The invention relates to a magnetic resonance apparatus which includes
a gradient system for generating L gradient fields in a measuring volume of the apparatus, which gradient system includes a number of N (N greater than L) mutually independent channels, each of which consists of a gradient amplifier with a signal input and an output and a gradient coil connected to the output of the gradient amplifier, and
a conversion unit
provided with N outputs which are connected to the N signal inputs of the N gradient amplifiers in a one-to-one association, and
provided with inputs, a number L of which is arranged to receive L gradient signals representing the gradient fields to be generated,
which conversion unit is arranged to convert, in conformity with a conversion algorithm stored in the conversion unit, at least the L gradient signals applied to the inputs into N control signals for controlling the N gradient amplifiers.
A magnetic resonance imaging (MRI) apparatus of this kind is known from U.S. Pat. No. 5,554,929. The gradient system in the known MRI apparatus is arranged to generate three gradient fields (so L=3) which, as is customary in this technique, form an x gradient field (Gx=∂Bx/∂x), a y gradient field (Gy=∂Bz/∂y) and a z gradient field (Gz=∂Bz/∂z). Therein, Bz is the magnetic field of the apparatus which is oriented in the z direction in the measuring volume. In this known apparatus the z gradient field is generated by means of a z channel which consists of a respective gradient amplifier; the gradient coil for the z gradient field is connected to the output of said amplifier. A z gradient signal which represents the z gradient field to be generated is applied to the input of the z channel.
The x gradient field and the y gradient field are generated by means of a number of channels (for example, four), each of which also consists of a respective gradient amplifier associated with the relevant channel; the associated gradient coils are connected to the outputs thereof. In an embodiment of this known gradient system one channel is intended to receive an x gradient signal which represents the x gradient field to be generated whereas another channel is intended to receive a y gradient signal which represents the y gradient field to be generated and two channels are intended to receive respective gradient signals which form a mix of the x gradient signal and the y gradient signal.
Said mix of the x gradient signal and the y gradient signal is obtained from a conversion unit which includes four outputs which are connected to the four signal inputs of the four gradient amplifiers in a one-to-one association. The conversion unit includes three inputs for receiving L=3 gradient signals, that is, the x gradient signal, the y gradient signal and the z gradient signal which represent the x gradient field, the y gradient field and the z gradient field to be generated, respectively. The conversion algorithm stored in the conversion unit thus converts the incoming x gradient signal and the incoming y gradient signal into said four signals, that is, one for each channel. Thus, in this embodiment the combination of x and y gradient fields is generated by means of four channels. This known configuration aims to enable inter alia a simpler structure of the gradient coils and easier impedance matching.
It is an object of the invention to provide a magnetic resonance imaging apparatus wherein the gradient system can be operated in a more flexible manner, that is, such that the operation of the apparatus can be optimized in respect of a parameter that can be freely chosen by the user.
To this end, the magnetic resonance imaging apparatus according to the invention is characterized in that
the conversion unit is provided with N inputs, the other Nxe2x88x92L inputs of which are arranged to receive Nxe2x88x92L other signals which can be chosen independently of the gradient signals,
and that the conversion unit is arranged to convert, in conformity with the conversion algorithm stored in the conversion unit and together with the L gradient signals applied to the first L inputs, the Nxe2x88x92L other signals applied to the other Nxe2x88x92L inputs into N control signals for controlling the N gradient amplifiers.
The number of N inputs of the conversion unit is always larger than the number of L gradient fields to be generated; this means that in addition to the L gradient signals representing the gradient fields to be generated there are Nxe2x88x92L inputs whereto additional signals which can be freely chosen can be applied. The choice of these additional signals is determined by the desired optimization which itself is determined, for example, on the basis of a type of image of the MRI apparatus to be selected by the user, for example, fast scanning with a comparatively low resolution or slow scanning with a high resolution.
In an embodiment of the invention the number of other Nxe2x88x92L inputs equals Nxe2x88x92L=1. This choice already enables a number of desired optimizations; the number of gradient channels can thus be kept as small as possible and the construction of the conversion unit can remain as simple as possible.
The gradient coils in a further embodiment of the invention are similar in shape. In combination with Nxe2x88x92L=1, this embodiment offers the advantage that only two types of coil need be manufactured and stored, i.e. a first coil shape and the mirror image thereof.
In another embodiment of the invention said one coil shape is derived by combining a saddle-shaped x gradient coil and a cylindrical z gradient coil in a given ratio Ix:Iz=1:2xcex1. This embodiment is based on a circular cylindrical z gradient coil and a conventional, saddle-shaped x gradient coil as are generally known from the state of the art. Said one coil shape is derived from said two conventional coil shapes by mapping them on the same cylindrical surface and applying the same current therethrough. Said one coil shape, i.e. the intended coil, is then obtained by summing, in each point of the common cylindrical surface, the current of the circular cylindrical z gradient coil Iz and the current of the saddle-shaped x gradient coil Ix in the given ratio Ix:Iz=1:2xcex1. Thus, this is actually a vectorial addition of the currents wherein one of the currents is first multiplied by a factor 2xcex1. The value of xcex1 may then be equal to xc2xd, so that in that case the currents Ix and Iz can be taken so as to be equal. Generally speaking, a number of desired optimizations can then be selected by a suitable choice of the value of the ratio number xcex1.
In another embodiment of the invention the other signal applied to the other input is formed by a signal of constant value, which is preferably equal to zero. The latter embodiment can be used when the desired gradient fields are to be generated with a minimum dissipation of energy. Because the four gradient coils are similar in shape (i.e. identical in respect of shape and dimensions except for the fact that they may be the mirror image of one another), they have the same resistance. For minimum energy dissipation the sum of the squares of the four currents must be minimum. It has been found that this is the case when said other signal has a constant value, notably when it is zero.
In another embodiment of the invention the ratio number xcex1 is:
xcex1=0.5(2xcex2),
wherein:
xcex2 equals (Lz/Lx)(k2x/k2z),
Lz and Lx are the inductance of the standard x gradient coil and the standard z gradient coil, respectively, and
kx and kz are the proportionality factor between the x gradient field (∂Bz/∂x) and the z gradient field on the one side and the current Ix through said x gradient coil and said z gradient coil, respectively, on the other side, so that Ix=kx(∂Bz/∂x) and Iz=kz(∂Bz/∂z).
Because of the given choice of the value of the parameters xcex1 and xcex2 in combination with the described structure of the gradient system, the degree of freedom thus obtained is used to distribute the total energy stored in the gradient coils uniformly among the associated gradient amplifiers. In the case of a conventional gradient system, that is, a gradient system in which one channel is provided for each of the gradient fields, there are always situations in which only one amplifier delivers the total energy (i.e. in the case where the pure orthogonal gradient fields are to be generated) whereas the remaining amplifiers do not deliver energy. In the present embodiment this xe2x80x9cworst casexe2x80x9d loading of a single amplifier does not occur as will be described in detail hereinafter with reference to FIG. 3.
In another embodiment of the invention a z gradient field (∂Bz/∂z) is generated by means of a cylindrical z gradient coil and an x gradient field (∂Bz/∂x) and a y gradient field (∂Bz/∂y) are generated by means of at least three further gradient channels with gradient coils which are similar in shape. This embodiment of the invention advantageously utilizes the fact that a cylindrical z gradient coil is usually efficient enough. Said additional degrees of freedom are then used to generate the gradients Gx=∂Bz/∂x and Gy=∂Bz/∂y with three (non-mirrored) gradient coils which are identical in shape and arranged so as to be rotated through an angle of 120xc2x0 relative to one another. For this case it can be demonstrated that the gradients Gx and Gy are also efficiently generated.
In another embodiment of the invention an x gradient field (∂Bz/∂x), a y gradient field (∂Bz/∂y) and a z gradient field (∂Bz/∂z) are generated by means of six gradient channels with gradient coils which are similar in shape. This embodiment of the invention utilizes the recognition of the fact that in this case three degrees of freedom are available for optimization. One degree can be used to make the sum of all currents equal to zero whereas the other two can be used to distribute the total energy stored in the gradient system as uniformly as possible among the amplifiers. Moreover, in this situation the gradient system according to the invention can be realized on the basis of a conventional gradient system without necessitating major modifications of the gradient amplifiers. In that case, however, the gradient coils should be given the appearance associated with the invention.
In another embodiment of the invention the number of connection cables between the gradient amplifiers and the gradient coils equals the number of gradient coils. This embodiment utilizes the possibility of optimizing the gradient system in such a manner that the sum of the currents through the gradient coils equals zero. In that case it suffices to use only one connection cable between the gradient amplifier and the associated gradient coil, the other ends of all gradient coils being interconnected in a common node. Because the sum of the currents is zero, no current is drained from the node.
In another version of the invention the number of connection cables between the gradient amplifiers and the gradient coils is one larger than the number of gradient coils. This situation occurs when the sum of the currents is not exactly equal to zero but deviates only slightly from zero. In that case a slight discharge of current occurs from the star point, so that it suffices to utilize only one discharge cable of small dimensions to an amplifier stage which is also of small dimensions.
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