient coil is directly proportional to the time variation of the value of the magnetic flux B, expressed by dB/dt, i.e., the time derivative of the magnitude of the magnetic field which is produced by the gradient coil. Because of the proportionality of the electrical field and dB/dt (chronological change of the magnetic flux B), it is sufficient merely to observe the time variation of the magnetic flux B.
On the basis of the proportionality of the magnetic flux B and the gradient field G for a given gradient coil, monitoring the time variation d/dt of the spatially dependent gradient field G (in mT/m) is equivalent to aforementioned monitoring the time variation d/dt of the spatially dependent magnetic flux B (in mT). Therefore, the following considers the time variation of the gradient signals.
A stimulation occurs when a characteristic threshold value of the electrical field is exceeded. The corresponding threshold value of dB/dt, or of dG/dt, depends for a fixed gradient schema on the anatomy and the physiology of the patient, his orientation in the MR device and the geometric and physical properties of the three gradient coils. The value dB/dt is defined by the amplitude of the gradient pulses and the rise times. In practice, however, the gradient profile is not constant from pulse to pulse, either with respect to the amplitudes or the timing, but rather, besides being dependent on the selection of the measuring sequence, it depends in particular on the selected measuring parameters (e.g. slice thickness, number of slices, field of view FOV, matrix size, repetition time T
R
, echo time T
E
, etc.). In this case, in addition to the aforementioned parameters, the threshold value for the stimulation particularly depends on the time configuration of the individual gradient pulses, their total number and repetition rate, and the superimposition of all three gradient coils G
x
, G
y
, and G
z
.
For whole-body gradient coils, not only the B
z
component of the magnetic field, which extends in a longitudinal direction, but also its transverse components B
x
and B
y
are responsible for the stimulation, the B
y
component being more critical with respect to stimulations, since the field lines penetrate the body frontally. Thus, given a prone or supine position of the patient, the stimulation limit value for the y-axis must be the smallest.
As an extreme simplification, from a physiological perspective, a consciously perceived stimulation by an external electrical field can be described in two steps. The electrical field can either act directly from outside or can be induced by a varying magnetic field.
In a first step, the electrical field generates an electrical potential at the cell wall of the stimulated nerve cells. The cell wall of the nerve cell can be approximately imagined as a capacitor which is charged by the electrical field. When the electrical potential exceeds a characteristic threshold, an action potential is triggered in the nerve cell and spreads over the entire nerve cell.
In the second step, at the connection of two nerve cells (a synapse), an action potential on the presynaptic side leads to a diffusion out of chemical messenger substances. These substances are absorbed on the postsyanptic side, i.e., they are absorbed in the nerve cell which is connected downstream, where they trigger another action potential. The stimulus spreads. The concentration of the messenger substances in the synapse is a measure of the number of postsynaptically triggered action potentials. In particular, the concentration of the messenger substances in the synapse subsides only gradually. The characteristic time constant is in the range of a few milliseconds. A more exact description of the neurophysiological processes can be found in the text
Neuro
-
und Sinnesphysiologie
(R. F. Schmidt, pub.; Springer, Second Edition 1995: Chapters 2 and 3).
In order to avoid such stimulations in the examined body given rapidly switched gradient fields of high amplitude, it is taught in German OS 42 25 592 to cover, with a closed conductor loop, those regions outside the examination region which are sensitive to stimulation. This results in a reduction of the currents induced in the covered region. This method is based on the fact that, given switched gradients, the highest current values are induced outside the examination region, so that the danger of stimulations is greatest there. The linearity of the gradients in the examination region, which is of importance for the image quality, is minimally compromised by the attachment of conductor loops outside the examination region. Given a change of examination region, however, the position of the conductor loops must also be adjusted.
There are also known methods which enable a prediction of magnetostimulations. One of these approaches to stimulation monitoring is what is known as the dB/dt model. This method involves checking and monitoring the pure dB/dt values that arise in a measurement. The maximum permissible dB/dt values derive from the result of a stimulation study with the corresponding gradient coil, or from the limit values which are strictly prescribed by the certification authorities. Further details can be from in the article “Peripheral Nerve Stimulation by Time-Varying Magnetic Fields” (J. Abart,
J. Computer Assisted Tomography
(1997); 21(4):532-38. The dB/dt model does not adequately consider the patient physiology; in particular, the dependency of the stimulation threshold on the timing of the gradient impulses is not taken into account. The dB/dt model is thus only a worst-case estimate, which, in many cases, allows the capability of modern gradient systems to be used only within certain limits.
Another known approach to stimulation monitoring is known as the “Irnich model”. This method describes the stimulation threshold value as a factor of the duration t
E
of the external influence. The duration t
E
is the time in which the gradient changes in one direction; i.e., dB/dt is permanently >0, or <0. A more detailed explanation can be found in the article “Electrostimulaticn by time-varying magnetic fields” (W. Irnich, MAGMA; 1994; 2:43-9. Represented as the dB/dt value, the threshold is therein proportional to (1+t
chron
/t
E
), i.e. is hyperbolically dependent on the duration of effect t
E
. The chronaxie time t
chron
is a physiologically defined characteristic time.
The experimental results in different studies can be described well with the Irnich model. The results of these studies are discussed in the article “Magnetostimulation in MRI” (W. Irnich, F. Schmitt; MRM; 1995;33:619-23), and in the article “Threshold and Pain Strength-Duration Curves for MRI Gradient Fields” (J. D. Bourland; Proc. SMRM; 1997:1974). Nevertheless, it is possible to apply the Irnich model with a fixed set of parameters only to one characteristic gradient pulse shape, given which the alteration of the duration of t
E
is performed globally, i.e. for each individual pulse in a like manner. A discrepancy thus arises when trapezoidal pulses with a corresponding duration of effect t
E
are used instead of sinusoidal pulses. The method which is based on the Irnich model is also not usable if mere single pulses within a long pu
Gebhardt Matthias
Hebrank Franz
Lenz Helmut
Oda Christine K.
Schiff & Hardin & Waite
Shrivastav Brij B.
Siemens Aktiengesellschaft
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