JP3040426B2 - MR device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an MR device, and more particularly, to an MR device that performs eddy current correction by digital calculation.

[Prior Art] FIG. 4 is a block diagram showing an example of a conventional MR apparatus.

In the MR device 51, the computer 52 controls the entire operation based on an instruction from the console 13.

The sequence controller 53 activates the magnetic field drive circuit 54 based on the stored sequence, and generates a static magnetic field and a gradient magnetic field using the static magnetic field coil and the gradient magnetic field coil of the magnet assembly 5. Further, it controls the gate modulation circuit 7 and modulates the RF signal generated by the RF oscillation circuit 6 to generate an RF signal.
The power is applied from the power amplifier 8 to the transmission coil of the magnet assembly 5.

NMR obtained with the receiving coil of the magnet assembly 5
The signal is input to the phase detector 10 via the preamplifier 9 and further to the computer 52 via the AD converter 11.

The computer 52 calculates image data based on the NMR signal data obtained from the AD converter 11, and displays the image on the display device 12.

FIG. 5 is a block diagram showing the sequence controller 53 and the magnetic field drive circuit 54 in detail from the viewpoint of gradient magnetic field drive.

Pulse sequence generator of sequence controller 53
61 is a gradient basic amount f based on a command from the computer 52.
(T) is output as a digital quantity. The DA converter 62 converts the gradient basic amount f (t) into an analog amount and outputs the analog amount to the magnetic field drive circuit 54.

The analog eddy current correction circuit 63 of the magnetic field drive circuit 54 adds the eddy current correction amount hn (t) (where n
= 1, 2,..., M) and the gradient drive amount h (t)
Is output. The gradient coil driver 64 drives the gradient coil of the magnet assembly 5 based on the gradient driving force h (t).

FIG. 6 shows a specific example of the analog eddy current correction circuit 63, where 73 is the main body of the eddy current correction circuit. 71 is a gain adjustment unit, and 72 is an offset adjustment unit. In this example, M = 3.

FIG. 7 generally shows the analog eddy current correction circuit 63, wherein the gradient drive amount h (t) is a gradient basic amount f.
(T) is replaced by a plurality of CR differentiating circuits (time constants τ1, τ2, ..., τM)
, A plurality of eddy current correction amounts h1 (t), h2 (t),..., HM (t) weighted (weighting constants α1, α2,..., ΑM)
Is added to the gradient basic amount f (t). That is, It is.

 Here, hn (t) is obtained as follows.

First, the transfer function G (s) of the system in FIG. , The Laplace transform of f (t) is
Assuming that (s) and the Laplace transform H (s) of the gradient drive amount h (t), H (s) = G (s) · F (s). Therefore, Is required. For simplicity, considering only one component of the eddy current correction amount, Becomes Therefore, Becomes

Related prior arts include the techniques disclosed in JP-A-62-189057 and Japanese Patent Application No. 63-234487.

[Problem to be Solved by the Invention] In the above-described conventional MR device 51, eddy current correction is performed by an analog eddy current correction circuit 63.

However, the correction by the analog method has a problem that it is limited in terms of reproducibility, stability, a settable time constant, a range of a weighting constant, and the like.

Therefore, it is conceivable to perform eddy current correction in a digital manner by digitally calculating the above equation and the above equation, but the calculation of the equation includes a convolution operation, and the number of terms is the longest time constant of the eddy current. (For example, 2 seconds)
And output data rate (for example, 20μsec / data)
It may be more than 100K points, which is not realistic.

Accordingly, an object of the present invention is to provide an MR apparatus that obtains an eddy current correction amount by digital calculation in a practical processing time by using an approximate recurrence formula.

[Means for Solving the Problems] The MR apparatus of the present invention provides a gradient drive in which an eddy current correction amount hn (t) (where n = 1, 2,..., M) is added to a gradient basic amount f (t). In the MR device that drives the gradient coil with the amount h (t), the eddy current correction amount hn (t) is represented by the following recurrence formula The eddy current correction amount hn (t) is represented by the following recurrence formula (Where αn is a weighting constant, τn is a time constant, and Δt is an output data rate).

[Operation] In the MR apparatus of the present invention, the eddy current correction amount hn (t) is calculated by the above recurrence formula. However, the digital calculation of this approximate recurrence formula does not require a long processing time unlike the convolution calculation. Eddy current correction amount hn within a practical processing time
(T) can be obtained.

[Example] Hereinafter, the present invention will be described in more detail based on an example shown in the drawings. It should be noted that the present invention is not limited by this.

FIG. 1 is a block diagram showing an embodiment of an MR apparatus according to the present invention.

In the MR device 1, the computer 2 controls the entire operation based on an instruction from the console 13.

The sequence controller 3 operates the magnetic field drive circuit 4 based on the stored sequence, and generates a static magnetic field and a gradient magnetic field using the static magnetic field coil and the gradient magnetic field coil of the magnet assembly 5. Further, it controls the gate modulation circuit 7 and modulates the RF signal generated by the RF oscillation circuit 6 to generate an RF signal.
The power is applied from the power amplifier 8 to the transmission coil of the magnet assembly 5.

NMR obtained with the receiving coil of the magnet assembly 5
The signal is input to the phase detector 10 via the preamplifier 9 and further to the computer 2 via the AD converter 11.

The computer 2 calculates image data based on the data of the NMR signal obtained from the AD converter 11 and displays the image on the display device 12.

FIG. 2 is a block diagram showing the sequence controller 3 and the magnetic field drive circuit 4 in the MR device 1 in detail from the viewpoint of gradient magnetic field drive.

Pulse sequence generator of sequence controller 3
61 is a gradient basic amount f based on a command from the calculator 2.
(T) is output.

The digital eddy current correction circuit 22 calculates the time constant τ
n and a weighting constant αn (where n = 1, 2,..., M), and based on these, an eddy current correction amount hn (t) is calculated by digital calculation, and added to the gradient basic amount f (t). Thus, the gradient driving amount h (t) is output. The digital eddy current correction circuit 22 is configured using, for example, a DSP (digital signal processor). The details of the operation will be described later with reference to FIG.

The gradient drive amount h (t) output from the digital eddy current correction circuit 22 is converted into an analog amount by the DA converter 23 of the magnetic field drive circuit 4 and input to the gradient coil driver 64.

The gradient coil driver 64 drives the gradient coil of the magnet assembly 5.

Next, the operation of the digital eddy current correction circuit 22 will be described in detail.

The digital eddy current correction circuit 22 calculates the time constant τ
When n and the weighting constant αn are received, β
Calculate n.

Here, the output data rate Δt is set in advance.

Then, the operation shown in the flowchart of FIG. 3 is executed at every output data rate Δt time.

In step S1, the gradient basic amount f (t) input from the pulse sequence generator 61 is read.

In step S2, the input gradient basic amount f (t) is provisionally set as the gradient drive amount h (t).

In step S3, the current gradient basic amount f (t) is stored for use in the next calculation.

 In step S4, n = 0.

 In step S5, n is incremented.

 In step S6, the following calculation is performed.

In the above equation, f (t−Δt) and hn (t−Δt) are the previous gradient basic amount and the eddy current correction amount.

 The above equation is derived as follows.

From the above equation, By setting x = y + Δt, Assuming that Δt is sufficiently small and that the gradient basic amount f (t) does not change significantly during that time, Therefore, hn (t) ≒ αn · f (t) −first term−second term, where f (t−Δt / 2) ≒ (f (t) + f (t−Δt)) / 2 By substituting and rearranging, the above equation is obtained.

Returning to FIG. 3, in step S7, the current eddy current correction amount
hn (t) is stored for use in the next calculation.

In step S8, the already calculated gradient drive amount h
(T) shows the eddy current correction amount hn calculated in step S6.
(T) is added to obtain a new gradient drive amount h (t).

In step S9, it is checked whether or not n has reached M. If M has not been reached, the process returns to step S5, and steps S6 to S8 are repeated. By repeating this, the eddy current correction amounts h1 (t), h of all the components are obtained.
.., HM (t) are added to the gradient basic amount f (t). When n exceeds M, the process proceeds to step S10.

 In step S10, the gradient drive amount h (t) is output.

As will be understood from the above description, the eddy current correction amount hn (t) is calculated in step S6, but this calculation involves three additions and subtractions and only four multiplications and divisions (actually, αn · ( 1
(+ Βn) / 2 is calculated in advance, so that addition and subtraction are performed twice and multiplication is performed only twice), so that processing can be performed in a much shorter time than in a convolution operation. Therefore, in the MR device 1, the eddy current correction amount can be calculated by digital calculation in a practical calculation time, and the eddy current correction can be performed. Therefore, characteristics superior to the conventional analog system can be obtained in terms of reproducibility, stability, a range that can be corrected, and the like.

In another embodiment, the pulse sequence generator 61 executes the function of the digital eddy current correction circuit 22 using the arithmetic function of the pulse sequence generator 61. In another embodiment, in addition to calculating the eddy current correction amount by digital calculation, the gain correction amount and offset correction amount are also calculated by digital calculation, and the sum of these is output as the gradient drive amount. To do.

[Effect of the Invention] According to the MR apparatus of the present invention, the eddy current correction amount can be calculated by a digital calculation within a practical processing time. For this reason, eddy current correction can be performed by a digital method, and the eddy current correction is more excellent in terms of reproducibility, stability, and the range in which correction is possible.

[Brief description of the drawings]

FIG. 1 is a block diagram of an MR apparatus according to an embodiment of the present invention, FIG. 2 is a detailed block diagram of a main part of a sequence controller and a magnetic field driving circuit shown in FIG. 1, and FIG. 3 is a digital vortex shown in FIG. Flowchart of operation of current correction circuit, fourth
FIG. 5 is a block diagram of an example of a conventional MR apparatus, FIG. 5 is a detailed block diagram of a main part of a sequence controller and a magnetic field drive circuit shown in FIG. 4, and FIG. FIG. 7 is a general model diagram of an eddy current correction circuit. (Description of reference numerals) 1 ... MR device, 2 ... Computer 3 ... Sequence controller 4 ... Magnetic field drive circuit 22 ... Digital eddy current correction circuit 61 ... Pulse sequence generator.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-62-152444 (JP, A) JP-A-62-27932 (JP, A) JP-A-62-189057 (JP, A) 173531 (JP, A) JP-A-2-211124 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 G01N 24/00

Claims (1)

(57) [Claims] 1. An eddy current correction amount hn is added to a gradient basic amount f (t).
(T) (however, n = 1, 2,..., M) plus the gradient drive amount h
In the MR device that drives the gradient coil in (t), the eddy current correction amount hn (t) is calculated by the following recurrence formula. (Where, αn is a weighting constant, τn is a time constant, and Δt is an output data rate) An MR apparatus comprising digital calculation means.