MRI apparatus with flexible gradient control

• US 20010024122A1
• Filed: 12/18/2000
• Published: 09/27/2001
• Est. Priority Date: 12/20/1999
• Status: Active Grant

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Mulder, Gerardus Bernardus Jozef, Peeren, Gerardus Nerius, Bunk, Paul Bernard
• Granted Patent: US 6,541,972 B2
• Time in Patent Office: Days
• US Class Current: 324/318
• CPC Class Codes: G01R 33/385 using gradient magnetic field coils
• CPC Class Codes: G01R 33/3852 Gradient amplifiers; means for controlling the application of a gradient magnetic field to the sample, e.g. a gradient signal synthesizer

Abstract

Gradient fields G_x=∂B_z/∂x etc. are generated in an MRI apparatus in order to indicate the position (x, y, z) of an object pixel to be imaged. In a known MRI system the x and y gradient fields are generated by a combination of four gradient coils, two of which are intended to receive the x and y gradient signals whereas the other two coils are intended to receive a mix of the x and y gradient signals in order to enable a simpler structure of the gradient coils and easier impedance matching. According to the invention, apart from the x, y and z gradient signals, other signals are applied to a conversion system (35) in order to realize the desired gradient fields but also to achieve degrees of freedom that can be used for optimizing system parameters such as heat dissipation or the uniformity of the distribution of power among the gradient amplifiers.

Claims

1. A magnetic resonance apparatus which includes:

a gradient system (3, 7) for generating L gradient fields in a measuring volume (29) of the apparatus, which gradient system includes a number of N (N>

L) mutually independent channels (3-i, 7-i), each of which consists of a gradient amplifier (7-i), with a signal input and an output and a gradient coil (3-i) connected to the output of the gradient amplifier, and

a conversion unit (35)

provided with N outputs which are connected to the N signal inputs of the N gradient amplifiers (3-i) in a one-to-one association, and

provided with inputs, a number L of which is arranged to receive L gradient signals (G_i) 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, 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 (λ

_i) 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 (G_i) applied to the first L inputs, the N-L other signals (λ

_i) applied to the other N-L inputs into N control signals for controlling the N gradient amplifiers.

2. A magnetic resonance apparatus as claimed in

claim 1

, wherein the number of other N-L inputs equals N−

L=1.

3. A magnetic resonance apparatus as claimed in

claim 2

, wherein the gradient coils are of a similar shape.

4. A magnetic resonance imaging apparatus as claimed in

claim 3

, wherein a gradient coil is derived from an assembly of a saddle-shaped x gradient coil and a cylindrical z gradient coil in a given ratio I_x:

I_z=1;

2α

.

5. A magnetic resonance imaging apparatus as claimed in

claim 4

, wherein the other signal applied to the other input is formed by a signal of constant value.

6. A magnetic resonance imaging apparatus as claimed in

claim 4

, wherein the ratio number α

is;

α

=0.5{square root}(2β

), wherein;

β

equals (L_z/L_x)(k²_x/k²_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).

7. A magnetic resonance apparatus as claimed in

claim 1

, wherein a z gradient field (∂

B_z/∂

z) is generated by means of a cylindrical z gradient coil and an x gradient field (∂

B_z/∂

x) and a y gradient field (∂

B_z/∂

y) are generated by means of at least three further gradient channels with gradient coils which are of a similar shape.

8. A magnetic resonance apparatus as claimed in

claim 1

, wherein an x gradient field (∂

B_z/∂

x), a y gradient field (∂

B_z/∂

y) and a z gradient field (∂

B_z/∂

z) are generated by means of six gradient channels with similar gradient coils.

9. A magnetic resonance imaging apparatus as claimed in

claim 1

, wherein the number of connection cables between the gradient amplifiers and the gradient coils equals the number of gradient coils.

10. A magnetic resonance imaging apparatus as claimed in

claim 1

, wherein the number of connection cables between the gradient amplifiers and the gradient coils is one larger than the number of gradient coils.