JP2638412B2 - MR imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR imaging apparatus for performing imaging by utilizing an NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging apparatus for obtaining an image by a fast spin echo method.

[0002]

2. Description of the Related Art Conventionally, in a MR imaging apparatus, one nutation RF pulse and a plurality of subsequent refocusing RF pulses are applied to a subject to sequentially generate echo signals, and each of the echo signals differs. There is known a high-speed spin echo method in which data of a plurality of lines in a raw data space is collected at once by performing phase encoding, and the number of repetitions of a nutation RF pulse is reduced to speed up imaging.

In this high-speed spin echo method, usually, the order of phase encoding is determined so that the data of each line is sequentially located symmetrically outward from the center in the phase encoding direction in the raw data space. The correction is performed by subtracting the estimated value of the DC offset obtained in advance by some method from each data.

[0004]

However, when the DC offset is simply corrected by subtracting the estimated value of the DC offset from each data as described above, if the estimated value of the DC offset is not accurately obtained, the error may occur. Appear as point noise at the center of the image, and it is difficult to estimate a DC offset value with such accuracy that point noise does not occur.

In view of the above, it is an object of the present invention to provide an MR imaging apparatus improved so that DC noise can be effectively removed.

[0006]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
RF transmitting means for sequentially generating a plurality of echo signals by irradiating a subject placed in a static magnetic field with one nutation RF pulse and a plurality of subsequent refocusing RF pulses, and a gradient magnetic field pulse for slice selection Generating means, readout and frequency encoding gradient magnetic field pulse generating means, phase encoding gradient magnetic field pulse generating means corresponding to each step of changing the phase encoding amount, and receiving the echo signal to obtain data Means, data acquisition means for adding data obtained from echo signals having the same phase encoding amount to acquire data, and discrete time of phase encoding amount changing step within one repetition period starting from one nutation RF pulse. Each of the typical multiple steps is applied to each echo signal, and in the next repetition period, the discrete All steps are repeated by applying each of a plurality of discrete steps adjacent to each of several steps to each echo signal, and a cycle corresponding to all the steps is repeated a plurality of times. In each repetition period, the phase of the carrier of the nutation RF pulse is alternately inverted, and the phase of the carrier of the nutation RF pulse is alternately inverted during each cycle. Means for controlling data added from echo signals of the same phase encoding amount to data obtained from the echo signal generated by the nutation RF pulse, by adding signs corresponding to the positive and negative signs thereof. It is a feature.

[0007]

When a plurality of echo signals are sequentially generated by irradiating the subject with one nutation RF pulse and a plurality of subsequent refocusing RF pulses, the phase of each of the plurality of echo signals is different. By applying the encoding amount, data of a plurality of lines can be collected in one repetition period. Then, within the one repetition period, each of a plurality of discrete steps of the phase encode amount change step is applied to each echo signal, and in the next repetition period, each of the plurality of discrete steps in the previous repetition period is applied. All steps are performed such that each of a plurality of adjacent discrete steps is applied to each echo signal, and a cycle corresponding to all the steps is repeated a plurality of times. In this one cycle, the phase of the carrier of the nutation RF pulse is alternately inverted for each repetition period. Also,
Even during each cycle, the phase of the carrier of the nutation RF pulse is alternately inverted. Then, a code corresponding to the sign of the carrier phase is attached to the data obtained from the echo signal generated by the nutating RF pulse having the positive and negative carrier phases, and the data obtained from the echo signals having the same phase encoding amount are added to each other. to add. The signal component included in the data is inverted correspondingly when the phase of the carrier wave is inverted.However, the DC offset is mainly caused by the drift of the signal detection system, etc., so the polarity is the same regardless of the phase of the carrier wave. Becomes Therefore, for example, data obtained from an echo signal generated by a nutation RF pulse having a positive carrier phase has a positive sign (or the opposite sign), and a nutation R having a negative carrier phase.
If the data obtained from the echo signal generated by the F pulse is given a negative sign (or the opposite sign), and if the data obtained from the echo signals of the same phase encoding amount are added together, the Are added in the same phase by coding, and the DC offset components are opposite in polarity by the coding and cancel each other. When this cycle is performed an even number of times, the DC offset components are all cancelled. When the cycle is performed an odd number of times, the last (odd-numbered) DC offset component remains without being canceled out, but the signal components are added and increased. Therefore, it is relatively equivalent to reducing the DC offset component, and
Since the DC offset component is alternately inverted for each line in the raw data space, the DC offset component becomes the highest frequency component by the two-dimensional Fourier transform, and the influence on the reconstructed image can be reduced. In other words, the effect of reducing the imaging time by collecting data of a plurality of lines in one repetition period while suppressing the DC offset component can be improved.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, a magnet 1 is for generating a static magnetic field, and a subject (not shown) is arranged in the static magnetic field, and a probe 2 is attached to the subject. A gradient magnetic field is superimposed on this static magnetic field by a gradient magnetic field coil not shown in the figure. A signal having a resonance frequency is generated from the high-frequency oscillator 3. The high-frequency signal is a carrier wave of a high-frequency excitation pulse such as a 90 ° pulse or a 180 ° pulse. The carrier is phase-shifted by the phase shifter 4 and sent to the probe 2 via the transmission amplifier 5 so that the high-frequency excitation pulse A pulse is emitted. A gradient magnetic field pulse is applied to the subject together with the high frequency excitation pulse, but the gradient magnetic field generation system is omitted.

An NMR signal generated from a subject is received by a probe 2, detected by a phase detector 8 via a receiving amplifier 7, converted into digital data by an A / D converter 9, and transmitted to an addition memory 10. And collect data. The collected data is processed by the image reconstruction calculation device 11,
The reconstructed image is displayed on the display 12. The control unit 6 controls the phase shifter 4 and adds
And the image reconstruction operation device 11, and also controls the A / D converter 9, the display 12, and the like. Further, the waveform and timing of the high frequency pulse and the gradient magnetic field pulse are also controlled by the control unit 6.

In such an MR imaging apparatus, a pulse sequence of the fast spin echo method as shown in FIG. 2 is performed. In this embodiment, one nutation pulse (90 ° pulse) has four refocusing pulses (180 pulses).
(エ コ ー pulse) to obtain four echo signals. Z
A pulse of Gz, which is a gradient magnetic field in the direction, is applied simultaneously with each of the high-frequency pulses as a pulse for slice selection.
The pulse of the gradient magnetic field Gx in the X direction is used to encode readout and position information in the X direction into a frequency. The pulse of the gradient magnetic field Gy in the Y direction is for encoding the position information in the Y direction into a phase. The Gy pulse is applied before each echo signal so that different amplitudes are obtained for each echo signal so as to obtain different lines of data from each of the echo signals. After each echo signal, a Gy pulse having the same waveform as the Gy pulse before the echo signal and having a different polarity is added and rewinding is performed (the effect of the Gy pulse for phase encoding before the echo signal is removed).

In this embodiment, as shown in FIG. 3, data is collected for the first to 256th lines. The data is collected on the positive side (upper side in the figure) and the negative side (lower side in the figure) from the center in the phase encoding direction. After the data collection on the negative side is completed in four blocks, the data on the positive side is collected. First, in the first repetition period (a period in which four echoes starting with one nutation pulse are obtained), the first echo signal
Gy so that data of the 33rd line is collected from the second echo signal, data of the 65th line is collected from the third echo signal, and data of the 97th line is collected from the fourth echo signal. Determine the pulse amplitude. In the next repetition period, data of the second, 34th, 66th, and 98th lines are collected, respectively. In this way, data for each line of blocks 1, 2, 3, and 4 are simultaneously collected in one repetition period, and all data collection on the negative side is completed in 32 repetition periods. After that, on the positive side, the first, second, third,
From the fourth echo signal, the 225th, 193rd, and 1st
Starting from collecting the data of the 61st and 129th lines, the data of each line of each of the blocks 8, 7, 6, and 5 are simultaneously collected in each repetition period, and all the positive side data are collected in the 32 repetition periods. End data collection.

The phase of the 90 ° pulse applied at the beginning of such a large number of repetition periods is alternately inverted by 180 ° by controlling the phase shifter 4 by the control unit 6. For example, in the first repetition period, the phase of the carrier is excited in the X direction as positive, and in the second repetition period, the phase is inverted by 180 ° and excited in the −X direction as negative, repeated 64 times. Data of the first to 256th lines required for image reconstruction is collected. The data of each line is arranged at the corresponding position in the raw data space while the sign of the data is inverted between the odd number and the even number. As a result, the phase of the DC offset is inverted between the odd-numbered line and the even-numbered line of each of the first to 256th lines. On the other hand, the phases of the signal components have the same polarity. This is because the DC offset component is mainly caused by the drift of the signal detection system or the like, so that it has the same polarity regardless of the phase of the carrier of the excitation signal, whereas the signal component is the phase of the carrier of the excitation signal. Is inverted in response to

Next, the same sequence is repeated to collect again the data of the first to 256th lines necessary for image reconstruction. In this case, the carrier wave of the 90 ° pulse is repeated in an odd number of repetition periods. Is negative, and is positive during an even number of repetition periods. Then, the data of the same line is integrated by the addition memory 10. Such first
The odd-numbered or even-numbered integration of data is performed by repeating the data collection of the 256th to 256th lines odd or even times.

According to the integration of the data even-numbered times, the DC offset is canceled by the odd-numbered times and the even-numbered times, so that the DC noise is completely removed. Also,
Even if the data is integrated an odd number of times, the influence of the DC offset on the reconstructed image can be small. Since the DC offset is inverted for each line arranged in the raw data space as shown in FIG. 3, it becomes the highest frequency component by two-dimensional Fourier transform, and appears at both ends of the image in the phase encoding direction (Y direction). This is because the image of the subject of interest located in the section is less affected.

Note that, as described above, the data collection on the positive side (or the negative side) is not performed after all the data collection on the negative side (or the positive side) is completed, but the data collection on the negative side (or the positive side) is performed. Data collection by two repetition periods,
After collecting data for two adjacent lines, such as lines 1, 2, 33, 34, 65, 66, and 97, 98, the process proceeds to data collection on the positive side (or negative side). , 225, 226 lines, 1st data collection by performing two repetition periods on the positive side (or the negative side).
93, 194 line, 161st, 162th line, 12th line
It is also possible to collect data of two adjacent lines, such as 9, 130 lines, and then repeat the operation of moving to the negative side again. Also in this case, by inverting the phase of the carrier of the 90 ° pulse for each repetition period,
Similarly to the above, the phase of the DC offset can be alternately inverted for each line. Furthermore, in these, the first
Each of the fourth echo signals from the echo signals is allocated from the end to the center in the phase encoding direction. This is because a signal generated by a long TE is arranged in the center of the raw data space, so that a T2-weighted image is obtained. To get Therefore, when obtaining a proton density weighted image, a short T
Since the signal generated in E needs to be located at the center of the raw data space, the first to fourth echo signals are conversely shifted from the center to the end in the phase encoding direction. To be assigned.

The NMR signal can be encoded by controlling the phase of the reference signal sent to the phase detector 8 or on the addition memory 10 when adding to the addition memory 10 after A / D conversion. It can also be done digitally.

Further, in the above description, four nutation RF pulses are used.
Although three echo signals are generated, the number of echo signals may be three or less or five or more. The number of lines is not limited to 256, and the division into blocks is not limited to eight. The relationship between these numbers can be variously changed without departing from the spirit of the present invention.

[0018]

According to the MR imaging apparatus of the present invention, each of a plurality of discrete steps of the phase encode amount change step is applied to each echo signal within one repetition period starting from one nutation RF pulse. In the next repetition period, all steps are repeated by applying each of the plurality of discrete steps adjacent to each of the plurality of discrete steps in the previous repetition period to each echo signal. Is repeated a plurality of times, and the phase of the carrier of the nutation RF pulse is alternately inverted for each repetition period within the one cycle, and the nutation RF pulse is alternately changed during each cycle. The phase of the carrier wave is inverted and obtained from the echo signal generated by the nutating RF pulse of the positive and negative carrier phases. Since the data obtained by adding the signs corresponding to the positive and negative signs to the data and adding the data obtained from the echo signals of the same phase encoding amount are added, a high-speed spin that collects data of a plurality of lines by one nutation pulse is used. The DC noise can be effectively removed at the same time while shortening the imaging time by using the echo method.

[Brief description of the drawings]

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

FIG. 2 is a time chart showing a pulse sequence of the embodiment.

FIG. 3 is a view showing a raw data space in the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Magnet 2 Probe 3 High frequency oscillator 4 Phase shifter 5 Transmission amplifier 6 Control part 7 Receiving amplifier 8 Phase detector 9 A / D converter 10 Addition memory 11 Image reconstruction arithmetic unit 12 Display

Claims (1)

(57) [Claims] 1. An RF transmitter for irradiating a subject placed in a static magnetic field with one nutation RF pulse and a plurality of subsequent refocusing RF pulses to sequentially generate a plurality of echo signals, and a slice. Selecting gradient magnetic field pulse generating means,
Readout and frequency encoding gradient magnetic field pulse generating means, means for generating a phase encoding gradient magnetic field pulse corresponding to each of the phase encoding amount change step, receiving means for receiving echo signals and obtaining data,
A data collecting means for adding data obtained from echo signals having the same phase encoding amount to collect data, and a plurality of discrete phase encoding amount changing steps within one repetition period starting from one nutation RF pulse. Applying each of the steps to each echo signal, and in the next repetition period, repeatedly applying each of the plurality of discrete steps adjacent to each of the plurality of discrete steps in the previous repetition period to each echo signal. All steps are performed, a cycle corresponding to all the steps is repeated a plurality of times, and the phase of the carrier of the nutation RF pulse is alternately inverted for each repetition period within the above-described one cycle. The phase of the carrier wave of the nutation RF pulse is alternately inverted, and the nutation of the positive and negative carrier wave phases is alternately performed. Means for controlling the data obtained from the echo signal generated by the F-pulse so that the data obtained from the echo signals having the same phase encode amount are added after adding signs corresponding to the positive and negative signs. MR imaging apparatus.