JPH0422338A - Mr device - Google Patents

Abstract

PURPOSE:To obtain the eddy current correcting amount hn(t) through digital computation by furnishing a digital computing means which computes the eddy current correcting amount using a specific recurrence formula. CONSTITUTION:A pulse sequence generator 61 of a sequence controller emits the gradient fundamental amount f(t) on the basis of a command given by a calculator. A digital eddy current correcting circuit 22 receives the time constant taun and weighting constant alphan (where (n) is 1, 2,<,M) from a computer, and on the basis thereof calculates the eddy current correcting amount hn(t) through digital computation, and emits the gradient drive amount h(t) in addition to the gradient fundamental amount f(t). This circuit 22 is formed, for ex., using a DSP(digital signal processor). The alphan is weighting constant, taun is time constant, and DELTAt is output data rate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] 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 j Fig. 4 is a block diagram showing an example of a conventional MR apparatus.

In this MR device 51, the computer 52 is connected to the operation console 13.
Controls overall operation based on instructions from

The sequence controller 53 operates the magnetic field drive circuit 54 based on the stored sequence, and causes the static magnetic field coil and the gradient magnetic field coil of the magnet assembly 5 to generate a static magnetic field and a gradient magnetic field.

It also controls the gate modulation circuit 7 to modulate the RF multiplied signal generated by the RF oscillation circuit 6 and applies it to the transmitting coil of the magnet assembly 5 from the RF power amplifier 8 .

NMR obtained with the receiving coil of magnet assembly 5
The signal is input to the phase detector 10 via the preamplifier 9, and further input 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 magnetic field drive circuit 54 in detail from the viewpoint of gradient magnetic field drive.

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

The analog eddy current correction circuit 63 of the magnetic field drive circuit 54 performs an analog addition of an eddy current correction amount hn(t) (where n=1.2...,M) to the gradient basic amount f(t). , outputs the gradient driving amount h (t). This gradient driving force h (
t), the gradient coil driver 64 drives the gradient coils of the magnet assembly 5.

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
72 is a gain adjustment section, and 72 is an offset adjustment section. In this example, M=3.

FIG. 7 generally shows the analog eddy current correction circuit 63, where the gradient drive amount h (t) is the gradient basic amount f
(t) using multiple CR differentiators (with a time constant of 1°τ2.
..., τM) with 1 weighting (weighting is constant α1.α
2. ..., αM) multiple eddy current correction amounts hl(t)
, h2(t),..., hM(t) to the gradient basic quantity f(t). That is, .

Here, hn(t) is obtained as follows.

First, since the transfer function G (s) of the system in Fig. 7 is expressed as
), and if the Laplace transform of the gradient driving amount h (t) is H (s), then H (s) = G (s) ・F (s)
−■. Therefore, h(t)=j−' [G(s)・F(s)]=g(
t) * f (t) = f2g(x)・f (t−x)dx −・■. For simplicity, considering only one component of the eddy current correction amount, gn(t)=f-' [αn-, s/(s+1/rn)
]t = αn・ (δ (t) −exp[L' 1 mouth)
...■τ n . Therefore, xf(t-x)dx·f(t-x)dx...■.

As related prior art, Japanese Patent Application Laid-Open No. 62-189057
There is a technique disclosed in Japanese Patent Application No. 63-234487.

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

However, correction using the analog method has problems in that it is limited in terms of reproducibility stability, settable time constant 9 weighting constant range, etc.

Therefore, it is possible to perform eddy current correction in a digital manner by digitally calculating the above formula It is conceivable that the maximum time constant (for example, 2 seconds) and the output data rate (for example, 20 μsee/data) will exceed 100K points, which is not realistic.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an MR apparatus in which an eddy current correction amount can be obtained by digital calculation in a practical processing time by using an approximate recurrence formula.

[Means for Solving the Problems] The MR device of the present invention uses gradient drive in which an eddy current correction amount hn(t) (where n=1.2.-, M) is added to a basic gradient amount f(t). In an MR device that drives a gradient coil with an amount h(t), the eddy current correction amount hn(t) is calculated using the following recurrence formula eddy current correction amount hn(t), and the following recurrence formula hn(t)= −
・an・ (1+exp[-Δt/rn])X(f(t
)-f(t-Δ1)) +exp[-Δt/rn(t-Δt)(However,
(an 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 h n (t) is calculated using the above recurrence formula, but digital calculation of this approximate recurrence formula requires a long processing time like convolution calculation. Therefore, the eddy current correction amount h n(t) can be obtained within a practical processing time.

[Example] Hereinafter, the present invention will be described in more detail based on the example shown in the drawings. Note that this invention is not limited to this.

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

In this MR system [1, the computer 2 controls the overall operation based on instructions 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 with the static magnetic field coil and the gradient magnetic field coil of the magnet assembly 5.
Generate a gradient magnetic field. It also controls the gate modulation circuit 7 and modulates the RF signal generated by the RF oscillation circuit 6 to
F power amplifier 8 is added to the transmitting coil of magnet assembly 5.

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

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

Pulse sequence generator 6 of sequence controller 3
1 is the gradient basic quantity f (
t) is output.

The digital eddy current correction circuit 22 has a time constant τ determined by the computer 2.
n and weighting are constant αn (however, n=1゜2,...
, M), and based on these, the eddy current correction amount hn
(t) is calculated by digital calculation, and the gradient basic amount f
In addition to (t), the gradient drive amount h (t) is output.

This digital eddy current correction circuit 22 is, for example, a DSP (
It is configured using a digital signal processor). Details of its 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 .

Gradient coil driver 64 drives the gradient coils of magnet assembly 5 .

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

The digital eddy current correction circuit 22 has a time constant τ determined by the computer 2.
If n and weighting are constant αn, then β
Calculate n.

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

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

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

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

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

In step S4, n==0.

In step S5, n is incremented.

In step S6, the following equation is calculated.

hn(t)=-・an・(1+βn)X(f(t)
-f(t-Δ1)) +βn-hn(t-Δt)... ■In the above equation, f(t-Δt) and hn(-Δt) are the previous gradient basic amount and eddy current correction amount. be.

The above equation 0 is derived as follows.

From the above equation 0, by setting an - f (t) -, hn (t) x = y + Δt, the first term Assuming that the quantity f (t) does not change significantly, the second term τ n is then: h n (t) = a n - f (t) - first term - second term, and f (t - Δt /2) 輯(f (t) + f (−Δt
))'/2 and rearranging, the above equation 0 is obtained.

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

In step S8, the gradient drive amount h (
t), the eddy current correction amount hn(
t) to obtain a new gradient drive amount h (t).

In step S9, it is checked whether n has reached M. If M has not been reached, the process returns to step S5 and steps 56 to $8 are repeated.

By repeating this process, the eddy current correction amount hl(
t), h2(t), . . . h M(t) will be added to the gradient basic quantity f (t). If n exceeds M, the process advances to step S10.

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

As can be understood from the above explanation, the eddy current correction amount h n(t) is calculated in step S6, but this operation involves only three additions and subtractions and four 1 multiplications and divisions (actually, an
By calculating (1+βn)/2 in advance, only two additions and subtractions and two multiplications are required), so the processing time is much shorter than that of convolution operations. Therefore, in the MR apparatus 1, the eddy current correction amount can be calculated by digital calculation and the eddy current correction can be performed in a practical calculation time. Therefore, characteristics superior to conventional analog systems can be obtained in terms of reproducibility, stability, and range of correction.

Another example is one in which the pulse sequence generator 61 performs the function of the digital eddy current correction circuit 22 by using the calculation function of the pulse sequence generator 61. In addition, 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 slope drive amount. There are things that do.

[Effects of the Invention] According to the MR apparatus of the present invention, the eddy current correction amount can be calculated by digital calculation within a practical processing time. This makes it possible to perform eddy current correction using a digital method, resulting in improved reproducibility and stability.

This is superior to the conventional method in terms of the range of correction.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an MR device according to an embodiment of the present invention.
Figure 2 is a detailed block diagram of the main parts of the sequence controller and magnetic field drive circuit shown in Figure 1, Figure 3 is a flowchart of the operation of the digital eddy current correction circuit shown in Figure 2, and Figure 4 is a diagram of the conventional MR device. An example block diagram, FIG. 5 is a detailed block diagram of main parts of the sequence controller and magnetic field drive circuit shown in FIG. 4, FIG. 6 is a circuit diagram of a specific example of the analog eddy current correction circuit shown in FIG. 5, and FIG. is a general model diagram of an eddy current correction circuit. (Explanation of symbols) l...MR device 2...Computer 3...
Sequence controller 4...Magnetic field drive circuit 22...Digital eddy current correction circuit 61...Pulse sequence generator. Applicant Yokogawa Medical System Co., Ltd. Agent
To patent attorney Shinshibe Aruchika■

Claims (1)

[Claims] 1. Gradient driving amount h (where n=1, 2, ..., M) is the sum of the basic gradient amount f(t) and the eddy current correction amount hn(t) (where n=1, 2, ..., M).
t), the eddy current correction amount hn(t) is calculated using the following recurrence formula hn(t)=1/2・αn・(1+exp[−Δt/τ
n])×(f(t)-f(t-Δt)) +exp[-Δt/τn]・hn(t-Δt) (However,
αn is a weighting constant, τn is a time constant, and Δt is an output data rate.