JPH0531096A - Mr imaging device - Google Patents

Abstract

(57) [Summary] [Purpose] To shorten the echo time TE and repetition time. [Structure] The rise of the gradient magnetic field for frequency encoding,
Data sampling is performed during the entire period in which the gradient magnetic field for frequency encoding is applied, including the falling period, and the data obtained when the gradient magnetic field strength during the rising and falling periods is insufficient is the normal gradient. Convert to the data at the magnetic field strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to nuclear magnetic resonance (NM).
The present invention relates to an MR imaging apparatus that performs imaging using R).

[0002]

2. Description of the Related Art In an MR imaging apparatus, an imaging sequence for obtaining one image is performed by repeating a sequence of sequentially applying various excitation pulses and gradient magnetic field pulses. For example, in the pulse sequence by the spin echo method, the generation timing and waveform of each of a series of RF pulses and gradient magnetic field pulses are controlled as shown in FIG. 90
A pulse of a gradient magnetic field Gs in the slice thickness direction (a gradient magnetic field for slice selection) Gs is applied at the same time as the application of the ° pulse to selectively excite a specific region in the slice thickness direction, and then a 180 ° pulse is applied after time TE Is applied together with the pulse of the gradient magnetic field Gs to selectively reverse the magnetization of nuclear spins in the same specific region in the slice thickness direction by 180 °. As a result, an echo signal is generated after a time TE from the 180 ° pulse. Then, a pulse of a gradient magnetic field (gradient magnetic field for phase encoding) Gp in one direction perpendicular to the slice thickness direction is applied to encode position information in that direction into the phase of the echo signal. A gradient magnetic field Gr (frequency encoding gradient magnetic field) Gr perpendicular to the gradient magnetic field Gp is added to encode position information in that direction into the frequency of the echo signal.

In such a pulse sequence, shortening the echo time TE enables imaging with less T2 relaxation. Further, if the repetition time of the pulse sequence is shortened, the time required for the entire imaging sequence can be shortened, and higher speed imaging can be performed.

However, each gradient magnetic field requires a time of about 1 ms as a rising time for generating the maximum magnetic field, and the time required for this rising is shortened when imaging with a short echo time TE or the repetition time. This is an obstacle to high-speed imaging.

Therefore, conventionally, as shown in FIG. 3, a slice selection gradient magnetic field Gs applied at the same time as the excitation pulse.
The pulse of the phase encoding gradient magnetic field Gp and the pulse of the readout gradient magnetic field Gr are started and started in synchronization with the trailing edge of the pulse, and devised so as to alleviate the obstacle due to the time required for the rising. However, in this case, as shown in FIG. 4, the data sampling pulse is generated only during the period when the frequency encoding gradient magnetic field Gr is constant. This is because the position information is encoded into the frequency when the gradient magnetic field is applied uniformly in time, and thereby the frequency encoding amount is uniformly given at each sampling point.

[0006]

However, when data sampling is performed only when the gradient magnetic field becomes constant as in the conventional case, data collection cannot be performed during the rising or falling of the gradient magnetic field. There is a problem that the time TE and the repetition time cannot be shortened.

In view of the above, it is an object of the present invention to provide an MR imaging apparatus improved so that the echo time TE and the repetition time can be shortened by performing data sampling even during the rising and falling periods of the gradient magnetic field. And

[0008]

In order to achieve the above object, in the MR imaging apparatus according to the present invention,
Data sampling is performed during the entire period in which the frequency-encoding gradient magnetic field is applied, including the rising and falling periods of the frequency-encoding gradient magnetic field, and the data sampled during the rising and falling periods is used as the gradient magnetic field. Is characterized by performing an operation for converting into data when the gradient magnetic field is constant. In this way, the data obtained in a state where the gradient magnetic field strength during the rising and falling periods is insufficient is used as a normal gradient magnetic field. Since the data is converted to intensity data, the
Data can be collected even during the falling period, the rising and falling periods are not wasted, and the echo time TE and the repetition time can be shortened.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing an MR imaging apparatus according to an embodiment of the present invention, in which a transmission coil 12 and a reception coil 13 are attached to a subject 11, and these are superposed by a static magnetic field formed by a main magnet 15 and the static magnetic field. Is arranged in the gradient magnetic field formed by the gradient coil 14. The gradient coil 14 is configured to be able to independently generate a gradient magnetic field whose magnetic field strength is inclined in each of the three orthogonal axes. The gradient magnetic fields of the three orthogonal axes are a slice selection gradient magnetic field Gs, a phase encoding gradient magnetic field Gp, and a frequency encoding gradient magnetic field Gr, respectively. Currents are supplied to the gradient coil 14 from the respective power sources 21, 22, and 23 for the gradient magnetic fields Gs, Gr, and Gp to form gradient magnetic fields in each direction. The gradient magnetic field control device 24 controls the waveforms of the current supplied from the gradient magnetic field power supplies 21 to 23 so that the gradient coil 14 forms each gradient magnetic field pulse having a predetermined waveform.

An RF pulse sent from a high frequency power supply 33 is supplied to the transmission coil 12. This RF pulse is
In the frequency converter 32, R from the synthesizer 34
RF waveform generator 3 using the F sine wave signal as a carrier signal
The sinc waveform from 1 is AM-modulated and then amplified by the high frequency power supply 33.

NMR generated after the subject 11 is irradiated with an RF pulse from the transmission coil 12 to excite its nuclear spin.
The signal is received by the receiving coil 13. Alternatively, the transmission coil 12 and the reception coil 13 may be used in common, and a high-frequency power supply 33 on the transmission side and a preamplifier 35 on the reception side may be switched by using a signal switcher (not shown). This reception NMR
The signal is amplified by the preamplifier 35, then detected by the quadrature detector 36, converted into digital data by the A / D converter 37, and taken into the host computer 41. The quadrature phase detector 36 outputs the difference between the frequencies of the two signals by mixing the reference signal and the received signal sent from the synthesizer 34.

Under the control of the host computer 41, the sequence controller 42 gives waveform information and generation timing information of each gradient magnetic field pulse to the gradient magnetic field controller 24, and R
The sinc waveform information and the generation timing information of the RF pulse are given to the F waveform generator 31, and the synthesizer 3
The information about the frequency of the carrier signal (corresponding to the resonance frequency) is sent to 4 to control the sample timing of the A / D converter 37.

The host computer 41 has a console 4 having a display device and an input device such as a keyboard device.
3 is connected. An image is reconstructed by two-dimensional Fourier transforming the data taken in the host computer 41, and the image is displayed on the display device of the console 43.

In the MR imaging apparatus according to this embodiment, for example, the spin echo method as shown in FIG. 3 and any other imaging sequence can be used, but the data sampling pulse of the A / D converter 37 is As shown in FIG. 2, the gradient magnetic field Gr for frequency encoding is generated in all periods including the rising and falling periods, and data sampling is performed in all the periods of the Gr pulse. There is. Then, the data obtained when the gradient magnetic field strength at the rising and falling edges is insufficient as compared with the normal time is converted into data when the gradient magnetic field strength is constant, and data interpolation is then performed.

This will be described in detail. It is assumed that M pieces of data including the rising and falling periods are obtained and the data is Am. However, m = -M / 2,-
(M / 2) +1, ..., 0, 1, ..., (M / 2) -1. m is sampled from I to J when the gradient magnetic field Gr is constant, m = −M / 2 to I−1 in the rising period, and J to (M / 2) −1 in the falling period. Each shall be sampled. Further, the time when m = 0 is t = 0, the sampling time interval is T, the waveform of the gradient magnetic field Gr is G (t), and when constant, G (t) = Go. Data Bn used for image reconstruction is created from the sampled original data Am as follows. However, n = -M /
2,-(M / 2) +1, ..., 0,1, ..., (M / 2)-
It is 1.

First, the data when n is I ≦ n ≦ J is Bn = Am for each n.

Next, when K is expressed by the following equation,

[Equation 1] For each n where n is J + 1 ≦ n ≦ K,
The following formula

[Equation 2] If there is m for which the equal sign holds, then Bn = Am, and if the equal sign does not hold,

[Equation 3] To obtain Bm.

When L is expressed by the following equation,

[Equation 4] For each n where n is L ≦ n ≦ I−1,
The following formula

[Equation 5] If there is m for which the equal sign holds, then Bn = Am, and if the equal sign does not hold,

[Equation 6] To obtain Bm.

Further, the data of n is n <L or n> K, and n <L is obtained by internally dividing the data of BL and BK.
In the case of L, Bn = [BL × (M−K + n) + BK × (L−n)] / (M−K + L), and in the case of n> K, Bn = [BL × (n−K) + BK × (M + L−n) )] / (M−K + L).

In the case of this embodiment, these calculations are performed by the host computer 41, and if the obtained data Bn is used to reconstruct an image by the host computer 41, sampling is performed when the conventional gradient magnetic field becomes constant. An image having an image quality comparable to the reconstructed image can be obtained from the obtained data.

[0021]

As described in the above embodiments, according to the MR imaging apparatus of the present invention, it becomes possible to collect data even during the rising and falling periods of the gradient magnetic field, and The period is not wasted in time, and the echo time TE
Or the repetition time can be shortened. In this way, because it becomes possible to image with a shorter echo time TE,
Imaging with less T2 relaxation is possible. Moreover, since the repetition time of the pulse sequence can be further shortened, the time required for the entire imaging sequence can be shortened, and higher-speed imaging can be performed. Further, the required number of data can be obtained in a short time while using the sampling pulse of the originally required frequency without increasing the sampling frequency.

[Brief description of drawings]

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

FIG. 2 is a time chart showing data sampling according to the embodiment.

FIG. 3 is a time chart showing a pulse sequence according to a conventional example.

FIG. 4 is a time chart showing data sampling according to a conventional example.

[Explanation of symbols]

 11 Subject 12 Transmitting coil 13 Receiving coil 14 Gradient coil 15 Main magnet 21 Slice selection gradient magnetic field power supply 22 Frequency encoding gradient magnetic field power supply 23 Phase encoding gradient magnetic field power supply 24 Gradient magnetic field control device 31 RF waveform generator 32 Frequency converter 33 high-frequency power supply 34 synthesizer 35 preamplifier 36 quadrature detector 37 A / D converter 41 host computer 42 sequence controller 43 console

Claims (1)

1. A means for generating an excitation pulse, a means for generating a gradient magnetic field for slice selection, a means for generating a gradient magnetic field for phase encoding, and a means for generating a gradient magnetic field for frequency encoding. And means for performing data sampling in the entire period in which the frequency encoding gradient magnetic field is applied, including the rising and falling periods of the frequency encoding gradient magnetic field, and the data sampled in the rising and falling periods, An MR imaging apparatus, comprising: an arithmetic means for converting data when the gradient magnetic field is constant.