Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-09-13
2004-02-24
Gutierrez, Diego (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06696837
ABSTRACT:
BACKGROUND
The invention relates to a coil system of an MR apparatus which is intended to generate gradient magnetic fields by means of a plurality of coils which are connected in series and each of which is provided with parallel coil turn configurations which are insulated from one another and connected to the coil turn configurations of the neighboring coil in the circuit arrangement so as to form conductive paths spanning coils.
In the context of a typical magnetic resonance imaging method the magnetic moment of the protons is oriented in one spatial direction by means of a strong, steady magnetic field of approximately 1.5 Tesla. The individual protons are excited to precession by means of brief, electromagnetic RF pulses, after which they become oriented again in conformity with the external, strong magnetic field. In particular the excitation and relaxation times and the frequencies of the precessional motions are dependent on the type of tissue and in the context of the measurement they provide, in conjunction with a position code of the excitation, information concerning the situation in space of various tissues. The position code utilizes position-dependent frequencies and phases of the precessional excitation and enables information as regards the location of the relevant emission to be derived via Fourier transformation of the measured MR signal.
In order to achieve a high image quality it makes sense to utilize several characteristics so as to distinguish the types of tissue being examined. A limiting factor in achieving a high image quality within an acceptable examination time consists in the speed at which the magnetic fields in the MR apparatus can be changed. Therefore, it is continuously being attempted to develop coil systems which enable high magnetic field strength transients to be obtained in conjunction with amplifiers and voltage sources.
In order to generate the magnetic field strength gradients desired for the position code, it is common practice to use different coil systems which extend in three mutually perpendicular spatial directions. Two Helmholz coils which face one another are usually arranged in the direction of the longitudinal axis of the body, that is, the axis which is usually referred to as the z direction. Generally speaking, in the direction perpendicular to the longitudinal axis of the body of the patient there are usually provided spiral-like planar coils which are arranged opposite one another so that they enclose the examination volume in a cylindrical fashion. Along the longitudinal axis of the body there are often provided two or more of such spiral-like coils which serve for a spatial direction extending perpendicularly to the longitudinal axis of the body. The two spatial directions which are oriented perpendicularly to the longitudinal axis of the body are generally referred to as the x direction and the y direction for which respective, separate coil systems of the latter kind are employed.
From British patent application GB 2 331 808 it is already known to configure a coil system with spiral-like, interleaved conductive paths of several series-connected identical coils in such a manner that the respective different conductive paths of each individual coil are connected to a dissimilar conductive path of the neighboring coil. The conductive paths resulting from the series connection of the individual coils are to be equalized in respect of their electrical properties in this manner.
The conventional coil systems have a drawback in that they lack flexibility in respect of the numbers of turns. The number of turns of the coil systems of, for example, an MR apparatus, should be adapted to the other components of the device or to the intended application. This is necessary, for example, in the case of active shielding of the magnetic stray fields. The design of the shielding coils is subject to the stray field arising, so that the design criterion to be satisfied in respect of the number of coil turns of the coil system for shielding is customarily a ratio of the numbers of turns of the coil system for the gradient field and the coil system for the shielding. This ratio is an integer number only rarely, so that this design criterion can usually be satisfied only by means of large numbers of turns of the shielding coil and the gradient coil. The quadratic contribution of the number of turns of a coil to the inductance makes that the design used thus far, for example, for active shields for stray fields is associated with high inductances and the voltages induced during the current strength transients impose severe requirements as regards the voltage source and the insulation.
SUMMARY
Considering the drawbacks and problems of the present state of the art, it is an object of the invention to provide a coil system which has a non-integer effective number of turns and nevertheless satisfies the requirements imposed as regards the quality of the magnetic field, notably as regards the symmetry and the homogeneity. It is also an object of the invention to provide a coil system for the active shielding of magnetic stray fields which has an as low as possible inductance and generates as low as possible voltages in phases of varying current flow.
The object is achieved in accordance with the invention by means of a coil system of the kind set forth in which the individual coils have at least two groups of coil turn configurations whose numbers of turns deviate from one another and which are connected to the coil turn configurations of the respective neighboring coils in such a manner that the number of turns summed over all coils is the same for each conductive path.
A special advantage of the coil system in accordance with the invention resides in the high degree of freedom as regards the choice of the number of turns of the coil turn configurations. The parallel coil turn configurations conduct identical currents because of their electrical properties which are substantially identical throughout the coil system and because of the parallel application of the operating voltage. The design of coil systems for generating gradient magnetic fields can be optimized in respect of inductance while utilizing non-integer effective numbers of turns.
In order to solve the described problems concerning the active shielding of magnetic stray fields, the invention proposes a coil system of the kind set forth which includes a first coil arrangement which generates the gradient magnetic field and a second coil arrangement which actively shields the stray field arising around the first coil arrangement, in which coil system the individual coils of at least the first or the second coil arrangement comprise at least two groups of coil turn configurations whose numbers of turns deviate from one another and which are connected to the coil turn configurations of the respective neighboring coil in such a manner that the number of turns summed over all coils is the same for each conductive path.
For the active shielding of magnetic stray fields generated by gradient magnetic field coils full benefit is derived from the advantages offered by coil systems in accordance with the invention. The ratio of the numbers of the turn configurations of the gradient magnetic field coil and the active stray field shielding coil, as defined by the configuration of the MR apparatus, is generally a random decimal number. Integer numbers of coil turn configurations as imposed thus far necessitated the choice of large numbers of turns in one or both coil systems in order to achieve an exact approximation in the interest of suitable shielding. Because in accordance with the invention the effective numbers of turn configurations of the coil systems are not necessarily integer numbers, it is possible to reduce the numbers of turns of the individual coils to a fraction while achieving the same shielding quality nevertheless. A ratio of, for example, 12.5 turn configurations of the gradient coil system to 7 coil turn configurations of the active shield as used thus far required a minim
Gutierrez Diego
Koninklijke Philips Electronics , N.V.
Lundin, Esq. Thomas M.
Shrivastav Brij B.
LandOfFree
Coil system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Coil system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Coil system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3305040