Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-12-18
2003-04-01
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06541972
ABSTRACT:
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>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 (G
x
=∂B
x
/∂x), a y gradient field (G
y
=∂B
z
/∂y) and a z gradient field (G
z
=∂B
z
/∂z). Therein, B
z
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 N−L inputs of which are arranged to receive N−L 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 N−L other signals applied to the other N−L 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 N−L 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 N−L inputs equals N−L=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 N−L=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 I
x
:I
z
=1:2&agr;. 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 I
z
and the current of the saddle-shaped x gradient coil I
x
in the given ratio I
x
:I
z
=1:2&agr;. Thus, this is actually a vectorial addition of the currents wherein one of the currents is first multiplied by a factor 2&agr;. The value of &agr; may then be equal to ½, so that in that case the currents I
x
and I
z
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 &agr;.
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 &agr; is:
&agr;=0.5(2&bgr;),
wherein:
&bgr; equals (L
z
/L
x
)(k
2
x
/k
2
z
),
L
z
and L
x
are the inductance of the standard x gradient coil and the standard z gradient coil, respectively, and
k
x
and k
z
are the proportionality factor between the x gradient field (∂B
z
/∂x) and the z gradient field on the one side and the current I
x
through said x gradient coil and said z gradient coil, respectively, on the other side, so that I
x
=k
x
(∂B
z
/∂x) and I
z
=k
z
(∂B
z
/∂z).
Because of the given choice of the value of the parameters &agr; and &bgr; in combination with the d
Bunk Paul Bernard
Mulder Gerardus Bernardus Jozef
Peeren Gerardus Nerius
Koninklijke Philips Electronics , N.V.
Lefkowitz Edward
Vargas Dixomara
Vodopia John
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